TRAM
Internet Engineering Task Force (IETF)                 M. Petit-Huguenin
Internet-Draft
Request for Comments: 8489                            Impedance Mismatch
Obsoletes: 5389 (if approved)                                             G. Salgueiro
Intended status:
Category: Standards Track                                          Cisco
Expires: September 22, 2019
ISSN: 2070-1721                                             J. Rosenberg
                                                                   Five9
                                                                 D. Wing
                                                                  Citrix
                                                                 R. Mahy
                                                            Unaffiliated
                                                             P. Matthews
                                                                   Nokia
                                                          March 21,
                                                          September 2019

               Session Traversal Utilities for NAT (STUN)
                       draft-ietf-tram-stunbis-21

Abstract

   Session Traversal Utilities for NAT (STUN) is a protocol that serves
   as a tool for other protocols in dealing with Network Address
   Translator (NAT) NAT traversal.  It can
   be used by an endpoint to determine the IP address and port allocated
   to it by a NAT.  It can also be used to check connectivity between
   two endpoints, endpoints and as a keep-alive protocol to maintain NAT bindings.
   STUN works with many existing NATs, NATs and does not require any special
   behavior from them.

   STUN is not a NAT traversal solution by itself.  Rather, it is a tool
   to be used in the context of a NAT traversal solution.

   This document obsoletes RFC 5389.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for a maximum publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of six months RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."
   This Internet-Draft will expire on September 22, 2019.
   https://www.rfc-editor.org/info/rfc8489.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5   4
   2.  Overview of Operation . . . . . . . . . . . . . . . . . . . .   6   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   8   7
   4.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   8   7
   5.  STUN Message Structure  . . . . . . . . . . . . . . . . . . .  10   9
   6.  Base Protocol Procedures  . . . . . . . . . . . . . . . . . .  12  11
     6.1.  Forming a Request or an Indication  . . . . . . . . . . .  12  11
     6.2.  Sending the Request or Indication . . . . . . . . . . . .  13  12
       6.2.1.  Sending over UDP or DTLS-over-UDP . . . . . . . . . .  14  13
       6.2.2.  Sending over TCP or TLS-over-TCP  . . . . . . . . . .  15  14
       6.2.3.  Sending over TLS-over-TCP or DTLS-over-UDP  . . . . .  16  15
     6.3.  Receiving a STUN Message  . . . . . . . . . . . . . . . .  17  16
       6.3.1.  Processing a Request  . . . . . . . . . . . . . . . .  18  17
         6.3.1.1.  Forming a Success or Error Response . . . . . . .  18  17
         6.3.1.2.  Sending the Success or Error Response . . . . . .  19  18
       6.3.2.  Processing an Indication  . . . . . . . . . . . . . .  19  18
       6.3.3.  Processing a Success Response . . . . . . . . . . . .  20  19
       6.3.4.  Processing an Error Response  . . . . . . . . . . . .  20  19
   7.  FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . .  21  20
   8.  DNS Discovery of a Server . . . . . . . . . . . . . . . . . .  21  20
     8.1.  STUN URI Scheme Semantics . . . . . . . . . . . . . . . .  22  21
   9.  Authentication and Message-Integrity Mechanisms . . . . . . .  23  22
     9.1.  Short-Term Credential Mechanism . . . . . . . . . . . . .  23
       9.1.1.  HMAC Key  . . . . . . . . . . . . . . . . . . . . . .  24  23
       9.1.2.  Forming a Request or Indication . . . . . . . . . . .  24  23
       9.1.3.  Receiving a Request or Indication . . . . . . . . . .  24  23
       9.1.4.  Receiving a Response  . . . . . . . . . . . . . . . .  25
       9.1.5.  Sending Subsequent Requests . . . . . . . . . . . . .  26  25
     9.2.  Long-Term Credential Mechanism  . . . . . . . . . . . . .  26  25
       9.2.1.  Bid Down  Bid-Down Attack Prevention  . . . . . . . . . . . . .  28  27
       9.2.2.  HMAC Key  . . . . . . . . . . . . . . . . . . . . . .  28  27
       9.2.3.  Forming a Request . . . . . . . . . . . . . . . . . .  29  28
         9.2.3.1.  First Request . . . . . . . . . . . . . . . . . .  29  28
         9.2.3.2.  Subsequent Requests . . . . . . . . . . . . . . .  29  28
       9.2.4.  Receiving a Request . . . . . . . . . . . . . . . . .  30  29
       9.2.5.  Receiving a Response  . . . . . . . . . . . . . . . .  32  31
   10. ALTERNATE-SERVER Mechanism  . . . . . . . . . . . . . . . . .  33  32
   11. Backwards Compatibility with RFC 3489 . . . . . . . . . . . .  34  33
   12. Basic Server Behavior . . . . . . . . . . . . . . . . . . . .  35  33
   13. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . .  36  34
   14. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . .  37  36
     14.1.  MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . .  38  37
     14.2.  XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . .  39  37
     14.3.  USERNAME . . . . . . . . . . . . . . . . . . . . . . . .  40  38
     14.4.  USERHASH . . . . . . . . . . . . . . . . . . . . . . . .  40  39
     14.5.  MESSAGE-INTEGRITY  . . . . . . . . . . . . . . . . . . .  40  39
     14.6.  MESSAGE-INTEGRITY-SHA256 . . . . . . . . . . . . . . . .  41  40
     14.7.  FINGERPRINT  . . . . . . . . . . . . . . . . . . . . . .  42  41
     14.8.  ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . .  42  41
     14.9.  REALM  . . . . . . . . . . . . . . . . . . . . . . . . .  44  43
     14.10. NONCE  . . . . . . . . . . . . . . . . . . . . . . . . .  44  43
     14.11. PASSWORD-ALGORITHMS  . . . . . . . . . . . . . . . . . .  44
     14.12. PASSWORD-ALGORITHM . . . . . . . . . . . . . . . . . . .  45  44
     14.13. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . .  46  45
     14.14. SOFTWARE . . . . . . . . . . . . . . . . . . . . . . . .  46  45
     14.15. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . .  46
     14.16. ALTERNATE-DOMAIN . . . . . . . . . . . . . . . . . . . .  47  46
   15. Operational Considerations  . . . . . . . . . . . . . . . . .  47  46
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  47  46
     16.1.  Attacks against the Protocol . . . . . . . . . . . . . .  47  46
       16.1.1.  Outside Attacks  . . . . . . . . . . . . . . . . . .  47  46
       16.1.2.  Inside Attacks . . . . . . . . . . . . . . . . . . .  48  47
       16.1.3.  Bid-Down Attacks . . . . . . . . . . . . . . . . . .  49  48
     16.2.  Attacks Affecting the Usage  . . . . . . . . . . . . . .  51  50
       16.2.1.  Attack I: Distributed DoS (DDoS) against a Target  .  51  50
       16.2.2.  Attack II: Silencing a Client  . . . . . . . . . . .  52  51
       16.2.3.  Attack III: Assuming the Identity of a Client  . . .  52  51
       16.2.4.  Attack IV: Eavesdropping . . . . . . . . . . . . . .  52  51
     16.3.  Hash Agility Plan  . . . . . . . . . . . . . . . . . . .  52  51
   17. IAB Considerations  . . . . . . . . . . . . . . . . . . . . .  53  52
   18. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  54  53
     18.1.  STUN Security Features Registry  . . . . . . . . . . . .  54  53
     18.2.  STUN Methods Registry  . . . . . . . . . . . . . . . . .  54  53
     18.3.  STUN Attribute Attributes Registry . . . . . . . . . . . . . . . .  54  53
       18.3.1.  Updated Attributes . . . . . . . . . . . . . . . . .  54  53
       18.3.2.  New Attributes . . . . . . . . . . . . . . . . . . .  55  54
     18.4.  STUN Error Code Codes Registry  . . . . . . . . . . . . . . . .  55  54
     18.5.  STUN Password Algorithm Algorithms Registry  . . . . . . . . . . . .  55
       18.5.1.  Password Algorithms  . . . . . . . . . . . . . . . .  56  55
         18.5.1.1.  MD5  . . . . . . . . . . . . . . . . . . . . . .  56  55
         18.5.1.2.  SHA-256  . . . . . . . . . . . . . . . . . . . .  56  55
     18.6.  STUN UDP and TCP Port Numbers  . . . . . . . . . . . . .  56  55
   19. Changes Since since RFC 5389  . . . . . . . . . . . . . . . . . . .  57  56
   20. References  . . . . . . . . . . . . . . . . . . . . . . . . .  57  56
     20.1.  Normative References . . . . . . . . . . . . . . . . . .  57  56
     20.2.  Informative References . . . . . . . . . . . . . . . . .  60  59
   Appendix A.  C Snippet to Determine STUN Message Types  . . . . .  62  61
   Appendix B.  Test Vectors . . . . . . . . . . . . . . . . . . . .  63  62
     B.1.  Sample Request with Long-Term Authentication with
           MESSAGE-INTEGRITY-SHA256 and USERHASH . . . . . . . . . .  63
   Appendix C.  Release notes  . . . . . . . . . . . . . . . . . . .  65
     C.1.  Modifications between draft-ietf-tram-stunbis-21 and
           draft-ietf-tram-stunbis-20  . . . . . . . . . . . . . . .  65
     C.2.  Modifications between draft-ietf-tram-stunbis-20 and
           draft-ietf-tram-stunbis-19  . .  62
   Acknowledgements  . . . . . . . . . . . . .  65
     C.3.  Modifications between draft-ietf-tram-stunbis-19 and
           draft-ietf-tram-stunbis-18 . . . . . . . . . . .  63
   Contributors  . . . .  65
     C.4.  Modifications between draft-ietf-tram-stunbis-18 and
           draft-ietf-tram-stunbis-17 . . . . . . . . . . . . . . .  65
     C.5.  Modifications between draft-ietf-tram-stunbis-17 and
           draft-ietf-tram-stunbis-16 . . . . . . .  64
   Authors' Addresses  . . . . . . . .  65
     C.6.  Modifications between draft-ietf-tram-stunbis-16 and
           draft-ietf-tram-stunbis-15 . . . . . . . . . . . . . . .  65
     C.7.  Modifications between draft-ietf-tram-stunbis-15  64

1.  Introduction

   The protocol defined in this specification, Session Traversal
   Utilities for NAT (STUN), provides a tool for dealing with Network
   Address Translators (NATs).  It provides a means for an endpoint to
   determine the IP address and
           draft-ietf-tram-stunbis-14  . . . . . . . . . . . . . . .  66
     C.8.  Modifications between draft-ietf-tram-stunbis-14 port allocated by a NAT that corresponds
   to its private IP address and
           draft-ietf-tram-stunbis-13  . . . . . . . . . . . . . . .  66
     C.9.  Modifications port.  It also provides a way for an
   endpoint to keep a NAT binding alive.  With some extensions, the
   protocol can be used to do connectivity checks between draft-ietf-tram-stunbis-13 and
           draft-ietf-tram-stunbis-12  . . . . . . . . . . . . . . .  67
     C.10. Modifications two endpoints
   [RFC8445] or to relay packets between draft-ietf-tram-stunbis-12 two endpoints [RFC5766].

   In keeping with its tool nature, this specification defines an
   extensible packet format, defines operation over several transport
   protocols, and
           draft-ietf-tram-stunbis-11  . . . . . . . . . . . . . . .  67
     C.11. Modifications between draft-ietf-tram-stunbis-11 and
           draft-ietf-tram-stunbis-10  . . . . . . . . . . . . . . .  67
     C.12. Modifications between draft-ietf-tram-stunbis-10 and
           draft-ietf-tram-stunbis-09  . . . . . . . . . . . . . . .  68
     C.13. Modifications between draft-ietf-tram-stunbis-09 and
           draft-ietf-tram-stunbis-08  . . . . . . . . . . . . . . .  68
     C.14. Modifications between draft-ietf-tram-stunbis-08 and
           draft-ietf-tram-stunbis-07  . . . . . . . . . . . . . . .  69
     C.15. Modifications between draft-ietf-tram-stunbis-07 and
           draft-ietf-tram-stunbis-06  . . . . . . . . . . . . . . .  69
     C.16. Modifications between draft-ietf-tram-stunbis-06 and
           draft-ietf-tram-stunbis-05  . . . . . . . . . . . . . . .  69
     C.17. Modifications between draft-ietf-tram-stunbis-05 and
           draft-ietf-tram-stunbis-04  . . . . . . . . . . . . . . .  69
     C.18. Modifications between draft-ietf-tram-stunbis-04 and
           draft-ietf-tram-stunbis-03  . . . . . . . . . . . . . . .  70
     C.19. Modifications between draft-ietf-tram-stunbis-03 and
           draft-ietf-tram-stunbis-02  . . . . . . . . . . . . . . .  70
     C.20. Modifications between draft-ietf-tram-stunbis-02 and
           draft-ietf-tram-stunbis-01  . . . . . . . . . . . . . . .  70
     C.21. Modifications between draft-ietf-tram-stunbis-01 and
           draft-ietf-tram-stunbis-00  . . . . . . . . . . . . . . .  71
     C.22. Modifications between draft-salgueiro-tram-stunbis-02 and
           draft-ietf-tram-stunbis-00  . . . . . . . . . . . . . . .  72
     C.23. Modifications between draft-salgueiro-tram-stunbis-02 and
           draft-salgueiro-tram-stunbis-01 . . . . . . . . . . . . .  72
     C.24. Modifications between draft-salgueiro-tram-stunbis-01 and
           draft-salgueiro-tram-stunbis-00 . . . . . . . . . . . . .  72
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  72
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  73
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  73

1.  Introduction

   The protocol defined in this specification, Session Traversal
   Utilities for NAT, provides a tool for dealing with NATs.  It
   provides a means for an endpoint to determine the IP address and port
   allocated by a NAT that corresponds to its private IP address and
   port.  It also provides a way for an endpoint to keep a NAT binding
   alive.  With some extensions, the protocol can be used to do
   connectivity checks between two endpoints [RFC8445], or to relay
   packets between two endpoints [RFC5766].

   In keeping with its tool nature, this specification defines an
   extensible packet format, defines operation over several transport
   protocols, and provides for two forms of authentication.

   STUN is intended to be used in the context of one or more NAT
   traversal solutions.  These solutions are known as STUN usages.  Each
   usage describes how STUN is utilized to achieve the NAT traversal
   solution.  Typically, a usage indicates when STUN messages get sent,
   which optional attributes to include, what server is used, and what
   authentication mechanism is to be used.  Interactive Connectivity
   Establishment (ICE) [RFC8445] is one usage of STUN.  SIP Outbound
   [RFC5626] is another usage of STUN.  In some cases, a usage will
   require extensions to STUN.  A STUN extension can be in the form of
   new methods, attributes, or error response codes.  More information
   on STUN usages can be found in Section 13.

2.  Overview of Operation

   This section is descriptive only.

                            /-----\
                          // STUN  \\
                         |   Server  |
                          \\       //
                            \-----/

                       +--------------+             Public Internet
       ................|     NAT 2    |.......................
                       +--------------+

                       +--------------+             Private NET 2
       ................|     NAT 1    |.......................
                       +--------------+

                            /-----\
                          // STUN  \\
                         |   Client  |
                          \\       //               Private NET 1
                            \-----/

                 Figure 1: One Possible STUN Configuration

   One possible STUN configuration is shown in Figure 1.  In this
   configuration, there are two entities (called STUN agents) that
   implement the STUN protocol.  The lower agent in the figure is the
   client, and is connected to private network 1.  This network connects
   to private network 2 through NAT 1.  Private network 2 connects to
   the public Internet through NAT 2.  The upper agent in the figure is
   the server, and resides on the public Internet.

   STUN is a client-server protocol.  It supports two types of
   transactions.  One is a request/response transaction in which a
   client sends a request to a server, and the server returns a
   response.  The second is an indication transaction in which either
   agent -- client or server -- sends an indication that generates no
   response.  Both types of transactions include a transaction ID, which
   is a randomly selected 96-bit number.  For request/response
   transactions, this transaction ID allows the client to associate the
   response with the request that generated it; for indications, the
   transaction ID serves as a debugging aid.

   All STUN messages start with a fixed header that includes a method, a
   class, and the transaction ID.  The method indicates which of the
   various requests or indications this is; this specification defines
   just one method, Binding, but other methods are expected to be
   defined in other documents.  The class indicates whether this is a
   request, a success response, an error response, or an indication.
   Following the fixed header comes zero or more attributes, which are
   Type-Length-Value extensions that convey additional information for
   the specific message.

   This document defines a single method called Binding.  The Binding
   method can be used either in request/response transactions or in
   indication transactions.  When used in request/response transactions,
   the Binding method can be used to determine the particular "binding"
   a NAT has allocated to a STUN client.  When used in either request/
   response or in indication transactions, the Binding method can also
   be used to keep these "bindings" alive.

   In the Binding request/response transaction, a Binding request is
   sent from a STUN client to a STUN server.  When the Binding request
   arrives at the STUN server, it may have passed through one or more
   NATs between the STUN client and the STUN server (in Figure 1, there
   were two such NATs).  As the Binding request message passes through a
   NAT, the NAT will modify the source transport address (that is, the
   source IP address and the source port) of the packet.  As a result,
   the source transport address of the request received by the server
   will be the public IP address and port created by the NAT closest to
   the server.  This is called a reflexive transport address.  The STUN
   server copies that source transport address into an XOR-MAPPED-
   ADDRESS attribute in the STUN Binding response and sends the Binding
   response back to the STUN client.  As this packet passes back through
   a NAT, the NAT will modify the destination transport address in the
   IP header, but the transport address in the XOR-MAPPED-ADDRESS
   attribute within the body of the STUN response will remain untouched.
   In this way, the client can learn its reflexive transport address
   allocated by the outermost NAT with respect to the STUN server.

   In some usages, STUN must be multiplexed with other protocols (e.g.,
   [RFC8445], [RFC5626]).  In these usages, there must be a way to
   inspect a packet and determine if it is a STUN packet or not.  STUN
   provides three fields in the STUN header with fixed values that can
   be used for this purpose.  If this is not sufficient, then STUN
   packets can also contain a FINGERPRINT value, which can further be
   used to distinguish the packets.

   STUN defines a set of optional procedures that a usage can decide to
   use, called mechanisms.  These mechanisms include DNS discovery, a
   redirection technique to an alternate server, a fingerprint attribute
   for demultiplexing, and two authentication and message-integrity
   exchanges.  The authentication mechanisms revolve around the use of a
   username, password, and message-integrity value.  Two authentication
   mechanisms, the long-term credential mechanism and the short-term
   credential mechanism, are defined in this specification.  Each usage
   specifies the mechanisms allowed with that usage.

   In the long-term credential mechanism, the client and server share a
   pre-provisioned username and password and perform a digest challenge/
   response exchange inspired by (but differing in details) to the one
   defined for HTTP [RFC7616].  In the short-term credential mechanism,
   the client and the server exchange a username and password through
   some out-of-band method prior to the STUN exchange.  For example, in
   the ICE usage [RFC8445] the two endpoints use out-of-band signaling
   to exchange a username and password.  These are used to integrity
   protect and authenticate the request and response.  There is no
   challenge or nonce used.

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  Definitions

   STUN Agent:  A STUN agent is an entity that implements the STUN
      protocol.  The entity can be either a STUN client or a STUN
      server.

   STUN Client:  A STUN client is an entity that sends STUN requests and
      receives STUN responses and STUN indications.  A STUN client can
      also send indications.  In this specification, the terms STUN
      client and client are synonymous.

   STUN Server:  A STUN server is an entity that receives STUN requests
      and STUN indications, and sends STUN responses.  A STUN server can
      also send indications.  In this specification, the terms STUN
      server and server are synonymous.

   Transport Address:  The combination of an IP address and port number
      (such as a UDP or TCP port number).

   Reflexive Transport Address:  A transport address learned by a client
      that identifies that client as seen by another host on an IP
      network, typically a STUN server.  When there is an intervening
      NAT between the client and the other host, the reflexive transport
      address represents the mapped address allocated to the client on
      the public side of the NAT.  Reflexive transport addresses are
      learned from the mapped address attribute (MAPPED-ADDRESS or XOR-
      MAPPED-ADDRESS) in STUN responses.

   Mapped Address:  Same meaning as reflexive address.  This term is
      retained only for historic reasons and due to the naming of the
      MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes.

   Long-Term Credential:  A username and associated password that
      represent a shared secret between client and server.  Long-term
      credentials are generally granted to the client when a subscriber
      enrolls in a service and persist until the subscriber leaves the
      service or explicitly changes the credential.

   Long-Term Password:  The password from a long-term credential.

   Short-Term Credential:  A temporary username and associated password
      that represent a shared secret between client and server.  Short-
      term credentials are obtained through some kind of protocol
      mechanism between the client and server, preceding the STUN
      exchange.  A short-term credential has an explicit temporal scope,
      which may be based on a specific amount of time (such as 5
      minutes) or on an event (such as termination of a Session
      Initiation Protocol (SIP [RFC3261]) dialog).  The specific scope
      of a short-term credential is defined by the application usage.

   Short-Term Password:  The password component of a short-term
      credential.

   STUN Indication:  A STUN message that does not receive a response.

   Attribute:  The STUN term for a Type-Length-Value (TLV) object that
      can be added to a STUN message.  Attributes are divided into two
      types: comprehension-required and comprehension-optional.  STUN
      agents can safely ignore comprehension-optional attributes they
      don't understand, but cannot successfully process a message if it
      contains comprehension-required attributes that are not
      understood.

   RTO:  Retransmission TimeOut, which defines the initial period of
      time between transmission of a request and the first retransmit of
      that request.

5.  STUN Message Structure

   STUN messages are encoded in binary using network-oriented format
   (most significant byte or octet first, also commonly known as big-
   endian).  The transmission order is described in detail in Appendix B
   of [RFC0791].  Unless otherwise noted, numeric constants are in
   decimal (base 10).

   All STUN messages comprise a 20-byte header followed by zero or more
   Attributes.  The STUN header contains a STUN message type, message
   length, magic cookie, and transaction ID.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0|     STUN Message Type     |         Message Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Magic Cookie                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                     Transaction ID (96 bits)                  |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: Format of STUN Message Header

   The most significant 2 bits of every STUN message MUST be zeroes.
   This can be used to differentiate STUN packets from other protocols
   when STUN is multiplexed with other protocols on the same port.

   The message type defines the message class (request, success
   response, error response, or indication) and the message method (the
   primary function) of the STUN message.  Although there are four
   message classes, there are only two types of transactions in STUN:
   request/response transactions (which consist of a request message and
   a response message) and indication transactions (which consist of a
   single indication message).  Response classes are split into error
   and success responses to aid in quickly processing the STUN message.

   The message type field is decomposed further into the following
   structure:

                       0                 1
                       2  3  4 5 6 7 8 9 0 1 2 3 4 5
                      +--+--+-+-+-+-+-+-+-+-+-+-+-+-+
                      |M |M |M|M|M|C|M|M|M|C|M|M|M|M|
                      |11|10|9|8|7|1|6|5|4|0|3|2|1|0|
                      +--+--+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: Format of STUN Message Type Field

   Here the bits in the message type field are shown as most significant
   (M11) through least significant (M0).  M11 through M0 represent a
   12-bit encoding of the method.  C1 and C0 represent a 2-bit encoding
   of the class.  A class of 0b00 is a request, a class of 0b01 is an
   indication, a class of 0b10 is a success response, and a class of
   0b11 is an error response.  This specification defines a single
   method, Binding.  The method and class are orthogonal, so that for
   each method, a request, success response, error response, and
   indication are possible for that method.  Extensions defining new
   methods MUST indicate which classes are permitted for that method.

   For example, a Binding request has class=0b00 (request) and
   method=0b000000000001 (Binding) and is encoded into the first 16 bits
   as 0x0001.  A Binding response has class=0b10 (success response) and
   method=0b000000000001, and is encoded into the first 16 bits as
   0x0101.

   Note:  This unfortunate encoding is due to assignment of values in
      [RFC3489] that did not consider encoding Indications, Success, and
      Errors using bit fields.

   The magic cookie field MUST contain the fixed value 0x2112A442 in
   network byte order.  In [RFC3489], this field was part of the
   transaction ID; placing the magic cookie in this location allows a
   server to detect if the client will understand certain attributes
   that were added to STUN by [RFC5389].  In addition, it aids in
   distinguishing STUN packets from packets of other protocols when STUN
   is multiplexed with those other protocols on the same port.

   The transaction ID is a 96-bit identifier, used to uniquely identify
   STUN transactions.  For request/response transactions, the
   transaction ID is chosen by the STUN client for the request and
   echoed by the server in the response.  For indications, it is chosen
   by the agent sending the indication.  It primarily serves to
   correlate requests with responses, though it also plays a small role
   in helping to prevent certain types of attacks.  The server also uses
   the transaction ID as a key to identify each transaction uniquely
   across all clients.  As such, the transaction ID MUST be uniformly
   and randomly chosen from the interval 0 .. 2**96-1, and MUST be
   cryptographically random.  Resends of the same request reuse the same
   transaction ID, but the client MUST choose a new transaction ID for
   new transactions unless the new request is bit-wise identical to the
   previous request and sent from the same transport address to the same
   IP address.  Success and error responses MUST carry the same
   transaction ID as their corresponding request.  When an agent is
   acting as a STUN server and STUN client on the same port, the
   transaction IDs in requests sent by the agent have no relationship to
   the transaction IDs in requests received by the agent.

   The message length MUST contain the size, in bytes, of the message
   not including the 20-byte STUN header.  Since all STUN attributes are
   padded to a multiple of 4 bytes, the last 2 bits of this field are
   always zero.  This provides another way to distinguish STUN packets
   from packets of other protocols.

   Following the STUN fixed portion of the header are zero or more
   attributes.  Each attribute is TLV (Type-Length-Value) encoded.  The
   details of the encoding, and of the attributes themselves are given
   in Section 14.

6.  Base Protocol Procedures

   This section defines the base procedures of the STUN protocol.  It
   describes how messages are formed, how they are sent, and how they
   are processed when they are received.  It also defines the detailed
   processing of the Binding method.  Other sections in this document
   describe optional procedures that a usage may elect to use in certain
   situations.  Other documents may define other extensions to STUN, by
   adding new methods, new attributes, or new error response codes.

6.1.  Forming a Request or an Indication

   When formulating a request or indication message, the agent MUST
   follow the rules in Section 5 when creating the header.  In addition,
   the message class MUST be either "Request" or "Indication" (as
   appropriate), and the method must be either Binding or some method
   defined in another document.

   The agent then adds any attributes specified by the method or the
   usage.  For example, some usages may specify that the agent use an
   authentication method (Section 9) or the FINGERPRINT attribute
   (Section 7).

   If the agent is sending a request, it SHOULD add a SOFTWARE attribute
   to the request.  Agents MAY include a SOFTWARE attribute in
   indications, depending on the method.  Extensions to STUN should
   discuss whether SOFTWARE is useful in new indications.  Note that the
   inclusion of a SOFTWARE attribute may have security implications; see
   Section 16.1.2 for details.

   For the Binding method with no authentication, no attributes are
   required unless the usage specifies otherwise.

   All STUN messages sent over UDP or DTLS-over-UDP [RFC6347] SHOULD be
   less than the path MTU, if known.

   If the path MTU is unknown for UDP, messages SHOULD be the smaller of
   576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
   IPv6 [RFC8200].  This value corresponds to the overall size of the IP
   packet.  Consequently, for IPv4, the actual STUN message would need
   to be less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
   UDP header, assuming no IP options are used).

   If the path MTU is unknown for DTLS-over-UDP, the rules described in
   the previous paragraph need to be adjusted to take into account the
   size of the (13-byte) DTLS Record header, the MAC size, and the
   padding size.

   STUN provides no ability to handle the case where the request is
   under the MTU but the response would be larger than the MTU.  It is
   not envisioned that this limitation will be an issue for STUN.  The
   MTU limitation is a SHOULD, and not a MUST, to account for cases
   where STUN itself is being used to probe for MTU characteristics
   [RFC5780].  See also [I-D.ietf-tram-stun-pmtud] for a framework that
   uses STUN to add Path MTU Discovery to protocols that lack one.
   Outside of this or similar applications, the MTU constraint MUST be
   followed.

6.2.  Sending the Request or Indication

   The agent then sends the request or indication.  This document
   specifies how to send STUN messages over UDP, TCP, TLS-over-TCP, or
   DTLS-over-UDP; other transport protocols may be added in the future.
   The STUN usage must specify which transport protocol is used, and how
   the agent determines the IP address and port of the recipient.
   Section 8 describes a DNS-based method of determining the IP address
   and port of a server that a usage may elect to use.

   At any time, a client MAY have multiple outstanding STUN requests
   with the same STUN server (that is, multiple transactions in
   progress, with different transaction IDs).  Absent other limits to
   the rate of new transactions (such as those specified by ICE for
   connectivity checks or when STUN is run over TCP), a client SHOULD
   limit itself to ten outstanding transactions to the same server.

6.2.1.  Sending over UDP or DTLS-over-UDP

   When running STUN over UDP or STUN over DTLS-over-UDP [RFC7350], it
   is possible that the STUN message might be dropped by the network.
   Reliability of STUN request/response transactions is accomplished
   through retransmissions of the request message by the client
   application itself.  STUN indications are not retransmitted; thus,
   indication transactions over UDP or DTLS-over-UDP are not reliable.

   A client SHOULD retransmit a STUN request message starting with an
   interval of RTO ("Retransmission TimeOut"), doubling after each
   retransmission.  The RTO is an estimate of the round-trip time (RTT),
   and is computed as described in [RFC6298], with two exceptions.
   First, the initial value for RTO SHOULD be greater or equal to 500
   ms.  The exception cases for this "SHOULD" are when other mechanisms
   are used to derive congestion thresholds (such as the ones defined in
   ICE for fixed rate streams), or when STUN is used in non-Internet
   environments with known network capacities.  In fixed-line access
   links, a value of 500 ms is RECOMMENDED.  Second, the value of RTO
   SHOULD NOT be rounded up to the nearest second.  Rather, a 1 ms
   accuracy SHOULD be maintained.  As with TCP, the usage of Karn's
   algorithm is RECOMMENDED [KARN87].  When applied to STUN, it means
   that RTT estimates SHOULD NOT be computed from STUN transactions that
   result in the retransmission of a request.

   The value for RTO SHOULD be cached by a client after the completion
   of the transaction, and used as the starting value for RTO for the
   next transaction to the same server (based on equality of IP
   address).  The value SHOULD be considered stale and discarded if no
   transactions have occurred to the same server in the last 10 minutes.

   Retransmissions continue until a response is received, or until a
   total of Rc requests have been sent.  Rc SHOULD be configurable and
   SHOULD have a default of 7.  If, after the last request, a duration
   equal to Rm times the RTO has passed without a response (providing
   ample time to get a response if only this final request actually
   succeeds), the client SHOULD consider the transaction to have failed.
   Rm SHOULD be configurable and SHOULD have a default of 16.  A STUN
   transaction over UDP or DTLS-over-UDP is also considered failed if
   there has been a hard ICMP error [RFC1122].  For example, assuming an
   RTO of 500ms, requests would be sent at times 0 ms, 500 ms, 1500 ms,
   3500 ms, 7500 ms, 15500 ms, and 31500 ms.  If the client has not
   received a response after 39500 ms, the client will consider the
   transaction to have timed out.

6.2.2.  Sending over TCP or TLS-over-TCP

   For TCP and TLS-over-TCP [RFC5246], the client opens a TCP connection
   to the server.

   In some usages of STUN, STUN is sent as the only protocol over the
   TCP connection.  In this case, it can be sent without the aid of any
   additional framing or demultiplexing.  In other usages, or with other
   extensions, it may be multiplexed with other data over a TCP
   connection.  In that case, STUN MUST be run on top of some kind of
   framing protocol, specified by the usage or extension, which allows
   for the agent to extract complete STUN messages and complete
   application layer messages.  The STUN service running on the well-
   known port or ports discovered through the DNS procedures in
   Section 8 is for STUN alone, and not for STUN multiplexed with other
   data.  Consequently, no framing protocols are used in connections to
   those servers.  When additional framing is utilized, the usage will
   specify how the client knows to apply it and what port to connect to.
   For example, in the case of ICE connectivity checks, this information
   is learned through out-of-band negotiation between client and server.

   Reliability of STUN over TCP and TLS-over-TCP is handled by TCP
   itself, and there are no retransmissions at the STUN protocol level.
   However, provides for a request/response transaction, if the client has not
   received a response by Ti seconds after it sent the request message,
   it considers the transaction to have timed out.  Ti SHOULD be
   configurable and SHOULD have a default two forms of 39.5s.  This value has been
   chosen to equalize the TCP and UDP timeouts for the default initial
   RTO.

   In addition, if the client authentication.

   STUN is unable intended to establish the TCP connection,
   or be used in the TCP connection is reset context of one or fails before a response is
   received, any request/response transaction in progress more NAT
   traversal solutions.  These solutions are known as "STUN Usages".
   Each usage describes how STUN is considered
   to have failed.

   The client MAY send multiple transactions over a single TCP (or TLS-
   over-TCP) connection, and it MAY send another request before
   receiving a response utilized to achieve the previous request.  The client SHOULD keep
   the connection open until it:

   o  has no further STUN requests or indications to send over that
      connection, and

   o  has no plans to use any resources (such as NAT
   traversal solution.  Typically, a mapped address
      (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
      [RFC5766]) that were learned though usage indicates when STUN requests sent over that
      connection, and
   o  if multiplexing other application protocols over that port, has
      finished using those other protocols, and

   o  if using that learned port with a remote peer, has established
      communications with that remote peer, as messages
   get sent, which optional attributes to include, what server is required by some TCP
      NAT traversal techniques (e.g., [RFC6544]).

   The details of an eventual keep-alive used,
   and what authentication mechanism are left is to each STUN
   Usage. be used.  Interactive
   Connectivity Establishment (ICE) [RFC8445] is one usage of STUN.  SIP
   Outbound [RFC5626] is another usage of STUN.  In any case if some cases, a transaction fails because an idle TCP
   connection doesn't work anymore the client SHOULD send an RST and try usage
   will require extensions to open a new TCP connection.

   At the server end, the server SHOULD keep the connection open, and
   let the client close it, unless STUN.  A STUN extension can be in the server has determined form
   of new methods, attributes, or error response codes.  More
   information on STUN Usages can be found in Section 13.

2.  Overview of Operation

   This section is descriptive only.

                           /-----\
                         // STUN  \\
                        |   Server  |
                         \\       //
                           \-----/

                      +--------------+             Public Internet
      ................|     NAT 2    |.......................
                      +--------------+

                      +--------------+             Private Network 2
      ................|     NAT 1    |.......................
                      +--------------+

                           /-----\
                         // STUN  \\
                        |   Client  |
                         \\       //               Private Network 1
                           \-----/

                 Figure 1: One Possible STUN Configuration

   One possible STUN configuration is shown in Figure 1.  In this
   configuration, there are two entities (called STUN agents) that
   implement the
   connection has timed out (for example, due to STUN protocol.  The lower agent in the client
   disconnecting from figure is the network).  Bindings learned by
   client, which is connected to private network 1.  This network
   connects to private network 2 through NAT 1.  Private network 2
   connects to the client will
   remain valid public Internet through NAT 2.  The upper agent in intervening NATs only while
   the connection remains
   open.  Only figure is the client knows how long it needs server, which resides on the binding.  The
   server SHOULD NOT close public Internet.

   STUN is a connection if client-server protocol.  It supports two types of
   transactions.  One is a request was received over
   that connection for request/response transaction in which a response was not sent.  A server MUST NOT
   ever open a connection back towards the
   client in order sends a request to send a
   response.  Servers SHOULD follow best practices regarding connection
   management in cases of overload.

6.2.3.  Sending over TLS-over-TCP or DTLS-over-UDP

   When STUN is run by itself over TLS-over-TCP or DTLS-over-UDP, the
   TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
   TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 cipher suites MUST be
   implemented and other cipher suites MAY be implemented.  Perfect
   Forward Secrecy (PFS) cipher suites MUST be preferred over non-PFS
   cipher suites.  Cipher suites with known weaknesses, such as those
   based on (single) DES server, and RC4, MUST NOT be used.  Implementations
   MUST disable TLS-level compression.

   These recommendations are just a part of the recommendations server returns a
   response.  The second is an indication transaction in
   [BCP195] which either
   agent -- client or server -- sends an indication that implementations and deployments generates no
   response.  Both types of transactions include a STUN Usage using
   TLS or DTLS MUST follow.

   When it receives the TLS Certificate message, transaction ID, which
   is a randomly selected 96-bit number.  For request/response
   transactions, this transaction ID allows the client MUST verify
   the certificate and inspect the site identified by the certificate.
   If the certificate is invalid or revoked, or if it does not identify to associate the appropriate party,
   response with the client MUST NOT send request that generated it; for indications, the
   transaction ID serves as a debugging aid.

   All STUN message or
   otherwise proceed messages start with a fixed header that includes a method, a
   class, and the STUN transaction. transaction ID.  The client MUST verify
   the identity method indicates which of the server.  To do that, it follows the
   identification procedures
   various requests or indications this is; this specification defines
   just one method, Binding, but other methods are expected to be
   defined in [RFC6125], with other documents.  The class indicates whether this is a certificate
   containing an identifier of type DNS-ID or CN-ID, optionally with
   request, a
   wildcard character as leftmost label, but not of type SRV-ID success response, an error response, or URI-
   ID.

   When STUN is run multiplexed with other protocols over a TLS-over-TCP
   connection an indication.
   Following the fixed header comes zero or a DTLS-over-UDP association, more attributes, which are
   Type-Length-Value extensions that convey additional information for
   the mandatory ciphersuites
   and TLS handling procedures operate as defined by those protocols.

6.3.  Receiving a STUN Message specific message.

   This section specifies the processing of document defines a STUN message. single method called "Binding".  The
   processing specified here is for STUN messages as defined Binding
   method can be used either in this
   specification; additional rules for backwards compatibility are
   defined request/response transactions or in Section 11.  Those additional procedures are optional, and
   usages
   indication transactions.  When used in request/response transactions,
   the Binding method can elect be used to utilize them.  First, a set of processing
   operations is applied that is independent of determine the class.  This is
   followed by class-specific processing, described particular binding a
   NAT has allocated to a STUN client.  When used in either request/
   response or in indication transactions, the subsections
   that follow.

   When Binding method can also
   be used to keep these bindings alive.

   In the Binding request/response transaction, a Binding request is
   sent from a STUN agent receives client to a STUN message, server.  When the Binding request
   arrives at the STUN server, it first checks that may have passed through one or more
   NATs between the STUN client and the STUN server (in Figure 1, there
   are two such NATs).  As the Binding request message obeys passes through a
   NAT, the rules NAT will modify the source transport address (that is, the
   source IP address and the source port) of Section 5.  It checks that the first two
   bits are 0, that packet.  As a result,
   the magic cookie field has source transport address of the correct value, that request received by the message length is sensible, server
   will be the public IP address and that port created by the method value NAT closest to
   the server.  This is called a
   supported method.  It checks "reflexive transport address".  The
   STUN server copies that source transport address into an XOR-MAPPED-
   ADDRESS attribute in the message class is allowed for STUN Binding response and sends the particular method.  If Binding
   response back to the message class is "Success Response" or
   "Error Response", STUN client.  As this packet passes back through
   a NAT, the agent checks that NAT will modify the transaction ID matches a
   transaction that is still destination transport address in progress.  If the FINGERPRINT extension
   is being used,
   IP header, but the transport address in the XOR-MAPPED-ADDRESS
   attribute within the body of the STUN response will remain untouched.
   In this way, the client can learn its reflexive transport address
   allocated by the agent checks that outermost NAT with respect to the FINGERPRINT attribute is
   present STUN server.

   In some usages, STUN must be multiplexed with other protocols (e.g.,
   [RFC8445] and contains the correct value.  If any errors are detected,
   the message is silently discarded. [RFC5626]).  In these usages, there must be a way to
   inspect a packet and determine if it is a STUN packet or not.  STUN
   provides three fields in the case when STUN is being
   multiplexed header with another protocol, an error may indicate fixed values that can
   be used for this purpose.  If this is not really a sufficient, then STUN message; in this case, the agent should try
   packets can also contain a FINGERPRINT value, which can further be
   used to
   parse distinguish the message as a different protocol.

   The packets.

   STUN agent then does any checks that are required by defines a
   authentication mechanism set of optional procedures that the a usage has specified (see
   Section 9).

   Once the authentication checks are done, the STUN agent checks can decide to
   use, called "mechanisms".  These mechanisms include DNS discovery, a
   redirection technique to an alternate server, a fingerprint attribute
   for
   unknown attributes demultiplexing, and known-but-unexpected attributes in the
   message.  Unknown comprehension-optional attributes MUST be ignored
   by the agent.  Known-but-unexpected attributes SHOULD be ignored by two authentication and message-integrity
   exchanges.  The authentication mechanisms revolve around the agent.  Unknown comprehension-required attributes cause
   processing that depends on use of a
   username, password, and message-integrity value.  Two authentication
   mechanisms, the message class long-term credential mechanism and is described below.

   At this point, further processing depends on the message class of short-term
   credential mechanism, are defined in this specification.  Each usage
   specifies the
   request.

6.3.1.  Processing a Request

   If mechanisms allowed with that usage.

   In the request contains one or more unknown comprehension-required
   attributes, long-term credential mechanism, the client and server replies with an error response with an error
   code of 420 (Unknown Attribute), share a
   pre-provisioned username and includes an UNKNOWN-ATTRIBUTES
   attribute password and perform a digest challenge/
   response exchange inspired by the one defined for HTTP [RFC7616] but
   differing in details.  In the response that lists short-term credential mechanism, the unknown comprehension-
   required attributes.

   Otherwise
   client and the server then does any additional checking that the exchange a username and password through some
   out-of-band method or prior to the specific usage requires.  If all STUN exchange.  For example, in the checks succeed,
   ICE usage [RFC8445], the server formulates two endpoints use out-of-band signaling to
   exchange a success response username and password.  These are used to integrity
   protect and authenticate the request and response.  There is no
   challenge or nonce used.

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described below.

   When run over UDP in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  Definitions

   STUN Agent:  A STUN agent is an entity that implements the STUN
      protocol.  The entity can be either a STUN client or DTLS-over-UDP, a request received by STUN
      server.

   STUN Client:  A STUN client is an entity that sends STUN requests and
      receives STUN responses and STUN indications.  A STUN client can
      also send indications.  In this specification, the terms "STUN
      client" and "client" are synonymous.

   STUN Server:  A STUN server
   could be is an entity that receives STUN requests
      and STUN indications and that sends STUN responses.  A STUN server
      can also send indications.  In this specification, the first request terms "STUN
      server" and "server" are synonymous.

   Transport Address:  The combination of an IP address and port number
      (such as a transaction, UDP or TCP port number).

   Reflexive Transport Address:  A transport address learned by a retransmission.
   The server MUST respond to retransmissions such client
      that the following
   property identifies that client as seen by another host on an IP
      network, typically a STUN server.  When there is preserved: if an intervening
      NAT between the client receives the response to the
   retransmission and not the response that was sent to the original
   request, other host, the overall state on reflexive transport
      address represents the client and server is identical mapped address allocated to the case where only client on
      the response to public side of the original retransmission is
   received, or where both responses NAT.  Reflexive transport addresses are received (in which case the
   client will use
      learned from the first).  The easiest way to meet this requirement mapped address attribute (MAPPED-ADDRESS or XOR-
      MAPPED-ADDRESS) in STUN responses.

   Mapped Address:  Same meaning as reflexive address.  This term is
      retained only for the server to remember all transaction IDs received over UDP
   or DTLS-over-UDP historic reasons and their corresponding responses in due to the last 40
   seconds.  However, this requires naming of the server to hold state,
      MAPPED-ADDRESS and will
   be inappropriate for any requests which XOR-MAPPED-ADDRESS attributes.

   Long-Term Credential:  A username and associated password that
      represent a shared secret between client and server.  Long-term
      credentials are not authenticated.
   Another way is generally granted to reprocess the request client when a subscriber
      enrolls in a service and recompute persist until the response. subscriber leaves the
      service or explicitly changes the credential.

   Long-Term Password:  The latter technique MUST only be applied to requests password from a long-term credential.

   Short-Term Credential:  A temporary username and associated password
      that represent a shared secret between client and server.  Short-
      term credentials are
   idempotent (a request is considered idempotent when the same request
   can be safely repeated without impacting the overall state obtained through some kind of protocol
      mechanism between the
   system) client and result in the same success response for server, preceding the same request. STUN
      exchange.  A short-term credential has an explicit temporal scope,
      which may be based on a specific amount of time (such as 5
      minutes) or on an event (such as termination of a Session
      Initiation Protocol (SIP) [RFC3261] dialog).  The Binding method specific scope
      of a short-term credential is considered to be idempotent.  Note defined by the application usage.

   Short-Term Password:  The password component of a short-term
      credential.

   STUN Indication:  A STUN message that there
   are certain rare network events does not receive a response.

   Attribute:  The STUN term for a Type-Length-Value (TLV) object that could cause the reflexive
   transport address value
      can be added to change, resulting in a different mapped
   address in different success responses.  Extensions to STUN MUST
   discuss the implications of request retransmissions on servers message.  Attributes are divided into two
      types: comprehension-required and comprehension-optional.  STUN
      agents can safely ignore comprehension-optional attributes they
      don't understand but cannot successfully process a message if it
      contains comprehension-required attributes that
   do are not store transaction state.

6.3.1.1.  Forming a Success or Error Response

   When forming the response (success or error), the server follows
      understood.

   RTO:  Retransmission TimeOut, which defines the
   rules of Section 6.  The method initial period of the response is the same as that
      time between transmission of the request, a request and the message class is either "Success Response" or
   "Error Response".

   For an error response, the server MUST add an ERROR-CODE attribute
   containing the error code specified first retransmit of
      that request.

5.  STUN Message Structure

   STUN messages are encoded in the processing above. binary using network-oriented format
   (most significant byte or octet first, also commonly known as big-
   endian).  The
   reason phrase transmission order is not fixed, but SHOULD be something suitable for the
   error code.  For certain errors, additional attributes are added to
   the message.  These attributes described in detail in Appendix B
   of [RFC0791].  Unless otherwise noted, numeric constants are spelled out in the description
   where the error code is specified.  For example, for an error code
   decimal (base 10).

   All STUN messages comprise a 20-byte header followed by zero or more
   attributes.  The STUN header contains a STUN message type, message
   length, magic cookie, and transaction ID.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0|     STUN Message Type     |         Message Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Magic Cookie                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                     Transaction ID (96 bits)                  |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: Format of
   420 (Unknown Attribute), the server STUN Message Header

   The most significant 2 bits of every STUN message MUST include an UNKNOWN-
   ATTRIBUTES attribute.  Certain authentication errors also cause
   attributes to be added (see Section 9).  Extensions may define other
   errors and/or additional attributes to add in error cases.

   If the server authenticated the request using an authentication
   mechanism, then the server SHOULD add the appropriate authentication
   attributes zeroes.
   This can be used to differentiate STUN packets from other protocols
   when STUN is multiplexed with other protocols on the response (see Section 9). same port.

   The server also adds any attributes required by message type defines the specific method message class (request, success
   response, error response, or usage.  In addition, indication) and the server SHOULD add message method (the
   primary function) of the STUN message.  Although there are four
   message classes, there are only two types of transactions in STUN:
   request/response transactions (which consist of a SOFTWARE attribute request message and
   a response message) and indication transactions (which consist of a
   single indication message).  Response classes are split into error
   and success responses to aid in quickly processing the STUN message.

   For the Binding method, no additional checking

   The STUN Message Type field is required unless decomposed further into the
   usage specifies otherwise.  When forming following
   structure:

                       0                 1
                       2  3  4 5 6 7 8 9 0 1 2 3 4 5
                      +--+--+-+-+-+-+-+-+-+-+-+-+-+-+
                      |M |M |M|M|M|C|M|M|M|C|M|M|M|M|
                      |11|10|9|8|7|1|6|5|4|0|3|2|1|0|
                      +--+--+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: Format of STUN Message Type Field

   Here the success response, bits in the
   server adds STUN Message Type field are shown as most
   significant (M11) through least significant (M0).  M11 through M0
   represent a XOR-MAPPED-ADDRESS attribute to the response, where the
   contents 12-bit encoding of the attribute are the source transport address method.  C1 and C0 represent a
   2-bit encoding of the
   request message.  For UDP or DTLS-over-UDP this class.  A class of 0b00 is the source IP
   address a request, a class
   of 0b01 is an indication, a class of 0b10 is a success response, and source UDP port
   a class of the request message. 0b11 is an error response.  This specification defines a
   single method, Binding.  The method and class are orthogonal, so that
   for each method, a request, success response, error response, and
   indication are possible for that method.  Extensions defining new
   methods MUST indicate which classes are permitted for that method.

   For TCP example, a Binding request has class=0b00 (request) and TLS-
   over-TCP, this is the source IP address
   method=0b000000000001 (Binding) and source TCP port of is encoded into the
   TCP connection first 16 bits
   as seen by the server.

6.3.1.2.  Sending the Success or Error Response

   The 0x0001.  A Binding response has class=0b10 (success or error) response) and
   method=0b000000000001 and is sent over encoded into the same transport first 16 bits as
   the request was received on.  If the request was received over UDP or
   DTLS-over-UDP the destination IP address and port of the response are
   the source IP address and port
   0x0101.

      Note: This unfortunate encoding is due to assignment of the received request message, values in
      [RFC3489] that did not consider encoding indication messages,
      success responses, and errors responses using bit fields.

   The Magic Cookie field MUST contain the source IP address and port of fixed value 0x2112A442 in
   network byte order.  In [RFC3489], the response are equal to 32 bits comprising the
   destination IP address and port Magic
   Cookie field were part of the received request message.  If transaction ID; placing the request was received over TCP or TLS-over-TCP, magic
   cookie in this location allows a server to detect if the response client will
   understand certain attributes that were added to STUN by [RFC5389].
   In addition, it aids in distinguishing STUN packets from packets of
   other protocols when STUN is
   sent back multiplexed with those other protocols
   on the same TCP connection as the request was received on. port.

   The server transaction ID is allowed to send responses in a different order than it
   received the requests.

6.3.2.  Processing an Indication

   If the indication contains unknown comprehension-required attributes, 96-bit identifier, used to uniquely identify
   STUN transactions.  For request/response transactions, the indication
   transaction ID is discarded and processing ceases.

   Otherwise the agent then does any additional checking that the method
   or chosen by the specific usage requires.  If all STUN client for the checks succeed, request and
   echoed by the agent
   then processes server in the indication.  No response is generated for an
   indication. response.  For the Binding method, no additional checking or processing indications, it is
   required, unless the usage specifies otherwise.  The mere receipt of
   the message chosen
   by the agent has refreshed the "bindings" in sending the
   intervening NATs.

   Since indications are not re-transmitted over UDP or DTLS-over-UDP
   (unlike requests), there is no need indication.  It primarily serves to handle re-transmissions of
   indications at the sending agent.

6.3.3.  Processing
   correlate requests with responses, though it also plays a Success Response

   If the success response contains unknown comprehension-required
   attributes, the response is discarded and small role
   in helping to prevent certain types of attacks.  The server also uses
   the transaction is
   considered ID as a key to have failed.

   Otherwise the client then does any additional checking that the
   method or the specific usage requires.  If identify each transaction uniquely
   across all clients.  As such, the checks succeed, transaction ID MUST be uniformly
   and randomly chosen from the client then processes interval 0 .. 2**96-1 and MUST be
   cryptographically random.  Resends of the success response.

   For same request reuse the Binding method, same
   transaction ID, but the client checks that MUST choose a new transaction ID for
   new transactions unless the XOR-MAPPED-ADDRESS
   attribute new request is present in bit-wise identical to the response.  The client checks
   previous request and sent from the same transport address
   family specified.  If it is an unsupported address family, to the
   attribute SHOULD be ignored.  If it is same
   IP address.  Success and error responses MUST carry the same
   transaction ID as their corresponding request.  When an unexpected but supported
   address family (for example, agent is
   acting as a STUN server and STUN client on the same port, the Binding
   transaction was IDs in requests sent over
   IPv4, but by the address family specified is IPv6), then agent have no relationship to
   the client MAY
   accept and use transaction IDs in requests received by the value.

6.3.4.  Processing an Error Response

   If agent.

   The message length MUST contain the error response contains unknown comprehension-required
   attributes, or if size of the message in bytes, not
   including the 20-byte STUN header.  Since all STUN attributes are
   padded to a multiple of 4 bytes, the last 2 bits of this field are
   always zero.  This provides another way to distinguish STUN packets
   from packets of other protocols.

   Following the error response does not contain an ERROR-CODE
   attribute, then STUN fixed portion of the transaction header are zero or more
   attributes.  Each attribute is simply considered to have failed.

   Otherwise TLV (Type-Length-Value) encoded.
   Details of the client then does any processing specified by encoding and the
   authentication mechanism (see attributes themselves are given in
   Section 9). 14.

6.  Base Protocol Procedures

   This may result in a new
   transaction attempt.

   The processing at this point depends on section defines the error code, base procedures of the method, STUN protocol.  It
   describes how messages are formed, how they are sent, and the usage; the following how they
   are processed when they are received.  It also defines the default rules:

   o  If detailed
   processing of the Binding method.  Other sections in this document
   describe optional procedures that a usage may elect to use in certain
   situations.  Other documents may define other extensions to STUN, by
   adding new methods, new attributes, or new error code is 300 through 399, the client SHOULD consider response codes.

6.1.  Forming a Request or an Indication

   When formulating a request or indication message, the transaction as failed unless agent MUST
   follow the ALTERNATE-SERVER extension
      (Section 10) is being used.

   o  If rules in Section 5 when creating the error code is 400 through 499, header.  In addition,
   the client declares message class MUST be either "Request" or "Indication" (as
   appropriate), and the
      transaction failed; method must be either Binding or some method
   defined in another document.

   The agent then adds any attributes specified by the method or the
   usage.  For example, some usages may specify that the case of 420 (Unknown Attribute), agent use an
   authentication method (Section 9) or the
      response should contain a UNKNOWN-ATTRIBUTES FINGERPRINT attribute that gives
      additional information.

   o
   (Section 7).

   If the error code agent is 500 through 599, the client MAY resend the
      request; clients that do so MUST limit the number of times they do
      this.  Unless a specific error code specifies sending a different value,
      the number of retransmissions request, it SHOULD be limited add a SOFTWARE attribute
   to 4.

   Any other error code causes the client to consider request.  Agents MAY include a SOFTWARE attribute in
   indications, depending on the transaction
   failed.

7.  FINGERPRINT Mechanism

   This section describes an optional mechanism for method.  Extensions to STUN that aids should
   discuss whether SOFTWARE is useful in
   distinguishing STUN messages from packets new indications.  Note that the
   inclusion of other protocols when a SOFTWARE attribute may have security implications; see
   Section 16.1.2 for details.

   For the
   two Binding method with no authentication, no attributes are multiplexed on
   required unless the same transport address.  This mechanism is
   optional, and a STUN usage must describe if and when it is used.  The
   FINGERPRINT mechanism is not backwards compatible with RFC3489, and
   cannot specifies otherwise.

   All STUN messages sent over UDP or DTLS-over-UDP [RFC6347] SHOULD be used in environments where such compatibility
   less than the path MTU, if known.

   If the path MTU is required.

   In some usages, STUN unknown for UDP, messages are multiplexed on SHOULD be the same transport
   address as other protocols, such as smaller of
   576 bytes and the Real Time Transport Protocol
   (RTP).  In order first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
   IPv6 [RFC8200].  This value corresponds to apply the processing described in Section 6, STUN
   messages must first be separated from overall size of the application packets.

   Section 5 describes three fixed fields in IP
   packet.  Consequently, for IPv4, the actual STUN header that can be
   used for this purpose.  However, in some cases, these three fixed
   fields may not message would need
   to be sufficient.

   When less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
   UDP header, assuming no IP options are used).

   If the FINGERPRINT extension path MTU is used, an agent includes unknown for DTLS-over-UDP, the
   FINGERPRINT attribute rules described in messages it sends
   the previous paragraph need to another agent.
   Section 14.7 describes be adjusted to take into account the placement and value
   size of this attribute.

   When the agent receives what it believes is a (13-byte) DTLS Record header, the Message Authentication
   Code (MAC) size, and the padding size.

   STUN message, then, in
   addition provides no ability to other basic checks, handle the agent also checks that case where the
   message contains a FINGERPRINT attribute and that request is
   smaller than the attribute
   contains MTU but the correct value.  Section 6.3 describes when in response is larger than the
   overall processing of MTU.  It is
   not envisioned that this limitation will be an issue for STUN.  The
   MTU limitation is a SHOULD, not a MUST, to account for cases where
   STUN message the FINGERPRINT check itself is
   performed.  This additional check helps being used to probe for MTU characteristics [RFC5780].
   See also [STUN-PMTUD] for a framework that uses STUN to add Path MTU
   Discovery to protocols that lack such a mechanism.  Outside of this
   or similar applications, the MTU constraint MUST be followed.

6.2.  Sending the Request or Indication

   The agent detect then sends the request or indication.  This document
   specifies how to send STUN messages of over UDP, TCP, TLS-over-TCP, or
   DTLS-over-UDP; other transport protocols that might otherwise seem to may be added in the future.
   The STUN messages.

8.  DNS Discovery Usage must specify which transport protocol is used and how
   the agent determines the IP address and port of a Server

   This section the recipient.
   Section 8 describes an optional procedure for STUN that allows a
   client to use DNS to determine DNS-based method of determining the IP address
   and port of a server.
   A STUN server that a usage must describe if and when this extension is used.  To
   use this procedure, the client must know may elect to use.

   At any time, a client MAY have multiple outstanding STUN URI [RFC7064]; the
   usage must also describe how requests
   with the client obtains this URI.  Hard-
   coding a same STUN URI into software is NOT RECOMMENDED server (that is, multiple transactions in case the domain
   name is lost or needs
   progress, with different transaction IDs).  Absent other limits to change
   the rate of new transactions (such as those specified by ICE for legal
   connectivity checks or other reasons.

   When when STUN is run over TCP), a client wishes SHOULD
   limit itself to ten outstanding transactions to locate a STUN server on the public Internet same server.

6.2.1.  Sending over UDP or DTLS-over-UDP

   When running STUN over UDP or STUN over DTLS-over-UDP [RFC7350], it
   is possible that accepts Binding request/response transactions, the STUN URI
   scheme is "stun".  When it wishes to locate a message might be dropped by the network.
   Reliability of STUN server that
   accepts Binding request/response transactions over a TLS, or DTLS
   session, the URI scheme is "stuns".

   The syntax accomplished
   through retransmissions of the "stun" and "stuns" URIs are defined in Section 3.1
   of [RFC7064]. request message by the client
   application itself.  STUN usages MAY define additional URI schemes.

8.1. indications are not retransmitted; thus,
   indication transactions over UDP or DTLS-over-UDP are not reliable.

   A client SHOULD retransmit a STUN URI Scheme Semantics

   If the <host> part request message starting with an
   interval of a "stun" URI contains RTO ("Retransmission TimeOut"), doubling after each
   retransmission.  The RTO is an IP address, then this
   IP address estimate of the round-trip time (RTT)
   and is used directly to contact computed as described in [RFC6298], with two exceptions.
   First, the server.  A "stuns" URI
   containing an IP address MUST initial value for RTO SHOULD be rejected.  A future STUN extension greater than or usage may relax equal to
   500 ms.  The exception cases for this requirement provided it demonstrates how "SHOULD" are when other
   mechanisms are used to
   authenticate derive congestion thresholds (such as the ones
   defined in ICE for fixed-rate streams) or when STUN server and prevent man is used in non-
   Internet environments with known network capacities.  In fixed-line
   access links, a value of 500 ms is RECOMMENDED.  Second, the middle attacks.

   If the URI does not contain an IP address, value of
   RTO SHOULD NOT be rounded up to the domain name contained
   in nearest second.  Rather, a 1 ms
   accuracy SHOULD be maintained.  As with TCP, the <host> part usage of Karn's
   algorithm is resolved RECOMMENDED [KARN87].  When applied to a transport address using the SRV
   procedures specified STUN, it means
   that RTT estimates SHOULD NOT be computed from STUN transactions that
   result in [RFC2782].  The DNS SRV service name is the
   content retransmission of the <scheme> part. a request.

   The protocol in value for RTO SHOULD be cached by a client after the SRV lookup is completion
   of the
   transport protocol transaction and used as the client will run STUN over: "udp" starting value for UDP and
   "tcp" RTO for TCP.

   The procedures of RFC 2782 are followed the
   next transaction to determine the same server (based on equality of IP
   address).  The value SHOULD be considered stale and discarded if no
   transactions have occurred to
   contact.  RFC 2782 spells out the details of how same server in the last 10 minutes.

   Retransmissions continue until a set of SRV records response is sorted received or until a
   total of Rc requests have been sent.  Rc SHOULD be configurable and then tried.  However, RFC 2782 only states that
   SHOULD have a default of 7.  If, after the
   client should "try to connect last request, a duration
   equal to Rm times the (protocol, address, service)" RTO has passed without giving any details on what happens in a response (providing
   ample time to get a response if only this final request actually
   succeeds), the event client SHOULD consider the transaction to have failed.
   Rm SHOULD be configurable and SHOULD have a default of failure.
   When following these procedures, 16.  A STUN
   transaction over UDP or DTLS-over-UDP is also considered failed if
   there has been a hard ICMP error [RFC1122].  For example, assuming an
   RTO of 500 ms, requests would be sent at times 0 ms, 500 ms, 1500 ms,
   3500 ms, 7500 ms, 15500 ms, and 31500 ms.  If the STUN transaction times out
   without receipt of client has not
   received a response, response after 39500 ms, the client SHOULD retry will consider the request
   transaction to have timed out.

6.2.2.  Sending over TCP or TLS-over-TCP

   For TCP and TLS-over-TCP [RFC5246], the next server in the ordered defined by RFC 2782.  Such client opens a retry TCP connection
   to the server.

   In some usages of STUN, STUN is the only possible for request/response transmissions, since indication
   transactions generate no response protocol over the TCP
   connection.  In this case, it can be sent without the aid of any
   additional framing or timeout. demultiplexing.  In addition, instead other usages, or with other
   extensions, it may be multiplexed with other data over a TCP
   connection.  In that case, STUN MUST be run on top of querying either some kind of
   framing protocol, specified by the A usage or the AAAA resource
   records extension, which allows
   for a domain name, a dual-stack IPv4/IPv6 client MUST query
   both the agent to extract complete STUN messages and try complete
   application-layer messages.  The STUN service running on the requests with all well-
   known port or ports discovered through the IP addresses received, as
   specified DNS procedures in [RFC8305].

   The default port for STUN requests
   Section 8 is 3478, for both TCP STUN alone, and UDP.
   The default port not for STUN over TLS multiplexed with other
   data.  Consequently, no framing protocols are used in connections to
   those servers.  When additional framing is utilized, the usage will
   specify how the client knows to apply it and what port to connect to.
   For example, in the case of ICE connectivity checks, this information
   is learned through out-of-band negotiation between client and server.

   Reliability of STUN over DTLS requests TCP and TLS-over-TCP is
   5349.  Servers can run STUN over DTLS on handled by TCP
   itself, and there are no retransmissions at the same port as STUN over
   UDP protocol level.
   However, for a request/response transaction, if the server software supports determining whether the initial
   message is client has not
   received a DTLS or STUN message.  Servers can run STUN over TLS on
   the same port as STUN over TCP if response by Ti seconds after it sent the server software supports
   determining whether request message,
   it considers the initial message is a TLS or STUN message.

   Administrators of STUN servers transaction to have timed out.  Ti SHOULD use these ports in their SRV
   records for UDP be
   configurable and TCP.  In all cases, SHOULD have a default of 39.5 s.  This value has
   been chosen to equalize the port in DNS MUST reflect TCP and UDP timeouts for the one on which default
   initial RTO.

   In addition, if the server client is listening.

   If no SRV records were found, unable to establish the client performs both an A and AAAA
   record lookup of TCP connection,
   or the domain name, as described TCP connection is reset or fails before a response is
   received, any request/response transaction in [RFC8305]. progress is considered
   to have failed.

   The
   result will be a list of IP addresses, each of which can be
   simultaneously contacted at the default port using UDP or TCP,
   independent of the STUN usage.  For usages that require TLS, the client connects MAY send multiple transactions over a single TCP (or TLS-
   over-TCP) connection, and it MAY send another request before
   receiving a response to the IP addresses using previous request.  The client SHOULD keep
   the default connection open until it:

   o  has no further STUN requests or indications to send over TLS
   port.  For usages that require DTLS, the client connects
      connection,

   o  has no plans to the IP
   addresses using the default use any resources (such as a mapped address
      (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
      [RFC5766]) that were learned though STUN requests sent over DTLS port.

9.  Authentication and Message-Integrity Mechanisms

   This section defines two mechanisms for STUN that
      connection,
   o  if multiplexing other application protocols over that port, has
      finished using those other protocols,

   o  if using that learned port with a client and server
   can use to provide authentication and message integrity; these two
   mechanisms are known remote peer, has established
      communications with that remote peer, as the short-term credential is required by some TCP
      NAT traversal techniques (e.g., [RFC6544]).

   The details of an eventual keep-alive mechanism and the
   long-term credential mechanism.  These two mechanisms are optional,
   and left to each usage must specify STUN
   Usage.  In any case, if a transaction fails because an idle TCP
   connection doesn't work anymore, the client SHOULD send a RST and when these mechanisms are used.
   Consequently, both clients and servers will know which mechanism (if
   any) try
   to follow based on knowledge of which usage applies.  For
   example, open a STUN server on the public Internet supporting ICE would
   have no authentication, whereas new TCP connection.

   At the STUN server functionality in an
   agent supporting connectivity checks would utilize short-term
   credentials.  An overview of these two mechanisms is given in
   Section 2.

   Each mechanism specifies end, the additional processing required to use server SHOULD keep the connection open and let
   the client close it, unless the server has determined that mechanism, extending the processing specified
   connection has timed out (for example, due to the client
   disconnecting from the network).  Bindings learned by the client will
   remain valid in Section 6. intervening NATs only while the connection remains
   open.  Only the client knows how long it needs the binding.  The
   additional processing occurs in three different places: when forming
   server SHOULD NOT close a message, when receiving connection if a message immediately after request was received over
   that connection for which a response was not sent.  A server MUST NOT
   ever open a connection back towards the basic
   checks have been performed, and when doing client in order to send a
   response.  Servers SHOULD follow best practices regarding connection
   management in cases of overload.

6.2.3.  Sending over TLS-over-TCP or DTLS-over-UDP

   When STUN is run by itself over TLS-over-TCP or DTLS-over-UDP, the detailed processing
   TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
   TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 ciphersuites MUST be
   implemented (for compatibility with older versions of
   error responses. this protocol),
   except if deprecated by rules of a specific STUN usage.  Other
   ciphersuites MAY be implemented.  Note that agents MUST ignore all attributes STUN clients and servers
   that follow MESSAGE-
   INTEGRITY, implement TLS version 1.3 [RFC8446] or subsequent versions are
   also required to implement mandatory ciphersuites from those
   specifications and SHOULD disable usage of deprecated ciphersuites
   when they detect support for those specifications.  Perfect Forward
   Secrecy (PFS) ciphersuites MUST be preferred over non-PFS
   ciphersuites.  Ciphersuites with the exception known weaknesses, such as those
   based on (single) DES and RC4, MUST NOT be used.  Implementations
   MUST disable TLS-level compression.

   These recommendations are just a part of the MESSAGE-INTEGRITY-SHA256 and
   FINGERPRINT attributes.  Similarly agents MUST ignore all attributes recommendations in
   [BCP195] that follow the MESSAGE-INTEGRITY-SHA256 attribute if the MESSAGE-
   INTEGRITY attribute is not present, with the exception implementations and deployments of the
   FINGERPRINT attribute.

9.1.  Short-Term Credential Mechanism

   The short-term credential mechanism assumes that, prior to the a STUN
   transaction, Usage using
   TLS or DTLS MUST follow.

   When it receives the TLS Certificate message, the client and server have used some other protocol to
   exchange a credential in MUST verify
   the form of a username certificate and password.  This
   credential is time-limited.  The time limit is defined inspect the site identified by the usage.

   As an example, in certificate.
   If the ICE usage [RFC8445], certificate is invalid or revoked, or if it does not identify
   the two endpoints use out-
   of-band signaling to agree on a username and password, and this
   username and password are applicable for appropriate party, the duration of client MUST NOT send the media
   session.

   This credential is used to form a message-integrity check in each
   request and in many responses.  There is no challenge and response as
   in STUN message or
   otherwise proceed with the long-term mechanism; consequently, replay is limited by virtue
   of STUN transaction.  The client MUST verify
   the time-limited nature identity of the credential.

9.1.1.  HMAC Key

   For short-term credentials the HMAC key is defined as follow:

                       key = OpaqueString(password)

   where server.  To do that, it follows the OpaqueString profile is
   identification procedures defined in [RFC8265].  The encoding
   used is UTF-8 [RFC3629].

9.1.2.  Forming [RFC6125], with a certificate
   containing an identifier of type DNS-ID or CN-ID, optionally with a Request
   wildcard character as the leftmost label, but not of type SRV-ID or Indication

   For
   URI-ID.

   When STUN is run multiplexed with other protocols over a request TLS-over-TCP
   connection or indication message, the agent MUST include a DTLS-over-UDP association, the
   USERNAME, MESSAGE-INTEGRITY-SHA256, mandatory ciphersuites
   and MESSAGE-INTEGRITY attributes
   in the message unless the agent knows from an external indication
   which message integrity algorithm is supported TLS handling procedures operate as defined by both agents.  In
   this case either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MUST
   be included in addition to USERNAME.  The HMAC for those protocols.

6.3.  Receiving a STUN Message

   This section specifies the MESSAGE-
   INTEGRITY attribute processing of a STUN message.  The
   processing specified here is computed for STUN messages as described defined in Section 14.5 and the
   HMAC this
   specification; additional rules for the MESSAGE-INTEGRITY-SHA256 attributes is computed as
   described backwards compatibility are
   defined in Section 14.6.  Note 11.  Those additional procedures are optional, and
   usages can elect to utilize them.  First, a set of processing
   operations is applied that is independent of the password class.  This is never included
   followed by class-specific processing, described in the request or indication.

9.1.3.  Receiving subsections
   that follow.

   When a Request or Indication

   After the STUN agent has done the basic processing of receives a STUN message, it first checks that the agent
   performs
   message obeys the rules of Section 5.  It checks listed below in order specified:

   o  If that the first two
   bits are 0, that the Magic Cookie field has the correct value, that
   the message does not contain 1) a MESSAGE-INTEGRITY or a
      MESSAGE-INTEGRITY-SHA256 attribute length is sensible, and 2) that the method value is a USERNAME attribute:

      *  If
   supported method.  It checks that the message class is a request, the server MUST reject allowed for
   the request
         with an error response.  This response MUST use an error code
         of 400 (Bad Request).

      * particular method.  If the message class is an indication, "Success Response" or
   "Error Response", the agent MUST silently
         discard the indication.

   o  If checks that the USERNAME does not contain transaction ID matches a username value currently valid
      within the server:

      *  If the message
   transaction that is a request, the server MUST reject the request
         with an error response.  This response MUST use an error code
         of 401 (Unauthenticated).

      * still in progress.  If the message FINGERPRINT extension
   is an indication, being used, the agent MUST silently
         discard the indication.

   o  If checks that the MESSAGE-INTEGRITY-SHA256 FINGERPRINT attribute is
   present compute and contains the
      value for correct value.  If any errors are detected,
   the message integrity as described in Section 14.6,
      using is silently discarded.  In the password associated case when STUN is being
   multiplexed with the username.  If the MESSAGE-
      INTEGRITY-SHA256 attribute another protocol, an error may indicate that this is
   not present, then use really a STUN message; in this case, the same
      password agent should try to compute the value for
   parse the message integrity as
      described in a different protocol.

   The STUN agent then does any checks that are required by a
   authentication mechanism that the usage has specified (see
   Section 14.5.  If 9).

   Once the resulting value does not match authentication checks are done, the STUN agent checks for
   unknown attributes and known-but-unexpected attributes in the
   message.  Unknown comprehension-optional attributes MUST be ignored
   by the agent.  Known-but-unexpected attributes SHOULD be ignored by
   the agent.  Unknown comprehension-required attributes cause
   processing that depends on the message class and is described below.

   At this point, further processing depends on the contents message class of the corresponding attribute (MESSAGE-INTEGRITY-
      SHA256 or MESSAGE-INTEGRITY):

      *
   request.

6.3.1.  Processing a Request

   If the message is a request, request contains one or more unknown comprehension-required
   attributes, the server MUST reject the request replies with an error response.  This response MUST use with an error
   code of 401 (Unauthenticated).

      *  If the message is 420 (Unknown Attribute) and includes an indication, UNKNOWN-ATTRIBUTES
   attribute in the agent MUST silently
         discard response that lists the indication. unknown comprehension-
   required attributes.

   Otherwise, the server then does any additional checking that the
   method or the specific usage requires.  If these all the checks pass, succeed,
   the agent continues to process server formulates a success response as described below.

   When run over UDP or DTLS-over-UDP, a request received by the server
   could be the first request of a transaction or
   indication.  Any response generated by could be a
   retransmission.  The server MUST respond to a request retransmissions such that
   contains a MESSAGE-INTEGRITY-SHA256 attribute MUST include
   the
   MESSAGE-INTEGRITY-SHA256 attribute, computed using following property is preserved: if the password
   utilized to authenticate client receives the request.  Any
   response generated by a
   server to a request the retransmission and not the response that contains only a MESSAGE-INTEGRITY attribute
   MUST include was sent to
   the MESSAGE-INTEGRITY attribute, computed using original request, the
   password utilized overall state on the client and server is
   identical to authenticate the request.  This means that case where only
   one of these attributes can appear in a response.  The the response MUST
   NOT contain to the USERNAME attribute.

   If any of original
   retransmission is received or where both responses are received (in
   which case the client will use the first).  The easiest way to meet
   this requirement is for the checks fail, a server MUST NOT include a MESSAGE-
   INTEGRITY-SHA256, MESSAGE-INTEGRITY, to remember all transaction IDs
   received over UDP or USERNAME attribute DTLS-over-UDP and their corresponding responses
   in the
   error response.  This is because, in these failure cases, last 40 seconds.  However, this requires the server
   cannot determine the shared secret necessary to compute the MESSAGE-
   INTEGRITY-SHA256 or MESSAGE-INTEGRITY attributes.

9.1.4.  Receiving a Response

   The client looks hold
   state and is inappropriate for any requests that are not
   authenticated.  Another way is to reprocess the MESSAGE-INTEGRITY or the MESSAGE-INTEGRITY-
   SHA256 attribute in the response.  If present request and if recompute
   the client only
   sent response.  The latter technique MUST only one of MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
   attributes in the be applied to requests
   that are idempotent (a request (because of is considered idempotent when the external indication in
   Section 9.1.2, or this being a subsequent same
   request as defined in
   Section 9.1.5) can be safely repeated without impacting the algorithm overall state of
   the system) and result in the same success response has for the same
   request.  The Binding method is considered to match otherwise be idempotent.  Note
   that there are certain rare network events that could cause the response
   reflexive transport address value to change, resulting in a different
   mapped address in different success responses.  Extensions to STUN
   MUST be discarded.

   The client then computes discuss the message integrity over implications of request retransmissions on servers
   that do not store transaction state.

6.3.1.1.  Forming a Success or Error Response

   When forming the response as
   defined in Section 14.5 (success or Section 14.6, respectively, using the same
   password it utilized for the request.  If error), the resulting value matches server follows the contents
   rules of Section 6.  The method of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
   attribute, respectively, the response is considered authenticated.
   If the value does not match, or if both MESSAGE-INTEGRITY and
   MESSAGE-INTEGRITY-SHA256 were absent, same as that
   of the processing depends on request, and the
   request been sent over a reliable message class is either "Success Response" or
   "Error Response".

   For an unreliable transport.

   If error response, the request was sent over server MUST add an unreliable transport, ERROR-CODE attribute
   containing the response
   MUST error code specified in the processing above.  The
   reason phrase is not fixed but SHOULD be discarded, as if it was never received.  This means that
   retransmits, if applicable, will continue.  If all something suitable for the responses
   received
   error code.  For certain errors, additional attributes are discarded then instead of signaling a timeout after
   ending added to
   the transaction message.  These attributes are spelled out in the layer MUST signal that description
   where the integrity
   protection was violated. error code is specified.  For example, for an error code of
   420 (Unknown Attribute), the server MUST include an UNKNOWN-
   ATTRIBUTES attribute.  Certain authentication errors also cause
   attributes to be added (see Section 9).  Extensions may define other
   errors and/or additional attributes to add in error cases.

   If the server authenticated the request was sent over a reliable transport, using an authentication
   mechanism, then the response MUST
   be discarded and server SHOULD add the layer MUST immediately end appropriate authentication
   attributes to the transaction and
   signal that response (see Section 9).

   The server also adds any attributes required by the integrity protection was violated.

9.1.5.  Sending Subsequent Requests

   A client sending subsequent requests specific method
   or usage.  In addition, the server SHOULD add a SOFTWARE attribute to
   the same server MUST send
   only message.

   For the MESSAGE-INTEGRITY-SHA256 or Binding method, no additional checking is required unless the MESSAGE-INTEGRITY
   usage specifies otherwise.  When forming the success response, the
   server adds an XOR-MAPPED-ADDRESS attribute
   that matches to the response; this
   attribute that was received in contains the response to source transport address of the
   initial request.  Here same server means same request
   message.  For UDP or DTLS-over-UDP, this is the source IP address and
   source UDP port
   number, not just the same URI or SRV lookup result.

9.2.  Long-Term Credential Mechanism

   The long-term credential mechanism relies on a long-term credential,
   in the form of a username and password that are shared between client
   and server.  The credential is considered long-term since it is
   assumed that it is provisioned for a user, and remains in effect
   until the user request message.  For TCP and TLS-over-TCP,
   this is no longer a subscriber of the system, or is
   changed.  This is basically a traditional "log-in" username and
   password given to users.

   Because these usernames source IP address and passwords are expected to be valid for
   extended periods source TCP port of time, replay prevention is provided in the form
   of a digest challenge.  In this mechanism, TCP
   connection as seen by the client initially sends
   a request, without offering any credentials server.

6.3.1.2.  Sending the Success or any integrity checks. Error Response

   The server rejects this request, providing response (success or error) is sent over the user a realm (used to
   guide same transport as
   the user request was received on.  If the request was received over UDP or agent in selection
   DTLS-over-UDP, the destination IP address and port of a username the response
   are the source IP address and password) port of the received request message,
   and
   a nonce.  The nonce provides a limited replay protection.  It is a
   cookie, selected by the server, source IP address and encoded in such a way as port of the response are equal to
   indicate a duration the
   destination IP address and port of validity the received request message.  If
   the request was received over TCP or client identity from which it TLS-over-TCP, the response is
   valid.  Only
   sent back on the same TCP connection as the request was received on.

   The server needs is allowed to know about send responses in a different order than it
   received the internal structure of requests.

6.3.2.  Processing an Indication

   If the cookie.  The client retries indication contains unknown comprehension-required attributes,
   the request, this time including its
   username indication is discarded and processing ceases.

   Otherwise, the realm, and echoing agent then does any additional checking that the nonce provided by
   method or the server.
   The client also includes one of specific usage requires.  If all the message-integrity attributes
   defined in this document, which provides checks succeed,
   the agent then processes the indication.  No response is generated
   for an HMAC over indication.

   For the entire
   request, including Binding method, no additional checking or processing is
   required, unless the nonce. usage specifies otherwise.  The server validates the nonce and
   checks mere receipt of
   the message integrity. by the agent has refreshed the bindings in the
   intervening NATs.

   Since indications are not re-transmitted over UDP or DTLS-over-UDP
   (unlike requests), there is no need to handle re-transmissions of
   indications at the sending agent.

6.3.3.  Processing a Success Response

   If they match, the request success response contains unknown comprehension-required
   attributes, the response is
   authenticated.  If discarded and the nonce is no longer valid, it transaction is
   considered
   "stale", and the server rejects the request, providing a new nonce.

   In subsequent requests to the same server, have failed.

   Otherwise, the client reuses then does any additional checking that the
   nonce, username, realm, and password it used previously.  In this
   way, subsequent requests are not rejected until
   method or the nonce becomes
   invalid by specific usage requires.  If all the server, in which case checks succeed,
   the rejection provides a new
   nonce to client then processes the client.

   Note that success response.

   For the long-term credential mechanism cannot be used to
   protect indications, since indications cannot be challenged.  Usages
   utilizing indications must either use a short-term credential or omit
   authentication and message integrity for them.

   To indicate Binding method, the client checks that it supports this specification, a server MUST
   prepend the NONCE XOR-MAPPED-ADDRESS
   attribute value with the character string composed
   of "obMatJos2" concatenated with the (4 character) Base64 [RFC4648]
   encoding of the 24 bit STUN Security Features as defined in
   Section 18.1.  The 24 bit Security Feature set is encoded as 3 bytes,
   with bit 0 as the most significant bit of the first byte and bit 23
   as present in the least significant bit of response.  The client checks the third byte. address
   family specified.  If no security
   features are used, then a byte array with all 24 bits set to zero
   MUST it is an unsupported address family, the
   attribute SHOULD be encoded instead.  For ignored.  If it is an unexpected but supported
   address family (for example, the remainder of this document Binding transaction was sent over
   IPv4, but the term
   "nonce cookie" will refer to address family specified is IPv6), then the complete 13 character string
   prepended to client MAY
   accept and use the NONCE attribute value.

   Since

6.3.4.  Processing an Error Response

   If the long-term credential mechanism is susceptible to offline
   dictionary attacks, deployments SHOULD utilize passwords that are
   difficult to guess.  In cases where error response contains unknown comprehension-required
   attributes, or if the credentials are error response does not entered
   by the user, but are rather placed on a client device during device
   provisioning, contain an ERROR-CODE
   attribute, then the password SHOULD transaction is simply considered to have at least 128 bits of
   randomness.  In cases where failed.

   Otherwise, the credentials are entered client then does any processing specified by the user,
   they should follow best current practices around password structure.

9.2.1.  Bid Down Attack Prevention
   authentication mechanism (see Section 9).  This document introduces two new security features that provide the
   ability to choose the algorithm used for password protection as well
   as the ability to use an anonymous username.  Both of these
   capabilities are optional may result in order to remain backwards compatible
   with previous versions of the STUN protocol.

   These a new capabilities are subject to bid-down attacks whereby an
   attacker in
   transaction attempt.

   The processing at this point depends on the message path can remove these capabilities and force
   weaker security properties.  To prevent these kinds of attacks from
   going undetected, error code, the nonce is enhanced with additional information.

   The value of method,
   and the "nonce cookie" will vary based on usage; the specific STUN
   Security Features bit values selected.  When this document makes
   reference to following are the "nonce cookie" in a section discussing a specific
   STUN Security Feature it default rules:

   o  If the error code is understood that 300 through 399, the corresponding STUN
   Security Feature bit in client SHOULD consider
      the "nonce cookie" is set to 1.

   For example, in Section 9.2.4 discussing transaction as failed unless the PASSWORD-ALGORITHMS
   security feature, it ALTERNATE-SERVER extension
      (Section 10) is implied that being used.

   o  If the "Password algorithms" bit,
   as defined in Section 18.1, error code is set to 1 400 through 499, the client declares the
      transaction failed; in the "nonce cookie".

9.2.2.  HMAC Key

   For long-term credentials that do not use a different algorithm, as
   specified by case of 420 (Unknown Attribute), the PASSWORD-ALGORITHM attribute,
      response should contain a UNKNOWN-ATTRIBUTES attribute that gives
      additional information.

   o  If the key is 16 bytes:

                key = MD5(username ":" OpaqueString(realm)
                  ":" OpaqueString(password))

   Where MD5 error code is defined in [RFC1321] and [RFC6151], and 500 through 599, the OpaqueString
   profile is defined in [RFC8265].  The encoding used is UTF-8
   [RFC3629].

   The 16-byte key is formed by taking client MAY resend the MD5 hash
      request; clients that do so MUST limit the number of times they do
      this.  Unless a specific error code specifies a different value,
      the result number of
   concatenating retransmissions SHOULD be limited to 4.

   Any other error code causes the following five fields: (1) client to consider the username, with any
   quotes and trailing nulls removed, as taken transaction
   failed.

7.  FINGERPRINT Mechanism

   This section describes an optional mechanism for STUN that aids in
   distinguishing STUN messages from packets of other protocols when the USERNAME
   attribute (in which case OpaqueString has already been applied); (2)
   a single colon; (3) the realm, with any quotes and trailing nulls
   removed
   two are multiplexed on the same transport address.  This mechanism is
   optional, and after processing using OpaqueString; (4) a single colon; STUN Usage must describe if and (5) the password, when it is used.  The
   FINGERPRINT mechanism is not backwards compatible with any trailing nulls removed RFC 3489 and after
   processing using OpaqueString.  For example, if the username was
   'user',
   cannot be used in environments where such compatibility is required.

   In some usages, STUN messages are multiplexed on the realm was 'realm', and same transport
   address as other protocols, such as the password was 'pass', then Real-Time Transport Protocol
   (RTP).  In order to apply the
   16-byte HMAC key would processing described in Section 6, STUN
   messages must first be separated from the result of performing an MD5 hash on the
   string 'user:realm:pass', the resulting hash being
   0x8493fbc53ba582fb4c044c456bdc40eb.

   The structure of application packets.

   Section 5 describes three fixed fields in the key when STUN header that can be
   used with long-term credentials
   facilitates deployment for this purpose.  However, in systems that also utilize SIP [RFC3261].
   Typically, SIP systems utilizing SIP's digest authentication
   mechanism do some cases, these three fixed
   fields may not actually store be sufficient.

   When the password FINGERPRINT extension is used, an agent includes the
   FINGERPRINT attribute in messages it sends to another agent.
   Section 14.7 describes the database.
   Rather, they store a placement and value called H(A1), which of this attribute.

   When the agent receives what it believes is equal a STUN message, then, in
   addition to other basic checks, the key
   defined above.  For example, this mechanism can be used with agent also checks that the
   authentication extensions defined in [RFC5090].

   When
   message contains a PASSWORD-ALGORITHM is used, the key length FINGERPRINT attribute and algorithm to
   use are described in that the attribute
   contains the correct value.  Section 18.5.1.

9.2.3.  Forming a Request

   There are two cases 6.3 describes when forming in the
   overall processing of a request.  In STUN message the first case, this FINGERPRINT check is
   performed.  This additional check helps the first request from the client agent detect messages of
   other protocols that might otherwise seem to the server (as identified by
   hostname, if the be STUN messages.

8.  DNS procedures Discovery of Section 8 are used, else a Server

   This section describes an optional procedure for STUN that allows a
   client to use DNS to determine the IP address and port of a server.
   A STUN Usage must describe if not).  In and when this extension is used.  To
   use this procedure, the second case, client must know a STUN URI [RFC7064]; the
   usage must also describe how the client obtains this URI.  Hard-
   coding a STUN URI into software is NOT RECOMMENDED in case the domain
   name is submitting lost or needs to change for legal or other reasons.

   When a
   subsequent request once client wishes to locate a previous STUN server on the public Internet
   that accepts Binding request/response transaction has
   completed successfully.  Forming a request as transactions, the STUN URI
   scheme is "stun".  When it wishes to locate a consequence of STUN server that
   accepts Binding request/response transactions over a 401 TLS or 438 error response is covered in Section 9.2.5 and is not
   considered a "subsequent request" and thus does not utilize DTLS
   session, the rules
   described in Section 9.2.3.2. URI scheme is "stuns".

   The difference between a first request syntax of the "stun" and a subsequent request "stuns" URIs is
   the presence or absence defined in Section 3.1
   of some attributes, so omitting or including
   them is a MUST.

9.2.3.1.  First Request [RFC7064].  STUN Usages MAY define additional URI schemes.

8.1.  STUN URI Scheme Semantics

   If the client has not completed <host> part of a successful request/response
   transaction with "stun" URI contains an IP address, then this
   IP address is used directly to contact the server, it server.  A "stuns" URI
   containing an IP address MUST omit the USERNAME, USERHASH,
   MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256, REALM, NONCE, PASSWORD-
   ALGORITHMS, and PASSWORD-ALGORITHM attributes.  In other words, the
   first request is sent as if there were no authentication be rejected.  A future STUN extension
   or message
   integrity applied.

9.2.3.2.  Subsequent Requests

   Once a request/response transaction has completed, the client will
   have been presented a realm and nonce by the server, and selected a
   username and password with which usage may relax this requirement, provided it authenticated.  The client SHOULD
   cache demonstrates how to
   authenticate the username, password, realm, STUN server and nonce for subsequent
   communications with prevent man-in-the-middle attacks.

   If the server.  When URI does not contain an IP address, the client sends a subsequent
   request, it MUST include either domain name contained
   in the USERNAME or USERHASH, REALM,
   NONCE, and PASSWORD-ALGORITHM attributes with these cached values.
   It MUST include a MESSAGE-INTEGRITY attribute or <host> part is resolved to a MESSAGE-INTEGRITY-
   SHA256 attribute, computed as described in Section 14.5 and
   Section 14.6 transport address using the cached password. SRV
   procedures specified in [RFC2782].  The choice between DNS SRV service name is the
   content of the <scheme> part.  The protocol in the two
   attributes depends on SRV lookup is the attribute received in
   transport protocol the response client will run STUN over: "udp" for UDP and
   "tcp" for TCP.

   The procedures of RFC 2782 are followed to the
   first request.

9.2.4.  Receiving a Request

   After determine the server has done to
   contact.  RFC 2782 spells out the basic processing details of how a request, it
   performs the checks listed below in the order specified.  Note that
   it set of SRV records
   is RECOMMENDED sorted and then tried.  However, RFC 2782 only states that the REALM value be
   client should "try to connect to the domain name of (protocol, address, service)"
   without giving any details on what happens in the
   provider event of failure.
   When following these procedures, if the STUN server:

   o  If the message does not contain transaction times out
   without receipt of a MESSAGE-INTEGRITY or MESSAGE-
      INTEGRITY-SHA256 attribute, response, the client SHOULD retry the request to
   the next server MUST in the order defined by RFC 2782.  Such a retry is
   only possible for request/response transmissions, since indication
   transactions generate an error no response with an error code or timeout.

   In addition, instead of 401 (Unauthenticated).  This
      response MUST include querying either the A or the AAAA resource
   records for a REALM value.  The response MUST include domain name, a
      NONCE, selected by the server.  The server dual-stack IPv4/IPv6 client MUST NOT choose query
   both and try the
      same NONCE requests with all the IP addresses received, as
   specified in [RFC8305].

   The default port for two STUN requests unless they have is 3478, for both TCP and UDP.
   The default port for STUN over TLS and STUN over DTLS requests is
   5349.  Servers can run STUN over DTLS on the same source IP
      address and port.  The server MAY support alternate password
      algorithms, in which case it can list them in preferential order
      in a PASSWORD-ALGORITHMS attribute.  If port as STUN over
   UDP if the server adds software supports determining whether the initial
   message is a
      PASSWORD-ALGORITHMS attribute it MUST set DTLS or STUN message.  Servers can run STUN over TLS on
   the same port as STUN Security
      Feature "Password algorithms" bit set to 1.  The over TCP if the server MAY
      support anonymous username, in which case it MUST set software supports
   determining whether the STUN
      Security Feature "Username anonymity" bit set to 1.  The response
      SHOULD NOT contain a USERNAME, USERHASH, MESSAGE-INTEGRITY or
      MESSAGE-INTEGRITY-SHA256 attribute.

   Note:  Reusing initial message is a NONCE for different source IP addresses TLS or STUN message.

   Administrators of STUN servers SHOULD use these ports was
      not explicitly forbidden in [RFC5389].

   o  If their SRV
   records for UDP and TCP.  In all cases, the message contains a MESSAGE-INTEGRITY or a MESSAGE-
      INTEGRITY-SHA256 attribute, but is missing either port in DNS MUST reflect
   the USERNAME or
      USERHASH, REALM, or NONCE attribute, one on which the server MUST generate an
      error response with is listening.

   If no SRV records are found, the client performs both an error code A and AAAA
   record lookup of 400 (Bad Request).  This
      response SHOULD NOT include a USERNAME, USERHASH, NONCE, or REALM. the domain name, as described in [RFC8305].  The response cannot contain
   result will be a MESSAGE-INTEGRITY list of IP addresses, each of which can be
   simultaneously contacted at the default port using UDP or MESSAGE-
      INTEGRITY-SHA256 attribute, as TCP,
   independent of the attributes required to generate
      them are missing.

   o  If STUN Usage.  For usages that require TLS, the NONCE attribute starts with
   client connects to the "nonce cookie" with IP addresses using the default STUN Security Feature "Password algorithms" bit set over TLS
   port.  For usages that require DTLS, the client connects to 1, the IP
   addresses using the default STUN over DTLS port.

9.  Authentication and Message-Integrity Mechanisms

   This section defines two mechanisms for STUN that a client and server performs
   can use to provide authentication and message integrity; these checks in two
   mechanisms are known as the order specified:

      *  If short-term credential mechanism and the request contains neither PASSWORD-ALGORITHMS nor
         PASSWORD-ALGORITHM, then
   long-term credential mechanism.  These two mechanisms are optional,
   and each usage must specify if and when these mechanisms are used.
   Consequently, both clients and servers will know which mechanism (if
   any) to follow based on knowledge of which usage applies.  For
   example, a STUN server on the request public Internet supporting ICE would
   have no authentication, whereas the STUN server functionality in an
   agent supporting connectivity checks would utilize short-term
   credentials.  An overview of these two mechanisms is processed as though
         PASSWORD-ALGORITHM were MD5 (Note given in
   Section 2.

   Each mechanism specifies the additional processing required to use
   that if mechanism, extending the PASSWORD-
         ALGORITHMS attribute is present but does not contain MD5, this
         will result processing specified in a 400 Bad Request Section 6.  The
   additional processing occurs in three different places: when forming
   a later step below).

      *  Otherwise, unless (1) PASSWORD-ALGORITHM and PASSWORD-
         ALGORITHMS are both present, (2) PASSWORD-ALGORITHMS matches
         the value sent in message, when receiving a message immediately after the response that sent this NONCE, basic
   checks have been performed, and (3)
         PASSWORD-ALGORITHM matches one of the entries in PASSWORD-
         ALGORITHMS, when doing the server MUST generate an detailed processing of
   error response responses.

   Note that agents MUST ignore all attributes that follow MESSAGE-
   INTEGRITY, with an
         error code the exception of 400 (Bad Request).

   o  If the NONCE is no longer valid MESSAGE-INTEGRITY-SHA256 and at
   FINGERPRINT attributes.  Similarly, agents MUST ignore all attributes
   that follow the same time MESSAGE-INTEGRITY-SHA256 attribute if the MESSAGE-
   INTEGRITY or a MESSAGE-INTEGRITY-SHA256 attribute is invalid, the
      server MUST generate an error response not present, with an error code the exception of 401.
      This response MUST include NONCE, REALM, and PASSWORD-ALGORITHMS
      attributes and SHOULD NOT include the USERNAME or USERHASH
   FINGERPRINT attribute.

9.1.  Short-Term Credential Mechanism

   The NONCE attribute value MUST be valid.  The response
      MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
      attribute, using the previous NONCE short-term credential mechanism assumes that, prior to calculate it.

   o  If the NONCE is no longer valid, STUN
   transaction, the client and server MUST generate an error
      response with an error code have used some other protocol to
   exchange a credential in the form of 438 (Stale Nonce). a username and password.  This response
      MUST include NONCE, REALM,
   credential is time-limited.  The time limit is defined by the usage.
   As an example, in the ICE usage [RFC8445], the two endpoints use out-
   of-band signaling to agree on a username and PASSWORD-ALGORITHMS attributes password, and
      SHOULD NOT include this
   username and password are applicable for the USERNAME, USERHASH attribute.  The NONCE
      attribute value MUST be valid.  The response MAY include a
      MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, using duration of the
      previous NONCE media
   session.

   This credential is used to calculate it.  Servers can revoke nonces form a message-integrity check in
      order to provide additional security.  See Section 5.4 each
   request and in many responses.  There is no challenge and response as
   in the long-term mechanism; consequently, replay is limited by virtue
   of
      [RFC7616] for guidelines.

   o  If the value time-limited nature of the USERNAME or USERHASH attribute credential.

9.1.1.  HMAC Key

   For short-term credentials, the Hash-Based Message Authentication
   Code (HMAC) key is not valid, defined as follow:

                       key = OpaqueString(password)

   where the server MUST generate an error response with an error code of
      401 (Unauthenticated).  This response MUST include a REALM value. OpaqueString profile is defined in [RFC8265].  The response MUST include encoding
   used is UTF-8 [RFC3629].

9.1.2.  Forming a NONCE, selected by Request or Indication

   For a request or indication message, the server.  The
      response agent MUST include a PASSWORD-ALGORITHMS attribute.  The
      response SHOULD NOT contain a the
   USERNAME, USERHASH attribute.  The
      response MAY include a MESSAGE-INTEGRITY-SHA256, and MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
      SHA256 attribute, using attributes
   in the previous key to calculate it.

   o  If message unless the agent knows from an external mechanism
   which message integrity algorithm is supported by both agents.  In
   this case, either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MUST
   be included in addition to USERNAME.  The HMAC for the MESSAGE-
   INTEGRITY attribute is present compute computed as described in Section 14.5, and the
      value
   HMAC for the message integrity MESSAGE-INTEGRITY-SHA256 attributes is computed as
   described in Section 14.6,
      using 14.6.  Note that the password associated with is never included
   in the username.  Else, using request or indication.

9.1.3.  Receiving a Request or Indication

   After the
      same password, compute agent has done the value for basic processing of a message, the message integrity as
      described agent
   performs the checks listed below in Section 14.5. the order specified:

   o  If the resulting value message does not match
      the contents of the contain 1) a MESSAGE-INTEGRITY attribute or a
      MESSAGE-INTEGRITY-SHA256 attribute and 2) a USERNAME attribute:

      *  If the MESSAGE-
      INTEGRITY-SHA256 attribute, message is a request, the server MUST reject the request
         with an error response.  This response MUST use an error code
         of
      401 (Unauthenticated).  It MUST include REALM and NONCE attributes
      and SHOULD NOT include the USERNAME, USERHASH, MESSAGE-INTEGRITY,
      or MESSAGE-INTEGRITY-SHA256 attribute. 400 (Bad Request).

      *  If these checks pass, the server continues to process the request.
   Any response generated by message is an indication, the server agent MUST include MESSAGE-INTEGRITY-
   SHA256 attribute, computed using the username and password utilized
   to authenticate the request, unless the request was processed as
   though PASSWORD-ALGORITHM was MD5 (because the request contained
   neither PASSWORD-ALGORITHMS nor PASSWORD-ALGORITHM).  In that case silently
         discard the MESSAGE-INTEGRITY attribute MUST be used instead of indication.

   o  If the MESSAGE-
   INTEGRITY-SHA256 attribute.  The REALM, NONCE, USERNAME and USERHASH
   attributes SHOULD NOT be included.

9.2.5.  Receiving does not contain a Response username value currently valid
      within the server:

      *  If the response message is a request, the server MUST reject the request
         with an error response.  This response with MUST use an error code
         of 401
   (Unauthenticated) or 438 (Stale Nonce), (Unauthenticated).

      *  If the client message is an indication, the agent MUST test if silently
         discard the
   NONCE indication.

   o  If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
      value starts for the message integrity as described in Section 14.6,
      using the password associated with the "nonce cookie". username.  If the test
   succeeds and the "nonce cookie" has MESSAGE-
      INTEGRITY-SHA256 attribute is not present, then use the STUN Security Feature
   "Password algorithms" bit set same
      password to 1 but no PASSWORD-ALGORITHMS compute the value for the message integrity as
      described in Section 14.5.  If the resulting value does not match
      the contents of the corresponding attribute (MESSAGE-INTEGRITY-
      SHA256 or MESSAGE-INTEGRITY):

      *  If the message is present, then a request, the client server MUST NOT retry reject the request
         with
   a new transaction.

   If the response is an error response.  This response with MUST use an error code
         of 401
   (Unauthenticated), (Unauthenticated).

      *  If the client SHOULD retry message is an indication, the request with a new
   transaction.  This request agent MUST contain a USERNAME or a USERHASH,
   determined by the client as the appropriate username for the REALM
   from silently
         discard the error response. indication.

   If these checks pass, the "nonce cookie" was present and had
   the STUN Security Feature "Username anonymity" bit set agent continues to 1 then the
   USERHASH attribute MUST be used, else the USERNAME attribute MUST be
   used.  The request MUST contain the REALM, copied from process the error
   response.  The request MUST contain the NONCE, copied from the error
   response.  If the or
   indication.  Any response contains generated by a server to a PASSWORD-ALGORITHMS attribute,
   the request MUST contain the PASSWORD-ALGORITHMS attribute with the
   same content.  If the response that
   contains a PASSWORD-ALGORITHMS
   attribute, and this MESSAGE-INTEGRITY-SHA256 attribute contains at least one algorithm that is
   supported by the client then the request MUST contain a PASSWORD-
   ALGORITHM attribute with include the first algorithm supported on
   MESSAGE-INTEGRITY-SHA256 attribute, computed using the list.
   If password
   utilized to authenticate the request.  Any response generated by a
   server to a request that contains only a PASSWORD-ALGORITHMS attribute, and this MESSAGE-INTEGRITY attribute does not contain any algorithm that is supported by
   MUST include the
   client, then MESSAGE-INTEGRITY attribute, computed using the client MUST NOT retry
   password utilized to authenticate the request with request.  This means that only
   one of these attributes can appear in a new
   transaction. response.  The client response MUST
   NOT perform this retry if it is not
   changing contain the USERNAME or USERHASH or REALM or its associated
   password, from the previous attempt. attribute.

   If the response is an error response with an error code any of 438 (Stale
   Nonce), the client checks fail, a server MUST retry the request, using the new NONCE NOT include a MESSAGE-
   INTEGRITY-SHA256, MESSAGE-INTEGRITY, or USERNAME attribute supplied in the 438 (Stale Nonce)
   error response.  This retry
   MUST also include either is because, in these failure cases, the USERNAME server
   cannot determine the shared secret necessary to compute the MESSAGE-
   INTEGRITY-SHA256 or USERHASH, REALM and either MESSAGE-INTEGRITY attributes.

9.1.4.  Receiving a Response

   The client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attributes.

   For all other responses, if the NONCE MESSAGE-INTEGRITY-
   SHA256 attribute starts with the
   "nonce cookie" with the STUN Security Feature "Password algorithms"
   bit set to 1 but PASSWORD-ALGORITHMS is not present, in the response
   MUST be ignored. response.  If present and if the response is an error response with an error code client only
   sent one of 400, and
   does not contains either the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
   SHA256 attribute then the response MUST be discarded, as if it was
   never received.  This means that retransmits, if applicable, will
   continue.

   Note:  In that case MESSAGE-INTEGRITY-SHA256
   attributes in the 400 will never reach request (because of the application,
      resulting external indication in
   Section 9.1.2 or because this is a timeout.

   The client looks for subsequent request as defined in
   Section 9.1.5), the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
   SHA256 attribute algorithm in the response (either success or failure).  If
   present, has to match;
   otherwise, the response MUST be discarded.

   The client then computes the message integrity over the response as
   defined in Section 14.5 for the MESSAGE-INTEGRITY attribute or
   Section 14.6, 14.6 for the MESSAGE-INTEGRITY-SHA256 attribute, using the
   same password it utilized for the request.  If the resulting value
   matches the contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MESSAGE-INTEGRITY-
   SHA256 attribute, respectively, the response is considered
   authenticated.  If the value does not match, or if both MESSAGE-INTEGRITY MESSAGE-
   INTEGRITY and MESSAGE-INTEGRITY-
   SHA256 were MESSAGE-INTEGRITY-SHA256 are absent, the processing
   depends on whether the request been was sent over a reliable or an
   unreliable transport.

   If the request was sent over an unreliable transport, the response
   MUST be discarded, as if it was had never been received.  This means that
   retransmits, if applicable, will continue.  If all the responses
   received are discarded discarded, then instead of signaling a timeout after
   ending the transaction transaction, the layer MUST signal that the integrity
   protection was violated.

   If the request was sent over a reliable transport, the response MUST
   be discarded discarded, and the layer MUST immediately end the transaction and
   signal that the integrity protection was violated.

   If the response contains a PASSWORD-ALGORITHMS attribute, all the
   subsequent requests MUST be authenticated using MESSAGE-INTEGRITY-
   SHA256 only.

10.  ALTERNATE-SERVER Mechanism

   This section describes a mechanism in STUN that allows a server to
   redirect a client to another server.  This extension is optional, and
   a usage must define if and when this extension is used.  The
   ALTERNATE-SERVER attribute carries an IP address.

9.1.5.  Sending Subsequent Requests

   A server using this extension redirects a client sending subsequent requests to another server by
   replying to a request message with an error response message with an
   error code of 300 (Try Alternate).  The the same server MUST include at least
   one ALTERNATE-SERVER send
   only the MESSAGE-INTEGRITY-SHA256 or the MESSAGE-INTEGRITY attribute
   that matches the attribute that was received in the error response, which MUST
   contain an IP address of response to the
   initial request.  Here, "same server" means same family as the source IP address of and port
   number, not just the request message. same URI or SRV lookup result.

9.2.  Long-Term Credential Mechanism

   The server SHOULD include an additional
   ALTERNATE-SERVER attribute, after the mandatory one, that contains an
   IP address of the other family than long-term credential mechanism relies on a long-term credential,
   in the source IP address form of the
   request message.  The error response message MAY be authenticated;
   however, there a username and password that are use cases shared between client
   and server.  The credential is considered long-term since it is
   assumed that it is provisioned for ALTERNATE-SERVER where
   authentication of a user and remains in effect until
   the response user is not possible or practical.  If no longer a subscriber of the
   transaction uses TLS system or DTLS and if the transaction until it is authenticated
   by
   changed.  This is basically a MESSAGE-INTEGRITY-SHA256 attribute traditional "log-in" username and if the server wants
   password given to
   redirect users.

   Because these usernames and passwords are expected to a server that uses a different certificate, then it MUST
   include an ALTERNATE-DOMAIN attribute containing the name inside the
   subjectAltName of that certificate.  This series be valid for
   extended periods of conditions on the
   MESSAGE-INTEGRITY-SHA256 attribute indicates that the transaction time, replay prevention is
   authenticated and that provided in the client implements form
   of a digest challenge.  In this specification and
   therefore can process mechanism, the ALTERNATE-DOMAIN attribute.

   A client using this extension handles initially sends
   a 300 (Try Alternate) error
   code as follows. request, without offering any credentials or any integrity checks.
   The client looks for an ALTERNATE-SERVER attribute
   in the error response.  If one is found, then server rejects this request, providing the client considers user a realm (used to
   guide the current transaction as failed, user or agent in selection of a username and reattempts the request with password) and
   a nonce.  The nonce provides a limited replay protection.  It is a
   cookie, selected by the server specified and encoded in such a way as to
   indicate a duration of validity or client identity from which it is
   valid.  Only the attribute, using server needs to know about the same transport
   protocol used for internal structure of
   the cookie.  The client retries the previous request.  That request, if
   authenticated, MUST utilize this time including its
   username and the same credentials that realm and echoing the nonce provided by the server.
   The client
   would have used also includes one of the message-integrity attributes
   defined in this document, which provides an HMAC over the request to entire
   request, including the nonce.  The server that performed validates the
   redirection.  If nonce and
   checks the transport protocol uses TLS or DTLS, then message integrity.  If they match, the
   client looks for an ALTERNATE-DOMAIN attribute. request is
   authenticated.  If the attribute nonce is
   found, no longer valid, it is considered
   "stale", and the domain MUST be used server rejects the request, providing a new nonce.

   In subsequent requests to validate the certificate using same server, the
   recommendations in [RFC6125].  The certificate MUST contain an
   identifier of type DNS-ID or CN-ID, eventually with wildcards, but
   not of type SRV-ID or URI-ID.  If client reuses the attribute is
   nonce, username, realm, and password it used previously.  In this
   way, subsequent requests are not found, rejected until the
   same domain that was used for nonce becomes
   invalid by the original request MUST be used server, in which case the rejection provides a new
   nonce to
   validate the certificate.  If client.

   Note that the client has been redirected long-term credential mechanism cannot be used to
   protect indications, since indications cannot be challenged.  Usages
   utilizing indications must either use a
   server to which short-term credential or omit
   authentication and message integrity for them.

   To indicate that it has already sent supports this request within the last five
   minutes, it specification, a server MUST ignore
   prepend the redirection and consider NONCE attribute value with the transaction
   to have failed.  This prevents infinite ping-ponging between servers
   in case character string composed
   of redirection loops.

11.  Backwards Compatibility "obMatJos2" concatenated with RFC 3489

   In addition to the backward compatibility already described (4-character) base64 [RFC4648]
   encoding of the 24-bit STUN Security Features as defined in
   Section 12 of [RFC5389], DTLS MUST NOT be used 18.1.  The 24-bit Security Feature set is encoded as 3 bytes,
   with [RFC3489] (also
   referred to bit 0 as "classic STUN").  Any STUN request or indication
   without the magic cookie (see Section 6 most significant bit of [RFC5389]) over DTLS MUST
   be considered invalid: all requests MUST generate a "500 Server
   Error" error response the first byte and indications MUST be ignored.

12.  Basic Server Behavior

   This section defines bit 23
   as the behavior least significant bit of a basic, stand-alone STUN
   server.

   Historically, "classic STUN [RFC3489]" only defined the behavior of third byte.  If no security
   features are used, then a
   server that was providing clients byte array with server reflexive transport
   addresses by receiving and replying all 24 bits set to STUN Binding requests.
   [RFC5389] redefined zero
   MUST be encoded instead.  For the protocol as an extensible framework and remainder of this document, the
   server functionality became
   term "nonce cookie" will refer to the sole STUN Usage defined in that
   document.  This STUN Usage is also known as Basic STUN Server.

   The STUN server MUST support complete 13-character string
   prepended to the Binding method.  It SHOULD NOT
   utilize NONCE attribute value.

   Since the short-term or long-term credential mechanism. mechanism is susceptible to offline
   dictionary attacks, deployments SHOULD utilize passwords that are
   difficult to guess.  In cases where the credentials are not entered
   by the user, but are rather placed on a client device during device
   provisioning, the password SHOULD have at least 128 bits of
   randomness.  In cases where the credentials are entered by the user,
   they should follow best current practices around password structure.

9.2.1.  Bid-Down Attack Prevention

   This is
   because document introduces two new security features that provide the work involved in authenticating
   ability to choose the request is more than algorithm used for password protection as well
   as the work ability to use an anonymous username.  Both of these
   capabilities are optional in simply processing it.  It SHOULD NOT utilize the
   ALTERNATE-SERVER mechanism for order to remain backwards compatible
   with previous versions of the same reason.  It MUST support UDP
   and TCP.  It MAY support STUN over TCP/TLS or STUN over UDP/DTLS;
   however, DTLS and TLS provide minimal security benefits protocol.

   These new capabilities are subject to bid-down attacks whereby an
   attacker in this basic
   mode of operation.  It does not require a keep-alive mechanism
   because a TCP or TLS-over-TCP connection is closed after the end message path can remove these capabilities and force
   weaker security properties.  To prevent these kinds of attacks from
   going undetected, the Binding transaction.  It MAY utilize the FINGERPRINT mechanism
   but MUST NOT require it.  Since the stand-alone server only runs
   STUN, FINGERPRINT provides no benefit.  Requiring it would break
   compatibility with RFC 3489, and such compatibility nonce is desirable in a
   stand-alone server.  Stand-alone STUN servers SHOULD support
   backwards compatibility enhanced with [RFC3489] clients, as described in
   Section 11.

   It is RECOMMENDED that administrators additional information.

   The value of the "nonce cookie" will vary based on the specific STUN servers provide DNS
   entries for those servers as described in Section 8.  If both A and
   AAAA Resource Records are returned then
   Security Feature bits selected.  When this document makes reference
   to the "nonce cookie" in a section discussing a specific STUN
   Security Feature it is understood that the client can simultaneously
   send corresponding STUN Binding requests
   Security Feature bit in the "nonce cookie" is set to 1.

   For example, when the IPv4 and IPv6 addresses (as
   specified PASSWORD-ALGORITHMS security feature (defined
   in [RFC8305]), as Section 9.2.4) is used, the Binding request corresponding "Password algorithms"
   bit (defined in Section 18.1) is idempotent.  Note
   that set to 1 in the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS attributes "nonce cookie".

9.2.2.  HMAC Key

   For long-term credentials that are
   returned will do not necessarily match use a different algorithm, as
   specified by the address family of PASSWORD-ALGORITHM attribute, the server
   address used.

   A basic STUN server key is not a solution for NAT traversal by itself.
   However, it can be utilized as part of a solution through STUN
   usages.  This 16 bytes:

                key = MD5(username ":" OpaqueString(realm)
                  ":" OpaqueString(password))

   Where MD5 is discussed further defined in Section 13.

13.  STUN Usages

   STUN by itself is not a solution to [RFC1321] and [RFC6151], and the NAT traversal problem.
   Rather, STUN defines a tool that can be OpaqueString
   profile is defined in [RFC8265].  The encoding used inside a larger
   solution. is UTF-8
   [RFC3629].

   The term "STUN usage" 16-byte key is used for formed by taking the MD5 hash of the result of
   concatenating the following five fields: (1) the username, with any solution that uses
   STUN
   quotes and trailing nulls removed, as taken from the USERNAME
   attribute (in which case OpaqueString has already been applied); (2)
   a component.

   A STUN usage defines how STUN is actually utilized -- when to send
   requests, what to do single colon; (3) the realm, with any quotes and trailing nulls
   removed and after processing using OpaqueString; (4) a single colon;
   and (5) the responses, password, with any trailing nulls removed and which optional
   procedures defined here (or in an extension to STUN) are to be used.
   A usage also defines:

   o  Which STUN methods are used.

   o  What transports are used.  If DTLS-over-UDP after
   processing using OpaqueString.  For example, if the username is used then
      implementing
   'user', the denial-of-service countermeasure described in
      Section 4.2.1 of [RFC6347] realm is mandatory.

   o  What authentication 'realm', and message-integrity mechanisms are used.

   o  The considerations around manual vs. automatic key derivation for the integrity mechanism, as discussed in [RFC4107].

   o  What mechanisms are used to distinguish STUN messages from other
      messages.  When STUN password is run over TCP or TLS-over-TCP, a framing
      mechanism may 'pass', then the
   16-byte HMAC key would be required.

   o  How a STUN client determines the IP address and port result of performing an MD5 hash on the STUN
      server.

   o  How simultaneous use
   string 'user:realm:pass', the resulting hash being
   0x8493fbc53ba582fb4c044c456bdc40eb.

   The structure of IPv4 and IPv6 addresses (Happy Eyeballs
      [RFC8305]) works with non-idempotent transactions the key when both
      address families are found for used with long-term credentials
   facilitates deployment in systems that also utilize SIP [RFC3261].
   Typically, SIP systems utilizing SIP's digest authentication
   mechanism do not actually store the STUN server.

   o  Whether backwards compatibility to RFC 3489 is required.

   o  What optional attributes defined here (such as FINGERPRINT and
      ALTERNATE-SERVER) or password in other extensions are required.

   o  If MESSAGE-INTEGRITY-SHA256 truncation the database.
   Rather, they store a value called "H(A1)", which is permitted, and equal to the
      limits permitted for truncation.

   o  The keep-alive key
   defined above.  For example, this mechanism if STUN is run over TCP or TLS-over-TCP.

   o  If Anycast addresses can be used for with the server in case TCP or
      TLS-over-TCP, or
   authentication are used.

   In addition, any STUN usage must consider the security implications
   of using STUN extensions defined in that usage.  A number of attacks against STUN are
   known (see [RFC5090].

   When a PASSWORD-ALGORITHM is used, the Security Considerations section in this document), key length and
   any usage must consider how these attacks can be thwarted or
   mitigated.

   Finally, algorithm to
   use are described in Section 18.5.1.

9.2.3.  Forming a usage must consider whether its usage of STUN is an
   example of Request

   The first request from the Unilateral Self-Address Fixing approach client to NAT
   traversal, and the server (as identified by
   hostname if so, the DNS procedures of Section 8 are used and by IP
   address if not) is handled according to the questions raised rules in RFC 3424
   [RFC3424].

14.  STUN Attributes

   After Section 9.2.3.1.
   When the STUN header are zero or more attributes.  Each attribute
   MUST be TLV encoded, with client initiates a 16-bit type, 16-bit length, and value.
   Each STUN attribute MUST end on subsequent request once a 32-bit boundary.  As mentioned
   above, all fields previous
   request/response transaction has completed successfully, it follows
   the rules in an attribute are transmitted most significant
   bit first.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Type                  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Value (variable)                ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: Format Section Section 9.2.3.2.  Forming a request as a
   consequence of STUN Attributes

   The value a 401 (Unauthenticated) or 438 (Stale Nonce) error
   response is covered in Section 9.2.5 and is not considered a
   "subsequent request" and thus does not utilize the length field MUST contain the length rules described in
   Section 9.2.3.2.  Each of the Value
   part these types of requests have a different
   mandatory attributes.

9.2.3.1.  First Request

   If the attribute, prior to padding, measured in bytes.  Since
   STUN aligns attributes on 32-bit boundaries, attributes whose content
   is client has not completed a multiple of 4 bytes are padded successful request/response
   transaction with 1, 2, the server, it MUST omit the USERNAME, USERHASH,
   MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256, REALM, NONCE, PASSWORD-
   ALGORITHMS, and PASSWORD-ALGORITHM attributes.  In other words, the
   first request is sent as if there were no authentication or 3 bytes of
   padding so that its value contains message
   integrity applied.

9.2.3.2.  Subsequent Requests

   Once a multiple of 4 bytes. request/response transaction has completed, the client will
   have been presented a realm and nonce by the server and selected a
   username and password with which it authenticated.  The
   padding bits MUST be set to zero on sending client SHOULD
   cache the username, password, realm, and nonce for subsequent
   communications with the server.  When the client sends a subsequent
   request, it MUST be ignored by include either the receiver.

   Any USERNAME or USERHASH, REALM,
   NONCE, and PASSWORD-ALGORITHM attributes with these cached values.
   It MUST include a MESSAGE-INTEGRITY attribute type MAY appear more than once or a MESSAGE-INTEGRITY-
   SHA256 attribute, computed as described in Sections 14.5 and 14.6
   using the cached password.  The choice between the two attributes
   depends on the attribute received in a STUN message.
   Unless specified otherwise, the order of appearance is significant:
   only response to the first occurrence needs to be processed by a receiver, and
   any duplicates MAY be ignored by
   request.

9.2.4.  Receiving a receiver.

   To allow future revisions Request

   After the server has done the basic processing of this specification to add new attributes
   if needed, a request, it
   performs the attribute space checks listed below in the order specified.  Note that
   it is divided into two ranges.
   Attributes with type values between 0x0000 and 0x7FFF are
   comprehension-required attributes, which means RECOMMENDED that the STUN agent
   cannot successfully process REALM value be the message unless it understands domain name of the
   attribute.  Attributes with type values between 0x8000 and 0xFFFF are
   comprehension-optional attributes, which means that those attributes
   can be ignored by
   provider of the STUN agent if it server:

   o  If the message does not understand them.

   The set contain a MESSAGE-INTEGRITY or MESSAGE-
      INTEGRITY-SHA256 attribute, the server MUST generate an error
      response with an error code of STUN attribute types is maintained by IANA. 401 (Unauthenticated).  This
      response MUST include a REALM value.  The initial
   set defined response MUST include a
      NONCE, selected by this specification is found in Section 18.3. the server.  The rest of this section describes server MUST NOT choose the format of
      same NONCE for two requests unless they have the various
   attributes defined same source IP
      address and port.  The server MAY support alternate password
      algorithms, in this specification.

14.1.  MAPPED-ADDRESS which case it can list them in preferential order
      in a PASSWORD-ALGORITHMS attribute.  If the server adds a
      PASSWORD-ALGORITHMS attribute, it MUST set the STUN Security
      Feature "Password algorithms" bit to 1.  The MAPPED-ADDRESS attribute indicates a reflexive transport address
   of server MAY support
      anonymous username, in which case it MUST set the client.  It consists of an 8-bit address family and STUN Security
      Feature "Username anonymity" bit set to 1.  The response SHOULD
      NOT contain a 16-bit
   port, followed by USERNAME, USERHASH, MESSAGE-INTEGRITY, or MESSAGE-
      INTEGRITY-SHA256 attribute.

      Note: Reusing a fixed-length value representing the NONCE for different source IP address. addresses or ports
      was not explicitly forbidden in [RFC5389].

   o  If the address family message contains a MESSAGE-INTEGRITY or a MESSAGE-
      INTEGRITY-SHA256 attribute, but is IPv4, the address MUST be 32 bits.  If missing either the
   address family is IPv6, USERNAME or
      USERHASH, REALM, or NONCE attribute, the address server MUST be 128 bits.  All fields
   must be in network byte order.

   The format generate an
      error response with an error code of the MAPPED-ADDRESS attribute is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 0 0 0 0 0|    Family     |           Port                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 Address (32 bits 400 (Bad Request).  This
      response SHOULD NOT include a USERNAME, USERHASH, NONCE, or 128 bits)                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 5: Format of MAPPED-ADDRESS Attribute REALM
      attribute.  The address family can take on response cannot contain a MESSAGE-INTEGRITY or
      MESSAGE-INTEGRITY-SHA256 attribute, as the following values:

   0x01:IPv4
   0x02:IPv6

   The first 8 bits of attributes required to
      generate them are missing.

   o  If the MAPPED-ADDRESS MUST be NONCE attribute starts with the "nonce cookie" with the
      STUN Security Feature "Password algorithms" bit set to 0 1, the
      server performs these checks in the order specified:

      *  If the request contains neither the PASSWORD-ALGORITHMS nor the
         PASSWORD-ALGORITHM algorithm, then the request is processed as
         though PASSWORD-ALGORITHM were MD5.

      *  Otherwise, unless (1) PASSWORD-ALGORITHM and MUST be
   ignored by receivers.  These bits PASSWORD-
         ALGORITHMS are present for aligning parameters
   on natural 32-bit boundaries.

   This attribute is used only by servers for achieving backwards
   compatibility with [RFC3489] clients.

14.2.  XOR-MAPPED-ADDRESS

   The XOR-MAPPED-ADDRESS attribute is identical to both present, (2) PASSWORD-ALGORITHMS matches
         the MAPPED-ADDRESS
   attribute, except value sent in the response that sent this NONCE, and (3)
         PASSWORD-ALGORITHM matches one of the reflexive transport address is obfuscated
   through entries in PASSWORD-
         ALGORITHMS, the XOR function.

   The format server MUST generate an error response with an
         error code of 400 (Bad Request).

   o  If the XOR-MAPPED-ADDRESS is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 0 0 0 0 0|    Family     |         X-Port                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                X-Address (Variable)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 6: Format value of XOR-MAPPED-ADDRESS Attribute the USERNAME or USERHASH attribute is not valid,
      the server MUST generate an error response with an error code of
      401 (Unauthenticated).  This response MUST include a REALM value.
      The response MUST include a NONCE, selected by the server.  The
      response MUST include a PASSWORD-ALGORITHMS attribute.  The
      response SHOULD NOT contain a USERNAME or USERHASH attribute.  The Family represents
      response MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
      SHA256 attribute, using the IP address family, and is encoded
   identically previous key to calculate it.

   o  If the Family in MAPPED-ADDRESS.

   X-Port MESSAGE-INTEGRITY-SHA256 attribute is computed by XOR'ing present, compute the mapped port
      value for the message integrity as described in Section 14.6,
      using the password associated with the most
   significant 16 bits of username.  Otherwise, using
      the magic cookie. same password, compute the value for the MESSAGE-INTEGRITY
      attribute as described in Section 14.5.  If the IP address family is
   IPv4, X-Address is computed by XOR'ing resulting value
      does not match the mapped IP address contents of the MESSAGE-INTEGRITY attribute or
      the MESSAGE-INTEGRITY-SHA256 attribute, the server MUST reject the
      request with an error response.  This response MUST use an error
      code of 401 (Unauthenticated).  It MUST include the
   magic cookie. REALM and
      NONCE attributes and SHOULD NOT include the USERNAME, USERHASH,
      MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY-SHA256 attribute.

   o  If the IP address family is IPv6, X-Address NONCE is
   computed by XOR'ing no longer valid, the mapped IP address server MUST generate an error
      response with the concatenation an error code of 438 (Stale Nonce).  This response
      MUST include NONCE, REALM, and PASSWORD-ALGORITHMS attributes and
      SHOULD NOT include the magic cookie USERNAME and USERHASH attributes.  The
      NONCE attribute value MUST be valid.  The response MAY include a
      MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, using the 96-bit transaction ID.  In all cases,
      previous NONCE to calculate it.  Servers can revoke nonces in
      order to provide additional security.  See Section 5.4 of
      [RFC7616] for guidelines.

   If these checks pass, the server continues to process the request.
   Any response generated by the
   XOR operation works on its inputs in network byte order (that is, server MUST include the
   order they will be encoded in MESSAGE-
   INTEGRITY-SHA256 attribute, computed using the message).

   The rules for encoding username and processing password
   utilized to authenticate the first 8 bits of request, unless the
   attribute's value, request was
   processed as though PASSWORD-ALGORITHM was MD5 (because the rules for handling multiple occurrences request
   contained neither PASSWORD-ALGORITHMS nor PASSWORD-ALGORITHM).  In
   that case, the MESSAGE-INTEGRITY attribute MUST be used instead of
   the MESSAGE-INTEGRITY-SHA256 attribute, and the rules for processing address families are the same
   as for MAPPED-ADDRESS.

   Note: XOR-MAPPED-ADDRESS REALM, NONCE,
   USERNAME, and MAPPED-ADDRESS differ only in their
   encoding of the transport address.  The former encodes USERHASH attributes SHOULD NOT be included.

9.2.5.  Receiving a Response

   If the transport
   address by exclusive-or'ing it response is an error response with an error code of 401
   (Unauthenticated) or 438 (Stale Nonce), the magic cookie.  The latter
   encodes it directly in binary.  RFC 3489 originally specified only
   MAPPED-ADDRESS.  However, deployment experience found that some NATs
   rewrite client MUST test if the 32-bit binary payloads containing
   NONCE attribute value starts with the NAT's public IP
   address, such as STUN's MAPPED-ADDRESS attribute, in "nonce cookie".  If so and the well-meaning
   but misguided attempt at providing a generic Application Layer
   Gateway (ALG) function.  Such behavior interferes with
   "nonce cookie" has the operation
   of STUN and also causes failure of STUN's message-integrity checking.

14.3.  USERNAME

   The USERNAME Security Feature "Password algorithms"
   bit set to 1 but no PASSWORD-ALGORITHMS attribute is used for message integrity.  It identifies present, then
   the username and password combination used in client MUST NOT retry the request with a new transaction.

   If the message-integrity
   check.

   The value of USERNAME response is a variable-length value containing an error response with an error code of 401
   (Unauthenticated), the
   authentication username.  It client SHOULD retry the request with a new
   transaction.  This request MUST contain a UTF-8 [RFC3629] encoded
   sequence of less than 509 bytes, USERNAME or a USERHASH,
   determined by the client as the appropriate username for the REALM
   from the error response.  If the "nonce cookie" is present and MUST have been processed using has
   the UsernameCasePreserved profile [RFC8265].  A compliant
   implementation MUST be able to parse UTF-8 encoded sequence of 763 or
   less bytes, to be compatible with [RFC5389] that mistakenly assumed
   up STUN Security Feature "Username anonymity" bit set to 6 bytes per characters encoded.

14.4.  USERHASH

   The 1, then the
   USERHASH attribute is used as a replacement for MUST be used; else, the USERNAME attribute when username anonymity is supported. MUST be
   used.  The value of USERHASH has a fixed length of 32 bytes. request MUST contain the REALM, copied from the error
   response.  The username request MUST have been processed using contain the UsernameCasePreserved profile
   [RFC8265] NONCE, copied from the error
   response.  If the response contains a PASSWORD-ALGORITHMS attribute,
   the request MUST contain the PASSWORD-ALGORITHMS attribute with the
   same content.  If the response contains a PASSWORD-ALGORITHMS
   attribute, and this attribute contains at least one algorithm that is
   supported by the realm client, then the request MUST have been processed using contain a PASSWORD-
   ALGORITHM attribute with the
   OpaqueString profile [RFC8265] before hashing.

   The following first algorithm supported on the list.
   If the response contains a PASSWORD-ALGORITHMS attribute, and this
   attribute does not contain any algorithm that is supported by the operation that
   client, then the client will perform to hash
   the username:

   userhash = SHA-256(UsernameCasePreserved(username)
     ":" OpaqueString(realm))

14.5.  MESSAGE-INTEGRITY

   The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of MUST NOT retry the STUN message. request with a new
   transaction.  The MESSAGE-INTEGRITY attribute can be present in
   any STUN message type.  Since client MUST NOT perform this retry if it uses is not
   changing the SHA-1 hash, USERNAME, USERHASH, REALM, or its associated password
   from the HMAC will
   be 20 bytes.

   The key for previous attempt.

   If the HMAC depends on which credential mechanism response is in use.
   Section 9.1.1 defines an error response with an error code of 438 (Stale
   Nonce), the key for client MUST retry the short-term credential mechanism
   and Section 9.2.2 defines request, using the key for new NONCE
   attribute supplied in the long-term credential
   mechanism.  Other credential mechanisms 438 (Stale Nonce) response.  This retry
   MUST define the key that is
   used for also include either the HMAC.

   The text used as input to HMAC is USERNAME or USERHASH, the STUN message, up to REALM, and
   including the attribute preceding
   either the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute.
   The length field of

   For all other responses, if the NONCE attribute starts with the
   "nonce cookie" with the STUN message header is adjusted to point Security Feature "Password algorithms"
   bit set to 1 but PASSWORD-ALGORITHMS is not present, the end response
   MUST be ignored.

   If the response is an error response with an error code of 400 (Bad
   Request) and does not contain either the MESSAGE-INTEGRITY or
   MESSAGE-INTEGRITY-SHA256 attribute, then the response MUST be
   discarded, as if it were never received.  This means that
   retransmits, if applicable, will continue.

      Note: In this case, the MESSAGE-INTEGRITY attribute.  The value of 400 response will never reach the
   MESSAGE-INTEGRITY attribute is set to
      application, resulting in a dummy value.

   Once the computation is performed, the value of timeout.

   The client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
   SHA256 attribute is filled in, and the value of the length in the STUN
   header is set to its correct value -- the length of response (either success or failure).  If
   present, the entire
   message.  Similarly, when validating client computes the MESSAGE-INTEGRITY, message integrity over the
   length field response
   as defined in Sections 14.5 or 14.6, using the STUN header must be adjusted to point to same password it
   utilized for the end request.  If the resulting value matches the
   contents of the MESSAGE-INTEGRITY attribute prior to calculating or MESSAGE-INTEGRITY-SHA256
   attribute, the HMAC over response is considered authenticated.  If the STUN message, up to value
   does not match, or if both MESSAGE-INTEGRITY and including MESSAGE-INTEGRITY-
   SHA256 are absent, the attribute preceding processing depends on the
   MESSAGE-INTEGRITY attribute.  Such adjustment is necessary when
   attributes, such as FINGERPRINT and MESSAGE-INTEGRITY-SHA256, appear
   after MESSAGE-INTEGRITY.  See also [RFC5769] for examples of such
   calculations.

14.6.  MESSAGE-INTEGRITY-SHA256

   The MESSAGE-INTEGRITY-SHA256 attribute contains request being sent
   over a reliable or an HMAC-SHA256
   [RFC2104] of unreliable transport.

   If the STUN message.  The MESSAGE-INTEGRITY-SHA256
   attribute can be present in any STUN message type.  The MESSAGE-
   INTEGRITY-SHA256 attribute contains request was sent over an initial portion of the HMAC-
   SHA-256 [RFC2104] of unreliable transport, the STUN message.  The value will be at most 32
   bytes, but MUST be at least 16 bytes, and response
   MUST be a multiple discarded, as if it had never been received.  This means that
   retransmits, if applicable, will continue.  If all the responses
   received are discarded, then instead of 4
   bytes.  The value must be signaling a timeout after
   ending the full 32 bytes unless transaction, the STUN Usage
   explicitly specifies layer MUST signal that truncation is allowed.  STUN Usages may
   specify a minimum length longer than 16 bytes.

   The key for the HMAC depends on which credential mechanism is in use.
   Section 9.1.1 defines integrity
   protection was violated.

   If the key for request was sent over a reliable transport, the short-term credential mechanism response MUST
   be discarded, and Section 9.2.2 defines the key for the long-term credential
   mechanism.  Other credential mechanism layer MUST define the key that is
   used for the HMAC.

   The text used as input to HMAC is immediately end the STUN message, up to transaction and
   including
   signal that the attribute preceding integrity protection was violated.

   If the MESSAGE-INTEGRITY-SHA256
   attribute.  The length field of response contains a PASSWORD-ALGORITHMS attribute, all the
   subsequent requests MUST be authenticated using MESSAGE-INTEGRITY-
   SHA256 only.

10.  ALTERNATE-SERVER Mechanism

   This section describes a mechanism in STUN message header is adjusted that allows a server to point
   redirect a client to the end of the MESSAGE-INTEGRITY-SHA256 attribute. another server.  This extension is optional, and
   a usage must define if and when this extension is used.  The
   value of the MESSAGE-INTEGRITY-SHA256
   ALTERNATE-SERVER attribute is set carries an IP address.

   A server using this extension redirects a client to another server by
   replying to a dummy
   value.

   Once the computation is performed, the value request message with an error response message with an
   error code of the MESSAGE-
   INTEGRITY-SHA256 300 (Try Alternate).  The server MUST include at least
   one ALTERNATE-SERVER attribute is filled in, and in the value error response, which MUST
   contain an IP address of the length
   in the STUN header is set to its correct value -- same address family as the length source IP
   address of the
   entire request message.  Similarly, when validating the MESSAGE-INTEGRITY-
   SHA256, the length field in the STUN header must be adjusted to point
   to  The server SHOULD include an
   additional ALTERNATE-SERVER attribute, after the end mandatory one, that
   contains an IP address of the MESSAGE-INTEGRITY-SHA256 attribute prior to
   calculating the HMAC over the STUN message, up to and including the
   attribute preceding address family other than the MESSAGE-INTEGRITY-SHA256 attribute.  Such
   adjustment is necessary when attributes, such as FINGERPRINT, appear
   after MESSAGE-INTEGRITY-SHA256.  See also Appendix B.1 for examples source IP
   address of such calculations.

14.7.  FINGERPRINT the request message.  The FINGERPRINT attribute error response message MAY be present in all STUN messages.

   The value
   authenticated; however, there are use cases for ALTERNATE-SERVER
   where authentication of the attribute response is computed as the CRC-32 of the STUN
   message up to (but excluding) the FINGERPRINT attribute itself,
   XOR'ed with not possible or practical.
   If the 32-bit value 0x5354554e.  (The XOR operation ensures
   that transaction uses TLS or DTLS, if the FINGERPRINT test will not report a false positive on a
   packet containing a CRC-32 generated by an application protocol.)
   The 32-bit CRC transaction is
   authenticated by a MESSAGE-INTEGRITY-SHA256 attribute, and if the one defined in ITU V.42 [ITU.V42.2002], which
   has
   server wants to redirect to a generator polynomial of x^32 + x^26 + x^23 + x^22 + x^16 + x^12
   + x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1.  See server that uses a different
   certificate, then it MUST include an ALTERNATE-DOMAIN attribute
   containing the sample
   code for name inside the CRC-32 in Section 8 subjectAltName of [RFC1952].

   When present, the FINGERPRINT attribute MUST be that certificate.
   This series of conditions on the last MESSAGE-INTEGRITY-SHA256 attribute in
   indicates that the message, transaction is authenticated and thus will appear after MESSAGE-INTEGRITY that the client
   implements this specification and
   MESSAGE-INTEGRITY-SHA256.

   The FINGERPRINT attribute therefore can aid in distinguishing STUN packets from
   packets of other protocols.  See Section 7.

   As with MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256, process the CRC used
   ALTERNATE-DOMAIN attribute.

   A client using this extension handles a 300 (Try Alternate) error
   code as follows.  The client looks for an ALTERNATE-SERVER attribute
   in the FINGERPRINT attribute covers error response.  If one is found, then the length field from client considers
   the STUN
   message header.  Therefore, this value must be correct current transaction as failed and include reattempts the CRC attribute as part of request with the message length, prior to computation
   of
   server specified in the CRC.  When attribute, using the FINGERPRINT attribute in a message, same transport protocol
   used for the
   attribute is first placed into previous request.  That request, if authenticated, MUST
   utilize the message with a dummy value, then same credentials that the CRC is computed, and then client would have used in the value of
   request to the attribute is updated. server that performed the redirection.  If the MESSAGE-INTEGRITY
   transport protocol uses TLS or MESSAGE-INTEGRITY-SHA256 attribute are
   also present, DTLS, then they must be present with the correct message-
   integrity value before the CRC is computed, since the CRC is done
   over the value of the MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256
   attributes as well.

14.8.  ERROR-CODE

   The ERROR-CODE client looks for an
   ALTERNATE-DOMAIN attribute.  If the attribute is used in error response messages.  It
   contains a numeric error code value in found, the range of 300 to 699 plus a
   textual reason phrase encoded in UTF-8 [RFC3629], and is consistent
   in its code assignments and semantics with SIP [RFC3261] and HTTP
   [RFC7231].  The reason phrase is meant for diagnostic purposes, and
   can domain
   MUST be anything appropriate for used to validate the error code.  Recommended reason
   phrases for certificate using the defined error codes are included recommendations in the IANA registry
   for error codes.
   [RFC6125].  The reason phrase certificate MUST be a UTF-8 [RFC3629] encoded
   sequence contain an identifier of less than 128 characters (which can be as long as 509
   bytes when encoding them type DNS-ID
   or 763 bytes when decoding them).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Reserved, should be 0         |Class|     Number    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Reason Phrase (variable)                                ..
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 7: ERROR-CODE Attribute

   To facilitate processing, the class CN-ID (eventually with wildcards) but not of type SRV-ID or URI-
   ID.  If the error code (the hundreds
   digit) attribute is encoded separately from not found, the rest of same domain that was used for
   the code, as shown in
   Figure 7.

   The Reserved bits SHOULD original request MUST be 0, and are for alignment on 32-bit
   boundaries.  Receivers used to validate the certificate.  If
   the client has been redirected to a server to which it has already
   sent this request within the last five minutes, it MUST ignore these bits.  The Class represents
   the hundreds digit of the error code.  The value MUST be between 3
   redirection and 6.  The Number represents consider the binary encoding transaction to have failed.  This
   prevents infinite ping-ponging between servers in case of redirection
   loops.

11.  Backwards Compatibility with RFC 3489

   In addition to the error code
   modulo 100, and its value backward compatibility already described in
   Section 12 of [RFC5389], DTLS MUST NOT be between 0 and 99.

   The following error codes, along used with their recommended reason
   phrases, are defined:

   300 Try Alternate:  The client should contact an alternate server for
      this request.  This error response MUST only be sent if the [RFC3489]
   (referred to as "classic STUN").  Any STUN request included either a USERNAME or USERHASH attribute and a
      valid MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute;
      otherwise, it indication
   without the magic cookie (see Section 6 of [RFC5389]) over DTLS MUST NOT
   be sent and error code 400 (Bad Request) is
      suggested.  This considered invalid: all requests MUST generate a 500 (Server
   Error) error response response, and indications MUST be protected with ignored.

12.  Basic Server Behavior

   This section defines the
      MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, and
      receivers MUST validate behavior of a basic, stand-alone STUN
   server.

   Historically, "classic STUN" [RFC3489] only defined the MESSAGE-INTEGRITY or MESSAGE-
      INTEGRITY-SHA256 behavior of this response before redirecting themselves a
   server that was providing clients with server reflexive transport
   addresses by receiving and replying to STUN Binding requests.
   [RFC5389] redefined the protocol as an alternate server.

   Note:  Failure to generate extensible framework, and validate message integrity for a 300
      response allows an on-path attacker to falsify a 300 response thus
      causing subsequent the
   server functionality became the sole STUN messages to be sent to a victim.

   400 Bad Request:  The request was malformed. Usage defined in that
   document.  This STUN Usage is also known as "Basic STUN Server".

   The client STUN server MUST support the Binding method.  It SHOULD NOT
      retry
   utilize the short-term or long-term credential mechanism.  This is
   because the work involved in authenticating the request without modification from is more than
   the previous attempt.
      The server may not be able to generate a valid MESSAGE-INTEGRITY
      or MESSAGE-INTEGRITY-SHA256 work in simply processing it.  It SHOULD NOT utilize the
   ALTERNATE-SERVER mechanism for this error, so the client same reason.  It MUST NOT
      expect a valid MESSAGE-INTEGRITY support UDP
   and TCP.  It MAY support STUN over TCP/TLS or MESSAGE-INTEGRITY-SHA256
      attribute on STUN over UDP/DTLS;
   however, DTLS and TLS provide minimal security benefits in this response.

   401 Unauthenticated:  The request did basic
   mode of operation.  It does not contain the correct
      credentials to proceed.  The client should retry the request with
      proper credentials.

   420 Unknown Attribute:  The server received require a STUN packet containing keep-alive mechanism
   because a comprehension-required attribute that it did not understand.
      The server MUST put this unknown attribute in TCP or TLS-over-TCP connection is closed after the UNKNOWN-
      ATTRIBUTE attribute end of its error response.

   438 Stale Nonce:  The NONCE used by
   the client was no longer valid.
      The client should retry, using Binding transaction.  It MAY utilize the NONCE provided in FINGERPRINT mechanism
   but MUST NOT require it.  Since the response.

   500 Server Error:  The stand-alone server has suffered only runs
   STUN, FINGERPRINT provides no benefit.  Requiring it would break
   compatibility with RFC 3489, and such compatibility is desirable in a temporary error.  The
      client should try again.

14.9.  REALM

   The REALM attribute may be present
   stand-alone server.  Stand-alone STUN servers SHOULD support
   backwards compatibility with clients using [RFC3489], as described in requests and responses.
   Section 11.

   It
   contains text is RECOMMENDED that meets the grammar administrators of STUN servers provide DNS
   entries for "realm-value" those servers as described in [RFC3261] but without Section 8.  If both A and
   AAAA resource records are returned, then the double quotes client can
   simultaneously send STUN Binding requests to the IPv4 and their surrounding
   whitespace.  That is, it IPv6
   addresses (as specified in [RFC8305]), as the Binding request is an unquoted realm-value (and
   idempotent.  Note that the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS
   attributes that are returned will not necessarily match the address
   family of the server address used.

   A basic STUN server is therefore not a sequence of qdtext or quoted-pair).  It MUST solution for NAT traversal by itself.
   However, it can be a UTF-8 [RFC3629]
   encoded sequence utilized as part of less than 128 characters (which a solution through STUN
   Usages.  This is discussed further in Section 13.

13.  STUN Usages

   STUN by itself is not a solution to the NAT traversal problem.
   Rather, STUN defines a tool that can be used inside a larger
   solution.  The term "STUN Usage" is used for any solution that uses
   STUN as long as
   509 bytes when encoding them and as long as 763 bytes a component.

   A STUN Usage defines how STUN is actually utilized -- when decoding
   them), to send
   requests, what to do with the responses, and MUST have been processed using which optional
   procedures defined here (or in an extension to STUN) are to be used.
   A usage also defines:

   o  Which STUN methods are used.

   o  What transports are used.  If DTLS-over-UDP is used, then
      implementing the OpaqueString profile
   [RFC8265].

   Presence denial-of-service countermeasure described in
      Section 4.2.1 of [RFC6347] is mandatory.

   o  What authentication and message-integrity mechanisms are used.

   o  The considerations around manual vs. automatic key derivation for
      the REALM attribute integrity mechanism, as discussed in a request indicates that long-term
   credentials [RFC4107].

   o  What mechanisms are being used for authentication.  Presence in certain
   error responses indicates that the server wishes the client to use distinguish STUN messages from other
      messages.  When STUN is run over TCP or TLS-over-TCP, a
   long-term credential in that realm for authentication.

14.10.  NONCE

   The NONCE attribute framing
      mechanism may be present in requests and responses.  It
   contains required.

   o  How a sequence of qdtext or quoted-pair, which are defined in
   [RFC3261].  Note that this means that STUN client determines the NONCE attribute will not
   contain IP address and port of the actual surrounding quote characters.  See [RFC7616],
   Section 5.4, for guidance on selection STUN
      server.

   o  How simultaneous use of nonce values in a IPv4 and IPv6 addresses (Happy Eyeballs
      [RFC8305]) works with non-idempotent transactions when both
      address families are found for the STUN server.
   It MUST be less than 128 characters (which can be as long

   o  Whether backwards compatibility to RFC 3489 is required.

   o  What optional attributes defined here (such as 509
   bytes when encoding them FINGERPRINT and a long as 763 bytes when decoding them).

14.11.  PASSWORD-ALGORITHMS
      ALTERNATE-SERVER) or in other extensions are required.

   o  If MESSAGE-INTEGRITY-SHA256 truncation is permitted, and the
      limits permitted for truncation.

   o  The PASSWORD-ALGORITHMS attribute may keep-alive mechanism if STUN is run over TCP or TLS-over-TCP.

   o  If anycast addresses can be present used for the server in requests and
   responses.  It contains case 1) TCP or
      TLS-over-TCP or 2) authentication is used.

   In addition, any STUN Usage must consider the list security implications
   of algorithms using STUN in that the server can
   use to derive the long-term password.

   The set usage.  A number of attacks against STUN are
   known algorithms is maintained by IANA.  The initial set
   defined by this specification is found (see the Security Considerations section in Section 18.5.

   The attribute contains this document), and
   any usage must consider how these attacks can be thwarted or
   mitigated.

   Finally, a list usage must consider whether its usage of algorithm numbers and variable
   length parameters.  The algorithm number STUN is a 16-bit value as defined
   in Section 18.5.  The parameters start with an
   example of the length (prior Unilateral Self-Address Fixing approach to
   padding) of NAT
   traversal and, if so, address the parameters as a 16-bit value, followed by questions raised in RFC 3424
   [RFC3424].

14.  STUN Attributes

   After the
   parameters that are specific to each algorithm.  The parameters STUN header are
   padded to zero or more attributes.  Each attribute
   MUST be TLV encoded, with a 16-bit type, 16-bit length, and value.
   Each STUN attribute MUST end on a 32-bit boundary, boundary.  As mentioned
   above, all fields in the same manner as an attribute. attribute are transmitted most significant
   bit first.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Algorithm 1           | Algorithm 1 Parameters Length |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Algorithm 1 Parameters (variable)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Algorithm 2         Type                  | Algorithm 2 Parameters            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Algorithm 2 Parameter                         Value (variable)                ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                             ...

                    Figure 8: 4: Format of PASSWORD-ALGORITHMS Attribute

14.12.  PASSWORD-ALGORITHM STUN Attributes

   The PASSWORD-ALGORITHM attribute is present only value in requests.  It
   contains the algorithms Length field MUST contain the length of the Value
   part of the attribute, prior to padding, measured in bytes.  Since
   STUN aligns attributes on 32-bit boundaries, attributes whose content
   is not a multiple of 4 bytes are padded with 1, 2, or 3 bytes of
   padding so that its value contains a multiple of 4 bytes.  The
   padding bits MUST be set to zero on sending and MUST be ignored by
   the server must use receiver.

   Any attribute type MAY appear more than once in a STUN message.
   Unless specified otherwise, the order of appearance is significant:
   only the first occurrence needs to derive be processed by a key from receiver, and
   any duplicates MAY be ignored by a receiver.

   To allow future revisions of this specification to add new attributes
   if needed, the long-term password. attribute space is divided into two ranges.
   Attributes with type values between 0x0000 and 0x7FFF are
   comprehension-required attributes, which means that the STUN agent
   cannot successfully process the message unless it understands the
   attribute.  Attributes with type values between 0x8000 and 0xFFFF are
   comprehension-optional attributes, which means that those attributes
   can be ignored by the STUN agent if it does not understand them.

   The set of known algorithms STUN attribute types is maintained by IANA.  The initial
   set defined by this specification is found in Section 18.5.

   The attribute contains an algorithm number and variable length
   parameters. 18.3.

   The algorithm number is a 16-bit value as rest of this section describes the format of the various
   attributes defined in
   Section 18.5. this specification.

14.1.  MAPPED-ADDRESS

   The parameters starts with the length (prior to
   padding) MAPPED-ADDRESS attribute indicates a reflexive transport address
   of the parameters as client.  It consists of an 8-bit address family and a 16-bit value,
   port, followed by the
   parameters that are specific to the algorithm.  The parameters are
   padded to a 32-bit boundary, in fixed-length value representing the same manner as an attribute.
   Similarly, IP address.
   If the padding bits address family is IPv4, the address MUST be set to zero on sending and 32 bits.  If the
   address family is IPv6, the address MUST be ignored by 128 bits.  All fields
   must be in network byte order.

   The format of the receiver. MAPPED-ADDRESS attribute is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 0 0 0 0 0|    Family     |          Algorithm           |  Algorithm Parameters Length           Port                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Algorithm Parameters (variable)                                                               |
     |                 Address (32 bits or 128 bits)                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 9: 5: Format of PASSWORD-ALGORITHM MAPPED-ADDRESS Attribute

14.13.  UNKNOWN-ATTRIBUTES

   The UNKNOWN-ATTRIBUTES address family can take on the following values:

   0x01:IPv4
   0x02:IPv6

   The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be
   ignored by receivers.  These bits are present for aligning parameters
   on natural 32-bit boundaries.

   This attribute is present used only in an error response
   when by servers for achieving backwards
   compatibility with [RFC3489] clients.

14.2.  XOR-MAPPED-ADDRESS

   The XOR-MAPPED-ADDRESS attribute is identical to the response code in MAPPED-ADDRESS
   attribute, except that the ERROR-CODE attribute reflexive transport address is 420. obfuscated
   through the XOR function.

   The attribute contains a list of 16-bit values, each format of which
   represents an attribute type that was not understood by the server. XOR-MAPPED-ADDRESS is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 0 0 0 0 0|    Family     |      Attribute 1 Type         |       Attribute 2 Type         X-Port                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Attribute 3 Type         |       Attribute 4 Type    ...                X-Address (Variable)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 10: 6: Format of UNKNOWN-ATTRIBUTES XOR-MAPPED-ADDRESS Attribute

   Note:  In [RFC3489], this

   The Family field was padded represents the IP address family and is encoded
   identically to 32 the Family field in MAPPED-ADDRESS.

   X-Port is computed by duplicating XOR'ing the
      last attribute. mapped port with the most
   significant 16 bits of the magic cookie.  If the IP address family is
   IPv4, X-Address is computed by XOR'ing the mapped IP address with the
   magic cookie.  If the IP address family is IPv6, X-Address is
   computed by XOR'ing the mapped IP address with the concatenation of
   the magic cookie and the 96-bit transaction ID.  In this version all cases, the
   XOR operation works on its inputs in network byte order (that is, the
   order they will be encoded in the message).

   The rules for encoding and processing the first 8 bits of the specification,
   attribute's value, the normal
      padding rules for attributes handling multiple occurrences of the
   attribute, and the rules for processing address families are used instead.

14.14.  SOFTWARE

   The SOFTWARE attribute contains a textual description the same
   as for MAPPED-ADDRESS.

   Note: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
   encoding of the software
   being used transport address.  The former encodes the transport
   address by XOR'ing it with the agent sending magic cookie.  The latter encodes it
   directly in binary.  RFC 3489 originally specified only MAPPED-
   ADDRESS.  However, deployment experience found that some NATs rewrite
   the 32-bit binary payloads containing the NAT's public IP address,
   such as STUN's MAPPED-ADDRESS attribute, in the well-meaning but
   misguided attempt to provide a generic Application Layer Gateway
   (ALG) function.  Such behavior interferes with the message.  It is used by clients
   and servers.  Its value SHOULD include manufacturer and version
   number.  The attribute has no impact on operation of the protocol, STUN
   and serves only as a tool also causes failure of STUN's message-integrity checking.

14.3.  USERNAME

   The USERNAME attribute is used for diagnostic message integrity.  It identifies
   the username and debugging purposes. password combination used in the message-integrity
   check.

   The value of SOFTWARE USERNAME is variable length. a variable-length value containing the
   authentication username.  It MUST be contain a UTF-8 UTF-8-encoded [RFC3629]
   encoded
   sequence of less fewer than 128 characters (which can be as long as 509 when encoding them and as long as 763 bytes when decoding them).

14.15.  ALTERNATE-SERVER

   The alternate server represents an alternate transport address
   identifying a different STUN server that the STUN client should try.

   It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a
   single server by IP address.

14.16.  ALTERNATE-DOMAIN

   The alternate domain represents the domain name that is used to
   verify the IP address in the ALTERNATE-SERVER attribute when MUST have been processed using
   the
   transport protocol uses TLS or DTLS.

   The value of ALTERNATE-DOMAIN is variable length.  It OpaqueString profile [RFC8265].  A compliant implementation MUST
   be able to parse a valid
   DNS name [RFC1123] (including A-labels [RFC5890]) UTF-8-encoded sequence of 255 763 or less
   ASCII characters.

15.  Operational Considerations

   STUN MAY fewer octets to
   be used compatible with anycast addresses, but [RFC5389].

      Note: [RFC5389] mistakenly referenced the definition of UTF-8 in
      [RFC2279].  [RFC2279] assumed up to 6 octets per characters
      encoded.  [RFC2279] was replaced by [RFC3629], which allows only 4
      octets per character encoded, consistent with UDP and changes made in
   STUN Usages where authentication
      Unicode 2.0 and ISO/IEC 10646.

14.4.  USERHASH

   The USERHASH attribute is used as a replacement for the USERNAME
   attribute when username anonymity is not used.

16.  Security Considerations

   Implementations and deployments supported.

   The value of USERHASH has a STUN Usage using TLS or DTLS fixed length of 32 bytes.  The username
   MUST follow have been processed using the recommendations in [BCP195].

   Implementations OpaqueString profile [RFC8265],
   and deployments of a STUN Usage using the Long-Term
   Credential Mechanism (Section 9.2) realm MUST follow the recommendations in
   Section 5 of [RFC7616].

16.1.  Attacks against have been processed using the Protocol

16.1.1.  Outside Attacks

   An attacker can try to modify STUN messages in transit, in order to
   cause a failure in STUN operation.  These attacks are detected for
   both requests and responses through OpaqueString profile
   [RFC8265] before hashing.

   The following is the message-integrity mechanism,
   using either a short-term or long-term credential.  Of course, once
   detected, operation that the manipulated packets client will be dropped, causing the STUN
   transaction perform to effectively fail.  This attack is possible only by hash
   the username:

   userhash = SHA-256(OpaqueString(username) ":" OpaqueString(realm))

14.5.  MESSAGE-INTEGRITY

   The MESSAGE-INTEGRITY attribute contains an
   on-path attacker.

   An attacker that can observe, but not modify, HMAC-SHA1 [RFC2104] of
   the STUN messages in-
   transit (for example, an attacker present on a shared access medium,
   such as Wi-Fi), message.  The MESSAGE-INTEGRITY attribute can see a STUN request, and then immediately send a
   STUN response, typically an error response, in order to disrupt STUN
   processing.  This attack is also prevented for messages that utilize
   MESSAGE-INTEGRITY.  However, some error responses, those related to
   authentication in particular, cannot be protected by MESSAGE-
   INTEGRITY.  When present in
   any STUN itself is run over a secure transport protocol
   (e.g., TLS), these attacks are completely mitigated.

   Depending on message type.  Since it uses the STUN usage, these attacks may SHA-1 hash, the HMAC will
   be of minimal
   consequence 20 bytes.

   The key for the HMAC depends on which credential mechanism is in use.
   Section 9.1.1 defines the key for the short-term credential
   mechanism, and thus do not require message integrity to mitigate.
   For example, when STUN Section 9.2.2 defines the key for the long-term
   credential mechanism.  Other credential mechanisms MUST define the
   key that is used for the HMAC.

   The text used as input to a basic HMAC is the STUN server message, up to discover a
   server reflexive candidate for usage with ICE, authentication and
   message integrity are not required since these attacks are detected
   during
   including the connectivity check phase. attribute preceding the MESSAGE-INTEGRITY attribute.
   The connectivity checks
   themselves, however, require protection for proper operation Length field of ICE
   overall.  As described in Section 13, the STUN usages describe when
   authentication and message integrity are needed.

   Since STUN uses header is adjusted to point to
   the HMAC end of the MESSAGE-INTEGRITY attribute.  The value of the
   MESSAGE-INTEGRITY attribute is set to a shared secret for authentication dummy value.

   Once the computation is performed, the value of the MESSAGE-INTEGRITY
   attribute is filled in, and
   integrity protection, it the value of the length in the STUN
   header is subject set to offline dictionary attacks.
   When authentication is utilized, it SHOULD its correct value -- the length of the entire
   message.  Similarly, when validating the MESSAGE-INTEGRITY, the
   Length field in the STUN header must be with a strong password
   that is not readily subject adjusted to offline dictionary attacks.
   Protection point to the end
   of the channel itself, using TLS or DTLS, mitigates these
   attacks. MESSAGE-INTEGRITY attribute prior to calculating the HMAC over
   the STUN supports both message, up to and including the attribute preceding the
   MESSAGE-INTEGRITY attribute.  Such adjustment is necessary when
   attributes, such as FINGERPRINT and MESSAGE-INTEGRITY-SHA256,
   which is subject to bid-down attacks by appear
   after MESSAGE-INTEGRITY.  See also [RFC5769] for examples of such
   calculations.

14.6.  MESSAGE-INTEGRITY-SHA256

   The MESSAGE-INTEGRITY-SHA256 attribute contains an on-path attacker that
   would strip HMAC-SHA256
   [RFC2104] of the STUN message.  The MESSAGE-INTEGRITY-SHA256
   attribute leaving only the
   MESSAGE-INTEGRITY can be present in any STUN message type.  The MESSAGE-
   INTEGRITY-SHA256 attribute and exploiting a potential vulnerability.
   Protection contains an initial portion of the channel itself, using TLS or DTLS, mitigates these
   attacks.  Timely removal HMAC-
   SHA-256 [RFC2104] of the support of MESSAGE-INTEGRITY in STUN message.  The value will be at most 32
   bytes, but it MUST be at least 16 bytes and MUST be a
   future version multiple of 4
   bytes.  The value must be the full 32 bytes unless the STUN Usage
   explicitly specifies that truncation is necessary.

   Note: allowed.  STUN Usages may
   specify a minimum length longer than 16 bytes.

   The use of SHA-256 key for password hashing does not meet modern
   standards, the HMAC depends on which are aimed at slowing down exhaustive password search
   by providing a relatively slow minimum time to compute credential mechanism is in use.
   Section 9.1.1 defines the hash.
   Although better algorithms such as Argon2 [I-D.irtf-cfrg-argon2] are
   available, SHA-256 was chosen key for consistency with [RFC7616].

16.1.2.  Inside Attacks

   A rogue client may try the short-term credential
   mechanism, and Section 9.2.2 defines the key for the long-term
   credential mechanism.  Other credential mechanism MUST define the key
   that is used for the HMAC.

   The text used as input to launch a DoS attack against a server by
   sending it a large number of STUN requests.  Fortunately, HMAC is the STUN
   requests can be processed statelessly by a server, making such
   attacks hard message, up to launch effectively.

   A rogue client may use a STUN server as a reflector, sending it
   requests with a falsified source IP address and port.  In such a
   case,
   including the response would be delivered to that source IP and port.
   There attribute preceding the MESSAGE-INTEGRITY-SHA256
   attribute.  The Length field of the STUN message header is no amplification adjusted
   to point to the end of the number MESSAGE-INTEGRITY-SHA256 attribute.  The
   value of packets with this attack
   (the STUN server sends one packet for each packet sent by the
   client), though there MESSAGE-INTEGRITY-SHA256 attribute is set to a small increase in dummy
   value.

   Once the amount computation is performed, the value of data,
   since the MESSAGE-
   INTEGRITY-SHA256 attribute is filled in, and the value of the length
   in the STUN responses are typically larger than requests.  This attack header is mitigated by ingress source address filtering.

   Revealing set to its correct value -- the specific software version length of the agent through
   entire message.  Similarly, when validating the
   SOFTWARE MESSAGE-INTEGRITY-
   SHA256, the Length field in the STUN header must be adjusted to point
   to the end of the MESSAGE-INTEGRITY-SHA256 attribute might allow them prior to become more vulnerable
   calculating the HMAC over the STUN message, up to
   attacks against software that and including the
   attribute preceding the MESSAGE-INTEGRITY-SHA256 attribute.  Such
   adjustment is known to contain security holes.
   Implementers SHOULD make usage necessary when attributes, such as FINGERPRINT, appear
   after MESSAGE-INTEGRITY-SHA256.  See also Appendix B.1 for examples
   of the SOFTWARE such calculations.

14.7.  FINGERPRINT

   The FINGERPRINT attribute a
   configurable option.

16.1.3.  Bid-Down Attacks

   This document adds the possibility MAY be present in all STUN messages.

   The value of selecting different algorithms
   for protecting the confidentiality of attribute is computed as the passwords stored on CRC-32 of the
   server side when using STUN
   message up to (but excluding) the Long-Term Credential Mechanism, while
   still ensuring compatibility FINGERPRINT attribute itself,
   XOR'ed with MD5, which was the algorithm used
   in 32-bit value 0x5354554e.  (The XOR operation ensures
   that the FINGERPRINT test will not report a previous version of this protocol.  It works false positive on a
   packet containing a CRC-32 generated by having the
   server send back to the client an application protocol.)
   The 32-bit CRC is the list of algorithms supported one defined in ITU V.42 [ITU.V42.2002], which
   has a
   PASSWORD-ALGORITHMS attribute, and having generator polynomial of x^32 + x^26 + x^23 + x^22 + x^16 + x^12
   + x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1.  See the client send back a
   PASSWORD-ALGORITHM attribute containing sample
   code for the algorithm selected.

   Because CRC-32 in Section 8 of [RFC1952].

   When present, the PASSWORD-ALGORITHMS FINGERPRINT attribute has to MUST be sent in an
   unauthenticated response, an on-path attacker wanting to exploit an
   eventual vulnerability in MD5 can just strip the PASSWORD-ALGORITHMS last attribute from in
   the unprotected response, message and thus making the server
   subsequently act as if the client was implementing a previous version
   of this protocol.

   To protect against this attack will appear after MESSAGE-INTEGRITY and MESSAGE-
   INTEGRITY-SHA256.

   The FINGERPRINT attribute can aid in distinguishing STUN packets from
   packets of other similar bid-down attacks,
   the nonce is enriched protocols.  See Section 7.

   As with a set of security bits which indicates
   which security features are MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256, the CRC used
   in use.  In the case of FINGERPRINT attribute covers the selection Length field from the STUN
   message header.  Therefore, prior to computation of the password algorithm CRC, this
   value must be correct and include the matching bit is set in CRC attribute as part of the nonce returned
   by
   message length.  When using the server FINGERPRINT attribute in a message,
   the same response that contains attribute is first placed into the PASSWORD-
   ALGORITHMS attribute.  Because message with a dummy value;
   then, the nonce used in subsequent
   authenticated transactions CRC is verified by computed, and the server to be identical
   to what was originally sent, it cannot be modified by an on-path
   attacker.  Additionally, value of the client attribute is mandated to copy updated.
   If the received
   PASSWORD-ALGORITHMS MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute in the next authenticated transaction
   to that server.

   An on-path attack that removes the PASSWORD-ALGORITHMS will be
   detected because the client will not be able to send is
   also present, then it back to the
   server in must be present with the next authenticated transaction.  The client will detect
   that attack because correct message-
   integrity value before the security bit CRC is set, but computed, since the matching
   attribute CRC is missing, ending done
   over the session.  A client using an older
   version value of this protocol will not send the PASSWORD-ALGORITHMS back
   but can only use MD5 anyway, so MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256
   attributes as well.

14.8.  ERROR-CODE

   The ERROR-CODE attribute is used in error response messages.  It
   contains a numeric error code value in the attack range of 300 to 699 plus a
   textual reason phrase encoded in UTF-8 [RFC3629]; it is inconsequential.

   The on-path attack may also try to remove the security bit together
   consistent in its code assignments and semantics with SIP [RFC3261]
   and HTTP [RFC7231].  The reason phrase is meant for diagnostic
   purposes and can be anything appropriate for the PASSWORD-ALGORITHMS attribute, but the server will discover
   that when error code.
   Recommended reason phrases for the next authenticated transaction contains an invalid
   nonce.

   An on-path attack that removes some algorithms from defined error codes are included
   in the PASSWORD-
   ALGORITHMS attribute will IANA registry for error codes.  The reason phrase MUST be equally defeated because that attribute
   will a
   UTF-8-encoded [RFC3629] sequence of fewer than 128 characters (which
   can be different from the original one as long as 509 bytes when encoding them or 763 bytes when
   decoding them).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Reserved, should be 0         |Class|     Number    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Reason Phrase (variable)                                ..
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 7: Format of ERROR-CODE Attribute

   To facilitate processing, the server verifies it
   in the subsequent authenticated transaction.

   Note that class of the bid-down protection mechanism introduced in this
   document error code (the hundreds
   digit) is inherently limited by encoded separately from the fact that it is not possible to
   detect an attack until rest of the code, as shown in
   Figure 7.

   The Reserved bits SHOULD be 0 and are for alignment on 32-bit
   boundaries.  Receivers MUST ignore these bits.  The Class represents
   the server receives hundreds digit of the second request after error code.  The value MUST be between 3
   and 6.  The Number represents the 401 response.

   SHA-256 was chosen as binary encoding of the new default for password hashing for error code
   modulo 100, and its
   compatibility value MUST be between 0 and 99.

   The following error codes, along with [RFC7616] but because SHA-256 (like MD5) is a
   comparatively fast algorithm, it does little to deter brute force
   attacks.  Specifically, their recommended reason
   phrases, are defined:

   300  Try Alternate: The client should contact an alternate server for
        this means that request.  This error response MUST only be sent if the user has
        request included either a weak
   password:

   o  An attacker who captures the server's password file can often
      determine USERNAME or USERHASH attribute and a
        valid MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute;
        otherwise, it MUST NOT be sent and error code 400 (Bad Request)
        is suggested.  This error response MUST be protected with the user's password
        MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, and thus impersonate
        receivers MUST validate the user MESSAGE-INTEGRITY or MESSAGE-
        INTEGRITY-SHA256 of this response before redirecting themselves
        to
      other servers where they have used that password.  Note that such an attacker can impersonate the user alternate server.

        Note: Failure to the server itself without
      any brute force attack.

   o  An generate and validate message integrity for a
        300 response allows an on-path attacker who captures to falsify a single exchange can brute force the
      user's password and 300
        response thus potentially impersonate causing subsequent STUN messages to be sent to a
        victim.

   400  Bad Request: The request was malformed.  The client SHOULD NOT
        retry the user to request without modification from the previous
        attempt.  The server and other servers where they have used the same password.

   A stronger (which is may not be able to say slower) algorithm, like Argon2
   [I-D.irtf-cfrg-argon2], would help both of these cases, but in the
   case of the first attack, only after until the database entry generate a valid
        MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 for this user is updated to exclusively use that stronger mechanism.

   The bid-down defenses in error, so
        the client MUST NOT expect a valid MESSAGE-INTEGRITY or MESSAGE-
        INTEGRITY-SHA256 attribute on this protocol prevent an attacker from
   forcing response.

   401  Unauthenticated: The request did not contain the correct
        credentials to proceed.  The client and should retry the request
        with proper credentials.

   420  Unknown Attribute: The server to complete received a handshake using weaker
   algorithms than they jointly support, but only if the weakest joint
   algorithm is strong enough STUN packet containing
        a comprehension-required attribute that it cannot be brute-forced.  However,
   this does did not defend against many attacks on those algorithms;
   specifically, an on-path attacker might perform a bid-down attack on understand.
        The server MUST put this unknown attribute in the UNKNOWN-
        ATTRIBUTE attribute of its error response.

   438  Stale Nonce: The NONCE used by the client was no longer valid.
        The client should retry, using the NONCE provided in the
        response.

   500  Server Error: The server has suffered a temporary error.  The
        client which supported both Argon2 [I-D.irtf-cfrg-argon2] should try again.

14.9.  REALM

   The REALM attribute may be present in requests and
   SHA-256 responses.  It
   contains text that meets the grammar for password hashing "realm-value" as described
   in [RFC3261] but without the double quotes and use that to collect a MESSAGE-
   INTEGRITY-SHA256 value which their surrounding
   whitespace.  That is, it uses for is an offline brute-force
   attack.  This would unquoted realm-value (and is therefore
   a sequence of qdtext or quoted-pair).  It MUST be detected a UTF-8-encoded
   [RFC3629] sequence of fewer than 128 characters (which can be as long
   as 509 bytes when encoding them and as long as 763 bytes when
   decoding them) and MUST have been processed using the server receives the second
   request, but that does not prevent the attacker from obtaining the
   MESSAGE-INTEGRITY-SHA256 value.

   Similarly, an attack against OpaqueString
   profile [RFC8265].

   Presence of the USERHASH mechanism will not succeed REALM attribute in establishing a session as the server will detect request indicates that long-term
   credentials are being used for authentication.  Presence in certain
   error responses indicates that the feature
   was discarded on-path, but server wishes the client would still have been convinced to send its username in clear use a
   long-term credential in the USERNAME attribute, thus
   disclosing it to the attacker.

   Finally, when the bid-down protection mechanism is employed that realm for authentication.

14.10.  NONCE

   The NONCE attribute may be present in requests and responses.  It
   contains a
   future upgrade sequence of qdtext or quoted-pair, which are defined in
   [RFC3261].  Note that this means that the HMAC algorithm used to protect message, it NONCE attribute will
   offer only a limited protection if the current HMAC algorithm is
   already compromised.

16.2.  Attacks Affecting not
   contain the Usage

   This section lists attacks that might actual surrounding quote characters.  The NONCE attribute
   MUST be launched against fewer than 128 characters (which can be as long as 509 bytes
   when encoding them and a usage long as 763 bytes when decoding them).  See
   Section 5.4 of
   STUN.  Each STUN usage must consider whether these attacks are
   applicable to it, and if so, discuss counter-measures.

   Most [RFC7616] for guidance on selection of the attacks nonce values in this section revolve around an attacker
   modifying the reflexive address learned by a STUN client through
   a
   Binding request/response transaction.  Since server.

14.11.  PASSWORD-ALGORITHMS

   The PASSWORD-ALGORITHMS attribute may be present in requests and
   responses.  It contains the usage list of algorithms that the
   reflexive address server can
   use to derive the long-term password.

   The set of known algorithms is maintained by IANA.  The initial set
   defined by this specification is found in Section 18.5.

   The attribute contains a function list of the usage, the applicability algorithm numbers and
   remediation of these attacks are usage-specific.  In common
   situations, modification variable
   length parameters.  The algorithm number is a 16-bit value as defined
   in Section 18.5.  The parameters start with the length (prior to
   padding) of the reflexive address parameters as a 16-bit value, followed by an on-path
   attacker is easy the
   parameters that are specific to do.  Consider, for example, each algorithm.  The parameters are
   padded to a 32-bit boundary, in the common situation
   where STUN same manner as an attribute.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Algorithm 1           | Algorithm 1 Parameters Length |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Algorithm 1 Parameters (variable)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Algorithm 2           | Algorithm 2 Parameters Length |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Algorithm 2 Parameters (variable)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                             ...

             Figure 8: Format of PASSWORD-ALGORITHMS Attribute

14.12.  PASSWORD-ALGORITHM

   The PASSWORD-ALGORITHM attribute is run directly over UDP.  In this case, an on-path
   attacker can modify present only in requests.  It
   contains the source IP address of algorithm that the Binding request
   before it arrives at server must use to derive a key from
   the STUN server. long-term password.

   The STUN server will then
   return set of known algorithms is maintained by IANA.  The initial set
   defined by this IP address specification is found in the XOR-MAPPED-ADDRESS Section 18.5.

   The attribute to the
   client, and send the response back to that (falsified) IP address contains an algorithm number and
   port.  If the attacker can also intercept this response, it can
   direct it back towards the client.  Protecting against this attack by
   using a message-integrity check variable length
   parameters.  The algorithm number is impossible, since a message-
   integrity 16-bit value cannot cover the source IP address, since the
   intervening NAT must be able to modify this value.  Instead, one
   solution to preventing the attacks listed below is for the client to
   verify the reflexive address learned, as is done defined in ICE [RFC8445].

   Other usages may use other means to prevent these attacks.

16.2.1.  Attack I: Distributed DoS (DDoS) against a Target

   In this attack, the attacker provides one or more clients
   Section 18.5.  The parameters starts with the
   same faked reflexive address that points to the intended target.
   This will trick the STUN clients into thinking that their reflexive
   addresses are equal length (prior to that
   padding) of the target.  If parameters as a 16-bit value, followed by the clients hand out
   parameters that reflexive address in order are specific to receive traffic on it (for
   example, the algorithm.  The parameters are
   padded to a 32-bit boundary, in SIP messages), the traffic will instead same manner as an attribute.
   Similarly, the padding bits MUST be sent set to zero on sending and MUST
   be ignored by the
   target.  This attack can provide substantial amplification,
   especially receiver.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Algorithm           |  Algorithm Parameters Length   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Algorithm Parameters (variable)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 9: Format of PASSWORD-ALGORITHM Attribute

14.13.  UNKNOWN-ATTRIBUTES

   The UNKNOWN-ATTRIBUTES attribute is present only in an error response
   when used with clients the response code in the ERROR-CODE attribute is 420 (Unknown
   Attribute).

   The attribute contains a list of 16-bit values, each of which
   represents an attribute type that are using STUN was not understood by the server.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Attribute 1 Type         |       Attribute 2 Type        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Attribute 3 Type         |       Attribute 4 Type    ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 10: Format of UNKNOWN-ATTRIBUTES Attribute

      Note: In [RFC3489], this field was padded to enable
   multimedia applications.  However, it can only be launched against
   targets for which packets from 32 by duplicating the
      last attribute.  In this version of the STUN server to specification, the target pass
   through normal
      padding rules for attributes are used instead.

14.14.  SOFTWARE

   The SOFTWARE attribute contains a textual description of the attacker, limiting software
   being used by the cases in which it agent sending the message.  It is possible.

16.2.2.  Attack II: Silencing a Client

   In this attack, used by clients
   and servers.  Its value SHOULD include manufacturer and version
   number.  The attribute has no impact on operation of the attacker provides a STUN client with protocol and
   serves only as a faked
   reflexive address. tool for diagnostic and debugging purposes.  The reflexive address it provides
   value of SOFTWARE is variable length.  It MUST be a UTF-8-encoded
   [RFC3629] sequence of fewer than 128 characters (which can be as long
   as 509 when encoding them and as long as 763 bytes when decoding
   them).

14.15.  ALTERNATE-SERVER

   The alternate server represents an alternate transport address that routes to nowhere.  As
   identifying a result, different STUN server that the STUN client won't
   receive any of should try.

   It is encoded in the packets it expects same way as MAPPED-ADDRESS and thus refers to receive when it hands out
   the reflexive a
   single server by IP address.  This exploitation

14.16.  ALTERNATE-DOMAIN

   The alternate domain represents the domain name that is not very interesting for used to
   verify the attacker. IP address in the ALTERNATE-SERVER attribute when the
   transport protocol uses TLS or DTLS.

   The value of ALTERNATE-DOMAIN is variable length.  It impacts MUST be a single client, which valid
   DNS name [RFC1123] (including A-labels [RFC5890]) of 255 or fewer
   ASCII characters.

15.  Operational Considerations

   STUN MAY be used with anycast addresses, but only with UDP and in
   STUN Usages where authentication is frequently not
   the desired target.  Moreover, any attacker that can mount the attack
   could also deny service to the client by other means, such as
   preventing the client from receiving any response from the used.

16.  Security Considerations

   Implementations and deployments of a STUN
   server, Usage using TLS or even DTLS
   MUST follow the recommendations in [BCP195].

   Implementations and deployments of a DHCP server.  As with STUN Usage using the attack long-term
   credential mechanism (Section 9.2) MUST follow the recommendations in
   Section 16.2.1,
   this attack is only possible when 5 of [RFC7616].

16.1.  Attacks against the Protocol

16.1.1.  Outside Attacks

   An attacker is on path can try to modify STUN messages in transit, in order to
   cause a failure in STUN operation.  These attacks are detected for
   both requests and responses through the message-integrity mechanism,
   using either a short-term or long-term credential.  Of course, once
   detected, the manipulated packets
   sent from will be dropped, causing the STUN server towards this unused IP address.

16.2.3.  Attack III: Assuming the Identity of a Client
   transaction to effectively fail.  This attack is similar to attack II.  However, the faked reflexive
   address points to the attacker itself.  This allows the possible only by an
   on-path attacker.

   An attacker to
   receive traffic that was destined for the client.

16.2.4.  Attack IV: Eavesdropping

   In this attack, the can observe, but not modify, STUN messages in-
   transit (for example, an attacker forces the client to use present on a reflexive
   address that routes to itself.  It shared access medium,
   such as Wi-Fi) can see a STUN request and then forwards any packets it
   receives immediately send a
   STUN response, typically an error response, in order to the client. disrupt STUN
   processing.  This attack would allow the attacker to
   observe all packets sent to the client. is also prevented for messages that utilize
   MESSAGE-INTEGRITY.  However, in order some error responses, those related to launch
   the attack,
   authentication in particular, cannot be protected by MESSAGE-
   INTEGRITY.  When STUN itself is run over a secure transport protocol
   (e.g., TLS), these attacks are completely mitigated.

   Depending on the attacker must have already been able STUN Usage, these attacks may be of minimal
   consequence and thus do not require message integrity to observe
   packets from the client mitigate.
   For example, when STUN is used to a basic STUN server to discover a
   server reflexive candidate for usage with ICE, authentication and
   message integrity are not required since these attacks are detected
   during the connectivity check phase.  The connectivity checks
   themselves, however, require protection for proper operation of ICE
   overall.  As described in Section 13, STUN server.  In most cases (such as Usages describe when
   authentication and message integrity are needed.

   Since STUN uses the attack HMAC of a shared secret for authentication and
   integrity protection, it is launched from an access network), this means that
   the attacker could already observe packets sent subject to the client.  This
   attack is, as offline dictionary attacks.
   When authentication is utilized, it SHOULD be with a result, only useful for observing traffic by
   attackers on the path from strong password
   that is not readily subject to offline dictionary attacks.
   Protection of the client channel itself, using TLS or DTLS, mitigates these
   attacks.

   STUN supports both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256,
   which makes STUN subject to bid-down attacks by an on-path attacker.
   An attacker could strip the STUN server, but not
   generally on MESSAGE-INTEGRITY-SHA256 attribute,
   leaving only the path MESSAGE-INTEGRITY attribute and thus exploiting a
   potential vulnerability.  Protection of packets being routed towards the client.

   Note that this attack can be trivially launched by the STUN server channel itself, so users using TLS
   or DTLS, mitigates these attacks.  Timely removal of STUN servers should have the same level support of trust
   MESSAGE-INTEGRITY in them as any other node that can insert themselves into the
   communication flow.

16.3.  Hash Agility Plan

   This specification uses both HMAC-SHA256 for computation a future version of the
   message integrity, sometimes in combination with HMAC-SHA1.  If, STUN is necessary.

   Note: The use of SHA-256 for password hashing does not meet modern
   standards, which are aimed at slowing down exhaustive password
   searches by providing a
   later time, HMAC-SHA256 is found relatively slow minimum time to be compromised, the following is compute the remedy that will be applied:

   o  Both a new message-integrity attribute and
   hash.  Although better algorithms such as Argon2 [Argon2] are
   available, SHA-256 was chosen for consistency with [RFC7616].

16.1.2.  Inside Attacks

   A rogue client may try to launch a new STUN Security
      Feature bit will be allocated in DoS attack against a Standard Track document.  The
      new message-integrity attribute will have its value computed using server by
   sending it a new hash.  The large number of STUN Security Feature bit will requests.  Fortunately, STUN
   requests can be used to
      simultaneously signal to processed statelessly by a STUN client using the Long Term
      Credential Mechanism that this server supports this new hash
      algorithm, and will prevent bid-down server, making such
   attacks on the new message-
      integrity attribute.

   o hard to launch effectively.

   A rogue client may use a STUN Clients server as a reflector, sending it
   requests with a falsified source IP address and Servers using port.  In such a
   case, the Short Term Credential Mechanism
      will need response would be delivered to update the external mechanism that they use to signal
      what message-integrity attributes are in use.

   The bid-down protection mechanism described in this document is new, source IP and thus cannot currently protect against a bid-down attack that
   lowers the strength port.
   There is no amplification of the hash algorithm to HMAC-SHA1.  This is why,
   after a transition period, a new document updating number of packets with this document will
   assign a new attack
   (the STUN Security Feature bit server sends one packet for deprecating HMAC-SHA1.
   When used, this bit will signal that HMAC-SHA1 is deprecated and
   should no longer be used.

   Similarly, if SHA256 each packet sent by the
   client), though there is found to be compromised, a new user-hash
   attribute and a new STUN Security Feature bit will be allocated small increase in a
   Standards Track document.  The new user-hash attribute will have its
   value computed using a new hash.  The the amount of data,
   since STUN Security Feature bit will
   be used responses are typically larger than requests.  This attack
   is mitigated by ingress source address filtering.

   Revealing the specific software version of the agent through the
   SOFTWARE attribute might allow them to simultaneously signal become more vulnerable to a STUN client using the Long Term
   Credential Mechanism
   attacks against software that this server supports this new hash
   algorithm for is known to contain security holes.
   Implementers SHOULD make usage of the user-hash attribute, and will prevent bid-down
   attacks on SOFTWARE attribute a
   configurable option.

16.1.3.  Bid-Down Attacks

   This document adds the new user-hash attribute.

17.  IAB Considerations

   The IAB has studied possibility of selecting different algorithms
   to protect the problem confidentiality of Unilateral Self-Address Fixing
   (UNSAF), which is the general process by which a client attempts to
   determine its address in another realm passwords stored on the other server
   side of a NAT
   through a collaborative protocol reflection when using the long-term credential mechanism ([RFC3424]).
   STUN can be while still
   ensuring compatibility with MD5, which was the algorithm used in
   [RFC5389].  This selection works by having the server send to perform this function using a Binding request/
   response transaction if one agent is behind a NAT and the other is on
   client the public side list of the NAT.

   The IAB has suggested that protocols developed for this purpose
   document algorithms supported in a specific set of considerations.  Because some STUN usages
   provide UNSAF functions (such as ICE [RFC8445] ), PASSWORD-ALGORITHMS
   attribute and others do not
   (such as SIP Outbound [RFC5626]), answers to these considerations
   need to be addressed by having the usages themselves.

18.  IANA Considerations

18.1.  STUN Security Features Registry

   A STUN Security Feature set defines 24 bit as flags.

   IANA is requested to create client send back a new registry PASSWORD-ALGORITHM
   attribute containing the STUN
   Security Features that are protected by algorithm selected.

   Because the bid-down attack
   prevention mechanism described PASSWORD-ALGORITHMS attribute has to be sent in section Section 9.2.1.

   The initial STUN Security Features are:

   Bit 0: Password algorithms
   Bit 1: Username anonymity
   Bit 2-23: Unassigned

   Bits are assigned starting an
   unauthenticated response, an on-path attacker wanting to exploit an
   eventual vulnerability in MD5 can just strip the PASSWORD-ALGORITHMS
   attribute from the most significant side of unprotected response, thus making the bit
   set, so Bit 0 is server
   subsequently act as if the leftmost bit client was implementing the version of
   this protocol defined in [RFC5389].

   To protect against this attack and Bit 23 other similar bid-down attacks,
   the rightmost bit.

   New Security Features are assigned by a Standards Action [RFC8126].

18.2.  STUN Methods Registry

   IANA nonce is requested to update enriched with a set of security bits that indicates
   which security features are in use.  In the name for method 0x002 and case of the
   reference from RFC 5389 to RFC-to-be for selection of
   the following STUN methods:

   0x000: (Reserved)
   0x001: Binding
   0x002: (Reserved; prior to [RFC5389] this was SharedSecret)

18.3.  STUN Attribute Registry

18.3.1.  Updated Attributes

   IANA password algorithm, the matching bit is requested to update set in the names for attributes 0x0002, 0x0004,
   0x0005, 0x0007, and 0x000B, and nonce returned
   by the reference from RFC 5389 to RFC-
   to-be for server in the following STUN methods:

   Comprehension-required range (0x0000-0x7FFF):
   0x0000: (Reserved)
   0x0001: MAPPED-ADDRESS
   0x0002: (Reserved; prior to [RFC5389] this was RESPONSE-ADDRESS)
   0x0004: (Reserved; prior to [RFC5389] this was SOURCE-ADDRESS)
   0x0005: (Reserved; prior to [RFC5389] this was CHANGED-ADDRESS)
   0x0006: USERNAME
   0x0007: (Reserved; prior to [RFC5389] this was PASSWORD)
   0x0008: MESSAGE-INTEGRITY
   0x0009: ERROR-CODE
   0x000A: UNKNOWN-ATTRIBUTES
   0x000B: (Reserved; prior to [RFC5389] this was REFLECTED-FROM)
   0x0014: REALM
   0x0015: NONCE
   0x0020: XOR-MAPPED-ADDRESS

   Comprehension-optional range (0x8000-0xFFFF)
   0x8022: SOFTWARE
   0x8023: ALTERNATE-SERVER
   0x8028: FINGERPRINT

18.3.2.  New Attributes

   IANA is requested to add same response that contains the following attribute to PASSWORD-
   ALGORITHMS attribute.  Because the STUN
   Attribute Registry:

   Comprehension-required range (0x0000-0x7FFF):
   0xXXXX: MESSAGE-INTEGRITY-SHA256
   0xXXXX: PASSWORD-ALGORITHM
   0xXXXX: USERHASH

   Comprehension-optional range (0x8000-0xFFFF)
   0xXXXX: PASSWORD-ALGORITHMS
   0xXXXX: ALTERNATE-DOMAIN

18.4.  STUN Error Code Registry

   IANA nonce used in subsequent
   authenticated transactions is requested to update verified by the reference from RFC 5389 server to RFC-to-be
   for be identical
   to what was originally sent, it cannot be modified by an on-path
   attacker.  Additionally, the Error Codes given in Section 14.8.

   IANA client is requested mandated to change copy the name of received
   PASSWORD-ALGORITHMS attribute in the 401 Error Code from
   "Unauthorized" to "Unauthenticated".

18.5.  STUN Password Algorithm Registry

   IANA is requested next authenticated transaction
   to create a new registry for Password Algorithm.

   A Password Algorithm is a hex number in that server.

   An on-path attack that removes the range 0x0000 - 0xFFFF.

   The initial Password Algorithms are:

   0x0000: Reserved
   0x0001: MD5
   0x0002: SHA-256
   0x0003-0xFFFF: Unassigned

   Password Algorithms in PASSWORD-ALGORITHMS will be
   detected because the first half of client will not be able to send it back to the range (0x0000 - 0x7FFF)
   are assigned by IETF Review [RFC8126].  Password Algorithms
   server in the
   second half of the range (0x8000 - 0xFFFF) are assigned by Designated
   Expert [RFC8126].

18.5.1.  Password Algorithms

18.5.1.1.  MD5

   This password algorithm is taken from [RFC1321]. next authenticated transaction.  The key length is 16 bytes and client will detect
   that attack because the parameters value is empty.

   Note:  This algorithm MUST only be used for compatibility with legacy
      systems.

                key = MD5(username ":" OpaqueString(realm)
                  ":" OpaqueString(password))

18.5.1.2.  SHA-256

   This password algorithm is taken from [RFC7616].

   The key length security bit is 32 bytes and set but the parameters value matching
   attribute is empty.

              key = SHA-256(username ":" OpaqueString(realm)
                ":" OpaqueString(password))

18.6.  STUN UDP and TCP Port Numbers

   IANA missing; this will end the session.  A client using an
   older version of this protocol will not send the PASSWORD-ALGORITHMS
   back but can only use MD5 anyway, so the attack is requested inconsequential.

   The on-path attack may also try to update remove the reference from RFC 5389 to RFC-to-be
   for security bit together
   with the following ports in PASSWORD-ALGORITHMS attribute, but the Service Name and Transport Protocol
   Port Number Registry.

   stun   3478/tcp   Session Traversal Utilities for NAT (STUN) port
   stun   3478/udp   Session Traversal Utilities for NAT (STUN) port
   stuns  5349/tcp   Session Traversal Utilities for NAT (STUN) port

19.  Changes Since RFC 5389

   This specification obsoletes [RFC5389].  This specification differs server will discover
   that when the next authenticated transaction contains an invalid
   nonce.

   An on-path attack that removes some algorithms from RFC 5389 the PASSWORD-
   ALGORITHMS attribute will be equally defeated because that attribute
   will be different from the original one when the server verifies it
   in the following ways:

   o  Added support for DTLS-over-UDP [RFC6347].

   o  Made clear subsequent authenticated transaction.

   Note that the RTO bid-down protection mechanism introduced in this
   document is considered stale if there inherently limited by the fact that it is no
      transactions with not possible to
   detect an attack until the server.

   o  Aligned server receives the RTO calculation with [RFC6298].

   o  Updated second request after
   the cipher suites for TLS.

   o  Added support 401 (Unauthenticated) response.

   SHA-256 was chosen as the new default for STUN URI [RFC7064].

   o  Added support password hashing for SHA256 message integrity.

   o  Updated its
   compatibility with [RFC7616], but because SHA-256 (like MD5) is a
   comparatively fast algorithm, it does little to deter brute-force
   attacks.  Specifically, this means that if the user has a weak
   password, an attacker that captures a single exchange can use a
   brute-force attack to learn the user's password and then potentially
   impersonate the PRECIS support user to [RFC8265].

   o  Added protocol the server and registry to choose other servers where the
   same password encryption
      algorithm.

   o  Added support for anonymous username.

   o  Added protocol and registry for preventing biddown attacks.

   o  Sharing a NONCE is no longer permitted.

   o  Added was used.  Note that such an attacker can impersonate
   the possibility user to the server itself without any brute-force attack.

   A stronger (which is to say, slower) algorithm, like Argon2 [Argon2],
   would help both of using a domain name these cases; however, in the alternate
      server mechanism.

   o  Added more C snippets.

   o  Added test vector.

20.  References

20.1.  Normative References

   [ITU.V42.2002]
              International Telecommunications Union, "Error-correcting
              Procedures first case, it would
   only help after the database entry for DCEs Using Asynchronous-to-Synchronous
              Conversion", ITU-T Recommendation V.42, 2002.

   [KARN87]   Karn, P. and C. Partridge, "Improving Round-Trip Time
              Estimates this user is updated to
   exclusively use that stronger mechanism.

   The bid-down defenses in Reliable Transport Protocols", August 1987.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application this protocol prevent an attacker from
   forcing the client and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <http://www.rfc-editor.org/info/rfc1123>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC2104]  Krawczyk, H., Bellare, M., server to complete a handshake using weaker
   algorithms than they jointly support, but only if the weakest joint
   algorithm is strong enough that it cannot be compromised by a brute-
   force attack.  However, this does not defend against many attacks on
   those algorithms; specifically, an on-path attacker might perform a
   bid-down attack on a client that supports both Argon2 [Argon2] and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words
   SHA-256 for password hashing and use in RFCs that to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR collect a MESSAGE-
   INTEGRITY-SHA256 value that it can then use for
              specifying an offline brute-
   force attack.  This would be detected when the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3629]  Yergeau, F., "UTF-8, server receives the
   second request, but that does not prevent the attacker from obtaining
   the MESSAGE-INTEGRITY-SHA256 value.

   Similarly, an attack against the USERHASH mechanism will not succeed
   in establishing a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates session as the server will detect that the feature
   was discarded on path, but the client would still have been convinced
   to send its username in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for clear in the MD5 Message-Digest and USERNAME attribute, thus
   disclosing it to the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <http://www.rfc-editor.org/info/rfc6151>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7064]  Nandakumar, S., Salgueiro, G., Jones, P., and M. Petit-
              Huguenin, "URI Scheme for attacker.

   Finally, when the Session Traversal Utilities
              for NAT (STUN) Protocol", RFC 7064, DOI 10.17487/RFC7064,
              November 2013, <https://www.rfc-editor.org/info/rfc7064>.

   [RFC7350]  Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as Transport for Session Traversal
              Utilities bid-down protection mechanism is employed for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
              August 2014, <https://www.rfc-editor.org/info/rfc7350>.

   [RFC7616]  Shekh-Yusef, R., Ahrens, D., and S. Bremer, "HTTP Digest
              Access Authentication", RFC 7616, DOI 10.17487/RFC7616,
              September 2015, <https://www.rfc-editor.org/info/rfc7616>.

   [RFC8174]  Leiba, B., "Ambiguity a
   future upgrade of the HMAC algorithm used to protect messages, it
   will offer only a limited protection if the current HMAC algorithm is
   already compromised.

16.2.  Attacks Affecting the Usage

   This section lists attacks that might be launched against a usage of
   STUN.  Each STUN Usage must consider whether these attacks are
   applicable to it and, if so, discuss countermeasures.

   Most of the attacks in this section revolve around an attacker
   modifying the reflexive address learned by a STUN client through a
   Binding request/response transaction.  Since the usage of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 8200, STD 86,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rf8200>.

   [RFC8265]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison the
   reflexive address is a function of Internationalized Strings
              Representing Usernames and Passwords", RFC 8265,
              DOI 10.17487/RFC8265, October 2017,
              <https://www.rfc-editor.org/info/rfc8265>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

20.2.  Informative References

   [BCP195]   Sheffer, Y., Holz, R., the usage, the applicability and P. Saint-Andre,
              "Recommendations for Secure Use
   remediation of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [I-D.ietf-tram-stun-pmtud]
              Petit-Huguenin, M. and G. Salgueiro, "Path MTU Discovery
              Using Session Traversal Utilities these attacks are usage-specific.  In common
   situations, modification of the reflexive address by an on-path
   attacker is easy to do.  Consider, for NAT (STUN)", draft-
              ietf-tram-stun-pmtud-10 (work in progress), September
              2018.

   [I-D.irtf-cfrg-argon2]
              Biryukov, A., Dinu, D., Khovratovich, D., and S.
              Josefsson, "The memory-hard Argon2 password hash and
              proof-of-work function", draft-irtf-cfrg-argon2-04 (work
              in progress), November 2018.

   [RFC1952]  Deutsch, P., "GZIP file format specification version 4.3",
              RFC 1952, DOI 10.17487/RFC1952, May 1996,
              <https://www.rfc-editor.org/info/rfc1952>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., example, the common situation
   where STUN is run directly over UDP.  In this case, an on-path
   attacker can modify the source IP address of the Binding request
   before it arrives at the STUN server.  The STUN server will then
   return this IP address in the XOR-MAPPED-ADDRESS attribute to the
   client and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3424]  Daigle, L., Ed. send the response back to that (falsified) IP address and IAB, "IAB Considerations for
              UNilateral Self-Address Fixing (UNSAF) Across Network
              Address Translation", RFC 3424, DOI 10.17487/RFC3424,
              November 2002, <https://www.rfc-editor.org/info/rfc3424>.

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C.,
   port.  If the attacker can also intercept this response, it can
   direct it back towards the client.  Protecting against this attack by
   using a message-integrity check is impossible, since a message-
   integrity value cannot cover the source IP address and R. Mahy,
              "STUN - Simple Traversal the
   intervening NAT must be able to modify this value.  Instead, one
   solution to prevent the attacks listed below is for the client to
   verify the reflexive address learned, as is done in ICE [RFC8445].

   Other usages may use other means to prevent these attacks.

16.2.1.  Attack I: Distributed DoS (DDoS) against a Target

   In this attack, the attacker provides one or more clients with the
   same faked reflexive address that points to the intended target.
   This will trick the STUN clients into thinking that their reflexive
   addresses are equal to that of the target.  If the clients hand out
   that reflexive address in order to receive traffic on it (for
   example, in SIP messages), the traffic will instead be sent to the
   target.  This attack can provide substantial amplification,
   especially when used with clients that are using STUN to enable
   multimedia applications.  However, it can only be launched against
   targets for which packets from the STUN server to the target pass
   through the attacker, limiting the cases in which it is possible.

16.2.2.  Attack II: Silencing a Client

   In this attack, the attacker provides a STUN client with a faked
   reflexive address.  The reflexive address it provides is a transport
   address that routes to nowhere.  As a result, the client won't
   receive any of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,
              DOI 10.17487/RFC3489, March 2003,
              <https://www.rfc-editor.org/info/rfc3489>.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines the packets it expects to receive when it hands out
   the reflexive address.  This exploitation is not very interesting for Cryptographic
              Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
              June 2005, <https://www.rfc-editor.org/info/rfc4107>.

   [RFC5090]  Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
              and W. Beck, "RADIUS Extension
   the attacker.  It impacts a single client, which is frequently not
   the desired target.  Moreover, any attacker that can mount the attack
   could also deny service to the client by other means, such as
   preventing the client from receiving any response from the STUN
   server, or even a DHCP server.  As with the attack described in
   Section 16.2.1, this attack is only possible when the attacker is on
   path for Digest Authentication",
              RFC 5090, DOI 10.17487/RFC5090, February 2008,
              <http://www.rfc-editor.org/info/rfc5090>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities packets sent from the STUN server towards this unused IP
   address.

16.2.3.  Attack III: Assuming the Identity of a Client

   This attack is similar to attack II.  However, the faked reflexive
   address points to the attacker itself.  This allows the attacker to
   receive traffic that was destined for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <https://www.rfc-editor.org/info/rfc5389>.

   [RFC5626]  Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
              "Managing Client-Initiated Connections the client.

16.2.4.  Attack IV: Eavesdropping

   In this attack, the attacker forces the client to use a reflexive
   address that routes to itself.  It then forwards any packets it
   receives to the client.  This attack allows the attacker to observe
   all packets sent to the client.  However, in order to launch the Session
              Initiation Protocol (SIP)", RFC 5626,
              DOI 10.17487/RFC5626, October 2009,
              <https://www.rfc-editor.org/info/rfc5626>.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions
   attack, the attacker must have already been able to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,
              <https://www.rfc-editor.org/info/rfc5766>.

   [RFC5769]  Denis-Courmont, R., "Test Vectors for Session Traversal
              Utilities observe packets
   from the client to the STUN server.  In most cases (such as when the
   attack is launched from an access network), this means that the
   attacker could already observe packets sent to the client.  This
   attack is, as a result, only useful for NAT (STUN)", RFC 5769, DOI 10.17487/RFC5769,
              April 2010, <https://www.rfc-editor.org/info/rfc5769>.

   [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using Session Traversal Utilities observing traffic by
   attackers on the path from the client to the STUN server, but not
   generally on the path of packets being routed towards the client.

   Note that this attack can be trivially launched by the STUN server
   itself, so users of STUN servers should have the same level of trust
   in the users of STUN servers as any other node that can insert itself
   into the communication flow.

16.3.  Hash Agility Plan

   This specification uses HMAC-SHA256 for NAT (STUN)",
              RFC 5780, DOI 10.17487/RFC5780, May 2010,
              <https://www.rfc-editor.org/info/rfc5780>.

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates computation of the message
   integrity, sometimes in combination with Interactive Connectivity
              Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
              March 2012, <https://www.rfc-editor.org/info/rfc6544>.

   [RFC7231]  Fielding, R. HMAC-SHA1.  If, at a later
   time, HMAC-SHA256 is found to be compromised, the following remedy
   should be applied:

   o  Both a new message-integrity attribute and R. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics a new STUN Security
      Feature bit will be allocated in a Standards Track document.  The
      new message-integrity attribute will have its value computed using
      a new hash.  The STUN Security Feature bit will be used to
      simultaneously 1) signal to a STUN client using the long-term
      credential mechanism that this server supports this new hash
      algorithm and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC8126]  Cotton, M., Leiba, B., 2) prevent bid-down attacks on the new message-
      integrity attribute.

   o  STUN clients and T. Narten, "Guidelines servers using the short-term credential mechanism
      will need to update the external mechanism that they use to signal
      what message-integrity attributes are in use.

   The bid-down protection mechanism described in this document is new
   and thus cannot currently protect against a bid-down attack that
   lowers the strength of the hash algorithm to HMAC-SHA1.  This is why,
   after a transition period, a new document updating this one will
   assign a new STUN Security Feature bit for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, May 2008,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8445]  Keranen, A., Holmberg, C., deprecating HMAC-SHA1.
   When used, this bit will signal that HMAC-SHA1 is deprecated and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

Appendix A.  C Snippet
   should no longer be used.

   Similarly, if HMAC-SHA256 is found to Determine STUN Message Types

   Given be compromised, a 16-bit new userhash
   attribute and a new STUN message type value in host byte order Security Feature bit will be allocated in msg_type
   parameter, below are C macros to determine the a
   Standards Track document.  The new userhash attribute will have its
   value computed using a new hash.  The STUN message types:

   <CODE BEGINS>
   #define IS_REQUEST(msg_type)       (((msg_type) & 0x0110) == 0x0000)
   #define IS_INDICATION(msg_type)    (((msg_type) & 0x0110) == 0x0010)
   #define IS_SUCCESS_RESP(msg_type)  (((msg_type) & 0x0110) == 0x0100)
   #define IS_ERR_RESP(msg_type)      (((msg_type) & 0x0110) == 0x0110)
   <CODE ENDS>

   A function Security Feature bit will
   be used to convert method and class into a message type:

   <CODE BEGINS>
   int type(int method, int cls) {
     return (method & 0x1F80) << 2 | (method & 0x0070) << 1
       | (method & 0x000F) | (cls & 0x0002) << 7
       | (cls & 0x0001) << 4;
     }
   <CODE ENDS>

   A function simultaneously 1) signal to extract a STUN client using the long-
   term credential mechanism that this server supports this new hash
   algorithm for the userhash attribute and 2) prevent bid-down attacks
   on the new userhash attribute.

17.  IAB Considerations

   The IAB has studied the method from problem of Unilateral Self-Address Fixing
   (UNSAF), which is the message type:

   <CODE BEGINS>
   int method(int type) {
     return (type & 0x3E00) >> 2 | (type & 0x00E0) >> 1
       | (type & 0x000F);
     }
   <CODE ENDS>

   A function general process by which a client attempts to extract
   determine its address in another realm on the class from other side of a NAT
   through a collaborative protocol reflection mechanism [RFC3424].
   STUN can be used to perform this function using a Binding request/
   response transaction if one agent is behind a NAT and the message type:

   <CODE BEGINS>
   int cls(int type) {
     return (type & 0x0100) >> 7 | (type & 0x0010) >> 4;
     }
   <CODE ENDS>

Appendix B.  Test Vectors

   This section augments other is on
   the list public side of test vectors defined in [RFC5769]
   with MESSAGE-INTEGRITY-SHA256.  All the formats NAT.

   The IAB has suggested that protocols developed for this purpose
   document a specific set of considerations.  Because some STUN Usages
   provide UNSAF functions (such as ICE [RFC8445]) and definitions
   listed others do not
   (such as SIP Outbound [RFC5626]), answers to these considerations
   need to be addressed by the usages themselves.

18.  IANA Considerations

18.1.  STUN Security Features Registry

   A STUN Security Feature set defines 24 bits as flags.

   IANA has created a new registry containing the STUN Security Features
   that are protected by the bid-down attack prevention mechanism
   described in Section 2 9.2.1.

   The initial STUN Security Features are:

   Bit 0: Password algorithms
   Bit 1: Username anonymity
   Bit 2-23: Unassigned

   Bits are assigned starting from the most significant side of [RFC5769] apply here.

B.1.  Sample Request with Long-Term Authentication with MESSAGE-
      INTEGRITY-SHA256 the bit
   set, so Bit 0 is the leftmost bit and USERHASH

   This request uses Bit 23 is the following parameters:

   Username: "<U+30DE><U+30C8><U+30EA><U+30C3><U+30AF><U+30B9>" (without
   quotes) unaffected rightmost bit.

   New Security Features are assigned by OpaqueString [RFC8265] processing

   Password: "The<U+00AD>M<U+00AA>tr<U+2168>" and "TheMatrIX" (without
   quotes) respectively before and after OpaqueString processing

   Nonce: "obMatJos2QAAAf//499k954d6OL34oL9FSTvy64sA" (without quotes)

   Realm: "example.org" (without quotes)
         00 01 00 9c      Request type and message length
         21 12 a4 42      Magic cookie
         78 ad 34 33   }
         c6 ad 72 c0   }  Transaction ID
         29 da 41 2e   }
         XX XX 00 20      USERHASH attribute header
         4a 3c f3 8f   }
         ef 69 92 bd   }
         a9 52 c6 78   }
         04 17 da 0f   }  Userhash value (32 bytes)
         24 81 94 15   }
         56 9e 60 b2   }
         05 c4 6e 41   }
         40 7f 17 04   }
         00 15 00 29      NONCE attribute header
         6f 62 4d 61   }
         74 4a 6f 73   }
         32 41 41 41   }
         43 66 2f 2f   }
         34 39 39 6b   }  Nonce value and padding (3 bytes)
         39 35 34 64   }
         36 4f 4c 33   }
         34 6f 4c 39   }
         46 53 54 76   }
         79 36 34 73   }
         41 00 00 00   }
         00 14 00 0b Standards Action [RFC8126].

18.2.  STUN Methods Registry

   IANA has updated the name for method 0x002 as described below as well
   as updated the reference from RFC 5389 to RFC 8489 for the following
   STUN methods:

   0x000: Reserved
   0x001: Binding
   0x002: Reserved; was SharedSecret prior to [RFC5389]

18.3.  STUN Attributes Registry

18.3.1.  Updated Attributes

   IANA has updated the names for attributes 0x0002, 0x0004, 0x0005,
   0x0007, and 0x000B as well as updated the reference from RFC 5389 to
   RFC 8489 for each the following STUN methods.

   In addition, [RFC5389] introduced a mistake in the name of attribute
   0x0003; [RFC5389] called it CHANGE-ADDRESS when it was actually
   previously called CHANGE-REQUEST.  Thus, IANA has updated the
   description for 0x0003 to read "Reserved; was CHANGED-REQUEST prior
   to [RFC5389]".

   Comprehension-required range (0x0000-0x7FFF):
   0x0000: Reserved
   0x0001: MAPPED-ADDRESS
   0x0002: Reserved; was RESPONSE-ADDRESS prior to [RFC5389]
   0x0003: Reserved; was CHANGED-REQUEST prior to [RFC5389]
   0x0004: Reserved; was SOURCE-ADDRESS prior to [RFC5389]
   0x0005: Reserved; was CHANGED-ADDRESS prior to [RFC5389]
   0x0006: USERNAME
   0x0007: Reserved; was PASSWORD prior to [RFC5389]
   0x0008: MESSAGE-INTEGRITY
   0x0009: ERROR-CODE
   0x000A: UNKNOWN-ATTRIBUTES
   0x000B: Reserved; was REFLECTED-FROM prior to [RFC5389]
   0x0014: REALM
   0x0015: NONCE
   0x0020: XOR-MAPPED-ADDRESS

   Comprehension-optional range (0x8000-0xFFFF)
   0x8022: SOFTWARE
   0x8023: ALTERNATE-SERVER
   0x8028: FINGERPRINT

18.3.2.  New Attributes

   IANA has added the following attribute header
         65 78 61 6d   }
         70 6c 65 2e   }  Realm value (11 bytes) and padding (1 byte)
         6f 72 67 00   }
         XX XX 00 20 to the "STUN Attributes"
   registry:

   Comprehension-required range (0x0000-0x7FFF):
   0x001C: MESSAGE-INTEGRITY-SHA256 attribute header
         c4 ec a2 b6   }
         24 6f 26 be   }
         bc 2f 77 49   }
         07 c2 00 a3   }  HMAC-SHA256 value
         76 c7 c2 8e   }
         b4 d1 26 60   }
         bb fe 9f 28   }
         0e 85 71 f2   }

   Note:  Before publication,
   0x001D: PASSWORD-ALGORITHM
   0x001E: USERHASH

   Comprehension-optional range (0x8000-0xFFFF)
   0x8002: PASSWORD-ALGORITHMS
   0x8003: ALTERNATE-DOMAIN

18.4.  STUN Error Codes Registry

   IANA has updated the reference from RFC 5389 to RFC 8489 for the
   error codes defined in Section 14.8.

   IANA has changed the name of the 401 error code from "Unauthorized"
   to "Unauthenticated".

18.5.  STUN Password Algorithms Registry

   IANA has created a new registry titled "STUN Password Algorithms".

   A password algorithm is a hex number in the range 0x0000-0xFFFF.

   The initial contents of the XX XX placeholder must be replaced "Password Algorithm" registry are as
   follows:

   0x0000: Reserved
   0x0001: MD5
   0x0002: SHA-256
   0x0003-0xFFFF: Unassigned

   Password algorithms in the first half of the range (0x0000-0x7FFF)
   are assigned by IETF Review [RFC8126].  Password algorithms in the value
   second half of the range (0x8000-0xFFFF) are assigned to MESSAGE-INTEGRITY-SHA256 and USERHASH by
      IANA. Expert
   Review [RFC8126].

18.5.1.  Password Algorithms

18.5.1.1.  MD5

   This password algorithm is taken from [RFC1321].

   The MESSAGE-INTEGRITY-SHA256 attribute key length is 16 bytes, and the parameters value will need to
      be updated after this.

Appendix C.  Release notes is empty.

      Note: This section must algorithm MUST only be removed before publication as an RFC.

C.1.  Modifications between draft-ietf-tram-stunbis-21 used for compatibility with
      legacy systems.

                key = MD5(username ":" OpaqueString(realm)
                  ":" OpaqueString(password))

18.5.1.2.  SHA-256

   This password algorithm is taken from [RFC7616].

   The key length is 32 bytes, and the parameters value is empty.

              key = SHA-256(username ":" OpaqueString(realm)
                ":" OpaqueString(password))

18.6.  STUN UDP and TCP Port Numbers

   IANA has updated the reference from RFC 5389 to RFC 8489 for the
   following ports in the "Service Name and draft-ietf-
      tram-stunbis-20 Transport Protocol Port
   Number Registry".

   stun   3478/tcp   Session Traversal Utilities for NAT (STUN) port
   stun   3478/udp   Session Traversal Utilities for NAT (STUN) port
   stuns  5349/tcp   Session Traversal Utilities for NAT (STUN) port

19.  Changes since RFC 5389

   This specification obsoletes [RFC5389].  This specification differs
   from RFC 5389 in the following ways:

   o  Added support for DTLS-over-UDP [RFC6347].

   o  Made clear that the RTO is considered stale if there are no
      transactions with the server.

   o  Aligned the RTO calculation with [RFC6298].

   o  Updated the ciphersuites for TLS.

   o  Added support for STUN URI [RFC7064].

   o  Final edits to clean up bid down protection text to address Eric
      Rescorla's DISCUSS and comments.

C.2.  Modifications between draft-ietf-tram-stunbis-20 and draft-ietf-
      tram-stunbis-19  Added support for SHA256 message integrity.

   o  Updates  Updated the Preparation, Enforcement, and Comparison of
      Internationalized Strings (PRECIS) support to address Eric Rescorla's DISCUSS [RFC8265].

   o  Added protocol and comments. registry to choose the password encryption
      algorithm.

   o  Addressed nits raised by Noriyuki Torii

C.3.  Modifications between draft-ietf-tram-stunbis-19  Added support for anonymous username.

   o  Added protocol and draft-ietf-
      tram-stunbis-18 registry for preventing bid-down attacks.

   o  Updates following Adam Roach DISCUSS  Specified that sharing a NONCE is no longer permitted.

   o  Added the possibility of using a domain name in the alternate
      server mechanism.

   o  Added more C snippets.

   o  Added test vector.

20.  References

20.1.  Normative References
   [ITU.V42.2002]
              International Telecommunication Union, "Error-correcting
              procedures for DCEs using asynchronous-to-synchronous
              conversion", ITU-T Recommendation V.42, March 2002.

   [KARN87]   Karn, P. and comments.

C.4.  Modifications between draft-ietf-tram-stunbis-18 C. Partridge, "Improving Round-Trip Time
              Estimates in Reliable Transport Protocols", SIGCOMM '87,
              Proceedings of the ACM workshop on Frontiers in computer
              communications technology, Pages 2-7,
              DOI 10.1145/55483.55484, August 1987.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <https://www.rfc-editor.org/info/rfc1123>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and draft-ietf-
      tram-stunbis-17

   o  Nits.

C.5.  Modifications between draft-ietf-tram-stunbis-17 Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5246]  Dierks, T. and draft-ietf-
      tram-stunbis-16

   o  Modifications following IESG, GENART E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and SECDIR reviews.

C.6.  Modifications between draft-ietf-tram-stunbis-16 Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [RFC6125]  Saint-Andre, P. and draft-ietf-
      tram-stunbis-15

   o  Replace "failure response" with "error response".

   o  Fix wrong section number.

   o  Use "Username anonymity" everywhere.

   o  Align with UTF-8 deprecation.

   o  Fix MESSAGE-INTEGRITY-256.

   o  Update references.

   o  Updates in the IANA sections.

   o  s/HMAC-SHA-1/HMAC-SHA1/, s/HMAC-SHA-256/HMAC-SHA256/, s/SHA1/SHA-
      1/, J. Hodges, "Representation and s/SHA256/SHA-256/.

   o  Fixed definitions
              Verification of STUN clients/servers.

   o  Fixed STUN message structure definition.

   o  Missing text.

   o  Add text explicitly saying that responses do not have to be in the
      same orders than requests.

   o  /other application/other protocol/

   o  Add text explicitly saying that the security feature encoding is 4
      character.

   o  Fixed discrepancy in section 9.2.3/9.2.3.1.

   o  s/invalidate/revoke/.

   o  Removed sentences about checking USERHASH Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in responses, as this
      should not happen.

   o  Specify that ALTERNATE-SERVER carries an IP address.

   o  More modifications following review...

C.7.  Modifications between draft-ietf-tram-stunbis-15 and draft-ietf-
      tram-stunbis-14

   o  Reverted the Context of Transport Layer
              Security (TLS)", RFC 2119 boilerplate to what was in RFC 5389.

   o  Reverted the V.42 reference to the 2002 version.

   o  Updated some references.

C.8.  Modifications between draft-ietf-tram-stunbis-14 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6151]  Turner, S. and draft-ietf-
      tram-stunbis-13

   o  Reorder the paragraphs in section 9.1.4.

   o  The realm is now processed through Opaque in section 9.2.2.

   o  Make clear in section 9.2.4 that it is an exclusive-xor.

   o  Removed text that implied that nonce sharing was explicitly
      permitted in RFC 5389.

   o  In same section, s/username/value/ L. Chen, "Updated Security Considerations
              for USERCASH.

   o  Modify the IANA requests to explicitly say that MD5 Message-Digest and the reserved
      codepoints were prior to HMAC-MD5 Algorithms",
              RFC 5389.

C.9.  Modifications between draft-ietf-tram-stunbis-13 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and draft-ietf-
      tram-stunbis-12

   o  Update references.

   o  Fixes some text following Shepherd review.

   o  Update co-author info.

C.10.  Modifications between draft-ietf-tram-stunbis-12 M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC6347]  Rescorla, E. and draft-ietf-
       tram-stunbis-11

   o  Clarifies the procedure to define a new hash algorithm for
      message-integrity.

   o  Explain the procedure to deprecate SHA1 as message-integrity.

   o  Added procedure for Happy Eyeballs (RFC 6555).

   o  Added verification that Happy Eyeballs works in the STUN Usage
      checklist.

   o  Add reference to Base64 RFC.

   o  Changed co-author affiliation.

C.11.  Modifications between draft-ietf-tram-stunbis-11 N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7064]  Nandakumar, S., Salgueiro, G., Jones, P., and draft-ietf-
       tram-stunbis-10

   o  Made clear that the same HMAC than received in response of short
      term credential must be used M. Petit-
              Huguenin, "URI Scheme for subsequent transactions.

   o  s/URL/URI/

   o  The "nonce cookie" is now mandatory to signal that SHA256 must be
      used in the next transaction.

   o  s/SHA1/SHA256/

   o  Changed co-author affiliation.

C.12.  Modifications between draft-ietf-tram-stunbis-10 Session Traversal Utilities
              for NAT (STUN) Protocol", RFC 7064, DOI 10.17487/RFC7064,
              November 2013, <https://www.rfc-editor.org/info/rfc7064>.

   [RFC7350]  Petit-Huguenin, M. and draft-ietf-
       tram-stunbis-09

   o  Removed the reserved value in the security registry, G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as it does
      not make sense in a bitset.

   o  Updated change list.

   o  Updated the minimum truncation size Transport for M-I-256 to 16 bytes.

   o  Changed the truncation order to match Session Traversal
              Utilities for NAT (STUN)", RFC 7518.

   o  Fixed bugs in truncation boundary text.

   o  Stated that STUN Usages have to explicitly state that they can use
      truncation.

   o  Removed truncation from the MESSAGE-INTEGRITY attribute.

   o  Add reference to C code 7350, DOI 10.17487/RFC7350,
              August 2014, <https://www.rfc-editor.org/info/rfc7350>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <https://www.rfc-editor.org/info/rfc7616>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1952.

   o  Replaced
              2119 Key Words", BCP 14, RFC 2818 reference to 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 6125.

C.13.  Modifications between draft-ietf-tram-stunbis-09 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8265]  Saint-Andre, P. and draft-ietf-
       tram-stunbis-08

   o  Packets discarded in a reliable or unreliable transaction triggers
      an attack error instead of a timeout error.  An attack error on a
      reliable transport is signaled immediately instead of waiting for
      the timeout.

   o  Explicitly state that a received 400 response without
      authentication will be dropped until timeout.

   o  Clarify the SHOULD omit/include rules in LTCM.

   o  If the nonce A. Melnikov, "Preparation,
              Enforcement, and the hmac are both invalid, then a 401 is sent
      instead Comparison of a 438.

   o  The 401 Internationalized Strings
              Representing Usernames and 438 error response to subsequent requests may use the
      previous NONCE/password to authenticate, if they are still
      available.

   o  Change "401 Unauthorized" to "401 Unauthenticated"

   o  Make clear that in some cases it is impossible to add a MI or MI2
      even if the text says SHOULD NOT.

C.14.  Modifications between draft-ietf-tram-stunbis-08 Passwords", RFC 8265,
              DOI 10.17487/RFC8265, October 2017,
              <https://www.rfc-editor.org/info/rfc8265>.

   [RFC8305]  Schinazi, D. and draft-ietf-
       tram-stunbis-07

   o  Updated list of changes since T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 5389.

   o  More examples are automatically generated.

   o  Message integrity truncation is fixed at a multiple of 4 bytes,
      because the padding will not decrease by more than this.

   o  USERHASH contains the 32 bytes of the hash, not a character
      string.

   o  Updated the example to use the USERHASH attribute 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

20.2.  Informative References

   [Argon2]   Biryukov, A., Dinu, D., Khovratovich, D., and the modified
      NONCE attribute.

   o  Updated ICEbis reference.

C.15.  Modifications between draft-ietf-tram-stunbis-07 S.
              Josefsson, "The memory-hard Argon2 password hash and draft-ietf-
       tram-stunbis-06

   o  Add USERHASH attribute to carry the hashed version of the
      username.

   o  Add IANA registry
              proof-of-work function", Work in Progress, draft-irtf-
              cfrg-argon2-04, November 2018.

   [BCP195]   Sheffer, Y., Holz, R., and nonce encoding P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security Features that
      need to be protected from bid-down attacks.

   o  Modified MESSAGE-INTEGRITY (TLS) and MESSAGE-INTEGRITY-SHA256 to support
      truncation limits (pending cryptographic review),

C.16.  Modifications between draft-ietf-tram-stunbis-06 Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, May 2015,
              <https://www.rfc-editor.org/info/bcp195>.

   [RFC1952]  Deutsch, P., "GZIP file format specification version 4.3",
              RFC 1952, DOI 10.17487/RFC1952, May 1996,
              <https://www.rfc-editor.org/info/rfc1952>.

   [RFC2279]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 2279, DOI 10.17487/RFC2279, January 1998,
              <https://www.rfc-editor.org/info/rfc2279>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and draft-ietf-
       tram-stunbis-05

   o  Changed I-D references to E.
              Schooler, "SIP: Session Initiation Protocol", RFC references.

   o  Changed CHANGE-ADDRESS to CHANGE-REQUEST (Errata #4233).

   o  Added test vector 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3424]  Daigle, L., Ed. and IAB, "IAB Considerations for MESSAGE-INTEGRITY-SHA256.

   o
              UNilateral Self-Address Fixing (UNSAF) Across Network
              Address additional review comments from Jonathan Lennox and
      Brandon Williams.

C.17.  Modifications between draft-ietf-tram-stunbis-05 Translation", RFC 3424, DOI 10.17487/RFC3424,
              November 2002, <https://www.rfc-editor.org/info/rfc3424>.

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and draft-ietf-
       tram-stunbis-04

   o R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address review comments from Jonathan Lennox Translators (NATs)", RFC 3489,
              DOI 10.17487/RFC3489, March 2003,
              <https://www.rfc-editor.org/info/rfc3489>.

   [RFC4107]  Bellovin, S. and Brandon Williams.

C.18.  Modifications between draft-ietf-tram-stunbis-04 R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
              June 2005, <https://www.rfc-editor.org/info/rfc4107>.

   [RFC5090]  Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
              and W. Beck, "RADIUS Extension for Digest Authentication",
              RFC 5090, DOI 10.17487/RFC5090, February 2008,
              <https://www.rfc-editor.org/info/rfc5090>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and draft-ietf-
       tram-stunbis-03

   o  Remove SCTP.

   o  Remove DANE.

   o  s/MESSAGE-INTEGRITY2/MESSAGE-INTEGRITY-SHA256/

   o  Remove Salted SHA256 password hash.

   o  The RTO delay between transactions is removed.

   o  Make clear that reusing NONCE will trigger a wasted round trip.

C.19.  Modifications between draft-ietf-tram-stunbis-03 D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <https://www.rfc-editor.org/info/rfc5389>.

   [RFC5626]  Jennings, C., Ed., Mahy, R., Ed., and draft-ietf-
       tram-stunbis-02

   o  SCTP prefix is now 0b00000101 instead of 0x11.

   o  Add SCTP at various places it was needed.

   o  Update the hash agility plan to take in account HMAC-SHA-256.

   o  Adds the bid-down attack on message-integrity F. Audet, Ed.,
              "Managing Client-Initiated Connections in the security
      section.

C.20.  Modifications between draft-ietf-tram-stunbis-02 Session
              Initiation Protocol (SIP)", RFC 5626,
              DOI 10.17487/RFC5626, October 2009,
              <https://www.rfc-editor.org/info/rfc5626>.

   [RFC5766]  Mahy, R., Matthews, P., and draft-ietf-
       tram-stunbis-01

   o  STUN hash algorithm agility (currently only SHA-1 is allowed).

   o  Clarify terminology, text J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,
              <https://www.rfc-editor.org/info/rfc5766>.

   [RFC5769]  Denis-Courmont, R., "Test Vectors for Session Traversal
              Utilities for NAT (STUN)", RFC 5769, DOI 10.17487/RFC5769,
              April 2010, <https://www.rfc-editor.org/info/rfc5769>.

   [RFC5780]  MacDonald, D. and guidance B. Lowekamp, "NAT Behavior Discovery
              Using Session Traversal Utilities for STUN fragmentation.

   o  Clarify whether it's valid to share nonces across TURN
      allocations.

   o  Prevent the server to allocate the same NONCE to clients NAT (STUN)",
              RFC 5780, DOI 10.17487/RFC5780, May 2010,
              <https://www.rfc-editor.org/info/rfc5780>.

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates with
      different IP address and/or different port.  This prevent sharing
      the nonce between TURN allocations in TURN.

   o  Add reference to draft-ietf-uta-tls-bcp

   o  Add a new attribute ALTERNATE-DOMAIN to verify the certificate of
      the ALTERNATE-SERVER after a 300 over (D)TLS.

   o  The RTP delay between transactions applies only to parallel
      transactions, not to serial transactions.  That prevents a 3RTT
      delay between the first transaction Interactive Connectivity
              Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
              March 2012, <https://www.rfc-editor.org/info/rfc6544>.

   [RFC7231]  Fielding, R., Ed. and the second transaction
      with long term authentication.

   o  Add text saying ORIGIN can increase a request size beyond the MTU J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and so require an SCTPoUDP transport.

   o  Move the Acknowledgments Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC8126]  Cotton, M., Leiba, B., and Contributor sections to the end of
      the document, T. Narten, "Guidelines for
              Writing an IANA Considerations Section in accordance with RFCs", BCP 26,
              RFC 7322 section 4.

C.21.  Modifications between draft-ietf-tram-stunbis-01 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8445]  Keranen, A., Holmberg, C., and draft-ietf-
       tram-stunbis-00

   o  Add negotiation mechanism J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for new password algorithms.

   o  Describe the MESSAGE-INTEGRITY/MESSAGE-INTEGRITY2 protocol.

   o  Add support Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [STUN-PMTUD]
              Petit-Huguenin, M. and G. Salgueiro, "Path MTU Discovery
              Using Session Traversal Utilities for SCTP NAT (STUN)", Work in
              Progress, draft-ietf-tram-stun-pmtud-10, September 2018.

Appendix A.  C Snippet to solve the fragmentation problem.

   o  Merge RFC 7350:

      *  Split the "Sending over..." sections Determine STUN Message Types

   Given a 16-bit STUN message type value in 3.

      *  Add DTLS-over-UDP as transport.

      *  Update host byte order in msg_type
   parameter, below are C macros to determine the cipher suites STUN message types:

   <CODE BEGINS>
   #define IS_REQUEST(msg_type)       (((msg_type) & 0x0110) == 0x0000)
   #define IS_INDICATION(msg_type)    (((msg_type) & 0x0110) == 0x0010)
   #define IS_SUCCESS_RESP(msg_type)  (((msg_type) & 0x0110) == 0x0100)
   #define IS_ERR_RESP(msg_type)      (((msg_type) & 0x0110) == 0x0110)
   <CODE ENDS>

   A function to convert method and cipher/compression restrictions.

      * class into a message type:

   <CODE BEGINS>
   int type(int method, int cls) {
     return (method & 0x1F80) << 2 | (method & 0x0070) << 1
       | (method & 0x000F) | (cls & 0x0002) << 7
       | (cls & 0x0001) << 4;
     }
   <CODE ENDS>

   A stuns uri with an IP address is rejected.

      *  Replace most of function to extract the RFC 3489 compatibility by a reference method from the message type:

   <CODE BEGINS>
   int method(int type) {
     return (type & 0x3E00) >> 2 | (type & 0x00E0) >> 1
       | (type & 0x000F);
     }
   <CODE ENDS>

   A function to extract the class from the message type:

   <CODE BEGINS>
   int cls(int type) {
     return (type & 0x0100) >> 7 | (type & 0x0010) >> 4;
     }
   <CODE ENDS>

Appendix B.  Test Vectors

   This section in RFC 5389.

      *  Update augments the STUN Usages list of test vectors defined in [RFC5769]
   with transport applicability.

   o  Merge RFC 7064:

      *  DNS discovery is done from the URI.

      *  Reorganized the text about default ports.

   o  Add more C snippets.

   o  Make clear that MESSAGE-INTEGRITY-SHA256.  All the cached RTO is discarded only if there is no
      new translations for 10 minutes.

C.22.  Modifications between draft-salgueiro-tram-stunbis-02 and draft-
       ietf-tram-stunbis-00

   o  Draft adopted as WG item.

C.23.  Modifications between draft-salgueiro-tram-stunbis-02 formats and draft-
       salgueiro-tram-stunbis-01

   o  Add definition definitions
   listed in Section 2 of MESSAGE-INTEGRITY2.

   o  Update text [RFC5769] apply here.

B.1.  Sample Request with Long-Term Authentication with MESSAGE-
      INTEGRITY-SHA256 and reference from RFC 2988 to RFC 6298.

   o  s/The IAB has mandated/The IAB has suggested/ (Errata #3737).

   o  Fix the figure for USERHASH

   This request uses the UNKNOWN-ATTRIBUTES (Errata #2972).

   o  Fix section number following parameters:

   Username: "<U+30DE><U+30C8><U+30EA><U+30C3><U+30AF><U+30B9>" (without
   quotes) unaffected by OpaqueString [RFC8265] processing

   Password: "The<U+00AD>M<U+00AA>tr<U+2168>" and make clear that the original domain name is
      used for the server certificate verification.  This is consistent
      with what RFC 5922 (section 4) is doing.  (Errata #2010)

   o  Remove text transitioning from RFC 3489.

   o  Add definition of MESSAGE-INTEGRITY2.

   o  Update text "TheMatrIX" (without
   quotes) respectively before and reference from RFC 2988 to RFC 6298.

   o  s/The IAB has mandated/The IAB has suggested/ (Errata #3737).

   o  Fix the figure for the UNKNOWN-ATTRIBUTES (Errata #2972).

   o  Fix section number after OpaqueString [RFC8265]
   processing

   Nonce: "obMatJos2AAACf//499k954d6OL34oL9FSTvy64sA" (without quotes)

   Realm: "example.org" (without quotes)
        00 01 00 9c      Request type and make clear that the original domain name is
      used for the server certificate verification.  This is consistent
      with what RFC 5922 (section 4) is doing.  (Errata #2010)

C.24.  Modifications between draft-salgueiro-tram-stunbis-01 message length
        21 12 a4 42      Magic cookie
        78 ad 34 33   }
        c6 ad 72 c0   }  Transaction ID
        29 da 41 2e   }
        00 1e 00 20      USERHASH attribute header
        4a 3c f3 8f   }
        ef 69 92 bd   }
        a9 52 c6 78   }
        04 17 da 0f   }  Userhash value (32 bytes)
        24 81 94 15   }
        56 9e 60 b2   }
        05 c4 6e 41   }
        40 7f 17 04   }
        00 15 00 29      NONCE attribute header
        6f 62 4d 61   }
        74 4a 6f 73   }
        32 41 41 41   }
        43 66 2f 2f   }
        34 39 39 6b   }  Nonce value and draft-
       salgueiro-tram-stunbis-00

   o  Restore the RFC 5389 text.

   o  Add list of open issues. padding (3 bytes)
        39 35 34 64   }
        36 4f 4c 33   }
        34 6f 4c 39   }
        46 53 54 76   }
        79 36 34 73   }
        41 00 00 00   }
        00 14 00 0b      REALM attribute header
        65 78 61 6d   }
        70 6c 65 2e   }  Realm value (11 bytes) and padding (1 byte)
        6f 72 67 00   }
        00 1c 00 20      MESSAGE-INTEGRITY-SHA256 attribute header
        e4 68 6c 8f   }
        0e de b5 90   }
        13 e0 70 90   }
        01 0a 93 ef   }  HMAC-SHA256 value
        cc bc cc 54   }
        4c 0a 45 d9   }
        f8 30 aa 6d   }
        6f 73 5a 01   }

Acknowledgements

   Thanks to Michael Tuexen, Tirumaleswar Reddy, Oleg Moskalenko, Simon
   Perreault, Benjamin Schwartz, Rifaat Shekh-Yusef, Alan Johnston,
   Jonathan Lennox, Brandon Williams, Olle Johansson, Martin Thomson,
   Mihaly Meszaros, Tolga Asveren, Noriyuki Torii, Spencer Dawkins, Dale
   Worley, Matthew Miller, Peter Saint-Andre, Julien Elie, Mirja
   Kuehlewind, Eric Rescorla, Ben Campbell, Adam Roach, Alexey Melnikov,
   and Benjamin Kaduk for the comments, suggestions, and questions that
   helped improve this document.

   The authors Acknowledgements section of RFC 5389 appeared as follows:

   The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
   Jennings, Bob Penfield, Xavier Marjou, Magnus Westerlund, Miguel
   Garcia, Bruce Lowekamp, and Chris Sullivan for their comments, and
   Baruch Sterman and Alan Hawrylyshen for initial implementations.
   Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
   Schulzrinne for IESG and IAB input on this work.

Contributors

   Christian Huitema and Joel Weinberger were original co-authors coauthors of
   RFC 3489.

Authors' Addresses

   Marc Petit-Huguenin
   Impedance Mismatch

   Email: marc@petit-huguenin.org

   Gonzalo Salgueiro
   Cisco
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   US
   United States of America

   Email: gsalguei@cisco.com

   Jonathan Rosenberg
   Five9
   Edison, NJ
   US
   United States of America

   Email: jdrosen@jdrosen.net
   URI:   http://www.jdrosen.net
   Dan Wing
   Citrix Systems, Inc.
   4988 Great America Pkwy
   Santa Clara, CA  95054
   United States of America

   Email: dwing-ietf@fuggles.com

   Rohan Mahy
   Unaffiliated

   Email: rohan.ietf@gmail.com

   Philip Matthews
   Nokia
   600 March Road
   Ottawa, Ontario  K2K 2T6
   Canada

   Phone: 613-784-3139
   Email: philip_matthews@magma.ca