TLS
Internet Engineering Task Force (IETF) H. Tschofenig, Ed.
Internet-Draft
Request for Comments: 9853 H-BRS
Updates: 9146, 9147 (if approved) A. Kraus
Intended status:
Category: Standards Track
Expires: 15 January 2026
ISSN: 2070-1721 T. Fossati
Linaro
14 July 2025
February 2026
Return Routability Check for DTLS 1.2 and DTLS 1.3
draft-ietf-tls-dtls-rrc-20
Abstract
This document specifies a return routability check Return Routability Check (RRC) for use in
the context of the Connection ID (CID) construct for the Datagram
Transport Layer Security (DTLS) protocol versions 1.2 and 1.3.
Implementations offering the CID functionality described in RFC RFCs 9146
and RFC 9147 are encouraged to also provide the return routability
check RRC functionality
described in this document. For this reason, this document updates RFC
RFCs 9146 and RFC 9147.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Transport Layer
Security Working Group mailing list (tls@ietf.org), which is archived
at https://mailarchive.ietf.org/arch/browse/tls/.
Source for this draft and an issue tracker can be found at
https://github.com/tlswg/dtls-rrc.
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
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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,
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This Internet-Draft will expire on 15 January 2026.
https://www.rfc-editor.org/info/rfc9853.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. RRC Extension . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. RRC and CID Interplay . . . . . . . . . . . . . . . . . . 4
4. Return Routability Check Message Types . . . . . . . . . . . 5
5. Path Validation Procedure . . . . . . . . . . . . . . . . . . 6
5.1. Basic . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Enhanced . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Path Challenge Requirements . . . . . . . . . . . . . . 8
5.4. Path Response/Drop Requirements . . . . . . . . . . . . . 9
5.5. Timer Choice . . . . . . . . . . . . . . . . . . . . . . 9
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Operational Considerations . . . . . . . . . . . . . . . . . 11
7.1. Logging Anomalous Events . . . . . . . . . . . . . . . . 12
7.2. Middlebox Interference . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8.1. Attacker Model . . . . . . . . . . . . . . . . . . . . . 13
8.1.1. Amplification . . . . . . . . . . . . . . . . . . . . 14
8.1.2. Off-Path Packet Forwarding . . . . . . . . . . . . . 14
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10.1. New TLS ContentType . . . . . . . . . . . . . . . . . . 19
10.2. New TLS ExtensionType . . . . . . . . . . . . . . . . . 19
10.3. New "TLS RRC Message Type" Registry . . . . . . . . . . 20
10.3.1. Designated Expert Instructions . . . . . . . . . . . 21
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1.
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2.
11.2. Informative References . . . . . . . . . . . . . . . . . 23
Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
A Connection ID (CID) is an identifier carried in the record layer
header of a DTLS datagram that gives the receiver additional
information for selecting the appropriate security context. The CID
mechanism has been specified in [RFC9146] for DTLS 1.2 and in
[RFC9147] for DTLS 1.3.
Section 6 of [RFC9146] describes how the use of CID increases the
attack surface of DTLS 1.2 and 1.3 by providing both on-path and off-
path attackers an opportunity for (D)DoS. DoS or DDoS. It also describes the
steps a DTLS principal must take when a record with a CID is received
that has a source address different from the one currently associated
with the DTLS connection. However, the actual mechanism for ensuring
that the new peer address is willing to receive and process DTLS
records is left open. To address the gap, this document defines a
Return Routability Check (RRC) sub-protocol subprotocol for DTLS 1.2 and 1.3 1.3,
inspired by the path validation procedure defined in Section 8.2 of
[RFC9000]. As such, this document updates [RFC9146] and [RFC9147].
The return routability check is performed by the receiving endpoint
before the CID-address binding is updated in that endpoint's session
state. This is done in order to give the receiving endpoint
confidence that the sending peer is in fact reachable at the source
address indicated in the received datagram. For an illustration of
the handshake and address validation phases, see Section 6.
Section 5.1 of this document explains the fundamental mechanism that
aims to reduce the DDoS attack surface. Additionally, in Section 5.2, 5.2
discusses a more advanced address validation mechanism is
discussed. mechanism. This
mechanism is designed to counteract off-path attackers trying to
place themselves on-path by racing packets that trigger address
rebinding at the receiver. To gain a detailed understanding of the
attacker model, please refer to Section 8.1.
Apart from of its use in the context of CID-address binding updates,
the path validation capability offered by RRC can be used at any time
by either endpoint. For instance, an endpoint might use RRC to check
that a peer is still reachable at its last known address after a
period of quiescence.
2. Conventions and 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.
This document assumes familiarity with the CID format and protocol
defined for DTLS 1.2 [RFC9146] and for DTLS 1.3 [RFC9147]. The
presentation language used in this document is described in Section 4
of [RFC8446].
In this document, the term "anti-amplification limit" means three
times the amount of data received from an unvalidated address. This
includes all DTLS records originating from that source address,
excluding those that have been discarded. This follows the pattern
of [RFC9000], applying a similar concept to DTLS.
The term "address" is defined in Section 1.2 of [RFC9000].
The terms "client", "server", "peer" "peer", and "endpoint" are defined in
Section 1.1 of [RFC8446].
3. RRC Extension
The use of RRC is negotiated via the rrc extension. The rrc
extension is only defined for DTLS 1.2 and DTLS 1.3. On connecting, a
client wishing to use RRC includes the rrc extension in its
ClientHello. If the server is capable of meeting this requirement,
it responds with a an rrc extension in its ServerHello. The
extension_type value for this extension is TBD1 61, and the extension_data
field of this extension is empty. A client offering the rrc
extension MUST also offer the connection_id extension [RFC9146]. If
the client includes the rrc extension in its ClientHello but omits
the connection_id extension, the server MUST NOT include the rrc
extension in its ServerHello. A client offering the connection_id
extension SHOULD also offer the rrc extension, unless the application
using DTLS has its own address validation mechanism. The client and
server MUST NOT use RRC unless both sides have successfully exchanged
rrc extensions.
3.1. RRC and CID Interplay
RRC offers an in-protocol mechanism to perform peer address
validation that complements the "peer address update" procedure
described in Section 6 of [RFC9146]. Specifically, when both CID
[RFC9146] and RRC have been successfully negotiated for the session,
if a record with CID is received that has the source address of the
enclosing UDP datagram different from what is currently associated
with that CID value, the receiver SHOULD perform a return routability
check as described in Section 5, unless an application-specific
address validation mechanism can be triggered instead (e.g., CoAP
Constrained Application Protocol (CoAP) Echo [RFC9175]).
4. Return Routability Check Message Types
This document defines the return_routability_check content type
(Figure 1) to carry Return Routability Check messages.
The RRC sub-protocol subprotocol consists of three message types: path_challenge,
path_response
path_response, and path_drop that path_drop. These message types are used for path
validation and selection as described in Section 5.
Each message carries a Cookie, an 8-byte field containing 64 bits of
entropy (e.g., obtained from the CSPRNG cryptographically secure
pseudorandom number generator (CSPRNG) used by the TLS
implementation,
implementation; see Appendix C.1 of [RFC8446]).
The return_routability_check message MUST be authenticated and
encrypted using the currently active security context.
enum {
invalid(0),
change_cipher_spec(20),
alert(21),
handshake(22),
application_data(23),
heartbeat(24), /* RFC 6520 */
tls12_cid(25), /* RFC 9146, DTLS 1.2 only */
return_routability_check(TBD2), /* NEW */
(255)
} ContentType;
uint64 Cookie;
enum {
path_challenge(0),
path_response(1),
path_drop(2),
(255)
} rrc_msg_type;
struct {
rrc_msg_type msg_type;
select (return_routability_check.msg_type) {
case path_challenge: Cookie;
case path_response: Cookie;
case path_drop: Cookie;
};
} return_routability_check;
Figure 1: Return Routability Check Message and Content Type
Future extensions to the RRC sub-protocol subprotocol may define new message
types. Implementations MUST be able to parse and understand the
three RRC message types defined in this document. In addition,
implementations MUST be able to parse and gracefully ignore messages
with an unknown msg_type.
5. Path Validation Procedure
A receiver that observes the peer's address change MUST stop sending
any buffered application data, data or limit the data sent to the
unvalidated address to the anti-amplification limit. It then
initiates the return routability check.
This document describes two kinds of checks: basic (Section 5.1) and
enhanced (Section 5.2). The choice of one or the other depends on
whether the off-path attacker scenario described in Section 8.1.2 is
to be considered. (The decision on what strategy to choose depends
mainly on the threat model, model but may also be influenced by other
considerations. Examples of impacting factors include: include the need to
minimise implementation complexity, privacy concerns, and the need to
reduce the time it takes to switch path. The choice may be offered
as a configuration option to the user of the TLS implementation.)
After the path validation procedure is completed, complete, any pending send
operation is resumed to the bound peer address.
Section
Sections 5.3 and Section 5.4 list the requirements for the initiator and
responder roles, broken down per protocol phase.
Please note that the presented algorithms are not designed to handle
nested rebindings, i.e. rebindings that may occur while a path is
being validated following a previous rebinding. If this happens
(which This should rarely occur),
occur, but if it happens, the path_response message is dropped, the
address validation times out, and the address will not be updated. A
new path validation will start when new data is received.
Also
Also, note that in the event of a NAT rebind, the initiator and
responder will have different views of the path: the The initiator will
see a new path, while the responder will still see the old one.
5.1. Basic
The basic return routability check comprises the following steps:
1. The receiver (i.e., the initiator) creates a
return_routability_check message of type path_challenge and
places the unpredictable cookie into the message.
2. The message is sent to the observed new address and a timer T
(see Section 5.5) is started.
3. The peer (i.e., the responder) cryptographically verifies the
received return_routability_check message of type path_challenge
and responds by echoing the cookie value in a
return_routability_check message of type path_response.
4. When the initiator receives the return_routability_check message
of type path_response and verifies that it contains the sent
cookie, it updates the peer address binding.
5. If T expires expires, the peer address binding is not updated.
5.2. Enhanced
The enhanced return routability check comprises the following steps:
1. The receiver (i.e., the initiator) creates a
return_routability_check message of type path_challenge and
places the unpredictable cookie into the message.
2. The message is sent to the previously valid address, which
corresponds to the old path. Additionally, a timer T is started,
see started
(see Section 5.5. 5.5).
3. If the path is still functional, the peer (i.e., the responder)
cryptographically verifies the received return_routability_check
message of type path_challenge. The action to be taken depends
on whether the path through which the message was received
remains the preferred one.
* If the path through which the message was received is
preferred, a return_routability_check message of type
path_response MUST be returned. (Note that, from the
responder's perspective, the preferred path and the old path
coincide in the event of a NAT rebind.)
* If the path through which the message was received is no
longer preferred, a return_routability_check message of type
path_drop MUST be returned. (Note that the responder must
have initiated a voluntary path migration in order to know
that this path is no longer the preferred one.)
In either case, the peer echoes the cookie value in the response.
4. The initiator receives and verifies that the
return_routability_check message contains the previously sent
cookie. The actions taken by the initiator differ based on the
received message:
* When a return_routability_check message of type path_response
was
is received, the initiator MUST continue using the previously
valid address, i.e., no switch to the new path takes place and
the peer address binding is not updated.
* When a return_routability_check message of type path_drop was is
received, the initiator MUST perform a return routability
check on the observed new address, as described in
Section 5.1.
5. If T expires expires, the peer address binding is not updated. In this
case, the initiator MUST perform a return routability check on
the observed new address, as described in Section 5.1.
5.3. Path Challenge Requirements
* The initiator MAY send multiple return_routability_check messages
of type path_challenge to cater for packet loss on the probed
path.
- Each path_challenge SHOULD go into different transport packets.
(Note that the DTLS implementation may not have control over
the packetization done by the transport layer.)
- The transmission of subsequent path_challenge messages SHOULD
be paced to decrease the chance of loss.
- Each path_challenge message MUST contain random data.
- In general, the number of "backup" path_challenge messages
depends on the application, since some are more sensitive than
others to latency caused by changes in the path than others. path. In the
absence of application-specific requirements, the initiator can
send a path_challenge message once per round-trip time (RTT),
up to the anti-amplification limit.
* The initiator MAY use padding using the record padding mechanism
available in DTLS 1.3 (and in DTLS 1.2, when CID is enabled on the
sending direction) up to the anti-amplification limit to probe if
the path Path MTU (PMTU) for the new path is still acceptable.
5.4. Path Response/Drop Requirements
* The responder MUST NOT delay sending an elicited path_response or
path_drop messages.
* The responder MUST send exactly one path_response or path_drop
message for each valid path_challenge it received.
* The responder MUST send the path_response or the path_drop to the
address from which the corresponding path_challenge was received.
This ensures that the path is functional in both directions.
* The initiator MUST silently discard any invalid path_response or
path_drop it receives.
Note that RRC does not cater for PMTU discovery on the reverse path.
If the responder wants to do PMTU discovery using RRC, it should
initiate a new path validation procedure.
5.5. Timer Choice
When setting T, implementations are cautioned that the new path could
have a longer RTT than the original.
In settings where there is external information about the RTT of the
active path (i.e., the old path), implementations SHOULD use T =
3xRTT.
If an implementation has no way to obtain information regarding the
RTT of the active path, T SHOULD be set to 1s. 1 second.
Profiles for specific deployment environments -- for example,
constrained networks [I-D.ietf-uta-tls13-iot-profile] [IOT-PROFILE] -- MAY specify a different, more
suitable value for T.
6. Example
Figure 2 shows an example of a DTLS 1.3 handshake in which a client
and a server successfully negotiate support for both the CID and RRC
extensions.
Client Server
Key ^ ClientHello
Exch | + key_share
| + signature_algorithms
| + rrc
v + connection_id=empty
-------->
ServerHello ^ Key
+ key_share | Exch
+ connection_id=100 |
+ rrc v
{EncryptedExtensions} ^ Server
{CertificateRequest} v Params
{Certificate} ^
{CertificateVerify} | Auth
<-------- {Finished} v
^ {Certificate}
Auth | {CertificateVerify}
v {Finished} -------->
[Application Data] <-------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
{} Indicates messages protected using keys
derived from a [sender]_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N.
Figure 2: Message Flow for Full DTLS Handshake
Once a connection has been established, the client and the server
exchange application payloads protected by DTLS with a unilaterally
used CID. In this case, the client is requested to use CID 100 for
records sent to the server.
At some point in the communication interaction, the address used by
the client changes and, changes, and thanks to the CID usage, the security context
to interpret the record is successfully located by the server.
However, the server wants to test the reachability of the client at
its new address.
Figure 3 shows the server initiating a "basic" basic RRC exchange (see
Section 5.1) that establishes reachability of the client at the new
address.
Client Server
------ ------
Application Data ========>
<CID=100>
Src-IP=A
Dst-IP=Z
<======== Application Data
Src-IP=Z
Dst-IP=A
<<------------->>
<< Some >>
<< Time >>
<< Later >>
<<------------->>
Application Data ========>
<CID=100>
Src-IP=B
Dst-IP=Z
<<< Unverified IP
Address B >>
<-------- Return Routability Check
path_challenge(cookie)
Src-IP=Z
Dst-IP=B
Return Routability Check -------->
path_response(cookie)
Src-IP=B
Dst-IP=Z
<<< IP Address B
Verified >>
<======== Application Data
Src-IP=Z
Dst-IP=B
Figure 3: "Basic" Basic Return Routability Example
7. Operational Considerations
7.1. Logging Anomalous Events
Logging of RRC operations at both ends of the protocol can be
generally useful for the users of an implementation. In particular,
for security information Security Information and event management Event Management (SIEM) and
troubleshooting purposes, it is strongly advised that implementations
collect statistics about any unsuccessful RRC operations, as they
could represent security-relevant events when they coincide with
attempts by an attacker to interfere with the end-to-end path. It is
also advisable to log instances where multiple responses to a single
path_challenge are received, as this could suggest an off-path attack
attempt.
In some cases, the presence of frequent path probes could indicate a
problem with the stability of the path. This information can be used
to identify any issues with the underlying connectivity service.
7.2. Middlebox Interference
Since the DTLS 1.3 encrypted packet's record type is opaque to on-
path observers, RRC messages are immune to middlebox interference
when using DTLS 1.3. In contrast, DTLS 1.2 RRC messages that are not
wrapped in the tls12_cid record (e.g., in the server-to-client
direction if the server negotiated a zero-length CID) have the
return_routability_check content type in plain text, making them
susceptible to interference (e.g., dropping of path_challenge
messages), which would hinder the RRC functionality altogether.
Therefore, when using RRC in DTLS 1.2 is used and middlebox interference is
a concern, it is recommended to enable CID in both directions.
8. Security Considerations
Note that the return routability checks do not protect against
flooding of third-parties third parties if the attacker is on-path, as the attacker
can redirect the return routability checks to the real peer (even if
those datagrams are cryptographically authenticated). On-path
adversaries can, in general, pose a harm to connectivity.
If the RRC challenger reuses a cookie that was previously used in the
same connection and does not implement anti-replay protection (see
Section 4.5.1 of [RFC9147] and Section 4.1.2.6 of [RFC6347]), an
attacker could replay a previously sent path_response message
containing the reused cookie to mislead the challenger into switching
to a path of the attacker's choosing. To prevent this, RRC cookies
must be _freshly_ generated using a reliable source of entropy
[RFC4086]. See Appendix C.1 of [RFC8446] for guidance.
8.1. Attacker Model
Two classes of attackers are considered, off-path and on-path, with
increasing capabilities (see Figure 4) partly following terminology
introduced in QUIC (Section 21.1 of [RFC9000]):
* An off-path attacker is not on the original path between the DTLS
peers, but it is able to observe packets on the original path and
has a faster forwarding path compared to the DTLS peers, which
allows it to make copies of the observed packets, race its copies
to either peer peer, and consistently win the race.
* An on-path attacker is on the original path between the DTLS peers
and is therefore capable, compared to the off-path attacker, to
also drop and delay records at will.
Note that, in general, attackers cannot craft DTLS records in a way
that would successfully pass verification, due to the cryptographic
protections applied by the DTLS record layer.
.--> .------------------------------------. <--.
| | Inspect un-encrypted unencrypted portions | |
| +------------------------------------+ |
| | Inject | |
off-path +------------------------------------+ |
| | Reorder | |
| +------------------------------------+ |
| | Modify un-authenticated unauthenticated portions | on-path
'--> +------------------------------------+ |
| Delay | |
+------------------------------------+ |
| Drop | |
+------------------------------------+ |
| Manipulate the packetization layer | |
'------------------------------------' <--'
Figure 4: Attacker capabilities Capabilities
RRC is designed to defend against the following attacks:
* On-path and off-path attackers that try to create an amplification
attack by spoofing the source address of the victim
(Section 8.1.1).
* Off-path attackers that try to put themselves on-path
(Section 8.1.2), provided that the enhanced path validation
algorithm is used (Section 5.2).
8.1.1. Amplification
Both on-path and off-path attackers can send a packet (either by
modifying it on the fly, fly or by copying, injecting, and racing it,
respectively) with the source address modified to that of a victim
host. If the traffic generated by the server in response is larger
compared to the received packet (e.g., a CoAP server returning an
MTU's worth of data from a 20-bytes 20-byte GET request
[I-D.irtf-t2trg-amplification-attacks]) [AMP-ATTACKS]), the
attacker can use the server as a traffic amplifier toward the victim.
8.1.1.1. Mitigation Strategy
When receiving a packet with a known CID that has a source address
different from the one currently associated with the DTLS connection,
an RRC-capable endpoint will not send a (potentially large) response
but instead a small path_challenge message to the victim host. Since
the host is not able to decrypt it and generate a valid
path_response, the address validation fails, which in turn keeps the
original address binding unaltered.
Note that in the case of an off-path attacker, the original packet
still reaches the intended destination; therefore, an implementation
could use a different strategy to mitigate the attack.
8.1.2. Off-Path Packet Forwarding
An off-path attacker that can observe packets might forward copies of
genuine packets to endpoints over a different path. If the copied
packet arrives before the genuine packet, this will appear as a path
change, like in a genuine NAT rebinding occurrence. Any genuine
packet will be discarded as a duplicate. If the attacker is able to
continue forwarding packets, it might be able to cause migration to a
path via the attacker. This places the attacker on-path, giving it
the ability to observe or drop all subsequent packets.
This style of attack relies on the attacker using a path that has the
same or better characteristics (e.g., due to a more favourable
service level agreements) as the direct path between endpoints. The
attack is more effective if relatively few packets are sent or if
packet loss coincides with the attempted attack.
A data packet received on the original path that increases the
maximum received packet number will cause the endpoint to move back
to that path. Therefore, eliciting packets on this path increases
the likelihood that the attack is unsuccessful. Note however However, note that,
unlike QUIC, DTLS has no "non-probing" packets so this would require
application specific
application-specific mechanisms.
8.1.2.1. Mitigation Strategy
Figure 5 illustrates the case where a receiver receives a packet with
a new source address. In order to determine that this path change
was not triggered by an off-path attacker, the receiver will send an
RRC message of type path_challenge (1) on the old path.
new old
path .----------. path
| |
.-----+ Receiver +-----.
| | | |
| '----------' |
| |
| |
| |
.----+------. |
/ Attacker? / |
'------+----' |
| |
| |
| |
| .----------. |
| | | |
'-----+ Sender +-----'
| |
'----------'
Figure 5: Off-Path Packet Forwarding Scenario
Three cases need to be considered:
Case 1: The old path is dead (e.g., due to a NAT rebinding), which
leads to a timeout of (1).
As shown in Figure 6, a path_challenge (2) needs to be sent on the
new path. If the sender replies with a path_response on the new path
(3), the switch to the new path is considered legitimate.
new old
path .----------. path
.------>| +-------.
| .-----+ Receiver +...... |
| | .---+ | . |
| | | '----------' . |
path- 3 | | . 1 path-
response | | | . | challenge
| | | . |
.--|-+-|----------------------v--.
/ | | NAT X / timeout
'----|-+-|-----------------------'
| | | .
| | 2 path- .
| | | challenge .
| | | .----------. .
| | '-->| | .
| '-----+ Sender +.....'
'-------+ |
'----------'
Figure 6: Old path is dead Path Is Dead
Case 2: The old path is alive but not preferred.
This case is shown in Figure 7 whereby the sender replies with a
path_drop message (2) on the old path. This triggers the receiver to
send a path_challenge (3) on the new path. The sender will reply
with a path_response (4) on the new path, thus providing confirmation
for the path migration.
new old
path .----------. path
.------>| |<------.
| .-----+ Receiver +-----. |
| | .---+ +---. | |
| | | '----------' | | |
path- 4 | | path- 1 | |
response | | | challenge | | |
| | | | | |
.---------|-+-|----. .--|-+-|-----------.
/ NAT A | | / / | | NAT B /
'-----------|---|--' '----|-+-|---------'
| | | | | |
| | 3 path- | | 2 path-
| | | challenge | | | drop
| | | .----------. | | |
| | '-->| |<--' | |
| '-----+ Sender +-----' |
'-------+ +-------'
'----------'
Figure 7: Old path is not preferred Path Is Not Preferred
Case 3: The old path is alive and preferred.
This is most likely the result of an off-path attacker trying to
place itself on path. The on-path. As shown in Figure 8, the receiver sends a
path_challenge on the old
path path, and the sender replies with a
path_response (2) on the old path.
The interaction is shown in Figure 8. This results in the connection
not being migrated to the new path, thus thwarting the attack.
new old
path .----------. path
| +-------.
.-----+ Receiver +-----. |
| | |<--. | |
| '----------' | | |
| | | 1 path-
| | | | challenge
| | | |
.---+------. .--|-+-|-----.
/ off-path / / |NAT| /
/ attacker / '----|-+-|---'
'------+---' | | |
| | | |
| path- 2 | |
| response | | |
| .----------. | | |
| | +---' | |
'-----+ Sender +-----' |
| |<------'
'----------'
Figure 8: Old path is preferred Path Is Preferred
Note that this defense is imperfect, but this is not considered a
serious problem. If the path via the attacker is reliably faster
than the old path despite multiple attempts to use that old path, it
is not possible to distinguish between an attack and an improvement
in routing.
An endpoint could also use heuristics to improve detection of this
style of attack. For instance, NAT rebinding is improbable if
packets were recently received on the old path. Endpoints can also
look for duplicated packets. Conversely, a change in connection ID CID is more
likely to indicate an intentional migration rather than an attack.
Note that changes in connection IDs CIDs are supported in DTLS 1.3 but not in DTLS
1.2.
9. Privacy Considerations
When using DTLS 1.3, peers SHOULD avoid using the same CID on
multiple network paths, in particular when initiating connection
migration or when probing a new network path, as described in
Section 5, as an adversary can otherwise correlate the communication
interaction across those different paths. DTLS 1.3 provides
mechanisms to ensure that a new CID can always be used. In general,
an endpoint should proactively send a RequestConnectionId message to
ask for new CIDs as soon as the pool of spare CIDs is depleted (or
goes below a threshold). Also, in case a peer might have exhausted
available CIDs, a migrating endpoint could include NewConnectionId in
packets sent on the new path to make sure that the subsequent path
validation can use fresh CIDs.
Note that DTLS 1.2 does not offer the ability to request new CIDs
during the session lifetime since CIDs have the same life-span lifespan of the
connection. Therefore, deployments that use DTLS in multihoming
environments SHOULD refuse to use CIDs with DTLS 1.2 and switch to
DTLS 1.3 if the correlation privacy threat is a concern.
10. IANA Considerations
// RFC Editor: please replace RFCthis with this RFC number and remove
// this note.
10.1. New TLS ContentType
IANA is requested to allocate has allocated an entry in the TLS ContentType "TLS ContentType" registry within
the "Transport Layer Security (TLS) Parameters" registry group
[IANA.tls-parameters] for the
return_routability_check(TBD2) return_routability_check (27) message
defined in this document. IANA is requested to set the DTLS_OK column to Y "Y" and to add
added the following note prior to the table:
NOTE: registry:
| Note: The return_routability_check content type is only applicable
| to DTLS 1.2 and 1.3.
10.2. New TLS ExtensionType
IANA is requested to allocate has allocated the extension code point (TBD1) (61) for the rrc extension to in the TLS "TLS
ExtensionType Values Values" registry as described in Table 1.
+=====+=========+===+===========+=============+===========+=======+
|Value|Extension|TLS| DTLS-Only | Recommended | Reference |Comment|
| |Name |1.3| | | | |
+=====+=========+===+===========+=============+===========+=======+
|TBD1
|61 |rrc |CH,| Y | N | RFCthis RFC 9853 | |
| | |SH | | | | |
+-----+---------+---+-----------+-------------+-----------+-------+
Table 1: rrc entry New Entry in the TLS ExtensionType Values registry Registry
10.3. New "TLS RRC Message Type" Registry
IANA is requested to create a new registry has created the "TLS RRC Message Types" registry within the Transport
"Transport Layer Security (TLS) Parameters Parameters" registry group
[IANA.tls-parameters]. This registry will be administered under the registration procedure is "Expert
Review" policy (Section 4.5 of [RFC8126]).
Follow
To submit registration requests, follow the procedures in Section 16
of [I-D.ietf-tls-rfc8447bis] to
submit registration requests. [RFC9847].
Each entry in the registry must include the following fields:
Value:
A (decimal) number in the range 0 to 253 253.
Description:
A brief description of the RRC message message.
DTLS-Only:
Whether
Indication of whether the message applies only applies to DTLS. Since RRC
is only available in DTLS, this column will be is set to Y "Y" for all the
current
initial entries in this registry. Future work may define new RRC
Message Types
message types that also apply to TLS.
Recommended:
Whether
Indication of whether the message is recommended for
implementations to support. The semantics for this field is
defined in Section 5 of [RFC8447] and updated in Section 3 of [I-D.ietf-tls-rfc8447bis]
[RFC9847].
Reference:
A reference to a publicly available specification for the value value.
Comment:
Any relevant notes or comments that relate to this entry
The entry.
Table 2 shows the initial state contents of this sub-registry is as follows: registry:
+=======+================+=========+=============+=========+=======+
|Value | Description |DTLS-Only| Recommended |Reference|Comment|
+=======+================+=========+=============+=========+=======+
|0 | path_challenge |Y | Y |RFCthis |RFC 9853 | |
+-------+----------------+---------+-------------+---------+-------+
|1 | path_response |Y | Y |RFCthis |RFC 9853 | |
+-------+----------------+---------+-------------+---------+-------+
|2 | path_drop |Y | Y |RFCthis |RFC 9853 | |
+-------+----------------+---------+-------------+---------+-------+
|3-253 | Unassigned | | | | |
+-------+----------------+---------+-------------+---------+-------+
|254-255| Reserved for |Y | |RFCthis |RFC 9853 | |
| | Private Use | | | | |
+-------+----------------+---------+-------------+---------+-------+
Table 2: Initial Entries in TLS RRC Message Type registry Registry
IANA is requested to add added the following note for to provide additional information
regarding the use of RRC message codepoints in experiments:
| Note: As specified in [RFC8126], assignments made in the Private
| Use space are not generally useful for broad interoperability.
| Those making use of the Private Use range are responsible for
| ensuring that no conflicts occur within the intended scope of use.
| For widespread experiments, provisional registrations
| (Section 4.13 of [RFC8126]) are available.
10.3.1. Designated Expert Instructions
To enable a broadly informed review of registration decisions, it is
recommended that multiple Designated Experts designated experts be appointed who are
able to
represent the perspectives of both the transport and security areas.
In cases where a registration decision could be perceived as creating
a conflict of interest for a particular Expert, expert, that Expert expert SHOULD
defer to the judgment of the other Experts.
12. experts.
11. References
12.1.
11.1. Normative References
[I-D.ietf-tls-rfc8447bis]
Salowey, J. A. and S. Turner, "IANA Registry Updates for
TLS and DTLS", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8447bis-14, 16 June 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8447bis-14>.
[IANA.tls-parameters]
IANA, "Transport Layer Security (TLS) Parameters",
<https://www.iana.org/assignments/tls-parameters>.
[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/rfc/rfc2119>.
<https://www.rfc-editor.org/info/rfc2119>.
[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/rfc/rfc6347>. <https://www.rfc-editor.org/info/rfc6347>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>. <https://www.rfc-editor.org/info/rfc8174>.
[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/rfc/rfc8446>.
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/rfc/rfc8447>.
<https://www.rfc-editor.org/info/rfc8447>.
[RFC9146] Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and
A. Kraus, "Connection Identifier for DTLS 1.2", RFC 9146,
DOI 10.17487/RFC9146, March 2022,
<https://www.rfc-editor.org/rfc/rfc9146>.
<https://www.rfc-editor.org/info/rfc9146>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
12.2. Informative References
[I-D.ietf-uta-tls13-iot-profile]
Tschofenig, H., Fossati, T.,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9847] Salowey, J. and M. Richardson, "TLS/DTLS
1.3 Profiles S. Turner, "IANA Registry Updates for the Internet of Things", Work in
Progress, Internet-Draft, draft-ietf-uta-tls13-iot-
profile-14, 5 May TLS
and DTLS", RFC 9847, DOI 10.17487/RFC9847, December 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-uta-
tls13-iot-profile-14>.
[I-D.irtf-t2trg-amplification-attacks]
<https://www.rfc-editor.org/info/rfc9847>.
11.2. Informative References
[AMP-ATTACKS]
Preuß Mattsson, J. P., J., Selander, G., and C. Amsüss,
"Amplification Attacks Using the Constrained Application
Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
irtf-t2trg-amplification-attacks-05, 18 June 2025,
<https://datatracker.ietf.org/doc/html/draft-irtf-t2trg-
amplification-attacks-05>.
[IOT-PROFILE]
Tschofenig, H., Fossati, T., and M. Richardson, "TLS/DTLS
1.3 Profiles for the Internet of Things", Work in
Progress, Internet-Draft, draft-ietf-uta-tls13-iot-
profile-18, 3 February 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-uta-
tls13-iot-profile-18>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
<https://www.rfc-editor.org/info/rfc4086>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
<https://www.rfc-editor.org/info/rfc9000>.
[RFC9175] Amsüss, C., Preuß Mattsson, J., and G. Selander,
"Constrained Application Protocol (CoAP): Echo, Request-
Tag, and Token Processing", RFC 9175,
DOI 10.17487/RFC9175, February 2022,
<https://www.rfc-editor.org/rfc/rfc9175>.
11.
<https://www.rfc-editor.org/info/rfc9175>.
Acknowledgments
We would like to thank Colin Perkins, Deb Cooley, Eric Rescorla, Éric
Vyncke, Erik Kline, Hanno Becker, Hanno Böck, Joe Clarke, Manuel
Pégourié-Gonnard, Marco Tiloca, Martin Thomson, Mike Bishop, Mike
Ounsworth, Mohamed Boucadair, Mohit Sahni, Rich Salz, Russ Housley,
Sean Turner, and Yaron Sheffer for their input to this document.
Authors' Addresses
Hannes Tschofenig (editor)
University of Applied Sciences Bonn-Rhein-Sieg
Email: Hannes.Tschofenig@gmx.net
Achim Kraus
Email: achimkraus@gmx.net
Thomas Fossati
Linaro
Email: thomas.fossati@linaro.org