wdiff rfc9869.alt-original rfc9869.txt

Internet Engineering Task Force (IETF) G. Fairhurst
Internet-Draft
Request for Comments: 9869 T. Jones
Intended status:
Category: Standards Track University of Aberdeen
Expires: 24 August 2025 20 February
ISSN: 2070-1721 September 2025

Datagram PLPMTUD Packetization Layer Path MTU Discovery (DPLPMTUD) for UDP
Options
draft-ietf-tsvwg-udp-options-dplpmtud-15

Abstract

This document specifies how a UDP Options sender implements Datagram
Packetization Layer Path Maximum Transmission Unit MTU Discovery (DPLPMTUD) as a robust method
for Path Maximum Transmission Unit
discovery. MTU Discovery (PMTUD). This method uses the UDP Options
packetization layer. It allows an application to discover the
largest size of datagram that can be sent across a network path. It
also provides a way to allow the application to periodically verify
the current maximum packet
size Maximum Packet Size (MPS) supported by a path and to
update this when required.

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,
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This Internet-Draft will expire on 24 August 2025.
https://www.rfc-editor.org/info/rfc9869.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DPLPMTUD for UDP Options . . . . . . . . . . . . . . . . . . 4
3.1. Packet Formats . . . . . . . . . . . . . . . . . . . . . 6
3.2. Sending Probe Packets with the Request Option . . . . . . 7
3.3. Receiving UDP-Options UDP Options Probe Packets and sending Sending the RES
Option . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. DPLPMTUD Sender Procedures for UDP Options . . . . . . . . . 8
4.1. Confirmation of Connectivity across Across a Path . . . . . . . 8
4.2. Sending Probe Packets to Increase the PLPMTU . . . . . . 8
4.3. Validating the Path with UDP Options . . . . . . . . . . 9
4.4. Probe Packets that do not include That Do Not Include Application Data . . . 9
4.5. Probe Packets that include That Include Application Data . . . . . . . 10
5. Receiving Events from the Network . . . . . . . . . . . . . . 10
5.1. Changes in the Path . . . . . . . . . . . . . . . . . . . 10
5.2. Validation of PTB Messages . . . . . . . . . . . . . . . 11
6. Examples with Different Receiver Behaviors . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9.
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10.
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1.
9.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2.
9.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Revision Notes . . . . . . . . . . . . . . . . . . . 15
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18

1. Introduction

The User Datagram Protocol (UDP) [RFC0768] offers a minimal transport
service on top of IP and is frequently used as a substrate for other
protocols. Section 3.2 of UDP Guidelines [RFC8085] recommends that
applications implement some form of Path MTU discovery Discovery (PMTUD) to
avoid the generation of IP fragments:

"Consequently,

| Consequently, an application SHOULD either use the path MTU
| information provided by the IP layer or implement Path MTU
| Discovery
(PMTUD)". (PMTUD) itself [RFC1191] [RFC1981] [RFC4821] to
| determine whether the path to a destination will support its
| desired message size without fragmentation.

The UDP API [RFC8304] offers calls for applications to receive ICMP
Packet Too Big (PTB) messages and to control the maximum size of
datagrams that are sent, but it does not offer any automated
mechanisms for an application to discover the maximum packet size MPS supported by a
path. Upper Layer protocols, which includes include applications, can
implement mechanisms for Path MTU discovery PMTUD above the UDP API.

Packetization Layer Path MTU Discovery (PLPMTUD) [RFC4821] describes
a method for a bi-directional bidirectional Packetization Layer (PL) to search for
the largest Packetization Layer PMTU (PLPMTU) supported on a path.
Datagram PLPMTUD (DPLPMTUD)
DPLPMTUD [RFC8899] specifies this support for datagram transports.
PLPMTUD and DPLPMTUD gain robustness by using a probing mechanism
that does not solely rely on ICMP PTB messages and works on paths
that drop ICMP PTB messages.

UDP Options [I-D.ietf-tsvwg-udp-options] [RFC9868] supplies functionality that can be used to
implement DPLPMTUD within the transport service or in an Upper Layer
protocol (including an application) that uses UDP Options. This
document specifies how DPLPMTUD using UDP Options is implemented
(Section 6.1 of [RFC8899]), [RFC8899]) and requires support to be enabled at both
the sender and receiver.

Implementing DPLPMTUD within the transport service above UDP Options
avoids the need for each Upper Layer protocol to implement the
DPLPMTUD method. It provides a standard method for applications to
discover the current maximum packet size MPS for a path and to detect when this changes.
It can be used with Equal-Cost Multipath (ECMP) routing and/or
multihoming. If multipath or multihoming is supported, a state
machine is needed for each path.

DPLPMTUD is not specified for multicast. The method requires
explicit acknowledgment acknowledgement of probe packets provided by UDP Options,
which is primarily intended for unicast use (see Section 23 of
[I-D.ietf-tsvwg-udp-options]).
[RFC9868]).

2. 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 uses the terms defined for DPLPMTUD (Sections 2 and 5
of [RFC8899]).

3. DPLPMTUD for UDP Options

A UDP Options sender implementing DPLPMTUD uses the method specified
in [RFC8899]. In this specification, DPLPMTUD is realised realized using a
pair of UDP Options: the Request (REQ) Option and the Response (RES)
Option (Section 11.7 of [I-D.ietf-tsvwg-udp-options]). [RFC9868]). The method also uses the End of
Options List (EOL) Option (Section 11.1 of
[I-D.ietf-tsvwg-udp-options]) [RFC9868]) to introduce
padding to set the size of a probe packet.

Use of DPLPMTUD MUST be explicitly enabled by the application, for
instance
instance, once an application has established connectivity and is
ready to exchange data with the remote Upper Layer protocol.
Similarly, a DPLPMTUD receiver MUST NOT respond to a UDP REQ Option
until DPLPMTUD has been enabled. This is to help protect from mis-
use misuse
of the mechanism for other forms of probing.

Probe packets consume network capacity and incur endpoint processing
(Section 4.1 of [RFC8899]). Implementations ought to send a probe
packet with a UDP REQ Option only when required by their local
DPLPMTUD state machine, i.e., when confirming the base PMTU for the
path, probing to increase the PLPMTU, or to confirm confirming the current
PLPMTU.

DPLPMTUD can be implemented over UDP Options in two ways:

* Implementation within the UDP transport service; service.

* Implementation in an Upper Layer protocol (or application) that
uses UDP Options.

When DPLPMTUD is implemented within the UDP transport service, the
DPLPMTUD state machine is responsible for sending probe packets to
determine a PLPMTU, as described in this document (and hence the
Maximum Packet Size (MPS), document. This determines
an MPS, the largest size of application data block that can be sent
across a network path using a single datagram). datagram. The Upper Layer
protocol is responsible for deciding when a session enables DPLPMTUD.

The discovered PLPMTU can be used to either:

* set the maximum datagram size for the current path; path or

* set the maximum fragment size when a sender uses the UDP
Fragmentation Option to divide a datagram into multiple UDP
fragments for transmission. The size of each UDP fragment is then
less than or equal to the size of the discovered largest IP packet
that can be received across the current path.

The figure below shows an implementation of DPLPMTUD within the UDP
transport service. It illustrates key interactions between the
layers. This design requires an API primitive to allow the
application to control whether the DPLPMTUD state machine is enabled
for a specific UDP port. By default, this API MUST disable DPLPMTUD
processing.

Note: UDP allows an Upper Layer Protocol protocol to send datagrams with or
without payload data (with or without UDP Options). These are
delivered across the network to the remote Upper Layer. When
DPLPMTUD is implemented within the UDP transport service, probe
packets that include a REQ or RES UDP Option can be sent with no UDP
payload. In this case, these probe packets were not generated by a
sending application and therefore application; therefore, the corresponding received datagrams
are not delivered to the remote application.

When DPLPMTUD is instead implemented by an Upper Layer protocol, the
format and content of probe packets are determined by the Upper Layer
protocol. This design is also permitted to use the REQ and RES
Options provided by UDP Options.

If DPLPMTUD is active at more than one layer, then the values of the
tokens used in REQ Options need to be coordinated with any values
used for other DPLPMTUD probe packets to ensure that each probe
packet can be identified by a unique token. When configurable, a
configuration ought to avoid performing such discovery both within
UDP Options and also by an upper Upper Layer protocol layer that sends and
receives probe packets via UDP Options. Section 6.1 of [RFC8899]
recommends that: "An application SHOULD avoid using DPLPMTUD when the
underlying transport system provides this capability."

3.1. Packet Formats

The UDP Options used in this document are described in
[I-D.ietf-tsvwg-udp-options] [RFC9868] and
are used in the following way: ways:

* The REQ Option is set by a sending PL to solicit a response from a
remote receiver. A four-byte (four octet) (four-octet) token identifies each
request.

* A sending PL can use the EOL option Option together with a minimum
datagram length to pad probe packets.

* The RES Option is sent by a UDP Options receiver in response to a
previously received REQ Option. Each RES Option echoes the last
received four-byte token.

* If a UDP Options endpoint creates and sends a datagram with a RES
option
Option solely as response to a received REQ Option, the responder
MUST limit the rate of these responses (e.g., limiting each pair
of ports to send 1 per measured RTT or 1 per second). This rate
limit is to mitigate the DoS vector, vector without significantly
impacting the operation of DPLPMTUD. An example in Section Section 6
describes a case where this might be used.

* Reception of a RES Option by the REQ sender confirms that a
specific probe packet has been received by the remote UDP Options
receiver.

The token allows a UDP Options sender to distinguish between
acknowledgements for initial probe packets and acknowledgements
confirming receipt of subsequent probe packets (e.g., travelling
along alternate paths with a larger round-trip time). RTT). Each probe packet MUST be
uniquely identifiable by the UDP Options sender within the Maximum
Segment Lifetime (MSL) [RFC8085]. The UDP Options sender MUST NOT
reuse a token value within the MSL. A four byte four-byte value for the token
field provides sufficient space for multiple unique probe packets to
be made within the MSL. Since UDP Options operates over UDP, the
token values only need to be unique for the specific 5-tuple over
which it is operating.

The value of the four-byte token field SHOULD be initialised initialized to a
randomised
randomized value to enhance protection from off-path attacks, as
described in Section 5.1 of [RFC8085].

3.2. Sending Probe Packets with the Request Option

DPLPMTUD relies upon sending a probe packet with a specific size.
Each probe packet includes the UDP Options area containing a REQ
Option and any other required options concluded with an EOL Option
(Section 11.1 of [I-D.ietf-tsvwg-udp-options]) [RFC9868]), followed by any padding needed to
inflate to the required probe size.

A probe packet can therefore be of size up to the maximum size supported by
the local interface (i.e., the Interface MTU). Item 2 in Section 3
of [RFC8899] (Section 3, item 2) requires the network interface below DPLPMTUD to provide
a way to transmit a probe packet that is larger than the current
PLPMTU. The size of this probe packet MUST NOT be constrained by the
maximum PMTU set by network layer mechanisms (such as discovered by
PMTUD [RFC1191][RFC8201] or the PMTU size held in the IP-layer
cache), as noted in bullet item 2 of in Section 3 in of [RFC8899]).

UDP datagrams used as DPLPMTUD probe packets, as described in this
document, MUST NOT be fragmented at the UDP or IP layer. Therefore,
Section 3 of [RFC8899] therefore requires: "In IPv4, a probe packet MUST be
sent with the Don't Fragment (DF) bit set in the IP header and
without network layer endpoint fragmentation."

3.3. Receiving UDP-Options UDP Options Probe Packets and sending Sending the RES Option

When DPLPMTUD is enabled, a UDP Options receiver responds by sending
a UDP datagram with the RES Option when it receives a UDP Options
datagram with the REQ Option.

The operation of DPLPMTUD can depend on the support at the remote UDP
Options endpoint, the way in which DPLPMTUD is implemented implemented, and in
some cases cases, the application data that is exchanged over the UDP
transport service. When UDP Options is not supported by the remote
receiver, DPLPMTUD will be unable to confirm the path or to discover
the PLPMTU. This will result in the minimum configured PLPMTU
(MIN_PLPMTU). More explanation of usage is provided in Section 6.

Note: A receiver that only responds when there is a datagram queued
for transmission by the Upper Layer could potentially receive
multiple datagrams with a REQ Option before it can respond. When
sent, the RES Option will only acknowledge the latest received token
value. A sender would then conclude that any earlier REQ Options
were not successfully received. However, DPLPMTUD does not usually
result in sending more than one probe packet per timeout interval,
and a delay in responding will already have been treated as a failed
probe attempt. Therefore, this does not significantly impact
performance, although a more prompt response would have resulted in
DPLPMTUD recording reception of all probe packets.

4. DPLPMTUD Sender Procedures for UDP Options

DPLPMTUD utilises utilizes three types of probe. These are described in the
following sections:

* Probes to confirm the path can support the BASE_PLPMTU
(Section 5.1.4 of [RFC8899]).

* Probes to detect whether the path can support a larger PLPMTU.

* Probes to validate that the path supports the current PLPMTU.

4.1. Confirmation of Connectivity across Across a Path

The DPLPMTUD method requires a PL to confirm connectivity over the
path (Section 5.1.4 of [RFC8899]), but UDP itself does not offer a
mechanism for this.

UDP Options can provide this required functionality. A UDP Options
sender implementing this specification MUST elicit a positive
confirmation of connectivity for the path, path by sending a probe packet, packet
padded to size BASE_PLPMTU. This confirmation probe MUST include the
REQ UDP Option to elicit a response from the remote DPLPMTUD.
Reception of a datagram with the corresponding RES Option confirms
the reception of a packet of the probed size that has successfully
traversed the path to the receiver. This also confirms that the
remote endpoint supports the RES Option.

4.2. Sending Probe Packets to Increase the PLPMTU

From time to time, DPLPMTUD enters the SEARCHING state, described in
Section 5.2 of [RFC8899], (e.g., after expiry of the
PMTU_RAISE_TIMER) to detect whether the current path can support a
larger PLPMTU. When the remote endpoint advertises a UDP Maximum
Datagram Size (MDS) option (see Section 11.5 of
[I-D.ietf-tsvwg-udp-options]), [RFC9868]), this
value MAY be used as a hint to
initialise initialize this search to increase the
PLPMTU.

Probe packets seeking to increase the PLPMTU SHOULD NOT carry
application data ("Probing (see "Probing using padding data" in Section 4.1 of
[RFC8899]), since they will be lost whenever their size exceeds the
actual PMTU. [RFC8899] requires a probe packet to elicit a positive
acknowledgement that the path has delivered a datagram of the
specific probed size and, size; therefore, when using
[I-D.ietf-tsvwg-udp-options] a probe packet MUST include the REQ
Option.
Option when using transport options for UDP [RFC9868].

At the receiver, a received probe packet that does not carry
application data does not form a part of the end-to-end transport
data and is not delivered to the Upper Layer protocol (i.e.,
application or protocol layered above UDP). A zero-length payload
notification could still be delivered to the application, see application (see
Section 5 of [RFC8085], [RFC8085]), although Section 18 of
[I-D.ietf-tsvwg-udp-options] [RFC9868] discusses
the implications when using UDP Options.

4.3. Validating the Path with UDP Options

A PL using DPLPMTUD MUST validate that a path continues to support
the PLPMTU discovered in a previous search for a suitable PLPMTU
value, as defined in Section 6.1.4 of [RFC8899]. This validation
sends probe packets in the DPLPMTUD SEARCH_COMPLETE state
(Section 5.2 of [RFC8899]) to detect black-holing of data
(Section 5.2 of [RFC8899], Section 4.3 of [RFC8899] defines a DPLPMTUD black-hole). black hole).

Path validation can be implemented within UDP Options by generating a
probe packet of size PLPMTU, which MUST include a REQ Option to
elicit a positive confirmation that the path has delivered this probe
packet. A probe packet used to validate the path MAY use either
"Probing using padding data" to construct a probe packet that does
not carry any application data (Section 4.1 of [RFC8899]) or "Probing using application data and
padding data", data"; see Section 4.1 of [RFC8899]. When using "Probing
using padding data", the UDP Options API does not indicate receipt of
the zero-length probe packet, see packet (see Section 6 of [I-D.ietf-tsvwg-udp-options]. [RFC9868]).

4.4. Probe Packets that do not include That Do Not Include Application Data

A simple implementation of the method might be designed to only use
probe packets in a UDP datagram that includes no application data.
The size of each probe packet is padded to the required probe size
including the REQ Option. This implements "Probing using padding
data" (Section 4.1 of [RFC8899]) and avoids having to retransmit
application data when a probe fails. This could be achieved by
setting a minimum datagram length, such that the options list ends in
EOL (Section 11.1 of [I-D.ietf-tsvwg-udp-options]) [RFC9868]) with any additional space is zero-filled
as needed (see Section 15 of
[I-D.ietf-tsvwg-udp-options]). [RFC9868]). In this use, the probe
packets do not form a part of the end-to-end transport data and a
receiver does not deliver them to the Upper Layer protocol.

4.5. Probe Packets that include That Include Application Data

An implementation always uses the format in Section 4.4 when DPLPMTUD
searches to increase the PLPMTU.

An alternative format is permitted for a probe packet that is used to
confirm the connectivity or to validate the path. These probe
packets MAY carry application data. (A UDP (UDP payload data is permitted
because these probe packets perform black-hole detection and will,
therefore, will
therefore usually have a higher probability of successful
transmission, similar to other packets sent by the Upper Layer
protocol.) Section 4.1 of [RFC8899] provides a discussion of the
merits and demerits of including application data. For example, this
reduces the need to send additional datagrams.

This type of probe packet MAY use a control message format defined by
the Upper Layer protocol, provided that the message does not need to
be delivered reliably. The REQ Option MUST be included when the
sending Upper Layer protocol performs DPLPMTUD. The DPLPMTUD method
tracks the transmission of probe packets (using the REQ Option
token).

A receiver that responds to DPLPMTUD MUST process the REQ Option and
include the corresponding RES Option with an Upper Layer protocol
message that it returns to the requester (examples of receiver
processing are provided in Section Section 6).

Probe packets that use this format form a part of the end-to-end
transport data and can be used to manage the PLPMTU in just one
direction or can be used for both directions.

5. Receiving Events from the Network

This specification does not rely upon reception of events from the
network, but an implementation can utilise utilize these events when they are
provided.

5.1. Changes in the Path

A change in the path or the loss of a probe packet can result in
DPLPMTUD updating the PLPMTU. DPLPMTUD [RFC8899] recommends that
methods are robust to path changes that could have occurred since the
path characteristics were last confirmed and to the possibility of
inconsistent path information being received. For example, a
notification that a path has changed could trigger path validation to
provide black-hole protection (Section 4.3 of [RFC8899]).

An Upper Layer protocol could trigger DPLPMTUD to validate the path
when it observes a high packet loss rate (or a repeated protocol
timeout) [RFC8899].

Section 3 of [RFC8899] requires any methods designed to share the
PLPMTU between PLs (such as updating the IP cache PMTU for an
interface/destination) to be robust to the wide variety of underlying
network forwarding behaviors. For example, an implementation could
avoid sharing PMTU information that could potentially relate to
packets sent with the same address over a different interface.

5.2. Validation of PTB Messages

Support for receiving ICMP PTB messages is OPTIONAL for DPLPMTUD. A
UDP Options sender can therefore ignore received ICMP PTB messages.

Before processing an ICMP PTB message message, the DPLPMTUD method needs to
perform two checks to ensure that the messgage message was received in
response to a sent probe packet:

* DPLPMTUD first utilises utilizes the quoted information in each PTB
message. The receiver MUST validate the protocol information in
the quoted packet carried in an ICMP PTB message payload to
validate the message originated from the sending node (see
Section 4.6.1 of [RFC8899]).

* The receiver SHOULD utilize information that is not simple for an
off-path attacker to determine (see Section 4.6.1 of [RFC8899]).
Specifically, a UDP Options receiver SHOULD confirm that the token
contained in the UDP REQ Option of the quoted packet has a value
that corresponds to a probe packet that was recently sent by the
current endpoint.

An implementation unable to support this validation MUST ignore
received ICMP PTB messages.

6. Examples with Different Receiver Behaviors

When enabled, a DPLPMTUD endpoint that implements UDP Options
normally responds with a UDP datagram with a RES Option when
requested by a sender.

The following examples describe various possible receiver behaviors:

* (No No DPLPMTUD receiver support) support: One case is when a sender supports
this specification, but no RES Option is received from the remote
endpoint. In this example, the method is unable to discover the
PLPMTU. This will result in using the minimum configured PLPMTU
(MIN_PLPMTU). MIN_PLPMTU. Such a remote
endpoint might be not configured to process UDP Options, Options or might
not return a datagram with a RES Option for some other reason (packet
(e.g., packet loss, insufficient space to include the option,
filtering on the path, etc). etc.).

* (DPLPMTUD DPLPMTUD receiver uses application datagrams) datagrams: In a second case,
both the sender and receiver support DPLPMTUD using the
specification, and the receiver only returns a RES Option with the
next UDP datagram that is sent to the requester. Therefore,
reception of a REQ Option does not systematically trigger a
response. This allows DPLPMTUD to operate when there is a flow of
datagrams in both directions, providing provided there is periodic feedback
(e.g., one acknowledgement packet per RTT). It requires the
PLPMTU at the receiver to be sufficiently large enough that the
RES
option Option can be included in the feedback packets that are sent
in the return direction. This method avoids opportunities to
misuse the method as a DoS attack. However, when there is a low
rate of transmission (or no datagrams are sent) in the return
direction, this will prevent prompt delivery of the RES Option.
At the DPLPMTUD sender, this results in probe packets failing to
be acknowledged in time, time and could result in a smaller PLPMTU than
is actually supported by the path or in using the minimum configured
PLPMTU (MIN_PLPMTU). MIN_PLPMTU.

* (Uni-directional transfer) Unidirectional transfer: Another case is where an application only
transfers data in one direction (or predominantly in one
direction). In this case case, the wait at the receiver for a datagram
to be queued before returning a RES Option could easily result in
a probe timeout at the DPLPMTUD sender. In this case, DPLPMTUD
could allow exchanging datagrams without a payload (as discussed
in earlier sections) to return the RES Option.

* (DPLPMTUD Receiver DPLPMTUD receiver permitted to send responses in UDP datagrams
with no payload) payload: A DPLPMTUD receiver can generate a datagram
(e.g., with zero payload data) solely to return a RES Option
(e.g., sent when no other datagrams are queued for transmission).
This would allow an endpoint to probe the set of UDP ports that
have been configured with support for this specification using a
DPLPMTUD probe packet. Although this results in some additional
traffic overhead, it has the advantage that it can ensure timely
progress of DPLPMTUD. Section Section 3.1 specifies: "If a UDP Options
endpoint creates and sends a datagram with a RES option Option solely as
response to a received REQ Option, the responder MUST limit the
rate of these responses (e.g., limiting each pair of ports to send
1 per measured RTT or 1 per second)". This rate limit is to
mitigate the DoS vector, without significantly impacting the
operation of DPLPMTUD.

7. IANA Considerations

This memo includes document has no requests to IANA.

9. IANA actions.

8. Security Considerations

The security considerations for using UDP Options are described in
[I-D.ietf-tsvwg-udp-options].
[RFC9868]. The method does not change the integrity protection
offered by the UDP options Options method.

The security considerations for using DPLPMTUD are described in
[RFC8899]. On path On-path attackers could maliciously drop or modify probe
packets to seek to decrease the PMTU, PMTU or to maliciously modify probe
packets in an attempt to black-hole traffic.

The specification recommends that the token value in the REQ Option
is initialised initialized to a randomised randomized value. This is to enhance protection
from off-path attacks. If a subsequent probe packet uses a token
value that is easily derived from the initial value, value (e.g.,
incrementing the value) value), a misbehaving on-path observer could then
determine the token values used for subsequent probe packets from
that sender, even if these probe packets are not transiting via the
observer. This would allow probe packets to be forged, with an
impact similar to other on-path attacks against probe packets. This
attack could be mitigated by using an unpredictable token value for
each probe packet.

The method does not change the ICMP PTB message validation method
described by DPLPMTUD: A UDP Options sender that utilises utilizes ICMP PTB
messages received to a probe packet MUST use the quoted packet to
validate the UDP port information in combination with the token
contained in the UDP Option, Option before processing the packet using the
DPLPMTUD method.

Upper Layer protocols or applications that employ encryption ought to
perform DPLPMTUD at a layer above UDP Options, Options and not enable UDP
Options support for DPLPMTUD. This allows the application to control
when DPLPMTUD is used to control the additional traffic that this
generates. This also ensures that DPLPMTUD probe packets are
encrypted.

10.

9. References

10.1.

9.1. Normative References

[I-D.ietf-tsvwg-udp-options]
Touch, J. D. and C. M. Heard, "Transport Options for UDP",
Work in Progress, Internet-Draft, draft-ietf-tsvwg-udp-
options-39, 13 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
udp-options-39>.

[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.

[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>.

[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/info/rfc8174>.

[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.

10.2.

[RFC9868] Touch, J. and C. Heard, Ed., "Transport Options for UDP",
RFC 9868, DOI 10.17487/RFC9868, September 2025,
<https://www.rfc-editor.org/info/rfc9868>.

9.2. Informative References

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.

[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.

[RFC8304] Fairhurst, G. and T. Jones, "Transport Features of the
User Datagram Protocol (UDP) and Lightweight UDP (UDP-
Lite)", RFC 8304, DOI 10.17487/RFC8304, February 2018,
<https://www.rfc-editor.org/info/rfc8304>.

Appendix A. Revision Notes

XXX Note to RFC-Editor: please remove this entire section prior to
publication. XXX

Individual draft-00.

* This version contains a description for consideration and comment
by the TSVWG.

Individual draft-01.

* Address Nits

* Change Probe Request and Probe Reponse options to Echo to align
names with draft-ietf-tsvwg-udp-options

* Remove Appendix B, Informative Description of new UDP Options

* Add additional sections around Probe Packet generation

Individual draft-02.

* Address Nits

Individual draft-03.

* Referenced DPLPMTUD RFC.

* Tidied language to clarify the method.

Individual draft-04

* Reworded text on probing with data a little

* Removed paragraph on suspending ICMP PTB suspension.

Working group draft-00

* -00 First Working Group Version

* RFC8899 call search_done SEARCH_COMPLETE, fixed.

Working group draft -01

* Update to reflect new fragmentation design in UDP Options.

* Add a description of uses of DPLPMTUD with UDP Options.

* Add a description on how to form probe packets with padding.

* Say that MSS options can be used to initialise the search
algorithm.

* Say that the recommended approach is to not use user data for
probes.

* Attempts to clarify and improve wording throughout.

* Remove text saying you can respond to multiple probes in a single
packet.

* Simplified text by removing options that don't yield benefit.

Working group draft -02

* Update to reflect comments from MED.

* More consistent description of DPLPMTUD with UDP Options.

* Clarify the nonce value (token) is intended per 5-tuple, not
interface.

* BASE_PLPMTU related to RFC8899.

* Probes with user data can carry application control data.

* Added that application data uses RES and REQ nonce (token) values
from the app.

* QUIC was intended as an informational reference to an example of
RFC8899.

Working group draft -03

* Update to reflect more comments from MED.

* Again more consistent description of DPLPMTUD with UDP Options.

* Clarify token/nonce to use token.

* Clarify any use of application data for black-hole detection.

* Minor changes to reflect update to UDP Options base spec.

Working group draft-04.

Update for WG Last Call

Working group draft-05.

Update following WG Last Call

Working group draft-06.

Tidy text after WG Last Call, based on review by Med.

Added text after WG Last Call, based on review by Magnus.

Added text after WG Last Call, based on comments by Joe and Mike.

Restructured to integrate the WGLC new text.

Working group draft-07.

Mention of UDP-Options in Intro, from a review by Med.

Resolve typo, from review by Magnus.

Working group draft-08.

Corrections following a review by Mike Heard.

Working group draft-09.

Corrections following a review by Erik Auerswald and others.

Working group draft-10.

Corrections following a review by Erik Auerswald.

Working group draft-11.

Revised data - waiting for UDP Options to complete.

Working group draft-11.

Editorial corrections to align section numbers in referenced RFCs/
I-Ds and minor editorial improvements.

Working group draft -12, -13.

Editorial corrections preparing for WGLC.

Working group draft -14.

Updated after INT and SEC reviews.

Working group draft -15.

Updated after IESG comments.

Acknowledgements

Gorry Fairhurst and Tom Jones are supported by funding provided by
the University of Aberdeen. The editors authors would like to thank Magnus
Westerlund and Mohamed Boucadair for their detailed comments and also
other people who contributed to completing this document.

Authors' Addresses

Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk

Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: tom@erg.abdn.ac.uk