Datagram PLPMTUD for UDP Options
draft-ietf-tsvwg-udp-options-dplpmtud-11

Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                                  T. Jones
Intended status: Standards Track                  University of Aberdeen
Expires: 7 July 2024                                      4 January 2024

                    Datagram PLPMTUD for UDP Options
                draft-ietf-tsvwg-udp-options-dplpmtud-11

Abstract

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

Status of This Memo

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Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  DPLPMTUD for UDP Options  . . . . . . . . . . . . . . . . . .   3
     3.1.  Packet Formats  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Sending Probe Packets with the Request Option . . . . . .   6
     3.3.  Receiving UDP-Options Probe Packets and sending the RES
           Option  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  DPLPMTUD Sender Procedures for UDP Options  . . . . . . . . .   7
     4.1.  Confirmation of Connectivity across a Path  . . . . . . .   8
     4.2.  Sending Probe Packets to Increase the PLPMTU  . . . . . .   8
     4.3.  Validating the Path with UDP Options  . . . . . . . . . .   8
     4.4.  Probe Packets that do not include Application Data  . . .   9
     4.5.  Probe Packets that include Application Data . . . . . . .   9
   5.  Receiving Events from the Network . . . . . . . . . . . . . .  10
     5.1.  Changes in the Path . . . . . . . . . . . . . . . . . . .  10
     5.2.  PTB Message Handling  . . . . . . . . . . . . . . . . . .  10
   6.  Examples with Different Receiver Behaviors  . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Revision Notes . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The User Datagram Protocol [RFC0768] offers a minimal transport
   service on top of IP and is frequently used as a substrate for other
   protocols.  Section 3.5 of UDP Guidelines [RFC8085] recommends that
   applications implement some form of Path MTU discovery to avoid the
   generation of IP fragments:

   "Consequently, an application SHOULD either use the path MTU
   information provided by the IP layer or implement Path MTU Discovery
   (PMTUD)".

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   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
   supported by a path.  Upper Layer protocols, which includes
   applications, can implement mechanisms for Path MTU discovery above
   the UDP API.

   Packetization Layer Path MTU Discovery (PLPMTUD) [RFC4821] describes
   a method for a Packetization Layer (PL) to search for the largest
   Packetization Layer PMTU (PLPMTU) supported on a path.  Datagram
   PLPMTUD (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] 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]).

   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 for a path and to detect
   when this changes.

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, this is realised using a pair
   of UDP Options: the Request (REQ) Option and the Response (RES)
   Option [I-D.ietf-tsvwg-udp-options].  The method also uses the the
   End of Options List (EOL) Option [I-D.ietf-tsvwg-udp-options] to
   introduce padding to set the size of a probe packet.

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   Use of DPLPMTUD MUST be explicitly enabled by the application, for
   instance once an application has established connectivity and is
   ready to exchange data with the remote Upper Layer protocol.
   Similarly, a receiver SHOULD NOT respond to a REQ Option until
   DPLPMTUD has been enabled.

   Probe packets consume network capacity and incur endpoint processing
   (Section 4.1 of [RFC8899]).  Implementations ought to send a probe
   packet with a 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 the current PLPMTU.

   There are two designs for using DPLPMTUD over UDP Options:

   *  Implementation within the UDP transport service;

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

   When DPLPMTUD is within the UDP transport service, the DPLPMTUD state
   machine is responsible for sending probe packets to determine a
   PLPMTU, as described in this document.  The Upper Layer protocol is
   responsible for deciding at what point in a session DPLPMTUD should
   be enabled.  Similarly a DPLPMTUD receiver ought to not respond to a
   REQ Option until this option is enabled.

   The discovered PLPMTU can be used to either:

   *  set the maximum datagram size for the current path;

   *  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 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.

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   +--------------------------------+
   |      Upper Layer Protocol      |
   |         or Application         |
   +---+----------------------------+
       |                       | Messages (with UDP Options)
       | receive         send  | Primitives for MPS, Min_PMTU etc.
   +---------------------------+----+
   | DPLPMTUD State Machine         |
   |  Maximum Packet Size (MPS)     |
   |  PLPMTU, Probed-Size,Min_PMTU  |
   |  Token Values & Probes, etc    |
   +---+----------------------------+
       |                       | Messages (with UDP Options)
       |                       | Send/Receive: Probes with Options
       | receive         send  | Events: ICMP, Interface MTU, etc
   +---------------------------+----+
   | UDP Options Transport          |
   +---+----------------------------+
       |                       | Datagrams (with UDP Options)
       |                       | Fragmented Datagrams with UDP Options
       | receive         send  | Events: ICMP, Interface MTU, etc

   Note: UDP allows an Upper Layer 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, DPLPMTUD
   can be permitted to generate probe packets with no UDP payload, and
   these include a REQ or RES UDP Option.  In this case, these probe
   packets were not generated by the sending application and therefore
   the corresponding 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
   design ought to avoid performing such discovery within UDP Options
   and also by upper protocol layers (that send and receive 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".

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3.1.  Packet Formats

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

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

   *  A sending PL can use the EOL 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.

   *  Reception of a RES Option by the 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).  Each probe
   packet MUST be uniquely identifiable by the UDP Options sender within
   the Maximum Segment Lifetime (MSL).  The UDP Options sender MUST NOT
   reuse a token value within the MSL.  A 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 to a
   randomised 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 9.1 of [I-D.ietf-tsvwg-udp-options]) followed by any padding
   needed to inflate to the required probe size.

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   A probe packet can therefore be of size up to the maximum for the
   size supported by the local interface.  [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 2 of Section 3 in [RFC8899]).

   UDP datagrams used as DPLPMTUD probe packets, as described in this
   document, MUST NOT be fragmented at the UDP layer.

3.3.  Receiving UDP-Options Probe Packets and 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 and in
   some 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 normally
   send 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 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.

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   *  Probes to validate the path supports the current PLPMTU.

4.1.  Confirmation of Connectivity 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, by sending a probe 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
   Segment Size (MSS) option, this value MAY be used as a hint to
   initialise this search to increase the PLPMTU.

   Probe packets seeking to increase the PLPMTU SHOULD NOT carry
   application data ("Probing using padding data" in Section 4.1 of
   [RFC8899]), since they will be lost whenever their size exceeds the
   actual PMTU.  A probe packet needs to elicit a positive
   acknowledgment that the path has delivered a datagram of the specific
   probed size and, therefore, MUST include the REQ Option.

   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).

4.3.  Validating the Path with UDP Options

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

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   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, as described in a previous section,
   or "Probing using application data and padding data", see Section 4.1
   of [RFC8899].  When using "Probing using padding data", the API does
   not indicate receipt of the zero-length probe packet, see
   Section 4.4.

4.4.  Probe Packets 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 and additional space is zero-filled as needed (Section 13 of
   [I-D.ietf-tsvwg-udp-options]).  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 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 used to confirm
   connectivity or that validates the path.  These probe packets are
   permitted to carry application data.  (A UDP payload data is
   permitted because these probe packets perform black-hole detection
   and 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 MAY utilise 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 a
   sending Upper Layer protocol performs DPLPMTUD.  The DPLPMTUD method
   tracks the transmission of probe packets (using the REQ Option) and
   reception of the corresponding RES Options to the Upper Layer
   protocol.

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   A receiver that responds to DPLPMTUD needs to process the REQ Option
   and include the corresponding RES Option in an Upper Layer protocol
   message that it returns to the requester.  DPLPMTUD can be used to
   manage the PLPMTU in just one direction or can be used for both
   directions.  Probe packets that use this format form a part of the
   end-to-end transport data.

5.  Receiving Events from the Network

   This specification does not rely upon reception of events from the
   network, but an implementation can utilise these events when
   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.  PTB Message Handling

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

   When DPLPMTUD utilises ICMP PTB messages received in response to a
   probe packet it MUST use the ICMP quoted packet to validate the UDP
   port information in combination with the token contained in the UDP
   Option, before processing the packet using the DPLPMTUD method.
   Section 4.6.1 of [RFC8899] specifies this validation procedure.  An
   implementation unable to support this validation needs to ignore
   received ICMP PTB messages.

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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 DPLPMTUD receiver support) One case is when a sender supports
      this specification, but the remote endpoint that does not return a
      RES Option.  In this example, the method is unable to discover the
      PLPMTU.  This will result in using the minimum configured PLPMTU
      (MIN_PLPMTU).  Such a remote endpoint might not process UDP
      Options, or might not return a datagram with a RES Option for some
      other reason (due to persistent packet loss, insufficient space to
      include the option, etc.)

   *  (DPLPMTUD receiver uses application datagrams) In a second case,
      both the sender and receiver support DPLPMTUD using the
      specification, and the receiver design only returns a RES Option
      with the next UDP datagram that is sent to the requester.  In this
      design, the reception of a REQ Option does not systematically
      trigger a response.  The design allows DPLPMTUD to operate when
      there is a flow of datagrams in both directions, providing there
      is periodic feedback (e.g., one acknowledgment packet per RTT).
      This also requires the PLPMTU at the receiver to be sufficiently
      large that the RES option can be added to the feedback packets
      that are sent in the return direction.  This is a simple method
      that also 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, and
      will result in a smaller PLPMTU than is actually supported by the
      path, or in using the minimum configured PLPMTU (MIN_PLPMTU).

   *  (Uni-directional transfer) Another case is where an application
      only transfers data in one direction (or predominantly in one
      direction).  In this 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 permitted to send responses in UDP datagrams
      with no payload) A DPLPMTUD receiver can generate a datagram
      (e.g., with zero payload data) solely to return a RES Option
      (e.g., when no other datagrams are queued for transmission).  This

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      design allows a UDP Options endpoint to probe the set of open UDP
      ports using DPLPMTUD probe packets.  It results in some additional
      traffic overhead but has the advantage that it can ensure timely
      progress of DPLPMTUD.  If a UDP Options endpoint creates and sends
      a datagram with a RES 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 RTT or 1 per
      second).  This rate limit is to mitigate the DoS vector, without
      significantly impacting the operation of DPLPMTUD.

7.  Acknowledgements

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

8.  IANA Considerations

   This memo includes no requests to IANA.

9.  Security Considerations

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

   The security considerations for using DPLPMTUD are described in
   [RFC8899].  On path attackers could maliciously drop or modify probe
   packets to seek to decrease the 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 to a randomised value.  This is designed to enhance
   protection from off-path attacks.  If a subsequent probe packet uses
   a token value that is easily derived from the initial value, (e.g.,
   incrementing the 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.

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   The method does not change the ICMP PTB message validation method
   described by DPLPMTUD: A UDP Options sender that utilises 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, 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, and not to 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.  References

10.1.  Normative References

   [I-D.ietf-tsvwg-udp-options]
              Touch, J. D., "Transport Options for UDP", Work in
              Progress, Internet-Draft, draft-ietf-tsvwg-udp-options-22,
              9 June 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-tsvwg-udp-options-22>.

   [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.  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>.

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   [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.

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   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.

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   *  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.

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      Corrections following a review by Erik Auerswald.

   Working group draft-11.

      Revised data - waiting for UDP Options to complete.

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

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