Datagram PLPMTUD for UDP Options
draft-ietf-tsvwg-udp-options-dplpmtud-01

Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                                  T. Jones
Intended status: Standards Track                  University of Aberdeen
Expires: 22 May 2022                                    18 November 2021

                    Datagram PLPMTUD for UDP Options
                draft-ietf-tsvwg-udp-options-dplpmtud-01

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 a datagram application to discover the largest size of
   datagram that can be sent across a network path.

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 of the Internet Engineering
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   This Internet-Draft will expire on 22 May 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  DPLPMTUD for UDP Options  . . . . . . . . . . . . . . . . . .   3
   4.  Sending UDP-Options Probe Packets . . . . . . . . . . . . . .   4
     4.1.  Packet Probes using the Echo Request Option Request
           Option  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  DPLPMTUD Procedures for UDP Options . . . . . . . . . . .   5
       4.2.1.  Confirmation of Connectivity across a Path  . . . . .   5
       4.2.2.  Sending Probe Packets to Increase the PLPMTU  . . . .   5
       4.2.3.  Validating the Path with UDP Options  . . . . . . . .   6
       4.2.4.  Sending Packet Probes that include Application
               Data  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  PTB Message Handling for this Method  . . . . . . . . . .   7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Revision Notes . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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)".

   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 does not offer any automated mechanisms
   for an application to discover the maximum packet size supported by a
   path.  Applications and upper layer protocols implement mechanisms
   for Path MTU discovery above the UDP API.

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   Packetization Layer PMTUD (PLPMTUD) [RFC4821] describes a method for
   a Packetization Layer (PL) (such as UDP Options) 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.

   In summary, UDP Options [I-D.ietf-tsvwg-udp-options] supplies
   functionality that can be used to implement DPLPMTUD within the UDP
   transport service.  This document specifies how an implementation can
   use this additional functionality to support DPLPMTUD.  Implementing
   DPLPMTUD using UDP Options avoids the need for each upper layer
   protocol or application to implement the DPLPMTUD method.  This
   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", "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.

3.  DPLPMTUD for UDP Options

   There are two ways an upper PL can perform DPLPMTUD:

   *  The UDP Options sender implementing DPLPMTUD uses the method
      specified in [RFC8899] and the upper PL or application does not
      perform PMTU discovery.  In this case, UDP Options processing is
      responsible for sending probes to determine a PLPMTU, as described
      in this document.  This discovered PLPMTU can be used by UDP
      Options to either:

      -  set the maximum datagram size for the current path (based on
         the discovered largest IP packet that can be received across
         the path).

      -  set the maximum fragment size when a sender uses the UDP
         Fragmentation Option to divide a datagram into multiple UDP
         fragments for transmission.  Each UDP fragment is then less
         than the discovered largest IP packet that can be received
         across the path.

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   *  An upper PL or application performs DPLPMTUD (e.g., QUIC
      [RFC9000]).  This upper PL then uses probes to determine a safe
      PLPMTU for the datagrams that it sends.  The contents of any probe
      is determined by the upper PL.  Such a design needs to avoid
      performing discovery at multiple levels, so, when when
      configurable, this upper PL SHOULD disable DPLPMTUD by UDP Options
      [RFC8899]).

   This section describe packet formats and procedures for DPLPMTUD
   using UDP Options.

4.  Sending UDP-Options Probe Packets

   DPLPMTUD relies upon the ability of a UDP Options sender to generate
   a probe with a specific size, up to the maximum for the size
   supported by the local interface.  The size of a DPLPMTUD probe
   packet MUST NOT be constrained by the maximum PMTU set by network
   layer mechanisms (such as PMTUD [RFC1063][RFC8201] or the IP Cache).

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

4.1.  Packet Probes using the Echo Request Option Request Option

   This section describes a format of probe consisting of an empty UDP
   datagram, UDP Options area and Padding.  The UDP Options area
   contains the Echo Request Option (RES), any other required options
   concluded with an EOL Option followed by any padding needed to
   inflate to the required probe size.  The reception of this option
   generates an Echo Response Option that confirms reception of a
   specific received probe.

   The UDP Options used in this method are described in section 6 of
   [I-D.ietf-tsvwg-udp-options]:

   *  The Echo Request Option (RES) is set by a sending PL to solicit a
      response from a remote UDP Options receiver.  A four-byte token
      identifies each request.

   *  The Echo Response Option (REQ) is generated by the UDP Options
      receiver in response to reception of a previously received Echo
      Request Option.  Each Echo Response Option echoes a previously
      received four-byte token.

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   The token value allows a sender to distinguish between
   acknowledgements for initial probes and acknowledgements confirming
   receipt of subsequent probes (e.g., travelling along alternate paths
   with a larger round trip time).  This needs each probe to be uniquely
   identifiable by the UDP Options sender within the Maximum Segment
   Lifetime (MSL).  The UDP Options sender therefore MUST NOT recycle
   token values until they have expired or have been acknowledged.  A
   four byte value for the token field provides sufficient space for
   multiple unique probes to be made within the MSL.

   The initial value of the four byte token field SHOULD be assigned to
   a randomised value to enhance protection from off-path attacks, as
   described in section 5.1 of [RFC8085]).

4.2.  DPLPMTUD Procedures for UDP Options

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

   *  A probe to confirm the path can support the base PLPMTU.

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

   *  A probe to validate the path supports the current PLPMTU.

4.2.1.  Confirmation of Connectivity across a Path

   The DPLPMTUD method requires a PL to confirm connectivity over the
   path using the base PLPMTU (see Section 5.1.4 of [RFC8899]), but UDP
   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, padded
   to size BASE_PLPMTU.  This confirmation probe MUST include a UDP
   Option that elicits a response from the remote endpoint (e.g., by
   including the ECHO Request/Response Option) to confirm that a packet
   of the size traversed the path.

4.2.2.  Sending Probe Packets to Increase the PLPMTU

   From time to time, DPLPMTUD searches 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 can be
   used as a hint to initialise this search to increase the PLPMTU.

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   Probe packets seeking to increase the PLPMTU SHOULD NOT carry
   application data (see "Probing using padding data" in Section 4.1 of
   [RFC8899]), since they will be lost whenever their size exceeds the
   actual PMTU.

   A probe seeking to increase the PLPMTU MUST elicit a positive
   confirmation that the path has delivered a Datagram of the specific
   probed size and therefore SHOULD include the Echo Request Option
   Request Option.

   Received probes that do not carry application data do not form a part
   of the end-to-end transport data and are not delivered to the upper
   layer protocol.

4.2.3.  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 (see Section 6.1.4 of [RFC8899]).  This validation sends
   probes in the DPLPMTUD SEARCH_COMPLETE state i.e., to detect black-
   holing of data (see Section 4.2 of [RFC8899]).

   This function can be implemented within UDP Options, by generating a
   probe of size PLPMTU which MUST include a UDP Option to elicit a
   positive confirmation that the path has delivered the probe.  This
   confirmation probe MAY use "Probing using padding data" or "Probing
   using application data and padding data" (see Section 4.1 of
   [RFC8899]) or can construct a probe packet that does not carry any
   application data, as described in a previous section.

4.2.4.  Sending Packet Probes that include Application Data

   The method can be designed to only use probes that are formed of a
   UDP Options datagram containing control information, padded to the
   required size.  This implements "Probing using padding data", and
   avoids having to retransmit application data when a probe fails.
   This type of probe must be used when searching to increase the
   PLPMTU.  These probes do not form a part of the end-to-end transport
   data and a receiver does not deliver these to the upper layer
   protocol.  A simple implementation of the method might be designed to
   only use this format for all probes.

   Probe used to confirm the connectivity or to validate support for the
   current PLPMTU are also permitted to carry application data, since
   this type of probe is expected to be successful.  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 an additional datagram when confirming that the current path

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   supports datagrams of size PLPMTU and could be designed to utilise a
   control message format defined by the PL that does not need to be
   delivered reliably.

4.3.  PTB Message Handling for this Method

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

   A UDP Options sender that utilises ICMP PTB messages received in
   response to a probe packet MUST use the quoted packet to validate the
   UDP port information in combination with the token and/or timestamp
   value contained in the UDP Option, before processing the packet using
   the DPLPMTUD method (see Section 4.4.1 of [RFC8899]).  An
   implementation unable to support this validation needs to ignore
   received ICMP PTB messages.

5.  Acknowledgements

   Gorry Fairhurst and Tom Jones are supported by funding provided by
   the University of Aberdeen.

6.  IANA Considerations

   This memo includes no requests to IANA.

7.  Security Considerations

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

   The specification recommends that the token in the REQ/RES message is
   initialised to a randomised value to enhance protection from off-path
   attacks.

   The security considerations for using DPLPMTUD are described in
   [RFC8899].  The proposed new method does not change the ICMP PTB
   message validation method described 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 and/or timestamp value contained in the UDP Option,
   before processing the packet using the DPLPMTUD method.

8.  References

8.1.  Normative References

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   [I-D.ietf-tsvwg-udp-options]
              Touch, J. D., "Transport Options for UDP", Work in
              Progress, Internet-Draft, draft-ietf-tsvwg-udp-options-13,
              19 June 2021, <https://www.ietf.org/archive/id/draft-ietf-
              tsvwg-udp-options-13.txt>.

   [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>.

   [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/info/rfc9000>.

8.2.  Informative References

   [RFC1063]  Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
              MTU discovery options", RFC 1063, DOI 10.17487/RFC1063,
              July 1988, <https://www.rfc-editor.org/info/rfc1063>.

   [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>.

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   [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, fix

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   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.

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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