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IPv6 Fragment Retransmission

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Author Fred Templin
Last updated 2021-11-08
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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Updates: RFC8200 (if approved)                         November 08, 2021
Intended status: Standards Track
Expires: May 12, 2022

                      IPv6 Fragment Retransmission


   Internet Protocol version 6 (IPv6) provides a fragmentation and
   reassembly service for end systems allowing for the transmission of
   packets that exceed the path MTU.  However, loss of just a single
   fragment requires retransmission of the original packet in its
   entirety, with the potential for devastating effects on performance.
   This document specifies an IPv6 fragment retransmission scheme that
   matches the loss unit to the retransmission unit.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 12, 2022.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  IPv6 Fragmentation  . . . . . . . . . . . . . . . . . . . . .   3
   4.  IPv6 Fragment Retransmission  . . . . . . . . . . . . . . . .   4
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Internet Protocol version 6 (IPv6) [RFC8200] provides a fragmentation
   and reassembly service similar to that found in IPv4 [RFC0791], with
   the exception that only the source host (i.e., and not routers on the
   path) may perform fragmentation.  When an IPv6 packet is fragmented,
   the loss unit (i.e., a single IPv6 fragment) becomes smaller than the
   retransmission unit (i.e., the entire packet) which under
   intermittent loss conditions could result in sustained retransmission
   storms with little or no forward progress [FRAG].

   The presumed drawbacks of fragmentation are tempered by the fact that
   greater performance can often be realized when the source sends large
   packets that exceed the path MTU.  This is due to the fact that a
   single large IPv6 packet produced by upper layers results in a burst
   of multiple fragment packets produced by lower layers with minimal
   inter-packet delays.  These bursts yield high network utilization for
   the burst duration, while modern reassembly implementations have
   proven capable of accommodating such bursts.  If the loss unit can
   somehow be made to match the retransmission unit, the performance
   benefits of IPv6 fragmentation can be realized.

   This document therefore proposes an IPv6 fragment retransmission
   service in which the source marks each fragment with an "Ordinal"
   number, and the destination may request retransmissions of any
   ordinal fragments that are lost.  This retransmission request service
   is intended only for short-duration and opportunistic best-effort
   recovery (i.e., and not true end-to-end reliability).  In this way,
   the service mirrors the Automatic Repeat Request (ARQ) function of

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   common data links [RFC3366] by considering an imaginary virtual link
   that extends from the IPv6 source to destination.  The goal therefore
   is for the destination to quickly obtain missing individual fragments
   of partial reassemblies before true end-to-end timers would cause
   retransmission of the entire packet.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  IPv6 Fragmentation

   IPv6 fragmentation is specified in Section 4.5 of [RFC8200] and is
   based on an IPv6 Fragment extension header formatted as shown below:

      |  Next Header  |   Reserved    |      Fragment Offset    |Res|M|
      |                         Identification                        |

   In this format:

   o  Next Header is a 1-octet IP protocol version of the next header
      following the Fragment Header.

   o  Reserved is a 1-octet reserved field set to 0 on transmission and
      ignored on reception.

   o  Fragment Offset is a 13-bit field that provides the offset (in
      8-octet units) of the data portion that follows from the beginning
      of the packet.

   o  Res is a 2-bit field set to 0 on transmission and ignored on

   o  M is the "more fragments" bit telling whether additional fragments

   o  Identification is a 32 bit numerical identification value for the
      entire IPv6 packet.  The value is copied into each fragment of the
      same IPv6 packet.

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   The fragmentation and reassembly specification in [RFC8200] can be
   considered as the default method which adheres to the details of that
   RFC.  This document presents an enhanced method that allows for
   retransmissions of individual fragments.

4.  IPv6 Fragment Retransmission

   Fragmentation implementations that obey this specification write an
   "Ordinal" value beginning with 0 and monotonically incrementing for
   each successive fragment in the (formerly) "Reserved" field of the
   IPv6 Fragment Header.  The Reserved field is then replaced with a
   6-bit "Ordinal" field followed by a 1-bit R(eserved) flag followed by
   a 1-bit A(RQ) flag as shown below:

      |  Next Header  |  Ordinal  |R|A|      Fragment Offset    |Res|M|
      |                         Identification                        |

   In particular, when a source that obeys this specification fragments
   an IPv6 packet it sets the Ordinal value for the first fragment to
   '0', the Ordinal value for the second fragment to '1', the Ordinal
   value for the third fragment to '2', etc. up to either the final
   fragment or the 64th fragment (whichever comes first).  The source
   also sets the A flag to 1 in each fragment to inform the destination
   that fragment retransmission is supported for this packet.

   When a destination that obeys this specification receives IPv6
   fragments with the A flag set to 1, it infers that the source
   participates in the protocol and maintains a checklist of all Ordinal
   numbered fragments received for a specific Identification number.

   If the destination notices one or more Ordinals missing after most
   other Ordinals for the same Identification have arrived, it can
   prepare a Fragmentation Report (FRAGREP) ICMPv6 message [RFC4443] to
   send back to the source.  The FRAGREP message is formatted as

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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |     Type      |     Code      |          Checksum             |
       |                        Identification (0)                     |
       |                    Ordinal Bitmap (0) (0-31)                  |
       |                    Ordinal Bitmap (0) (32-63)                 |
       |                        Identification (1)                     |
       |                    Ordinal Bitmap (1) (0-31)                  |
       |                    Ordinal Bitmap (1) (32-63)                 |
       |                                ...                            |
       |                                ...                            |

   In this format, the destination prepares the FRAGREP message as a
   list of 12-octet (Identification(i), Bitmap(i)) pairs.  The first 4
   octets in each pair encode the Identification value for the IPv6
   packet that is subject of the report, while the remaining 8 octets
   encode a 64-bit Bitmap of Ordinal fragments received for this
   Identification.  For example, if the destination receives Ordinals 0,
   1, 3, 4, 6, and 8 it sets Bitmap bits 0, 1, 3, 4, 6 and 8 to '1' and
   sets all other bits to '0'.  The destination may include as many
   (Identification, Bitmap) pairs as necessary without the entire
   FRAGREP message exceeding the minimum IPv6 MTU of 1280 bytes.  (If
   additional pairs are necessary, the destination may prepare and send
   multiple FRAGREPs.)

   After the destination has assembled the FRAGREP it transmits the
   message to the IPv6 source.  When the source receives the FRAGREP, it
   examines each entry to determine the per-Identification Ordinal
   fragments that require retransmission.  For example, if the source
   receives a Bitmap for Identification 0x12345678 with bits 0, 1, 3, 4,
   6 and 8 set to '1', it would retransmit Ordinal fragments
   (0x12345678, 2), (0x12345678, 5) and (0x12345678, 7).

   This implies that the source should maintain a cache of recently
   transmitted fragments for a time period known as the "link
   persistence interval" [RFC3366].  Then, if the source receives a
   FRAGREP requesting retransmission of one or more Ordinals, it can
   retransmit if it still holds the Ordinal in its cache.  Otherwise,
   the Ordinal will incur a cache miss and the original source will
   eventually retransmit the original packet in its entirety.

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   Note that the maximum-sized IPv6 packet that a source can submit for
   fragmentation is 64KB, and the minimum IPv6 path MTU is 1280B.
   Assuming the minimum IPv6 path MTU as the nominal size for non-final
   fragments, the number of Ordinals for each IPv6 packet should
   therefore fit within the allotted 64 Bitmap bits when the fragments
   are transmitted over IPv6-only network paths.

   However, when the path may traverse one or more IPv4 networks (e.g.,
   via tunneling) the path MTU may be significantly smaller.  In that
   case, the number of IPv6 fragments needed may exceed the maximum
   number of Ordinal candidates for retransmission (i.e., 64).

   When the number of IPv6 fragments exceeds 64, the source assigns an
   Ordinal value and sets A to 1 in the first 64 fragments, but sets
   both Ordinal and A to 0 in all remaining fragments then transmits all
   fragments.  When the destination receives the fragments, it may
   return a FRAGREP to request retransmission of any of the first 64
   fragments, but may not request retransmission of any additional
   fragments for which the default behavior of best-effort delivery
   applies.  (However, all fragments are presented equally to the
   reassembly cache where successful reassembly is likely.)

   For this reason, when an IPv6 tunnel endpoint acting as the source
   forwards a fragmented packet with more than 64 fragments it also
   returns an ICMPv6 Packet Too Big (PTB) "soft error" to the original
   source as specified in [AERO][OMNI].  When the original source
   receives the PTB soft error, it should reduce the size of the packets
   it sends.  Either IPv6 tunnel endpoint may also return PTB soft
   errors if the frequency of retransmissions or reassembly failures
   exceeds acceptable thresholds.

   Finally, transmission of IPv6 fragments over IPv6-only paths can
   safely proceed without a fragmentation-layer integrity check since
   IPv6 includes a 32-bit Identification value and reassembly
   safeguards.  On the other hand, transmission of IPv6 fragments over
   IPv4-only or mixed IPv6/IPv4 paths requires a fragmentation-layer
   integrity check inserted by the source before fragmentation and
   verified by the destination following reassembly since IPv4 provides
   only a 16-bit Identification and no reassembly safeguards.  (In cases
   where the full path cannot be determined a priori, an integrity check
   should always be included as specified in [AERO][OMNI].)

5.  Implementation Status


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6.  IANA Considerations

   A new ICMPv6 Message Type code for "Fragmentation Report (FRAGREP)"
   is requested.

7.  Security Considerations

   Communications networking security is necessary to preserve
   confidentiality, integrity and availability.

8.  Acknowledgements

   This work was inspired by ongoing AERO/OMNI/DTN investigations.


9.  References

9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,

   [RFC5326]  Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
              Transmission Protocol - Specification", RFC 5326,
              DOI 10.17487/RFC5326, September 2008,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

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9.2.  Informative References

   [FRAG]     Mogul, J. and C. Kent, "Fragmentation Considered Harmful,
              ACM Sigcomm 1987", August 1987.

              Burleigh, S., Fall, K., and E. J. Birrane, "Bundle
              Protocol Version 7", draft-ietf-dtn-bpbis-31 (work in
              progress), January 2021.

              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-49 (work in progress), October 2021.

   [MPPS]     Majkowski, M., "How to Receive a Million Packets Per
              million-packets/", June 2015.

   [QUIC]     Ghedini, A., "Accelerating UDP Packet Transmission for
              udp-packet-transmission-for-quic/", December 2019.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,

   [RFC8899]  Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
              T. Voelker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,

Author's Address

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   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124


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