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IPv6 Fragment Retransmission and Path MTU Discovery Soft Errors
draft-templin-6man-fragrep-02

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Author Fred Templin
Last updated 2021-11-17 (Latest revision 2021-11-08)
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draft-templin-6man-fragrep-02
Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Updates: RFC8200, RFC8201, RFC4443,                    November 17, 2021
         RFC1191 (if approved)
Intended status: Standards Track
Expires: May 21, 2022

    IPv6 Fragment Retransmission and Path MTU Discovery Soft Errors
                     draft-templin-6man-fragrep-02

Abstract

   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 potentially devastating effects on performance.  This
   document specifies an IPv6 fragment retransmission scheme that
   matches the loss unit to the retransmission unit.  The document
   further specifies an update to Path MTU Discovery that distinguishes
   hard link size restrictions from reassembly congestion events.

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 https://datatracker.ietf.org/drafts/current/.

   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 21, 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
   (https://trustee.ietf.org/license-info) in effect on the date of

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   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
   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.  Common Use Cases  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  IPv6 Fragmentation  . . . . . . . . . . . . . . . . . . . . .   4
   5.  IPv6 Fragment Retransmission  . . . . . . . . . . . . . . . .   5
   6.  Packet Too Big (PTB) Soft Errors  . . . . . . . . . . . . . .   7
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 [RFC8900].

   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"

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   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
   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.

   When conditions suggest that original sources should begin sending
   smaller packets, the fragmentation source and/or reassembly
   destination can return a new type of ICMPv6 Packet Too Big or ICMPv4
   Fragmentation Needed message termed a PTB "soft error" that is
   distinguished from classic "hard errors" by including a non-zero
   value in the PTB Code (ICMIPv6) or unused (ICMPv4) field.  The
   fragmentation source can return soft errors (subject to rate
   limiting) suggesting a smaller packet size while fragmentation of
   large packets is producing excessive numbers of fragments.
   Similarly, the reassembly destination can return soft errors (via the
   fragmentation source) while reassembly of large packets is causing
   excessive reassembly congestion.  Original sources that receive these
   soft errors should reduce the size of packets they send for the short
   term, but can again begin to increase their packet sizes without
   delay as long as no further soft or hard errors arrive.

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.

3.  Common Use Cases

   A common use case of interest is to improve the state of affairs for
   IPv6 encapsulation (i.e., "tunneling") [RFC2473] when the original
   source may be many IP hops away from the tunnel ingress, and the
   tunnel packet may be fragmented following encapsulation.  The tunnel
   is seen as a "link" on the path from the original source to the final
   destination, and the goal is to increase the reliability of that link
   in order to minimize wasteful end-to-end retransmissions.

   When the original source and IPv6 fragmentation source are located on
   the same platform (physical or virtual) the window of opportunity for
   successful retransmission of individual fragments may be narrow

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   unless the link persistence timeframe is carefully coordinated with
   upper layer retransmission timers.  (In an uncoordinated case, upper
   layers may retransmit the entire packet before or at roughly the same
   time the IPv6 fragmentation source retransmits individual fragments,
   leading to increased congestion and wasted retransmissions.)

4.  IPv6 Fragmentation

   IPv6 fragmentation is specified in Section 4.5 of [RFC8200] and is
   based on the 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
      reception.

   o  M is the "More Fragments" bit telling whether additional fragments
      follow.

   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.

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

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5.  IPv6 Fragment Retransmission

   Fragmentation implementations that obey this specification write an
   "Ordinal" value beginning with 0 and monotonically incremented for
   each successive fragment in the (formerly) "Reserved" field of the
   IPv6 Fragment Header, which is redefined as 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 an ICMPv6 Fragmentation Report (FRAGREP) message [RFC4443] to
   send back to the source.  The message is formatted as follows:

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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 causing the
   entire message to exceed the minimum IPv6 MTU of 1280 bytes.  (If
   additional pairs are necessary, the destination may prepare and send
   multiple messages.)

   The destination next transmits the FRAGREP message to the IPv6
   fragment source.  When the source receives the message, 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 interval known as "link persistence"
   [RFC3366].  The link persistence should be at least as long as the
   round-trip time from the fragmentation source to the reassembly
   destination, plus an additional small delay to allow for reassembly
   processing overhead.  Then, if the source receives a FRAGREP message
   requesting retransmission of one or more Ordinals, it can retransmit

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   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.  After processing all
   entries in the FRAGREP, the source discards the message.

   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.)

   Finally, transmission of IPv6 fragments over IPv6-only paths can
   safely proceed without a fragmentation-layer integrity check since
   IPv6 includes reassembly safeguards and a 32-bit Identification
   value.  Conversely, 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 [I-D.templin-6man-aero] and
   OMNI [I-D.templin-6man-omni].)

6.  Packet Too Big (PTB) Soft Errors

   When an IPv6 fragmentation source forwards packets that produce what
   it considers as excessive numbers fragments (e.g., 32, 48, 64, more),
   the fragmentation source can also return PTB "soft errors" to the
   original source (subject to rate limiting).  Either the fragmentation
   source or reassembly destination may also return PTB soft errors if
   the frequency of retransmissions or reassembly failures exceeds
   acceptable thresholds.

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   PTB soft errors are distinguished from ordinary "hard errors" through
   a non-zero value in the ICMPv6 "Code" field [RFC8201][RFC4443] or
   ICMPv4 "unused" field [RFC1191].  The following values are currently
   defined:

   o  0 - "PTB hard error" - Original sources that receive these
      messages obey the classic Path MTU Discovery (PMTUD)
      specifications found in [RFC8201][RFC1191].

   o  1 - "PTB soft error (packet lost)" - Original sources that receive
      these messages should reduce their packet sizes while
      retransmitting the data from the lost packet, but need not wait
      the prescribed 10 minutes before attempting to again increase
      packet sizes.

   o  2 - "PTB soft error (packet forwarded)" - Original sources that
      receive these messages should reduce their packet sizes without
      invoking retransmission, and also need not wait the prescribed 10
      minutes before attempting to again increase packet sizes.

   o  3-255 - reserved for future use.

   PTB soft errors include as much of the invoking packet as possible
   without the message exceeding the minimum MTU (i.e., 1280 bytes for
   IPv6 or 576 bytes for IPv4).  Original sources that recognize PTB
   soft errors should follow common logic to dynamically tune their
   packet sizes to obtain the best performance.  In particular, an
   original source can gradually increase the size of packets it sends
   while no or few PTB soft errors are arriving then again reduce packet
   sizes when excessive soft errors arrive.

   Original sources that do not recognize PTB soft errors (i.e., that do
   not examine the Code/unused field value) follow the same standards as
   for hard errors as described above.  These sources may miss
   opportunities to realize improved performance.

7.  Implementation Status

   TBD.

8.  IANA Considerations

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

   The IANA is instructed to create new registries for "ICMPv6 Packet
   Too Big Code field" and "ICMPv4 Fragmentation Needed unused field"
   values.  Both registries should have the following initial values:

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      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        PTB hard error                 [RFCXXXX]
      1        PTB soft error (loss)          [RFCXXXX]
      2        PTB soft error (no loss)       [RFCXXXX]
      3-252    Unassigned
      253-254  Reserved for Experimentation   [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

                Figure 1: Packet Too Big Code/unused Values

9.  Security Considerations

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

10.  Acknowledgements

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

   .

11.  References

11.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

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

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc4443>.

   [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>.

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   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

11.2.  Informative References

   [I-D.templin-6man-aero]
              Templin, F. L., "Automatic Extended Route Optimization
              (AERO)", draft-templin-6man-aero-37 (work in progress),
              November 2021.

   [I-D.templin-6man-omni]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-51 (work in progress), November 2021.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [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,
              <https://www.rfc-editor.org/info/rfc8900>.

Author's Address

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

   Email: fltemplin@acm.org

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