IPv6 Fragment Retransmission

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Table of Contents

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.¶

This document proposes 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 not received. 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 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", "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. 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:¶

• Next Header is a 1-octet IP protocol version of the next header following the Fragment Header.¶
• Reserved is a 1-octet reserved field set to 0 on transmission and ignored on reception.¶
• 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.¶
• Res is a 2-bit field set to 0 on transmission and ignored on reception.¶
• M is the "more fragments" bit telling whether additional fragments follow.¶
• 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 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 Number" beginning with 1 and monotonically incrementing for each successive fragment in the one-octet "Reserved" field of the IPv6 Fragment Header. The Reserved field is then renamed as "Ordinal" as shown below:¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |    Ordinal    |      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 '1', the Ordinal value for the second fragment to '2', the Ordinal value for the third fragment to '3', etc. up to the total number of IPv6 fragments. When a destination that obeys this specification receives an IPv6 fragment with the Reserved/Ordinal field set to non-zero, 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 Ordinal numbers 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 follows:¶

       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 Map (0) (0-31)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Ordinal Map (0) (32-63)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Identification (1)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Ordinal Map (1) (0-31)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Ordinal Map (1) (32-63)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                ...                            |
      |                                ...                            |

In this format, the destination prepares the Fragrep message as a list of ordered-triples of 4-octet fields. The first field in each triple includes the Identification value for the IPv6 packet that is subject of the report, while the second and third fields include a 64-bit bitmap of the Ordinal values received for this Identification. For example, if the destination receives Ordinals 1, 2, 4, 5, 7, 9, then 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 ordered triples as necessary without the entire Fragrep message exceeding the minimum IPv6 MTU of 1280 bytes.¶

After the destination has assembled the Fragrep message, it transmits the message to the IPv6 source. When the source receives the message, it examines each ordered triple to determine the (Identification, Ordinal) pairs that require retransmission. For example, if the source receives an Ordinal bitmap for Identification 0x12345678 with bits 0, 1, 3, 4, 6 and 8 set to '1', it would retransmit Ordinal fragments (0x12345678, 3), (0x12345678, 6) and (0x12345678, 8).¶

This implies that the source should maintain a cache of recently transmitted fragments for a time period known as the "link persistence interval". Then, if the source receives a Fragrep that requests 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.¶

Note: The maximum-sized IPv6 packet that can undergo 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 be significantly less than the allotted 64 bitmap bits.¶

5. Implementation Status

TBD.¶

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.¶

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