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 [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 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", "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" 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 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 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.¶

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

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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