IPv6 Fragment Retransmission
draft-templin-6man-fragrep-00
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| Last updated | 2021-10-25 | ||
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draft-templin-6man-fragrep-00
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational October 25, 2021
Expires: April 28, 2022
IPv6 Fragment Retransmission
draft-templin-6man-fragrep-00
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 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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 28, 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
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. IPv6 Fragmentation . . . . . . . . . . . . . . . . . . . . . 3
4. IPv6 Fragment Retransmission . . . . . . . . . . . . . . . . 3
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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
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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
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 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:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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,
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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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9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[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>.
[RFC5326] Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
Transmission Protocol - Specification", RFC 5326,
DOI 10.17487/RFC5326, September 2008,
<https://www.rfc-editor.org/info/rfc5326>.
[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>.
[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>.
9.2. Informative References
[FRAG] Mogul, J. and C. Kent, "Fragmentation Considered Harmful,
ACM Sigcomm 1987", August 1987.
[I-D.ietf-dtn-bpbis]
Burleigh, S., Fall, K., and E. J. Birrane, "Bundle
Protocol Version 7", draft-ietf-dtn-bpbis-31 (work in
progress), January 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-48 (work in progress), October 2021.
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[MPPS] Majkowski, M., "How to Receive a Million Packets Per
Second, https://blog.cloudflare.com/how-to-receive-a-
million-packets/", June 2015.
[QUIC] Ghedini, A., "Accelerating UDP Packet Transmission for
QUIC, https://calendar.perfplanet.com/2019/accelerating-
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,
<https://www.rfc-editor.org/info/rfc4963>.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013,
<https://www.rfc-editor.org/info/rfc6864>.
[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, <https://www.rfc-editor.org/info/rfc8899>.
[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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