Internet DRAFT - draft-templin-6man-linkadapt
draft-templin-6man-linkadapt
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational February 27, 2015
Expires: August 31, 2015
IPv6 Path MTU Interactions With Link Adaptation
draft-templin-6man-linkadapt-02.txt
Abstract
IPv6 intentionally deprecates fragmentation by routers in the
network. Instead, links with restricting Maximum Transmission Units
(MTUs) must either drop each too-large packet and return an ICMPv6
Packet Too Big (PTB) message or perform link-specific fragmentation
and reassembly (also known as "link adaptation") at a layer below
IPv6. This latter category of links is often performance-challenged
to accommodate steady-state link adaptation. This document therefore
proposes an update to the base IPv6 specification to better
accommodate links that require link-specific adaptation.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 31, 2015.
Copyright Notice
Copyright (c) 2015 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2
3. Link Adaptation Signaling and Accommodation . . . . . . . . . 3
3.1. Accommodating Legacy Nodes . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
IPv6 intentionally deprecates fragmentation by routers in the
network. Instead, links with restricting Maximum Transmission Units
(MTUs) must either drop each too-large packet and return an ICMPv6
Packet Too Big (PTB) message or perform link-specific fragmentation
and reassembly (also known as "link adaptation") at a layer below
IPv6. This latter category of links is often performance-challenged
to accommodate steady-state link adaptation. This document therefore
proposes an update to the base IPv6 specification to better
accommodate links that require link-specific adaptation.
2. Problem Statement
The current "Internet cell size" is effectively 1500 bytes, i.e., the
minimum MTU configured by the vast majority of links in the Internet.
IPv6 constrains this even further by specifying a minimum link MTU of
1280 bytes [RFC2460]. However, due to operational issues with Path
MTU Discovery (PMTUD) [RFC1981] these sizes can often only be
accommodated when links with smaller link-layer segment sizes are
configured to perform link adaptation.
Unfortunately, link adaptation can present a significant burden to
the link endpoints, i.e., especially when the link supports high data
rates and/or is located nearer the "middle" of the network instead of
nearer the "edge". An alternative therefore is to ask the
originating IPv6 node to either reduce the size of the packets it
sends or perform host-based fragmentation, in which case reassembly
would be performed by the final destination.
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In addition to the above considerations, it is becoming more and more
evident that PMTUD uncertainties can be encountered even when there
are no links in the path that must perform link adaptation. This is
due to the fact that the PTB messages required for PMTUD can be lost
due to network filters that block ICMPv6 messages
[RFC2923][WAND][SIGCOMM]. Originating IPv6 node are therefore
advised to take precautions to avoid path MTU related failure modes.
This document updates the IPv6 protocol specification [RFC2460] to
better accommodate paths with various MTUs as described in the
following sections.
3. Link Adaptation Signaling and Accommodation
Section 5 of [RFC2460] states:
"IPv6 requires that every link in the Internet have an MTU of 1280
octets or greater. On any link that cannot convey a 1280-octet
packet in one piece, link-specific fragmentation and reassembly must
be provided at a layer below IPv6."
and:
"A node must be able to accept a fragmented packet that, after
reassembly, is as large as 1500 octets.".
This document does not propose to change these requirements, but
notes that link adaptation can be burdensome for some links to the
point that it would be highly desirable to signal the MTU limitation
to the IPv6 communication endpoints. In order to accommodate this,
when the router at the link ingress performs link adaptation on a
packet it should also send an ICMPv6 PTB message back to the original
source (subject to rate limiting) with a Next-Hop MTU set to the link
adaptation threshold and with Code field set to 1 [RFC4443]. (Note
that these PTB messages are advisory in nature and do not necessarily
indicate packet loss.)
As a result, the originating IPv6 node may receive this "new kind" of
PTB message and should modify its behavior accordingly. This is
accomplished by adding a new final paragraph to Section 5 of
[RFC2460] as follows:
"In response to an IPv6 packet that is sent to a destination located
beyond an IPv6 link that must perform link adaptation, the
originating IPv6 node may receive an ICMP Packet Too Big message with
Code=1. In that case, the IPv6 node can either reduce the size of
subsequent packet it sends or perform IPv6 fragmentation on packets
no larger than 1500 bytes by breaking the packet into N roughly
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equal-length pieces (where N is minimized and the length of each
piece is smaller than the Next-Hop MTU). These fragments will be
reassembled by the destination."
3.1. Accommodating Legacy Nodes
Legacy IPv6 nodes observe the current final paragraph of Section 5 of
[RFC2460]:
"In response to an IPv6 packet that is sent to an IPv4 destination
(i.e., a packet that undergoes translation from IPv6 to IPv4), the
originating IPv6 node may receive an ICMP Packet Too Big message
reporting a Next-Hop MTU less than 1280. In that case, the IPv6 node
is not required to reduce the size of subsequent packets to less than
1280, but must include a Fragment header in those packets so that the
IPv6-to-IPv4 translating router can obtain a suitable Identification
value to use in resulting IPv4 fragments. Note that this means the
payload may have to be reduced to 1232 octets (1280 minus 40 for the
IPv6 header and 8 for the Fragment header), and smaller still if
additional extension headers are used."
For such legacy nodes, the receipt of a PTB message with a Next-Hop
MTU less than 1280 will result in the above behavior regardless of
the value in the Code field. As a result, a link ingress node that
returns this new kind of PTB message may receive future packets
containing a Fragment header with the More Fragments (MF) bit and
Offset field set to 0. The link ingress node should process these
packets as an indication that the originating IPv6 node is a legacy
node, and should not send further PTB messages. Instead, the link
ingress node should use the fragment header supplied by the source to
fragment the original packet to a size that would avoid link
adaptation. These fragments are then reassembled by the final
destination.
4. IANA Considerations
There are no IANA considerations for this document.
5. Security Considerations
The security considerations for [RFC2460] apply also to this
document.
6. Acknowledgments
This method was inspired through discussion on the IETF v6ops and
NANOG mailing lists in the May through July 2012 timeframe. Further
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discussion occurred on the Intarea list in the February 2015
timeframe.
7. References
7.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
7.2. Informative References
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
2923, September 2000.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[SIGCOMM] Luckie, M. and B. Stasiewicz, "Measuring Path MTU
Discovery Behavior", November 2010.
[WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and
Debugging Path MTU Discovery Failures", October 2005.
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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