IPv6 Path MTU Updates
draft-generic-6man-tunfrag-07

Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                          January 14, 2013
Expires: July 18, 2013

                         IPv6 Path MTU Updates
                   draft-generic-6man-tunfrag-07.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.  A common case that
   exhibits these link characteristics is seen for IPv6-in-IP tunnels.
   Additionally, IPv6 nodes can avoid path MTU discovery issues even
   when no link adaptation is necessary by performing a small amount of
   fragmentation and/or by probing the path as necessary.  This document
   therefore proposes an update to the base IPv6 specification to better
   accommodate path MTU issues.

Status of this Memo

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Copyright Notice

   Copyright (c) 2013 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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Considerations for Small MTU Paths  . . . . . . . . . . . . . . 4
     3.1.  Accommodating Legacy Nodes  . . . . . . . . . . . . . . . . 4
   4.  Considerations for Medium MTU Paths . . . . . . . . . . . . . . 5
   5.  Considerations for Large MTU Paths  . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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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.  A common case that
   exhibits these link characteristics is seen for IPv6-in-IP tunnels
   [I-D.generic-v6ops-tunmtu].  Additionally, IPv6 nodes can avoid path
   MTU discovery issues even when no link adaptation is necessary by
   performing a small amount of fragmentation and/or by probing the path
   as necessary.  This document therefore proposes an update to the base
   IPv6 specification to better accommodate path MTU issues.

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 perform fragmentation on the packets it
   sends, in which case reassembly would be performed by the final
   destination.

   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.

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3.  Considerations for Small MTU Paths

   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 push the fragmentation and
   reassembly responsibility 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 less than 1280 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
   reporting a Next-Hop MTU less than 1280 and with Code=1.  In that
   case, the IPv6 node is not required to reduce the size of subsequent
   packets to less than 1500 bytes, but must perform IPv6 fragmentation
   on those packets by breaking the packet into N roughly 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

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   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 up to
   1280 bytes in length and 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 fragment these packets (again,
   by splitting the packet into N roughly equal-length pieces).

4.  Considerations for Medium MTU Paths

   Regardless of whether there is a link in the path that performs link
   adaptation, when an originating IPv6 node receives a PTB message
   reporting a Next-Hop MTU value between 1280 and 1500 bytes, the node
   need not reduce the size of the packets it sends.  The node may
   instead invoke fragmentation for packets between 1281 and 1500 bytes
   (again, by splitting the packet into N roughly equal-length pieces)
   before submitting each fragment for link-layer framing.  These
   fragments will be reassembled by the final destination.

   A more interesting situation arises when PTB messages are lost on the
   return path to the originating IPv6 node.  Since the node has no way
   of discerning which paths may exhibit this condition, it may be
   better served to assume the worst case for all paths and take
   precautionary measures to avoid silent packet loss.  For example, an
   originating IPv6 node that wishes to ensure that packets between 1281
   and 1500 bytes will reach the final destination can use "proactive
   fragmentation" to fragment each packet (again, by splitting the
   packet into N roughly equal-length pieces).  Alternatively, the node
   can use Packetization Layer Path MTU Discovery (PLPMTUD) per
   [RFC4821] to ensure that packets of this size are being delivered to
   the final destination.

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5.  Considerations for Large MTU Paths

   An originating IPv6 node connected to a link that supports an MTU of
   1500 bytes or larger is permitted to send packets larger than 1500
   bytes without fragmentation, but should implement PLPMTUD [RFC4821]
   to verify that these larger packets are reaching the final
   destination.

6.  IANA Considerations

   There are no IANA considerations for this document.

7.  Security Considerations

   The security considerations for [RFC2460] apply also to this
   document.

8.  Acknowledgments

   This method was inspired through discussion on the IETF v6ops and
   NANOG mailing lists in the May through July 2012 timeframe.

9.  References

9.1.  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.

9.2.  Informative References

   [I-D.generic-v6ops-tunmtu]
              Templin, F., "Operational Considerations for Tunnel
              Fragmentation and Reassembly",
              draft-generic-v6ops-tunmtu-12 (work in progress),
              November 2012.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

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   [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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