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IPv6 Path MTU Updates
draft-generic-6man-tunfrag-05

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Author Fred Templin
Last updated 2012-07-13
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draft-generic-6man-tunfrag-05
Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                             July 13, 2012
Expires: January 14, 2013

                         IPv6 Path MTU Updates
                   draft-generic-6man-tunfrag-05.txt

Abstract

   IPv6 intentionally deprecates fragmentation by routers in the
   network.  Instead, links with restricting 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-within-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

   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 14, 2013.

Copyright Notice

   Copyright (c) 2012 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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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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  . . . . . . . . . . . . . . 5
   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 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-within-IP tunnels
   [I-D.generic-v6ops-tunmtu].  Additionally, IPv6 nodes can avoid path
   MTU discovery issues even when no link adapation 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
   permitted to perform link adaptation.  A common example of such links
   is seen for IPv6-within-IP tunnels.

   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 for 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 tunnels nor other 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."

   This document does not propose to change this requirement, but notes
   that link adaptation can be burdensome for some links (e.g., IPv6-
   within-IP tunnels) 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 a 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 modifying the final paragraph of Section 5 of
   [RFC2460] as follows:

   "In response to an IPv6 packet that is sent to a destination located
   beyond an IPv6-within-IP tunnel or 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, but must perform IPv6
   fragmentation on those packets using the Next-Hop MTU as the maximum
   fragment size.  These fragments will be reassembled by the
   destination."

   An example tunnel protocol that invokes this behavior appears in:
   [I-D.templin-intarea-seal].

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

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   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 those 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, an IPv6 link that returns this
   new kind of PTB message may receive future packets up to 1280 bytes
   in length and containing a Fragment header with MF=0 and Offest=0.
   The link should process these packets as an indication that the
   originating IPv6 node is a legacy node, and should not send further
   PTB messages.

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 but may instead
   invoke fragmentation for packets up to 1500 bytes using a maximum
   fragment size of 1280 bytes.  These fragments will again 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.

   In one approach, an originating IPv6 node that wishes to ensure that
   packets between 1281 and 1500 bytes in length will reach the
   destination can use "proactive fragmentation" to fragment the packets
   into two pieces that are no larger than 1280 bytes.  In a second
   approach, the node can use Packetization Layer Path MTU Discovery
   (PLPMTUD) [RFC4821] without fragmentation to verify that packets
   larger than 1280 bytes are reaching the final destination.

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 [RFC4821] to verify
   that these larger packets are reaching the final destination.

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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 Issues with Tunnel Maximum
              Transmission Unit (MTU)", draft-generic-v6ops-tunmtu-09
              (work in progress), July 2012.

   [I-D.templin-intarea-seal]
              Templin, F., "The Subnetwork Encapsulation and Adaptation
              Layer (SEAL)", draft-templin-intarea-seal-42 (work in
              progress), December 2011.

   [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.

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   [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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