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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                             July 11, 2012
Expires: January 12, 2013


                         IPv6 Path MTU Updates
                   draft-generic-6man-tunfrag-04.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 (also known as "link adaptation")
   at a layer below IPv6.  This latter category of links is often
   performance-challenged to accommodate steady-state link-specific
   fragmentation and reassembly to the point that it would be highly
   desirable to push the workload back to the IPv6 communication
   endpoints.  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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 12, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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Internet-Draft            IPv6 Path MTU Updates                July 2012


   This document is subject to BCP 78 and the IETF Trust's Legal
   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
   4.  Considerations for Medium MTU Paths . . . . . . . . . . . . . . 4
   5.  Considerations for Large MTU Paths  . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6


























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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 (also known as "link adaptation")
   at a layer below IPv6.  This latter category of links is often
   performance-challenged to accommodate steady-state link-specific
   fragmentation and reassembly to the point that it would be highly
   desirable to push the workload back to the IPv6 communication
   endpoints.  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-layer fragmentation 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 packet before
   sending it out, 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



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   better accommodate paths with various MTUs as described in the
   following sections.


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-specific fragmentation can be burdensome for some links
   (e.g., IPv6-within-IP tunnels), to the point that it would be highly
   desirable for the fragmentation and reassembly to be offloaded 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 a PTB message back to the original source (subject
   to rate limiting) with a Next-Hop MTU less than 1280.  Note that
   these PTB messages are advisory in nature and do not necessarily
   indicate packet loss.

   As a result, in response to an IPv6 packet that is sent to an IPv6
   destination located beyond a link that must perform link-specific
   fragmentation, the originating IPv6 node may receive a PTB message
   reporting a Next-Hop MTU less than 1280.  In that case, the IPv6 node
   should perform IPv6 fragmentation on any subsequent packets that are
   larger than this MTU value but no larger than 1500 bytes using the
   MTU value as the maximum fragment size.  These fragments will be
   reassembled by the final destination, which is required by Section 5
   of [RFC2460] to configure a minimum reassembly buffer size of 1500
   bytes.

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


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.



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   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.


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.





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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.

   [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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