IPv6 Network Fragmentation for Tunnels
draft-generic-6man-tunfrag-00

Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                             July 03, 2012
Expires: January 4, 2013

                 IPv6 Network Fragmentation for Tunnels
                   draft-generic-6man-tunfrag-00.txt

Abstract

   IPv6 intentionally deprecates fragmentation by routers in the
   network.  An exception to this requirement occurs when there is an
   IPv6/IPv4 protocol translator in the path, where in-the-network
   fragmentation may be unavoidable when the IPv4 network path includes
   a restricting link.  Unfortunately, recent investigations have shown
   that IPv6 Path MTU Discovery (PMTUD) interacts poorly with tunnels to
   the point that tunneled packets can be best accommodated only when
   the tunnel ingress is permitted to perform fragmentation.  This
   document therefore updates the IPv6 protocol specification to enable
   in-the-network fragmentation by routers that configure tunnel
   endpoints.

Status of this Memo

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   This Internet-Draft will expire on January 4, 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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   (http://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  IPv6 Updates  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5

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1.  Introduction

   IPv6 intentionally deprecates fragmentation by routers in the
   network.  An exception to this requirement occurs when there is an
   IPv6/IPv4 protocol translator in the path, where in-the-network
   fragmentation may be unavoidable when the IPv4 network path includes
   a restricting link.  Unfortunately, recent investigations have shown
   that IPv6 Path MTU Discovery (PMTUD) interacts poorly with tunnels
   [I-D.generic-v6ops-tunmtu] to the point that tunneled packets can be
   best accommodated only when the tunnel ingress is permitted to
   perform fragmentation.  This document therefore asserts an additional
   case in which in-the-network IPv6 fragmentation is permitted.

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)
   and should therefore also be the minimum MTU assigned to tunnels.
   However, due to issues with PMTUD this size can only be accommodated
   when the tunnel ingress is permitted to perform fragmentation.  The
   tunnel ingress can perform fragmentation on the outer packet
   following encapsulation and can instead (or in addition) perform
   "tunnel fragmentation" via an encapsulation mid-layer inserted
   between the inner and outer header.  In both cases reassembly would
   be performed by the tunnel egress.

   Unfortunately, the tunnel ingress may not always know the size of the
   reassembly buffer configured by the egress, and the burden for
   reassembly on the egress may be excessive - especially if the egress
   must service many destinations.  The third alternative therefore is
   to permit the tunnel ingress to perform fragmentation on the inner
   packet before encapsulation in which case reassembly would be
   performed by the final destination.  This document therefore updates
   the IPv6 protocol specification [RFC2460] to enable in-the-network
   fragmentation by tunnel routers.

3.  IPv6 Updates

   Section 4.5 of [RFC2460] includes the clause:

   "(Note: unlike IPv4, fragmentation in IPv6 is performed only by
   source nodes, not by routers along a packet's delivery path -- see
   section 5.)"

   This text is already ignored by IPv6/IPv4 translators which must
   perform fragmentation when the IPv4 path beyond the translator

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   includes a restricting link.  This document proposes that this text
   also be relaxed in the case of tunnels that must perform
   fragmentation.

   Section 5 of [RFC2460] states:

   "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."

   This document proposes to add the following text immediately after
   the above:

   "In response to an IPv6 packet that is sent to an IPv6 destination
   that is located beyond a tunnel, 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, but must include a Fragment header in any
   packets that are larger than this MTU value but no larger than 1500
   bytes.  Note that the node is permitted to continue to send packets
   larger than 1500 bytes without including a Fragment header, but
   should implement [RFC4821] to ensure that the packets are reaching
   the final destination."

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

4.  IANA Considerations

   There are no IANA considerations for this document.

5.  Security Considerations

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

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6.  Acknowledgments

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

7.  References

7.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

7.2.  Informative References

   [I-D.generic-v6ops-tunmtu]
              Templin, F., "Operational Issues with Tunnel Maximum
              Transmission Unit (MTU)", draft-generic-v6ops-tunmtu-08
              (work in progress), June 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.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

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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