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Network Working Group                                             Y. Cui
Internet-Draft                                                    C. Liu
Intended status: Standards Track                     Tsinghua University
Expires: April 24, 2014                                 October 21, 2013


                     IPv6 Tunnel MTU Configuration
                  draft-liu-6man-tunnel-mtu-config-00

Abstract

   It is not specific about how to decide IPv6 tunnel MTU in IPv6
   tunneling mechanisms in some situations.  This document describes the
   problem and provides a general solution to decide tunnel MTU value.

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
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   This Internet-Draft will expire on April 24, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   5.  Tunnel MTU Configuration  . . . . . . . . . . . . . . . . . .   3
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   IPv6 tunneling mechanism defined in [RFC2473] provides support for
   various protocols to work in IPv6-only network.  Because IPv6
   intermediate routers do not support fragmentation, a IPv6 tunnel
   packet may be discarded by an intermediate router if the packet size
   exceeds the next-hop MTU.  Thus, the tunnel MTU of IPv6 tunnel nodes
   should be well decided and managed in order to eliminate data loss.

   But [RFC2473] is not specific about how to decide tunnel MTU, when a
   tunnel entry-point connects to multiple tunnel exit-points.  It is
   also not clear about how to decide tunnel MTU without Path MTU
   Discovery [RFC1981].  This document describes the problems and
   proposes a solution to specify the behavior of tunnel entry-point to
   configure its tunnel MTU.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Terminology

   Terminology defined in [RFC2473] is used extensively in this
   document.

4.  Problem Statement

   Section 6.7 of [RFC2473] defines the behavior of the IPv6 tunnel
   entry-point to set its tunnel MTU that:

     "The tunnel MTU is set dynamically to the Path MTU between the
     tunnel entry-point and the tunnel exit-point nodes, minus the size



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     of the tunnel headers"

     "The tunnel entry-point node performs Path MTU discovery on the
     path between the tunnel entry-point and exit-point nodes"

   However, it is unspecific about how a tunnel entry-point sets its
   tunnel MTU without performing Path MTU discovery.  As IPv6 tunneling
   is the foundation of several IPv6 transition mechanisms, but some of
   these mechanisms require tunnel end-nodes not to perform Path MTU
   discovery.  DS-Lite [RFC6333] handles this by increasing the MTU size
   of all the links in the path by at least 40 bytes.  MAP-E
   [I-D.ietf-softwire-map] strongly recommends to well manage the MTU of
   links in the whole MAP domain, which is similar to what DS-Lite does,
   and specifies that "A MAP BR SHOULD NOT by default use Path MTU
   discovery across the MAP domain" in section 8.3.1 of
   [I-D.ietf-softwire-map].

   In [RFC2473], tunnel MTU is defined as the Path MTU between the
   tunnel entry-point and the tunnel exit-point nodes minus the size of
   the tunnel header.  In some cases, a single tunnel entry-point may
   connect to multiple tunnel exit-points, e.g. the IPv6 address of the
   tunnel exit-point is a multicast or an anycast address, or the tunnel
   entry-point works as an AFTR element [RFC6333].

   Figure 1 shows an example that a tunnel entry-point is connecting to
   2 tunnel exit-points.  When the tunnel entry-point sends a tunnel
   packet of length 1500 to tunnel exit-point1, the packet is discarded
   by the intermediate router and the router sends an ICMPv6 "Packet Too
   Big"(PTB) message to tunnel entry-point with the MTU field equals
   1280.  After received the ICMPv6 PTB message, the tunnel entry- point
   sets its tunnel MTU to 1280-40=1240 according to section 6.7 of
   [RFC2473].  After that, when the tunnel entry-point sends tunnel
   packets to tunnel exit-point2, the tunnel packet size will be
   restricted to 1280.  This is inefficient.

                                                 +-------------+  +-------+
                                        MTU=1280 | IPv6 Tunnel |  | IPv4  |
                                          +------+ exit-point1 +--+ host2 |
          MTU=1500             MTU=1500   |      +-------------+  +-------+
              |                   |       |
   +-------+  |  +-------------+  |  +----+---+          +-------------+
   | IPv4  |  v  | IPv6 Tunnel |  v  |  IPv6  | MTU=1500 | IPv6 Tunnel |
   | host1 +-----+ entry-point +-----+ router +----------+ exit-point2 |
   +-------+     +-------------+     +--------+          +-------------+

              Figure 1: An Example of IPv6 Tunneling Scenario

5.  Tunnel MTU Configuration



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   An IPv6 tunnel entry-point node SHOULD perform Path MTU discovery
   dynamically on the paths between the tunnel entry-point itself and
   each exit-point node it connects to.  The PMTU information SHOULD be
   stored in a table indexed by the destination IPv6 address as is
   described in section 5.2 of [RFC1981].  When a packet enters the
   tunnel, the tunnel entry-point looks up the PMTU table for the
   corresponding PMTU value.  If not found, it uses the MTU of IPv6
   next-hop link as the default value.  This PMTU value minus the size
   of the tunnel headers is set as the tunnel MTU value.

   If the IPv6 tunnel entry-point node does not perform Path MTU
   discovery to decide its tunnel MTU, the network operator MUST
   estimate a safe MTU value and configure this value as the tunnel MTU.
   This safe MTU value should be no larger than the minimum Path MTU
   between the tunnel node and every potential tunnel exit-point, minus
   the size of tunnel headers.  If the operator can not decide this
   value, the tunnel MTU SHOULD be set to 1280 minus the size of tunnel
   headers.

6.  Security Considerations

   TBD

7.  IANA Considerations

   This document does not include an IANA request.

8.  References

8.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, 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.

8.2.  Informative References

   [I-D.ietf-softwire-map]
              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S.,



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              Murakami, T., and T. Taylor, "Mapping of Address and Port
              with Encapsulation (MAP)", draft-ietf-softwire-map-08
              (work in progress), August 2013.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

Authors' Addresses

   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R.China

   Phone: +86-10-6260-3059
   Email: yong@csnet1.cs.tsinghua.edu.cn


   Cong Liu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R.China

   Phone: +86-10-6278-5822
   Email: gnocuil@gmail.com




















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