Internet DRAFT - draft-liu-6man-tunnel-mtu-config
draft-liu-6man-tunnel-mtu-config
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
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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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