Internet DRAFT - draft-bar-v6ops-ismtu
draft-bar-v6ops-ismtu
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational May 18, 2012
Expires: November 19, 2012
ISATAP Tunnel MTU
draft-bar-v6ops-ismtu-00.txt
Abstract
The ISATAP tunnel MTU is currently recommended to be set to 1480 or
less. This is to avoid IPv4 fragmentation within the tunnel, but
requires the ISATAP tunnel ingress interface to drop any IPv6 packet
larger than 1480 bytes and return an ICMPv6 Packet Too Big (PTB)
message. Concerns for operational issues with both IPv4 and IPv6
Path MTU Discovery point to the possibility of MTU-related black
holes when a packet is dropped due to an MTU restriction, so dropping
packets is considered highly undesirable. Fortunately, the "Internet
cell size" is 1500 bytes, i.e., the minimum MTU configured by the
vast majority of links in the Internet. This document therefore
presents a method to boost the ISATAP MTU to 1500 bytes.
Status of this Memo
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Setting the ISATAP MTU to 1500 Bytes . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
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
The ISATAP tunnel MTU is currently recommended to be set to 1480
[RFC5214]. This is to avoid IPv4 fragmentation within the tunnel
[RFC0791], but requires the ISATAP tunnel ingress interface to drop
any IPv6 packet larger than 1480 bytes and return an ICMPv6 Packet
Too Big (PTB) message [RFC2460]. Concerns for operational issues
with both IPv4 and IPv6 Path MTU Discovery [RFC1191][RFC1981] point
to the possibility of MTU-related black holes when a packet is
dropped due to an MTU restriction, so dropping packets is considered
highly undesirable. Fortunately, the "Internet cell size" is 1500
bytes, i.e., the minimum MTU configured by the vast majority of links
in the Internet, such that 1500 byte or smaller packets are likely to
be delivered without loss to MTU limitations in the vast majority of
cases. This document therefore presents a method to boost the ISATAP
tunnel MTU to 1500 bytes.
Pushing the ISATAP tunnel MTU to 1500 bytes is met with the challenge
that the addition of the IPv4 encapsulation header would cause a 1500
byte IPv6 packet to appear as a 1520 byte IPv4 packet on the wire.
This can result in the packet being either fragmented or dropped by
an IPv4 router that configures a 1500 byte link, depending on the
setting of the "Don't Fragment" (DF) bit in the IPv4 header. Using
the approach outlined in this document, the ISATAP tunnel avoids this
issue by performing IPv6 fragmentation on the inner IPv6 packet
before IPv4 encapsulation. The approach is outlined in the following
sections.
2. Setting the ISATAP MTU to 1500 Bytes
Setting the ISATAP MTU to 1500 bytes is accomplished via the
following algorithm:
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1. set the ISATAP tunnel interface MTU to 1500
2. for IPv6 packets to be admitted into the ISATAP tunnel,
do the following:
a) drop the packet and send an ICMPv6 PTB if it is
1501 or more
b) encapsulate the packet in an IPv4 header and send
it to the tunnel far end if it is 1280 or less
c) if the packet is between 1281 - 1500:
- break it into 2 equal-sized pieces and insert a
fragment header on both pieces
- choose a random 32-bit value and write the value
in the Identification field in both pieces
- encapsulate both pieces in an IPv4 header and
send them to the tunnel far end
3. the IPv6 destination host gets to reassemble if
necessary
3. Discussion
In the algorithm given in Section 2, the ISATAP tunnel interface MTU
for ISATAP hosts and/or routers can be set to 1500 bytes
independently of other ISATAP interfaces within the site, i.e., the
approach is incrementally deployable.
The algorithm in Section 2 further ignores the IPv6 requirement that
routers in the network must not fragment IPv6 packets, i.e.
fragmentation must be performed only by hosts. However, we observe
that the ISATAP host is allowed to perform IPv6 fragmentation. We
further observe that the ISATAP router is tied through stateless
address mapping to a single ISATAP host such that the fragments would
not be delivered to multiple hosts.
The algorithm in Section 2 requires that ISATAP ingress tunnel
endpoint perform IPv6 fragmentation (when necessary), but the ISATAP
egress tunnel endpoint does not perform reassembly; hence, the ISATAP
tunnel endpoints remain stateless. Instead, the final destination
IPv6 host may be obliged to reassemble IPv6 packets up to 1500 bytes
in length, but hosts are required to reassemble at least that much by
the IPv6 standard [RFC2460].
Since the ISATAP tunnel endpoints set the Identification field in
fragmented IPv6 packets to a random 32-but value, there is no
possibility for "serialization" in which an IPv6 host might find the
number of distinct outstanding Identification values reduced due to
the ISATAP tunnel endpoint applying a single serial Identification
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value for all IPv6 hosts. And, the use of a random 32-bit value
would still allow for far more than enough outstanding IPv6 packet
reassemblies without risk of colliding Identification values.
4. IANA Considerations
There are no IANA considerations for this document.
5. Security Considerations
The security considerations for ISATAP apply also to this document.
6. Acknowledgments
This method was inspired through discussion on the IETF v6ops list in
the May 2012 timeframe.
7. References
7.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
7.2. Informative References
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
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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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