One document matched: draft-generic-6man-tunfrag-06.txt
Differences from draft-generic-6man-tunfrag-05.txt
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational July 16, 2012
Expires: January 17, 2013
IPv6 Path MTU Updates
draft-generic-6man-tunfrag-06.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 and reassembly (also known as
"link adaptation") at a layer below IPv6. This latter category of
links is often performance-challenged to accommodate steady-state
link adaptation. 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 17, 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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Provisions Relating to IETF Documents
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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. Considerations for Small MTU Paths . . . . . . . . . . . . . . 4
3.1. Accommodating Legacy Nodes . . . . . . . . . . . . . . . . 4
4. Considerations for Medium MTU Paths . . . . . . . . . . . . . . 5
5. Considerations for Large MTU Paths . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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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 and reassembly (also known as
"link adaptation") at a layer below IPv6. This latter category of
links is often performance-challenged to accommodate steady-state
link adaptation. 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 adaptation 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 for the packets it
sends, 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
better accommodate paths with various MTUs as described in the
following sections.
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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."
and:
"A node must be able to accept a fragmented packet that, after
reassembly, is as large as 1500 octets.".
This document does not propose to change these requirements, but
notes that link adaptation can be burdensome for some links (e.g.,
IPv6-within-IP tunnels) to the point that it would be highly
desirable to push the fragmentation and reassembly responsibility 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 an ICMPv6 PTB message back to the original source
(subject to rate limiting) with a Next-Hop MTU less than 1280 and
with a Code field set to 1 [RFC4443]. (Note that these PTB messages
are advisory in nature and do not necessarily indicate packet loss.)
As a result, the originating IPv6 node may receive this "new kind" of
PTB message and should modify its behavior accordingly. This is
accomplished by adding a new final paragraph to Section 5 of
[RFC2460] as follows:
"In response to an IPv6 packet that is sent to a destination located
beyond an IPv6-within-IP tunnel or an IPv6 link that must perform
link adaptation, the originating IPv6 node may receive an ICMP Packet
Too Big message reporting a Next-Hop MTU less than 1280 and with
Code=1. In that case, the IPv6 node is not required to reduce the
size of subsequent packets to less than 1500, but must perform IPv6
fragmentation on those packets using the Next-Hop MTU as the maximum
fragment size. These fragments will be reassembled by the
destination."
An example tunnel protocol that invokes this behavior appears in:
[I-D.templin-intarea-seal].
3.1. Accommodating Legacy Nodes
Legacy IPv6 nodes observe the current final paragraph of Section 5 of
[RFC2460]:
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"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."
For those nodes, the receipt of a PTB message with a Next-Hop MTU
less than 1280 will result in the above behavior regardless of the
value in the Code field. As a result, an IPv6 link that returns this
new kind of PTB message may receive future packets up to 1280 bytes
in length and containing a Fragment header with MF=0 and Offest=0.
The link should process these packets as an indication that the
originating IPv6 node is a legacy node, and should not send further
PTB messages. Instead, the node should break these packets into two
pieces that are each significantly smaller than 1280 bytes in case
there may be additional layers of encapsulation in the path.
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 between 1281 and 1500 bytes using a
maximum fragment size of 1280 bytes. These fragments will again be
reassembled by the final destination.
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 will reach the destination can
use "proactive fragmentation" to fragment the packets using a maximum
fragment size of 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.
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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.
[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.
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.
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[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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