draft-generic-6man-tunfrag-07:
IPv6 Path MTU Updates
Author(s): Fred Templin
IPv6 intentionally deprecates fragmentation by routers in the network. Instead, links with restricting Maximum Transmission Units (MTUs) must either drop each too-large packet and return an ICMPv6 Packet Too Big (PTB) message or perform link-specific fragmentation and reassembly...
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational January 14, 2013
Expires: July 18, 2013
IPv6 Path MTU Updates
draft-generic-6man-tunfrag-07.txt
Abstract
IPv6 intentionally deprecates fragmentation by routers in the
network. Instead, links with restricting Maximum Transmission Units
(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-in-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.
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This Internet-Draft will expire on July 18, 2013.
Copyright Notice
Copyright (c) 2013 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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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 Maximum Transmission Units
(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-in-IP tunnels
[I-D.generic-v6ops-tunmtu]. 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.
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
configured to perform link adaptation.
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 on 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 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 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 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 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 bytes, but must perform IPv6 fragmentation
on those packets by breaking the packet into N roughly equal-length
pieces (where N is minimized and the length of each piece is smaller
than the Next-Hop MTU). These fragments will be reassembled by the
destination."
3.1. Accommodating Legacy Nodes
Legacy IPv6 nodes observe the current final paragraph of Section 5 of
[RFC2460]:
"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
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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 such legacy 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, a link ingress node that
returns this new kind of PTB message may receive future packets up to
1280 bytes in length and containing a Fragment header with the More
Fragments (MF) bit and Offset field set to 0. The link ingress node
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 link ingress node should fragment these packets (again,
by splitting the packet into N roughly equal-length pieces).
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. The node may
instead invoke fragmentation for packets between 1281 and 1500 bytes
(again, by splitting the packet into N roughly equal-length pieces)
before submitting each fragment for link-layer framing. These
fragments will 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. For example, an
originating IPv6 node that wishes to ensure that packets between 1281
and 1500 bytes will reach the final destination can use "proactive
fragmentation" to fragment each packet (again, by splitting the
packet into N roughly equal-length pieces). Alternatively, the node
can use Packetization Layer Path MTU Discovery (PLPMTUD) per
[RFC4821] to ensure that packets of this size are being delivered to
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 PLPMTUD [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", RFC2460, December 1998.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC4443, March 2006.
9.2. Informative References
[I-D.generic-v6ops-tunmtu]
Templin, F., "Operational Considerations for Tunnel
Fragmentation and Reassembly",
draft-generic-v6ops-tunmtu-12 (work in progress),
November 2012.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC1981, August 1996.
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[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC2923, September 2000.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC4821, 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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