draft-ietf-ipngwg-pmtuv6-01.txt   draft-ietf-ipngwg-pmtuv6-02.txt 
INTERNET-DRAFT J. McCann, Digital Equipment Corporation INTERNET-DRAFT J. McCann, Digital Equipment Corporation
February 21, 1996 S. Deering, Xerox PARC April 22, 1996 S. Deering, Xerox PARC
J. Mogul, Digital Equipment Corporation J. Mogul, Digital Equipment Corporation
Path MTU Discovery for IP version 6 Path MTU Discovery for IP version 6
draft-ietf-ipngwg-pmtuv6-01.txt draft-ietf-ipngwg-pmtuv6-02.txt
Abstract Abstract
This document describes Path MTU Discovery for IP version 6. It is This document describes Path MTU Discovery for IP version 6. It is
largely derived from RFC-1191, which describes Path MTU Discovery for largely derived from RFC-1191, which describes Path MTU Discovery for
IP version 4. IP version 4.
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
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To learn the current status of any Internet-Draft, please check the To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
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ftp.isi.edu (US West Coast). ftp.isi.edu (US West Coast).
Distribution of this document is unlimited. Distribution of this document is unlimited.
Expiration Expiration
August 21, 1996 October 22, 1996
Contents Contents
Abstract........................................................1 Abstract........................................................1
Status of this Memo.............................................1 Status of this Memo.............................................1
Contents........................................................2 Contents........................................................2
1. Introduction.................................................3 1. Introduction.................................................3
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When one IPv6 node has a large amount of data to send to another When one IPv6 node has a large amount of data to send to another
node, the data is transmitted in a series of IPv6 packets. It is node, the data is transmitted in a series of IPv6 packets. It is
usually preferable that these packets be of the largest size that can usually preferable that these packets be of the largest size that can
successfully traverse the path from the source node to the successfully traverse the path from the source node to the
destination node. This packet size is referred to as the Path MTU destination node. This packet size is referred to as the Path MTU
(PMTU), and it is equal to the minimum link MTU of all the links in a (PMTU), and it is equal to the minimum link MTU of all the links in a
path. IPv6 defines a standard mechanism for a node to discover the path. IPv6 defines a standard mechanism for a node to discover the
PMTU of an arbitrary path. PMTU of an arbitrary path.
IPv6 nodes SHOULD implement Path MTU Discovery in order to discover
and take advantage of paths with PMTU greater than the IPv6 minimum
link MTU [IPv6-SPEC]. A minimal IPv6 implementation (e.g., in a boot
ROM) may choose to omit implementation of Path MTU Discovery.
Nodes not implementing Path MTU Discovery use the IPv6 minimum link Nodes not implementing Path MTU Discovery use the IPv6 minimum link
MTU defined in [IPv6-SPEC] as the maximum packet size. In most MTU defined in [IPv6-SPEC] as the maximum packet size. In most
cases, this will result in the use of smaller packets than necessary, cases, this will result in the use of smaller packets than necessary,
because most paths have a PMTU greater than the IPv6 minimum link because most paths have a PMTU greater than the IPv6 minimum link
MTU. A node sending packets much smaller than the Path MTU allows is MTU. A node sending packets much smaller than the Path MTU allows is
wasting network resources and probably getting suboptimal throughput. wasting network resources and probably getting suboptimal throughput.
2. Terminology 2. Terminology
node - a device that implements IPv6. node - a device that implements IPv6.
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multicast destination. multicast destination.
Note that Path MTU Discovery must be performed even in cases where a Note that Path MTU Discovery must be performed even in cases where a
node "thinks" a destination is attached to the same link as itself. node "thinks" a destination is attached to the same link as itself.
In a situation such as when a neighboring router acts as proxy [ND] In a situation such as when a neighboring router acts as proxy [ND]
for some destination, the destination can to appear to be directly for some destination, the destination can to appear to be directly
connected but is in fact more than one hop away. connected but is in fact more than one hop away.
4. Protocol Requirements 4. Protocol Requirements
As discussed in section 1, IPv6 nodes are not required to implement
Path MTU Discovery. The requirements in this section apply only to
those implementations that include Path MTU Discovery.
When a node receives a Packet Too Big message, it MUST reduce its When a node receives a Packet Too Big message, it MUST reduce its
estimate of the PMTU for the relevant path, based on the value of the estimate of the PMTU for the relevant path, based on the value of the
MTU field in the message. The precise behavior of a node in this MTU field in the message. The precise behavior of a node in this
circumstance is not specified, since different applications may have circumstance is not specified, since different applications may have
different requirements, and since different implementation different requirements, and since different implementation
architectures may favor different strategies. architectures may favor different strategies.
After receiving a Packet Too Big message, a node MUST attempt to After receiving a Packet Too Big message, a node MUST attempt to
avoid eliciting more such messages in the near future. The node MUST avoid eliciting more such messages in the near future. The node MUST
reduce the size of the packets it is sending along the path. Using a reduce the size of the packets it is sending along the path. Using a
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Jeffrey Mogul Jeffrey Mogul
Digital Equipment Corporation Western Research Laboratory Digital Equipment Corporation Western Research Laboratory
250 University Avenue 250 University Avenue
Palo Alto, CA 94301 Palo Alto, CA 94301
Phone: +1 415 617 3304 Phone: +1 415 617 3304
Email: mogul@pa.dec.com Email: mogul@pa.dec.com
Expiration Expiration
August 21, 1996 October 22, 1996
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