Subject: WG Review: Path Maximum Transmission Unit Discovery (pmutd)

From: The IESG <[email-masked]>
Date: Tue, 17 Jun 2003 11:13:16 -0400

A new IETF working group has been proposed in the Transport Area.
The IESG has not made any determination as yet. The following description
was submitted, ans is provided for informational purposes only:

 Path Maximum Transmission Unit Discovery (pmutd)
 ------------------------------------------------

 Current Status: Proposed Working Group

 Chair(s):

 Matt Mathis <[email-masked]>
 TBD

 Transport Area Directors(s):

 Allison Mankin <[email-masked]>
 Jon Peterson <[email-masked]>

 Transport Area Advisor:

 Allison Mankin <[email-masked]>

 Mailing List (temporary):

 General Discussion: [email-masked]
 Subscribe: [email-masked] with "subscribe mtu" in the body
 Archive: http://www.psc.edu/~mathis/MTU/mbox.txt

 (This is to be moved to the IETF as soon as chartered).

 Description of Working Group:

 The goal of the PMTUD working group is to specify a robust method for
 determining the IP Maximum Transmission Unit supported over an
 end-to-end path. This new method is expected to update most uses of
 RFC1191 and RFC1981, the current standards track protocols for this
 purpose. Various weakness in the current methods are documented in
 RFC2923, and have proven to be a chronic impediment to the deployment
 of new technologies that alter the path MTU, such as tunnels and new
 types of link layers.

 The proposed new method does not rely on ICMP or other messages from
 the network. It finds the proper MTU by starting a connection using
 relatively small packets (e.g. TCP segments) and searching upwards by
 probing with progressively larger test packets (containing application
 data). If a probe packet is successfully delivered, then the path MTU
 is raised. The isolated loss of a probe packet (with or without an
 ICMP can't fragment message) is treated as an indication of a MTU
 limit, and not a congestion indicator.
 
 The working group will specify the method for use in TCP, SCTP, and
 will outline what is necessary to support the method in transports
 such as DCCP. It will particularly describe the precise conditions
 under which lost packets are not treated as congestion indications.
 The work will pay particular attention to details that affect
 robustness and security.

 Path MTU discovery has the potential to interact with many other parts
 of the Internet, including all link, transport, encapsulation and
 tunnel protocols. Thereforethis working group will particularly
 encourage input from a wide cross section of the IETF to help to
 maximize the robustness of path MTU discovery in the presence of
 pathological behaviors from other components.