Path MTU Discovery (pmtud)
working group status page

This is the unofficial status page for the Path MTU Discovery (pmtud) working group. For official IETF information please see the IETF charter page.

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, without relying on ICMP or other messages from the network. This method 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. This new method is not subject to the weakness in the current standards (RFC1191 and RFC1981) as documented in RFC2923.

Working Group Status:

The official WG charter and mailing list subscription page.

We are not planing to meet at IETF 66.

Document Status:

Unless we have missed something, the document is ready for WGLC.

The most recent IETF document is draft-ietf-pmtud-method-07.txt, [Local txt copy]

See below for earlier document versions.

Implementation Status:

  • Linux release expected ca 2.6.27

  • The following implementation were written to a pre-IETF draft [mathis-MSS-discovery] (Nov 2002).

  • Kevin Lahey has a preliminary implementation of Path MSS Discovery for netBSD. His web page includes a patch file and a sample tcpdump. The dump is really cool, because is it a CPU bound LAN trace (older CPU's), that noticeably speeds up as it raises the MTU. Kevin's implementation is subject to the standard BSD license language.

  • John Heffner has a preliminary implementation of Path MSS Discovery for Linux. John and Kevin's implementations are completely independent per the IETF rules on standards promotion.

  • Yoshifumi Nishida has a preliminary implementation of Path MSS Discovery for the ns2 simulator. This is likely to be useful for testing MTU searching heuristics.

    This page is It is maintained as part of Matt Mathis' project to push up the Internet MTU.

    For additional information check out these pages: Raising the Internet MTU, Pittsburgh Supercomputing Center, Network research at PSC, or Matt Mathis. Please send comments and suggestions to