Path MTU Discovery (pmtud)

Last Modified: 2007-02-16

Additional information is available at tools.ietf.org/wg/pmtud

Chair(s):

  • Matt Mathis <mathis@psc.edu>

  • Matthew Zekauskas <matt@internet2.edu>

    Transport Area Director(s):

  • Magnus Westerlund <magnus.westerlund@ericsson.com>
  • Lars Eggert <lars.eggert@nokia.com>

    Transport Area Advisor:

  • Lars Eggert <lars.eggert@nokia.com>

    Mailing Lists:

    General Discussion: pmtud@ietf.org
    To Subscribe: pmtud-request@ietf.org
    In Body: In Body: subscribe email_address
    Archive: http://www.ietf.org/mail-archive/web/pmtud/index.html

    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.
                                                                         
                       
    Input draft:
                                                                         
                       
                    Packetization Layer Path MTU Discovery
                    draft-mathis-plpmtud-00.txt

    Goals and Milestones:

    Done  Reorganized Internet-Draft. Solicit implementation and field experience.
    Done  Update Internet-Draft incorporating implementers experience,
    Done  Submit completed Internet-draft and a PMTUD MIB draft for Proposed Standard.

    No Current Internet-Drafts

    Request For Comments:

    Packetization Layer Path MTU Discovery (RFC 4821) (75665 bytes)
    IETF Secretariat - Please send questions, comments, and/or suggestions to ietf-web@ietf.org.

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