Path MTU Discovery (pmtud)

Last Modified: 2007-02-16

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

Chair(s):

Transport Area Director(s):

Transport Area Advisor:

Mailing Lists:

General Discussion: pmtud@ietf.org
To Subscribe: pmtud-request@ietf.org
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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)