Technical Summary
This document describes a robust method for Path MTU Discovery
("Packetization Layer Path MTU Discovery" or PLPMTUD) that
relies on TCP or some other Packetization Layer to probe an Internet
path with progressively larger packets. This method is described as
an extension to RFC 1191 and RFC 1981, which specify ICMP based Path
MTU Discovery for IP versions 4 and 6, respectively.
The general strategy of the new algorithm is to start with a small
MTU and search upward, testing successively larger MTUs by probing
with single packets. If a probe is successfully delivered then the
MTU can be raised. If the probe is lost, it is treated as an MTU
limitation and not as a congestion signal.
PLPMTUD introduces some flexibility in the implementation of
classical Path MTU discovery. If can be configured to perform just
ICMP black hole recovery to increase the robustness of classical Path
MTU Discovery, or at the other extreme, all ICMP processing can be
disabled and PLPMTUD can completely replace classical Path MTU
Discovery.
Working Group Summary
The working group feels that an update to path MTU discovery is needed
to rectify problems with current "classical" path MTU discovery that
occur in today's network deployments (which result in Path MTU "black
holes", and the failure of not only the algorithm but often the entire
connection). The document has had considerable review by the working
group over its eight revisions, and the comments have been
incorporated in the current draft. Reviewers have had background in
TCP, SCTP, DCCP, the use of tunnels (ipsec and other) and IPv6. The
WGLC version has had four careful reviews that only revealed nits and
clarifications that are fixed in this version.
Protocol Quality
The current protocol has one implementation in Linux, by a co-author,
and another independent implementation in a user-space transport
protocol. Previous versions have had implementation in Linux, NetBSD,
and FreeBSD, by different people, resulting in comments that contributed
to document changes. Other operating systems vendors and tunnel vendors
have reviewed the document.
Matt Zekauskas (matt@internet2.edu) acted as Document Shepherd.
Lars Eggert (eggert@netlab.nec.de) has reviewed this document for the
IESG.