Network Working Group S. Knowles
Request for Comments: 1435
IESG Advice from Experience with Path MTU Discovery
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
In the course of reviewing the MTU Discovery protocol for possible
elevation to Draft Standard, a specific operational problem was
uncovered. The problem results from the optional suppression of ICMP
messages implemented in some routers. This memo outlines a
modification to this practice to allow the correct functioning of MTU
Advice on the Deployment of Path MTU Discovery Protocol
While reviewing the Path MTU Discovery Protocol for Draft Standard
], the Internet Engineering Steering Group (IESG) became
aware from the reports of various implementors that some vendors have
added to their routers the ability to disable ICMP messages generated
by the router. This is to protect older BSD hosts, which would drop
all connections to a host it found an ICMP message on any of the
connections, even if it was a non-fatal ICMP message. While this
protects older BSD hosts, it causes MTU discovery to fail in a
silent, hard to diagnose way.
From the descriptions the IESG has obtained, adjusting the routers to
continue to send ICMP message Type 3 code 4 (destination unreachable,
don't fragment (DF) bit sent and fragmentation required) even when
they have their "don't send ICMP messages" switch turned on would
allow path MTU discovery to work but not effect older BSD hosts,
since they never set the DF bit in their packets.
This document was the result of an IESG meeting discussing MTU
Discovery. This author was chosen to write the document as the
Internet Engineering Task Force (IETF) Internet Area Director.
] Mogul, J., and S. Deering, S., "Path MTU Discovery", RFC 1191
, DECWRL, Stanford University, November 1990.
Security issues are not discussed in this memo.
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