Network Working Group S. Knowles
Request for Comments: 1435 FTP Software
March 1993

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
unlimited.

Abstract

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
Discovery.

Advice on the Deployment of Path MTU Discovery Protocol

While reviewing the Path MTU Discovery Protocol for Draft Standard
[RFC1191], 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.

Author's Note

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.

References

[RFC1191] Mogul, J., and S. Deering, S., "Path MTU Discovery",
RFC1191, DECWRL, Stanford University, November 1990.

Security Considerations

Security issues are not discussed in this memo.

Author's Address

Stev Knowles
ftp Software
2 High Street
North Andover, Ma, 01845

EMail: stev@ftp.com