IESG Advice from Experience with Path MTU Discovery
Routers that suppress ICMP messages silently break Path MTU Discovery, and the IESG says how to fix it.
Knowles, S., "IESG Advice from Experience with Path MTU Discovery", RFC 1435, DOI 10.17487/RFC1435, March 1993, <https://www.rfc-editor.org/info/rfc1435>.
@misc{rfc1435,
series = {Request for Comments},
number = 1435,
howpublished = {RFC 1435},
publisher = {RFC Editor},
doi = {10.17487/RFC1435},
url = {https://www.rfc-editor.org/info/rfc1435},
author = {S. Knowles},
title = {{IESG Advice from Experience with Path MTU Discovery}},
pagetotal = 2,
year = 1993,
month = mar,
} RFC text
Contents
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.
RFC 1435 IESG Advice from Experience with Path MTU Discovery March 1993
References
[RFC1191] Mogul, J., and S. Deering, S., "Path MTU Discovery",
RFC 1191, 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
AI ELI5
On this page
§1How the Problem Was Found
While reviewing the Path MTU Discovery protocol (RFC 1191) for Draft Standard, the IESG learned from implementors that some vendors had added to their routers the ability to disable ICMP messages generated by the router.
The reason was to protect older BSD hosts, which would drop all connections to a host if they found an ICMP message on any of the connections, even a non-fatal one.
Some routers were told to stop sending warning notes, because some old computers overreacted to every note.
§2Why Path MTU Discovery Breaks
Suppressing ICMP messages protects the older BSD hosts, but it causes MTU discovery to fail in a silent, hard to diagnose way. The sender never learns that its packets are too large for some link along the path.
If nobody tells you the package is too big, you keep sending packages that never arrive and you do not know why.
§3The Recommended Change
Routers should continue to send ICMP message Type 3 code 4 (destination unreachable, don't fragment bit sent and fragmentation required) even when their "don't send ICMP messages" switch is turned on.
This lets path MTU discovery work without affecting the older BSD hosts, since those hosts never set the DF bit in their packets.
The fix: even in quiet mode, a router should still send the one note that says "this package is too big". The old computers never send such packages, so they are not bothered.