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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


 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
 [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.

Knowles [Page 1] RFC 1435 IESG Advice from Experience with Path MTU Discovery March 1993


 [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

Knowles [Page 2]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1435.txt · Last modified: 1993/03/16 22:39 (external edit)