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Network Working Group                                        S. KnowlesRequest for Comments: 1435                                 ftp Software                                                             March 1993IESG Advice from Experience with Path MTU DiscoveryStatus 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.Knowles                                                         [Page 1]

RFC 1435  IESG Advice from Experience with Path MTU Discovery March 1993References   [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.comKnowles                                                         [Page 2]