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]