RFC 1435

IESG Advice from Experience with Path MTU Discovery

📅 published March 1993, from ietf

RFC 1435: IESG Advice from Experience with Path MTU Discovery 🎀

Okay bestie, this is RFC 1435 🦋 It's a little memo from the Network Working Group written by Stev Knowles of ftp Software, dated March 1993 ✨ Its status is that it provides information for the Internet community and does not specify an Internet standard. The header has no Obsoletes or Updates lines, so it doesn't replace or modify any other RFC 🌸

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 ✨

Okay so while people were reviewing the MTU Discovery protocol to see if it could be promoted to Draft Standard, a specific operational problem popped up 😭 It comes from routers that have an optional feature to suppress ICMP messages. This memo describes a change to that practice so MTU Discovery can actually work 💖

Advice on the Deployment of Path MTU Discovery Protocol 🧚

So while the IESG was reviewing the Path MTU Discovery Protocol for Draft Standard [RFC1191], it heard from implementors that some vendors had added a way for their routers to disable the ICMP messages those routers generate 👀 The reason is to protect older BSD hosts. Those hosts would drop all of their connections to a host whenever they found an ICMP message on any one of those connections, even a non-fatal one 💀 Protecting those hosts is great, but the side effect is that MTU discovery fails in a silent, hard-to-diagnose way 🙈

Based on the descriptions the IESG collected, if routers keep sending ICMP message Type 3 code 4 (destination unreachable, don't fragment (DF) bit sent and fragmentation required) even when their "don't send ICMP messages" switch is on, then path MTU discovery can work 🔥 And it wouldn't affect older BSD hosts, since they never set the DF bit in their packets 💅

Author's Note 📝

This document came out of an IESG meeting that discussed MTU Discovery. The Internet Engineering Task Force (IETF) Internet Area Director picked this author to write it 🫶

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 💜

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