RFC 1191 - Path MTU Discovery

Document Information

• RFC Number: 1191
• Obsoletes: RFC 1063
• Authors: J. Mogul (DECWRL), S. Deering (Stanford University)
• Published: November 1990
• Status: Standards Track

Contents

• Abstract
• Acknowledgements
• 1. Introduction
• 2. Protocol Overview
• 3. Host Specification
  • 3.1 TCP MSS Option
• 4. Router Specification
• 5. Host Processing of Old-Style Messages
• 6. Host Implementation
  • 6.1 Layering
  • 6.2 Storing PMTU Information
  • 6.3 Purging Stale PMTU Information
  • 6.4 TCP Layer Actions
  • 6.5 Issues for Other Transport Protocols
  • 6.6 Management Interface
• 7. Likely Values for Path MTUs
  • 7.1 A Better Way to Detect PMTU Increases
• 8. Security Considerations
• References
• Authors' Addresses

Abstract

This memo describes a technique for dynamically discovering the Maximum Transmission Unit (MTU) of an arbitrary internet path. It specifies a small change to the way routers generate one type of ICMP message. For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice.

Acknowledgements

This proposal is a product of the IETF MTU Discovery Working Group.

The mechanism proposed here was first suggested by Geof Cooper [2], who in two short paragraphs set out all the basic ideas that took the working group several months to reinvent.