Packetization Layer Path MTU Discovery
RFC 4821

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Network Working Group                                          M. Mathis
Request for Comments: 4821                                    J. Heffner
Category: Standards Track                                            PSC
                                                              March 2007

                 Packetization Layer Path MTU Discovery

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes a robust method for Path MTU Discovery
   (PMTUD) that relies on TCP or some other Packetization Layer to probe
   an Internet path with progressively larger packets.  This method is
   described as an extension to RFC 1191 and RFC 1981, which specify
   ICMP-based Path MTU Discovery for IP versions 4 and 6, respectively.

Mathis & Heffner            Standards Track                     [Page 1]
RFC 4821         Packetization Layer Path MTU Discovery       March 2007

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Layering . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Accounting for Header Sizes  . . . . . . . . . . . . . . . 10
     5.2.  Storing PMTU Information . . . . . . . . . . . . . . . . . 11
     5.3.  Accounting for IPsec . . . . . . . . . . . . . . . . . . . 12
     5.4.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Common Packetization Properties  . . . . . . . . . . . . . . . 13
     6.1.  Mechanism to Detect Loss . . . . . . . . . . . . . . . . . 13
     6.2.  Generating Probes  . . . . . . . . . . . . . . . . . . . . 13
   7.  The Probing Method . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Packet Size Ranges . . . . . . . . . . . . . . . . . . . . 14
     7.2.  Selecting Initial Values . . . . . . . . . . . . . . . . . 16
     7.3.  Selecting Probe Size . . . . . . . . . . . . . . . . . . . 17
     7.4.  Probing Preconditions  . . . . . . . . . . . . . . . . . . 18
     7.5.  Conducting a Probe . . . . . . . . . . . . . . . . . . . . 18
     7.6.  Response to Probe Results  . . . . . . . . . . . . . . . . 19
       7.6.1.  Probe Success  . . . . . . . . . . . . . . . . . . . . 19
       7.6.2.  Probe Failure  . . . . . . . . . . . . . . . . . . . . 19
       7.6.3.  Probe Timeout Failure  . . . . . . . . . . . . . . . . 20
       7.6.4.  Probe Inconclusive . . . . . . . . . . . . . . . . . . 20
     7.7.  Full-Stop Timeout  . . . . . . . . . . . . . . . . . . . . 20
     7.8.  MTU Verification . . . . . . . . . . . . . . . . . . . . . 21
   8.  Host Fragmentation . . . . . . . . . . . . . . . . . . . . . . 22
   9.  Application Probing  . . . . . . . . . . . . . . . . . . . . . 23
   10. Specific Packetization Layers  . . . . . . . . . . . . . . . . 23
     10.1. Probing Method Using TCP . . . . . . . . . . . . . . . . . 23
     10.2. Probing Method Using SCTP  . . . . . . . . . . . . . . . . 25
     10.3. Probing Method for IP Fragmentation  . . . . . . . . . . . 26
     10.4. Probing Method Using Applications  . . . . . . . . . . . . 27
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 28
     12.2. Informative References . . . . . . . . . . . . . . . . . . 29
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 31

Mathis & Heffner            Standards Track                     [Page 2]
RFC 4821         Packetization Layer Path MTU Discovery       March 2007

1.  Introduction

   This document describes a method for Packetization Layer Path MTU
   Discovery (PLPMTUD), which is an extension to existing Path MTU
   Discovery methods described in [RFC1191] and [RFC1981].  In the
   absence of ICMP messages, the proper MTU is determined by starting
   with small packets and probing with successively larger packets.  The
   bulk of the algorithm is implemented above IP, in the transport layer
   (e.g., TCP) or other "Packetization Protocol" that is responsible for
   determining packet boundaries.

   This document does not update RFC 1191 or RFC 1981; however, since it
   supports correct operation without ICMP, it implicitly relaxes some
   of the requirements for the algorithms specified in those documents.

   The methods described in this document rely on features of existing