Packetization Layer Path MTU Discovery
RFC 4821
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RFC - Proposed Standard
(March 2007; Errata)
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2015-10-14
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IETF
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RFC 4821 (Proposed Standard)
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Unknown
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Lars Eggert
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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
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