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Path MTU discovery (RFC1191)

IP.com Disclosure Number: IPCOM000002004D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2019-Feb-11
Document File: 19 page(s) / 28K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

J.C. Mogul: AUTHOR [+1]

Related Documents

10.17487/RFC1191: DOI

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. [STANDARDS-TRACK]

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 10% of the total text.

Network Working Group J. Mogul Request for Comments: 1191 DECWRL Obsoletes: RFC 1063 S. Deering Stanford University November 1990

Path MTU Discovery

Status of this Memo

This RFC specifies a protocol on the IAB Standards Track for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Table of Contents

Status of this Memo 1 Abstract 2 Acknowledgements 2 1. Introduction 2 2. Protocol overview 3 3. Host specification 4 3.1. TCP MSS Option 5 4. Router specification 6 5. Host processing of old-style messages 7 6. Host implementation 8 6.1. Layering 9 6.2. Storing PMTU information 10 6.3. Purging stale PMTU information 11 6.4. TCP layer actions 13 6.5. Issues for other transport protocols 14 6.6. Management interface 15 7. Likely values for Path MTUs 15 7.1. A better way to detect PMTU increases 16 8. Security considerations 18 References 18 Authors’ Addresses 19

List of Tables

Table 7-1: Common MTUs in the Internet 17

Mogul & Deering [page 1]

RFC 1191 Path MTU Discovery November 1990

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 months to reinvent.

1. Introduction

When one IP host has a large amount of data to send to another host, the data is transmitted as a series of IP datagrams. It is usually preferable that these datagrams be of the largest size that does not require fragmentation anywhere along the path from the source to the destination. (For the case against fragmentation, see [5].) This datagram size is referred to as the Path MTU (PMTU), and it is equal to the minimum of the MTUs of each hop in the path. A shortcoming of the current Internet protocol suite is the lack of a standard mechanism for a host to discover the PMTU of an arbitrary path.

Note: The Path MTU is what in [1] is called the "Effective MTU for sending" (EMTU_S). A PMTU is associated with a path, which is a particular combination of IP source and destination address and perhaps a Type-of-service (TOS).

The current practice [1] is to use the lesser of 576 and the first-hop MTU as the PMTU for any destination that is not connected to the same network or subnet as the source. In many cases, this results in...

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