Browse Prior Art Database

IP MTU discovery options (RFC1063)

IP.com Disclosure Number: IPCOM000001871D
Original Publication Date: 1988-Jul-01
Included in the Prior Art Database: 2000-Sep-12
Document File: 10 page(s) / 25K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

J.C. Mogul: AUTHOR [+4]

Abstract

Although the Internet Protocol allows gateways to fragment packets that are too large to forward, fragmentation is not always desirable. It can lead to poor performance or even total communication failure in circumstances that are surprisingly common. (For a thorough discussion of this issue, see [1]).

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 12% of the total text.

Network Working Group J. Mogul

Request For Comments: 1063 C. Kent

DEC

C. Partridge

BBN

K. McCloghrie

TWG

July 1988

IP MTU Discovery Options

STATUS OF THIS MEMO

A pair of IP options that can be used to learn the minimum MTU of a

path through an internet is described, along with its possible uses.

This is a proposal for an Experimental protocol. Distribution of

this memo is unlimited.

INTRODUCTION

Although the Internet Protocol allows gateways to fragment packets

that are too large to forward, fragmentation is not always desirable.

It can lead to poor performance or even total communication failure

in circumstances that are surprisingly common. (For a thorough

discussion of this issue, see [1]).

A datagram will be fragmented if it is larger than the Maximum

Transmission Unit (MTU) of some network along the path it follows.

In order to avoid fragmentation, a host sending an IP datagram must

ensure that the datagram is no larger than the Minimum MTU (MINMTU)

over the entire path.

It has long been recognized that the methods for discovering the

MINMTU of an IP internetwork path are inadequate. The methods

currently available fall into two categories: (1) choosing small MTUs

to avoid fragmentation or (2) using additional probe packets to

discover when fragmentation will occur. Both methods have problems.

Choosing MTUs requires a balance between network utilization (which

requires the use of the largest possible datagram) and fragmentation

avoidance (which in the absence of knowledge about the network path

encourages the use of small, and thus too many, datagrams). Any

choice for the MTU size, without information from the network, is

likely to either fail to properly utilize the network or fail to

avoid fragmentation.

Probe packets have the problem of burdening the network with

unnecessary packets. And because network paths often change during

the lifetime of a TCP connection, probe packets will have to be sent

on a regular basis to detect any changes in the effective MINMTU.

Implementors sometimes mistake the TCP MSS option as a mechanism for

learning the network MINMTU. In fact, the MSS option is only a

mechanism for learning about buffering capabilities at the two TCP

peers. Separate provisions must be made to learn the IP MINMTU.

In this memo, we propose two new IP options that, when used in

conjunction will permit two peers to determine the MINMTU of the

paths between them. In this scheme, one option is used to determine

t...