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Default IP MTU for use over ATM AAL5 (RFC1626) Disclosure Number: IPCOM000002461D
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2000-Sep-12
Document File: 5 page(s) / 11K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

R. Atkinson: AUTHOR


Status of this Memo

This text was extracted from a ASCII document.
This is the abbreviated version, containing approximately 25% of the total text.

Network Working Group R. Atkinson

Request for Comments: 1626 Naval Research Laboratory

Category: Standards Track May 1994

Default IP MTU for use over ATM AAL5

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.

Default Value for IP MTU over ATM AAL5

Protocols in wide use throughout the Internet, such as the Network

File System (NFS), currently use large frame sizes (e.g. 8 KB).

Empirical evidence with various applications over the Transmission

Control Protocol (TCP) indicates that larger Maximum Transmission

Unit (MTU) sizes for the Internet Protocol (IP) tend to give better

performance. Fragmentation of IP datagrams is known to be highly

undesirable. [KM87] It is desirable to reduce fragmentation in the

network and thereby enhance performance by having the IP Maximum

Transmission Unit (MTU) for AAL5 be reasonably large. NFS defaults

to an 8192 byte frame size. Allowing for RPC/XDR, UDP, IP, and LLC

headers, NFS would prefer a default MTU of at least 8300 octets.

Routers can sometimes perform better with larger packet sizes because

most of the performance costs in routers relate to "packets handled"

rather than "bytes transferred". So there are a number of good

reasons to have a reasonably large default MTU value for IP over ATM


RFC 1209 specifies the IP MTU over SMDS to be 9180 octets, which is

larger than 8300 octets but still in the same range. [RFC-1209] There

is no good reason for the default MTU of IP over ATM AAL5 to be

different from IP over SMDS, given that they will be the same

magnitude. Having the two be the same size will be helpful in

interoperability and will also help reduce incidence of IP


Therefore, the default IP MTU for use with ATM AAL5 shall be 9180

octets. All implementations compliant and conformant with this

specification shall support at least the default IP MTU value for use

over ATM AAL5.

Permanent Virtual Circuits

Implementations which only support Permanent Virtual Circuits (PVCs)

will (by definition) not implement any ATM signalling protocol. Such

implementations shall use the default IP MTU value of 9180 octets

unless both parties have agreed in advance to use some other IP MTU