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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: 2019-Feb-12
Document File: 5 page(s) / 7K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

R. Atkinson: AUTHOR

Related Documents

10.17487/RFC1626: DOI


There are a number of good reasons to have a reasonably large default MTU value for IP over ATM AAL5. This paper presents the default IP MIU for use over ATM AAL5. [STANDARDS-TRACK]

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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 AAL5.

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 fragmentation.

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.

Atkinson [Page 1]

RFC 1626 Default IP MTU for use over ATM AAL5 May 1994

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 value via some mechanism not specified here.

Switched Virtual Circuits

Implementations that support Switched Virtual Circuits (SVCs) MUST attempt to negotiate the AAL CPCS-SDU size using the ATM signalling protocol. The industry standard ATM signalling protocol uses two different parts of the Information Element named "AAL Parameters" to exchange information on the MTU over the ATM circuit being setup [ATMF93a]. The Forward Maximum CPCS-SDU Size f...