Default IP MTU for use over ATM AAL5 (RFC1626)
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2000-Sep-12
Internet Society Requests For Comment (RFCs)
Status of this Memo
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
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 Par...