Default IP MTU for use over ATM AAL5
RFC 1626
| Document | Type |
RFC - Proposed Standard
(May 1994; No errata)
Obsoleted by RFC 2225
Was draft-ietf-ipatm-mtu (ipatm WG)
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|---|---|---|---|
| Last updated | 2013-03-02 | ||
| Stream | IETF | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 1626 (Proposed Standard) | |
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| Telechat date | |||
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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 field contains the
value over the path from the calling party to the called party. The
Backwards Maximum CPCS-SDU Size Identifier field contains the value
over the path from the called party to the calling party. The ATM
Forum specifies the valid values of this identifier as 1 to 65535
inclusive. Note that the ATM Forum's User-to-Network-Interface (UNI)
signalling permits the MTU in one direction to be different from the
MTU in the opposite direction, so the Forward Maximum CPCS-SDU Size
Identifier might have a different value from the Backwards Maximum
CPCS-SDU Size Identifier on the same connection.
If the calling party wishes to use the default MTU it shall still
include the "AAL Parameters" information element with the default
values for the Maximum CPCS-SDU Size as part of the SETUP message of
the ATM signalling protocol [ATMF93b]. If the calling party desires
to use a different value than the default, it shall include the "AAL
Parameters" information element with the desired value for the
Maximum CPCS-SDU Size as part of the SETUP message of the ATM
Signalling Protocol. The called party will respond using the same
information elements and identifiers in its CONNECT message response
[ATMF93c].
If the called party receives a SETUP message containing the "Maximum
CPCS-SDU Size" in the AAL Parameters information element, it shall
handle the Forward and Backward Maximum CPCS-SDU Size Identifier as
follows:
a) If it is able to accept the ATM MTU values proposed by the
SETUP message, it shall include an AAL Parameters information
element in its response. The Forward and Backwards Maximum
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