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.

RFC1209 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.

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
CPCS-SDU Size fields shall be present and their values shall be
equal to the corresponding values in the SETUP message.

b) If it wishes a smaller ATM MTU size than that proposed, then
it shall set the values of the Maximum CPCS-SDU Size in the AAL
Parameters information elements equal to the desired value in the
CONNECT message responding to the original SETUP message.

c) If the calling endpoint receives a CONNECT message that does
not contain the AAL Parameters Information Element, but the
corresponding SETUP message did contain the AAL Parameters
Information Telement (including the forward and backward CPCS-SDU
Size fields), it shall clear the call with cause "AAL Parameters
cannot be supported".

d) If either endpoint receives a STATUS message with cause
"Information Element Non-existent or Not Implemented" or cause
""Access Information Discarded", and with a diagnostic field
indicating the AAL Parameters Information Element identifier, it
shall clear the call with cause "AAL Parameters cannot be
supported."

e) If either endpoint receives CPCS-SDUs in excess of the
negotiated MTU size, it may use IP fragmentation or may clear the
call with cause "AAL Parameters cannot be supported". In this
case, an error has occurred either due to a fault in an end
system or in the ATM network. The error should be noted by ATM
network management for human examination and intervention.

If the called endpoint incorrectly includes the Forward and Backward
Maximum CPCS-SDU Size fields in the CONNECT messages (e.g. because
the original SETUP message did not include these fields) or it sets
these fields to an invalid value, then the calling party shall clear
the call with cause "Invalid Information Element Contents".

Path MTU Discovery Required

The Path MTU Discovery mechanism is an Internet Standard [RFC-1191]
and is an important mechanism for reducing IP fragmentation in the
Internet. This mechanism is particularly important because new
subnet ATM uses a default MTU sizes significantly different from
older subnet technologies such as Ethernet and FDDI.

In order to ensure good performance throughout the Internet and also
to permit IP to take full advantage of the potentially larger IP
datagram sizes supported by ATM, all routers implementations that
comply or conform with this specification must also implement the IP
Path MTU Discovery mechanism as defined in RFC-1191 and clarified by
RFC-1435. Host implementations should implement the IP Path MTU
Discovery mechanism as defined in RFC-1191.

Applicability Statement

The Multiprotocol Encapsulation over ATM AAL5 defined in RFC-1483 is
not specific to any model of IP and ATM interaction. [RFC-1483]
Similarly, this specification is general enough to apply to all
models for use of IP over ATM AAL5. Use of this specification is
recommended for all implementatons of IP over ATM AAL5 in order to
increase interoperability and performance. This specification does
not conflict with the Classical IP over ATM specification and may be
used as a conforming extension to that specification. [RFC-1577]
Applicability of this draft is not limited to the Classical IP over
ATM model.

Security Considerations

Security issues are not discussed in this memo.

References

[RFC-791] Postel, J., "Internet Protocol - DARPA Internet Program
Protocol Specification", STD 5, RFC791, DARPA, September
1981.

[RFC-793] Postel, J., "Transmission Control Protocol - DARPA
Internet Program Protocol Specification", STD 7, RFC793,
DARPA, September 1981.

[RFC-1122] Braden, R., Editor, Requirements for Internet Hosts --
Communications Layers, STD 3, RFC1122, USC/Information Sciences
Institute, October 1989, pp.58-60.

[RFC-1191] Mogul, J., and S. Deering, "Path MTU Discovery",
RFC1191, DECWRL, Stanford University, November 1990.

[RFC-1209] Piscitello, D., and J. Lawrence, "The Transmission of
IP Datagrams over the SMDS Service", RFC1209, Bell Communications
Research, March 1991.

[RFC-1435] Knowles, S., "IESG Advice from EXPerience with Path MTU
Discovery, RFC-1435, IESG, March 1993.

[RFC-1483] Heinanen, J., "Multiprotocol Encapsulation over ATM
Adapatation Layer 5", RFC1483, Telecom Finland, July 1993.

[RFC-1577] Laubach, M., "Classical IP and ARP over ATM", RFC1577,
Hewlett-Packard Laboratories, January 1994.

[ATMF93a] Breault, R., Grace, J., Jaeger, J., and L. Wojnaroski,
Editors, "ATM Forum User Network Interface Specification", Version
3.0, Section 5.4.5.5, p. 194-200, 10 September 1993, ATM Forum.

[ATMF93b] Breault, R., Grace, J., Jaeger, J., and L. Wojnaroski,
Editors, "ATM Forum User Network Interface Specification", Version
3.0, Section 5.3.1.7, p. 171-172, 10 September 1993, ATM Forum.

[ATMF93c] Breault, R., Grace, J., Jaeger, J., and L. Wojnaroski,
Editors, "ATM Forum User Network Interface Specification", Version
3.0, Section 5.3.1.3, p. 168, 10 September 1993, ATM Forum.

[KM87] Kent C., and J. Mogul, "Fragmentation Considered Harmful",
Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers in
Computer Communications Technology, August 1987.

Acknowledgements

While all members of the IETF's IP over ATM Working Group have been
helpful, Vern Schryver, Rob Warnock, Craig Partridge, Subbu
Subramaniam, and Bryan Lyles have been especially helpful to the
author in analysing the host and routing implications of the default
IP MTU value. Similarly, Dan Grossman provided significant review
and help in ensuring alignment of this text with the related work in
the ATM Forum and ITU.

Disclaimer

Author's organisation provided for identification purposes only.
This document presents the author's views and is not necessarily the
official opinion of his employer.

Author's Address

Randall J. Atkinson
Information Technology Division
Naval Research Laboratory
Washington, DC 20375-5320
USA