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rfc:rfc1626

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

Atkinson [Page 2] RFC 1626 Default IP MTU for use over ATM AAL5 May 1994

  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.

Atkinson [Page 3] RFC 1626 Default IP MTU for use over ATM AAL5 May 1994

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, RFC 791, DARPA, September
 1981.
 [RFC-793] Postel, J., "Transmission Control Protocol - DARPA
 Internet Program Protocol Specification", STD 7, RFC 793,
 DARPA, September 1981.
 [RFC-1122] Braden, R., Editor, Requirements for Internet Hosts --
 Communications Layers, STD 3, RFC 1122, USC/Information Sciences
 Institute, October 1989, pp.58-60.
 [RFC-1191] Mogul, J., and S. Deering, "Path MTU Discovery",
 RFC 1191, DECWRL, Stanford University, November 1990.
 [RFC-1209] Piscitello, D., and J. Lawrence, "The Transmission of
 IP Datagrams over the SMDS Service", RFC 1209, 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", RFC 1483, Telecom Finland, July 1993.
 [RFC-1577] Laubach, M., "Classical IP and ARP over ATM", RFC 1577,
 Hewlett-Packard Laboratories, January 1994.

Atkinson [Page 4] RFC 1626 Default IP MTU for use over ATM AAL5 May 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
 EMail: atkinson@itd.nrl.navy.mil

Atkinson [Page 5]

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