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.





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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.




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   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.




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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.





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  [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: [email protected]










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