Network Working Group Randall J. Atkinson Request for Comments: DRAFT Naval Research Laboratory 11 June 1993 Default IP MTU for use over ATM Adaptation Layer 5 (AAL 5) Status of this Memo Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. This particular draft is a working document of the IETF's "IP over ATM" working group. Distribution of this memo is unlimited. Internet Drafts are draft documents valid for a maximum of six months. This Internet Draft expires on 11 December 1993. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress". Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of this or any other Internet Draft. 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. Empirical evidence with various applications over TCP indicates that larger MTU sizes tend to give better performance. 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 costs in routers relate to "packets handled" rather than "bytes transferred". Some recent research into gigabit IP routers appears to indicate routing performance is much better with larger MTU sizes (e.g. near 8K octets) rather than smaller MTU sizes (e.g. 1500 octets). 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 Atkinson [Page 1] Internet Draft 11 June 1993 different from IP over SMDS, given that they will be the same magnitude. Fragmentation between IP over SMDS and IP over AAL5 will be reduced in the most common case by selecting the same default MTU value for both. Therefore, the default MTU for IP over ATM AAL5 shall be 9180 octets. All implementations compliant and conformant with this specification shall support this default IP MTU value for use over ATM AAL5. Minimum Value for IP MTU over ATM AAL5 The smallest acceptable value for the IP MTU for use over ATM AAL5 is 576 octets, which is necessary to conform with the requirements of the Host Requirements RFC and is consistent with the IP specification. [RFC-1122, RFC-791] Use of such a small IP MTU value is not generally recommended. Before such an MTU value may be used, the requirements described in the following section must be adhered to. MTU Negotation for ATM AAL5 MTU Negotiation is an optional procedure that may be used to establish an MTU other than the default by mututal agreement of the two endpoints of the ATM connection. This optional procedure uses the standard ATM signalling mechanisms (e.g. ITU's Q.93B protocol) rather than some IP-specific mechanism. In this RFC, an ATM endpoint may be a host or a device which performs IP routing ("router"). The "Maximum SDU Length" field of the AAL Parameters Information Element is used in the SETUP and CONNECT messages of the industry- standard ATM signalling protocol (e.g. Q.93B) to negotiate the MTU for the Virtual Channel, when negotiation is used. [CCITT92, ATMF93] If the calling endpoint wishes to negotiate an MTU other than the default, it includes the "Maxiumum SDU Length" field in the AAL Parameters Information Element in the SETUP message. The value of the Maximum SDU Length may range from 576 to 65535 (octets) for use with IP. If it is only willing to use the default MTU value, the "Maximum SDU Length" field shall not be included. If the called endpoint receives a SETUP message containing the "Maximum SDU Length Field" in the AAL Parameters Information Element, it may either: Atkinson [Page 2] Internet Draft 11 June 1993 a) If it is able to accept the MTU value proposed by the calling endpoint, set the value of the "Maximum SDU Length Field" equal to that received in the SETUP message. b) If it wishes an MTU value less than that proposed in the SETUP message but greater than or equal to 576 octets, set the value of the "Maximum SDU Length" field in the CONNECT message to the desired value. c) If it does not wish to negotiate the MTU, shall not include the "Maximum SDU Length" field in the connect message. If a called endpoint receives a SETUP message containing no "Maxiumum SDU Length" field in the AAL Parameters Information Element, it shall not include the "Maximum SDU Length" field in the CONNECT message that it sends (i.e. the called party shall not require the calling party to negotiate the MTU). If the called endpoint incorrectly includes the "Maximum SDU Length" field in the CONNECT messages or indicates a value greater than that indicated by the calling endpoint in the SETUP message, the calling endpoint shall clear the call with cause "Invalid Information Element Contents" being indicated. Security Considerations Security Considerations are not discussed in this memo. Acknowledgements While all members of the IETF's IP over ATM Working Group have been helpful, Vern Schryver, Rob Warnock, Craig Partridge, and Subbu Subramaniam have been especially helpful to the author in analysing host and router implications of the default IP MTU value. Similarly, Dan Schnackenberg provided significant assistance in clarifying the optional ATM signalling procedure used to negotiate the IP MTU value. References [RFC-791] Information Sciences Institute, Internet Protocol Specification, RFC-791, DDN Network Information Center, September 1981. Atkinson [Page 3] Internet Draft 11 June 1993 [RFC-1122] Braden, R. (Ed.), Requirements for Internet Hosts -- Communications Layers, RFC-1122, DDN Network Information Center, October 1989, pp.58-60. [RFC-1209] Piscitello, D & J. Lawrence, The Transmission of IP Datagrams over the SMDS Service, RFC-1209, DDN Network Information Center, March 1991. [CCITT92] CCITT Study Group XI/Working Party 6, Draft Text for Q.93B, Document TD XI/6-37, Revision 1, March 1992, Geneva, Switzerland. (This is most assuredly out of date but is the best I have handy...) [ATMF93] Grace, Jim (ed.), Signalling Specification Draft, Document 93-265R5, pp. 40-43, 16 April 1993, ATM Forum. (This will be replaced with a reference to ATM Forum's "User Network Interface" specification Version 3, once that document is published.) 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 Information Randall J. Atkinson Information Technology Division Naval Research Laboratory Washington, DC 20375 USA Atkinson [Page 4]