Network Working Group Randall J. Atkinson INTERNET DRAFT Naval Research Laboratory 16 November 1993 Default IP MTU for use over ATM AAL5 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. It is intended to eventually submit this draft to the IESG for possible release as a standards-track RFC. Internet Drafts are draft documents valid for a maximum of six months. This Internet Draft expires on 9 May 1994. 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. 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 Atkinson [Page 1] Internet Draft 16 November 1993 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 Atkinson [Page 2] Internet Draft 16 November 1993 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 some other unforeseen problem occurs (e.g. the AAL Parameters are removed by a 3rd party because of transit through some 3rd party network), then the connection is created, then the default IP MTU value shall be used by both parties over the circuit. 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 Atkinson [Page 3] Internet Draft 16 November 1993 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. Security Considerations Security Considerations are not discussed in this memo. References [RFC-791] J. Postel, Internet Protocol, RFC-791, DDN Network Information Center, September 1981. [RFC-793] J. Postel, Transmission Control Protocol, RFC-793, DDN Network Information Center, September 1981. [RFC-1122] R. Braden (Ed.), Requirements for Internet Hosts -- Communications Layers, RFC-1122, DDN Network Information Center, October 1989, pp.58-60. [RFC-1191] J. Mogul & S. Deering, Path MTU Discovery, RFC-1191, DDN Network Information Center, November 1990. [RFC-1209] D. Piscitello, D & J. Lawrence, The Transmission of IP Datagrams over the SMDS Service, RFC-1209, DDN Network Information Center, March 1991. [RFC-1435] S. Knowles, IESG Advice from Experience with Path MTU Discovery, RFC-1435, DDN Network Information Center, March 1993. [ATMF93a] R. Breault, J. Grace, J. Jaeger, & L. Wojnaroski(eds.), ATM Forum User Network Interface Specification, Version 2.4 (clean), Document 93-620R3, Section 5.4.5.5, p. 174, 5 August 1993, ATM Forum. [ATMF93b] R. Breault, J. Grace, J. Jaeger, & L. Wojnaroski(eds.), ATM Forum User Network Interface Specification, Version 2.4 (clean), Document 93-620R3, Section 5.3.1.7, p. 149-150, 5 August 1993, ATM Forum. [ATMF93c] R. Breault, J. Grace, J. Jaeger, & L. Wojnaroski(eds.), ATM Forum User Network Interface Specification, Version 2.4 (clean), Document 93-620R3, Section 5.3.1.3, p. 146, 5 August 1993, ATM Forum. Atkinson [Page 4] Internet Draft 16 November 1993 [KM87] C. Kent & 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 help in clarifying the ATM signalling procedure used to negotiate the IP MTU value for Switched Virtual Circuits. 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 20375-5320 USA Atkinson [Page 5]