Network Working Group Randall J. Atkinson INTERNET DRAFT DRAFT Naval Research Laboratory 26 October 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 26 April 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. 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 Atkinson [Page 1] Internet Draft 26 October 1993 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 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. This value conforms with the requirements of the Host Requirements RFC and is consistent with the IP specification. [RFC- 793, RFC-1122]. All ATM Forum compliant implementations of ATM technology are capable of handling this restriction without difficulty. This restriction is placed in order to minimise the occurence for IP fragmentation, which is known to be harmful, and to ensure support for future versions of IP that are currently in development. Note that this minimum MTU does not place any lower bound on the size of the IP datagram that may be transmitted by any system. Rather, it ensures that all systems will handle IP datagrams of size less than or equal to 576 octets without IP fragmentation. Before such a small MTU value may be used, the requirements described in the MTU NEGOTIATION section must be adhered to. MTU Negotation for ATM AAL5 The 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 component named "Forward Maximum CPCS-SDU Size Identifier" contains the value over the path from the calling party to the called party. The component named "Backwards Maximum CPCS-SDU Size Identifier" 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 65534 inclusive. Not all of those values make sense in the context of IP over ATM as is explained in the preceding section, so not all of those values may be used by implementations of this specification. 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 ATM information elements might have Atkinson [Page 2] Internet Draft 26 October 1993 different values on the same connection. Other MTU values for ATM AAL5 (i.e. other than the default value specified above) shall not be used unless use of the other MTU value was successfully negotiated using industry-standard ATM-specific mechanisms (e.g. ATM Signalling Protocol) prior to being attempted. If negotiation of the MTU value is attempted but fails, then the default MTU value shall be used. If either of the above cited information elements is not present in the ATM Signalling Protocol's "SETUP" message, then the default MTU value shall be used for each missing value. If the calling device erroneously sets the value of either information element to 0, then either the default MTU value shall be used or the party receiving the invalid value shall clear the call with cause "Invalid Information Element Contents" being indicated. If the calling party wishes to negotiate an MTU size other than the default, it must include the "AAL Parameters" information element with the desired values for the Maximum CPCS-SDU Size Identifier as part of the SETUP message of the ATM signalling protocol [ATMF93b]. The value of these identifiers may range from 576 to 65535 octets in an implementation of this specification. If the calling party is only willing to use the default MTU value, the Maximum CPCS-SDU components must not be specified in the SETUP message. 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 a "Maximum CPCS-SDU Size Identifier" in the "AAL Parameters" information element, it shall handle the Maximum CPCS-SDU Size Identifier as follows: a) If it is able to accept the proposed MTU values, it shall set the values of the Maximum CPCS-SDU Size Identifier equal to the proposed values and include that information in its CONNECT message responding to the original SETUP message. b) If it wishes a smaller MTU size than that proposed but greater than or equal to 576, then it shall set the values of the Maximum CPCS-SDU Size Identifier equal to the desired value in its CONNECT message responding to the original SETUP message. c) If it does not wish to negotiate the MTU, it shall not include the Maximum CPCS-SDU Size Identifiers in its CONNECT message responding to the original SETUP message. Atkinson [Page 3] Internet Draft 26 October 1993 If a called endpoint receives a SETUP message containing no "Maximum CPCS-SDU Size Identifier" in the "AAL Parameters" information element, it shall not include such an identifier in the CONNECT message sent in response to the original SETUP message and the Default MTU shall be used by both parties. If the called endpoint incorrectly includes the "Maximum CPCS-SDU Size Identifier" in the CONNECT messages (e.g. because the original SETUP message did not include such information) or it sets such an identifier to a value less than 576 or more than the value of the original SETUP message, then the calling party shall clear the call with cause "Invalid Information Element Contents" being indicated. Path MTU Discovery Required The Path MTU Discovery mechanism is an Internet Standard and is an important mechanism for reducing IP fragmentation in the Internet. This mechanism is particularly important because new subnet technologies, such as ATM, use default MTU sizes 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 implementations that comply or conform with this specification must 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. Atkinson [Page 4] Internet Draft 26 October 1993 [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. [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. [KM87] C. Kent & J.Mogul, "Fragmentation Considered Harmful", Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology, August 1987. 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]