Movatterモバイル変換

Network Working Group                                        R. AtkinsonRequest for Comments: 1626                     Naval Research LaboratoryCategory: Standards Track                                       May 1994Default IP MTU for use over ATM AAL5Status 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 1994Permanent 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 inRFC-1191 and clarified byRFC-1435.  Host implementations should implement the IP Path MTU   Discovery mechanism as defined inRFC-1191.Atkinson                                                        [Page 3]

RFC 1626          Default IP MTU for use over ATM AAL5          May 1994Applicability Statement   The Multiprotocol Encapsulation over ATM AAL5 defined inRFC-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.milAtkinson                                                        [Page 5]