Internet Draft Transport Working Group L. Ong Internet-Draft Nortel Networks draft-ietf-sigtran-sctptunnel-00.txt R. Stewart March 2000 Q. Xie Expires: August 2000 Motorola Tunneling of SCTP over Single UDP Port Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract The Simple Control Transmission Protocol (SCTP) [1] provides a standard method for transporting messages over IP, with features such as multi-homing and multi-stream built into the protocol for additional reliability and real-time delivery. This memo defines a method for tunneling SCTP over a single UDP port in order to simplify initial implementations. This memo is intended to become an Experimental document and is not intended as a standard of any kind. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2 2. Brief background to the SCTP Protocol. . . . . . . . . . . 2 3. Interim Implementation over UDP. . . . . . . . . . . . . . 2 4. Use of Single UDP Port . . . . . . . . . . . . . . . . . . 2 5. Impacts on SCTP Protocol . . . . . . . . . . . . . . . . . 3 6. Possible applications . . . . . . . . . . . . . . . . . . 4 7. Implementation considerations . . . . . . . . . . . . . . 4 8. Security considerations . . . . . . . . . . . . . . . . . 4 9. References . . . . . . . . . . . . . . . . . . . . . . . . 5 10. Author's Addresses . . . . . . . . . . . . . . . . . . . . 5 Ong,Stewart & Xie draft-ietf-sigtran-sctptunnel-00.txt [Page 1] Internet Draft Tunneling of SCTP Over UDP March 2000 1. Introduction This memo describes the methods for tunneling of Simple Control Trans- mission Protocol (SCTP) over UDP. Tunneling of SCTP can be done without change to the protocol formats and basic procedures. Since SCTP provides a means of de-multiplexing sessions using the SCTP source and destination port numbers, only a single UDP port is necessary for tunneling. 2. Background SCTP is designed to transport PSTN signaling messages over IP networks, but is capable of broader applications. SCTP is a reliable datagram transfer protocol operating on top of an unreliable routed packet network such as IP. It offers the following services to its users: -- acknowledged error-free non-duplicated transfer of user data, -- data segmentation to conform to discovered path MTU size, -- sequenced delivery of user datagrams within multiple streams, with an option for order-of-arrival delivery of individual datagrams, -- optional multiplexing of user messages into SCTP datagrams, and -- network-level fault tolerance through supporting of multi-homing at either or both ends of an association. 3. Interim Implementation over UDP Implementation of SCTP directly over IP requires use of the raw_IP socket interface and/or modification to the kernel. To allow more rapid implementation and implementation on systems that do not at this time support the raw_IP socket interface, this memo defines a method of tunneling SCTP over a single registered UDP port number. Tunneling of SCTP over UDP adds some unnecessary processing to the protocol, including redundant source and destination port fields and greater interprocess communication. It is therefore provided only as an interim measure until implementation of SCTP over IP is available. 4. Use of Single UDP Port UDP port 9899 has been registered with IANA for SCTP Tunneling. The use of a single port number still allows multiplexing of SCTP sessions, using the SCTP's own source and destination port numbers. At the same time, the use of a single registered port number allows intermediate systems such as routers and firewalls to immediately identify SCTP packets and provide any associated treatment that is desired. Ong,Stewart & Xie draft-ietf-sigtran-sctptunnel-00.txt [Page 2] Internet Draft Tunneling of SCTP Over UDP March 2000 5. Impacts on SCTP Protocol There are minimal impacts on SCTP fields and basic procedures from the use of tunneling over UDP. In general SCTP functions such as initiation, data transfer, congestion control and heartbeat operate exactly as defined in the standard SCTP specification. 5.1 Impact on MTU discovery SCTP requires the use of path MTU discovery [2] [3] . However many operating systems do not allow a user to implement changes to the DF bit (in the IP header) without opening a RAW IP socket. In some operating systems facilities are available to allow a UDP user to enable MTU discovery, these require some minor modification to the SCTP procedures. 5.1.1 Where MTU discovery is NOT allowed In cases where MTU discovery requires a RAW IP socket to implement, it is expected that the UDP tunnel would NOT be capable of supporting MTU discovery. This feature should remain disabled when implementing on these systems. 5.1.2 Where MTU discovery IS allowed In cases where a MTU discovery feature is enabled by a kernel interface, the following modifications need to be made to the SCTP path MTU discovery procedures: 1) Whenever the kernel reports an unreachable destination due to an ICMP unreachable destination message. Within the message a size value is passed to indicate the size of the largest MTU value that the reporting network element can send. This size must be reduced by the size of the UDP header before passing it to the SCTP stack. 2) When a report is made and SCTP is unable to re-fragment the datagram by changing the bundling, the UDP interface layer must re-enable IP fragmentation by clearing the DF bit until the larger datagram is retransmitted. Once the larger, datagram has been retransmitted the IP fragmentation must be disabled. 5.2 Impact on Multi-homing SCTP explicitly uses multi-homing to provide for fast recovery from network error conditions (when the peer is multi-homed). In cases where a tunneled endpoint is multi-homed, the endpoint MUST bind all addresses to the reserved UDP port numbers. This will allow the SCTP multi-homing to work seamlessly. Ong,Stewart & Xie draft-ietf-sigtran-sctptunnel-00.txt [Page 3] Internet Draft Tunneling of SCTP Over UDP March 2000 6. Possible Applications Tunneled SCTP has the same range of applications as SCTP over IP, such as transport of Signaling System 7, ISUP [4] or other telephony signaling protocols. The only new consideration is that tunneled SCTP should eventually be deprecated in favor of SCTP over IP for transport. It must be noted that there is NO inter-operability between a tunneled SCTP endpoint and a SCTP endpoint running directly on top of IP. This provides a limitation that when one endpoint moves directly on top of IP all endpoints it communicates with must also move at the same time. 7. Implementation considerations As discussed in section 5.2, a daemon providing tunneling of SCTP MUST bind all addresses to the SCTP reserved port. Other considerations such as: A) How the application process(es) communicate with the SCTP tunneling point. B) How the tunneling point de-multiplexes the SCTP port number to find the real endpoint to send a datagram to. are beyond the scope of this document and are implementation specific. 8. Security Considerations Tunneled SCTP is subject to the same security concerns as SCTP over IP. However, the same security mechanisms can be used, i.e., application of IPSEC to provide authentication and/or encryption of the transported signaling and control information. In this case, IPSEC could be applied as for any UDP stream, whereas for SCTP over IP, application to an SCTP protocol type is required. Ong,Stewart & Xie draft-ietf-sigtran-sctptunnel-00.txt [Page 4] Internet Draft Tunneling of SCTP Over UDP March 2000 9. References [1] Simple Control Transport Protocol, Mar. 2000, Work in Progress [2] Mogul, J., and Deering, S., "Path MTU Discovery", RFC 1191, November 1990. [3] McCann, J., Deering, S., and Mogul, J., "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [4] ITU-T Recommendations Q.761 to Q.767, 'Signalling System No.7 (SS7) - ISDN User Part (ISUP)' 10. Author's Addresses Lyndon Ong Tel: +1-408-495-5072 Nortel Networks EMail: long@nortelnetworks.com 4401 Great America Pkwy Santa Clara, CA, USA 95054 Randall R. Stewart Tel: +1-847-632-7438 Motorola, Inc. EMail: rstewar1@email.mot.com 1501 W. Shure Drive, #2315 Arlington Heights, IL 60004 USA Qiaobing Xie Tel: +1-847-632-3028 Motorola, Inc. EMail: qxie1@email.mot.com 1501 W. Shure Drive, #2309 Arlington Heights, IL 60004 USA This draft expires in August 2000. 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