Network Working Group Q. Xie INTERNET-DRAFT Motorola Category: Informational C. Sharp CISCO I. Rytina Ericsson R.R. Stewart Motorola Expires in six months March 2000 Use SCTP as MEGACO Transport 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 This document discusses the use of the Simple Control Transmission Protocol (SCTP) [1], for carrying MEGACO [2] messages between an MGC and an MG. SCTP is currently being developed by IETF SIGTRAN Working Group. The framework architecture defined in RFC 2719 [3] can be used as a guideline for transporting MEGACO messages using SCTP. MEGACO implementations targeting for high capacity and high availability deployment can especially benefit from the stream capability, redundant network support, congestion avoidance, and strong security features provided by SCTP. 1. Introduction MEGACO protocol messages may be transmitted over the Simple Control Transmission Protocol (SCTP) [1]. The MEGACO implementation may take advantage of the following services provided by SCTP: o Datagram-based transport o Reliable delivery --- As a reliable transport protocol, SCTP provides recovery mechanisms for transmission loss and duplicate packet receipt. This simplifies the design of application level repetition and timer control. o Ordered and unordered reliable message delivery --- Settable on a per-message basis by the application, SCTP allows high priority transactions be sent through unordered delivery for possible expedited treatment. o Stream capability --- SCTP can provide up to 65536 unidirectional streams in each direction of an MGC-MG association. SCTP transmits messages and processes received messages in one stream independent to the order or status of messages in any other streams. The application may effectively avoid head-of-line blocking by transmitting unrelated transactions on different streams. o Protection against "SYN" attacks --- The encryption cookie mechanism built into the SCTP provides protection against the equivalent of TCP "SYN" attacks on a MG or MGC node. o Network congestion management --- SCTP provides effective means for detecting and handling network congestion. o Redundant path management --- It may become strongly desirable for a large MG to have fault resilient network-level connectivity towards an MGC. SCTP supports multi-homed IP nodes for redundant path deployment. SCTP provides reachability monitoring, fast switch- over/fail-over, and potentially load balancing over redundant paths. In a transaction-oriented protocol like MEGACO, there are still ways for transaction requests or responses to be lost, e.g., caused by entity/component failure. As such, it is recommended that entities using SCTP transport implement application level timers for each request. 2. Providing the At-Most-Once functionality SCTP is designed to recover from transport losses or duplications, but loss of a transaction request or its reply may nonetheless be noted in real implementations. In the absence of a timely response, MEGACO may repeat commands. Most MEGACO commands are not idempotent. The state of the MG would become unpredictable if, for example, Add commands were executed several times. To guard against such losses, it is recommended that entities follow the procedures in Annex D.1.1 of document [2], with the exception LONG-TIMER or the use of the TransactionResponseAck parameter, which shall not be used. 3. Transaction identifiers and three way handshake For the same reasons as discussed above, it is possible that transaction replies may be lost even with a reliable delivery protocol such as SCTP. It is recommended that entities use transaction identifers following the procedures in Annex D.1.2.1 of document [2]. Three way handshake is not applicable when SCTP is used. 4. Computing retransmission timers With reliable non-duplicate delivery guaranteed by SCTP, application level timers are only used to guard against entity/component failure. Therefore, only simple timer mechanisms are required. Exponential back-off algorithms shall not be necessary. The first retransmission of a request can occur after a short interval. If additional retransmissions are required a longer time interval is recommended between the retransmissions. 5. Provisional responses The basic procedures in section 8.2.3 of document [2] apply. 6. Ordering of commands SCTP provides both ordered and unordered reliable delivery, settable on a per-transaction basis. Therefore, MEGACO can take advantage of the ordered capability of SCTP. High priority transactions can get expedited treatment by properly using unordered delivery. No special procedures are therefore required. 7. Stream independence SCTP can provide up to 65536 unidirectional streams in each direction of an MGC-MG association. SCTP transmits messages and processes received messages in one stream independent to the order or status of messages in any other streams. MEGACO may avoid head-of-line blocking by transmitting unrelated transactions on different streams. Reliability is still provided. Ordering of messages is available per-stream. It is recommended that transactions related to one context are transported over the same stream. 8. Authors' Addresses Qiaobing Xie Tel: +1-847-632-3028 Motorola, Inc. EMail: qxie1@email.mot.com 1501 W. Shure Drive, #2309 Arlington Heights, IL 60004 USA Chip Sharp Tel: Cisco Systems Inc. EMail:chsharp@cisco.com 7025 Kit Creek Road Research Triangle Park, NC 27709 USA 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 Ian Rytina Tel: Ericsson Australia EMail:ian.rytina@ericsson.com 37/360 Elizabeth Street Melbourne, Victoria 3000 Australia 9. References [1] R.R. Stewart, Q. Xie, K. Morneault, C. Sharp, H.J. Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson, "Simple Control Transmission Protocol," , IETF SIGTRAN Working Group, March, 2000. [2] F. Cuervo, B. Hill, N. Greene, C. Huitema, A. Rayhan, B. Rosen, and J. Segers "Megaco Protocol," , IETF MEGACO Working Group, February, 2000. [3] L. Ong, I. Rytina, M. Garcia, H. Schwarzbauer, L. Coene, H. Lin, I. Juhasz, M. Holdrege, and C. Sharp, "Framework Architecture for Signaling Transport," RFC 2719, IETF, Oct. 1999. This Internet Draft expires in 6 months from March 2000.