Media Gateway Control (Megaco) Alf Heidermark Internet Draft Ericsson Document: draft-ietf-megaco-h248h-00.txt July 2000 Category: Standards Track H.248 Annex H (Pre-Decision White Document) Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. 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. 1. Abstract This document reproduces the content of the ITU-T Study Group 16 White Document draft of H.248 Annex H, which is scheduled for decision in Geneva in November 2000. H.248 Annex H describes procedures for transport of the Megaco protocol over SCTP [4]. This document is submitted for IETF comment prior to ITU-T decision, in accordance with procedures currently being negotiated between ITU-T Study Group and ISOC on behalf of the IETF. 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [2]. 3. Overview Megaco protocol messages may be transmitted over the Simple Control Transmission Protocol (SCTP) [4]. Heidermark Standards Track - Expires January 2001 1 H.248 Annex H (White Document draft) July 2000 The implementation may take advantage of the following services provided by SCTP: . Datagram-based transport . 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. . 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. . 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 . . 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 . Network congestion management --- SCTP provides effective means for detecting and handling network congestion. . 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/H.248, 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. 4. 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/H.248 may repeat commands. Most Megaco/H.248 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 Megaco/H.248 Annex D.1.1. with the exception LONG- TIMER or the use of the TransactionResponseAck parameter, which shall not be used. Heidermark Standards Track - Expires January 2000 2 H.248 Annex H (White Document draft) July 2000 5. Transaction identifiers and three way handshake 5.1 Transaction identifiers Megaco/H.248 Section D.1.2.1 is recommended to be followed. 5.2 Three way handshake It is not applicable. 6. 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. 7. Provisional responses The basic procedures in section 8.2.3 of this document apply. 8. Ordering of commands SCTP provides both ordered and unordered reliable delivery, settable on a per-transaction basis. Therefore, Megaco/H.248 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. 9. 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/H.248 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. 10. Security Considerations Security considerations regarding media gateway control are discussed in section 10 of [3]. Heidermark Standards Track - Expires January 2000 3 H.248 Annex H (White Document draft) July 2000 11. References 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 3 ITU-T Recommendation H.248, "Gateway Control Protocol", Geneva, June 2000. Also to appear as RFC xxxx (currently draft-ietf- megaco-merged-01.txt). 4 R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, V. Paxon, "Stream Control Transmission Protocol", draft-ietf-sigtran-sctp-11.txt, Internet Engineering Task Force, 6 July 2000. 6. Authors' Addresses Alf Heidermark (editor) Ericsson Tel:+46 87273894 E-mail: alf.heidermark@uab.ericsson.se Heidermark Standards Track - Expires January 2000 4