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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
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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].



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   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.




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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].



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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




























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