Session Announcement Protocol
RFC 2974

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Network Working Group                                         M. Handley
Request for Comments: 2974                                         ACIRI
Category: Experimental                                        C. Perkins
                                                                 USC/ISI
                                                               E. Whelan
                                                                     UCL
                                                            October 2000

                     Session Announcement Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes version 2 of the multicast session directory
   announcement protocol, Session Announcement Protocol (SAP), and the
   related issues affecting security and scalability that should be
   taken into account by implementors.

1  Introduction

   In order to assist the advertisement of multicast multimedia
   conferences and other multicast sessions, and to communicate the
   relevant session setup information to prospective participants, a
   distributed session directory may be used.  An instance of such a
   session directory periodically multicasts packets containing a
   description of the session, and these advertisements are received by
   other session directories such that potential remote participants can
   use the session description to start the tools required to
   participate in the session.

   This memo describes the issues involved in the multicast announcement
   of session description information and defines an announcement
   protocol to be used.  Sessions are described using the session
   description protocol which is described in a companion memo [4].

Handley, et al.               Experimental                      [Page 1]
RFC 2974             Session Announcement Protocol          October 2000

2  Terminology

   A SAP announcer periodically multicasts an announcement packet to a
   well known multicast address and port.  The announcement is multicast
   with the same scope as the session it is announcing, ensuring that
   the recipients of the announcement are within the scope of the
   session the announcement describes (bandwidth and other such
   constraints permitting).  This is also important for the scalability
   of the protocol, as it keeps local session announcements local.

   A SAP listener learns of the multicast scopes it is within (for
   example, using the Multicast-Scope Zone Announcement Protocol [5])
   and listens on the well known SAP address and port for those scopes.
   In this manner, it will eventually learn of all the sessions being
   announced, allowing those sessions to be joined.

   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 [1].

3  Session Announcement

   As noted previously, a SAP announcer periodically sends an
   announcement packet to a well known multicast address and port.
   There is no rendezvous mechanism - the SAP announcer is not aware of
   the presence or absence of any SAP listeners - and no additional
   reliability is provided over the standard best-effort UDP/IP
   semantics.

   That announcement contains a session description and SHOULD contain
   an authentication header.  The session description MAY be encrypted
   although this is NOT RECOMMENDED (see section 7).

   A SAP announcement is multicast with the same scope as the session it
   is announcing, ensuring that the recipients of the announcement are
   within the scope of the session the announcement describes. There are
   a number of possibilities:

   IPv4 global scope sessions use multicast addresses in the range
      224.2.128.0 - 224.2.255.255 with SAP announcements being sent to
      224.2.127.254 (note that 224.2.127.255 is used by the obsolete
      SAPv0 and MUST NOT be used).

Handley, et al.               Experimental                      [Page 2]
RFC 2974             Session Announcement Protocol          October 2000

   IPv4 administrative scope sessions using administratively scoped IP
      multicast as defined in [7].  The multicast address to be used for
      announcements is the highest multicast address in the relevant
      administrative scope zone.  For example, if the scope range is
      239.16.32.0 - 239.16.33.255, then 239.16.33.255 is used for SAP
      announcements.

   IPv6 sessions are announced on the address FF0X:0:0:0:0:0:2:7FFE
      where X is the 4-bit scope value.  For example, an announcement
      for a link-local session assigned the address