RFC 2974 (rfc2974) - Page 2 of 18

Session Announcement Protocol

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