RFC 2974 (rfc2974) - Page 2 of 18
Session Announcement Protocol
Alternative Format: Original Text Document
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