Editing Session Announcement Protocol

The '''Session Announcement Protocol''' ('''SAP''') is an experimental [[protocol (computing)|protocol]] for advertising [[multicast]] session information. SAP typically uses [[Session Description Protocol]] (SDP) as the format for [[Real-time Transport Protocol]] (RTP) session descriptions. Announcement data is sent using [[IP multicast]] and the [[User Datagram Protocol]] (UDP).

Under SAP, senders periodically transmit SDP descriptions to a well-known [[multicast address]] and [[Port (computer networking)|port number]] (9875).<ref>{{cite web |url=http://www.javvin.com/protocolSAP.html |title=SAP (v1 & v2): Session Announcement Protocol |access-date=2012-04-06 |archive-url=https://archive.today/20130126204537/http://www.javvin.com/protocolSAP.html |archive-date=2013-01-26 |url-status=dead }}</ref> A listening application constructs a guide of all advertised multicast sessions.

SAP was published by the [[IETF]] as RFC 2974.<ref>{{cite IETF |RFC=2974 |title=Session Announcement Protocol |author1=M. Handley |author2=C. Perkins |author3=E. Whelan |date=October 2000}}</ref>

==Announcement interval==
The announcement interval is cooperatively modulated such that all SAP announcements in the multicast delivery scope, by default, consume 4000 bits per second. Regardless, the maximum announce interval is 300 seconds (5 minutes). Announcements automatically expire after 10 times the announcement interval or one hour, whichever is greater. Announcements may also be explicitly withdrawn by the original issuer.

==Authentication, encryption and compression==
SAP features separate methods for [[authentication|authenticating]] and [[encryption|encrypting]] announcements. Use of encryption is not recommended. Authentication prevents unauthorized modification and other DoS attacks. Authentication is optional. Two authentication schemes are supported:
*[[Pretty Good Privacy]] as defined in RFC 2440
*[[Cryptographic Message Syntax]] as defined in RFC 5652

The message body may optionally be [[Lossless data compression|compressed]] using the [[zlib]] format as defined in RFC 1950.

==Applications and implementations==
[[VLC media player]] monitors SAP announcements and presents the user a list of available streams.<ref>{{citation |url=https://www.epiphan.com/userguides/pearl/Content/UserGuides/Streaming/stream/streamSAPView.htm |title=Viewing with Session Announcement Protocol (SAP) |access-date=2019-03-02}}</ref>

SAP is one of the optional discovery and connection management techniques described in the [[AES67]] audio-over-Ethernet interoperability standard.<ref name=aessc>{{cite web |url=http://www.aes.org/publications/standards/search.cfm?docID=96 |title=AES67-2013: AES standard for audio applications of networks - High-performance streaming audio-over-IP interoperability |publisher=[[Audio Engineering Society]] |date=2013-09-11 |access-date=2014-02-11}}</ref>

==References==
{{reflist}}

==External links==
*[https://www.cl.cam.ac.uk/~jac22/books/mm/book/node184.html Session Announcement Protocol (SAP)]
*[https://www.cs.columbia.edu/~hgs/research/projects/sap/Report/Report.htm SAP/SDP Listener]

[[Category:Internet protocols]]
[[Category:Internet Standards]]