The Session Announcement Protocol (SAP) is an experimental

protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...

for advertising

multicast In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused with ...

session information. SAP typically uses

Session Description Protocol The Session Description Protocol (SDP) is a format for describing multimedia communication sessions for the purposes of announcement and invitation. Its predominant use is in support of streaming media applications, such as voice over IP (VoIP) ...

(SDP) as the format for

Real-time Transport Protocol The Real-time Transport Protocol (RTP) is a network protocol for delivering audio and video over IP networks. RTP is used in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applicatio ...

(RTP) session descriptions. Announcement data is sent using

IP multicast IP multicast is a method of sending Internet Protocol (IP) datagrams to a group of interested receivers in a single transmission. It is the IP-specific form of multicast and is used for streaming media and other network applications. It uses speci ...

and the

User Datagram Protocol In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages (transported as datagrams in packets) to other hosts on an Internet Protocol (IP) network. ...

(UDP). Under SAP, senders periodically transmit SDP descriptions to a well-known

multicast address A multicast address is a logical identifier for a group of hosts in a computer network that are available to process datagrams or frames intended to be multicast for a designated network service. Multicast addressing can be used in the link laye ...

and

port number In computer networking, a port is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within an operating system, a port is a logical construct that identifies a specific ...

(9875). A listening application constructs a guide of all advertised multicast sessions. SAP was published by the

IETF The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...

as RFC 2974.

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

authenticating Authentication (from ''authentikos'', "real, genuine", from αὐθέντης ''authentes'', "author") is the act of proof (truth), proving an Logical assertion, assertion, such as the Digital identity, identity of a computer system user. In ...

and

encrypting In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can deci ...

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 Pretty Good Privacy (PGP) is an encryption program that provides cryptographic privacy and authentication for data communication. PGP is used for signing, encrypting, and decrypting texts, e-mails, files, directories, and whole disk partition ...

as defined in RFC 2440 *

Cryptographic Message Syntax The Cryptographic Message Syntax (CMS) is the IETF's standard for cryptographically protected messages. It can be used by cryptographic schemes and protocols to digitally sign, digest, authenticate or encrypt any form of digital data. CMS is base ...

as defined in RFC 5652 The message body may optionally be compressed using the

zlib zlib ( or "zeta-lib", ) is a software library used for data compression. zlib was written by Jean-loup Gailly and Mark Adler and is an abstraction of the DEFLATE compression algorithm used in their gzip file compression program. zlib is also a ...

format as defined in RFC 1950.

Applications and implementations

VLC media player VLC media player (previously the VideoLAN Client and commonly known as simply VLC) is a free and open-source, portable, cross-platform media player software and streaming media server developed by the VideoLAN project. VLC is available for desk ...

monitors SAP announcements and presents the user a list of available streams. SAP is one of the optional discovery and connection management techniques described in the

AES67 AES67 is a technical standard for audio over IP and audio over Ethernet (AoE) interoperability. The standard was developed by the Audio Engineering Society and first published in September 2013. It is a layer 3 protocol suite based on existing ...

audio-over-Ethernet interoperability standard.

References

{{reflist

External links

Session Announcement Protocol (SAP)
Internet protocols Internet Standards