Session Announcement Protocol

from Wikipedia, the free encyclopedia

The Session Announcement Protocol ( SAP ) is a network protocol in the field of IP telephony , which Multicast - multimedia sessions and other multicast sessions supported and forwards the corresponding session configuration in question participants. It was developed by the IETF and is described in RFC 2974 .

Working principle

SAP announcer is sending the periodic announcement packets to the well-known ( "well known") multicast - address (sap.mcast.net = 224.2.127.254) on the corresponding port (default: 9875). The SAP client listens on this port in order to be able to recognize and accept possible requests for multicast sessions.

Structure of a SAP package

Structure of a SAP package

  • V - 3 bit version number, must be set to 1
  • A - 1 bit - Address Type, can be 0 or 1
    • 0 - Originating Source field contains IPv4 address (32 bit)
    • 1 - Originating Source field contains IPv6 address (128 bit)
  • R - 1 bit - Reserved, is set to 0 by SAP advertisers and ignored by SAP clients
  • T - 1 bit message type, can be 0 or 1
    • 0 - package to announce a session
    • 1 - Package for deleting a session
  • E - 1 bit - Encryption bit, can be 0 or 1
    • 0 - The payload is not encrypted, the timeout field may be missing
    • 1 - The payload is encrypted, the timeout field must be added to the header
  • C - 1 Bit - Compressed Bit, if set to 1, the payload is compressed
  • Authentication Length - 8 bit - Specifies the number of 32-bit words that follow the header and contain authentication data. If 0, then there is no authentication header
  • Message Identifier Hash - 16 Bit - Is used in conjunction with the Origination Source field to obtain a globally unique identifier for this message.
  • Origination Source - 32-bit or 128-bit - The IP address of the originator of the message. If field A 0 is set, then there is an IPv4 address, if 1, then an IPv6 address.
  • Timestamp - 32 bit - If the payload is encrypted, this field contains a timestamp that specifies how long the message is valid. It is saved in NTP format and specified in seconds.
  • Payload Type - Contains the MIME type (i.e. the data format) of the payload. It is specified as an ASCII string and is terminated by a zero byte (ASCII NUL).
  • Payload - Contains the actual data (payload) and is divided into various subfields.

See also

Web links

Commons : Session Announcement Protocol  - collection of pictures, videos and audio files