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SDP

SDP. Session Description Protocol (SDP) an application-layer protocol intended to describe multimedia sessions a text-based protocol when describing a session the caller and callee indicate their respective "receive" capabilities, media formats and receive address/port.

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SDP

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  1. SDP

  2. Session Description Protocol (SDP) • an application-layer protocol intended to describe multimedia sessions • a text-based protocol • when describing a session the caller and callee indicate their respective "receive" capabilities, media formats and receive address/port

  3. capability exchange can be performed • during session set-up or • during the session itself (while the session is in progress)

  4. 9.1 SDP message contents 9.2 SDP message format 9.3 Selected SDP lines

  5. 9.1 SDP message contents • An SDP message contains three levels of information • Session-level description • includes the session identifier and other session level parameters, such as the IP address, subject, contact info about the session and/or creator • Timing description • start and stop times, repeat times, one or more media-level descriptions

  6. Media type and format • transport protocol and port number, other media-level parameters • The three levels of information must appear in the order described above • The SDP message is a collection of SDP lines

  7. 9.1.1 Session description 9.1.2 Time description 9.1.3 Media description

  8. 9.1.1 Session description • Table 9.1 lists all the session-level description lines and indicates their mandate and the letter used as the line name

  9. 9.1.2 Time description • Table 9.2 lists all the time-level description lines and indicates their mandate and the letter used as the line name

  10. 9.1.3 Media description • Table 9.3 lists all the media-level description lines and indicates their mandate and the letter used as the line name

  11. 9.2 SDP message format • The SDP syntax is very strict and all lines follow the same format • Every SDP line has the format <character>=<value> value=parameter1 parameter2 . . . parameterN • Each SDP line ends with a carriage return line feed (CRLF) and each line has a defined number of parameters

  12. 9.3 Selected SDP lines 9.3.1 Protocol version line 9.3.2 Connection information line 9.3.3 Media line 9.3.4 Attribute line 9.3.5 The rtpmap attribute

  13. 9.3.1 Protocol version line • The SDP protocol version is 0 and, therefore, the v-line in an SDP message must always be set to 0 v=0

  14. 9.3.2 Connection information line • The c-line must be either present at the session level or media level • It must be present at the media level if it is not present at the session level • If it is present at both levels, then media-level connection information overrides session-level information c=<network type> <address type> <network address>

  15. The c-line has three parameters • network type • the only currently defined network type is the Internet • the value appears as "IN" • address type • there are two address types, IP4 or IP6 • network address • this parameter identifies the IP address or domain name where media are received

  16. 9.3.3 Media line • The m-line carries information about the media, including transport information • The syntax is as follows m=<media> <port> <transport> <format-list> • The m-line has four parameters • Media • the type of media (e.g., audio, video, game) • Port • contains the port number where these media can be received

  17. Transport • the transport protocol to use, either the User Datagram Protocol (UDP) or Real-time Transport Protocol Audio and Video Profile (RTP/AVP) • Format-list • contains more information about the media, usually payload types defined in RTP/AVPs

  18. If the transport is RTP/AVP, then the port number for the RTP Control Protocol (RTCP) = RTP port + 1 • RTCP is assumed to be sent whenever RTP is carrying the media • The RTP port number must always be an even number and, therefore, the RTCP port is an odd number

  19. 9.3.4 Attribute line • The a-line defines the attributes of the media • Attributes can be session-level attributes, media-level attributes or both • Attribute interpretation depends on the media tool being invoked

  20. Syntax a=<attribute field> [" : "<attribute value>] • attribute field • contains the name of the attribute. • attribute value • optional, if present is separated from the attribute field by a colon • Table 9.4 shows a list of the most commonly used attributes

  21. 9.3.5 The rtpmap attribute • For RTP-transported media, SDP can be used to bind a media-encoding codec to the media's RTP payload type • this is done using a payload-type number • Payload types • static payload types • the payload-type number is sufficient for the binding • the payload-type number is carried in the format-list parameter of the media line

  22. dynamic payload types • the payload-type number is not sufficient and additional encoding information is needed • this is achieved using the rtpmap attribute

  23. Syntax of the rtpmap attribute a=rtpmap: <payload type> <encoding name>/<clock rate>[/<encoding parameters>] • payload type • carries the payload-type number as indicated in the m-line • encoding name • the codec name

  24. clock rate • bits per second • encoding parameters • media-specific parameters, including the number of channels, but not codec-specific parameters

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