Application of SDP in SIP protocol

SDP used to build INVITE , 200OK and the ACK the message body of the message, for the master to be called the user to Exchange media information.

1 . configuration of media streams

( 1 ) the master called Media Description must correspond exactly to the main called N a media stream (m=) corresponding, both contain A=rtpmap. The purpose of this is to easily adapt the static payload type to the dynamic net load type conversion.

( 2 ) The port number for which the media stream is set in the response is called not to receive a media stream that is raised by the caller. 0. and must return the corresponding media popularity.

2 . setting of unicast SDP values

( 1 ) only media streaming, the port number is meaningless, should be set to 0 .

( 2 ) The net Load Type example table for each media stream should transmit two messages : can accept / and The code that is used to identify the compiler code. RTP Type of net load type.

( 3 ) / called No public media format, called still required to return the media stream m= line, Port good 0

( 4 ) called loopback if none of the media streams have a public media format 415 Response ( Media type not supported ) , and join 304 Warning Header field ( Media type not supported ) .

3 . Multicast Operations

( 1 ) The multicast addresses that are accepted and sent are the same.

( 2 ) is called not allowed to change the media stream only hair, just accept or receive / To the hair characteristics.

( 3 ) If the call does not support multicasting, the loopback - Response and the Warning ( multicast not available ) .

4 . delayed Media streaming

because the main call could actually be one and other protocols ( e.g. , a ) similar to the protocol of the Gateway, and called to request the call after the establishment of media consultation. The caller can first send the INVITEwithout the SDP , carrying the SDP by ACK Or resend a INVITE request to modify the called Session Description (SDP).

5 . Media Stream hold

If you ask the other person to enter hold , which temporarily stops sending one or more media streams, at which time you can use the Re-invite Request Carry SDP , the description is the same as described in the original request or response, just " c=0.0.0.0 "and there is Re_invite in the CSeq to increase.

6 . corresponds to 3 entity fields in a SIP

( 1 ) Content-type: indicates the message body type, for example: APPLICATION/SDP says yes SDP The session description.

( 2 ) content-encoding: A supplemental description of the message body type allows the user to edit the body of the message in a compressed encoding.

(3)content-length:gives the number of bytes of the message body.

Application of SDP in SIP protocol