Practical VoIP Using VOCAL

SDP Messages

The Session Description Protocol (SDP, RFC 2327 ) is used to describe the components of the communication channel that are under negotiation when an endpoint is attempting to set up a call. These components include codecs, ports, and the streaming protocol that will enable one-way or two-way transmission. In a voice call originating from a SIP-based device, SDP headers are usually sent with the INVITE and the 200 OK message and normally describe how a data stream is going to be supported through the Real-time Transport Protocol (RTP: RFC 1889).

It is also possible to send an INVITE without SDP headers and , instead, send these headers with the 200 OK and ACK messages. This is sometimes useful when the originating device must receive data about the quality or capabilities of the party it is trying to contact before it can finish setting up the audio stream.

The INVITE

Here is the INVITE that was described earlier. To bring more attention to the SDP portion, most of the SIP headers have been replaced with an ellipsis (...).

    -----------------------------------------------------------------
    INVITE sip:6713@192.168.26.180:6060;user=phone SIP/2.0
    ...
    Content-Type: application/sdp
    Content-Length: 168

    v=0
    o=- 238540244 238540244 IN IP4 192.168.22.36
    s=VOVIDA Session
    c=IN IP4 192.168.22.36
    t=3174844751 0
    m=audio 23456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=ptime:20
    -----------------------------------------------------------------

Breakdown of the Lines

The following is a brief ...