Practical VoIP Using VOCAL

Message Headers

In the previous section, the message flows began with an INVITE message. Earlier in this chapter, we looked at how the INVITE message contains SIP headers and an SDP attachment. In this section, we will look at these components more closely.

The INVITE

The following is an INVITE that has been generated by a User Agent and sent to a Proxy:

    -----------------------------------------------------------------
    INVITE sip:6713@192.168.26.180:6060;user=phone SIP/2.0
    Via: SIP/2.0/UDP 192.168.22.36:6060
    From: UserAgent<sip:6710@192.168.22.36:6060;user=phone>
    To: 6713<sip:6713@192.168.26.180:6060;user=phone>
    Call-ID: 96561418925909@192.168.22.36
    CSeq: 1 INVITE
    Subject: VovidaINVITE
    Contact: <sip:6710@192.168.22.36:6060;user=phone>
    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 Headers

The INVITE is divided into two sections. The top section contains the SIP headers, and the bottom section contains the body of the SIP message, in this case, an SDP session. The headers are separated from the body by a blank line. SDP session description information is discussed later in this chapter.

INVITE

The first line identifies the message type, the address of the called party, and the version of SIP that the caller supports. In the example, it is: ...