Figure 7-7 illustrates a simple call flow with two users, one Marshal server, and one Redirect server.
The following text is a listing of the messages from the call flow illustrated in Figure 7-7. Many of the message headers are repeated throughout the transactions.
In this scenario, the calling phone is extension 5121 and the called phone is local 5120. There is only one Marshal server,
192.168.36.180:5060, and one Redirect server,
192.168.36.200:5060. For the sake of brevity, in this setup, the Marshal server acts as both ingress and egress proxy. You will notice some messages being sent to and from the same IP address. We have also used the informal term, call, instead of the proper term, dialog, to express the setup, media negotiation, establishment, and teardown of a conversation.
An INVITE message is sent by a calling party (5121) to the VoIP service, intended for a called party (5120). The key fields in this message have been highlighted in bold text.
INVITE sip:email@example.com SIP/2.0 Via: SIP/2.0/UDP 192.168.6.21:5060 From: sip:firstname.lastname@example.org To: <sip:email@example.com> Call-ID: firstname.lastname@example.org CSeq: 10 INVITE Expires: 180 User-Agent: Cisco IP Phone/ Rev. 1/ SIP enabled Accept: application/sdp Contact: sip:email@example.com:5060 Content-Type: application/sdp Content-Length: 219 v=0 o=CiscoSystemsSIP-IPPhone-UserAgent 16264 18299 IN IP4 192.168.6.21 s=SIP Call ...