In this illustration, we see Alice on the left with her computer and Bob on the right with his computer. Dotted arrows extend vertically downward from each, representing the passage of time. Alice's IP address is given as 167.180.112.24, and Bob's is given as 193.64.210.89.

A blue arrow extends diagonally downward to the right from Alice's timeline to Bob's. At Alice's timeline we see the following information:

INVITE bob@193.64.210.89

c=IN IP4 167.180.112.24

m=audio 38060 RTP/AVP 0

At Bob's timeline we see: port 5060. At the point where the arrow intersects, a note explains: Bob's terminal rings." Bob's response includes the following information:

200 OK

c=In IP4 193.64.210.89

m=audio 48753 RTP/AVP 3

At Alice's timeline we see: port 38060. Alice's response is labeled "ACK" and extends to Bob's port 5060.

A gray arrow extends horizontally leftward from Bob's timeline to Alice's. The arrow includes blue sound waves that are labeled "? Law audio." It arrives in Alice's port 38060. A second gray arrow-Alice's response-extends horizontally from Alice's timeline to Bob's arriving at port 48753. This arrow also includes sound waves that are labeled "GSM."