Dialog (Call leg) Information

INVITE sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com:5060;branch=z9hG4bK776asdhds
To: Bob <sip:bob@biloxi.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
>Contact: <sip:alice@pc33.atlanta.com>
Content-Type: application/sdp
Content-Length: 142
(Alice’s SDP not shown)

Dialog^† (formerly “call leg”) information is in headers:

To tag, From tag, and Call-ID -- all requests and responses in this call will use this same dialog information.

“To” specifies the logical recipient of the message, “From” the logical sender – the string “Bob” is called a “display name”

Call-ID is unique identifier

The Call-ID is an arbitrary number, but it uniquely identifies this call (i.e., session), hence all future references to this session refer to this Call-ID
usually composed of a pseudo-random string@hostname or IP Address

^†A Dialog formally begins upon receipt of a response containing a tag. It is called an “Early dialog” when the response was a 18x provisional response.

Transcript

[slide132] We have a dialogue field, formerly called the call leg, it includes the To, From, and Call ID tags. The call ID is just a unique identifier within this session, and we just keep incrementing it. Why do we need that? To tell the difference between the first time and a repeat of it. Right? Because if we retransmit it because our timer's gone off, we'd like to know, is it the same command again, or is it a new command?