IMS

SIP Header : From/To

From/To would sound very simple to you, but this was one of the most confusing item to me when I first study SIP. It is a little different from what you see in your email communication.

For example, let's suppose John send an email to Alice and Alice respond to John. In this case, the from/to part of the John's email and Alice's email would be as follows. You see from/to changes depending on who is sending the email.

John's 'send' email :

     From : John

     To : Alice

Alice's 'response' email :

     From : Alice

     To : John

But in case of SIP. From/To is not determined by who is sending the specific SIP message. It is determined by who initiated the whole dialog. In John/Alice case, the from/to would be as follows. You see Alice is also using exactly same From/To as John used. Alice just copies John's From/To because John is the one who initiated the dialog.

John's Dialog initiating SIP message:

     From : John

     To : Alice

Alice's Response (or Status) SIP message :

     From : John

     To : Alice

See the addresses in SIP message page and follow the arrows showing From/To header in various situation. It can be a good practice for you.