When Alice places a call to Bob, Bob sends a 180 ringing message to Alice. Then why does the from field contains the uri of Alice and to field contains the uri of Bob? I am referring to the RFC 3261.
F2 180 Ringing Bob -> Alice
SIP/2.0 180 Ringing
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 ;received=192.0.2.101
From: Alice ;tag=9fxced76sl
To: Bob ;tag=8321234356
Call-ID: [email protected]
CSeq: 1 INVITE
Contact:
Content-Length: 0
According to RFC 3261, From Header identifies the originator of the REQUEST. To Header indicates the recipient of the REQUEST. And so, there wont be any change in From and To headers while Response is received.