Basic Call Flow
In Figure A, Caller A completes a call to User B using two proxies: Proxy 1 and Proxy 2. The initial INVITE (F1) does not contain the Authorization credentials that Proxy 1 requires, so an Authorization response is sent containing the challenge information.A new INVITE (F4) is then sent containing the correct credentials and the call proceeds. The call terminates when User B disconnects by initiating a BYE message.
F1 INVITE A -> Proxy 1
The call begins, as always, with an INVITE message that contains information on caller and called party as well as the session description request (2nd part).
INVITE sip:UserB@ss1.wcom.com SIP/2.0
Via: SIP/2.0/UDP here.com:5060
From: BigGuy
To: LittleGuy
Call-ID: 12345600@here.com
CSeq: 1 INVITE
Contact: BigGuy
Content-Type: application/sdp
Content-Length: 147
v=0
o=UserA 2890844526 2890844526 IN IP4 here.com
s=Session SDP
c=IN IP4 100.101.102.103
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F2 407 Proxy Authorization Required Proxy 1 -> User A
SIP always works in a request-response mode and in this example Proxy 1 challenges Caller A for authentication
SIP/2.0 407 Proxy Authorization Required
Via: SIP/2.0/UDP here.com:5060
From: BigGuy
To: LittleGuy
Call-ID: 12345600@here.com
CSeq: 1 INVITE
Proxy-Authenticate: Digest realm="MCI WorldCom SIP",
domain="wcom.com", nonce="wf84f1ceczx41ae6cbe5aea9c8e88d359",
opaque="", stale="FALSE", algorithm="MD5"
Content-Length: 0
As we move further down the call flow, the actual voice call begins , using Realtime Transport Protocol (RTP) to move the voice stream.
F17 ACK Proxy 2 -> B
ACK sip: UserB@there.com SIP/2.0
Via: SIP/2.0/UDP ss2.wcom.com:5060
Via: SIP/2.0/UDP ss1.wcom.com:5060
Via: SIP/2.0/UDP here.com:5060
From: BigGuy
To: LittleGuy ;tag=314159
Call-ID: 12345601@here.com
CSeq: 1 ACK
Content-Length: 0
Calls are then terminated with a BYE request to the caller.
F18 BYE User B -> Proxy 2
BYE sip: UserA@ss2.wcom.com SIP/2.0
Via: SIP/2.0/UDP there.com:5060
Route: ,
From: LittleGuy ;tag=314159
To: BigGuy
Call-ID: 12345601@here.com
CSeq: 1 BYE
Content-Length: 0
