In this chapter we shall see SIP Registration and SIP Invite call flow.
SIP REGISTRATION Call flow:
User1 SIP Server
| |
| REGISTER M1 |
|------------------------------>|
| 401 Unauthorized M2 |
|<------------------------------|
| REGISTER M3 |
|------------------------------>|
| 200 OK M4 |
|<------------------------------|
| |
In the above diagram, User 1 is trying to register to SIP server.
User1 will first send registration request to SIP server (M1).
Then SIP server will challenge the Registration request by sending 401 Unauthorized response (M2).
Then User1 will fill the user information and sends one more Registration Request (M3).
SIP server will check HSS database and validates user’s credentials.
Then SIP server will send “200 OK” (M4).
SIP INVITE Call flow through two proxies.
A Proxy 1 Proxy 2 B
| | | |
| INVITE M1 | | |
|--------------->| | |
| 407 M2 | | |
|<---------------| | |
| ACK M3 | | |
|--------------->| | |
| INVITE M4 | | |
|--------------->| INVITE M5 | |
| 100 M6 |--------------->| INVITE M7 |
|<---------------| 100 M8 |--------------->|
| |<---------------| |
| | | 180 M9 |
| | 180 M10 |<---------------|
| 180 M11 |<---------------| |
|<---------------| | 200 M12 |
| | 200 M13 |<---------------|
| 200 M14 |<---------------| |
|<---------------| | |
| ACK M15 | | |
|--------------->| ACK M16 | |
| |--------------->| ACK M17 |
| | |--------------->|
| Both Way RTP Media |
|<================================================>|
| | | BYE M18 |
| | BYE M19 |<---------------|
| BYE M20 |<---------------| |
|<---------------| | |
| 200 M21 | | |
|--------------->| 200 M22 | |
| |--------------->| 200 M23 |
| | |--------------->|
| | | |
From the above image we can see that, User Agent “A” is calling User Agent “B”.
“A” will initiate a SIP session by sending “INVITE” request (M1) to the proxy server.
The proxy server will challenge “A” by sending “407” response (M2). “A” will acknowledge it by sending “ACK” (M3) message.
Then “A” will send “INVITE” request with authentication details. Proxy 1 will verify the changes and forwards INVITE to Proxy 2. (M5).
As soon as “Proxy 1” forwards message to “Proxy 2”, “Proxy 1” will send “100 trying” to “A”.
When the “INVITE” is reached to “B” (M7), “B” UA will send “180 Ringing” to “A”.
If “B” picks the call, it will send “200Ok” to “A” (M12).
“A” will acknowledge the final response to INVITE message by sending “ACK” (M15).
Then the session has been established, and 2 way audio communication will start.
If “B” wants to terminate the current session, then “BYE” will be sent (M19).
“A” responds to it by sending “200OK” (M21) to “B.”
Reference: RFC 3261
