4 types
1) Registrar Server 
2) Proxy Serve
3) Redirect Server
4) UA Server

1) Registrar Server  :





Step 1: For this situation User A  sends a REGISTER ask for, where the fields "from" and "to" compare to the enrolled client and Request-URI contains the location of "Recorder". The Proxy server, who goes about as" Registrar Server".

Step 2:The Registrar server Challange the  User A  by including "WWW-Authenticate" header field in 401 Unauthorized Response.

WWW-Authenticate: Digest realm="atlanta.com", qop="auth", opaque="", nonce="f4dfjkdfh6jfjdf7fdfdf7fdf8dfd4", algorithm=MD5, stale=false

Step 3:User A send Register ask for with Credentials containing the confirmation data of client specialists for the intermediary included header field " Authorization".

Approval: Digest Username="alice",realm="atlanta.com",nonce="f4dfjkdfh6jfjdf7fdfdf7fdf8dfd4",, response="f8fjfhgf8ffgf8gfg9egt9gt9gtsfs9",cnonce="gtgt5kuyk6jujuj8",opaque="",qop=auth.

Step 4:

Enlistment center server sends,  Registration successfull -200 OK reaction back to User operators . Enrollment Unsuccessfull - 401 Unauthorized back to User specialists.



2) Proxy Server :




Step 1: For this situation User An and User B sends a REGISTER ask for, where the fields "from" and "to" compare to the enrolled client and Request-URI contains the location of "Recorder". The Proxy server, who goes about as" Registrar Server", checks if the client can be verified and sends an OK message if all is well.

Step 2:User A sends an INVITE appeal to the intermediary server. Quickly, the intermediary sends a TRYING 100 to stop the retransmission and reroute the solicitation to the User B by sending an INVITE to it.

Step 3:The User B on getting the INVITE sends a TRYING 100 to intermediary took after by a Ringing 180. It is right now the phone starts to ring and it is additionally reroute by the intermediary to the User A.

Step 4:Finally, User B sends the  200 OK message to the intermediary which is sent to User A which is recognized by User An and the ACK is sent to User B.

Step 5:

The call is presently establised and the RTP transport convention begins with the parameters (ports, addresses, codecs, and so on.) of the SDP convention.

Step 6:This exchange relates to a session end . This is completed with a just BYE solicitation to the Proxy, and later reroute to User B. This client answers with an  200OK message to affirm that the last message has been gotten effectively.

3) Redirect Server:


4) UA Server: