Session Initiation Protocol - SIP

Today I’ll try to remind what SIP is, how it works, and how to integrate it with my Cisco voice Lab.

Originally designed by Shulzrinne and Handley, SIP, has been standardized in 1996 by IETF with RFC 3261. It is a widely utilized peer-to-peer telecommunications protocol, and differently from others, it is open, that is the reason why it is widely used and implemented.

SIP introduces some network elements, here explained really shortly :

·         User Agent (UA) : This is a logical SIP endpoint used to create and manage a SIP session. It can act either as a client (UAC) or as a server (UAS). A sip phone, or a Softphone can be UAC

·         Proxy Server : An intermediary agent that acts as UAC or UAS on behalf of other UA in order to establish a SIP session

·         Registrar/Location Server : It’s the server that accepts REGISTER messages and places the information it receives in those requests into the location service for the domain it handles.

·        Redirect Server : A UA server that accepts requests and redirect them to contact an alternate set of URIs.

·         Session Border Controller (SBC) : This is a server usually placed at the border of a SIP network, used as a bridge between different networks

Basic messages sent in the SIP environment

• INVITE – connection establishing request
• ACK – acknowledgement of INVITE by the final message receiver
• BYE – connection termination
• CANCEL – termination of non-established connection
• REGISTER – UA registration in SIP proxy
• OPTIONS – inquiry of server options

Answers to SIP messages are in the digital format like in the http protocol. Here are the most important ones:

• 1XX – information messages (100 – trying, 180 – ringing, 183 – progress)
• 2XX – successful request completion (200 – OK)
• 3XX – call forwarding, the inquiry should be directed elsewhere (302 – temporarily moved, 305 – use proxy)
• 4XX – error (403 – forbidden)
• 5XX – server error (500 – Server Internal Error, 501 – not implemented)
• 6XX – global failure (606 – Not Acceptable)

Let’s see now how a session is established, in order to see which network elements are involved and which messages are exchanged :

User@ibm.com is the first UA, user@example.com the second one. As we can see, between them there can be on or more proxy server, that are used to route the messages from the UAC to the UAS. Further and more detailed explanations of this flow Is out from the scope of this article.