Introduction to SIP

The Session Initiation Protocol (SIP) is the dominant signaling protocol used in VoIP today. It is responsible for the establishment, control and termination of sessions by exchanging ASCII-text-based messages between the endpoints. This post goes through the basic components of SIP: messages and logical entities.

There are two types of SIP messages: requests and responses. The table below summarizes the most important SIP requests and the types of SIP responses:

SIP Requests

SIP Responses

INVITE It initiates the session. 1xx Provisional The request is received and is being processed.
REGISTER It requests the registration of the user. 2xx Success The request was successfully received and accepted.
ACK It confirms final responses. 3xx Redirection Further process is needed for this request.
BYE It terminates the session. 4xx Client Error The request has bad syntax or cannot be processed.
CANCEL It cancels the current signaling process. 5xx Server Error The server cannot process the request.
OPTIONS It negotiates the capabilities of endpoints. 6xx Global Failure Request cannot be processed by any server.

There are various logical entities in a SIP environment that are responsible for processing the SIP messages and they serve different roles. The table below provides an overview of logical entities:

SIP Logical Entities Description
User Agent (UA) It is the endpoint of a SIP session and can function as a Client (UAC) or a Server (UAS). The clients are sending new requests (e.g. to initiate or terminate sessions) while the servers are processing and responding to the requests.
Proxy Server It acts on behalf of the end users functioning both as UAC and UAS.
Registrar Server It is responsible for registering the contact and location information of the users (processing the REGISTER requests).
Redirect Server It returns information about the location of the called party.
Back-To-Back User Agent (B2BUA) Acts as an intermediary in a SIP session. It can process SIP messages, alter their content and regenerate them to the other party. Typical example of B2BUA is the Session Border Controller (SBC).

In a call scenario all these entities are working together and exchanging SIP messages during the signaling phase. SIP is also co-operating with other protocols in a VoIP environment such as the Session Description Protocol (SDP). All the above VoIP entities and protocols, as well as the various call flows and troubleshooting procedures will be analysed extensively in later posts of this blog.

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About TelcoNotes

IP & VoIP networking

Posted on February 17, 2013, in VoIP and tagged , . Bookmark the permalink. 3 Comments.

  1. Thank you. I read your article, it is very interesting.

  2. You’re welcome. You will find quite a few articles on SIP and VoIP on my blog. Feel free to stop by anytime. I promise to do the same with yours. 🙂

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