SIP Signaling

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SIP is based on the request-response paradigm. The following sequence is a simple example of a call set-up procedure:

1. To initiate a session, the caller (or User Agent Client) sends a request with the SIP URL of the called party.

2. If the client knows the location of the other party it can send the request directly to their IP address; if not, the client can send it to a locally configured SIP network server.

3. The server will attempt to resolve the called user's location and send the request to them. There are many ways it can do this, such as searching the DNS or accessing databases. Alternatively, the server may be a redirect server that may return the called user location to the calling client for it to try directly. During the course of locating a user, one SIP network server can proxy or redirect the call to additional servers until it arrives at one that definitely knows the IP address where the called user can be found.

4. Once found, the request is sent to the user and then several options arise. In the simplest case, the user's telephony client receives the request, that is, the user's phone rings. If the user takes the call, the client responds to the invitation with the designated capabilities* of the client software and a connection is established. If the user declines the call, the session can be redirected to a voice mail server or to another user.

* "Designated capabilities" refers to the functions that the user wants to invoke. The client software might support videoconferencing, for example, but the user may only want to use audio conferencing. Regardless, the user can always add functions - such as videoconferencing, white-boarding, or a third user - by issuing another invite request to other users on the link.

SIP has two additional significant features. The first is a stateful SIP server's ability to split or "fork" an incoming call so that several extensions can be rung at once. The first extension to answer takes the call. This feature is handy if a user is working between two locations (a lab and an office, for example), or where someone is ringing both a boss and their secretary.

The second significant feature is SIP's unique ability to return different media types within a single session. For example, a customer could call a travel agent, view video clips of possible holiday destinations, complete an on-line booking form and order currency - all within the same communication session.

SIP Methods

The commands that SIP uses are called methods. SIP defines the following methods:



SIP Method Description
INVITE Invites a user to a call
ACK Used to facilitate reliable message exchange for INVITEs
BYE Terminates a connection between users or declines a call
CANCEL Terminates a request, or search, for a user
OPTIONS Solicits information about a server's capabilities
REGISTER Registers a user's current location
INFO Used for mid-session signalling


SIP responses
The following are SIP responses:
•1xx Informational (e.g. 100 Trying, 180 Ringing)
•2xx Successful (e.g. 200 OK, 202 Accepted)
•3xx Redirection (e.g. 302 Moved Temporarily)
•4xx Request Failure (e.g. 404 Not Found, 482 Loop Detected)
•5xx Server Failure (e.g. 501 Not Implemented)
•6xx Global Failure (e.g. 603 Decline)

They closely resemble HTTP responses


Inside a SIP message


The Request line and header field define the nature of the call in terms of services, addresses, and protocol features. The message body is independent of the SIP protocol and can contain anything.