Dialogic 6.2 Manuale Utente

Understanding the SIP Protocol

November 2009

247

This section introduces the basic operations of SIP using simple 

examples. This section is tutorial in nature and does not contain any 

normative statements.

The first example shows the basic functions of SIP: 

„

Locating an end point

„

Signaling a desire to communicate

„

Negotiating session parameters to establish the session

„

Tearing down the established session

 shows a typical example of a SIP message 

exchange between two users, Caller A and Caller B. (Each message 

is labeled with the letter “F” and a number for reference by the text.) 

In this example, Caller A uses a SIP application on a PC (referred to 

as a softphone) to call Caller B on the SIP phone over the internet. 

Also shown are two SIP proxy servers that act on behalf of Caller A 

and Caller B to facilitate the session establishment. This typical 

arrangement is often referred to as the “SIP trapezoid” as shown by 

the geometric shape of the dotted lines in 

Caller A “calls” Caller B using Caller B’s SIP identity, a type of 

Uniform Resource Identifier (URI) called a SIP URI. It has a similar 

form to an email address, typically containing a user name and a 

host name. In this case, it is sip:callerb@biloxi.com, where biloxi.com 

is the domain of Caller B's SIP service provider. Caller A has a SIP 

URI of sip:callera@atlanta.com. Caller A might have typed in 

Caller B's URI or perhaps clicked on a hyperlink or an entry in an 

address book. SIP also provides a secure URI, called a SIPS URI. An 

example would be sips:callerb@biloxi.com. A call made to a SIPS URI 

guarantees that secure, encrypted transport (namely TLS) is used to 

carry all SIP messages from the caller to the domain of the callee. 

From there, the request is sent securely to the callee, but with 

security mechanisms that depend on the policy of the domain of the 

callee.

SIP is based on an HTTP-like request/response transaction model. 

Each transaction consists of a request that invokes a particular 

method, or function, on the server and at least one response. In this 

example, the transaction begins with Caller A's softphone sending 

an INVITE request addressed to Caller B's SIP URI. INVITE is an 

example of a SIP method that specifies the action that the requester 

(Caller A) wants the server (Caller B) to take. The INVITE request