Needless to say, SIP as it has been redefined for use within the IMS adds much better
security to the network than a simple VoIP deployment would (or could). This is
the primary reason that any service provider that is moving to an IP-based network
should seriously consider SIP and the IMS as its best means of success. The business
case is sometimes more difficult when it is risk-based, but it is the best business case
there is for deploying IP in the network. Anything less will leave the service provider
with serious vulnerability issues placing revenues at risk.
This chapter is by no means an exhaustive description of SIP, nor is it intended to
provide all of the details according to the IETF RFC 3261. That is covered in another
book, Session Initiation Protocol (SIP): Controlling Convergent Networks (McGraw-Hill,
forthcoming). The purpose of this chapter is to provide an overview of the SIP protocol
as it relates to the IMS.
SIP Protocol Structure
The SIP protocol comes in three parts:
?– Start/status line
?– Message headers
?– Message body
The start line is the first line within a SIP request message. It contains an address
referred to as the Request-URI, as well as other information regarding the version of
SIP creating the message and the SIP method (think of the method as the message
type). Here is an example of a standard request:
INVITE SIP:travis.
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