Electro/Telecom Industry Trends
February 2005
The World According to SIP
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Once only a small piece of the Internet multimedia communications puzzle, the Session Initiation Protocol (SIP) is quickly taking on a larger role in helping to enable some of the latest developments in Internet-based communications. Besides being adopted by many as the protocol of choice for Voice over IP (VoIP) communications, SIP is also demonstrating its potential in enabling other types of applications such as interactive gaming, instant messaging, music and video on demand, and voice, video, and Web-based conferencing.
Despite SIP's high profile these days, the protocol initially had a much smaller role to play when it was first developed in 1996. At that time, it was created as a way to initiate sessions on the Multicast Backbone (MBone), an experimental network developed in the early 1990s as an overlay to the Internet that could handle IP multicasting. While MBone has since given way to newer technologies, SIP has lived on.
The importance of SIP grew rapidly as interest in Internet telephony blossomed in the late 1990s. Then, in 1999, it received its stamp of approval when the Internet Engineering Task Force (IETF) accepted SIP as an official standard. Since its release, more and more VoIP vendors have been relying on the protocol to facilitate the execution of their applications, but interest hasn't stopped there. It's also being adopted for a number of other uses, and has also demonstrated its potential to become a key part of future Internet-based communication applications.
The attraction of SIP is easy to see. First of all, SIP was developed based on another commonly used Internet protocol, HTTP (HyperText Transfer Protocol). Both are request-response protocols, and both use text-based messages with a similar syntax to transfer information. These similarities have ensured the popularity of SIP among developers already familiar with HTTP.
In addition, SIP is simple. It doesn't define the session or the application being used — its only job is to initiate sessions. Instead, SIP relies on other protocols, such as the Session Description Protocol (SDP) to describe the sessions. In effect, the protocol stands far removed from the applications used during a session, allowing SIP to be used to initiate sessions for any number of applications.
According to the SIP Center, a portal designed to encourage the commercial development of SIP, the protocol is responsible for only a few functions:
· Finding the recipient of a call
· Allowing call participants to agree on the features to be used during the call
· Helping callers to manage participation by allowing them to invite new participants into the call or manage functions such as call transfers or holding
· Allowing users to change the features being used during a call
Despite the widespread enthusiasm about SIP, it does have some drawbacks. Since it doesn't regulate the applications being used to administer the sessions it is initiating, it doesn't offer any control over quality of service. In addition, the SIP specification doesn't define how other protocols must work with SIP to enable applications — in other words, it's up to the individual vendors to work out these details.
However, for many, these problems are insignificant compared to the potential that SIP offers to both current and future Internet-based communications applications. And with more vendors embracing the protocol, it looks like SIP is here to stay.