QuickStudy: Session Initiation Protocol

Computerworld - Session Initiation Protocol (SIP), with its promise of serving as a single global signaling standard, has mushroomed in importance for networking in the past year. But it may be years from adoption because of technical barriers still to be surmounted, including problems with device interoperability and concerns that SIP will make networks more vulnerable, experts say.

The idea behind SIP is to provide a simple, lightweight means for creating and ending connections for real-time interactive communications over IP networks—mainly for voice, but also for videoconferencing, chat, gaming or even application sharing.

Since the Internet Engineering Task Force launched SIP in 1999, hundreds of vendors have started to sell SIP-enabled phones and proxy servers globally. In one significant move, Microsoft Corp. built support for SIP into the Windows XP operating system.

A typical corporate scenario using SIP for an IP phone call would go something like this:

Caller X needs to speak to Caller Y. Each of their companies has a SIP proxy server. X and Y can be using any of a variety of clients, including a PC software phone, or "softphone"; a SIP hardware phone; an analog phone with an adapter; or a SIP-enabled cell phone.

When it was turned on, X's client automatically sent a register message to his company's SIP proxy server, telling it to route calls to a specific IP address. X initiates a call to Y via a PC softphone by typing a text request that's sent to her company's SIP proxy server, which uses the Domain Name System to look up Y's domain. The invite request is forwarded to Y's company's SIP proxy server, which sees that X wants to call Y and forwards the invite request to Y's IP address.

Y's phone rings, or a screen pops up, and Y is asked if he wants to accept the call. His affirmative response, called a 200 OK, is sent to his company's proxy server, which forwards it to X's company's SIP proxy server, which sends the 200 OK to X's client.

An acknowledgment message, or ACK, is sent directly to Y's client, and the communication begins

SIP is designed to be a key component for integrated data and voice IP networks. For example, companies can run a cost-effective single wire to a desktop using IP (replacing the second line to a traditional phone) and have the PC operate as a softphone that enables a user to click on a name in a PC directory. The name is associated with a SIP URL, sending a message into a network cloud. Then, when a connection is established, the softphone user can communicate via a headset connected to the PC.