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.

Industry Inroads

"SIP already has a tremendous stronghold in a multitude of areas," says David Fraley, an analyst at Gartner Inc. in Stamford, Conn. "Lately, SIP is the protocol of choice for new 3G wireless networks and phones."

Moreover, Cisco Systems Inc. and other manufacturers of IP public branch exchange (PBX) equipment are putting SIP into that hardware, while media gateway makers are adding it to network cores, Fraley says. Microsoft, Yahoo Inc. and America Online Inc. have made SIP a part of instant messaging sessions.

"What we'll have in the future is a single signaling protocol across all IP networks, and 10 years out all networks are going to be IP," Fraley predicts.

The principal intention, and advantage, of SIP is, of course, having a common signal across a multitude of devices, Fraley says.

But Tim McCracken, business development manager at Cisco, points out that interoperability isn't always as good as proposed. He says that for basic person-to-person calls, SIP works fine. However, beyond the basic connection and call waiting and call holding, there are hundreds of features, such as call transferring and call billing, that are being delayed due to interoperability problems.

Craig Cotton, a manager of product marketing at Cisco, says his company is "bullish" on SIP but questions whether it can evolve to deliver all the functionality enterprises want.

Cisco officials worry that SIP, written as a peer-to-peer protocol, could be inadequate for organizations that need a signaling protocol for client/server networks. But Fraley says SIP proxy servers can be created to overcome this problem.

At WorldCom Inc., SIP "has opened entire lines of business," says Teresa Hastings, director of multimedia services engineering. In fact, the company is already working with Microsoft on a beta version of a Windows XP server supporting SIP, says Henry Sinnreich, a distinguished member of engineering at WorldCom. The company in August launched a commercial IP telephony service called Connection that depends on SIP.

Despite such high hopes, there are concerns that SIP could pose network security problems as it becomes more universal. "If you have a single signaling technology running from telephones over the Internet into core networks and everywhere else, there's a lot more room for malicious behavior," says Fraley.

The peer-to-peer nature of SIP also raises related concerns about management and control in general, Cotton says.

"With the traditional client/server, [the datacom manager] is in control and you know all the users, but with peer-to-peer, you have a lot of features on a device and you don't go through a central repository," says Cotton. "Eventually, with a SIP proxy server, we'll get that control, but how long will it take in a pure SIP environment to get pure management and control and security?"

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