devices

SIP network elements Hardware endpoints — devices with the look, feel, and shape of a traditional telephone, but that use SIP and RTP for communication — are commercially available from several vendors. Some of these can use Electronic Numbering (ENUM) or DUNDi to translate existing phone numbers to SIP addresses using DNS, so calls to other SIP users can bypass the telephone network, even though your service provider might normally act as a gateway to the PSTN network for traditional phone numbers (and charge you for it.)

Today, software SIP endpoints are common. Microsoft Windows Messenger uses SIP. iChat AV, Apple Computer's AOL Instant Messenger compatible client, has supported audio and video chat through SIP, first in the 2003 public beta and now in the production version. On Linux, Ekiga supports SIP.

SIP also requires proxy and registrar network elements to work as a practical service. Although two SIP endpoints can communicate without any intervening SIP infrastructure, which is why the protocol is described as peer-to-peer, this approach is impractical for a public service. There are various softswitch implementations (by Nortel, Sonus and many more) that can act as proxy and registrar. Other companies, led by Ubiquity Software and previously Dynamicsoft (who are now no more, after being acquired by Cisco in 2004) have implemented products based on the proposed standards, building on the Java JAIN specification. These follow the SIP Servlet API, JSR 116. These products allow deploying applications of arbitrary complexity onto a telephony network. In particular, they play the role of Application Servers in the IMS architecture.

From the RFCs:

"SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users." "SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. " "Since registrations play an important role in SIP, a User Agent Server that handles a REGISTER is given the special name registrar." "It is an important concept that the distinction between types of SIP servers is logical, not physical."

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