SIP Proxy Servers Explained
What is a SIP Proxy Server?
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SIP Proxy Server - a definition A SIP Proxy Server acts as an intermediate router for SIP requests and is always located somewhere in the path between two end points. A SIP Proxy Server would never be the end point itself. When it receives a SIP request it will examine it and decide whether to pass it on unmodified, change it in some way and forward it, re-route it to an alternative destination (or destinations - see "Forking Proxy below") or reject it. When it rejects a request, it must send back a response giving a reason. One of the core functions of a Proxy Server is to ensure that a SIP INVITE request is routed to the correct onward destination. It would normally have a "dial plan" for this purpose. The actual destination might be a PSTN gateway, another Proxy Server, a SIP phone (often referred to as a User Agent) or a Media Server. Most SIP Proxy Servers include additional functionality to allow them to support remote or non-static SIP devices. The actual component services that do this are explained in more detail below. |
How does a Proxy Server support remote SIP devices?
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Registration and location services No matter where your IP phone is physically located, you will want it to be able to receive calls. The registration process, which is normally initiated as soon as the phone is switched on, allows the phone to register its location in a central database called a Location Server. The Proxy Server is closely integrated with the Location Server and is able to route inbound calls to the correct endpoint or user device based on the number called. For a more detailed explanation of the registration and location server processes, click here. Far-end NAT Traversal and Media Proxies Servers that offer far-end NAT traversal often use an integrated Media Proxy. They act as a Proxy Server, but modify the contents of the SIP packets (in both directions) to make it look as if the terminating device requires the media stream to be opened at the address of the Media Proxy rather than at its own address. This means the device that initiated the call and the one that answered it have a media stream connection that goes via the Media Proxy which is sitting between them (transparently proxying the media). The Media Proxy is always on a public IP address which means devices behind NAT are able to initiate connections to it where they would not have been able to initiate connections to each other. |
What other special functions can a SIP Proxy Server offer?
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Forking Proxy A Proxy Server is said to be acting as a Forking Proxy if it duplicates the SIP request and forwards it to multiple locations. The Forking Proxy must also handle the multiple responses and filter them to ensure that the device that sent the original request is not confused. Redirect Server This is another specialised function that can be enabled within a SIP Proxy Server. A Redirect Server responds to SIP INVITE requests by returning a 3xx response. Alternatively, it may reject the call and send a response to this effect. The main 3xx responses are 300, 301 and 302. Along with with the 3xx response, the Redirect Server sends back one or more alternative addresses using the SIP Contact header. The device that initiated the call and sent the original INVITE receives the 3xx response and knows that it must re-try the call using the address (or addresses) provided by the Redirect Server. |
