About Session Initiation Protocol (SIP) proxy services
The Smoothwall supports a proxy to manage SIP traffic. SIP is often used to set up calls in Voice over Internet Protocol (VoIP) systems. SIP normally operates on port 5060 and is used to set up sessions between two parties. In the case of VoIP, it's a Real-Time Protocol (RTP) session that's set up, and it's the RTP stream that carries voice data. RTP operates on random unprivileged ports, and, as such, isn't NAT friendly. For this reason, the Smoothwall’s SIP proxy ensures that RTP is also proxied, allowing VoIP products to work correctly. The Smoothwall’s SIP proxy is also able to proxy RTP traffic, solving some of the problems involved in setting up VoIP behind NAT.
There are two types of SIP proxy: a registering SIP proxy, and a pass-through proxy. A registering proxy or registrar means that SIP clients can register so that they might be looked up and contacted by external users. A pass-through proxy merely rewrites the SIP packets such that the correct IP addresses are used and the relevant RTP ports can be opened. Some clients allow users to configure one SIP proxy – this is invariably the registering proxy, others allow for two proxies, one to which the client registers, and one to which the client users for access, a pass-through.
As with many types of proxy, the SIP proxy can be used in transparent mode. In transparent mode, the proxy is only useful as a pass-through. This mode is useful for those clients that don't support a second proxy within their configuration. If all your clients can be properly configured with a second proxy, you don't need transparent mode. If the proxy is operating in transparent mode, the nontransparent proxy is still available, so you can have a mixture of operation.