Protocol helpers and Session Initiation Protocols
The TORRENT project team set out to build a test-bed for networks carrying a variety of services whether using connection-based or connectionless architectures. The aim was to use the test bed to investigate different technologies for delivering a range of services to residential users. The framework developed by the project uses proxy servers, placed between the network and the user or web server, to route the packets belonging to different services. Proxies can be complemented by using a protocol helper; software that transparently extracts dynamic information from packets passing through the proxy. A connection-tracking protocol helper can assist in the management of connections set up between servers, or between client and server without involving the proxy server that is situated between them. Proxy servers usually terminate the signalling relation, meaning they can reply on behalf of the server 'behind' them, whereas the protocol helper does not. The SIP protocol is a service provisioning mechanism that uses location-independent addresses much like e-mail. In this, and other aspects such as signalling options, protocol extensions and application scenarios that suit it for sophisticated services, it is a relatively complex protocol. The project team investigated the viability of using a connection-tracking protocol helper with the SIP protocol but found that this could only be achieved in simple cases and with great difficulty. The researchers successfully implemented the protocol helper in a single client test case involving one control and one media connection, but consider that it is not valid for multiple related traffic streams. Whereas protocol helpers are of use to simpler protocols such as Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP) the TORRENT project concludes that they do not offer a solution for a complex protocol like SIP. The limiting factor appears to be that the protocol helper cannot handle more than two related data streams or features such as network address translation and complex calls.