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SIP interoperability and testing

Session Initiation Protocol (SIP) is a signaling protocol typically used for telephony, instant messaging and Internet conferencing. This article discusses the interoperability and testing being done to support this protocol.

Session Initiation Protocol (SIP) is a signaling protocol typically used for telephony, instant messaging and Internet conferencing. It was developed by IETF in 1999. The text-based protocol is similar in many respects to HTTP and SMTP. This article will discuss the interoperability and testing being done to support this protocol.

There is a portal which lists all the SIP interoperability test events which you should definitely take a look at. These special tests take place over the course of a week, and are open to anyone that has a working type SIP product, but unfortunately not to the general public. These tests, aptly named Session Initiation Protocol Interoperability tests (SIPITS) are done with the goal of increasing the usage and functionality of SIP and its implementations. The next test is on April 17-21, 2006, in Tokyo, Japan. The SIPITs are held bi-yearly with different companies hosting each event. One example of tests they do is peer-to-peer tests, which focus on scenarios that require the coordination of two or three implementations. A summary of one test done at a recent conference is a multi-proxy forking. This test stresses, among other things:

  • via formation (including branch) and parsing
  • hop-hop non-200 final responses and ACKs
  • e-e 200 final responses and ACKs
  • propagation of client-initiated CANCEL

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They encourage only people with endpoint implementation (including gateways) to join. In this case, any example capable of parallel forking is acceptable. Parallel forking enables a user to be called simultaneously on several SIP devices. The faint-hearted need not apply!

One product I have looked at is sipX, which is marketed as an open source enterprise PBX. The SIP-based VoIP product combines call routing using proxy servers from other products that are distributed through SIPfoundry, a Massachusetts based not-for-profit organization (founded in 2004).

The Linux-based product is a 100% SIP implementation, intended for end-users, OEMs and developers. It combines a PBX with voicemail, auto-attendant and a SIP proxy. Here is a screen print of what their GUI software looks like.

I also was impressed with the features of the voice mail system, as well as the interface:

This foundation clearly illustrates the importance of the interoperability and testing of their products. As a result of these initiatives, they have established a test framework (SFTF) which allows SIP vendors to test their devices for common errors. The testing is done with the intent to improve the interoperability of these devices.

There are two parts to the framework. The first allows programmers to write their own tests for SIP devices. The second is a group of implemented tests, which use the framework to test SIP user agents for errors. They promote the standardization of SIP and the interoperability of SIP products, which is important to the future growth of SIP.

Through this project, SIPfoundry is working towards an achievable goal of ensuring that all products subscribe to certain standards that will drive the technology. Too often today, companies will give lip service to the importance of standards, testing and working together. This is definitely the case with more traditional profit-based companies. Where open source has the advantage here is that much of what is done is non-profit, so there is more of an incentive to work with one another, rather then compete for profits. That bodes well for the success of the industry moving forward.

Ken Milberg is the founder of Unix-Linux Solutions. He is also a board member of Unigroup of NY, the oldest Unix users group in NYC. Ken regularly answers user questions on Unix and Linux interoperability issues as a site expert on

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