SIP Sessions

Can SIP be a successful protocol?

Some time ago my colleague Jiri Kuthan recommended me to read RFC5218. In it the authors discuss what makes protocols succeed or fail. A successful protocol is defined as one that meets its design goals and is widely deployed.  The authors present some factors which they believe to be crucial for the success of a protocol and present some use cases in which they apply these factors to some successful and failed protocols. Among these factors the authors list the design, extensibility and openness of networking protocols.

While reading the RFC I started thinking, what would be the result of applying these factors on SIP:

Initial Success factors: These are the factors that help a protocol to become successful in the initial phase of their deployment

• Positive net value: SIP obviously solves a problem; namely that of establishing a session in IP networks. While SIP bears the promise of enabling all kinds of sessions it is mostly used for establishing voice calls. In this context it does not offer more functionality than traditional SS7 signaling, H.323 or Skype. The real positive net value of SIP is hence demonstrated when operators start deploying more SIP-based services such as presence and application servers that offer more flexible and intelligent communication services than we have today.
• Incremental deployment: SIP can be deployed without having to update the network routers. However, unlike the arguably most successful Internet protocol, HTTP, it is not sufficient to provide a server and a client. For a communication service to be of use there must be a lot of clients and users available. While there are already different providers offering VoIP services using SIP with millions of users, these providers act as islands that are connected over the PSTN. Hence, in order for SIP to excel on this point, more SIP-based peering between providers is needed.
• Open code availability: There are already different open source components needed for a SIP service. The SIP Express Router is an excellent and widely used SIP proxy. Asterisk and SEMS offer flexible and easy to use media services such as IVR or conferencing. On the user agent side, there are also different implementations of different quality.
• Restriction free: SIP is a provided as a patent free technology for all.
• Open specifications: The SIP specifications are provided by IETF and are open.
• Open maintenance: SIP is maintained by the IETF and is extended and fixed continuously. While this is surely a good thing, this has also led to a load of specifications that some might claim are too much.
• Good technical design: While SIP was being hailed at the beginning as the simpler alternative to H.323, it has gained a lot of weight over the years. Taking the same comparison factors used in RFC5218 - namely security and congestion control - then SIP does not seem so perfect as congestion control is not considered and it does not have a powerful concept for identity management. Also, deployment issues such as NAT traversal were only added at later stages.

Wild success factors: These are the factors that contribute to success and wide deployment:

• Extensible: While designed in the early stage for simple calls, SIP is now used for multi-party calls, presence and trunking scenarios. Also, the integration of new applications and services should be rather straightforward as SIP is not restricted to a certain usage scenario.
• Scalability: While we still do not have any experience regarding the cost and complexity of building a SIP infrastructure for hundreds of millions of users. I do not see a real reason why this could not be done.
• Security: SIP has different mechanisms for authenticating users and protecting the signaling traffic. However, it does not have explicit mechanisms for protection against DoS attacks or fraud.

Discussion

Looking at the points above it looks like SIP has more or less a positive result on the discussed factors. However, getting positive marks on the evaluation factors does not mean that a protocol will be a success. If we evaluate Skype based on these parameters then we should conclude that Skype should fail. There is no open source code or open specifications and the net value is not much higher than PSTN or SIP. However, the number of users of Skype is higher than that of SIP.

So does this mean that SIP will become a wild success? Well, I guess the answer is a very definite maybe! The success or failure of a protocol can only be judged 5 to 10 years after finishing the standardization - so we still have a few years in front of us. But, it has the needed success factors, and with more applications, peering relations and clearer business models, the chance that SIP will be wildly successful are pretty good.