SIP Myths

On this page, we try to present some of the myths surrounding SIP and provide the real truth. We hope that this might help in your fairly judging the technology and deciding whether this is the right solution for you, either short-term or long-term.

1. SIP is an Emerging Protocol

The first drafts of SIP were developed in 1995 and first published for public review in February 1996! How long does it take for a technology to emerge? Planets emerge. Stars emerge. Should it take so long for a VoIP technology to emerge?

As of today, SIP is , counting from the date the first specification was published for public comment.

SIP is hardly new and it has been updated and revised for years. SIP is really quite an old protocol, yet people continue to apply the word "emerging" to the technology. Even after all of these years, the capabilities that SIP delivers are not exceptional. At this point, SIP is a very mature protocol that delivers basic voice and video services.

2. SIP is Simple

This is false. Telephony, by its very nature, is not simple. In order to implement the most basic and practically useless call flow, sure it looks simple. However, if you want to actually create something useful, there is an inordinate amount of complexity involved. SIP was originally designed to simply make a "black phone" ring. From there, SIP was expanded and extended to allow for more general-purpose telephony. Further, it was discovered later that SIP over UDP was not a very workable solution, so TCP was recommended for most calls. To implement a basic SIP phone, one needs to support TCP and UDP, one needs to support most all of the features in RFC 3261 and many of the features from dozens of other RFCs. Further, because there are reliability issues with transmitting messages, the concept of reliable provisional responses was introduced and is necessary to ensure proper functionality. We could go on, but to say the least, SIP is hardly simple and is, in many respects, more complex than more capable video conferencing standards like H.323.

The charts at VoIP RFC Watch are useful.

3. SIP Can Do Many Things that Were Never Possible Before

There is nothing that SIP can do that other IP-based communication protocols cannot do. In fact, much of the functionality that SIP has been trying to deliver to the market for years was delivered by competitors like Skype in a very short time. Further, SIP is still very weak in terms of basic audio/visual communications, something at which H.323 excels. SIP has the potential to enable other forms of communication, of course, and instant messaging is one. But, it is important to note that H.323, Skype, or anything else can also deliver those same capabilities. So, where is SIP's advantage here? It does not have one.

4. SIP Will Enable All Kinds of New Applications

SIP or any other IP-based system could enable a host of new applications. The fact is, SIP is primarily used only as a PSTN replacement and really offers nothing more than basic voice and video service. In short, SIP has often been "over-sold" in terms of capabilities.

SIP followed the design model of H.323 significantly, though some will argue with that. The fact remains that the SIP design leads to the creation of monolithic applications where the only features you get are whatever the manufacturer put into the phone or video terminal. It is not possible with SIP, for example, to associate your phone with a PC or tablet so that the phone might be used for voice and your PC or tablet used for data conferencing or file transfers.

Of course, SIP could be expanded with more capabilities and interconnected with other devices. Some have used proprietary methods to do just that. However, such topics have been the study of newer multimedia communication platforms and is really beyond the scope of SIP.

The challenge of adding more sophisticated functionality to SIP is also complicated by the fact that SIP proxies, softswitches, session border controllers and other devices in the network would likely not support any new functionality without also being upgraded.

5. SIP Is Extensible, Whereas H.323 is Not

Both SIP and H.323 have defined mechanisms for extending the protocol in standard and non-standard ways. In fact, both protocols have a number of vendor-specific extensions that exist in real-world deployments. It is not clear how this rumor ever got started, but it is patently false.