Since RFC 2543 was released in March 1999 as an Internet standards track protocol, SIP has attracted enormous interest. Many companies, including those from the traditional telecommunications environment and others such as cable TV providers and Internet Service Providers (ISPs), have recognized that SIP offers great potential. In the process, however, several individuals and companies have found that SIP would be better if some enhancements or extensions were added. A large number of extensions to SIP have been proposed and are being handled in the SIP working group of the IETF.

As of this writing, numerous revisions have been made to the SIP specification as part of its progress along the standards track from proposed standard to standard. In addition, a number of Internet drafts have been prepared that we can expect to be included in the SIP specification in the future or that may become RFCs in their own right. The following descriptions address some of the most significant extensions to SIP.

One example of an extension that has been included within the revised SIP specification is the 183 (session progress) response code, which is typically used to open a one-way audio path from called end to calling end. The 183 response is often used when interworking with the SS7 network, where the Address Complete Message is used to open such a one-way path so that ring-back tone can be relayed from the called end to the calling end.

Another addition to the latest...