Both protocols can be extended, but differ significantly in method. H.323 defines protocol elements called nonStandardParams, which are sprinkled throughout the protocol. H.323 can also be extended through a version change. Versions must be completely backward compatible, ensuring that each version of the protocol takes up more room than its predecessor. SIP, on the other hand, allows for standards-based extensions to perform specific functions.
Implementations can implement different sets of extensions depending on their needs.
SIP takes care of interoperability in this environment by providing a powerful set of tools for indicating and negotiating the set of extensions used in requests and responses. SIP can be extended in numerous ways, including adding headers, new methods, new bodies, and parameters to existing headers. Therefore, it is less restrictive than H.323 in where new things can be placed. Support for this kind of evolution is critical for long-term success. The protocol allows the controller to instruct the controlled to send and receive media from specific addresses, generate tones, and modify configuration.