ll SIP extensions are designed to be modular SO that their use can be individually negotiated. I can pick one particular set of extensions for the first session I initiate and a completely different set for the next one. Negotiation guarantees real interoperability between all the SIP users in the network. That's something new in many voice applications where pro- tocols (ISDN User Part [ISUP], for instance) may have many incompatible flavors, necessitating the implementation of gateways between networks that fail to speak the same ISUP. Any implementation of gateways for pro- tocol translation is undesirable because it breaks the end-to-end model and because some features present in a protocol flavor can be lost in the trans- lation process. In contrast, SIP is a genuinely global protocol. Chapter 4 Scalability SIP pushes the intelligence to the end system and obviates the need to store state information inside the network during a session. Once a user is located at session establishment, end-to-end communication is possible between end systems without the server's assistance. Servers that do not need to monitor signalling for the duration of the session can handle a larger number of sessions. Some State in the Network In the next chapters, we'll see how SIP servers can be classified by the amount of state information that they store. But even when stateful, SIP servers that store state are used in the periph- ery of the network, leaving stateless SIP servers in the core of the network, where they have to handle a larger number of sessions. In part, SIP net- works are highly scalable because they shift stateless operation to the points where the network is stressed. SIP as a Platform for Service Creation This is undeniably the most important feature of SIP. All of the SIP features explained SO far are useful only insofar as they convert SIP into a good plat- form for user services. Reuse of Components SIP (deliberately) makes use of many Internet components also exploited by other Internet applications. This mak