SIP protocol is designed group of MMUSIC (Multiparty Multimedia Session Control) Committee, a specification of the protocol presented in RFC 2543 is IETF (Internet Engineering Task Force), . The basis of the protocol working group MMUSIC laid down the following principles:
Personal mobility users: Users can move without restrictions within the network, so the services should be available to them anywhere in the network. The user is assigned a unique identifier, a network of services it provides, regardless of where it is located. To do this, a user with a special message – REGISTER – informed about their movements server location.
Scalable network: It is characterized, first of all, the possibility of increasing the number of network elements in its expansion. Server network structure, built on the basis of the protocol SIP, to fully meet this requirement.
Extensibility Protocol: It is characterized by the possibility of supplementing the protocol with new functions when introducing new services and adapt it to work with different applications.
An example is the situation where the SIP protocol is used to establish a connection between the gateways that interact with the PSTN by means of signaling OKS7 or DSS1. Currently, SIP does not support the transparent transmission of signaling information telephone signaling systems. Consequently, the additional ISDN services are inaccessible to users of IP networks.
Enhanced SIP protocol functions can be accomplished through the introduction of new message headers, which must be registered with the previously mentioned organizations IANA. If the SIP server accepts a message from him to unknown fields, it simply ignores them, and handles only those fields that it knows.