SIP Protocol Structure

This chapter focuses on the structure of the session initiation protocol (SIP) and how its functionality is internally organized and achieved. RFC 3261 structures the SIP functionality in several layers that obtain services from the layer below and provide services to the layer above. SIP core sublayer is the highest layer where the specific functionalities of SIP entities are implemented and the SIP cores implemented in this layer can be split into two types, transaction users and transport users. The SIP transaction sublayer is utilized by the transaction user irrespective of the used network transport protocol. A SIP transaction consists of a single request and any response to that request and includes zero or more provisional responses and one or more final responses. The transaction sublayer assures reliable message delivery within each transaction and it contains the logic required to handle transactions and retransmit messages. The SIP transport sublayer is responsible for the actual transmission/reception of requests and responses over/from network transports and the network transport-layer functionality is normally exposed. Finally the concept of the SIP dialog is presented in this chapter, which is used for building user agents (UAs) and advanced SIP applications on top of UAs or back-to-back user agents (B2BUA).