Chapter 13 - SIP Proxies

Abstract

This chapter focuses on session initiation protocol (SIP) proxies, which are defined in the base SIP spec RFC 3261, and explains the critical role played by it in the routing of SIP messages in the SIP architecture. SIP proxies can be classified into inbound and outbound proxies, according to the functions they fulfill. Outbound and inbound proxies are typically implemented together in the same piece of equipment with the registrar for the corresponding domain and such an aggregated entity is often referred to as the SIP server for the particular domain. Another classification of SIP proxies is based on the amount of state information stored by them, and in this case, the SIP proxies can be split into transaction stateful and transaction stateless proxies. The transaction stateful proxy, also called the stateful proxy, has a server transaction associated with one or more client transactions. A stateless proxy does not have a notion of a transaction and thus it stores no information and does not keep any response context. A stateless proxy routes requests and responses based only on the parameters present in the received message. In this chapter, a simple SIP proxy is built, which also acts as a simple registrar and location service. In other words, this is a simplified SIP, which is used here to create a more realistic end-to-end scenario composed of two communicating soft phones and a SIP server that acts as the home server for both of them.