Prototyping SIP-based VoIP services in Java

Next to the binary-based H.323 signaling protocol for IP telephony developed by the telecommunication community (ITU-T), a more lightweight text-based protocol was deemed necessary to address the specific needs of the Internet community. The session initiation protocol (SIP) (see Handley, M. et al., IETF-RFC 2543, March 1999) was introduced in the IETF conference control working group and has meanwhile gained maturity and support. This paper presents a Java-based implementation of the SIP protocol stack, based on a layered SW architecture, reusable object oriented software components and application programming interfaces (API). This protocol stack has been used as a basis to realize the main network components necessary for SIP based call setup: user agent, proxy server, redirect server, and registration server. The interaction between these components is illustrated through a number of service scenarios, focusing on PC-to-PC communication within the context of a virtual private network (VPN) service.