Contemplating some open challenges in SIP

Session Initiation Protocol (SIP), despite its beginnings as a protocol for multimedia session signaling over the Internet, has rapidly evolved as a prominent signaling protocol for Internet telephony. While most efforts to evolve the protocol have resulted in greater simplicity, flexibility, and extensibility, some uncoordinated efforts have engendered a few open challenges for the protocol implementers. In this paper, we contemplate some of these challenges, including the 3rd Generation Partnership Project's (3GPP's) use of SIP, flexibility in the service model, lack of ratification of user/application interaction model, and network address translator (NAT) traversal. In analyzing these issues, it is important to recognize two facts early. First, some of these issues are inherent in Internet telephony in general and are not endemic to SIP. Second, although standards bodies are attempting to address most of these issues, competing and proprietary implementations are emerging to the detriment of the entire SIP community.