Adapting SIP for application server requirements in 3G networks

The IP Multimedia Subsystem Service Control (ISC) protocol has been adopted by the 3rd Generation Partnership Project (3GPP) in Revision 5 for all signaling in a 3G IP Multimedia Subsystem (IMS) network. While the core protocol is essentially the Session Initiation Protocol (SIP) with extensions, 3GPP has adapted many of the functional elements in slightly different ways. The distinction of the roles (the user agent client [UAC], user agent server [UAS], proxy, redirect, and registrar), as defined in RFC 3261, blurs in the 3G architecture, giving way to more generalized SIP entities such as the proxy, interrogating, and serving call session control functions (P-CSCF, I-CSCF, and S-CSCF) and the SIP application server (AS). In this paper, we suggest that neither of the standard SIP entities, back-to-back user agent (B2BUA) nor proxy, meets the IMS needs of having the SIP AS maintain complete control over the session. Rather, a blend of these two functions is required for a certain class of applications. Using a new abstraction called the pseudo∗∗∗dialog, we show how to augment a B2BUA to provide both the proxy and B2BUA modes of service on a per-call basis.