ABSTRACT
We describe how Session Initiation Protocol (SIP) signaling messages and media streams can cross intermediaries like session border controllers (SBCs) and network address translators (NATs), providing end-to-end seamless interworking services among the SIP end points. SBCs are being deployed between the two network boundaries by network operators. We address how interoperability can be offered, avoiding proprietary implementations. A whole new set of protocols have been created for NAT crossing, making NATs a kind of SIP application-aware back-to-back user agent devices, which can be used by SIP user agents for NAT crossing. The NAT crossing by SIP itself is a rather big area and requires a stand-alone separate discussion. We only briefly summarize this topic for the sake of brevity. In addition, we address the SIP public switched telephone network/Integrated Services Digital Network (PSTN/ISDN) protocol interworking that enables real-time communications between Internet Protocol (IP) and PSTN/ISDN end points. Again, the interworking between the IP packet switching-based IP telephony and circuit-switched-based telephony is also a huge area, and we only briefly address this topic here by providing some basic ideas of interworking.
