Handover-aware SIP-based VoIP provided by a Roaming-Enabled Architecture (REACH)

The protocols of voice-over-IP (SIP, RTP and RTCP) have several issues, for example the NAT and the firewall problem. They lead to solutions such as "back-to-back user agents" (B2BUA), "session border controllers" and helper- protocols such as "simple traversal of UDP through NATs" (STUN) and "traversal using relay NAT" (TURN). This paper focusses on a different challenge, a problem that arises when a user becomes mobile and his IP-enabled telephone is forced to change its network access technology due to network coverage issues ("perform a vertical handover"). We present a middleware-based solution that makes SIP-based VoIP handover-aware and effectively solves the NAT and firewall problem. This approach builds on the "roaming-enabled architecture" (REACH), a plugin-driven middleware that uses proxy servers. REACH offers relay plugins for multiple data capturing schemes required to provide an easy-to-use handover-aware solution, but VoIP was not supported yet. Here, we present a self- implemented "session border controller" for SIP and RTP that was divided into two relay plugins designed to be used within the infrastructure of REACH. This mechanism adds full support for VoIP to the REACH software suite, allowing telephony in combination with any kind of vertical handover such as hard-, soft and softer handovers in IP-based networks of any kind.