Support of end points on IPv6 networks

PortaSIP can correctly process calls sent to and received from endpoints registered on IPv6 networks. This allows service providers to support communication between IPv4 and IPv6 networks and deliver services to users on either network type.

Since IPv6 hosts are not directly reachable by the IPv4 network and vice versa, technologies such as DNS64/NAT64 are used for interactions between networks.

This is how it works: When a IPv6-only user agent tries to reach a IPv4-only resource by their hostname, a DNS64 server creates a synthetic IPv6 address for the destination resource that the IPv6-only user agent is capable of addressing. NAT64 then translates this synthetic IPv6 address into an IPv4 address.

However, for SIP calls between IPv6-only and IPv4-only devices, not only the host address of the SIP server needs translation. A SIP message might contain IP addresses in various headers, and for the call to be correctly handled, they may also need to be translated.

Consider a user whose softphone is connected to an IPv6 network calling a colleague whose IP phone is connected to an IPv4 network, registered with PortaSIP as a SIP server. The softphone sends an INVITE request. The DNS64/NAT64 communication routes this INVITE outside the IPv6 network to PortaSIP. PortaSIP processes the call involving DNS64/NAT64 networks in the following way:

1. PortaSIP checks the Via SIP header (added by the calling device) to define whether a call comes from an IPv6 endpoint and checks the Route SIP header to see if the destination is also on IPv6.
   The internal flow between the PortaSIP components, Dispatching SBC, and SIPProxy, uses IPv4.
2. When an IPv6 endpoint is detected (either in the Via or Route header), PortaSIP records its own address in the Record-Route header in the form appropriate for the next hop (IPv4 or synthetic IPv6), so that subsequent in-dialog requests reach it correctly.
3. When a media stream must be conveyed via a RTPProxy, PortaSIP inserts the IPv6 address into the c attribute of the SDP field.

This PortaSwitch feature helps service providers keep up with the ongoing internet transition to the IPv6 network.