You could have problems with NAT, but SIP protocol has STUN and ICE which are responsible for dealing with this. Check that out. Btw i had idea SIP + JSON/HTML

You could have problems with NAT, but SIP protocol has STUN and ICE which are responsible for dealing with this. Check that out. Btw i had idea SIP + JSON/HTML + DHT which would not suffer with "man in the middle attack" that HTTP has as designed decision. You could connect, for example, with HTTP/FTP/... protocols to your buddies directly. You just have to make right SDP packet when calling them ...