1. A full cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port. Any external host can send a packet to the internal host, by sending a packet to the mapped external address.
2. A restricted cone NAT is the same […]

Real time media protocol based on UDP for voice. Separate from SIP.
SIP establish connection and port numbers for RTP. Not necessary using the same path.
Audio Quality depends on codec. ILBC used for google talk and Gizmo, Skype.

When a SIP phone starts, it first register itself with the SIP registrar.
1. SIP phone send HTTP request to SIP registrar
2. SIP registrar send SIP phone info to a location database (ip address etc)
3. ACK SIP phone
Basic call flow:
1. Invite from a SIP phone to SIP registrar (Proxy)
2. Proxy query location database and finds target […]