Media (RTP/RTCP/T.38)

When a remote UA initiates a call and is not located behind a NAT server, the device sends the media (RTP, RTCP, and T.38) packets to the remote UA using the IP address:port (UDP) indicated in the SDP body of the SIP message received from the UA. However, if the UA is located behind NAT, the device sends the RTP with the IP address of the UA (i.e., private IP address) as the destination instead of that of the NAT server. Thus, the RTP will not reach the UA. To resolve this NAT traversal problem, the device offers the following features:

■

First Incoming Packet Mechanism - see First Incoming Packet Mechanism

■

RTP No-Op packets according to the avt-rtp-noop draft - see No-Op Packets

The figure below illustrates a typical network architecture where the remote UA is located behind NAT: