Media (Rtp/Rtcp/T.38) - AudioCodes Mediant 4000 SBC User Manual

User's Manual

11.7.2.2 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'' on page
145

RTP No-Op packets according to the avt-rtp-noop draft - see ''No-Op Packets'' on
page 146
The figure below illustrates a typical network architecture where the remote UA is located
behind NAT:
11.7.2.2.1 First Incoming Packet Mechanism
In scenarios where the remote user agent (UA) resides behind a NAT server, it's possible
that the device, if not configured for NAT traversal, will send the media (RTP, RTCP and
T.38) streams to an invalid IP address and UDP port. In other words, it will send the media
to the private IP address:port of the UA and not the public address (of the NAT server) and
therefore, the media will not reach the UA. When the UA is located behind NAT, although
the UA sends its private IP address:port in the original SIP message (INVITE), the device
receives the media packets with a source address of a public IP address:port (i.e.,
allocated by the NAT server). Therefore, to ensure that the media reaches the UA, the
device must send it to the public address.
The device identifies whether the UA is located behind NAT by comparing the source IP
address of the first received media packet with the IP address and UDP port of the first
received SIP message (INVITE) when the SIP session was started. This is done for each
media type--RTP, RTCP and T.38--and therefore, they can have different destination IP
addresses and UDP ports than one another.
You can configure the device's NAT feature to operate in one of the following modes:

[0] Enable NAT Only if Necessary: NAT traversal is performed only if the UA is located
behind NAT:
•
UA behind NAT: The device sends the media packets to the IP address:port
obtained from the source address of the first media packet received from the UA.
•
UA not behind NAT: The device sends the packets to the IP address:port
specified in the SDP 'c=' line (Connection) of the first received SIP message.
Version 7.2
Figure 11-8: Remote UA behind NAT
145
11. Network
Mediant 4000 SBC