Media (Rtp/Rtcp/T.38) - AudioCodes Mediant 800B User Manual

reason) is unable to identify NAT using its regular NAT Detection mechanism. This feature
is enabled per specific calls using IP Groups. To configure this feature, use the 'Always
Use Source Address' parameter in the IP Group table (see ''Configuring IP Groups'' on
page 248). If this feature is disabled, the device's NAT detection is according to the settings
of the global parameter, 'SIP NAT Detection' parameter.
12.11.2.2

Media (RTP/RTCP/T.38)

When a remote UA initiates a call and is not located behind a NAT server, the device
sends the RTP (or RTCP, T.38) packets to the remote UA using the IP address / UDP port
in the SIP message (Contact header). 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
136

RTP No-Op packets according to the avt-rtp-noop draft - see ''No-Op Packets'' on
page 137
The figure below illustrates a typical network architecture where the remote UA is located
behind NAT:
12.11.2.2.1 First Incoming Packet Mechanism
If the remote UA resides behind a NAT device, it's possible that the device can activate the
RTP/RTCP/T.38 streams to an invalid IP address / UDP port. To avoid such cases, the
device automatically compares the source address of the first received incoming
RTP/RTCP/T.38 stream from the remote UA with the IP address and UDP port of the
remote UA when the session was initially opened (i.e., originally received INVITE). If they
are different, the device sets the destination IP address of the outgoing RTP packets to the
source IP address of the first incoming packet. The RTP, RTCP and T.38 can thus have
independent destination IP addresses and UDP ports.

To enable NAT resolution using the First Incoming Packet mechanism:
1. Open the General Media Settings page (Configuration tab > VoIP menu > Media >
General Media Settings).
2. Set the 'NAT Traversal' parameter to Enable.
3. Click Submit.
User's Manual
Figure 12-16: Remote UA behind NAT
136
Mediant 800B Gateway and E-SBC
Document #: LTRT-10278