AudioCodes Mediant 800B User Manual page 158

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:

Auto-Detect: NAT is performed only if necessary. If the UA is identified as being
located behind NAT, the device sends the media packets to the public IP address:port
obtained from the source address of the first media packet received from the UA.
Otherwise, the packets are sent using the IP address:port obtained from the first
received SIP message. Note also that if the SIP session is established (ACK) and the
device (not the UA) sends the first packet, it sends it to the address obtained from the
SIP message and only after the device receives the first packet from the UA, does it
determine whether the UA is behind NAT.

NAT Is Not Used: (Default) NAT feature is disabled. The device considers the UA as
not located behind NAT and always sends the media packets to the UA using the IP
address:port obtained from the first received SIP message.

NAT Is Used: NAT is always performed. The device considers the UA as located
behind NAT and always sends the media packets to the UA using the source address
obtained from the first media packet received from the UA. In this mode, the device
does not send any packets until it receives the first packet from the UA (in order to
obtain the IP address).

To enable NAT resolution using the First Incoming Packet mechanism:
1. Open the General Settings page (Configuration tab > VoIP menu > Media > General
Media Settings).
2. Set the 'NAT Mode' parameter (NATMode) to one of the following:
•
[0] Auto-Detect
•
[1] NAT Is Not Used
•
[2] NAT Is Used
3. Click Submit.
12.10.2.2.2 No-Op Packets
The device's No-Op packet support can be used to verify Real-Time Transport Protocol
(RTP) and T.38 connectivity, and to keep NAT bindings and Firewall pinholes open. The
No-Op packets are available for sending in RTP and T.38 formats.
You can control the activation of No-Op packets by using the ini file parameter
NoOpEnable. If No-Op packet transmission is activated, you can control the time interval in
which No-Op packets are sent in the case of silence (i.e., no RTP or T.38 traffic). This is
done using the ini file parameter NoOpInterval. For a description of the RTP No-Op ini file
parameters, see ''Networking Parameters'' on page 923.

RTP No-Op: The RTP No-Op support complies with IETF Internet-Draft draft-wing-
avt-rtp-noop-03 ("A No-Op Payload Format for RTP"). This IETF document defines a
No-Op payload format for RTP. The draft defines the RTP payload type as dynamic.
You can control the payload type with which the No-Op packets are sent. This is
performed using the RTPNoOpPayloadType ini parameter (see ''Networking
Parameters'' on page 923). The default payload type is 120.

T.38 No-Op: T.38 No-Op packets are sent only while a T.38 session is activated. Sent
packets are a duplication of the previously sent frame (including duplication of the
sequence number).
User's Manual
Mediant 800B Gateway and E- SBC
158 Document #: LTRT-10296