AudioCodes Mediant 4000 SBC User Manual page 146

Note: 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.

[1] Disable NAT: (Default) The device considers the UA as not located behind NAT
and sends media packets to the UA using the IP address:port specified in the SDP 'c='
line (Connection) of the first received SIP message.

[2] Force NAT: The device always considers the UA as behind NAT and sends the
media packets to the IP address:port obtained from the source address of the first
media packet received from the UA. The device only sends packets to the UA after it
receives the first packet from the UA (to obtain the IP address).

[3] NAT by Signaling = The device identifies whether or not the UA is located behind
NAT based on the SIP signaling. The device assumes that if signaling is behind NAT
that the media is also behind NAT, and vice versa. If located behind NAT, the device
sends media as described in option [2] Force NAT; if not behind NAT, the device
sends media as described in option [1] Disable NAT. This option is applicable only to
SBC calls. If the parameter is configured to this option, Gateway calls use option [0]
Enable NAT Option, by default.

To enable NAT resolution using the First Incoming Packet mechanism:
1. Open the Media Settings page (Setup menu > Signaling & Media tab > Media folder
> Media Settings), and then from the 'NAT Traversal' drop-down list (NATMode),
select the required NAT option:
2. Click Apply.
11.7.2.2.2 No-Op Packets
The device can send No-Op packets to verify Real-Time Transport Protocol (RTP) and
T.38 connectivity, and to keep NAT bindings and Firewall pinholes open. The No-Op
packets can be sent in RTP and T.38 formats:

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"). The IETF document defines a
No-Op payload format for RTP. The draft defines the RTP payload type as dynamic.
You can configure the payload type as described in the following procedure (default 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).

To configure the No-Op packet feature:
1. Enable the feature, using the NoOpEnable ini file parameter.
2. Configure the time interval during which the device sends No-Op packets when
silence occurs (i.e., no RTP or T.38 traffic), using the NoOpInterval ini file parameter.
3. For RTP No-Op packets, configure the payload type of the No-Op packets, using the
RTPNoOpPayloadType ini file parameter.
User's Manual
Figure 11-9: Configuring NAT Traversal for Media
146
Mediant 4000 SBC
Document #: LTRT-40203