AudioCodes Mediant 500 MSBR, User Manual

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Version 7.2 145 Mediant 500 MSBR
User's Manual 15. Network
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.
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:
Figure 15-7: Configuring NAT Traversal for Media
2. Click Apply .