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8 • Overview
[ S N O M 4 S N A T F I L T E R ]
• The first exception is a REGISTER request. When a user agent tries
to register and needs the support of the filter, the filter will set up a
local data structure representing the user agents. It will make sure
that the connection to the user agents stays alive. It will also make
sure that requests destined to the user agents will be forwarded
properly.
• The second exception is an SDP attachment. The filter checks if the
user agent needs support (or must be recorded) and will in that case
add a local contact to the SDP that can be used for media relay.
• The third exception occurs when the filter queries a web server for
routing information. In this case, it will send a provisional response
to stop the UAC from repeating messages.
These three exceptions make sure that all user agents will work
behind NAT, no matter what NAT-type or how many NAT-levels are being
used. If user agents support ICE, they will automatically find the shortest
media path to the other party (peer-to-peer).
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