Optimizing The Media Path For Symmetrical Nat; Filter Behaviour; Registering Without Ua Support - Snom 4S NAT Filter Admin Manual

office in a private network), the media would have to flow first from Tokyo
via New York then via Sydney and then back to Tokyo. Considering the
speed of light, the delay would at least be around one second; practically
it would be much higher although the user agents are located in the same
network.
Unfortunately, it is not trivial to make the media path shorter.
There have been some attempts to reduce the problem, but it is much
easier to address the problem from the user agent. If the user agent uses
ICE, it will try all addresses listed in the SDP attachment, including the
port allocated by the NAT Filter. If there should be a shorter path, it will
switch to this shorter path. If there is no other way of if the other side
does not support ICE, it will fall back to the NAT Filter-allocated port which
will work in all cases.
2.2.8 Optimizing the Media Path for Symmetrical
NAT
In the case when both user agents are behind symmetrical NAT
the NAT Filter approach will ensure that media will flow between the
user agents. However, the Tokyo example shows that this might result in
intolerable media delay.
To address this problem, TURN [5] comes into play. The idea
behind this approach is to allocate identities on several places in the
Internet and to propose all of the allocated ports to the other user
agent. If the ports are allocated on all continents, the other user agent
will automatically pick the TURN server with the shortest delay. In the
Tokyo example, a TURN server located in Japan will reduce the delay
to a tolerable level (if there is not even a direct path between the user
agents).

2.3 Filter Behaviour

2.3.1 Registering without UA Support

When a user agent registers, it puts its IP address in the top
Via. If the user agent is on public Internet or properly supports NAT, this
Via will match the perceived IP address. In this case the filter does not
interfere with the registering process and just forwards this packet.
12 • Architecture
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