Detecting The Right Nat Filter - Snom 4S NAT Filter Admin Manual

The following table shows an example configuration for Linux
named(8):
_sip._udp IN SRV
_sip._udp IN SRV
_sip._udp IN SRV
_sip._udp IN SRV
_sip._udp IN SRV
_sip._udp IN SRV
If one of the servers should become unavailable, the SRV algorithm
will make sure that the other servers will be contacted. The user agents
that are refreshed by that particular server will become unreachable until
the user agents initiate a new REGISTER request. Therefore, you should
make sure that you servers have a high uptime probability and that the
registration period is not too long. We think that registration periods of
thirty minutes up to one hour are a good balance between service failure
time and performance.

2.5 Detecting the right NAT Filter

User agents must detect which server in the server farm is
nearest to the user agent. This is an important feature for a company or
operator that has user agents scattered around the globe. Example: A
company has offices in Berlin, Tokyo and Dallas and locally operates NAT
Filter servers. When a user agents is located in Tokyo, it should use the
Tokyo server. This could be set up manually; however it is also possible to
automatically pick the best server.
To detect the nearest server the user agent sends STUN packets
to all possible servers (the servers with the lowest priority in the SRV list).
The user agent picks the server that responds first. Alternatively, the user
agent could send more test packets and take the mean response time for
making the decision.
The snom 4S NAT Filter includes a STUN server that operates on
the SIP UDP port. User agents should send their test packets to the SIP
port.
18 • Architecture
[ 4 S N A T F
S N O M
3 5 5082 frankfurt1
3 5 5082 newyork1
3 5 5082 newyork2
3 5 5082 newyork3
3 5 5082 tokyo2
3 5 5082 tokyo1
]
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