snom technology AG • 21
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S N O M
4 S N A T F
I L T E R
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2.5 Detecting the right NAT Filter
User agents must detect which server in the server farm is near-
est 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 agent is located in Tokyo, it should use the
Tokyo server. This could be set up manually; howeve, 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.
2.6 Requirements on User Agents
Generally, there are two categories of user agents: The non NAT
aware user agents and the STUN/ICE capable user agents.
2.6.1 Non NAT-Aware User Agents
Non-NAT aware user agents must have at least the following fea-
tures:
1.
Must send SIP UDP packets from the port where they receive SIP
traffic
2.
Must start sending media immediately after receiving SDP
Requirement 1 is not fulfilled by the default configuration of the
Cisco 7960; however there is a setting that enables this feature. Most
known user agents support this feature, however.
Requirement 2 sometimes creates problems with user agents
who don’t send media during silent periods. In this case, users have to
start speaking before two-way audio can be established.
2.
