Snom 4S - Manual (Page 20)

20 • Architecture
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S N O M
4 S N A T F
I L T E R
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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.
In any case, customers are asked to contact their vendor in case
of problems and explanations. In general, snom recommends using NAT-
aware user agents to reduce the network and support overhead.
2.6.2 STUN/ICE-Aware User Agents
STUN/ICE-Aware User Agents must implement the two IETF stan-
dards. It is ok if the user agents use the built-in STUN server for refresh-
ing the bindings and learning the public IP address.
snom phones starting with version 2.05a fall into this category.
2.