Requirements On User Agents; Non Nat-Aware User Agents; Stun/Ice-Aware User Agents - Snom 4S NAT Filter Admin Manual

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 at least fulfill the following
features:
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 gives sometimes problems with user agents that
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. As a general remark, snom recommends to
use NAT-aware user agents to reduce the network overhead and support
overhead.

2.6.2 STUN/ICE-Aware User Agents

STUN/ICE-Aware User Agents must implement the two IETF
standards. It is ok if the user agents use the built-in STUN server for
refreshing the bindings and learning the public IP address.
snom phones fall into this category starting with version 2.05a.
snom phones starting with version 2.05g support the self-refresh feature
using the P-NAT-Refresh header.
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