Stun/Ice-Aware User Agents; Defining The Maximum Session Time - Snom 4S NAT Filter Admin Manual

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
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 starting with version 2.05a fall into this category.
snom phones starting with version 2.05g support the self-refresh feature
using the P-NAT-Refresh header.
2.7 Defining the Maximum Session Time
There are a couple of timeout-related settings that terminate a
call when certain events fire (see below). However, when prepaid cards are
being used, operators want to limit the call duration to a certain time.
The filter has a mechanism to terminate calls anyway. It does
not only send BYE messages to both sides of the call, it also cuts media
relaying which in practice will be used in most cases when the call is ter-
minated via PSTN. This feature can be used to tear down calls when a
card expires.
The remaining call duration depends not on a static setting, but
on a dynamically provisioned parameter. This parameter is usually pro-
vided in the AAA procedure in the proxy. The proxy needs a simple way to
tell the filter how many seconds this call can stay up.
We decided to add a proprietary header called "P-Session-
Timeout" to the filter. When this filter is detected in a message that
belongs to an existing call, the filter sets the timeout for this call to the
value provided in the header (in seconds). After this time the filter will
terminate the call with the reason "Maximum Session Duration" (see
below, Call History).
With the setting for trusted IP addresses, the filter will accept
these headers only from explicitly listed addresses. After the header has
been used, the filter removes it from the packet so that the user agent
will not see this header.
22 • Architecture
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