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. 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.

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 practically will be used in most cases when the call is
terminated 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 a
dynamically provisioned parameter. This parameter is usually provided 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).
By the setting for trusted IP addresses, the filter will accept these
headers only from explicitly listed addresses. After the header has been
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