snom technology AG • 43
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4.4.1 Register Timeouts
The
Refresh Interval
for the UA
is the number of seconds
between NAT refreshes from the NAT Filter. The NAT Filter keeps track of
registered user agents and keeps their NAT port binding alive with pack-
ets. Typically, a port timeout is 60 seconds; however, because this value
must handle the smallest timeout, a value of 15 seconds seems more ap-
propriate. The filter sends the keep-alive packets regardless of the trans-
port layer. Although on TCP most implementations keep the connection
alive for a much longer time, some implementations close their ports after
a short timeout. Therefore, TCP connections must also be refreshed.
You can use two different methods for refreshing. If you use “
OP-
TIONS for refresh”
, the filter sends out OPTIONS SIP requests to the
registered user agents. This method has much more traffic overhead than
other methods. But it is the only one ensuring compatibility with existing
implementations and guarantees that the user agent sends something
back from the inside NAT. This is important in some NAT types as they do
not allow refreshing NAT bindings from the outside. If you turn the flag
off, the filter will send short white space packets to the registered user
agent.
The value directly influences the keep-alive traffic caused by the
NAT Filter.
4.
