Architecture; The Nat Filter And Sip - Snom 4S NAT Filter Admin Manual

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2.1 The NAT Filter and SIP

In the SIP architecture, the filter acts as the first proxy that is
contacted by user agents. There are two ways to make sure that the
relevant traffic gets routed trough the filter:
• User agents can be set up to use the filter as outbound proxy. When
using this method, all SIP traffic will flow through the filter, weather
it is destined to the operator or not. That means that also service for
calls outside of the operator's domain may be serviced by the filter.
However, by redirecting all outgoing traffic of the filter to a proxy
the operator can make sure that the authentication, authorization
and accounting (AAA) requirements for requiring the service are
fulfilled.
• User agents resolve the filter though the RFC3263 DNS resolving
process. That means that only the traffic that is destined to the
operator's domain will use the service of the NAT Filter. However,
users might be annoyed if they place a call to a domain that does
not properly support NAT services. In this case, the filter can also
redirect the traffic to another proxy for AAA.
We recommend using the first alternative and only choose the
second alternative if it is too difficult to provision the user agents with the
outbound proxy or there are concerns about providing service for foreign
operators.
Usually, the filter acts as stateless proxy. That means, by default
it just forwards the packets and does not change the content of the
attachments or the headers themselves. That means, the filter will not
interfere with applications (instant messaging, presence, weather report,
etc).
There are three exceptions to this rule:
• The first exception is a REGISTER request. When a user agent tries

Architecture

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