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2 Architecture
2.1 The NAT Filter and SIP
In the SIP architecture, the SBC acts as the first proxy that is
contacted by user agents. There are two ways to make sure that the rel-
evant 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 SBC, whether
it is destined to the operator or not. That means that service for calls
outside of the operator’s domain may also be serviced by the SBC.
However, by redirecting all outgoing traffic of the SBC to a proxy
the operator can make sure that the authentication, authorization
and accounting (AAA) requirements for requiring the service are
fulfilled. Alternatively, you can use the application server interface
to do the job on the SBC itself.
• User agents resolve the SBC 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 SBC can also
redirect the traffic to another proxy.
We recommend using the first alternative and to only choose the
second alternative if it is too difficult to provision user agents with the
outbound proxy or when there are concerns about providing service for
foreign operators.
Usually, the SBC acts as stateless proxy. This means, that it just
forwards the packets by default and that it does not change the content
of the attachments or the headers themselves. The SBC 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
2.