2.
[ S N O M 4 S N A T F I L T E R ]
to register and needs the support of the SBC, the SBC will set up a local data structure representing the user agents. It will make sure that the connection to the user agents stays alive. It will also make sure that requests destined to the user agents will be forwarded properly.
•The second exception is an SDP attachment. The SBC checks if the user agent needs support (or must be recorded) and, in that case, will add a local contact to the SDP that can be used for media re- lay.
•The third exception occurs when the SBC queries a web server for routing information. In this case, it will send a provisional response to stop the UAC from repeating messages.
These three exceptions make sure that all user agents will work behind NAT, no matter what
2.2 NAT
Network Address Translation (NAT) is a reality in today’s networks. Many operators save IP addresses by providing only one IP address for a number of devices, sometimes companies. Firewall manufacturers make NAT a feature by performing inspection of packets that go though NAT. Even for IPv6 networks, the fundamental problem will remain as there will also be a need for firewalls and private networks.
The Session Initiation Protocol (SIP) has neglected this problem in the beginning. However, in some recent RFC there have been useful proposals on how to deal with the problem. This document shows how the snom 4S NAT Filter can be used to solve the problems.
Although snom also makes user agents, the snom 4S NAT Filter works with most SIP user agents from other companies. The require- ments on these user agents are described below.
If you want to use the SBC just for recording purposes, you don’t need to bother about NAT. The SBC also works when no NAT is present.
10 • Architecture