Comrex BRIC-Link Product Manual page 64

A good way to think of this is that an outgoing packet "punches a hole" in the router, through which authorized
reply packets may be returned to the codec for a limited time (Figure
FigUre 55 bidirectionAL commUnicAtion
Switchboard aids in breaking through these different types of routers for incoming calls. Because it is in constant
contact with all subscribed codecs, it can send and receive test patterns to determine whether one or more NAT
routers exist on a link and what type they are. It can then choose a connection method to be used to circumvent
any issues. Switchboard can:
• Instruct the calling codec to make a normal connection (no NAT detected).
• Use the hole punched by connection to the Directory Server for incoming connections from other codecs.
• Instruct the called codec to make the connection in the reverse direction.
The second option, which utilizes the outgoing Directory Server "ping" described earlier, is very useful. The interval
of this ping is adjustable, but defaults to about one minute, which is short enough to keep a hole punched through
the majority of NAT routers.
These techniques are based loosely, with enhancements, on a generic Internet protocol called STUN (Simple
Traversal of UDP through NAT). The system works well in all environments except one: when both users are sitting
behind a symmetric NAT. In this situation, calls will fail even with Switchboard. The only option in that environment
is to resort to port forwarding on one side of the link.
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