Manual

72
Switchboard aids in breaking through these dierent types of routers for incoming calls. Because it is in constant contact
with all subscribed codecs, it can send and receive test paerns to determine whether one or more NAT routers exist on
a link and what type they are. It can then choose a connecon method to be used to circumvent any issues. Switchboard
can:
• Instruct the calling codec to make a normal connecon (No NAT detected).
• Use the hole punched by connecon to the Directory Server for incoming connecons from other
codecs.
• Instruct the called codec to make the connecon in the reverse direcon.
The second opon, which ulizes 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 sing behind a
symmetric NAT. In this situaon, calls will fail even with Switchboard. The only opon in that environment is to resort to
port forwarding on one side of the link.