Solutions; Stunning Success; Fix Of Last Resort - Comrex ACCESS MultiRack Product Manual

solutIons

Many times the "return address" field issue is fixed by the SIP server (in Registered mode) and no compensation
measures are necessary. Often, the server insists on acting as a "proxy" and handles all the traffic itself. Outgoing
and incoming streams are relayed directly by the server, solving any router issues.
In point-to-point connections this isn't possible, and some hacks are required to make this work. The first place to
look is the router, as many modern routers are aware of this issue and may be configured to ease connectivity. If a
router supports the SIP Application Layer Gateway (ALG), then enabling this option can fix the issue. The router will
read the SIP handshake, find the outgoing address field, and replace it with the public IP. This is a valuable solution
in environments where the router supports ALG. In environments where ALG is not available, STUN is a valuable
alternative.

stunnIng success

Another technique for working around the SIP-Router issue is by using a protocol called STUN. This can be enabled
in Comrex codecs in the Advanced EBU3326/SIP options and allows for the codec to learn its public IP address. It
does this by contacting a STUN server on the Internet (the default one is maintained by Comrex) and requesting its
Public IP. If this option is enabled, the codec itself will handle the address switching.
Be aware of the "battling work-arounds" issue, as ports are being translated by the router as well as IP addresses. If
the ALG-enabled router receives an unexpected result in the SIP address field (as it might if using STUN), it may not
translate ports as expected, and it's likely that the call will fail. When in doubt, the best technique is to try a SIP call
with STUN turned off, and if the return channel fails, try enabling STUN.

FIX oF lAst resort

Finally, there's a brute-force option available on Comrex Codecs when STUN ports are blocked by a firewall, or
are unusable for some reason. Under Advanced System Settings, a field is available called Public IP Override. Any
address put into that field will be pasted into the address SIP field. A user can thus place their Public IP address
(obtainable from many websites via a browser) in this location. Keep in mind, the Public IP Address is often subject
to change over time, so it's important to remember that this change has been made on a codec.
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