ACCESS Product Manual
95
Many times the “return address” field issue is fixed by the SIP server (in
Registered
mode) and no compensation measures are necessary. Often,
in fact, 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.
But in point-to-point connections, this isn’t possible. All is not lost here,
since we can find some hacks to make this work. The first place to look is
your router, since many modern routers are aware of this issue and have
taken steps to relieve the pain. If your router supports a SIP Application
Layer Gateway (ALG), then enabling this option can fix the issue. Essen-
tially, the router will get smart enough to read your SIP handshake, find the
outgoing address field, and replace it with your public IP. This is a pretty
slick solution, but there may be environments when you are not aware
whether this option is supported on your router, or have the ability to en-
able it. So on to solution two:
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
Ad-
vanced N/ACIP SIP
options and essentially allows for the codec to learn
what its public IP address is. It does this by contacting a STUN server out
on the Internet (the default one is maintained by Comrex) and simply
asking. If this option is enabled, the codec itself will handle the address
switching.
Be aware of the dreaded “battling workarounds” issue. In our simple de-
scription, we left out the fact that 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 trans-
late 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 re-
turn channel fails, try enabling STUN.
Finally, there’s a brute-force option available on Comrex Codecs when
STUN ports are blocked by a firewall, or it can’t be used for some other
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. So if you know what your public IP address is (can be
gotten from many websites via a browser) you can manually paste it here.
Keep in mind, this is often subject to change over time (and obviously if
you use a different network) so it’s important to remember this change has
been made on your codec.
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