The FiOS G1100 "Quantum" router and various combo modem/routers provided by Comcast and other ISPs are all designed with SIP-ALG enabled and with no way to disable. (Many of us believe this is by design, as a sneaky way to get around Net Neutrality rules). As a provider/reseller of VoIP services, and as more and more of these devices are popping up, SIP-ALG is becoming a bigger and bigger challenge, and so I think it would behoove us to compile some possible fixes/workarounds to get VoIP to work properly with SIP-ALG enabled.
We have come up with two work-arounds so far. While not perfect, they have, for the most part, helped substantially:
1: Switch SIP transport from UDP to TCP. TCP can cause latency issues, but we are finding that TCP's error correction usually reassembles packets fragmented by SIP-ALG. It's been working most of the time as long as the server is nearby, although some calls still do drop or have audio issues.
2: Shorten the SIP headers by forcing the SIP device into using one codec. Enabling multiple codecs on the device lengthens the SIP headers, which increases the chances of headers being fragmented by SIP-ALG. (i.e.: If you use PCMU, make sure the device is set ONLY for PCMU.) This has also been working most (not all) of the time.
Additional suggestions welcome and appreciated ...
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