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Forum Discussion
Fragmented UDP packets dropped
I'm installing a VoIP switch on the LAN. It connects to the provider (Flowroute) through the Nighthawk. I finally worked around a problem that was initially a mystery, where calls from a Polycom phone worked, but calls from an ATA (Analog Telephone Adapter used to connect our analog FAX machine) would never connect.
It was finally determined that the ATA added extra information to the VoIP packet that would make it exceed the MTU of 1500 bytes, meaning the UDP packed would get fragmented at the phone system. This large UDP packet never seemed to reach the provider. I suspect that the UDP NAT code in the Nighthawk drops the 2nd fragment because it can't handle a fragment that doesn't contain the UDP header. Could this be the case? If so, is there a fix?
2 Replies
There are so many "network" devices out there that it is unlikely that Netgear has tested them all and knows how they should be configured.
Maybe someone else has used this kit and hangs out here. (Did you try a community search for your unnamed VoIP switch?) But a much better source of information should be the people who made whatever you are trying to set up. The network settings will not depend on the brand of the router, so it is up to them to tell everyone how to configure these things.
The VoIP switch is FreeSWITCH, a very well known and supported software.
I found a solution to the problem by changing enough settings on the ATA to reduce the size of the SIP INVITE UDP packet to less than the MTU, thereby avoiding fragmentation.
It appears that the Netgear router cannot handle fragmented UDP packets.
I was using the same configuration (the ATA before I changed its options and FreeSWITCH) with an Xfinity router, which worked, leading me to the conclusion that the Xfinity router will correctly route fragmented UDP packets, while the Netgear router will not.
