I thought I’d ask NANOG this, since somebody may be from a large cable operator and may know. I am a Comcast customer and don’t want to call this in through tech support, as I’ve tried that before without any success.

I use my cable modem normally – to surf the web, to play a game or two, and to listen to stream casts. I have a buddy who often needs tech support, so I use RAdmin (http://www.famatech.com/radmin/) to login to his computer and go through stuff with him. I use large TCP packets, small UDP packets – ICMP, ESP – large and small.

RAdmin can transfer files from desktop to desktop. Ethereal reveals this is a standard TCP based protocol used on the RAdmin port tcp/4899. If I want him to have a file, I push it that way.

Last night – I decided to start IIS FTP server, and have him use his SmartFTP (www.smartftp.com) client to pull several larger files from me. I poked a hole in the Linksys router’s port forwarding for FTP and off we went. He grabs the first file, and it’s going along at roughly the correct uplink speed for Cable (20 KB/s or so) and all is fine. Then, my cable modem reboots. Stops dead. All the lights go off and start blinking, a full reload. It grabs the same IP address via DHCP and he restarts. Again, about 15 seconds into the download, it crashes the cable modem. We do this about 5 times to really make sure it’s just FTP doing this.

I have a DOCSIS 1.1 modem from RCA (http://www.rca.com/product/viewdetail/0,2588,PI700502-CI700094,00.html?)

I have a Linksys BEFSR81 (http://www.linksys.com/products/product.asp?prid=155&grid=5) with the latest firmware

FTP is the only protocol that does this. Using RAdmin file transfer, I push files to him over tcp/4899 and it doesn’t happen.  

The cable modem is somewhat old, perhaps it needs to be flashed with a newer image, but I am not sure what could cause it.

Ideas?