08-15-2013 9:36 AM - edited 08-15-2013 9:40 AM
I have recently swtiched from AT&T DSL to U-verse Internet. I have a Motorola NVG510 modem/router.
When I FTP a large file to my web hosting server, it causes all other ports to shutdown. I cannot connect to web pages, I am disconnected from chat rooms and disconnected from instant messenger services. As soon as the file transfer completes, everything returns to normal. (This is true not only on the computer from which I am transferring files, but every other computer on the home network).
While files are being transfered, if I go to the modem interface and "Run Full Diagnostics," I get an error message that IP failed. Under details, it says Ping Default Gateway failed. Again, as soon as the file transfer ends, the diagnostic tool shows that everything passed.
I used the same FTP client (WS_FTP) for years with DSL transferring large files in the background, without interferring with any other functions. This problem only showed up when I installed the new U-verse modem.
The Level 1 tech support folks at AT&T have not been able to help with this issue. Does anyone have any suggestions for a fix?
08-16-2013 6:52 PM
It sounds to me like you have a common problem known to some as "buffer bloat". The IP buffers on your router are way to large, so the router is buffering all the outbound FTP packed from your upload. Since you are probably connected at 56Mbps wireless or 100Mbps wired connection, but probably only have a 1.5Mbps upload link on UVerse, your FTP upload is sending packets out way faster than the router can send them to the next hop upstream, so it buffers them. The problem is that if the buffers are too large, and you saturate them too much with outbound FTP traffic, then all other ourbound traffic on your router is affected too because they share that same upload buffer that your FTP is using on your router. You can go to this link: http://netalyzr.icsi.berkeley.edu/ to test your router for this problem. If you want to get more details, you can look up the "Security Now!" podcast episode 345 where this issue is discussed in a good bit of detail.
08-16-2013 7:33 PM - edited 08-16-2013 7:33 PM
I ran the Netalyzr test and got this result: "We estimate your uplink as having 2400 ms of buffering. This is quite high, and you may experience substantial disruption to your network performance..."
My problem now is, I can't find any documentation on how to change the buffering size on the NVG510.
What are my options to solve this problem? The Netalyzr suggestion was buy a more expensive modem or get a faster Internet connection, neither of which is guaranteed to work.
08-16-2013 10:34 PM
I found a work around to fix this Buffer Bloat problem with the Motorola NVG510 modem, in case anyone else has the same situation.
I unplugged the ethernet cable from my PC and connected to the router wirelessly. I can now FTP large files and not experience any other outages or delays.
It doesn't "fix" the problem with the too much buffering, but it works.
08-17-2013 1:38 PM - edited 08-17-2013 1:39 PM
I don't happen to have the NVG510 modem/router myself, so I'm not familiar with it, but you could alleviate much of the issue by using the QoS settings in the router. You would just go there and configure FTP to have a lower priority than anything else, and the router should move those packets to the back of the queue allowing the other traffic to flow properly.
The problem with the buffers being so large is that you really need a firmware update for the router to change the buffering. You might try looking for the term CODL (I believe that is how it is spelled, it was referenced in a slightly more recent episode of the Security Now podcast I referenced before). This was a proposed change to the traffic handling which would alleviate much of the buffering problems, but it would still require a firmware update for your router to apply it. I am not aware of any software or configuration changes that can be made by us end-users to solve this problem, it is all in the router's hardware & firmware.
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