I suspect the realtek gigE on my laptop crashes my cable modem. Is that even a thing?

[Kdawg]

I've noticed that when I saturate the builtin realtek gigabit ethernet on my asus laptop with multithreaded downloads, or try to max out my internet connection, occasionally the modem will crash, requiring hard reboot of modem.

when using several different computers with Intel gigE, I haven't crashed the modem yet despite saturating the connection.

Does the realtek send bad packets or something?

 

[toast0]

I have many questions. I wouldn't expect a NIC to send really bad packets that could crash anything else on the network, but you know how computers are. Also, most(all?) Intel GigE nics can do multiple rx/tx queues and Realtek support for that is more limited, so packet ordering could be different and that could cause problems.

Is this a modem you own, or a modem you rent? If you rent, is the device old enough that you can get a new replacement with no questions; if so, I'd do that cause investigating this is going to be a big PITA. If you own, are there any firmware updates available --- bonus if they have a changelog (double bonus if they have a change log that's like 'don't crash when weird shit happens')

Is this easily reproducible? Can you run the same download/speed test/whatever and crash it every time or does it vary?

If it's easy to reproduce, there's a couple things to try. Obviously, trying the exact same test case on a different computer. It'd be great if you could put an intel nic in your laptop to test with that, but it's a laptop. If you can put a realtek nic in a desktop and test with that, you might be able to say for sure it's realtek vs Intel, but maybe it's a specific model, etc.

Anyway, if it's repeatable, I'd try fiddling with settings on the realtek nic. I'm assuming this is Windows, but if you're a FreeBSD or Linux or etc kind of person, let me know. In the adapter properties (the control panel one), go to advanced and turn off all these things (if listed) and see if makes a difference. Advanced EEE, Energy-Efficient Ethernet, Flow Control, Green Ethernet, Large Send Offload (there may be many settings here), Power Saving Mode. I'd turn them all off (make a note of default settings), and test and see if it helps; if it does help, turn them all back to default except leave Large Send Offload off and see if you still have a problem. IMHO, if something is going to break things, it's probably large send offload. If it's not Large Send Offload, but it's one of those, then fiddle around until you find it.

If that doesn't pinpoint the issue, you're going to need to be good with Wireshark. Run a capture (best if large send offload is off for this, so you're seeing closer to what's sent on the wire; if you turn off checksum offload you'll probably get proper checksums in wireshark, but you can also just ignore invalid checksum messages) and break the modem, find the last packet the modem sent and then see what the 10ish packets you sent before that were. Do it several times and see if there's a pattern. The trick is, it might not be something you send, but something from the internet. But if you find a pattern, we can discuss what's weird about that packet and how dumb your cable modem is. Of course, getting anybody to actually fix it is another story.

 

[SamirD]

In addition to those great suggestions, I'd try swapping the cable and port on the cable modem (if it has multiple). I'd also try putting a simple unmanaged gigabit switch in between the modem and your laptop and see if that fixes it. I've seen some weird nic/switch incompatibilites before, but they're ultra rare and I've only seen it once with a device and a modem. You can also try booting up a linux live cd and make sure you can replicate the problem under linux to eliminate some sort of weird windows issue.