[ALUG] Mostly-OT: Strange ADSL issues

[Mark Rogers]

Adam Bower wrote:
> On Sat, Oct 07, 2006 at 11:04:44PM +0100, Mark Rogers wrote:
>   
>> Explain that! Snide comments about W2k vs Linux I can come up with
>> myself! I need to bring a Linux box back home to try, but when I've sent
>> this email I'll reboot this PC with a LiveCD and see whether that's better.
>>     
>
> Perhaps the W2k box is setting a DNF (do not fragment) or something so
> the router is just dropping the packet? Was that ping running over the
> vpn?
>   

No, I've not done any tests over the VPN, I've just used the VPN to give
me VNC access to a PC in the office (or vice versa) to do remote tests.

Right, some more test results...

I rebooted to a Mepis LiveCD (closest to hand) and got identical
results. So I changed MTU back to 1500 and tried that, and that worked.

So I rebooted to W2k and it also seemed pretty reliable at 1500, so I
left it overnight and found the router "dead" in the morning. (I could
ping the router but not access its web front end, SSH into it, etc; no
traffic was going through it either.) So I think I have a router issue
at home, so have switched back to my old router (which is the same as
the one in the office, an unbadged Connexant unit from Solwise). I left
that at 1500 for the time being and it seems sort of OK, but not what
I'd expect from a 8Mbps connection (based on what I had before). I need
therefore to do some proper tests when I get chance, probably tomorrow
evening or Tuesday.

Meanwhile, I've just popped into the office and (with 1458 as MTU) it
seems OK right now, but that's almost meaningless; I'll have a better
idea after its been in use for a day tomorrow.

> Anything Netgear, Speedtouch USB modems all seem to default to 1458 with
> Windows, a couple of brands I forget but I think possibly Dlink, Dynamode 
> and 3com. It would be quite easy I guess given how many consumer routers
> are out there to find plenty that are set to 1500 by default.
>   

Interesting, I'll get my colleagues to check which brands use what
default in future.

>> Of-course I have to be open to the possibility that the problems at home
>> and at work have completely different causes, even if the symptoms are
>> similar.
>>     
> Very possibly, try perhaps using ping and setting do not fragment bits
> and see what happens. I don't really have too much recent experience of
> fiddling lots with the innards of tcp/ip so i'm a bit out of ideas for
> now. I'll try re-reading the thread when I feel less ill perhaps.
>   

OK, I'm going to do nothing for a day (mostly because I don;t have much
choice) and get a proper idea of what is really going on while I'm not
changing anything, then try some more tests and document what happens
properly this time around. I'm determined to work this out!!

I hope you feel less ill soon, and only in a small part for selfish
reasons. Thanks for your help.

PS: Any good TCP/IP book recommendations? I know a fair bit but this
exercise is highlighting just how much I don't know (or thought I know
but don't).

-- 
Mark Rogers
More Solutions Ltd :: 0845 45 89 555