[ALUG] Mostly-OT: Strange ADSL issues

[Mark Rogers]

Wayne Stallwood wrote:
> Some of the things you mention below almost sound like a MTU miss-match
> problem, but why this would be intermittent I am not sure. MTU problems
> could explain why a ping works until the packet size is increased and
> could also explain why traffic encapsulated by your VPN may work while
> other connections fail.
>   

That was what I thought which is what lead me in that direction, but I
can't really say it's got me anywhere.

The MTU is set on the router to 1500. If I ping with packets of
1472bytes disabling fragmentation (ie to allow for the 28-byte TCP/IP
overhead) I get consistent good results, with 1473byte packets I fail on
fragmentation, so the path does seem to support 1500 MTU. But if I
increase the packet size beyond 1472 whilst allowing fragmentation I get
intermittent results. I don't really know what that "means", though.
Maybe some routers are just dropping some ICMP packets which would allow
normal packets (although it's odd that its not consistent). For example,
I can ping "www.lug.org.uk" with packets of 1472 and below, but not
1473+, even though the connection seems fine right now. So I assume that
something is dropping the large packets somewhere (consistently). If I
try the same to my own server (www.more-solutions.co.uk) I get some good
results and some bad ones. (If someone else can try pinging those two
with 1473-byte packets and tell me if my results differ from theirs that
would be useful. "ping -s 1473 www.more-solutions.co.uk" under Linux
should do it. NB I've been using a 2sec timeout, I should probably
increase that to see if the packets are just very slow coming back.)

> What are the line stats on your routers showing ? It doesn't sound
> completely like an ADSL/Line problem but the intermittent nature sort of
> points in that direction. 
>   

All numbers Downstream/Upstream:
    SNR Margin    26.0/28.0dB
    Line Attenuation    57.4/31.5dB
    Errored Seconds    74/174   
    Loss of Signal    0/0   
    Loss of Frame    0/0   
    CRC Errors    210/273   
    Data Rate    1152/288kbps

Whilst those CRC errors aren't great (uptime is 3days 4hrs) they're not
atypical for my office ADSL line. My home one is at 8MB (nice and close
to the exchange!) but I can't get at the stats remotely.

NB: Restarting the router sometimes makes a difference for a minute or
two after the restart, but no more than that. And of-course it means any
established SSH connections get dropped, so it's not really worthwhile.
Both routers are pretty cheap-n-cheerful things (office one is Connexant
chipset, not sure about home one but it's Safecom branded and I think
they're usually Conexant too). I've updated firmware without effect. If
I get chance I'll build an IPCop box, stick a USB ADSL modem in it, and
see if that works any better.

> When you are in a state of not being able to start a new ssh connection
> can you still telnet to port 22 of an external ssh server and see the
> server banner (or do you get connection timed out even doing that)
>   

Next time it fails I'll try that. My guess would be that the connection
will timeout even though an established connection to the same server
will continue just fine, but I'm not 100% sure I've actually tried it to
be sure.

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