[Sderby] ADSL Problems - gah!

[Mike Hemstock]

Kris,

As you'll soon learn, I'm not really sure what I'm going on about!  But,=20
thinking aloud, from what I don't know, MTU has something to do with pack=
et=20
sizes.  Routers and firewalls can be configured to restrict the size of t=
he=20
packets of data that travels through them.  If it's too big they can send=
 an=20
ICMP packet which effectivly says "make that packet smaller and try again=
=2E" =20
If you have a firewall blocking those ICMP packets, it'll never know and =
the=20
packets will carry on getting dumped in the bit bucket.  Given that you a=
re=20
having probelms losing 64 byte ping packets, I think we can safely discar=
d=20
the theory about MTU.  The reason that web, ftp, etc aren't working is du=
e to=20
the data disappearing!  The more data you transfer, the bigger the proble=
m,=20
which is why you can do listing, etc, but not transfer files.  In theory,=
 I=20
think that packet loss shouldn't really matter as if packets don't arrive=
=20
they are asked for again, but maybe the requests for sends go missing?  I=
=20
don't know!  The most common problem I know of that causes this kind of=20
packet loss is when NICs fail to autonegociate the speed and duplex with =
the=20
kit that the connect to.  This means that they lose packets as they are n=
ot=20
properly configured.  The only thing that I can think is that the etherne=
t=20
port on the modem is failing to autonegociate with the kit that you plug =
into=20
it, so try forcing the speed of the NIC and the duplex.  As I said, I'm n=
ot=20
really sure about any of this, but that is the best I can think of.  Any=20
other offers?  :-)

Mike.

On Saturday 29 March 2003 15:55, Kris and Laura Adcock wrote:
> (I've posted this on ADSL-related newsgroups, but not had any replies.
> Thinking about it, though, I reckon I'd probably get more
> technically-minded answers here.)
>
> Right,
>
> I am trying to set up Broadband access for the office of a small
> church/charity. The exchange has been switched over, the Zoom x3
> Ethernet modem is up and running, and I have my laptop connected to it.
> The modem's log shows that things are positive - IP address assigned,
> netmask, DNS and gateway settings are all fine.
>
> However ...
>
> If I leave Linux pinging an arbitrary address and then stop after a
> minute or so, it informs me that 5-10% of packets have been lost.
> Webpages refuse to transfer - the browser informs me that it has found
> the site, then just stops. I can telnet to an NTP server and get the
> time fine. If I ftp over to an ftp site, things go fine until I actuall=
y
> try and transfer a file, or if text of a substantial quantity needs to
> be transferred (so 'ls' tends to work, but 'ls -al' screws up).
> Basically, any transfer of any real quantity tends to grind to a halt.
>
> Now, this is the second modem. I took the first one back to the shop
> because I believed the modem had a 'dodgy-buffer', or something. But as
> the new one has exactly the same symptoms, I'm thinking that the fault
> is elsewhere.
>
> So, has anyone else had this problem? How did you fix it? It could be
> possible that there's a setting in the modem which limits bandwidth, bu=
t
> I don't think that's where the fault lies. Who should I be talking to t=
o
> faultfind this situation?
>
> Now, I've had some very unproductive telephone calls with freenetname's
> supposed 'technical support', who basically charged me 50p per minute t=
o
> be put on hold and then refused to help me. To confuse matters further,
> freenetname's ADSL access is provided by a company called Bulldog DSL.
> But it turns out that Bulldog are really only the admin side, and
> actually the service is being provided by BT. So I just KNOW that
> whoever I ring will blame one of the other parties ... and so on, until
> the end of time.
>
> Originally I was using a Windows ME laptop, but after the tech-support
> tried to fob me of with "blah blah MTU sizes blah blah" I've installed
> Smoothwall 'twixt the two. No difference, but then I didn't think there
> would be.
>
> Sigh ...
>
> Cheers, and thanks for any help in advance,
>
> Kris.
>
>
> _______________________________________________
> Sderby mailing list
> Sderby@mailman.lug.org.uk
> http://mailman.lug.org.uk/mailman/listinfo/sderby
> Web site: http://sderby.lug.org.uk/
> wiki: http://www.sderby.lug.org.uk/cgi-bin/wiki.pl