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Re: Strange MTU-ish problem



On Tue, Jan 06, 2009 at 12:20:21AM +0100, Timothy Murphy wrote:
> 
> I'm presently away from home, in Italy,
> using an alice.it (Telecom Italia's broadband service) ADSL modem.
> 
> This works fine, except when googling, or using wget.
> When I start a search, I only get half a page of links,
> even though I am told there are several million hits.
> Again, I just tried downloading a 750k PDF file;
> it downloaded 6%, and then hung.
> 
> The behaviour is the same using WiFi or ethernet.
> 
> What is galling is that there is no such problem with Windows XP,
> while the Linux problem occurs both on my Thinkpad T43
> and on my grand-daughter's EeePC (running Xandros).
> 
> I tried changing the MTU with "ifconfig eth1 mtu 1492"
> but this did not seem to improve matters.
> (I tried various bigger and smaller mtu's.)
> 
> I found with ipconfig under windows
> that fragmentation occurred with packet length > 1464
> (I didn't find an equivalent Fedora application),
> so on adding 28 for the header as advised,
> I got the figure of 1492.
> 
> It's a slightly theoretical problem,
> as I am going home soon,
> and anyway can always google under Windows if necessary.
> But I would be interested to know the cause,
> and if possible a cure.
> 
> Incidentally, as far as I could see from the 192.168.1.1 web-page,
> it is not possible to change the MTU of the modem.
> 

As Frank indicated 'just ping something' but set data size and
also set the -M flag to control fragmentation.  Doing your own
MTU discovery.  BTW: You will find that 1492 is a common magic number.

You should snoop packets to see if this is the issue because you get
enough downloaded that something else is going on.

Since fragmentation is expensive discovering the minimum MTU in
any end to end network connection has value.  The expense is multi
fold.   It costs you local resources to reassemble fragments
and it costs remote resources that you have no control over to
fragment and or reassemble packets.

It can be educational to capture 10MB of packets when you are having
problems.   Tools like wireshark are clever and can make it easy
to figure stuff out.

Good luck.


-- 
	T o m  M i t c h e l l 
	Found me a new hat, now what?


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