Re: Strange throughput anomaly: gateway fast, behind gateway slow

From: Ralf Gerlich
Date: Fri May 06 2005 - 03:48:11 EST


On Fri, May 06, 2005 at 09:07:40AM +0200, Ralf Gerlich wrote:
> Hi,
> On Thu, May 05, 2005 at 01:34:58PM -0500, Jason Clark wrote:
> > Silly question, but have you checked your speed/duplex settings on the lan
> > side of the gateway to make sure you dont have a mismatch with your
> > switch/hub?
> Not that silly, though it's not the solution :-/ The gateway has only one NIC. The DSL modem and the rest of the network are on the same hub - which is probably not the cleanest solution but it had worked the last 2-3 years. Therefore a mismatch is impossible - or am I misunderstanding your question?

And before you ask: I don't know what exactly might have changed in my configuration from when the connection worked flawlessly to now. At first I attributed it to a shortage at my provider until I found out about the difference between the gateway and the other boxes on the intranet. So I don't quite know when this whole thing started.

I should probably add: The fact that the DSL-modem and all computers are on the same physical network allows me to open the DSL-connection from another computer for experimentation without changing anything else. If I open the connection from a box which normally is behind the gateway, I get the same download rates - about 100kbyte/s - there as I get on the gateway when the connection is opened from the gateway. To me this seems to imply that this is not a matter of the specific network configuration of the gateway but probably rather an issue of the routing configuration.

Every host on the intranet - in addition to the internal route entries - just has a default route pointing to the gateway and the gateway has a default route pointing to the PPPoE-interface. In between there's a masquerading/firewall setup.

Yet aside from the differences I listed in the original post I see no differences in the packets going out to the net which could explain the timing differences. And to me from the differences I see - MSS and window growth - I'd rather think that the throughput should be higher "inside" the intranet than on the gateway. Yet the reality is exactly the opposite. Perhaps I misunderstand TCP :-/

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