[pptp-server] PPTP performance again: malfunction singled out

Gianluca Insolvibile g.insolvibile at cpr.it
Wed Dec 19 02:22:29 CST 2001


Thanks Steve. 

I tried changing the DefaultRcv-Window (tried 32768 and 65536 - which is the
default on Linux), with no changes in throughput. I admit I did not collect new
tcpdump traces to analyze, but I do not think they would have changed. 

Enough is enough. With great disappointment (and shame on the Windows 98 PPTP
client), I will abandon PPTP in favor of other solutions (e.g. FreeSWAN).

Many thanks to all who tried to help me and to those who endured my lengthy
mails. ;-)

Gianluca

Steve Jorgensen wrote:
> 
> Yes, it's a registry key.  Here's a pretty good article -
> http://www.smartcomputing.com/editorial/article.asp?article=articles%2F2
> 000%2Fs1107%2F31s07%2F31s07%2Easp
> 
> On Monday, December 17, 2001 10:58 AM, Gianluca Insolvibile
> [SMTP:g.insolvibile at cpr.it] wrote:
> > I would be glad to try it. Where can I find that parameter? Is it a
> registry
> > key?
> >
> > Thanks,
> > Gianluca
> >
> > Steve Jorgensen wrote:
> > >
> > > Doh!  Try mucking with the Receive Window settings in Windows 98.
>  Windows 2000 is supposed to tune settings in real time, but W9x does not.
> > >
> > > On Monday, December 17, 2001 3:45 AM, Gianluca Insolvibile
> [SMTP:g.insolvibile at cpr.it] wrote:
> > > > Hi,
> > > >
> > > > to whom it may concern: I finally located the problem with the sloppy
> > > > performance I am experiencing. Unfortunately, this does not mean that
> I
> > > > succeeded in fixing it.
> > > >
> > > > The facts:
> > > >  - any Windows 98/98SE client connects to pptpd but has very low
> performance
> > > > (exactly 1.5 Mbps server->client, exactly 0.75 Mbps client->server);
> > > >  - the same 98/98SE client performs reasonably when using the LAN
> (Ethernet)
> > > > connection;
> > > >  - any Windows 2000 client connects to pptpd and has a throughput
> around 25-30
> > > > Mbps.
> > > >
> > > > This leads me to the unimaginable ;-) conclusion that 98/98SE PPTP
> support is
> > > > severely bugged (I already tried all the patches available from
> micro$oft, of
> > > > course). Further investigation on tcpdump traces revealed that ACKs
> sent by the
> > > > 98 client are being sent with a 10 msec granularity (that is, no more
> than 1 ACK
> > > > is sent in 10 msec, and ACKs are evenly spaced by 10 msec);
> furthermore, each
> > > > TCP segment is ACKed with a 40 msec delay. This behaviour fools the
> TCP
> > > > congestion algorithm on the server and causes the drop in throughput
> (1500 bytes
> > > > per packets at 100 packets per second roughly gives 1.2 Mbps, which
> is near the
> > > > value I measured).
> > > >
> > > > Lacking source code for the client side ;-), I tried to set up some
> > > > countermeasures on the server: I modified the Linux kernel to expose
> via
> > > > /proc/sys/net/ipv4 the most sensible parameters of TCP congestion
> control (i.e.
> > > > min and max retransmit timeouts, min and max delayed ACKs, and so on)
> and tried
> > > > to tweak them, with no result. I tried to have a look at the pptpd
> source code,
> > > > and it seems to me that it implements no window flow control at all;
> anyway, I
> > > > can't imagine why this could be the cause of client side malfunction.
> > > >
> > > > I just wonder how does a Windows 98 client perform with a Windows
> PPTP server,
> > > > in order to assess whether the real cause is something in pptpd or
> the braindead
> > > > implementation in Windows. I have strong suspects on the latter,
> anyway...
> > > >
> > > > In conclusion, I'm really stuck now. Any ideas, hints or comments
> will be
> > > > appreciated. In any case, I hope my experience will be of help if
> somebody ever
> > > > falls into this problem again.
> > > >
> > > >
> > > > Best regards,
> > > >       Gianluca
> > > > _______________________________________________
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