Re: PPTPD upload speed bottleneck

Hi Christoph,

> [ re-sent ]
> 
> Gustavo Homem wrote...
> 
> > Command line scp transfer performance between end points WITHOUT
> > using the VPN is:
> > 
> > up  ~ 700KB/s
> > down ~ 1100MB/s
> 
> That should be "KB" I presume (but it shouldn't matter).

Exactly.

> 
> (...)
> 
> > So, either the PPPD client or PPTPD is limiting the traffic rate to
> > something way below the connection rate and, therefore, queuing
> > happens. Which one would it be and why?
> 
> Wild guessing: Some rate limiting is still in effect. Either on the
> client, hidden deeply in an ip-up script, or in the network, applied
> to ecapsulated (i.e. GRE) traffic and in client->server direction
> only.
> 

No, the traffic shaping was off in the router, which is the pptp client, during the tests. I tried the transfers both directly from the router and also from two network machines that go through the router to connect to a VPN host. Those machines have no rate limiting either.

Unless you are suggesting that the ISP is targeting GRE packets specifically, which would seem very odd. 

> To find out, I'd run tcpdump on both sides and compare the latency
> for
> encapsulated and other packets under load. If there's no significant
> difference, it might be local. Then comparing the timestamps on
> client
> side's dumps (when did a packet leave ppp0, when the related one
> eth1?) should give an indication.
> 

I did compare the latency: the latency for direct network to network pings is perfect (7-15 ms) whereas the latency for encapsulated packets fluctuates between 7 and 700ms. So the virtual pipe for VPN packets gets congested at 160KB/s whereas the real pipe has lots of room available. At 160KB/s there is no relevant load in the real connection. Buffers must be filling up somehwere.

Following your suggestion I ran tcpdump on ppp0 and eth1 at the same time and compared to the ping console output:

http://pastebin.com/RZNNSTTj

It would seem that the delay is not local to the pptp client router.

> FWIW, I was not able to reproduce your scenario. I am however quite
> sure that pppd, pptpd and pptp-client are innocent. Blaming the
> network instead is always a good start :->
> 

Network related problems are usually not obvious. I suspected of pptp or pptpd because there are lots of people complaining about similar situations. But you may be right. Let's see what else I can discover.

Cheers
Gustavo

-- 
Angulo Sólido - Tecnologias de Informação
http://angulosolido.pt

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