Re: PPTPD upload speed bottleneck

Hi Greg,

The mtu is 996 on both ends - I havent changed it, that is the default. It would fragment each large packet in two adding the overhead of headers. I can push it two 1400 but do you think this could explain such a large discrepance of upload rates between network and vpn? Why doesn't the same discrepance show for download rates?

Can you share what values you obtain?

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

Greg Scott <[email protected]> wrote:

>MTU issues maybe?  The PPTP tunnel will gobble a few bytes for tunnel
>overhead so you don't get the full MTU of the physical interfaces.  If
>both endpoints "think" the MTU size is 1500 for the raw physical
>interfaces and they're sending large packets, maybe the tunnel is
>fragmenting the packets and putting them back together on the receiving
>end.  Just a guess.
>
>- Greg
>
>-----Original Message-----
>From: Gustavo Homem [mailto:[email protected]] 
>Sent: Saturday, November 16, 2013 4:32 PM
>To: [email protected]
>Subject: [Poptop-server] PPTPD upload speed bottleneck
>
>Hi,
>
>We are using PPTPD with a high performance connection and the upload
>speed appears to be artificially limited. This was not an issue in the
>past with ADSL connections but now that fiber is becoming available it
>became noticeable.
>
>The network connection between PPTP client and PPTPD is as follows:
>
>upstream  10Mbps
>downstream 100Mbps
>latency ~ 7-15ms
>packet loss 0%
>
>Command line scp transfer performance between end points WITHOUT using
>the VPN is:
>
>up  ~ 700KB/s
>down ~ 1100MB/s
>
>Command line scp transfer performance using the VPN is:
>
>up ~ 160 KB/s
>down ~ 1000 KB/s
>
>The values are in KBytes and were measured by the scp command. 
>
>So clearly, there is a problem with the upstream transfers (client ->
>server) through the VPN.
>
>We've debugged this enough to know that CPU is not an issue (neither or
>the router, nor on the server), that there is no packet loss and that
>there is no other bottleneck. We tested this with traffic shapping
>turned off. We have read on the docs that the speed option does not
>affect pppd on Linux. We tried --nobuffer on the client, which made no
>difference. We also made transfers using time+netcat just to be sure
>scp wasn't screwing things - no difference either.
>
>Interestingly, when an upstream transfer is done, using scp trough the
>VPN, and the rate stabilizes at 160 KB/s the pings between the client
>and the VPN server interface behave as if a packet queue was saturated,
>with rtt fluctuations between 14ms and 700ms, but at the exact same
>time we can ping the public server interface - through which the VPNn
>traffic flows as well - and the pings retain their normal values of
>between 7 and 15ms. When pinging the VPN server interface with an
>interval of 0.1 (ping -i 0.1) we can see the latency varying like a
>cosine between normal and large values, which suggests a queue is
>filled and then emptied in a loop. There is absolutely no packet loss
>that would justify this behaviour.
>
>In short, in the router where the pptp client runs interface ppp0
>appears to get saturated, but interface eth1 where the ppp0 traffic is
>tunneled through doesn't show any problem.
>
>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?
>
>On the server side we are using pptpd-1.4.0-1.rhel5 and on the client
>pptp-1.7.2-8.1.el5.rf.
>
>Cheers
>Gustavo
>
>Cheers
>Gustavo
>
>--
>Angulo Sólido - Tecnologias de Informação http://angulosolido.pt
>
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