Network slowing down by masquerade

Network slowing down by masquerade

[Glen Huang]

I have a pptp client connection (ppp1) on a gateway. If I directly using curl --interface ppp1 to download a file, I get full download speed very quickly (2m/s). But if I route my lan host to ppp1 and -o ppp1 -j MASQUERADE, running curl to download the same file on the host starts very slow (less than 100k/s), then the speed *slowly* increases(about 50k per second), until it reach about 1.8m/s. While downloading the file on the host, the gateway's cpu usage never reach 1 from the output of top.

If I directly establish the pptp client connection on host, I quickly get full speed again.

I wonder what might slow down the network when the packets are forwarded. I'm currently guess it's the masquerade target, but I'm not sure. How do I test it? If it's masquerade, is it possible to speed it up?

I'm using iptables v1.4.21, kernel v3.18.14.

Thanks in advance.

Re: Network slowing down by masquerade

[Pascal Hambourg]

Glen Huang a écrit :
> I have a pptp client connection (ppp1) on a gateway. If I directly
> using curl --interface ppp1 to download a file, I get full download
> speed very quickly (2m/s).

Why do you have to use --interface ?

> But if I route my lan host to ppp1 and -o ppp1 -j MASQUERADE, running
> curl to download the same file on the host starts very slow (less than
> 100k/s), then the speed *slowly* increases(about 50k per second), until
> it reach about 1.8m/s. While downloading the file on the host, the
> gateway's cpu usage never reach 1 from the output of top.
> 
> If I directly establish the pptp client connection on host, I quickly
> get full speed again.
> 
> I wonder what might slow down the network when the packets are
> forwarded. I'm currently guess it's the masquerade target, but I'm not sure.

I don't think MASQUERADE is the culprit. I would suspect first MTU
issues (fragmentation, path MTU discovery).

> How do I test it?

Lower the MTU of the client host LAN interface below ~1460.

Re: Network slowing down by masquerade

[Glen Huang]

> Why do you have to use --interface ?

Because ppp1 is not the default route's output device. I'm running that command on the gateway to test the connection. Forgot to mention it, Sorry.

> I would suspect first MTU issues

It's indeed a MTU issue. After enabling MSS clamping, I get full speed on the host. Thank you so much for the help. :)

> On Jul 12, 2015, at 5:05 AM, Pascal Hambourg <pascal@plouf.fr.eu.org> wrote:
> 
> Glen Huang a écrit :
>> I have a pptp client connection (ppp1) on a gateway. If I directly
>> using curl --interface ppp1 to download a file, I get full download
>> speed very quickly (2m/s).
> 
> Why do you have to use --interface ?
> 
>> But if I route my lan host to ppp1 and -o ppp1 -j MASQUERADE, running
>> curl to download the same file on the host starts very slow (less than
>> 100k/s), then the speed *slowly* increases(about 50k per second), until
>> it reach about 1.8m/s. While downloading the file on the host, the
>> gateway's cpu usage never reach 1 from the output of top.
>> 
>> If I directly establish the pptp client connection on host, I quickly
>> get full speed again.
>> 
>> I wonder what might slow down the network when the packets are
>> forwarded. I'm currently guess it's the masquerade target, but I'm not sure.
> 
> I don't think MASQUERADE is the culprit. I would suspect first MTU
> issues (fragmentation, path MTU discovery).
> 
>> How do I test it?
> 
> Lower the MTU of the client host LAN interface below ~1460.