ppp max speed

Martijn van Oosterhout kleptog at svana.org
Wed Mar 20 01:28:57 EST 2002

On Wed, Mar 20, 2002 at 12:57:33AM +1100, Grant Morphett wrote:
> I played a little more and the largest speed setting I could configure in pppd
> was 921600 so I am assuming ppp can't go any faster then that.

Why set a speed at all? The given speed is only used in an ioctl to set the
port speed of the serial port. If you're not using a serial port I don't
think it has any effect whatsoever.

Put it another way, pppd does not do speed limiting. It's the serial ports
that don't go any faster.

> Yes I am running ppp over ssh but don't think its the compression/encryption.  On
> the slow box ssh and pppd are using 18% cpu each and there is nothing else
> running on that box.  Plenty of spare horses.

Are you sure? I have a box here where disabling compression quadripled the
transfer speed. Also, that 18% may be misleading since that's 18% of the
time since the program started, not current CPU usage. Other issues:

1. You're running TCP-over-TCP, welcome to backoff-hell.

2. Think of kernel transitions. When a program sends a packet it goes like

ping -> kernel -> pppd -> kernel -> sshd -> kernel -> network

The same on the receiving end. So each block of data transferred will be
copied 6 times before it goes out the network card. That kills performance
more than you might think. (I think the ppp bit may be skipped in some
situations, but you get the idea).

The CIPE page has more details.


> On Tue, 19 Mar 2002 23:00:44 +1100 Martijn van Oosterhout scribbled:
> >On Tue, Mar 19, 2002 at 10:29:13PM +1100, Grant Morphett wrote:
> >> What is the max speed a ppp connection go?  i.e. if you run one over ethernet?
> >>  I can't get more then 1Mbit.
> >
> >Considering that DSL is available over 1.5Mb, I'd say that's not the
> >problem. Is your CPU up to the job? What problems are you actually seeing?
> >
> >(If you're doing PPP over SSH, it's the compression/encryption that's
> >killing you. Don't do that, use CIPE).
> >
> >HTH,

Martijn van Oosterhout <kleptog at svana.org>   http://svana.org/kleptog/
> Ignorance continues to thrive when intelligent people choose to do
> nothing.  Speaking out against censorship and ignorance is the imperative
> of all intelligent people.

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