[CLUE-Tech] PPP serial link performance tuning (MTU)?

[Chris Schock]

If NAGLE is in use, you might want to turn it off. If you are not familiar
with NAGLE, it essentially chunks a couple smaller pieces of data together
into one larger piece so there's less overhead on the link. NAGLE is
usually pretty good about interactive traffic, but it's something to try.

What kind of bandwidth monitor are you using? The problem with things like
MRTG is they take 5 minute averages, and if you're dealing with a highly
interactive application a 5 minute average is glossing over a lot of
detail. Try to use something more granular like 5 second averages, if
you're not already. You may find that the link is being saturated over
short bursts.

> Hi everyone,
>
> I am trying to improve the performance (bandwidth usage) of
> a pointtopoint serial link using PPP.  Right now the MTU is
> 1500, but the link bandwidth is 115Kbps, so that MTU might
> be high.
>
> How can I figure out what the best MTU should be?  Shouldn't
> the kernel do this automatically (path MTU discovery)?  How
> can I tell if the kernel is doing PMTUD?  Isn't there a
> formula for calculating the best MTU for a given link?
>
> We have Linux 2.4.22 kernels on both ends of the link, with
> standard serial.c driven hardware.  Netfilter is not in use.
> It's point-to-point only, not much communication with other
> hosts.
>
> We have used a bandwidth monitor to verify that the link is
> not close to being saturated, but for some reason it won't
> go any faster.  The interactive application is not responsive
> enough.
>
> Linux pppd already does MTU/MRU negotiation during LCP or IPCP,
> and it is possible to set the MTU in the PPP options file:
>
>        mtu n  Set the MTU [Maximum Transmit Unit] value to n.  Unless the
> peer  requests  a
>               smaller  value via MRU negotiation, pppd will request that
> the kernel network-
>               ing code send data packets of no more than n bytes  through
> the  PPP  network
>               interface.  (Note that for IPv6 MTU must be at least 1280)
>
> Shouldn't there be an "mtu auto" setting?  That'd be good.  :)
>
> I am a bit surprised I'm not finding this in any of the
> Linux PPP or networking HOWTOs or FAQs, other than passing
> references.  Even the mailing lists aren't too useful.
>
> I'd appreciate any suggestions or pointers from the network
> gurus out there.  I remember using a MTU of 576 or smaller
> back in the olden days of ~14.4Kbps modems.
>
> Thanks,
> Jim
>
> PS Cisco says that their stuff does PTMUD for PPP links as long
> as the DF (don't fragment) flag is not set on the interfaces.  Hm!
>
> --
> Jim Ockers, P.Eng. (ockers at ockers.net)
> Contact info: please see http://www.ockers.net/
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