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

[Jim Ockers]

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/