Re: Config of my USERS FILE - ?? MTU ??

John Storms (jstorms@livingston.com)
Wed, 06 Aug 1997 08:38:54 -0700

At 11:52 AM 8/3/97 -0600, Jawaid Bazyar wrote:
>On Sun, 3 Aug 1997, Jeff Halper wrote:
>> I was told i could increase the user's speed on the net by changing the
>> MTU to 576. Any one have any feedback on this??
>
>Only under the following conditions would decreasing the MTU result in an
>improvement in performance:
>1) User's phone line is noisy
>2) User does not have an error correcting modem
>
>Otherwise, you've just tripled the IP header overhead, *decreasing*
>performance.

MTU is the Maximum Transfer Unit for a ppp frame. If a ppp frame is
damaged in transit due to noise, low flying UFOs whatever, then that packet
will have to be retransmitted. If you have a noisy line, ppp frames will
have to be retransmitted more often so reducing the MTU will mean that
smaller frames will have to be retransmited. On a noisy line this would
tend to increase performace.

The trade off for a lower MTU is increased overhead of packaging, sending,
receiving, packaging and processing of an increased number of frames.

If line noise was measurable according to noise frequency and duration (I
doubt it can accurately be done) then you could come up with an equation
that would optimize performance by balancing MTU size vs. additional overhead.

---
jstorms@livingston.com
Diplomacy:  The art of saying good doggie
while seaching for a big rock.