Recommended MTU values?

Recommended MTU values?

Post by Bill Kearn » Thu, 18 Jul 1996 04:00:00



I'm running Solaris 2.4 using aspppd on my IPC.  It's connecting my local
network to the internet via a 28.8 dial-up link.  What's the recommended
MTU for such a connection?  The reason I ask is that terminal IO is rather
'lumpy' and I've heard that a lower MTU will improve the responsiveness.

Since the default of 1500 is the same as ethernet I can't imagine it's
efficient.  There are only a few machines on the local net and I have
verfied that the behavior is not because of other activity.  No blinkin'
lights on the modem until I press keys and it's still sluggish.

Also, would a 4/300 make a better PPP server?  The IPC does little more
than run sendmail, popper and aspppd.  The modem only get's 21.6k
connections (it's rural) and I'm using the port at 38400 now.  It would be
a simple matter to move the modem to the 4/300 if it would improve
throughput.  The 4/300 is usually rather idle as well.

Thanks and email replies appreciated.
Bill

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Recommended MTU values?

Post by Bryan O'Sulliv » Thu, 18 Jul 1996 04:00:00



Quote:(Bill Kearney) writes:

b> I'm running Solaris 2.4 using aspppd on my IPC.  It's connecting my
b> local network to the internet via a 28.8 dial-up link.  What's the
b> recommended MTU for such a connection?

Depends on what you want to do.  If you want fair interactive
response, but you also want large data transfers to fare pretty well,
then an MTU of something like 300 should be okay.

b> The reason I ask is that terminal IO is rather 'lumpy' and I've
b> heard that a lower MTU will improve the responsiveness.

b> There are only a few machines on the local net and I have verfied
b> that the behavior is not because of other activity.

If your link is otherwise idle (as this seems to imply), then changing
the MTU won't make any difference.

b> No blinkin' lights on the modem until I press keys and it's still
b> sluggish.

It should be more sluggish than typing locally, but not annoyingly
so.  You mgiht want to use snoop (run it as root, something like
"snoop -d ipdptp" should do the trick) to find out whether you are
getting lots of retransmits.  If you are, then you need to watch out
for the TCP retransmit fix coming out soonish (see other postings in
this group for details).

b> Also, would a 4/300 make a better PPP server?

No.

        <b

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