[H-GEN] PPP problems

[Ben Carlyle]

[ Humbug *General* list - semi-serious discussions about Humbug and ]
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Michael Anthon wrote:

> I've never really understood this statement when it comes to modern modems.
> Pretty much most modem connections established these days would (I guess..
> no hard facts to support me here) be using error correction on the modem
> connection itself.  In this situation, setting the MTU/MRU to a lower value
> has only one effect as far as I can tell and that is to increase the
> overhead on the link by generating more packets.

An important effect of the MTU can be felt when communicating
many things at once.  If the MTU is high and you are transmitting
to only one host then you will get less packet overhead and therefore
a more efficient connection, and even if you are transmitting to
more than one host you will feel this effect.  The troubles come when
you want interactive media to be passing over the link, especially
when this interactive media is fighting with non-interactive media
as in the fight between a telnet session and an ftp transfer.  The
ftp transfer will use as much of the connection as possible, and
the few telnet packets that pass through must slip in-between the
ftp packets.  The MTU is (roughly) defined as the number of octets
per largest packet, but at each particular transmission speed it
implicity defines the time the largest packet takes to transmit.
If you are doing anything interactive, then you probably want this
to be considerably less than a second, despite the extra packet
overhead that will be introduced by this.  If the MTU is high, then
the telnet packets will slip in less often, so despite the more
efficient transfer it will look and feel slow.

If you are dealing only with small packet lengths which won't
ever reach the MTU anyway, or dealing only with large packets
that need to be efficient instead of interactive, then the MTU
can be high.  If you are combining the two, then the interactive
performance will suffer at high MTUs even if you can provide a
scheme to set telnet packets at a higher priority as they will
be in IPv6.


Clear?  :)

Benjamin.

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