Verizon DSL question

[jj at lcs.mit.edu (John Jannotti)]

"Derek D. Martin" <ddm at pizzashack.org> writes:
> > I know the conventional wisdom is to diss PPPoE but I can only think
> > of one technical reason to dislike it - IP fragmentation. This can be
> > sovled by just turning down the MTU on any home LAN links you might
> > have.
> 
> ...to the detriment of your local LAN's performance.  Thanks, but no
> thanks.  This doesn't solve any problem; it only creates a new one.
> Essentially you're causing your local LAN to fragment packets.

While you're right that a smaller MTU doesn't help get your usable bits to
header bits ratio up, you're wrong that it "doesn't solve any problem".  In
fact, it can improve your bandwidth.

One problem with chronic IP fragmentation is with TCP.  Suppose you are
splitting (at the IP level) all your packets into two frames.  Now two
frames have to get through instead of one in order to get a packet through.
So, roughly speaking a 1% (Ethernet frame) loss rate becomes a 2% (IP
packet) loss rate.  TCP gets unhappy quickly as loss rate climbs.

Matching your IP packet size to the true MTU size of the path gets you down
to the true loss rate, and TCP works better.

  jj