Why does the connection speed drop when using a router?

In article <110820032322570432%jon@nonof.bz>, Jon <jon DeleteThis @nonof.bz> wrote:

> I have 3 Macs connected to a 10 Mbit/s LAN using a D-Link DI 604
> router. Two of the Macs are running Mac OS X 10.2 and one is running
> 9.2.2.
>
> If I connect my main Mac directly, without any router, I get almost
> 9.6 Mbit/s, but if the signal passes through the router I only get
> about 1 Mbit/s.
>
> I was told that if I tell my "PC" to use Full Duplex I would get back
> to 9+ Mbit/s. The trouble is I have no idea how that can be done on a
> Mac. Obviously that is possible on a PC running Win XP.
>

One possibility is that MTU discovery by the Mac is being broken by a
paranoid router not passing the needed ICMP packets back to the Mac (if
the router is doing NAT it really doesn't have a choice except to be
paranoid if not configured to forward the packets).

If that's the problem, then forcing the MTU to a smaller value would be
something to try...that's been discussed in the newsgroup
recently...since I didn't care I didn't pay attention.

--John

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In article <140820031232254167%jon@nonof.bz>, Jon <jon.TakeThisOut@nonof.bz> wrote:

> In article John Baxter wrote:
>
> > One possibility is that MTU discovery by the Mac is being broken by a
> > paranoid router not passing the needed ICMP packets back to the Mac (if
> > the router is doing NAT it really doesn't have a choice except to be
> > paranoid if not configured to forward the packets).
> >
> > If that's the problem, then forcing the MTU to a smaller value would be
> > something to try...that's been discussed in the newsgroup
> > recently...since I didn't care I didn't pay attention.
> >
> > --John
>
> Thanks John and Jerry for answering my question.
>
> I am not a regular reader of this board. Maybe that is why I don't
> know what MTU stands for. I could not find any reference to any thing
> like MTU in the router's software. Does it go by some other name too?

MTU is maximum transmission unit. It is shown in one of the lines
output by
ifconfig -a
at the command line. Sample:
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500

If, somewhere between me and the world, there is a piece of network
which can't handle that much of my data in one packet, then I'm forcing
hardware to break up packets and reassemble them. (Which is impossible
for a packet marked as "do not fragment".)

Left to itself, the software running the interface will discover a good
MTU by setting do not fragment on packets of decreasing size and seeing
what happens. But if ICMP (or the wrong parts of ICMP) is filtered out,
that technique doesn't work.

>
> Perhaps I should buy another router, but I would feel very silly if
> that did not solve the problem because of some value or flag not being
> set correctly in the Macs.

What brand do you have? (You likely don't need to buy
another...particularly if there is someone you can borrow from.)

>
> Again, does anyone know if I can force my Macs to use full duplex?
> Any command line tip? (With its undo command, please)

It's probably in the ifconfig command. But I'm not familiar enough with
the FreeBSD form of ifconfig (it varies a LOT) to know how. Nor am I
familiar enough with Mac OS X startup to know how to induce it to happen
as the interface is started.

And you may not want to force full duplex...if you do, you have to force
both ends of the wire to full duplex...the standards require that if one
end wants to negotiate and the other refuses, the negotiating end has to
set up half duplex. (Which can ruin speed and cause large error rates
on a network segment, as we learned the hard way several years ago. [HP
vs Cisco in that particular case])

>
> It really bugs me that they can do it in Windows.

They can do Blaster in Windows, too. Cheer up. (And yes...that's
unfair.)

--John

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