>From: "Andrew" <ac.macc@ihug.co.nz>
>So are you saying I don't have an MTU issue because the MTU is set by the
>Speedtouch Pro? I guess I know enough about this stuff to say the wrong
>thing! ;) Can you explain briefly why you say this?

Briefly?

One topic is whether a 1500 byte packet leaving NAB's server will make it to
your web browser.
We (this list) have already found out that if you have a SpeedTouch Home or
3com Home Connect then the packet won't make it for two reasons:

Both these modems have an MTU of around 147x bytes, therefore the packet
would need fragmentation from modem to user's system. If the packet has the
Don't Fragment flag set then it can't be delivered to the user. The 3com
doesn't fragment on a good day so it will never get delivered.

You have a Speedtouch Pro running in router mode, it should have a 1500 byte
MTU, therefore the Speedtouch Pro should not be causing you grief.

However there remains a possibility that another device between you and NAB
(e.g. At Telecom or your ISP) has a lower MTU.

>I tested my ADSL connection using the site:
http://www.dslreports.com/tweaks
>which reports back your MTU. The changed MTU on the XP machine
>was reported back to me, so I was assuming it was passed through the
router?

Changing the MTU on your system does not change the MTU of the router or any
other device between you and say NAB's server.
Your MTU is used in TCP as a Maximum Segment Size (MSS), setting it to 750
tells the Web server's TCP to limit the packet size to 750 bytes, these
packets will easily fit through devices between you and the server.

>Anyhow I set the MTU to 1476 and while the site mostly worked it wasn't as
>reliable as when the MTU was 750. I then decided it was time to get some
>sleep!

I requested that you test NAB with an MTU of 1450 and perhaps 1400, I would
be interested in the results.
When you get a failure (page doesn't load) wait at least 3 minutes to see if
it eventually arrives.

>I presume that there is more to this problem that just the MTU as Jetstart
>seems to be the confounding variable. Perhaps it is the rate-limiting
system
>in combination with the blocking of ICMP responses that breaks things?

Good observation :)

On Jetstream the Web server's TCP can probably pump data faster than the
Webserver can produce it - result, small packets. On Jetstart, data produced
by the Webserver will queue in TCP - result, large full MTU packets.

But who said you have an MTU issue...

Cheers

BG.

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