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Re: airport on tibook: surfing vs. ssh/scp

On Mon, Feb 24, 2003 at 05:37:02PM +0100, Michel Lanners wrote:
> Well, I can explain in a few words what's up with MTU-related problems.
> MTU is the maximum size of a frame you can send out on the network (be
> it Ethernet, wireless, whatever). This is a limit on the physical (or
> rather, MAC)-layer. IP can handle larger packets, but then the IP packet
> needs to be fragmented into multiple (for Ethernet as example) Ethernet
> frames, which is not that efficient. There are other reasons, but in
> general, you try to avoid fragmentation.
> Maximum MTU is almost always 1500 bytes everywhere, even on media other
> than Ethernet (think your ISP's big backbone pipes). The problem arises
> when you put more protocol stacks one above the other, as is the case
> for example for PPPoE: now your Ethernet frame doesn't contain an IP
> packet, but rather contains a PPP frame that contains an IP packet. and
> suddenly, the medium just below IP, which is now PPP, doesn't have
> 1500 bytes MTU anymore, but (1500 - PPP header size).
> If IP continued to send (1500 - Ethernet header) sized packets, they
> would all get fragmented into a big (1500 - Ethernet header - PPP
> header) and a small chunk (PPP header size).this is obviously very
> inefficient: on your PPPoE link, for every IP packet, you get double the
> overhead: two Ethernet headers and two PPP headers instead of just one
> of each if you made your IP packets just a little bit smaller.
> That's the reason to do path MTU discovery, a mechanism for hosts to try
> and determine the largest IP packet they can send _without_ triggering
> fragmentation. Sometimes, this mechanism breaks, and that's what the
> page posted here is about.
> Now, after this (rather long, after all :-) explanation, questions to
> those with this Airport problem: are you using PPPoE (i.e. ADSL or some
> other broadband medium) to connect to the Internet?

Nice explanation. Thanks.

I have a DSL connexion (My provider is dca.net using a verizon line). I
don't know if the DSL modem does PPPoE but at least not on my side. On my
side it is talking plain TCP/IP (even with fix IPs). 
I don't think that the modem is talking PPPoE on the other side but I
have nearly no knowledge of the DSL technilogy.

But my understanding is that WEP Encryption looks like a kind of
encapsulation to me. Again I have no idea on how this is implemented.

Now what I don't understand is why this problem depends on the airport
card. I first thought it was caused by the airport base station but I
see the problem (again only when encryption is on) on another Wlan. I
don't know what kind of base station they use but I doubt it is from
Apple (this is the wlan of an university).

So to summary, If I understand correctly, the problem is that the MTU
path discovery is broken by something between the browser and the web

Open questions:
. Why not everybody with the last firmware see the problem? 
. Why the problem doesn't exist with the same hardware and the same network
when using MacOS.
. Why some browser doesn't have this problem (for example encompass
loads all pages without difficulties)?


Christophe Barbé <christophe.barbe@ufies.org>
GnuPG FingerPrint: E0F6 FADF 2A5C F072 6AF8  F67A 8F45 2F1E D72C B41E

People that hate cats will come back as mice in their next life.
--Faith Resnick

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