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 server. 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 -- Christophe Barbé <[EMAIL PROTECTED]> 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
