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urban_alchemist · Jul 26, 2003, 05:49 AM

I have a 17" G4 PB, and a PowerMac G4 Dual Processor (both running Jaguar - 10.2.6), connected to an Airport Network (last of the non-Extreme basestations).

I need to change the MTU of the network to a higher value, and I was wandering how to do this. I have no idea where to start, so any help would be really appreciated...

car1son · Jul 26, 2003, 09:21 AM

Are you sure you want to increase the MTU size? OS X ships with a default size of 1500 characters for both the built-in ethernet port and theAirport. In my experience this is actually slightly too large for some broadband ISPs and FTP servers, and 1492 (Earthlink) or 1458 are more appropriate values for many users. I.e., most people want to adjust this to a smaller size. (Perhaps you can mention WHY you want to adjust the MTU size?)

The Airport Base Station doesn't seem to have a settable MTU value (some, such as the LinkSys routers, do); you can change them on the individual Macs, however.

Quickest thing is to use the Terminal (Applications/Utilities folder).

Before you start, use the Unix command:
/sbin/ifconfig -l

to list your current interfaces' MTU values, and make a note of them in case you want to go back.

To change the MTU size for the Airport connection, enter the command:

/usr/bin/sudo /sbin/ifconfig en1 mtu 1458

(This sets it to 1458 characters; pick the size you want. "en1" specifies the Airport interface. "en0" sets the built-in ethernet interface instead. You will be asked to provide an administrator password to execute this command.)

This setting only lasts until you reboot.

(I suggest using copy & paste to enter those commands unless you are comfortale with Unix command-line syntax to get the spaces right. Type a RETURN at the end of each line to execute the command.)

You can also set this value more easily in the GUI with the Shareware IPNetTunerX .

Apple offers this Knowledge Base article on how to make an MTU size change permanent (set it at boot.) It's a rather daunting set of instructions.

urban_alchemist · Jul 26, 2003, 04:16 PM

I bought up an X-box and connected it by Ethernet (direct) to the spare ethernet port on the Airport Basestation.

It connects fine, but after about five minutes of playing, it disconnects me for no apparent reason. I've called Customer Care (bunch of retards) and they say that the only time this has happened is because of the MTU setting. They gave me a list to try: 1365, 1465m 1500.

I'm totally stumped, and considering I just shelled out �40 for this LIVE cr@p, I'm just a little pissed off...

What do you think I should do?

Scotttheking · Jul 26, 2003, 06:16 PM

1500 is the max MTU for ethernet, and that's what the computer should be set at.

Angus_D · Jul 27, 2003, 09:31 AM

Originally posted by Scotttheking:
1500 is the max MTU for ethernet, and that's what the computer should be set at.

Not really - for BT's (as in British Telecom) ATM network (which almost all ADSL goes through in the UK, unless you're on a LLU exchange served by, say, EasyNet or Bulldog Communications), you should be at 1458 or lower at the moment, because they suck and you get large amounts of packet fragmentation and stuff on your way out to the internet. However, I've only heard of that causing you to get slower than expected speeds on ADSL, not disconnection.

John Strung · Jul 27, 2003, 10:50 AM

See: THIS THREAD and in particular the last post in that thread.

urban_alchemist · Jul 28, 2003, 06:55 AM

Thanks guys... downloaded the utility, and I'm going to try the adjustments now.

Just a note: I'm not using DSL, but instead a 2MB cable modem from Telewest. I refuse to accept that lowering the MTU value will change anything, but I can try.

Will keep you posted.

And thanks again!

ghporter · Jul 28, 2003, 09:18 AM

Depending on how they have their system configured, a small reduction in MTU could radically improve your throughput. Typical cable MTU setting is 1500, but if they frame their packets with something non-standard, a slightly smaller MTU would be appropriate-your MTU would be 1500 minus the number of overhead bytes.

With PPPoE on DSL, that's almost always 8 bytes of overhead, for a standard MTU of 1492 (pretty easy for us Yanks to recall, as that's the year Columbus stumbled across Hispanola on his "trip to India" for Ferdinand and Isabela).

car1son · Jul 28, 2003, 11:30 AM

I'm confused, but then I don't know anything about Xbox and gaming. What is the Xbox talking to, the Mac or some internet Game Server? Because if it's the latter, changing the MTU size in the Mac isn't going to make any difference that I can see.

Wouldn't an Xbox (or the specific game CD or DVD) come with an MTU size pre-configured for the servers it runs with?

I don't think there's any way to reach into the Airport base station and change its MTU. Does the Xbox work if you plug it directly to the cable service?

What am I not understanding?

ghporter · Jul 28, 2003, 01:01 PM

Originally posted by car1son:
I'm confused, but then I don't know anything about Xbox and gaming. What is the Xbox talking to, ...What am I not understanding?

There are a few XBox hacks out there-it's a specialized PC that runs some odd OS Microsoft came up with. There are some interesting things you can do with it, once you get it to stop thinking that all it is is a game console.

car1son · Jul 28, 2003, 05:23 PM

Yeah, I've seen the XBox hacks demoed on TechTV's Screen Savers ("kids, don't try this at home.") But as near as I can tell, urban_alchemist's "LIVE" is a stock MS game for playing on a stock MS Xbox with a stock MS gaming server. (I assume that game is more elaborate than Pong.)

You'd think that stock setup would work (certainly urban_alchemist wants it to) - at least if the Airport Base Station wasn't a router in his path. But (correct me if I'm wrong) tweaking the MTU parameter on a Mac that's also on that router isn't going to change anything on the gaming part of his setup. (Assuming the MTU size is the real reason his connection is dropping, if the Airport won't re-packetize to a different MTU size, he'll need a different router - like the LinkSys - that does. I assume no one knows a trick to get the Airport to change its MTU size. It's not a parameter I've seen exposed in my Airport Admin Utility.)

I assume he'd have to hack the Xbox to change the MTU size on that end. Or did MS expose that in the stock Xbox interface?

(I really ought to buy some new toys.)

urban_alchemist · 06:57 PM

OK, another update, and this one has phased me as much as you guys.

I changed the MTU setting on my powerbook, from 1500 to 1458. Don't know how this was relayed to the Airport, but maybe the network takes the lowest MTU setting for all computers on the network.

The X-Box now works (LIVE is the standard broadband muliplayer network from microsoft). Have been looking around some hack sites, and it looks quite interesting. I've only had it for a few days, though, so I'll probably wait a little while longer before fiddling with it.

Oh, and, even more surprisingly, my download speeds from BitTorrent have quadrupled as well (up from 20k/sec average to over 100).

Stumpted as to why this worked, buy hey, I'm not complaining. Oh, and RMAC is a great little app for changing network setting on your mac (far more complete than the Network Preferences, and with a nice little GUI instead of having to work through the console)...

Thanks again guys, and take care...

ghporter · Jul 28, 2003, 07:45 PM

That is screwy. I had sort of lost track of what machine you were messing with, and had thought it was a router. I have to agree that it looks like the PB's setting "osmosed" into the base station; I don't know how, but that's what it looks like. Sigh. Anyone else have a clue?

Signed-Confused.