Nice Computer, Nasty Details

by Chris Adamson

I wasn't surprised when my dad's laptop died on him - for months he'd been getting error messages at startup, and getting all the way to the Windows laptop was a dicey proposition. So when it finally seized up and the display died, the third of my family's WinTel laptops to die a horrid hardware death over five years, it was great that he decided to go down the road to his local Mac dealer (indeed, the only one in northern Michigan) and get a new iMac. I figured that all the support calls I'd been getting would soon be a thing of the past.

Uh... no. Not yet, anyways.

Getting up and running with Mail and IE was easy enough (no, it didn't have Safari pre-installed), but we kept getting dropped by Earthlink's dial-up. I was out of my element here, since I haven't relied on dial-up for years. Unfortunately, in remote Torch Lake, Michigan, the cable company is in no hurry to offer cable-modem service, and DSL is almost certainly impossible. Still, a few hours of phone help from Earthlink and Apple seemed to clear things up.

But then he called me to say that when he sent a big (1.5 MB) PDF file as an e-mail attachment, he'd constantly get a server failure part-way through and the file would go back to his out-box.

This is where having a supportive Apple community helps out. I posted a message to Apple's support boards, which quickly generated a response suggesting I look into resetting the MTU (Maximum Transmission Unit) value, a low-level networking config. Researching this, I found an article on Mac OS X Hints that described a one-line call to ifconfig to reset the value, and in the feedback, someone posted a good tip for using different packet sizes for ping to find an optimal value.

Ordinarily, investigating this over the phone would be a nightmare, but that's where the Unixness of Mac OS X pays off. I had my dad create a user for me on his iMac, dial-in, start an ssh server, and tell me his IP address. That allowed me to ssh into his iMac and get a command-line. Pinging with common MTU values (576, 1452), I found we were getting massive packet loss, up to 70% at higher MTU values. I guess that's what happens when you're out in the woods and can only get 24Kbps dial-up.

I finally found that I could eliminate packet loss with an MTU of 200, so I set that temporarily with ifconfig and had him send his attachments. Everything started working.

Granted, now I have to find a way to make that MTU value permanent, and maybe I can find a slightly more optimal value, but the process went a lot faster by allowing me to get in with a command-line.

I also wonder why Mac OS X fails so badly off the shelf - does Apple not test a low-bandwidth dial-up configuration? And why can't a proper MTU be auto-determined at the time a dial-up connection is established? And why is any end-user ever responsible for this value - that's like expecting me to calculate torque ratios for my car's gearbox before driving to work. I think I'll file it as a bug.

Still, for such an obscure problem, this wasn't too bad to work through. Apple community, unix toolset. Nice combination.

Surely someone has something important and useful to say about this hideous little MTU config. Let's have it.


2003-09-17 12:39:31
Great article. I too have found myself in similar situations, providing IT support to dear ol' mom and dad who are stuck w/ dial up internet. I'm curious how you solved your connection being dropped. I remember seeing an error message something to the effect of "connection terminated due to no response from host" after a minute or two connected. I wonder if this is similar?
2003-09-17 12:47:24
Re: Question
Try the support discussions at . There are lots of people eager to share info there.

For what it's worth, have you tried switching the modem script (system prefs > networking > show internal modem > modem) from v.90 or v.92 to v.34? That's an OS 8/9 era bit of advice that sacrifices speed for stability. Didn't help my folks, but might help yours.


2003-09-17 12:50:08
I get that, but only when i'm connecting via bluetooth to my mobile. In-built modem was never a problem. Might have to try sending a ping to google to look for packet loss. Not sure what traffic is actually carried across GPRS though because iChat doesn't seem to work over it.
2003-09-17 12:50:11
Maybe the phone service?
My significant other has been using her iBook & earthlink for over a year now, and she's never complained once. Always used the stock configuration.

I had issues like what you mention when I lived in a house with a bad phone connection. You might want to check into that. Maybe a lot of noise on the line. Or gremlins.

2003-09-17 13:16:41
Maybe the phone service?
...or underpants gnomes. :-)

Seriously, you're right to wonder about line noise. I think that's reflected in the low connection speed that gets established. After all, we've tried multiple computers (including 2 iBooks) with 56K modems and never gotten better than 24Kbps out of the house. Not too surprising, considering how remote it is, thus how long the phone lines must be to get there.

Still, the goal here was that since this was replacing a wintel laptop that had no such problems sending large attachments, there seemed to be something unique to the Mac. I can't retroactively determine the default MTU of the dead laptop, but adjusting it on the Mac clearly made a big difference.


2003-09-17 14:02:57
Maybe the phone service?
I'd also bet on the phone lines. Several friends are all earthlink and live in the same basic area and all dialing into the same number. All have fairly reliable dialup and no problems with uploads/downloads of any size except for one of them. It is reliable a few blocks away at his office but not at home.

Another friend in a little town north of me (also earthlink) had to throttle his modem back to a slower speed as the phone system couldn't handle it. But that's all he had to do, no ssh involved :-)

PS: Earthlink...It seems they are dropping a lot lately...if you can manage to keep the connection for several minutes, your golden but it can drop, and drop, and drop before you hit magic. Never used to be that way and it's on all four of their local (Mpls) numbers.

2003-09-17 14:19:15
MTU on restart
I'm not sure if this is a help, but there is the following article in the knowledge base: "Mac OS X 10.2: How to Set the MTU Value During Startup" You can find ist here:
2003-09-17 17:29:19
It's a dirty little secret that Apple's software modems are extremely flaky and may not work with some dial-up lines. This problem first cropped up in the original Flat Panel iMac and there are extensive discussions on Apple's Discussion Boards about it. When I first got my iMac I was having a heck of a time getting and keeping a connection with my ISP (which I never had on the same phone line with my old PowerMac and GV modem). Apple never figured out the problem which I "solved" by switching to a different dial-up number. (It seems that the v.92 numbers were the one's giving my iMac fits.) I'm now on broadband so it's not an issue but AFAIK, Apple has never owned up to this problem and never come up with a solution (albeit for one Australian ISP). Supposedly, the problem isn't quite as widespread with Jaguar with speculation being that the modem drivers were changed.
2003-09-17 21:49:52
That's a severely broken network
This sounds like the problem is on the other end - you normally never need to worry about this sort of thing as a) it's not supposed to be that low on non-pathological networks and b) their server is supposed to provide your machine with the proper values during the PPP negotiation. Not only that, they must have broken path MTU discovery, which is turned on by default in OS X (sysctl net.inet.tcp.path_mtu_discovery).
2003-09-18 09:35:48
Cable Modem at Torch Lake
Just FYI...

My dad lives on Torch Lake and he has a cable modem connection for his iMac running OS X. Everything works great.

2003-09-18 11:30:20
The Line Quality
I would say that the ISP or the line quality is probably the biggest factor. I have had to live with dial-up for quite a while and I have found my PowerMac to be about the best at getting quality connections (compared to other Windows systems). Of course, this is a qualitative observation. I would just say across the board, Windows or Mac, modems are iffy (at least the cheap ones that come pre-installed -- I know you can get great high-end modems). Sometimes they work, and sometimes they don't -- at which time you are left to adjusting esoteric properties, praying to Greek gods, or doing a little dance.
2003-09-18 15:15:47
Troubleshooting Macs using SSH is great
I've had to do this a couple of times with friends of mine. Especially in earlier versions of OS X when it was more difficult to delete files with the wrong permissions/ownership. Have them flip on SSH server in sharing, log in, and do whatever you need to. Very, very useful.
2003-09-19 06:01:40
Cable Modem at Torch Lake
What side of the lake is he on? Charter keeps advertising it but saying it's not available for us... Charter took over some local twinkie little cable company and I guess they've been slow to upgrade.

I think there's also fixed-point wireless covering the south end of the lake (like Elk Rapids up to Clam River)... not sure that would reach.

Wow. Small world, meeting someone else on this site with iMac-toting parents on Torch.


2003-09-19 10:01:51
Opposite experience
I've got a Dell Inspiron 3800 with Windows XP and a Power Mac G4 with 10.2.5. In my experience, the Mac's dialup connection is much more reliable (same phone line, same dialup account). The Windows system seems to get disconnected about once every 10 minutes - the Mac never has any issues.
2003-09-19 13:28:20
Re: Cable Modem at Torch Lake
Hi Chris,
If you want, send me a message at with your parents address and I can find out if we can get the Pipeline service to them.

Andy - Charter Communications Online Technical Support

Dial-Up E-mail Support:
Pipeline E-mail Support:

1-800-968-5221 Residential Dial-Up Technical Support
1-800-545-8926 Residential Pipeline Technical Support and Sales

2003-09-20 16:41:02
Cable Modem at Torch Lake
He's on the east side of Torch just north of Clam River. He's got the whole digital cable/pipeline package through Charter. But from the looks of the other posts, looks like maybe you've got that base covered by now.

Yeah, it's lonely for a Mac user up there. I've definitely become the primary source of tech support -- I dread the day when it's something more serious that I can't solve over the phone.