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User profile for user: larrynyc
larrynyc Author
User level: Level 1
0 points

A fix for random, slow, intermittent internet problem

My '06 Mac Mini would intermittently pause and hang while loading websites. I mean, slow as molasses. Sometimes it would be slower than dialup, or wouldn't even load pages at all. Other times, the internet worked fine like nothing was wrong. I was puzzles.

I thought it was the cable company. So I swapped the cable modem box. Still had the problem.

I've been scouring the internet, trying to find the answer. I kept finding the same standard responses, such as fix permissions, reinstall the OS, call the Apple Store. Nothing worked.

I'm sure you've double-checked these to no avail:

+Outside wire (cable or DSL). Cable or DSL modem. Ethernet cable from modem to router. The router. Ethernet cord from router to computer. AirPort antenna. PRAM, PMU and SMC. IPv6 off. Different DNS servers. Reinstalling Safari. Clearing the fonts caches. Clearing all caches. Running everything on Onyx. Permissions repair. Fresh installation of Snow Leopard. On different hard drives.+

I tried these. None of these worked. Drove me nuts for months. My roommate's Mac worked fine. I honestly though my computer went bad.

*Then, I finally found the culprit: The automatic MTU!!*
*I couldn't believe it. Such a stupid, basic cause of such a big problem. This is most certainly a BUG.*

*To fix the MTU:*

1. Apple Menu > System Preferences... then Network.

2. On the left, choose Ethernet , then Advanced... on the bottom.

3. Choose Ethernet tab:
. . . Configure: Change from Automatically to MANUALLY.
. . . MTU: Change from Standard to CUSTOM. Put 1452 then click OK. Then Apply at the bottom.

4. Repeat step 3 for all services listed on the left, such as AirPort, Parallels, etc. Press Apply.

5. Turn off your computer, unplug the router, then unplug the cable/DSL modem. Wait 60 seconds, turn on the cable modem (patiently wait for it to come on with full lights active), then the router (wait for its lights, be patient), then your computer.

Voilà!

Mac Mini, Mac OS X (10.6.3)

Posted on Apr 4, 2010 3:57 PM

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38 replies
User profile for user: BDAqua
BDAqua
User level: Level 10
239,375 points

Apr 4, 2010 4:44 PM in response to larrynyc

Thanks for the report Larry! 🙂

Sounds like a Modem/Router, or ISP problem.

• 1500. The largest Ethernet packet size; it is also the default value. This is the typical setting for non-PPPoE, non-VPN connections. The default value for most routers, adapters and switches.
• 1492. The size PPPoE prefers.
• 1472. Maximum size to use for pinging. (Bigger packets are fragmented.)
• 1468. The size DHCP prefers.
• 1460. Usable by AOL if you don't have large email attachments, etc.
• 1430. The size VPN and PPTP prefer.
• 1400. Maximum size for AOL DSL.
• 576. Typical value to connect to dial-up ISPs.
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User profile for user: larrynyc
larrynyc Author
User level: Level 1
0 points

Apr 4, 2010 5:38 PM in response to BDAqua

Thank you BDAqua!

I've been spreading the word about this, because it's been happening all over, to many friends of mine. I bet Apple has been receiving quite a few AirPort replacement services because of this.

My ISP is TIme Warner Cable, so it wouldn't be surprising if it's partially their fault for having a service that trips up the Automatic MTU sensing ability of Snow Leopard. Still, Apple should learn about this phenomenon.
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User profile for user: larrynyc
larrynyc Author
User level: Level 1
0 points

Apr 4, 2010 9:23 PM in response to tbux

1452 is the MSS (maximum segment size) when the MTU is at 1492 for PPPoE (the 40 comes from the headers). Thought it seemed like a good, round safe number.

I've been playing around with even lower MTUs, such as 1000, 1300, etc. My peak download speed slows somewhat as the MTU gets lower.

To give you an idea, 1470 MTU gives me a peak of 9.6 megabits of speed. 1000 MTU gives me 7 mbits. 1300 gives me 9 mbits. 1452 gives me 9.5 mbits.

Now, that's just raw speed. In the real world, actual download speed is rarely that fast unless there's no traffic to the site you're at. Interestingly enough, when there's congestion, a lower MTU helps because it takes less time to retransmit the lost segments.

If your MTU is too high, even by a value of just 1, it will trip everything up and slow you down to a crawl or no transmission at all.

Essentially, the trick is finding the sweet spot where the MTU value is just high enough to get the speed you want, low enough to prevent fragmenting, and at a size that has a quick retransmission. Usually I see people literally put the MTU all the way up until it starts to choke and then drop it down by 1, but that's too close for comfort for me.

It's more of an art than a formula. I suspect Apple tweaked this formula somewhat recently for the mechanism that detects the ideal MTU.
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User profile for user: larrynyc
larrynyc Author
User level: Level 1
0 points

Apr 4, 2010 9:33 PM in response to BDAqua

I agree, I thought it was a problem with those too.

I swapped the modem box for a new one. Still, the router (an AirPort Extreme) worked fine with other computers. Still, for testing, I tried plugging in straight to the cable modem. Still had the problem. Yet other computers worked while plugged in to the router and straight to the mode.

So this ruled out the modem, the router and the ISP.

Then, after doing a full low-level format and reinstallation, I still had a problem. Thought it was my hardware but then I noticed that the problem didn't exist when I booted into Windows via Boot Camp.

And then, when I took my computer to a friend's place, the internet worked fine in OS X. Weird right?

It truly was a mystery. Took me months to figure this one out. I almost bought a new computer! The culprit was half my ISP, and half Apple.
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User profile for user: Jeffrey Schneider1
Jeffrey Schneider1
User level: Level 1
9 points

Apr 6, 2010 10:50 AM in response to larrynyc

Hi All!

Went though the process and everything was fine for a day... now my internet is slow again.

I checked today and my Airport was configured to AUTOMATICALLY.

I've tried to change the setting to CUSTOM, 1452. I hit apply, go back and check and Airport has not saved the setting... the ETHERNET is back to being configured AUTOMATICALLY.

*Does anyone know why my Airport won't apply the Manual changes?*

Thank you!
Jeff
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User profile for user: Kristin Kassela
Kristin Kassela
User level: Level 1
2 points

Apr 7, 2010 6:41 AM in response to Jeffrey Schneider1

Yes, same here. My internet suddenly is running ssllloowww. I recently did the lastest software update and am wondering if that has something to do with the slow internet. It did have an airport base station update and I am wondering if it messed up my settings somehow. I tried to do the "manual, 1452" but it keeps going back to automatic. HELP! I am losing my mind over this one. I am not using snow leopard yet, do you think that will help if I upgrade?
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User profile for user: M4R4G4T0
M4R4G4T0
User level: Level 1
10 points

Apr 12, 2010 4:40 AM in response to Kristin Kassela

I have also faced the "going back to automatic" problem. I could manage to fix it at manual setting by creating another location called "Home" and then adjusting it's settings. It seems the "Automatic" location is really meant to be automatic.

I have also read somewhere that automatic optimal MTU detection relies on ICMP, so I started to think the stealth firewall mode could prevent the automatic setting to work, but I couldn't test it for real, yet.
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A fix for random, slow, intermittent internet problem

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