Saturday, November 29, 2014

"Why Does My Internet Connection Keep Dropping?" -- A Few Tips About IPv6, DSL, MTU, and Apple AirPort


We are told that IPv6 -- an updated and advanced system of network addressing -- is the future of the Internet. That may be true, but IPv6 is also an immature and treacherous technology.

For maximum reliability, my wife and I have two Internet connections, one cable and one DSL. Both are routed through wireless networks running on Apple's latest generation of AirPort base station. Generally, I use the cable Internet, while my wife uses the DSL, though we switch as needed.

For the past few months, my wife has switched to the cable more often than intended. With her DSL, she kept complaining that Web pages were slow to load or stopped partway. I tried a number of possible solutions and managed to make some improvements in network operation, but nothing fixed the original problem. And then she complained it was getting worse.

I began to wonder if our old DSL modem was at fault. So, I started researching modems and uncovered some interesting facts. One was that our DSL provider, CenturyLink, was in the process of switching its systems to IPv6. Another was that our old modem could only handle IPv4. Could that be the source of the problem?

I thought I should get CenturyLink to replace our modem, but their monthly rental charges were high, so instead I looked into buying. But when I checked the IPv6 models that CenturyLink recommended, I found they had bad customer ratings on Amazon because of poor reliability -- even though the previous-generation IPv4 modems from the same company had good ratings!

So, I changed course again and just bought a new IPv4 modem. But when I finally got it to work, I found that our connection was slower than with our old one! I wound up switching back and keeping the new modem as a backup.

In the course of trying to set up the new modem, though, I noticed something interesting about our AirPort settings. At some point I must have decided to "prepare" for IPv6 by setting up automatic configuration for it in our base stations. I must have figured that the stations' operations would be adjusted as appropriate for the connected modem.

But why, then, had the station on the DSL network been automatically set for "tunnel" mode? Was CenturyLink somehow bypassing the limitation of the IPv4 modem, or trying to? And was it botching the job and screwing up our connection -- or at the least, hitting an incompatibility with the base station?

Or was the base station itself screwing up? At about this time, I was reading about problems with dropped wireless connections in the first versions of Mac OS X 10.10, Yosemite. Why now? Was Apple having trouble with its own developing IPv6 technology?

Where the fault lay, I'll never be sure. Quite possibly with both CenturyLink and Apple. But the solution turned out to be simple: Completely turn off IPv6 from the Internet connection. I did this in the AirPort Utility by going to the Internet tab, choosing Internet Options, and on the Configure IPv6 drop-down menu, selecting "Link-local only." My wife's connection has been fine ever since.

Moral: Leave the future of the Internet to the future, and for now, reside safely and securely in the present.

Update: It turned out there was another piece to the puzzle. Because my wife's Internet connection was DSL with PPPoE, we needed to adjust the Maximum Transmission Unit -- "MTU" -- in the Mac's Network settings. This is something CenturyLink never bothers to tell you. (To check whether your DSL uses PPPoE, open AirPort Utility, select your base station, click on Edit, then on Internet, and look at Connect Using.)

Normally, the MTU for DSL with PPPoE is 1492. That's the largest packet of data your computer should send out if you want everything to run smoothly. But I noticed that part of CenturyLink's IPv6 setup called for an MTU of 1472. So, for future proofing, that's what I set.

To set the MTU in Yosemite, go to System Preferences > Network > Wi-Fi > Advanced > Hardware. Change Configure to "Manually" then lower the MTU from the standard 1500. To get the best number, you might want to talk to your DSL provider; or you can start at 1492 and go down in steps to maybe 1400, if needed. Don't worry about overshooting a bit. Though that does slow down the connection, it shouldn't be enough to notice.