Spotty Internet connection? Maybe change the MTU

At this point, I’ll try anything. We’re having a bit of a problem with network drop-outs here. Most of the time, it only affects websites (browser traffic). It’s not DNS, as it happens with both OpenDNS and Comcast’s name servers. From time to time, out of nowhere I can’t connect to any website for a few minutes, while mail and IM work fine. Or IM drops out completely, but everything else is fine. Very rarely do I get completely thrown off.

Our current setup: Motorola 5100 cable modem -> Vonage VT1005 phone adapter -> Netgear WPN824 router -> All the wireless devices throughout the house, WPA Personal security

The only wired connection to the router right now is my MacBook Pro, and for no other reason than it’s sitting in the same room as the router so might as well take advantage of any wired boost.

Other devices include our Wii, the girls’ Mac Mini, Eric’s iBook, Eric’s Dell Latitude laptop that he only uses for work and access via VPN, and my Cingular 8125 when I choose to turn wifi on.

If I take Vonage and the router out of the mix and connect directly to the cable modem, it seems to work fine but I can’t really test like that. I need the phone, and I need to let the rest of the house online.

So is it the phone adapter or the router? No way of knowing for sure without going offline.

I turned to Netgear’s site. I like the way their knowledge base lets you dig down to where the problem might be.

Start with “No Internet Through the Router” which isn’t really the problem but a good place to begin. Not relevant if “Sometimes your network connects to the Internet”, click leads here. Which is not relevant if “you can access some programs and not others” which leads hereto an article on MTU and partial loss of performance. Ah….now we may be getting somewhere…

Could my problems be caused by packet sizes that are too large? Hmmm….worth a shot…

Setting MTU size is a process of trial-and-error: start with the maximum value of 1500, then reduce the size until the problem goes away. Using one of these values is likely to solve problems caused by MTU size:

* 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 NETGEAR 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.

Eric uses VPN for work and who knows how much I’m trying to squeeze through with the phone and everything else.

So I dialed the MTU back to 1492 (easily done in the router’s configuration screens) and followed the directions here to set my MacBook Pro to the same MTU speed. This is something that has to be done on all computers. We’ll see if there’s improvement. If not, I’ll keep throttling back slowly to around 1400. If it’s still messed up then, I’ll look in another direction.

I have no problem with slower download speeds (upload can’t get that much worse)…I want consistency.

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3 thoughts on “Spotty Internet connection? Maybe change the MTU

  1. Hi Judi,

    I’m also in New Jersey, and I have the same ISP. The rest of our setup is different — different Motorola modem, Asante router, Airport base station…

    Same issue. On occasion, we lose all browser traffic, but everything else continues to work — even sending email, which I would think involves a DNS lookup of some sort, too.

    I have no solution, but I’m eager to follow-up on your MTU test results.

  2. Anonymous says:

    This may be off the mark but how about this: from Steve Bass who writes for PC World

    AntiSpyware Gripe

    The Hassle: After running an anti-spyware program, I now have only sporadic access to the Web.

    The Fix: The culprit is a Layered Service Provider that went kaflooey or vanished. You may be able to Web surf, but you may not have e-mail, or vice versa. The fix is easy.

    If you’re using Windows 98 or Me, download and run LSP-Fix. XP users with Service Pack 2 should go to the Start menu, choose Run, type CMD, click OK, type netsh winsock reset, and reboot. No luck? Borrow a buddy’s PC and download WinSock XP Fix, a free tool that makes the LSP behave. Then read the tutorial.

  3. Judi Sohn says:

    Thanks for the comment, but that’s not it. My Internet connection is through a computer running Mac OS X, so I don’t think LSP is at play. Besides, it’s an intermittent problem, not a constant. Thanks, anyway!

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