Apples drop packets wifi and LAN, Win7 is OK

I think Apple needs to decrease the default MTU setting on its devices - just a tad, but still...

Turns out something here on this network at thie location is limiting packets to 1500 MTU. Research has led me to find out that many routers default are 1500.

The Macs needed to have their MTU decreased to 1492 to accomodate 8 bytes overhead to clear the 1500 limit here. User still unable to use his iphone - doesn't seem to be a way to adjust the MTU on it ?

I think Apple should lower the default MTU they built into the MACs and iphones to 1472 (28 bytes overhead needed with some auth schemes). By contract, checking my Windows interfaces, they are set at 1300.

[ DOS command: netsh interface ipv4 show subinterface ]

Hence, none of the Windows users here had any issues because they were asking for packets well below the threshold.

On the one hand, you may think it makes sense for us to find the one device that is limiting here on this network and increase it so that any future Apple users who come in won't have a problem. On the other hand, they may still run up against this issue bringing their devices onto other networks. It may be better for them to adjust their devices so they will be compatible on more networks.

Seeing the number of forum posts all over now referring to wifi and MTU with MACs, I think Apple should fix their default! Obviously they are too forward thinking - many networks haven't caught up with being able to handle larger packets yet.

"It isn't a problem for tens of millions of Macs and hundreds of millions of iOS devices."

Well, it's a problem for _enough_ of them that forums all over are littered with MAC/wif-fi issues being solved with adjusting the MTU on their MAC or ipad or telling them to turn off "Jumbo frames" in their router. I think many of those users would actually experience the same issues on LAN, but they are only using wi-fi and hadn't even thought to check to rule out being strictly a wi-fi issue.

"The fact that you have already set your MTU to a very unusual value seems to indicate a problem on your end."

I don't connect to that network. In fact, though my Windows 7 interfaces are set at 1300 [NOT BY ME], here my router can ping at 60000 without fragmentation = no limit here = no reason why they would ever have been adjusted. My windows pings fragment anything above 1272 = it's allowing 28 bytes for overhead with its 1300 limit and falling way below most of the default standard MTU for different data types.

[ DOS command: ping yahoo.com -f -l 1273 ] "packet needs to be fragmented"

ping yahoo.com -f -l 1272 (the ping succeeds)

On my router there is an area I can run pings from and it allows me to set the packet size - 60000 doesn't fragment. That tells me my network here is not limited. My computer's interfaces are set with low enough MTU to requestand handle small enough packets to respect any limits it may encounter which cover many standard data types. Whether that's Windows or Toshiba that set it?????

Commands for ping test to determine MTU on OS X and linux can be seen here: http://www.dslreports.com/faq/695

or here http://kb.linksys.com/Linksys/ukp.aspx?pid=80&vw=1&articleid=386

Where my Windows interfaces state 1300, but actually only handle 1272 (leaving room for up to 28 bytes overhead)... I wonder if Apple's default MTU of 1500 is not accounting for any overhead... since adjusting it to 1492 works + 8 bytes for UDP overhead = 1500.

For sure our recent changes to that network introduced a piece of equipment which is now limiting them to 1500. But all of the Windows, Androids, and BBs had no issues. I have found several manufacturers whose networ devices are set at 1500. I'll be investigating if any of the Cisco units are miscalculating for overhead. Can any of yoru MAC users post your hardware MTU settings along with results of the ping test to find your MTU size?

It's an aDSL line - it is correctly configured to accept max packets @ 1492 to allow 8 bytes for pppoe = total 1500 for internet traffic.

And yes, Linc Davis, you are correct! Windows is also using 1500 as the default MTU. It seems mine are set low at 1300 to accomodate VPN...

Regardless, there is also supposed to be a 'conversation' of sorts. If any device in the path between source and destination cannot handle the packet size being sent - there's something called "path mtu discovery" whereby a bit of back&forth communication occurs to determine the optimal size.

I found an extremely "laymen-friendly" explanation of what should be going on.

http://www.netheaven.com/pmtu.html

Now the Windows also set with 1500 by default, yet appear to be successfully negotiating to find a value that works. I found an MS kb which stated it would use a manually set MTU only if it was lower than the optimal MTU determined by discovery. i.e. you can not force Windows to use a higher MTU that is incompatible for the network.

So my next question is whether recent MACs implement "path mtu discovery" by default? Or is Apple's philosophy that it's better to manually adjust your MTU to match the network you are using? There's an outdated Apple kb article [TA36590] about how to disable it. And there's a few current KB entries teaching how to alter your MTU if instructed by your network admin. Now, lowering it manually shouldn't really be necessary if discovery works unless you're seeing performance hits due to the extra time 'negotiating'? Yet they've got it at the highest "common" value. So what's its current default state of pMTUd?

And if it should be happening by default, where can we check in the MAC to see whether something had been done to disable this on these MACs? The above article references some back end voodoo akin to fiddling in Windows registry. It would seem plausible that some firewall adjustments or tweaking for video watching or gaming could be the culprit.