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help me out with MTU and DSL / wireless router


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hl5's Avatar
hl5 hl5 is offline
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03-Jul-2006, 11:53 PM #1
help me out with MTU and DSL / wireless router
I've looked at MTU before but never had this sort of issue. I remember in the past changing the MTU settings would speed up or slow down a connection (basically, optimize the data transfer or make it less efficient).

I just got a wireless router which I have connected to a DSL modem, and the default settings on the router were for an MTU size of 1500.

I talked to the tech support and was told to reduce it to 1400 (or 1450 or something).

I remember that 1500 isn't that efficient a size for DSL, since 8 bytes are used by PPPoE, making the maximum packet size before that 1492.

I didn't realize, though, that using an MTU over 1492 would actually mess up (rather than just slow down) some things. What I originally noticed was that Yahoo Mail was acting strange and wouldn't let me upload file attachments. With a little testing, I've found that it works fine with 1492 and under but not with 1493 and over.

My question is what's the ideal size to use? I'm not sure why he told me 1400, since there seems to be no rhyme or reason to it. Since 1492 works, should I go with that or is there a good recent to bump it back to 1450 or 1400 like the guy said?

Thanks for your help.


p.s. And my other question is, shouldn't everything have worked fine even with 1500? I thought it was supposed to break a large packet up, so if it's 1500 but the top packet size is 1492 it just gets one 1492 and then one 8 -- which is inefficient but at least works. And I'm still trying to figure out why Yahoo Mail in particular had problems. I tried another HTTP upload which worked fine.
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etaf   (Wayne) etaf is offline
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04-Jul-2006, 05:32 AM #2
i found this posted here - and I usually make a note of the original author and thread to give credit where its due - but ddint in this case

anyway - i'll try and find the thread again

EDIT found it

http://forums.techguy.org/networking...eed-sucks.html

TERRYNET

Quote:
The following was obviously written for a router, but it works if you just have PC connected directly to your DSL modem. I don't know offhand how you set the MTU (if it needs to be changed) in the case of just PC and modem. Most times, I think, the MTU should be 1500 or 1492.

On your router there should be an option for mtu. use this formula.

To find the proper MTU Size, you'll have to do a special ping of the
destination you're trying to go to. A destination could be another
computer, or a URL.



1. Click on the Start button, then select the Run option.



2. In the "Open" field type in command (If you're using Windows
2K/NT/XP use CMD instead)



3. Once the window opens, you'll need to do a special ping. It
should be formatted:



ping [url] [-f] [-l] [MTU value] [Enter] an example would be:



ping yahoo.com -f -l 1472



ping yahoo.com -f -l 1462



ping yahoo.com -f -l 1452



You should always start at 1472 and work your way down by 10 each
time. Once you get a reply, go up by 2 until you get a fragmented
packet. Take that value and add 28 to the value to account for the
various TCP/IP headers. For example, lets say that 1452 was the
proper value, the actual MTU size would be 1480, which is the optimum
for the network we're working with.



4. Once you've found the optimum MTU size open up your browser,
usually Internet Explorer, which is located on your desktop (Or on
the Start menu in Windows XP)

Enter the MTU you get from this into the MTU settings on the router. Or on the modem.
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JohnWill's Avatar
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Location: South Eastern PA, USA
04-Jul-2006, 10:20 AM #3
FWIW, most DSL connections require 1492. If that works properly, setting it smaller will only reduce performance.
hl5's Avatar
hl5 hl5 is offline
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04-Jul-2006, 08:20 PM #4
Thanks, guys. That's why this forum is such a useful tool.

I tried the trick etaf mentioned. I think that for a DSL modem that is using the PPPoE protocol (which most do I think), you would have to add 20 rather than 28 to the value.

In other words, I tried 1472 and it worked, but 1473 didn't. (Yet I ran into problems at the 1493 mark, not the 1501 mark.) I think the logic here is that PPPoE needs an extra 8 bytes compared to a regular router using regular ethernet.

Right now I think I'm at 1450, which has been working fine. I think I'll up it to 1492 however and see how that works.

I guess I was wondering whether a wireless network would use any additional overhead, like PPPoE does. Maybe one of these days I'll try speed tests using the different settings. But anyway, the 1400-1492 range seems to work pretty well. I'll probably go with 1492 or 1450.


I'm still not sure why Yahoo in particular had problems. And I'm also not totally clear on what happens when the setting is too high. Do the packets get broken up into weird sizes, including one very tiny packet, or do the packets get sent too large and confuse some routers or something -- like, say, 1500 + the 8 bytes of PPPoE overhead = 1508, which is larger than the standard 1500 and could confuse something along the network...

Anyway, it is working OK for now. Thanks for the help.
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