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| reply to Da Geek Kid
Re: [Other] MTU quandry! Thanks for the great explanation. So, in a perfect world, the router is set to auto (or 1500) so it does not become a bottleneck. Then the sending computer sends a 1500 byte packet and if it goes through, this continues maximizing your throughput.
If the packet has to travel along a path that has a lower MTU for one or more links...
1) The packets are split which doubles the transmission overhead. In the case above this overhead is just 8/1492 which doubles to 16/1492 so you would presumably only double the 0.5% overhead adding 0.5% to your max throughput speed. Not a big deal for a typical user like me.
or
2) The router asks the sending computer to resend the data with a smaller packet size that will fit through the pipe. The sending computer then splits the packet and you end up with the situation in #1 above, except that now each piece of data may have to be sent twice, which could presumably double the transmission time. A big deal!
It also sounds like the sending computer should "learn" that 1500 byte packets are too big and begin reducing the packet size until it no longer gets resend requests. In this case the situation is self-correcting and so there may be numerous re-transmissions at first, throughput is quickly maximized.
Soooooooo...
You need to set your MTU to a value below 1500 only if 1) You KNOW that there is a bottleneck in your transmission path (presumably because of your protocol or infrastructure) 2) either the auto-negotiation described above isn't working or you don't want the small amount of lost packets resulting from the auto-negotiation process.
For our 3 different requirements (Ethernet, TCP/IP, VPN) it seems like a no-brainer to let the transmissions auto-negotiate because we will use the highest throughput much of the time, and we don't want to limit that. It also sounds like the auto-negotiation has a small overhead (1% or so) which is well-worth the price of simplicity!
Does that sound like a relatively correct understanding?
Should I be worried about my 1006 byte limit at home? Is this a sign of a hardware problem? Based on my new understanding the worst-case is that OVERHEAD will increase by 33% due to the packet size but since overhead is small anyhow this shouldn't make a noticable difference anyhow.
Thanks again for all your help!
Steve | | |
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 cacrollEventually, Prozac becomes normalPremium
join:2002-07-25
Martinez, CA | said by stevemayman:If the packet has to travel along a path that has a lower MTU for one or more links... 1) The packets are split which doubles the transmission overhead. In the case above this overhead is just 8/1492 which doubles to 16/1492 so you would presumably only double the 0.5% overhead adding 0.5% to your max throughput speed. Not a big deal for a typical user like me. or 2) The router asks the sending computer to resend the data with a smaller packet size that will fit through the pipe. The sending computer then splits the packet and you end up with the situation in #1 above, except that now each piece of data may have to be sent twice, which could presumably double the transmission time. A big deal!
Steve,
There is one additional possibility here.
3) Some servers absolutely can't handle fragmented transmissions (split packets). They drop fragmented packets, and you lose the connection. You see a white screen with no error, just no data.
»www.netheaven.com/pmtulist.html
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Cheers,
Chuck
MS-MVP [Windows - Networking]
PChuck's Network | |
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