Burbie
@comcast.net
| What MTU is optimal for your NIC?
I'm using Comcast with a DLINK 4300. The MTU inside the DLINK is set to 1500.
The reason I ask is when my NIC is set to 1500MTU I get 1800kbps on speedtest sites and 275KB/s UPloads in real world FTP tests. When I change my NICs MTU to 1472 I get 3000kbps on speedtest sites and 400KB/s UPloads in real world FTP tests (which drops off to 265KB/s after powerboost runs out.
The only downside I have found with 1472MTU is occasionally a 3rd party POP server times out and gets stuck on the same email. |
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swhitney2003
quack.
Premium
join:2003-06-13
Weare, NH | You should be using 1500 on both your router and nic. Have you checked your TCPIP receive window settings? That could prevent your downloads from achieving maximum speeds. |
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Burbie
@comcast.net | This post is not regarding download speed but UPload speeds. My TCPIP recieve window settings are fine and I get around 33000kbit DL on speedtest sites. |
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EG
The wings of love
Premium
join:2006-11-18
Union, NJ
| reply to Burbie
There really isn't any *one size fits all* optimal MTU value in the way that you seem to be thinking.
As was stated, 1500 is what CC recommends for their network.
If your chosen value seems to function better for you, go for it. But there may be other consequences such as you have seen.. |
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Network 2
@comcast.net
| MTU is not a fixed value and the optimum setting depends on many things. If you are tunneling any protocol the additional header and encryption affects MTU. For instance if you run a GRE Tunnel and IPSEC on a router the best practice is to reduce the MTU to accommodate the additional overhead to avoid fragmentation. That said, it is unlikely, and I have not read up on all the implementation, a VOIP protocol would generate 1500 byte packets but smaller packets is more the norm. Reducing MTU typically will not affect your throughput in a negative manner. For straight IP I don't see any need to reduce the MTU size. Well behaving tunneling protocols run from host to host will also not exceed 1500 bytes. 1500 is the default MTU for ethernet and other interfaces may have varying MTU sizes. If you set your router to 1300 and your PCs are still putting out 1500 byte packets your packets will be fragmented by the router so you may want to be a little careful there. But there is not best size and if there is a lot of congestion a smaller packet size it usually better. |
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Johkal
Cool Cat
Premium,MVM
join:2002-11-13
Happy Valley
clubs: | So, to summarize: Use an MTU of 1500  Geez! |
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Burbie
@comcast.net | You are ignoring the fact that it leads to 66% more upload bandwidth across the board. |
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EG
The wings of love
Premium
join:2006-11-18
Union, NJ
edit:
July 27th, @11:24AM
| Be that as it may, it is still working around something that is wrong in the network.
»en.wikipedia.org/wiki/Maximum_tr···ion_unit
»/tweaks/MTU |
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toolkit
join:2001-01-16
Crystal Lake, IL
edit:
July 27th, @11:28AM
| reply to Burbie
Because you noticed such a big improvement in performance, I would guess that somewhere in the chain, packets with MTU=1500 are getting fragmented.
That's easy enough to test. In Windows, run the following command:
ping -f -l 1472 www.somewebsite.com
The -f flag means don't let the packet get fragmented. The -l sets the size of the packet. Make sure you use a web site that responds to pings  .
For other OS's a similar command is available:
Linux: ping -s 1472 www.somewebsite.com
OS-X: ping -D -s 1472 www.somewebsite.com
If the ping reports that the packet had to be fragmented, try a smaller size. I suggest dropping by multiples of 4. So try 1468, 1464, etc.
When you find the highest number that does NOT lead to fragmented packets, then add 28 to that number, and that is your optimal MTU. So, if 1472 worked, then 1500 is you best MTU.
You could investigate further to find out why your optimal MTU is not 1500 - but why bother? Something in the chain is causing the fragment (maybe your NIC, maybe your router, maybe something at Comcast), and as long as you don't have to set the MTU too low (say below 1400), then it's probably not worth searching for the bottleneck. |
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EG
The wings of love
Premium
join:2006-11-18
Union, NJ | This may be an o/k work around on a smaller LAN, but on the internet WAN, there may be black hole routers anywhere in the many many paths that may be traversed to any particular remote server. |
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