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TOPIC: MTU level

MTU level 10 years 7 months ago #13258

  • dwyane
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8) Hi guys can any one tell me how to change MTU level on my win XP machine, i have cable dsl, current MTU of my computer is set to 1420 i checked that by the following command
ping < gateway ip of our network > -f -l 1420 ( as pinging different sites is blocked on the server ), above 1420 it starts giving a message saying " packet needs to be fragmented ",
just to inform u that we are getting a private ip and what i understand that its a ethernet connection between the hosts and the router so max MTU shd be set to 1492 on the host ,
& should i change the MTU settigns will there be any change in my browsing speed... :?:

thanx in advance :wink:
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Re: MTU level 10 years 7 months ago #13259

  • nske
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Hello, unless you suffer from low quality connections or your real bandwidth is too small, the best would be to keep the MTU at it's maximum (as it is already). A smaller MTU would only save you bandwidth if a lot of packets needed to be retransmited (because they were lost or currupted on the way) -or if your connection speed was so limited that even a normal number of retransmitions of this size caused delay problems.
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Re: MTU level 10 years 7 months ago #13264

  • dwyane
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:) hi nske,
i understand that we shld not play with the MTU setting of the computer, reason why i want to change the MTU level on my computer is because its currently set to 1420 which is ideal when there is VPN running with ipsec, Now because i have a simple cable dsl and most probably the router im hooked to shld be set to 1500 by default,
:idea: so why not i change the MTU on my computer to some where aroud 1492 compared to 1420 current MTU of my computer, so that lesser number of packets with bigger size can travell that will make the speed more faster,

I'd really appreciate if u can give me more info on this...
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Re: MTU level 10 years 7 months ago #13270

  • MezzUp
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If I recall correctly your MTU is reallu 1500 bytes all-in-all. The 1420 bytes you tell 'ping' to send do not include the nessecary overhead for an IP packet.
Am I correct?
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Re: MTU level 10 years 7 months ago #13271

  • dwyane
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so according to u Mezzup if i do a ping on the gateway ip the pakets dont fragment at 1420 so u mean the exact MTU includes 80 bytes of header + 1420 that makes a total of 1500 bytes is this what u want to say ??
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Re: MTU level 10 years 7 months ago #13272

  • nske
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Hello :)

In windows XP you can change the mtu through registry, as described here.

The message that "packet needs to be fragmented & DF set" was an ICMP message from your gateway, it is not caused necessarily because your PC's MTU is too low, but because you tried to send a packet marked with the "Do not Fragment" flag, that "wouldn't fit the pipe". Under tcp connections, this ICMP message (which should also contain the maximum allowed MTU) would cause your pc's TCP stack to continue the connection sending packets with the reduced MTU mentioned, so there would be no problem. Setting your system's mtu to the actual useable is not a bad idea, it might even save a pointless negotiation. However under windows the setting can only be set on a per-NIC basis, so it will inevitably affect connections inside your network -which is not optimal.

Also, like MezzUp said, ping's size argument doesn't include the headers, only the ICMP payload. The headers for ICMP though, would add to 28 bytes not 80!
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