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TOPIC: different mtus in computer and router??

different mtus in computer and router?? 11 years 10 months ago #5951

  • carmatic
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i always wondered, why i can set the mtu value both in my computer and in my dsl router/modem... which will come into effect? most likely the one in the modem, right....
what happens when they are different values???
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Re: different mtus in computer and router?? 11 years 10 months ago #5953

  • nske
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hmm in short, both will come into effect, each will pose a bottleneck to the other if lower. That is the reason for the existense of packet fragmentation by the routers and also the PMTU, because every intermediate link in a connection between two hosts must be able to handle packets of the specific length, but that is not always the case, as internet is a large and non-homogeneous (as far as it concerns the quality and capacity of the inks) network. So it is inevitable that the optimal MTU for each network of the internet might vary. But one thing is for sure, there is no reason to create this anomaly at your own network and it can also degrade the performance in many cases ;)

- When MTU between two links is different, then there are two methods to complete your connection sucessfully: Packet Segmentation and Path MTU Discovery.

The first method, Packet Segmentation, is to send the packet according to the MTU that is reported by the remote host you wish to contact, which means that if some router inbetween uses a smaller MTU for an interface, the previous router will have to segment the packet, cat it into pieces according to the new MTU, before forwarding it. This means potentially extra work for lots of routers, which is ineficient and, basically in a network such as the internet, sucks badly.

The second method is the smart way and the way it works almost everywhere today. This time you send again the packet according to the MTU that is reported by the remote host, but now you have set a flag on it's header, "Don't Fragment", which tells each router that instead of fragmenting the packet if the next link has a lower MTU, just deny it returning an ICMP error message to you (the sender) containing the reason for the denial ("fragmentation needed") and the length in which the packet will need to be fragmented in order to pass through the next router. You, the sender of the packet, receive this message and fragment the packet accordingly. This procedure may take place more than once, if a later router uses an even smaller MTU, in which case the MTU gets smaller and smaller each time until you find the maximum supported by all the routers. In any case, the actual MTU is cached for each session, so that the MTU path needs to be discovered only once. ;)
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Re: different mtus in computer and router?? 11 years 8 months ago #6574

  • carmatic
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ah ok... so like the mtu setting in each device will grate the packets coming through if the packets are going to be segmented ...

hopefully my router is using the auto discovery thing , theres something inside the router/modem's settings to control the auto discovery, but maybe its for the WAN side instead of the LAN side, like hopefully it will lower my ping times in games etc...
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