Setting MTU via DHCP

kiokoman

AFAIK ,DHCP Server's only give what the client ask for. you can configure all the options you want but if the other side don't ask for it, it will not apply
you can change MTU for windows with regedit
under
HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces
find the interface you need and create a DWORD named MTU and with the value you want

Nischi

@kiokoman Okay, thanks. Didn't know that.

johnpoz

You can also adjust it via netsh command on your windows boxes..

But to be honest this rarely makes any sense on local network.. Jumbo frames not going to get you much to be honest, and not worth the hassle.. Do all of your devices actually support jumbo? Printers? Any IOT devices? Your TV for example, etc. etc. Your internet sure doesn't

While it might be worth while on an isolated storage vlan where PC talks to File storage.. It almost never makes any sense elsewhere. I would suggest if you ran into something say hey jumbo going to get your XYZ performance increase.. I would suggest you do a bit more research..

Here is a good read
https://www.netcraftsmen.com/just-say-no-to-jumbo-frames/

JKnott

@johnpoz said in Setting MTU via DHCP:

While it might be worth while on an isolated storage vlan where PC talks to File storage.. It almost never makes any sense elsewhere.

Internet2 and similar networks (CANARIE in Canada) use 9000 byte jumbo frames. They also may be used in data centres, where a lot of data is moved around.

johnpoz

Yeah sure - that is not your typical local network was my point ;)

JKnott

@kiokoman said in Setting MTU via DHCP:

AFAIK ,DHCP Server's only give what the client ask for. you can configure all the options you want but if the other side don't ask for it, it will not apply
you can change MTU for windows with regedit

I just tried it here. I specified the MTU as a 16 bit integer. It worked with Linux, but not Windows 10. I would consider this a bug in Windows (no surprise there), as a client is expected to use the LANs MTU without any manual configuration.

Incidentally, I was reading an article, the other day, about a proposal to move the internet to jumbo frames, as things have changed so much since the dark ages when the Internet began. Back when I was at IBM, in the late '90s, we ran 4000 MTU, on the token ring LAN, IIRC. This change would just require increasing the MTU on the various equipment, which pretty much everything can handle these days. Path MTU discovery would take care of any differences along the way.

kiokoman

yes, i was talking about windows client.
windows only ask for this:

1 Subnet Mask
15 Domain Name
3 Router
6 Domain Name Server
44 NetBIOS over TCP/IP Name Server
46 NetBIOS over TCP/IP Node Type
47 NetBIOS over TCP/IP Scope
31 Perform Router Discovery
33 Static Route
121 Classless Static Route
249 Classless Static Route (Microsoft)
43 Vendor-Specific Information

it's not a bug, it's how it is (shitty) designed

JKnott

@kiokoman said in Setting MTU via DHCP:

it's not a bug, it's how it is (shitty) designed

Well, we are talking about Windows.

Is there a setting that will allow Windows to ask for MTU?

JKnott

One thing I've noticed is if the device has a valid lease, it will not do a full discover/request, but does a request only. Make sure that's not confusing things.

kiokoman

@JKnott
there is no settings that i know of

As johnpoz say it's not a typical situation, usualy you set jumbo frames inside a lan but not to the wan, even corporate probably use Group Policy to run a script that would apply the changes.

johnpoz

I can tell you that for all the corps had to support over the years.. Nobody is doing this on their local lan that client machines are connected to.. There have been a few companies that had jumbo setup on specific segments (waste of time if you ask me)..

This just makes no sense on your typical lan, especially a home network.. If want to play with it in your "lab" have at it.. Its not going to get you anything other then headaches when something doesn't work ;)

But yeah if windows shop was going to do it - they would prob set via group policy sure.

Here is a simple test... Set your storage nas/server with the larger mtu you want... Now set your machine your going to move files back and forth in your test... Move some files.. Did you speed increase by X percent? Did your cpu usage lower by Y percent.. Do the % gained in either of those two measurements make it worth the hassle? My bet is NO! ;)

JKnott

@kiokoman

It might not be common today, but years ago, when DHCP was starting out, different MTUs were common. I recall 576 used to be frequently used and, as I mentioned, we used 4000 at IBM, but it could have been configured for other. Still, a DHCP client that doesn't configure correctly for the network it's connected to is defective.

BTW, a Google search shows many others asking about why Windows doesn't support option 26. I guess this is more of Microsoft's habit of ignoring specs and doing their own thing.

In an earlier post I mentioned jumbo frames are used in the Internet2, which is used by a lot of researchers. This means they can't plug a Windows notebook directly into the network and then into other networks, without reconfiguration.

JKnott

@johnpoz said in Setting MTU via DHCP:

I can tell you that for all the corps had to support over the years.. Nobody is doing this on their local lan that client machines are connected to..

I'll have to ask my cousin what he does. He's a nuclear physicist, who works with neutrinos. He spends a lot of time on supercomputers and runs Red Hat Linux on his notebook computer. He's currently at the Sudbury Neutrino Observatory, but had previously been at Fermilab and a couple of other places.

kiokoman

I wasn't trying to find an excuse for the crap behavior of windows
for example now that i have my stratum 1 running in a raspberry i would love to see option 42 but even that is not possible

johnpoz

Yeah sure there are questions about why windows doesn't ask for mtu in dhcp.. Not really sure what that has to do with the just plain FACT that its not worth the effort to change this on your local home network... If your a geek/nerd and into this sort of thing and you want to play with it - have at it on your lab network.

I went through my lets play with jumbo years ago when gig was somewhat new to the market, jumbo was all the rage - lower the cpu needed to move stuff, etc. etc.. Yeah back then a lot of the stuff was having to be done by the cpu, and not offloaded onto the nic, etc.

CPUs were far less powerful back then... Take it or leave it - this is my personal and professional opinion, with 30 some years in the biz, and plenty of geeking and nerding about network both at work, in my home, labbing this and that, etc.. etc..

Playing around with jumbo isn't worth the effort.. its not going to get you anything but headaches.. There just is no point to changing from the default.. You want to shave off your cpu from doing a couple of cycles?? So it can just do more work when wanting to talk to anything else on your network that is not capable of changing the mtu to jumbo.

Internet 3 going to use super jumbo frames ;) Doesn't mean anything when your talking about some local work/home network..

JKnott

@johnpoz said in Setting MTU via DHCP:

But yeah if windows shop was going to do it - they would prob set via group policy sure.

Does Active Directory support assigning MTU size?

As I mentioned, there has been some discussion about moving the Internet to jumbo frames, as the conditions that drove the smaller MTUs no longer exist. Also, users are moving a lot more data than we were 20 - 30 years ago.

From a network/ISP point of view, larger MTUs mean a lot less work for routers, which is a major consideration for many companies.

johnpoz

@JKnott said in Setting MTU via DHCP:

Does Active Directory support assigning MTU size?

It does via making changes to the registry.. Which yeah pretty simple to roll out any sort of reg entry you want.

larger MTUs mean a lot less work for routers

If their routers are over worked moving their data around their network, their routers are under sized ;)

JKnott

@johnpoz said in Setting MTU via DHCP:

Not really sure what that has to do with the just plain FACT that its not worth the effort to change this on your local home network...

Perhaps not on a home network, but what about places like Google, where they have to move a huge amount of data? Each packet requires the router to handle it and that's done with something called interrupts, where the NIC taps the CPU on the shoulder and says handle this. The CPU then has to stop what it's doing, save the current state, handle the interrupt and then go back to what it was doing. With 1500 MTU, that happens 6x more often than with 9000. Then there's also the reduced bandwidth demand by having 1 header in place of 6, though that's a minor consideration. The Internet is getting faster (some carriers and ISPs are now moving to 100 Gb) and they need some way to reduce the load on routers, Jumbo frames help with that.

JKnott

@johnpoz said in Setting MTU via DHCP:

It does via making changes to the registry.. Which yeah pretty simple to roll out any sort of reg entry you want.

What happens with a notebook, where the user takes it to a another network? Does that registry setting still apply?

BTW, my only experience with AD is resetting passwords.

JKnott

@JKnott said in Setting MTU via DHCP:

As I mentioned, there has been some discussion about moving the Internet to jumbo frames, as the conditions that drove the smaller MTUs no longer exist.

Jumbo Frame Deployment at Internet Exchange Points (IXPs)

Hurricane Electric is a major Internet backbone provider.