[SOLVED] MTU size

olive · 2010-10-02 08:15:58

I connect to the internet by a computer with an ethernet cable connected to an ADSL modem (some people say DSL, broadband connection using the phone wire) that itself is connected to the internet using pppoe. The adsl modem has an MTU size of 1492 (the default) while computer has 1500 (the default). Both can normally be changed (but I am unsure about the modem: it accepts size as big as 9000 that should normally not be accepted, I do not know if this is really effective). I have read that it can pose various problems but is it really the case? Should I change one of these settings? How can I reliably test this?

Last edited by olive (2010-10-05 18:07:51)

tomk · 2010-10-02 08:23:21

Don't change the modem - it will adversely affect your network performance. You can try changing the computer if you want, but if everything's working for you as is, I'd leave it alone.

fukawi2 · 2010-10-02 08:25:04

Do you actually have a problem, or are you just playing?

MTU is set on each interface, so the DSL side of the modem will have 1 MTU (probably 1492 at you said), and the LAN side will have it's own. The MTU of all machine on your LAN need to match (normally 1500).

You can increase it up to 9000 which can increase throughput between machine on your LAN, since each packet is bigger, it can fit more data with one header. This decreases your data to header ratio.

It's rarely worth changing, especially on home networks.

Last edited by fukawi2 (2010-10-02 08:25:40)

olive · 2010-10-02 12:40:22

fukawi2 wrote:

Do you actually have a problem, or are you just playing?

Put me in the second category -:)

fukawi2 wrote:

MTU is set on each interface, so the DSL side of the modem will have 1 MTU (probably 1492 at you said), and the LAN side will have it's own. The MTU of all machine on your LAN need to match (normally 1500).
You can increase it up to 9000 which can increase throughput between machine on your LAN, since each packet is bigger, it can fit more data with one header. This decreases your data to header ratio.

I have a single computer connected to the modem. This is with 100Mb ethernet cable (compare to my 6Mb adsl connection). So the connection between my computer and the modem is already much faster than the rest, I do think I can gain something here. It's just that I have read that sending 1500 packet through a link under a MTU of 1492 obliges the modem to split the packets and can cause various problems, including decreased performance. To be honest I do no have seen any problems so far.

fukawi2 wrote:

It's rarely worth changing, especially on home networks.

That makes sense.

faelar · 2010-10-02 13:24:32

It's just that I have read that sending 1500 packet through a link under a MTU of 1492 obliges the modem to split the packets and can cause various problems, including decreased performance.

There's more operations so yes, it reduce your speed in theory. Try to change it on the computer to match 1492.

I don't think that really matters on practice, Your packets probably travel through many network devices with different settings...

broch · 2010-10-03 03:45:58

optimal MTU value has nothing to do with modem or connected computer. It depends on your ISP

you can check optimal MTU value pretty easy by pinging google with different MTU value starting from lower going up to first ping error:
start with lets say value 1460 and go up:

In my case I have got first error with the value of 1465:

$ ping -c 1 -M do -s 1465 www.google.com
PING www.l.google.com (66.102.7.99) 1465(1493) bytes of data.
From 192.168.1.65 icmp_seq=1 Frag needed and DF set (mtu = 1492)

--- www.l.google.com ping statistics ---
0 packets transmitted, 0 received, +1 errors

and last good ping was with the value 1464
$ ping -c 1 -M do -s 1464 www.google.com
PING www.l.google.com (66.102.7.104) 1464(1492) bytes of data.
72 bytes from lax04s01-in-f104.1e100.net (66.102.7.104): icmp_seq=1 ttl=56 (truncated)

--- www.l.google.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 29.205/29.205/29.205/0.000 ms

so there you have it: best value is (in my case 1464
no set this (temporary):
sudo ifconfig wlan0 mtu 1464

and then if you like it make it permanent.

Remember though that this value very likely will change each time you switch network.

Whether it is worth 5min of resting is up to you, definitely though has nothing to do with blind setup based on modem/cable/net cart capabilities. Of course as usual there is plenty of syscalls to play with for network optimization.

Last edited by broch (2010-10-03 03:55:58)

olive · 2010-10-03 08:22:47

broch wrote:

optimal MTU value has nothing to do with modem or connected computer. It depends on your ISP

Yes but the modem connect to the ISP with pppoe. I believe it can negotiate with the pppd on the other side (provided by the ISP) custom MTU size.

broch wrote:

you can check optimal MTU value pretty easy by pinging google with different MTU value starting from lower going up to first ping error:
start with lets say value 1460 and go up:
In my case I have got first error with the value of 1465:

Me too, it appears to be standard pppoe size (but please read below).

broch wrote:

$ ping -c 1 -M do -s 1465 www.google.com
PING www.l.google.com (66.102.7.99) 1465(1493) bytes of data.
From 192.168.1.65 icmp_seq=1 Frag needed and DF set (mtu = 1492)
--- www.l.google.com ping statistics ---
0 packets transmitted, 0 received, +1 errors
and last good ping was with the value 1464
$ ping -c 1 -M do -s 1464 www.google.com
PING www.l.google.com (66.102.7.104) 1464(1492) bytes of data.
72 bytes from lax04s01-in-f104.1e100.net (66.102.7.104): icmp_seq=1 ttl=56 (truncated)
--- www.l.google.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 29.205/29.205/29.205/0.000 ms
so there you have it: best value is (in my case 1464
no set this (temporary):
sudo ifconfig wlan0 mtu 1464

Are you sure that the correct value is not 1492 in your case? Ping add 28 byte for its headers. If you see the output of your ping command you see this 1492 after the 1464 in bracket. I am still a little confused, it would be good if other people can confirm.

Last edited by olive (2010-10-03 08:39:14)

Anonymo · 2010-10-03 14:16:49

Usually it's okay to leave MTU at 1500. You'll know if you have to change it because certain pages won't load, like Hotmail and some pages that use https like banks. It's a really weird issue having to do with packet fragmentation. That's why you would then use the test that brochs mentioned. You take that number that and add 28 according to some websites but some don't mention that at all, so I'm not sure. The usual practice is if you have such a problem, to change the MTU on the device that handles the PPPoE connection. I do this anyways, always change it to 1492 in my router, because I think I have a modem that tends to have this problem, so my router is set to PPPoE. Works fine. No problems with websites.

eldragon · 2010-10-03 14:53:27

the mtu size of 1492 is related to max fragmet size over an ethernet link. if you increase that number over ethernet, it will be fragmented at the link level, and you will end up with a lower performance overall.

i dont know what the limits are on a pppoe. but im almost sure that if you increase it, the packet will end up being fragmented anyway on its way to destination, which will eventually lead to slower speeds too (if one of those fragments is lost, you will end up retransmitting the entire packet again).

my advice, leave it at 1492, and let the adsl company handle its mtu (they should know better).

broch · 2010-10-05 02:54:33

Are you sure that the correct value is not 1492 in your case? Ping add 28 byte for its headers. If you see the output of your ping command you see this 1492 after the 1464 in bracket. I am still a little confused, it would be good if other people can confirm.

you are correct.
sorry forgot about overhead (28 in US):
1464 +28 = 1492

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#1 2010-10-02 08:15:58

[SOLVED] MTU size

#2 2010-10-02 08:23:21

Re: [SOLVED] MTU size

#3 2010-10-02 08:25:04

Re: [SOLVED] MTU size

#4 2010-10-02 12:40:22

Re: [SOLVED] MTU size

#5 2010-10-02 13:24:32

Re: [SOLVED] MTU size

#6 2010-10-03 03:45:58

Re: [SOLVED] MTU size

#7 2010-10-03 08:22:47

Re: [SOLVED] MTU size

#8 2010-10-03 14:16:49

Re: [SOLVED] MTU size

#9 2010-10-03 14:53:27

Re: [SOLVED] MTU size

#10 2010-10-05 02:54:33

Re: [SOLVED] MTU size

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