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Author Topic: Lower MTU users out there?  (Read 1717 times)

Weaver

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Lower MTU users out there?
« on: February 13, 2020, 03:31:33 PM »

Are you using an IP MTU below 1492 bytes? Is that for IPv4, IPv6 or both? What are the circumstances ? Any problems encountered in the Internet as a result?

I ask because if I were to start combining technologies, like DSL and LTE then I assume that would mean using L2TP and hence reduced MTU.
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neil

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Re: Lower MTU users out there?
« Reply #1 on: February 13, 2020, 08:41:05 PM »

Are you using an IP MTU below 1492 bytes? Is that for IPv4, IPv6 or both? What are the circumstances ? Any problems encountered in the Internet as a result?

I ask because if I were to start combining technologies, like DSL and LTE then I assume that would mean using L2TP and hence reduced MTU.
i am using 1452 i tried using 1492 but i was getting pkt loss in my games and i also faced websites not loading properly when i tried different MTU sizes dont know it is related or not
and in my modem and router i can select 1492 as max and also i changed MTU size in windows settings too
« Last Edit: February 13, 2020, 09:22:35 PM by neil »
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Weaver

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Re: Lower MTU users out there?
« Reply #2 on: February 14, 2020, 12:03:17 AM »

Thanks Neil. You had certain problems with particular website before with MTU 1492 ?
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burakkucat

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Re: Lower MTU users out there?
« Reply #3 on: February 14, 2020, 12:20:56 AM »

Replying on behalf of neil . . .

He started a thread, "modem/router/PC MTU size", late last November (2019) and in Reply #10 posted the following --

i have turned on ping and other stats in game settings it shows pkt loss for upstream 1-10-15% and sometimes it hit 50% during start of game for downstream it is all fine

and i am getting fragmented after 1452
>ping bbc.co.uk -f -l 1453

Pinging bbc.co.uk [151.101.0.81] with 1453 bytes of data:
Reply from 192.168.1.1: Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
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Weaver

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Re: Lower MTU users out there?
« Reply #4 on: February 14, 2020, 01:03:17 AM »

Thank you my friends. One adds on 20+8 to that, is that correct? So what is the significance of 1452 + 20 + 8 = 1480 ?
« Last Edit: February 14, 2020, 01:11:31 AM by Weaver »
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burakkucat

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Re: Lower MTU users out there?
« Reply #5 on: February 14, 2020, 02:59:35 PM »

I'm not sure as to the significance but the evidence is clear . . . with a MTU set greater than 1452 fragmentation would occur. Of course we do not have any knowledge of how the ISP/CP deploys a service in Pakistan. 
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Weaver

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Re: Lower MTU users out there?
« Reply #6 on: February 14, 2020, 07:27:44 PM »

Indeed
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neil

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Re: Lower MTU users out there?
« Reply #7 on: February 15, 2020, 02:03:44 AM »

Thanks Neil. You had certain problems with particular website before with MTU 1492 ?
it was default 1400 in my modem before any changes. And in windows it was 1500. I will test again with 1492. Not just 1492 i tried different values randomly i think i also set very low value too less than 1000 Websites were not loading properly had to refresh a few times.

I am using 1452+28 header because it is default in my router. 1480 is mentioned in router settings
« Last Edit: February 15, 2020, 06:40:58 AM by neil »
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neil

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Re: Lower MTU users out there?
« Reply #8 on: February 19, 2020, 01:08:00 AM »

now i am using 1492 MTU size in router settings. And after windows update MTU size of windows has been reset to 1500
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Weaver

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Re: Lower MTU users out there?
« Reply #9 on: February 19, 2020, 04:10:14 AM »

You should leave it set to the default (1500) in your PC and use one of the online MTU test tools to find your internet connection’s MTU but you might have to temporarily increase the router MTU value to 1500 too to find out what the true bottleneck MTU is (MTU of the internet connection itself, not just your router). Now, someone can remind me what is wrong with the IPv6 function on that test?
« Last Edit: February 19, 2020, 04:13:21 AM by Weaver »
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neil

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Re: Lower MTU users out there?
« Reply #10 on: February 20, 2020, 08:24:27 AM »

You should leave it set to the default (1500) in your PC and use one of the online MTU test tools to find your internet connection’s MTU but you might have to temporarily increase the router MTU value to 1500 too to find out what the true bottleneck MTU is (MTU of the internet connection itself, not just your router). Now, someone can remind me what is wrong with the IPv6 function on that test?
this test is showing 1500 MTU size but when i test on windows cmd using ping 8.8.8.8 -f -l 1464 then i get packets need to be fragmented after 1464
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Weaver

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Re: Lower MTU users out there?
« Reply #11 on: February 20, 2020, 10:29:07 AM »

So 1464 + 28 = 1492 which is your IP MTU for PPPoEoE, which is the normal value you would expect for anyone using a PPPoEoE modem. The standard value for two-box systems with separate modem and router.

Now you probably know all this anyway, but if not: the value 1492 = 1500 - 8 is the IP MTU less the PPPoEoE header overhead of 8 bytes, an overhead which reduces the size of IP packets down from the standard 1500 bytes on the LAN. So you can set your MTU in your router to 1492 or lower, higher is better clearly. You probably don’t need to do anything as 1492 should by the default for any PPPoE user.

Unless that is you are fortunate to have a ‘baby jumbo’, larger MTU link with a PPP MTU of 1500+8=1508, as I do (my IP MTU is 1500=1508-8, not 1492=1500-8, for this reason). I can send/receive full size 1500 byte IP packets, not just 1492 bytes long. Gives me a microscopic edge in performance and a theoretical advantage on the internet which is rather unrealistic.  :angel:



Neil’s MTU doesn’t have to be so low surely, at 1492 bytes it should be fine. I seem to remember some home routers that have really low default MTU settings for god knows what reason.

I was wanting to ask about people’s experience who have an MTU rather lower than 1492 though. If I start to consider a situation where I’m forced to live with a lower MTU because of L2TP say, then I would like to know what I’m getting into.  ;D  My 3G USB NIC ‘dongle’ has a really really low MTU.
« Last Edit: February 20, 2020, 10:34:24 AM by Weaver »
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burakkucat

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Re: Lower MTU users out there?
« Reply #12 on: February 20, 2020, 06:07:35 PM »

Many years ago, TalkTalk required a silly-low MTU setting. Fortunately that has long been consigned to history.

As PPPoA user I can, with my ZyXEL VMG1312-B10A, have a full 1500 bytes MTU --

[bcat ~]$ ping -c1 -Mdo -s1473 forum.kitz.co.uk
PING forum.kitz.co.uk (185.24.98.37) 1473(1501) bytes of data.
ping: local error: Message too long, mtu=1500

--- forum.kitz.co.uk ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 999ms

[bcat ~]$ ping -c1 -Mdo -s1472 forum.kitz.co.uk
PING forum.kitz.co.uk (185.24.98.37) 1472(1500) bytes of data.
1480 bytes from kitz.servers.eqx.misp.co.uk (185.24.98.37): icmp_seq=1 ttl=55 time=56.5 ms

--- forum.kitz.co.uk ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 99ms
rtt min/avg/max/mdev = 56.592/56.592/56.592/0.000 ms
[bcat ~]$
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ejs

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Re: Lower MTU users out there?
« Reply #13 on: February 20, 2020, 08:02:58 PM »

I use an MTU of 1478, set on the wlan0 interface of my computer. It results in very slightly more usable bandwidth for larger bulk downloads due to less wastage on ATM cell padding. I've done this for the past few years and not noticed any problems due to it. I don't think an MTU of 1500 is advantageous on ADSL due to the aforementioned ATM cell padding.
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Weaver

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Re: Lower MTU users out there?
« Reply #14 on: February 20, 2020, 10:48:40 PM »

@ejs Agreed. I would do better by using an optimal ATM cell multiple, but from my calculations it would seem that the difference is not that much. I do use an optimal value for ATM with IPv6 traffic of 1408, 1408+32 = 1440 = 48 * 30. This small IPv6 MTU is to do with failover to 3G using my USB 3G NIC which has a very low MTU, so I lowered it even further to get cell optimality.

I presume you’re using 1478+10=1488=48*31, am I correct ?

I haven’t noticed any weirdness anywhere from using this low value for IPv6 (only, IPv4 is 1500).

* I should have pointed out that in the above calculation regarding my own choice for packet size, 32 is the overhead in bytes that I have with PPPoEoA, and 1440 happens to be the MTU of my 3G NIC iirc, and coincidentally that value is also a perfect cell multiple.
« Last Edit: February 21, 2020, 12:18:00 AM by Weaver »
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