MTU problems

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MTU problems

suse-8
Hi all,

I noticed that my ISP requires me to reduce my MTU to 1472.
As my ethernet devices are used via bridge devices, I wonder if:

A) it is enough to lower the MTU on the physical ethernet device on my
linux box..
B) also lower the MTU of the related bridge device
C) also required to lower the MTU of the other (virtual) devices
connected to this bridge?


Kind regards, Hans.

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Re: MTU problems

James Knott
On 01/04/2018 10:54 AM, [hidden email] wrote:

> Hi all,
>
> I noticed that my ISP requires me to reduce my MTU to 1472.
> As my ethernet devices are used via bridge devices, I wonder if:
>
> A) it is enough to lower the MTU on the physical ethernet device on my
> linux box..
> B) also lower the MTU of the related bridge device
> C) also required to lower the MTU of the other (virtual) devices
> connected to this bridge?

I assume you have a router/firewall between the ISP and your network.
If so, you just change the WAN interface MTU to 1472 (that's an unusual
size).  Anything behind the firewall can continue to use 1500, if you
wish, though there's no harm in reducing the MTU there as well.  You can
often do that with DHCP.  Regardless, 1500 on the local network is not a
problem, as IPv4 can use Path MTU Discovery or fragmentation to deal
with the smaller MTU.  IPv6 uses PMTUD only.


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Re: MTU problems

Per Jessen
In reply to this post by suse-8
[hidden email] wrote:

> Hi all,
>
> I noticed that my ISP requires me to reduce my MTU to 1472.
> As my ethernet devices are used via bridge devices, I wonder if:
>
> A) it is enough to lower the MTU on the physical ethernet device on my
> linux box..
> B) also lower the MTU of the related bridge device
> C) also required to lower the MTU of the other (virtual) devices
> connected to this bridge?

It's enough to lower the MTU on the uplink device.



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Re: MTU problems

Carlos E. R.-2
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On Thursday, 2018-01-04 at 17:05 +0100, Per Jessen wrote:

> [hidden email] wrote:
>
>> Hi all,
>>
>> I noticed that my ISP requires me to reduce my MTU to 1472.
>> As my ethernet devices are used via bridge devices, I wonder if:
>>
>> A) it is enough to lower the MTU on the physical ethernet device on my
>> linux box..
>> B) also lower the MTU of the related bridge device
>> C) also required to lower the MTU of the other (virtual) devices
>> connected to this bridge?
>
> It's enough to lower the MTU on the uplink device.

Yes, but that will cause a 1500 byte packet inside to be divided in two
packets when going outside. More work for the router, and slowdown.

The 1472 figure rings a fuzzy bell with old modems. :-?

- --
Cheers,
        Carlos E. R.
        (from openSUSE 42.2 x86_64 "Malachite" at Telcontar)

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Re: MTU problems

Dave Howorth-3
On Thu, 4 Jan 2018 20:51:30 +0100 (CET)
"Carlos E. R." <[hidden email]> wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
>
>
> On Thursday, 2018-01-04 at 17:05 +0100, Per Jessen wrote:
>
> > [hidden email] wrote:
> >  
> >> Hi all,
> >>
> >> I noticed that my ISP requires me to reduce my MTU to 1472.
> >> As my ethernet devices are used via bridge devices, I wonder if:
> >>
> >> A) it is enough to lower the MTU on the physical ethernet device
> >> on my linux box..
> >> B) also lower the MTU of the related bridge device
> >> C) also required to lower the MTU of the other (virtual) devices
> >> connected to this bridge?  
> >
> > It's enough to lower the MTU on the uplink device.  
>
> Yes, but that will cause a 1500 byte packet inside to be divided in
> two packets when going outside. More work for the router, and
> slowdown.
>
> The 1472 figure rings a fuzzy bell with old modems. :-?

1472 is the maximum size ping packet that will fit inside a
1500 byte network packet, given the 20 byte IP header and 8 byte ICMP
header (according to stackoverflow or https://support.aa.net.uk/MTU).
1492 for PPPoE, BTW

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Re: MTU problems

James Knott
In reply to this post by Carlos E. R.-2
On 01/04/2018 02:51 PM, Carlos E. R. wrote:
>> > It's enough to lower the MTU on the uplink device.
> Yes, but that will cause a 1500 byte packet inside to be divided in two
> packets when going outside. More work for the router, and slowdown.

With path MTU discovery, the packet size is automatically adjusted to
accommodate smaller MTUs.  It does this by a router sending a "too big"
ICMP message back to the source, with the recommended MTU.  Linux
appears to use it for all traffic, but Window for TCP only.  Of course,
with IPv6, PMTUD is mandatory.
>
> The 1472 figure rings a fuzzy bell with old modems. :-?

I remember 576 from the dial up modem days and 1492 for ADSL.  I don't
ever recall hearing 1472 until now.


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Re: MTU problems

Carlos E. R.-2
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On Thursday, 2018-01-04 at 15:18 -0500, James Knott wrote:

> On 01/04/2018 02:51 PM, Carlos E. R. wrote:
>>>> It's enough to lower the MTU on the uplink device.
>> Yes, but that will cause a 1500 byte packet inside to be divided in two
>> packets when going outside. More work for the router, and slowdown.
>
> With path MTU discovery, the packet size is automatically adjusted to
> accommodate smaller MTUs.  It does this by a router sending a "too big"
> ICMP message back to the source, with the recommended MTU.  Linux
> appears to use it for all traffic, but Window for TCP only.  Of course,
> with IPv6, PMTUD is mandatory.

Interesting.

>>
>> The 1472 figure rings a fuzzy bell with old modems. :-?
>
> I remember 576 from the dial up modem days and 1492 for ADSL.  I don't
> ever recall hearing 1472 until now.

Maybe I got the figure wrong.

- --
Cheers,
        Carlos E. R.
        (from openSUSE 42.2 x86_64 "Malachite" at Telcontar)

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Re: MTU problems

James Knott
On 01/04/2018 05:34 PM, Carlos E. R. wrote:
>> > With path MTU discovery, the packet size is automatically adjusted to
>> > accommodate smaller MTUs.  It does this by a router sending a "too big"
>> > ICMP message back to the source, with the recommended MTU.  Linux
>> > appears to use it for all traffic, but Window for TCP only.  Of course,
>> > with IPv6, PMTUD is mandatory.
> Interesting.

https://en.wikipedia.org/wiki/Path_MTU_Discovery

>>> >>
>>> >> The 1472 figure rings a fuzzy bell with old modems. :-?
>> >
>> > I remember 576 from the dial up modem days and 1492 for ADSL.  I don't
>> > ever recall hearing 1472 until now.
> Maybe I got the figure wrong.

Perhaps it was 1492, as that is the usual for PPPoE, which is used for ADSL.


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Re: MTU problems

Toshi Esumi-2
In reply to this post by suse-8
On 01/04/2018 07:54 AM, [hidden email] wrote:
> A) it is enough to lower the MTU on the physical ethernet device on my
> linux box..
> B) also lower the MTU of the related bridge device
> C) also required to lower the MTU of the other (virtual) devices
> connected to this bridge?
>
I think you got enough info already through those all replies. So I'll
just add some that was missed.

1) You want to adjust mtu size at the device interface where the size is
not the default size, like 1500 for copper eth (BTW you can increase it
to more than 9000 for fiber GigE) for below 2) reason. PMTUD is supposed
to find out the smallest MTU on the path between end to end and set the
packet size below it before sending packets down to the network.
However, as described at Wikipedia
[https://en.wikipedia.org/wiki/Path_MTU_Discovery] IPv4 based method
often doesn't work due to the fact ICMP messages are blocked by some
hops/network devices between end to end. Therefore effective PMTUD needs
to rely on TCP based discovery. It most unlikely works for any UDP traffic.

2) If you set MTU lower at the link interface where it's not necessary,
like at your PC, all packets use that link need to be fragmented if
larger than the MTU size, such as packets to your printer, network
storage device, etc. You should avoid that as much as possible not to
slow your LAN down. Router/FW are designed to perform it most
effectively (by hardware/ASIC) than anything else. Let them do their job.

Toshi


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Re: MTU problems

suse-8
On 2018-01-05 06:59, Toshi Esumi wrote:

> On 01/04/2018 07:54 AM, [hidden email] wrote:
>> A) it is enough to lower the MTU on the physical ethernet device on my
>> linux box..
>> B) also lower the MTU of the related bridge device
>> C) also required to lower the MTU of the other (virtual) devices
>> connected to this bridge?
>>
> I think you got enough info already through those all replies. So I'll
> just add some that was missed.
>
> 1) You want to adjust mtu size at the device interface where the size
> is not the default size, like 1500 for copper eth (BTW you can
> increase it to more than 9000 for fiber GigE) for below 2) reason.
> PMTUD is supposed to find out the smallest MTU on the path between end
> to end and set the packet size below it before sending packets down to
> the network. However, as described at Wikipedia
> [https://en.wikipedia.org/wiki/Path_MTU_Discovery] IPv4 based method
> often doesn't work due to the fact ICMP messages are blocked by some
> hops/network devices between end to end. Therefore effective PMTUD
> needs to rely on TCP based discovery. It most unlikely works for any
> UDP traffic.
>
> 2) If you set MTU lower at the link interface where it's not
> necessary, like at your PC, all packets use that link need to be
> fragmented if larger than the MTU size, such as packets to your
> printer, network storage device, etc. You should avoid that as much as
> possible not to slow your LAN down. Router/FW are designed to perform
> it most effectively (by hardware/ASIC) than anything else. Let them do
> their job.
>
> Toshi

Yes I certainly got quite some responses.
And I do remember that old modems required a value of 1492 (just 8
bytes), but my max payload is 1472.

Reason for asking, is thatI used a SuSE box as a firewall.
One leg to cable-modem (the one with the 1472 MTU)
others to internal servers, clients or wifi-modem
Besides that, on the machine itself I have some virtualized machines
doing common tasks like dns

So all other (internal) interfaces (and bridges) I kept at the default
MTU 1500 bytes,
But one physical interface should be changed for certain, but I wonder
if the corresponding virtual-bridge device should also lower its mtu


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Re: MTU problems

Per Jessen
[hidden email] wrote:

> Reason for asking, is thatI used a SuSE box as a firewall.
> One leg to cable-modem (the one with the 1472 MTU)
> others to internal servers, clients or wifi-modem
> Besides that, on the machine itself I have some virtualized machines
> doing common tasks like dns
>
> So all other (internal) interfaces (and bridges) I kept at the default
> MTU 1500 bytes,
> But one physical interface should be changed for certain, but I wonder
> if the corresponding virtual-bridge device should also lower its mtu

Your WAN interface is part of bridge?  I am not sure, but I expect it
should be sufficient to change the bridge MTU.  



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Re: MTU problems

suse-8
On 2018-01-05 11:10, Per Jessen wrote:

> [hidden email] wrote:
>
>> Reason for asking, is thatI used a SuSE box as a firewall.
>> One leg to cable-modem (the one with the 1472 MTU)
>> others to internal servers, clients or wifi-modem
>> Besides that, on the machine itself I have some virtualized machines
>> doing common tasks like dns
>>
>> So all other (internal) interfaces (and bridges) I kept at the default
>> MTU 1500 bytes,
>> But one physical interface should be changed for certain, but I wonder
>> if the corresponding virtual-bridge device should also lower its mtu
>
> Your WAN interface is part of bridge?  I am not sure, but I expect it
> should be sufficient to change the bridge MTU.

eth1 is connected to the cable modem;
fw6:~ # ifconfig eth1
eth1      Link encap:Ethernet  HWaddr 00:08:C7:E6:9A:3D
           inet6 addr: fe80::208:c7ff:fee6:9a3d/64 Scope:Link
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:8531668002 errors:0 dropped:0 overruns:1 frame:1
           TX packets:4249592124 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:1000
           RX bytes:11815554011517 (11268190.3 Mb)  TX bytes:416408316189
(397117.9 Mb)

fw6:~ #
fw6:~ # ifconfig br1
br1       Link encap:Ethernet  HWaddr 00:08:C7:E6:9A:3D
           inet addr:172.16.11.1  Bcast:172.16.11.255  Mask:255.255.255.0
           inet6 addr: 2001:xyz:1f01:3785:1::1/80 Scope:Global
           inet6 addr: fe80::208:c7ff:fee6:9a3d/64 Scope:Link
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:8498737383 errors:0 dropped:0 overruns:0 frame:0
           TX packets:4249592140 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:0
           RX bytes:11695013314138 (11153233.8 Mb)  TX bytes:416408323489
(397117.9 Mb)

fw6:~ # brctl show br1
bridge name     bridge id               STP enabled     interfaces
br1             8000.0008c7e69a3d       no              eth1

Currently both eth1 AND br1 have a MTU of 1500

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Re: MTU problems

Per Jessen
[hidden email] wrote:

> On 2018-01-05 11:10, Per Jessen wrote:
>> [hidden email] wrote:
>>
>>> Reason for asking, is thatI used a SuSE box as a firewall.
>>> One leg to cable-modem (the one with the 1472 MTU)
>>> others to internal servers, clients or wifi-modem
>>> Besides that, on the machine itself I have some virtualized machines
>>> doing common tasks like dns
>>>
>>> So all other (internal) interfaces (and bridges) I kept at the default
>>> MTU 1500 bytes,
>>> But one physical interface should be changed for certain, but I wonder
>>> if the corresponding virtual-bridge device should also lower its mtu
>>
>> Your WAN interface is part of bridge?  I am not sure, but I expect it
>> should be sufficient to change the bridge MTU.
>
> eth1 is connected to the cable modem;
> fw6:~ # ifconfig eth1
> eth1      Link encap:Ethernet  HWaddr 00:08:C7:E6:9A:3D
>           inet6 addr: fe80::208:c7ff:fee6:9a3d/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           RX packets:8531668002 errors:0 dropped:0 overruns:1 frame:1
>           TX packets:4249592124 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:1000
>           RX bytes:11815554011517 (11268190.3 Mb)  TX bytes:416408316189
> (397117.9 Mb)
>
> fw6:~ #
> fw6:~ # ifconfig br1
> br1       Link encap:Ethernet  HWaddr 00:08:C7:E6:9A:3D
>           inet addr:172.16.11.1  Bcast:172.16.11.255  Mask:255.255.255.0
>           inet6 addr: 2001:xyz:1f01:3785:1::1/80 Scope:Global
>           inet6 addr: fe80::208:c7ff:fee6:9a3d/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           RX packets:8498737383 errors:0 dropped:0 overruns:0 frame:0
>           TX packets:4249592140 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:0
>           RX bytes:11695013314138 (11153233.8 Mb)  TX bytes:416408323489
> (397117.9 Mb)
>
> fw6:~ # brctl show br1
> bridge name     bridge id               STP enabled     interfaces
> br1             8000.0008c7e69a3d       no              eth1
>
> Currently both eth1 AND br1 have a MTU of 1500

Hi Hans

Having thought about it, I think you need to change the MTU of the physical
device, eth1.
Assuming you have physical access, I would just try it out

ip link set mtu xxxx dev eth1

then check the device see if the mtu changes propagate to br1.

The MTU is just as easily reset.


/Per

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Re: MTU problems

James Knott
In reply to this post by Toshi Esumi-2
On 01/05/2018 12:59 AM, Toshi Esumi wrote:

> On 01/04/2018 07:54 AM, [hidden email] wrote:
>> A) it is enough to lower the MTU on the physical ethernet device on
>> my linux box..
>> B) also lower the MTU of the related bridge device
>> C) also required to lower the MTU of the other (virtual) devices
>> connected to this bridge?
>>
> I think you got enough info already through those all replies. So I'll
> just add some that was missed.
>
> 1) You want to adjust mtu size at the device interface where the size
> is not the default size, like 1500 for copper eth (BTW you can
> increase it to more than 9000 for fiber GigE) for below 2) reason.
> PMTUD is supposed to find out the smallest MTU on the path between end
> to end and set the packet size below it before sending packets down to
> the network. However, as described at Wikipedia
> [https://en.wikipedia.org/wiki/Path_MTU_Discovery] IPv4 based method
> often doesn't work due to the fact ICMP messages are blocked by some
> hops/network devices between end to end. Therefore effective PMTUD
> needs to rely on TCP based discovery. It most unlikely works for any
> UDP traffic.

You can set MTU much bigger than 1500 on a lot of hardware.  Most
gigabit gear supports 9K or more on copper.  Also, when PMTUD fails,
another mechanism kicks in where it's assumed the failure is caused by a
small MTU somewhere and so the MTU is set smaller so that it works.
There is also provision for testing larger MTUs from that point.
>
> 2) If you set MTU lower at the link interface where it's not
> necessary, like at your PC, all packets use that link need to be
> fragmented if larger than the MTU size, such as packets to your
> printer, network storage device, etc. You should avoid that as much as
> possible not to slow your LAN down. Router/FW are designed to perform
> it most effectively (by hardware/ASIC) than anything else. Let them do
> their job.

If you set a smaller MTU on the local link, you won't get fragmentation,
which you shouldn't see anyway.  You'll just have a less efficient network.

>
> Toshi
>
>


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Re: MTU problems

James Knott
In reply to this post by suse-8
On 01/05/2018 04:49 AM, [hidden email] wrote:
> So all other (internal) interfaces (and bridges) I kept at the default
> MTU 1500 bytes,
> But one physical interface should be changed for certain, but I wonder
> if the corresponding virtual-bridge device should also lower its mtu

Only if it's on the same physical interface as the WAN.  Anything behind
the firewall/router, no.


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Re: MTU problems

James Knott
In reply to this post by suse-8
On 01/05/2018 05:32 AM, [hidden email] wrote:

> On 2018-01-05 11:10, Per Jessen wrote:
>> [hidden email] wrote:
>>
>>> Reason for asking, is thatI used a SuSE box as a firewall.
>>> One leg to cable-modem (the one with the 1472 MTU)
>>> others to internal servers, clients or wifi-modem
>>> Besides that, on the machine itself I have some virtualized machines
>>> doing common tasks like dns
>>>
>>> So all other (internal) interfaces (and bridges) I kept at the default
>>> MTU 1500 bytes,
>>> But one physical interface should be changed for certain, but I wonder
>>> if the corresponding virtual-bridge device should also lower its mtu
>>
>> Your WAN interface is part of bridge?  I am not sure, but I expect it
>> should be sufficient to change the bridge MTU.
>
> eth1 is connected to the cable modem;
> fw6:~ # ifconfig eth1
> eth1      Link encap:Ethernet  HWaddr 00:08:C7:E6:9A:3D
>           inet6 addr: fe80::208:c7ff:fee6:9a3d/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           RX packets:8531668002 errors:0 dropped:0 overruns:1 frame:1
>           TX packets:4249592124 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:1000
>           RX bytes:11815554011517 (11268190.3 Mb)  TX
> bytes:416408316189 (397117.9 Mb)
>
> fw6:~ #
> fw6:~ # ifconfig br1
> br1       Link encap:Ethernet  HWaddr 00:08:C7:E6:9A:3D
>           inet addr:172.16.11.1  Bcast:172.16.11.255  Mask:255.255.255.0
>           inet6 addr: 2001:xyz:1f01:3785:1::1/80 Scope:Global
>           inet6 addr: fe80::208:c7ff:fee6:9a3d/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           RX packets:8498737383 errors:0 dropped:0 overruns:0 frame:0
>           TX packets:4249592140 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:0
>           RX bytes:11695013314138 (11153233.8 Mb)  TX
> bytes:416408323489 (397117.9 Mb)
>
> fw6:~ # brctl show br1
> bridge name     bridge id               STP enabled     interfaces
> br1             8000.0008c7e69a3d       no              eth1
>
> Currently both eth1 AND br1 have a MTU of 1500
>

That's a strange situation.  Does that bridge device actually talk to
the ISP???  If not, get it off of there.  Also, you shouldn't be using
RFC1918 addresses on the WAN side, unless your ISP provides them.

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Re: MTU problems

James Knott
In reply to this post by James Knott
On 01/05/2018 07:16 AM, James Knott wrote:
> You can set MTU much bigger than 1500 on a lot of hardware.  Most
> gigabit gear supports 9K or more on copper.  Also, when PMTUD fails,
> another mechanism kicks in where it's assumed the failure is caused by a
> small MTU somewhere and so the MTU is set smaller so that it works.
> There is also provision for testing larger MTUs from that point.

You may be wondering about that 1500 byte limit on Ethernet.  That only
applies to 802.3, not Ethernet II, which IP uses.  There is no hard
limit to Ethernet II, so the MTU can be as big as the hardware supports.

https://en.wikipedia.org/wiki/Ethernet_frame

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