Shaper Quantum value

[Melroy vd Berg]

Hi,

After reading the following topic about how-to setup a Shaper to get rid of bufferbloat: https://docs.opnsense.org/manual/how-tos/shaper_bufferbloat.html#quantum

Regarding the correct value for "Quantum" its stated:

The proper value of Quantum should be no more or less than is the WAN MTU.

However, while this might sound clear. This is still too ambiguous for me.

I have fiber connection from my ISP using PPPoE, which is 1500 bytes.
However PPPoE has a connection overhead of 8 Bytes, so the WAN interface with VLAN 6 tag has a MTU of 1508.
So I have set 1508 MTU on my WAN interface (Identifier: opt1, device: ppoe0).

But that is not all, I also have a 'WAN RAW' interface configured as well (Identifier: opt6, device: ax0). Which has the overhead of the VLAN of 4 bytes, meaning that MTU setting is 1512.

Meaning "WAN MTU" term is too ambiguous for me. So what value should I use now for quantum; 1500, 1508 or 1512?

I guess 1512 in my case. Right or not? This could be written more clearly.

Thanks in advance!

Kind regards,
Melroy van den Berg

[Seimus]

should be no more or less than is the WAN MTU

This means it should be exact or close enough to the real WAN MTU used by your device on that Interface.

If you check the default value of Quantum >

quantum
Maximum number of bytes to dequeue for transmission at one time. Should be set to the value of Interface MTU. (Default: 1514 bytes, 1500+14B hardware header, max 9000)

Default: 1514 bytes, 1500+14B hardware header this accounts for the Ethernet Frame.

So if you want to use the real value used on WAN add the extra Byte overhead created by an additional header to these 1514 bytes. But usually you don't need to do that and can use the default value, because the ISPs provide usually MTU max to 1500B and any extra header like VLAN TAG is subtracted from the MTU anyway. So in reality your real MTU is less than 1500B due to all the overheads added.

Regards,
S.

[Melroy vd Berg]

Now I'm actually more confused.. What is your final verdict here?

To which value should I set it, considering all the information I gave you?

[Seimus]

In theory It should match your Interface MTU + the overhead.

So in theory;
A. MTU is 1500 but you have an overhead of 8B PPPoE + 14B Ethernet > than set it to 1522
B. MTU is 1500 but you have an overhead of 8B PPPoE + 14B Ethernet + 4B VLAN > than set it to 1526

But the reality is, you can not go over your set 1500B on the Interface, that's the maximum sized packet that can pass not-fragmented. If you are using PPPoE and VLANs you should anyway accommodate your MTU on the Interface, cause if you have PPPoE and VLANs and set MTU to 1500B you already surpassed what is theoretically possible.

https://forum.opnsense.org/index.php?topic=45658.msg228347#msg228347

The "should be no more or less than is the WAN MTU" is vague a bit cause here and there a Byte should not affect FQ_Codel much. Most of the time you don't need to deal with it on a base 1500B MTU sized Interface and just use the default value. I would advice let the default value (blank field) test it and than try to change it if you see undesirable outcomes.

To keep the rule of thumb "have quantum at the same size as is your interface on which you are doing shaping"

Regards,
S.

[stanps]

1526!  We have 1526!  Do we have 1527?  1527?  Going once at 1526....