[Cake] overhead and mpu

Sebastian Moeller moeller0 at gmx.de
Thu Sep 7 03:58:35 EDT 2017

Hi Jonathan,

please let me elaborate below.

> On Sep 7, 2017, at 06:25, Jonathan Morton <chromatix99 at gmail.com> wrote:
> Because 1514 is the normal maximum size of an Ethernet frame.

	This might or might not be true (depending on the defnition of ethernet frame one uses), but it certainly is not overly helpful to novices, as e.g. https://en.wikipedia.org/wiki/Ethernet_frame will show the FCS as being part of the layer2 ethernet frame and 1514 does not contain the FCS nor any potential VLAN tag(s). 
What is probably correct to say is that 1514 is the kernel's internal view of the size of an ethernet frame (most likely the FCS will be added by the NIC, so 1514 is the true size of the data the kernel shuttles around at that abstraction layer, but certainly not a real complete ethernet frame).

> This can easily differ from the size actually used on the wire

	Not easily, but certainly with 1500 bytes payload an ethernet packet will take at least 1518 bytes on the L2-wire and effectively 1518+20 = 1538 bytes on L1 (here I account for the silent inter-frame gap by the 12 bytes worth of data that could be transmitted in the same time)

> and from the size used by the provisioning shaper.

	And that is the thing that makes specifying overhead hard to automate, instead of the actually connected interface one needs to figure out the overhead (and bandwidth) applicable on the slowest part of the internet connection... (Which in theory is variable, but in practice it luckily is often the link to the ISP).

Best Regards

> - Jonathan Morton
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