Help untangling CAKE settings for FTTH

[Thibaut]

Hi,

A few questions for the CAKE experts here:

I’m trying to untangle the information available in the openwrt wiki:
https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm
https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm-details
and for the latter especially the part here: https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm-details#sqmlink_layer_adaptation_tab

For ADSL/VDSL, I believe the instructions are clear and the Luci interface is too. However for ethernet/fiber, I’m confused:

In the first list of this paragraph it is first suggested to use « Ethernet with overhead » and set the per-packet overhead to 44 for FFTH (which seems like a large value for this use case), then later in the second list (« the details »), it is suggested to use « None », directly contradicting the above.
The 44 byte overhead for ethernet/FTTH is also mentioned here: https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm

More specifically, there are two (increasingly common I think) use cases I would like to document with your help:

- FTTH with plain DHCP, no VLAN
- FTTH with PPPoE, no VLAN
(And adding a footnote for the extra overhead to consider if the above include a VLAN tag)

In the latter case (FTTH with PPPoE), another point that needs to be clarified is: do we apply CAKE to the ethernet interface, or to the PPP interface? (and I assume that depending on the answer, the overhead settings will have to be adjusted). Also in that case, what about ISPs that allow sending a full 1500 frame over PPPoE (using 1508 MTU on the underlying ethernet interface)?

Thanks for your input,
T