So the 80 MPU includes 4 bits for VLAN ID/tag also which is handled by your DSL-AX82U ?
Correct… however when looking at stuff it gets more complicated than I originally thought.
The PPPoE header actually lives inside the ethernet payload (46-1500) so when I was doing the original calculation, I assumed this wasn’t the case. Thus the actual mpu should be 72. I was accounting for the full PPPoE when I didn’t need to.
So why 72? The minimum packet size is 64 (above post screenshot from wikipedia, if less then it gets auto padded to 64). However you still need to add 4 for ptm transmission and again this pesky vlan tag (4) – note the 64 number does not include the vlan tag in its minimum, why it’s in brackets. The PPPoE is already accounted for in the 46-1500 as stated above so no +8 (my original mistake).
Although different technologies, the above Sky user in that openwrt wiki is under the exact same constraints as us. They still had to add 4 bytes for briged-ptm transmission and 4 for the vlan tag needed by Sky. In this case there is no difference between non pppoe and pppoe mpu because of the payload.
As for second question: All the values I’ve looked at should be the same regardless of if it’s a modem+router combo or just router. I did all the reading and prep looking at pure router openwrt command line configuration of cake. If anything the DSL-AX82U should have been the one to suffer in bufferbloat tests but since it’s holding 0~1ms under load on waveform we can safely assume it’s identical.
Note: My original testing of 80 mpu didn’t show much issue because it was still a low enough packet size to not immediately show a problem and since it was still above the minimum mpu, no issue. If I had set something like 50 then it would have been more apparent. Just like they say with overhead before it was figured out, it’s always better to slightly overestimate than underestimate if you don’t know the exact number.
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