[Cake] Ethernet frame overheads

Kevin Darbyshire-Bryant kevin at darbyshire-bryant.me.uk
Wed Sep 30 06:53:01 EDT 2015

Greetings fellow cake eaters :-)

Sebastian raised a little flag in the brain cell yesterday in his
questioning/discussion of cake stats & kernel accounted for overheads. 
As he stated the kernel considers the overhead of an ethernet packet to
be 14 bytes but forgets about the 4 byte frame check sequence and
pre/post ambles which in the end make a packet on the wire worth 1538
bytes  (must use octets, must use octets)

Jonathan pointed out that cake's overhead parameter/s are used for
timing purposes, which to me makes it all the more important that
overheads are got right.

As much as I'm loath to suggest yet another option to cake, should it
not by default assume ethernet 'on-the-wire' framing overheads
(preamble& start of frame=8, FCS=4, interpacket gap=12) totalling 24
octets, not forgetting VLAN tag/s at 4 octets each and the already
accounted for 14 octets of MAC source,dest & frame type/length for
timing purposes?  Ref: https://en.wikipedia.org/wiki/Ethernet_frame

For ethernet links this is going to be important.  For other links
'transporting' ethernet at slower rates (I'm thinking VDSL2 modems & the
like) I suspect their overheads and pure slowness of link swamp the
timing discrepancy.

'ethernet' & 'ethernetotw' flags?

What don't I understand properly here?


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