[Cake] Ethernet frame overheads

Benjamin Cronce bcronce at gmail.com
Wed Sep 30 08:36:11 EDT 2015

I'm pretty sure Ethernet rates are at the Layer 2, so we only need to care
about Layer 2 sizes, not Layer 1. At Layer 1, Ethernet is faster than the
listed 10/100/1000/10000 rates. You are correct about worrying about VLAN,
but I would hope they actually include these.

On Wed, Sep 30, 2015 at 5:53 AM, Kevin Darbyshire-Bryant <
kevin at darbyshire-bryant.me.uk> wrote:

> 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?
> Kevin
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