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From: Benjamin Cronce <bcronce@gmail.com>
To: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>
Cc: cake@lists.bufferbloat.net
Subject: Re: [Cake] Ethernet frame overheads
Date: Wed, 30 Sep 2015 07:36:11 -0500	[thread overview]
Message-ID: <CAJ_ENFHD+4OmuO-rVquzNuHNawwg2tdP5jF_ESB307GppYZiUg@mail.gmail.com> (raw)
In-Reply-To: <560BBF0D.2080802@darbyshire-bryant.me.uk>

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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@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
>
>
> _______________________________________________
> Cake mailing list
> Cake@lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/cake
>
>

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      parent reply	other threads:[~2015-09-30 12:36 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-09-30 10:53 Kevin Darbyshire-Bryant
2015-09-30 12:11 ` Sebastian Moeller
2015-09-30 12:36 ` Benjamin Cronce [this message]

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