On 04/07/2016 12:16 PM, moeller0 wrote:
Hi Richard,
For these tests I had the inbound and outbound limits set to 975000
kbps. 975000 was somewhat arbitrary. I wanted it below 1Gbps
enough that I could be sure it was the router as the limit but yet
fast enough that I would be able to see the peak transfer rates.
All of the following might be old news to you, but please let me
elaborate for others on this list (well, most folks here know way
more about these things than I do). I believe Gbit ethernet is
trickier than one would guess, the 1 Gbit rate contains some overhead
that one typically does not account for. Here is the equivalent
on-the-wire size of a full MTU non-jumbo ethernet frame: Layer “1+":
1500 (payload pMTU) + 6 (dest MAC) + 6 (src MAC) + 2 (ethertype) + 4
(FCS) + 7 (preamble) + 1 (start of frame delimiter) + 12 (interframe
gap)) = 1500+6+6+2+4+7+1+12 = 1538 ”Equivalent” in that the
interframe gap is not really used, but filled with silence but it has
the duration one would need for 12 bytes.
I knew there was a bit of overhead but thanks for laying out the details
and why it matters.
975000 * (1538/1514) = 990455.746367 (which still is below the 1GBit
Layer1+ ceiling that GbE has). At 985000 * (1538/1514) =
1000614.26684 you would already have slowly caused the NIC’s buffer
to fill ;)
So by luck I just barely made it. :)
Luckily sqm-scripts will allow you to specify any
additional per-packet overhead so just set this to 24 and things
should just work out I believe.
I knew that there might need to be some overhead accounting and I looked
at Link Layer Adaption tab in the options when I set up the SQM but the
info in that menu isn't quite descriptive enough for for my setup.
It has a box for "Ethernet with overhead, select for eg VSDL2". I'm
using Ethernet but I don't know about VSDL2. If I select it then I get
a 2nd box that asks for the per packet overhead. Even if I had tried to
fill that out then I would have gotten it wrong. :)
The other option is "ATM" which I know I'm not using.
Not knowing what I really should select I just left it at None.
Perhaps there are too many combinations to add items for everything but
a few more options and more descriptions of scenarios might be helpful.
So now that I have that set at 24 is it worth re-running some of the tests?
--
Richard A. Smith
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