[Cerowrt-devel] Linksys wrt1900acs rrul traces

moeller0 moeller0 at gmx.de
Thu Apr 7 12:16:25 EDT 2016

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.

The kernel, if left to its own devices, will only account for 14 of 38 overhead bytes. But that means that each packet will carry an additional 24 bytes of unaccounted for size that still needs to be transferred:

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 ;)
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.

Best Regards

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