how do I make shapers count FCS (CRC) bytes

gabyz · 01-19-2014, 03:26 AM

Hi All,
From testing that I've done with TC HTC shaping I noticed that the shaper doesn't count packet FCS, so effectively where I expect traffic to be X Mbps I get a bit more. How much more depends on the packet length.
Is there a way to make the shaper count the packet's FCS?

Thanks
Gaby

Ser Olmy · 01-19-2014, 08:45 PM

If you're talking about the Ethernet FCS, then I believe the answer is no. The FCS is discarded by the NIC itself, and is never visible to applications. I don't even think the NIC driver sees that part of the frame.

gabyz · 01-20-2014, 01:20 AM

Nevertheless, from my perspective, a shaper configured to Xmbit/s should transmit Xmbit/s on the line. So I'm looking for a way to make shapers count the 4 bytes of the FCS when they check if they have enough tokens in their buckets, even if it means modifying kernel code.

Ser Olmy · 01-20-2014, 01:28 AM

I can't say whether or not the shaper takes into account the size of the FCS (it would be a simple matter of adding 32 to the bit counter for every frame), only that the FCS isn't available to anything above the layer 1/2 logic circuitry in the NIC itself.

gabyz · 01-20-2014, 04:02 AM

I measured the shaper configuration using IXIA and it doesn't count the FCS.
Actually, for a shaper you don't modify the bit counter, but deficit the token bucket by 4 additional bytes.
I'm looking to modify the HTB code to accommodate that. (reduce by configuration, to enable also support to accommodate for the IPG in Ethernet)
Any pointer will be appreciated.

Thanks