[Cerowrt-devel] SQM Question #2: How does CeroWrt use info gleaned from the link layer adaptation?

[Sebastian Moeller]

Rich Brown <richb.hanover at gmail.com> wrote:
>QUESTION #2: How does CeroWrt use info gleaned from the link layer
>adaptation?

      The link layer adaptations work in correcting the kernels estimate of a packets behavior on the wire. In the tc_stab case the kernel calculates the effective size of the packet on the wire, that is it pretends the packet is larger than it really is, so for a given bandwidth it estimates the correct time it takes for that packet to be actually transmitted. In the htb_private case the kernel keeps the packet's size (more or less) intact but adjusts its estimate of the packets transmit rate. Both methods boil down to the same idea, make sure the packet scheduler will only send packet N+1 after packet N has just cleared the wire.

>
>Specifically, the link layer adaptation all seem to be designed to
>compute the actual time it takes to transmit a packet, accounting for
>Ethernet & PPPoE header bytes, other overhead, and ATM 48-in-53
>framing.

       And the annoying size dependent padding of the last ATM cell.

>
>How does CeroWrt use this time calculation? Does it simply make sure
>that the target time doesn’t get too low for a particular flow’s queue?

        Thanks to the link layer adjustments (lla) cero now estimates the correct time each packet takes and will not send any faster than the shaped rate allows. If no lla is performed cero would overestimate the link capacity, send more than expected and potentially fill the modems bloated buffers. Traditionally people tried to reduce their shaped rate by >10% to at least account for the 48 in 53 framing, but failed miserably for small packets since overhead and padding can more than double the wire size of a packet. Note that ACQ packets typically are small as are voice over IP packets.

I hope this helps
        Sebastian

>(I could imagine that a short packet over ATM would take 2x the (naive)
>expected/calculated time for a packet of that length, and that flow
>would be penalized. Is there more to it?)
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Hi Rich
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