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

Fred Stratton fredstratton at imap.cc
Sun Dec 29 05:52:02 EST 2013

Providers differ across countries.

European competition laws are interpreted quite differently in different 
countries. AFAIK, Deutsche Telekom and France Telecom are monopolies. 
ISPs there therefore use the same infrastructure. BT is not. Here there 
are differences between ISPs.


Shows service availability in the area where I am, immediately to the 
south of the Manchester conurbation.

Recommendations are also affected by service cost. In this country, BT 
increases its prices once or even twice a year by just below the ten per 
cent threshold where the regulator takes an interest. Sky and TalkTalk 
then follow. Individuals then have to make decisions based not just on 
shaping policy, the type of CPE supplied, and TV channels specific to 
the provider, but on cost.

Negotiation on cost involves a yearly conversation with the provider, 
and cashback offers available elsewhere. Recommendations are therefore 

On 29/12/13 08:54, Dave Taht wrote:
> I would like it if we had a couple per-provider recomendations and
> relevant discussion.
> On Sat, Dec 28, 2013 at 11:36 PM, Sebastian Moeller <moeller0 at gmx.de> wrote:
>> 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?)
>>> _______________________________________________
>>> Cerowrt-devel mailing list
>>> Cerowrt-devel at lists.bufferbloat.net
>>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>> Hi Rich
>> --
>> Sent from my Android phone with K-9 Mail. Please excuse my brevity.
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