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

[Sebastian Moeller]

Hi Dave,


On Dec 29, 2013, at 09:54 , Dave Taht <dave.taht at gmail.com> wrote:

> I would like it if we had a couple per-provider recomendations and
> relevant discussion.

	I think this is a can of worms we should open carefully ;). In Germany several providers serve part of their area with their own infrastructure while reselling bitstream access provided by other carriers in other parts of their area, with potential differences in encapsulation. Also in the US at&t is in the process of relabeling their ADSL2 access as U-Verse High Speed Internet (the same label they use for VDSL as well) making it quite complicated to give proper recommendations… But to make a start here are only connections I actually measured (there is no guarantee that all connections of the same ISP use the same technology):
Date			Country			ISP					nominal  Speed D/U	line type		link layer	access type						overhead	comment
2013-12-29		Germany		Deutsche Telekom	16Mbit/s 2.8Mbit/s	ADSL2+		ATM		PPPoE, LLC/SNAP RFC-2684		40bytes		16M down also offered as VDSL version without ATM
2013-12-29		Germany		Deutsche Telekom	 2Mbit/s 0.2Mbit/s		ADSL1		ATM		PPPoE, LLC/SNAP RFC-2684		40bytes		connections below 16M down are always ADSL, either v1 or v2+
2013-12-29		Germany		Netaachen			18Mbit/s 1.0Mbit/s	ADSL2+		ATM		PPPoE, LLC/SNAP RFC-2684		40bytes		

General Notes: 
Deutsche Telekom; the network is slowly moved to fiber to the node/curb using VDSL2 (PTM) for speeds >= 16M down and ADSL2+ (ATM) for slower speeds . These new DSLAMs replacements are called MSANs, customers on 
ADSL ports are still using ATM on the last mile and need the link layer adjustments.

Best Regards
	Sebastian


> 
> 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?)
>>> _______________________________________________
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>> 
>> Hi Rich
>> --
>> Sent from my Android phone with K-9 Mail. Please excuse my brevity.
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> 
> 
> 
> -- 
> Dave Täht
> 
> Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html