AW: why ALTQ must be supported by interface drivers?

[Spenst, Aleksej]

Hi Ricky,

Thanks for your explanations. 

>If, in-fact, a driver happens to 
>implement another queue underneath the AltQ queue (sometimes 
>happens in sophisticated hardware) then this 'lower' queue 
>will probably be a FIFO queue and may wind up destroying the 
>value of AltQ.

If the driver supports only one FIFO queue, I don't think it can destroy ALTQ functionality, since prioritization of traffic is done at layer 3 and this FIFO queue at layer 2 can be used to send packets in the order according to classification performed at layer 3. However, if the driver supports its own queuing mechanism I see that there can be problems. I don't think that pf has to be aware of layer 2 queuing mechanism to make any kind of mapping between two queuing systems at layers 2 and 3. So, I suppose, either pf disables the queuing mechanism at layer 2 or pf makes nothing (but in this case one wouldn't need the driver to support pf). So, I'm still not sure why pf needs the drivers to support ALTQ.

>In conclusion, AltQ enforcement must be implemented at 
>the lowest layer, and AltQ classification should maintain 
>ability to examine low level headers even though it is much 
>more common to examine layer3 /layer4 headers.

Ok, this would answer my question, but what kind of layer 2 classification is supported by pf? I don't remember any pf rules that can set layer 2 tags or filter all packets based on layer 2 information.

Thanks,
Aleksej.


>-----Ursprüngliche Nachricht-----
>Von: Ricky Charlet [mailto:RCharlet at adaranet.com] 
>Gesendet: Mittwoch, 22. Dezember 2010 17:55
>An: Spenst, Aleksej; 'freebsd-pf at freebsd.org'
>Betreff: RE: why ALTQ must be supported by interface drivers?
>
>Hi Aleksej,
>
>        I happen to have recently been 'playing' with altq and 
>trying to (successfully eventually) get to work well in the 
>100Mb utilization range.
>
>        So you need to think of it like this, AltQ is 
>replacing a current queueing system... that of the driver. 
>Each driver has a simple FIFO queue. AltQ is just a more 
>sophisticated queue for the driver to use when sending out 
>packets (enforcement). If, in-fact, a driver happens to 
>implement another queue underneath the AltQ queue (sometimes 
>happens in sophisticated hardware) then this 'lower' queue 
>will probably be a FIFO queue and may wind up destroying the 
>value of AltQ.
>
>        Now a piece of that added sophistication is 
>classification. Site administrators set policy on which 
>packets are more or less preferential to drop during 
>congestion. AltQ's classification is implemented via the 
>packet-filters.
>
>        A very common use case for classification is to 
>distinguish traffic based on layer3 and layer4 (IP and 
>TCP/UDP/ICMP/SCTP...) headers. However this is not the only 
>use case and classification based on layer2 headers should 
>continue to be supported for those few sites that wish it.
>
>        In conclusion, AltQ enforcement must be implemented at 
>the lowest layer, and AltQ classification should maintain 
>ability to examine low level headers even though it is much 
>more common to examine layer3 /layer4 headers.
>
>
>        So much for philosophy. About compiling, yes you 
>understood quite correctly. I have found these web sites to 
>have helpful AltQ info (in no particular order):
>
>http://www.sonycsl.co.jp/~kjc/software/altq-new-design.txt
>http://www.sonycsl.co.jp/person/kjc/software.html#ALTQ
>http://www.sonycsl.co.jp/person/kjc/software/TIPS.txt
>http://www.openbsd.org/faq/pf/index.html
>http://openbsd.org/faq/pf/queueing.html
>http://loquefaltaba.com/documentacion/pf/en/altqintro.html
>http://system-tribudi.blogspot.com/2006/08/pf-hfsc-part-i.html
>http://www.probsd.net/pf/index.php/Hednod%27s_HFSC_explained
>http://www.icir.org/floyd/cbq.html
>
>
>
>---
>Ricky Charlet
>
>
>
>
>
>> -----Original Message-----
>> From: owner-freebsd-pf at freebsd.org [mailto:owner-freebsd- 
>> pf at freebsd.org] On Behalf Of Spenst, Aleksej
>> Sent: Wednesday, December 22, 2010 4:00 AM
>> To: 'freebsd-pf at freebsd.org'
>> Subject: why ALTQ must be supported by interface drivers?
>>
>> Hi All,
>>
>> at what network level is ALTQ (QoS) implemented? At the IP 
>level or at 
>> the driver level?
>>
>> I would think that all queuing functionality is probabliy working at 
>> the IP level and should not depend on underlying interfaces. Is that 
>> correct? If this is true, I don't understand why ALTQ must be 
>> supported by interface drivers? If I understand it 
>correctly: (1) the 
>> driver has to support ALTQ. (2) the driver has to be compiled as 
>> ALTQ-aware with some special flags.
>>
>> Thanks,
>> Aleksej.
>>
>>
>>
>>
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