Re: DSCP AF class

Ravi/Jay,

AF classification is meant to standardize QoS service offering. It is
well understood that EF class is known to every engineer that it meant
LLQ/Priority while Best Effort is DSCP 0 marking.

Of course you can choose to reverse the logic and implement the other
way round but you won't be integrating very well with other DSCP
compliant networks.

That's why, you don't have the luxury of control the entire domain
but following the standards help to improve the QoS treatment of
properly marked DSCP packets.

Engineering is fun. :)

On Wed, Apr 22, 2009 at 9:27 AM, Ravi Singh <way2ccie_at_googlemail.com> wrote:
> Hi Jay,
>
> As per my understanding, while inherently there is no advantage of a
> packet in one class over a packet in another, the AF PHB states that
> packets with a higher value of the AF class should be given queuing
> preference over packets in a lower value AF class . For example,
> packets in class AF4 should be given better queuing treatment than
> packets in AF1.In other words, when you classify and mark packets and
> assign them the AF PHB, a packet with a PHB of AF4 is not bound to
> receive better queuing treatment than the AF1 packet. It is totally
> upto you on what queuing treatment to give packets in different AF
> classes . If you have a network domain totally under your control, you
> may wish to provide packets in the AF1 class to be given the best
> treatment,whatsoever.
>
> And while I am writing this, Bryan has already responded on how you
> could achieve this configuration. Thanks Bryan.
>
> Regards,
> Ravi
>
>
> On Tue, Apr 21, 2009 at 11:40 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:
>> Hi Ravi,
>> In this case I have the same understanding then 'all AF class have the same
>> priority'.
>>
>> Is there anyway to make AF4 with higher priority than AF1?
>>
>> thanks
>> Jay
>>
>> On Tue, Apr 21, 2009 at 11:32 PM, Ravi Singh <way2ccie_at_googlemail.com>
>> wrote:
>>>
>>> Hi Jay,
>>>
>>> Below is a paste of some relevant lines from the Cisco QoS Exam Cert
>>> Guide which will make this clear
>>>
>>> "An individual PHB describes what happens in a single hop, most
>>> typically a router. In the case of AF,
>>> each PHB contains two separate QoS function, typically performed by
>>> two different QoS tools. The
>>> first function is queuing. Each router classifies the packets into
>>> four different classes, and packets
>>> from each class are placed in a separate queue.
>>>
>>> The AF PHB defines Congestion Avoidance as the second behavior that
>>> comprises the AF PHB.
>>>
>>> Unlike the CS PHB, AF does not follow the bigger-is- better
>>> logic for the AF DSCPs. First, AF11, AF12, and so on are names for
>>> DSCP values, not the binary
>>> of decimal equivalent. Given the names, at least you can
>>> think of the first digit after the AF to be the queuing
>>> classification for example, all AF4x code
>>> points are in the same class for queuing. No specific queuing
>>> parameters are implied for any of these
>>> classes, so there is no inherent advantage to being in class 4 versus
>>> class 1.
>>>
>>> Similarly, the second numeric digit in the AF DSCP names imply the
>>> drop preference with
>>> 3 meaning highest likelihood of being dropped, and 1 meaning the least
>>> likelihood. In other words,
>>> inside a single class, an AFx3 DSCP would mean that these packets
>>> would be dropped more quickly
>>> (more aggressively) than AFx2, which would be dropped more
>>> aggressively than AFx1 packets. In
>>> the actual DSCP names, a bigger number for the second numeric digit
>>> actually implies a less desirable
>>> QoS behavior.
>>>
>>> HTH,
>>> Ravi
>>>
>>>
>>> On Tue, Apr 21, 2009 at 2:23 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:
>>> > ok. I am clear on the drop probability. what about the first numerical
>>> > value. Does AF4 have preference over AF1?
>>> >
>>> > On Tue, Apr 21, 2009 at 9:19 PM, GAURAV MADAN
>>> > <gauravmadan1177_at_gmail.com>wrote:
>>> >
>>> >> correct !
>>> >>
>>> >> first integer is "class selector" and second is " drop precedence"
>>> >>
>>> >> for drop prec : 1,2,3 .... 1 means low 2 means medium and 3 means high
>>> >>
>>> >> HTH
>>> >> Gaurav Madan
>>> >>
>>> >> On Tue, Apr 21, 2009 at 6:43 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:
>>> >>
>>> >>> Actually, drop preference is the second numeric value in a particular
>>> >>> class
>>> >>> like AF11 (low drop), AF12 (medium drop) and AF13 (high drop).
>>> >>>
>>> >>> I was talking more on AF class (AF1, AF2, AF3 and AF4).
>>> >>>
>>> >>> On Tue, Apr 21, 2009 at 9:07 PM, Divin Mathew John
>>> >>> <divinjohn_at_gmail.com
>>> >>> >wrote:
>>> >>>
>>> >>> > AF1, AF2 etc... 1,2 is the drop preference. like AF1 will be dropped
>>> >>> > less than AF2 and all
>>> >>> > Thanking You
>>> >>> >
>>> >>> > Yours Sincerely
>>> >>> >
>>> >>> > Divin Mathew John
>>> >>> > divinjohn_at_gmail.com
>>> >>> > divin_at_dide3d.com
>>> >>> > http://www.dide3d.com
>>> >>> > +91 9945430983
>>> >>> > +91 9846697191
>>> >>> > +974 5008916
>>> >>> > PGP PUBLIC KEY BLOCK @
>>> >>> > http://www.dide3d.com/divin_Public_PGP_key.txt
>>> >>> > Sent from Bangalore, KA, India
>>> >>> >
>>> >>> >
>>> >>> > On Tue, Apr 21, 2009 at 6:31 PM, Jay Pal <jay.b.pal_at_gmail.com>
>>> >>> > wrote:
>>> >>> > > Hi all,
>>> >>> > >
>>> >>> > > Just to make sure my understanding is correct on DSCP AF class.
>>> >>> > > AF1,
>>> >>> AF2,
>>> >>> > > AF3 and AF4 have the same priority when using DSCP and there is
>>> >>> > > no
>>> >>> > > advantage of using AF4 to AF1 or AF2.
>>> >>> > >
>>> >>> > > If this is correct is there a way to make one of the AF class with
>>> >>> more
>>> >>> > > priority than other?
>>> >>> > >
>>> >>> > > thanks
>>> >>> > > Jay
>>> >>> > >
>>> >>> > >
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>>> >>> > >
>>> >>> > >
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Received on Wed Apr 22 2009 - 09:42:10 ART