Re: [RPRWG] A problem about shperD in Draft 2.2

[Fredrik Davik <bjornfd@xxxxxxxxx>]

Chwang

In draft 2.2, the dual queue transmit flow chart (fig 6.28, page 151)
specifies that stage queue entries shall have transmit priority over
stq entries when stq space >= MTU.

As a consequence, the stq will cross exceed stqLowThreshold and the
IsCongested() function will return true

Best Regards, 
Fredrik 
---------------------------------------------
Fredrik Davik
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"chwang" <chwang@xxxxxxxxxxxxxxxxxxx> writes:

> Hi, all,
>
> RPR Draft 2.2 states that shaperD's credit is decreased by the sizes of
> all add and transit frames that are not subclassA0. All traffic except for
> subclassA0 is monitored by shaperD, but only client traffic is backpressured
> (via the passD indication). (See Page 110, D2.2)
>
> D2.2 also states that, in DualQueueMac datapath, local congestion status is
> indicated by either of the following conditions being ture.
> 1. stqDepth > stqLowThreshold
> 2. lpNrXmitRate > rateLowThreshold (optional, checked when AccessDelay
> option is set)
> 3. classBAccessDelayTimerExpired or classCAccessDelayTimerExpired (optional,
> checked when checkRateThreshold option is set)
>
> In clause 9.3.8, D2.2 further defines the IsCongested() function as
> following:
>  IsCongested()
>  {
>  if ((dualQueueMAC && ((lpNrXmitRate > unreservedRate) || (stqDepth() >
> stqLowThreshold)))
>   return TRUE;
>  else if ((!dualQueueMAC && ((lpNrXmitRate > unreservedRate)
>    || (lpNrXmitRate > rateLowThreshold) || classBAccessDelayTimerExpired
>    || classCAccessDelayTimerExpired)))
>   return TRUE;
>  else
>   return FALSE;
>  }
>
>>From above we conclude that we can implement a dualQueue RPR MAC without the
> AccessDelay and checkRateThreshold option.
> But I found a problem when I test it on my OPNet model.
>
> The congifurtion scenario is as fllowing:
>
> flow 1:  ------------------->   800Mbps
> flow 2:            --------->   800Mbps
>
>          A ------> B ------> C ------> D
>          |                             |
>          -------------------------------
>
> LinkRate = 622Mbps, unreservedRate 500Mbps,
> ShaperD incSize = 500Mbps, (equal to unreservedRate)
> station A add in traffic(sepcified as flow1): A->C, 800Mbps, Service Class C
> station B add in traffic(sepcified as flow2): B->C, 800Mbps
>
> Ideal bandwidth received by C should be: flow1 --> 250Mbps, flow2 -->
> 250Mbps
>
>
> But the simulation result show that the bandwidth received by C is:
> flow1: about 500Mbps
> flow2: about 0Mbps
>
>
> When flow1 traffic being transited by B, it consumes up the credits of
> shaperD. B's add traffic can't be sent due to lack of credits. Total traffic
> of flow1 and flow2 sent by B (nrXmitRate) doesn't exceed unreservedRate
> because they are both control by shaperD. At the same time, stqDepth is
> little enough. So station B is not considered to be congested for all the
> time, and flow1's traffic isn't limited. In this case the fairness control
> protocol doesn't work.
>
>
> I think there are two ways to solve this problem:
> 1. Change the AccessDelay check from optional to mandatory in RPR standard.
> 2. Or let shaperD credit decreased only by add frames, not by transit
> frames, just as D2.0 specified.
>
>
> Can someone explain the right usage of shaperD for me?
>
>
>
> Regards,
>
> Wang Chao
> ==================================
> Photonic Bridges Co., Ltd.
> E-mail: chwang@xxxxxxxxxxxxxxxxxxx