Traffic Management for the Available Bit Rate (ABR) Service in ...
Traffic Management for the Available Bit Rate (ABR) Service in ... Traffic Management for the Available Bit Rate (ABR) Service in ...
all eligible sources can also get this high allocation. To do this, we add a vari- able MaxAllocP revious which stores the maximum allocation given in the previous interval, and another variable MaxAllocCurrent which accumulates the maximum allocation given during the current switchaveraging interval. The step 9 of the basic algorithm is replaced by the ow chart shown in gure C.2 (see appendix C). Basically, for z > 1 + , where is a small fraction, we use the basic ER- ICA algorithm and allocate the source Max (FairShare, VCShare). But, for z 1+ , we attempt to make all the rate allocations equal. We calculate the ER as Max (FairShare, VCShare, MaxAllocPrevious). The key point isthattheV CShare is only used to achieve e ciency. The fairness can be achieved only by giving the contending sources equal rates. Our solution attempts to give the sources equal allocations during underload and then divide the (equal) CCRs by the same z during the subsequent overload to bring them to their max-min fair shares. The system is considered to be in a state of overload when its load factor, z, is greater than 1+ . The aim of introducing the quantity is to force the allocation of equal rates when the overload is uctuating around unity, thus avoiding unnecessary rate oscillations. The next subsection examines one further modi cation to the ERICA algorithm. 6.3 Fairshare First to Avoid Transient Overloads The inter-RM cell time determines how frequently a source receives feedback. It is also a factor in determining the transient response time when load conditions change. With the basic ERICA scheme, it is possible that a source which receives feedback rst can keep getting rate increase indications, purely because it sends more RM cells 157
efore competing sources can receive feedback. This results in unnecessary spikes (sudden increases) in rates and queues with the basic ERICA scheme. The problem arises when the Backward RM (BRM) cells from di erent sources arrive asynchronously at the switch. Consider a LAN con guration of two sources (A and B), initially sending at low rates. When the BRM arrives, the switch calculates the feedback for the current overload. Without loss of generality, assume that the BRM of source A is encountered before that of source B. Now it is possible that the BRM changes the rate of source Aandthe new overload due to the higher rate of A is experienced at the switch before the BRM from the source B reaches the switch. The transient overload experienced at the switch may still be below unity, and the ACR of source A is increased further (BRMs for source A are available since source A sends more RM cells at higher rates). This e ect is observed as an undesired spike in the ACR graphs and sudden queue spikes when the source B gets its fair share. This problem can be solved by incorporating the following change to the ERICA algorithm. When the calculated ER is greater than the fair share value, and the source is increasing from a CCR below F airShare, we limit its increase to F airShare. Alternatively, the switch could decide not to give new feedback to this source for one measurement interval. The following computation is added to the switch algorithm. After \ER Calculated" is computed: IF ((CCR < FairShare) AND (ER Calculated FairShare)) THEN ER Calculated FairShare We can also disable feedback to this source for one measurement interval. \ER in RM Cell" is then computed as before. 158
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e<strong>for</strong>e compet<strong>in</strong>g sources can receive feedback. This results <strong>in</strong> unnecessary spikes<br />
(sudden <strong>in</strong>creases) <strong>in</strong> rates and queues with <strong>the</strong> basic ERICA scheme.<br />
The problem arises when <strong>the</strong> Backward RM (BRM) cells from di erent sources<br />
arrive asynchronously at <strong>the</strong> switch. Consider a LAN con guration of two sources (A<br />
and B), <strong>in</strong>itially send<strong>in</strong>g at low rates. When <strong>the</strong> BRM arrives, <strong>the</strong> switch calculates<br />
<strong>the</strong> feedback <strong>for</strong> <strong>the</strong> current overload. Without loss of generality, assume that <strong>the</strong><br />
BRM of source A is encountered be<strong>for</strong>e that of source B. Now it is possible that <strong>the</strong><br />
BRM changes <strong>the</strong> rate of source Aand<strong>the</strong> new overload due to <strong>the</strong> higher rate of A<br />
is experienced at <strong>the</strong> switch be<strong>for</strong>e <strong>the</strong> BRM from <strong>the</strong> source B reaches <strong>the</strong> switch.<br />
The transient overload experienced at <strong>the</strong> switch may still be below unity, and <strong>the</strong><br />
ACR of source A is <strong>in</strong>creased fur<strong>the</strong>r (BRMs <strong>for</strong> source A are available s<strong>in</strong>ce source<br />
A sends more RM cells at higher rates). This e ect is observed as an undesired spike<br />
<strong>in</strong> <strong>the</strong> ACR graphs and sudden queue spikes when <strong>the</strong> source B gets its fair share.<br />
This problem can be solved by <strong>in</strong>corporat<strong>in</strong>g <strong>the</strong> follow<strong>in</strong>g change to <strong>the</strong> ERICA<br />
algorithm. When <strong>the</strong> calculated ER is greater than <strong>the</strong> fair share value, and <strong>the</strong><br />
source is <strong>in</strong>creas<strong>in</strong>g from a CCR below F airShare, we limit its <strong>in</strong>crease to F airShare.<br />
Alternatively, <strong>the</strong> switch could decide not to give new feedback to this source <strong>for</strong> one<br />
measurement <strong>in</strong>terval. The follow<strong>in</strong>g computation is added to <strong>the</strong> switch algorithm.<br />
After \ER Calculated" is computed:<br />
IF ((CCR < FairShare) AND (ER Calculated FairShare)) THEN<br />
ER Calculated FairShare<br />
We can also disable feedback to this source <strong>for</strong> one measurement <strong>in</strong>terval.<br />
\ER <strong>in</strong> RM Cell" is <strong>the</strong>n computed as be<strong>for</strong>e.<br />
158
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