Fair queue servicing using dynamic weights (dwfq)

Description of equivalent WO 9714240 A1

0001Fair Queue Servicing using dynamic weights (DWFQ)

0002This invention relates to the field of telecommunications, and more particularly to a method of fair queue servicing in asynchronous data networks, such as Asynchronous Transfer Mode (ATM) networks or more generally any packet switched network that supports more than one class of service.

0003The use of ATM by a continually increasing number of applications is driving a requirement to increase the number of service classes and to allow more flexibility in the service offerings. To support the application requirements, the ATM Forum is adding new service categories in new releases of ATM specifications. Furthermore, network providers are looking for the flexibility of defining multiple service classes for a given service category. The service classes are differentiated by their Quality-Of-Service requirements (QoS). The QoS requirements are configurable in accordance with a bi- dimensional matrix describing loss and delay. The delay jitter is another factor which needs to be bounded for some service classes.

0004Previously, three service categories were supported on an ATM network element, namely constant bit rate (CBR), variable bit rate (VBR) and unspecified bit rate (UBR). The CBR service is the only service that guarantees a bound on delay. It is used for time sensitive data, such as voice and video.

0005These various services can be supported by traditional exhaustive round-robin queuing among two priority queues. However, this simple technique cannot be used when the number of queues increases beyond two, because ofthe high potential of starvation for lower priority queues. Furthermore, the exhaustive round robin can only guarantee bounds on delay and delay variation for the highest priority queue. The support of multiple service class in an ATM switching product or multiplexer requires a minimum of one queue per class.

0006A queue scheduling algorithm. Weighted Fair Queuing (WFQ), has been recently proposed in the literature (see S. Golestani, A self-clocked Fair Queuing scheme for broadband applications. INFOCOM 1994. June 1994). This scheduling scheme allows any number queues (service classes) to be serviced, while providing fair and work conserving access to bandwidth. One of the key features of WFQ is that the CDV (Cell Delay Variation) is bounded for any service class, as long as it is given a minimum weight.

0007This proposed scheme can be implemented in ATM products. However, it has not been determined how to set the servicing weights efficiently to take into account the dynamically changing bandwidth requirement of each service class (connection addition/removal. ABR flow control, Early packet Discard).

0008An object ofthe invention is to provide a framework that ensures that the weights are set appropriately to guarantee the desired Quality of Service and modified in real-time to ensure that the dynamic allocation of bandwidth across the classes is optimized.

0009According to the present invention there is provided a method of fair queue servicing at a queuing point in a multi-service class packet switched network, wherein ijicoming packets are received in buffers and outgoing packets are scheduled by a weighted fair queue scheduler characterized in that real-time information of buffer usage along with the minimum bandwidth requirement is used to dynamically modify the weights of the weighted fair queue scheduler.

0010Preferably tlie minimum bandwidth requirement is extracted during connection admission control.

0011The method is particularly suitable for use in ATM networks.

0012The DWFQ (Dynamic Weighted Fair Queuing) can be implemented at any queuing point which arbitrates servicing between n queues (n>2).

0013The invention also provides a fair queue servicing arrangement in a multi-service class packet switched network, comprising a weighted fair queuing controller, and buffer means for receiving incoming packets in queues, characterized in that further comprises means for monitoring buffer usage for each queue, means for determining the bandwidth requirements of each class of service, and a service weights manager for dynamically modifying the weights of said weighted fair queuing controller means in response to said buffer usage and bandwidth requirements. Preferably, the means for monitoring buffer usage a queue growth monitor which performs real-time estimation ofthe queue growth in said buffer means.

0014The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:-

0015Figure 1 is a diagram depicting the high level queuing scheme at an ATM switch;

0016Figure 2 shows the information provided by the Queue Growth Monitor;

0017Figure 3 illustrates the data flow between the key components ofthe system and the action of the Service Weight Manager (SWM); and

0018Figure 4 describes the process performed by the SWM.

0019Referring now to Figure 1, ATM cells 2 arrive at buffer 1 and are placed in queues 1 ', l², ... 1 ". From there the cells are passed to a weighted fair queuing unit 3. The buffer 1 is also connected to a queue growth monitor 4, which in turn is connected to congestion control unit 5, congestion analyzer 6, and connection admission controller 8, which in turn is connected to SVC & PVC (Switched Virtual Circuit and Permanent Virtual Circuit) connection handling unit 7, and service class manager 10. Queue growth monitor 4, connection admission controller 8 and service class manager 9 are connected to service weights manager 9, which is connected to weighted fair queuing scheduler 3.

0020The key element of the Dynamic Weighted Fair Queuing (DWFQ) scheme is the service weight manager (SWM) 9, which dynamically modifies the service weights to be used by the WFQ Scheduler 3. It uses real-time information from the service class manager 10. the connection admission controller 8, and the Queue growth monitor 4.

0021The service class manager 10 configures the service classes. A service class is configured with a given value of delay (CTD - Cell Transfer Delay) and loss (CLR - Cell Ratio Loss) requirements. These parameters represent the maximum nodal delay and loss allowed in order to meet the end-to-end QoS requirements of the connection. The service classes are mapped into a priority table as exemplified in Table 1. The priority table is used later by the service weight manager to allocate remaining bandwidth. The priority table is updated when a service class definition is modified. The service class manager also dictates which traffic descriptors are used to compute the minimum bandwidth required by a connection of a given class.

0022<img file="WO9714240A1_D0001.tif" />

0023Table 1 • Example o a Queue Service Priority Mapping.

0024The connection admission controller (CAC) 8 computes the minimum bandwidth required for each service" class. The minimum bandwidth is updated each time a connection of a given class is established or disconnected, based on its traffic descriptor.

0025Table 2 shows a typical example of which traffic descriptors that can be used to compute the minimum bandwidth for each basic service category relative to the queue service rate (SR). The CAC 8 communicates the minimum Weight table to the SWM ever}' time the value of the minimum weights have changed by a factor of ξ.

0026<img file="WO9714240A1_D0002.tif" />

0027Table 2 • Example of minimum weight table.

0028The CTD is further taken into account in the target queue size (TQS) table, which is the maximum queue size allowed to limit the CTD. An example of TQSj computation is shown in Table 3, for typical service categories. A zero TQS indicates that the queue can grow without limitation. This table is computed by the CAC. <img file="WO9714240A1_D0003.tif" />

0029The Queue Growth Monitor (QGM) 4 performs real-time estimation of the queue growth every T_s cell slots (sampling interval). The information provided by the Queue Growth monitor 4 to the SWM 9 consists of ΔQi, the Queue Growth Rate of output queue i during an interval of duration Ts, Qi, the length of output queue i at the sampling time, and Ai, the arrival rate during the same interval of time.

0030The corresponding parameters: queue size Qi, queue growth ΔQi, number of arrivals Ai are collected or each queue by the QGM 4 for each Ts interval. From these parameters, auxiliary parameters such as average arrival rate λi and service rate μi can be -derived by the SWM 9: λ, : average arrival rate, λ, = A, - T_s

0031μ, : average service rate, μ, = S, - T_s, where S, = A, - ΔQ, is the number of cells served during T_s.

0032Figure 2 shows the information provided by the Queue Growth Monitor 4. Using this information for tlie CAC 8, the service class manager 9 and the queue growth monitor 4, the SWM computes the service eight for each queue (W.) to be used during the next sampling interval.

0033As can be seen in Figure 3. which shows the data flow between the key components ofthe system and the action ofthe SWM 9, the queue weights, Wi. are updated using information provided by the Queue Growth monitor 4.

0034If λ, denotes the arrival rate of cells in queue / in the coming T_s interval, then ideally, the target service rate μ_: can be calculated as: lu, - λ_t V T_s = Q_L - TQS . This means at the end of next Ts interval, the queue size Qi will reach the target queue size TQSi. On the assumption that λ, remains unchanged from λ, , the service weight

0035W , = μ , • T_s can be approximated as

0036W₁=μ_ι.T. = λ₁-T_ + Q_ι-TQS₁=A_ι+Q₁-TQS₁.

0037However, the assumption on the stable arrival rate may not hold, and also the actual number of serviced cells Si could be less than Wi; therefore a more conservative approach is ΔQ, > 0, then Q.+ΔQ,. the predicted queue size at the end ofthe next Ts interval, is used to calculate the target service rate and weight. That is:

0038(μ,-λ,).T. =Q,+ΔQ,-TQS, and W,Ξλ,.T_s+Q_ι+ΔQ_ι-TQS_l = A_ι + Q_ι + ΔQ,-TQS,

0039The detailed algorithm performed by the service weights manager 9 is shown in Figure 4. The queue size Q, at the end of each interval T_s, the number of arrivals A, during the previous interval T_s. and the change in queue size ΔQ, are input at step 20. Step 21 ^•determines whether the queue growth is positive: if yes. the service weight W, is conservatively adjusted to bring the queue size to TQSi at step 22; if no, the service weight W, is adjusted to bring the queue size to TQSi at step 23. The difference ΔW is determined in step 24.

0040Step 25 determines whether the shared weights pool is empty: if yes, Wi is set to min Wi in step 26; if no. step 27 determines whether Ws > ΔW,: if yes, step 28 sets W, = min_W, + ΔW, and W_s = W_s + ΔW,; if no, step 29 sets W,= min_W, + W_s and W_s = 0.

0041Step 30 runs through all the W, in the ordered list L and step 31 updates the weight table used by the Weighted Fair Queuing scheduler 3.

0042The described technique complies with ITU and ATM Forum standards and can be applied to any switching equipment which supports more than a single service class of service.