FAIR QUEUE SERVICING USING DYNAMIC WEIGHTS (DWFQ)

Abstract: In a method of fair queue servicing at a queuing point in a multi-service class packet switched network, incoming packets are received in buffers and outgoing packets are scheduled by a weighted fair queue scheduler. 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.

Description: Fair Queue Servicing using dynamic weights (DWFQ) This 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. The 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. Previously, 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. These 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. A 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. This 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). An 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. According 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. Preferably tlie minimum bandwidth requirement is extracted during connection admission control. The method is particularly suitable for use in ATM networks. The DWFQ (Dynamic Weighted Fair Queuing) can be implemented at any queuing point which arbitrates servicing between n queues (n>2). The 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. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a diagram depicting the high level queuing scheme at an ATM switch; Figure 2 shows the information provided by the Queue Growth Monitor; Figure 3 illustrates the data flow between the key components ofthe system and the action of the Service Weight Manager (SWM); and Figure 4 describes the process performed by the SWM. Referring now to Figure 1, ATM cells 2 arrive at buffer 1 and are placed in queues 1 ', l 2 , ... 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. The 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. The 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. Table 1 • Example o a Queue Service Priority Mapping. The 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. Table 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. Table 2 • Example of minimum weight table. The 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. The 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. The 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 μ, : average service rate, μ, = S, - T s , where S, = A, - ΔQ, is the number of cells served during T s . Figure 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. As 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. If 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 W , = μ , • T s can be approximated as W 1 =μ ι .T. = λ 1 -T_ + Q ι -TQS 1 =A ι +Q 1 -TQS 1 . However, 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: (μ,-λ,).T. =Q,+ΔQ,-TQS, and W,Ξλ,.T s +Q ι +ΔQ ι -TQS l = A ι + Q ι + ΔQ,-TQS, The 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. Step 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. Step 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. The 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.

Claims: Claims: 1. A method of fair queue servicing at a queuing point in a multi-service class packet switched network, wherein incoming 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 ofthe weighted fair queue scheduler. 2. A method as claimed in claim 1 , the minimum bandwidth requirement is extracted during connection admission control. 3. A method as claimed in claim 1, characterized in that said weights are also modified in accordance with real-time service class information. 4. A method as claimed in claim 1, characterized in that buffer usage is monitored by a queue growth monitor, which performs real-time estimation ofthe queue growth every sampling interval T s . 5. A method as claimed in any of claims 1 to 4, wherein said packet switched network is an ATM network and said packets are ATM cells. 6. A method as claimed in any of claims 1 to 5, wherein the weights ofthe weighted fair queue scheduler are modified in accordance with the following algorithm: 7. 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. 8. A fair queue servicing arrangement as claimed in claim 7, characterized in that it further comprises a service class manager which stores the cell transfer delay cell loss ratio requirements for each class of service, and said service weights manager is also responsive to said class of service requirements stored in said service class manager to dynamically modifythe weights of said weighted fair queuing controller means. 9. A fair queue servicing arrangement as claimed in claim 8, characterized in that said means for monitoring buffer usage comprises a queue growth monitor which performs real-time estimation of the queue growth in said buffer means. 10. A fair queue servicing arrangement as claimed in claim 8, characterized in that said queue growth monitor outputs to said service weights manager the queue growth rate of each output queue / during a sampling interval, the length ofthe output queue / at sampling time, and the arrival rate of cells A, during the sampling interval. 1 1. A fair queue servicing arrangement as claimed in any of claims 8 to 10, characterized in that said network is an ATM network.

Inventors: AISSAOUI MUSTAPHA, GIROUX NATALIE, LIAO RAYMOND RUI-FENG, AISSAOUI, MUSTAPHA, GIROUX, NATALIE, LIAO, RAYMOND, RUI-FENG
Applicants: AISSAOUI MUSTAPHA, GIROUX NATALIE, LIAO RAYMOND RUI FENG, NEWBRIDGE NETWORKS CORP, AISSAOUI, MUSTAPHA, GIROUX, NATALIE, LIAO, RAYMOND, RUI-FENG, NEWBRIDGE NETWORKS CORPORATION
Application year: 1996
Main classification: H04L12
IPC codes: H04L12, H04Q11