Abstract:
The present invention focuses on the aggregation of flows belonging to different classes of non-guaranteed-delay traffic into a single FIFO queue in a downstream stage of the multi-stage switch. These include the guaranteed flows requiring bandwidth reservation, the best-effort flows that require a fair share of the excess bandwidth, and the flows that require both types of guarantee. We disclose a credit-based backpressure scheme which selectively controls the traffic originating from the previous stage of the system while achieving the goal of meeting the requirements of the individual flows. The credit function is maintained for each controlled traffic component in the aggregate session, and its objective is to balance the actual arrival rate of the component with the service rate dynamically granted by the downstream scheduler. The number of flows that can be aggregated is related to the complexity of maintaining the credit functions for the different traffic components.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to the filed of communication and switching systems and in particular, to a method of selective feedback control in multi-module multi-stage packet switches. 
   BACKGROUND OF THE INVENTION 
   As the demand for aggregate switching capacity increases, multiple-module switching systems are being adopted as a scalable alternative to a single-node output-buffered switch (see, for example, F. M. Chiussi, J. G. Kneuer, and Kumar V. P., “Low-cost scalable switching solutions for broadband networking: The ATLANTA architecture and chipset”, IEEE Communications Magazine, 35(12):44–53, December 1997.) At the same time, a significant amount of effort has been spent in designing complex scheduling algorithms which provide diversified Quality of Service (QoS) guarantees such as bounded delay, guaranteed bandwidth and fairness to individual flows or virtual connections. However, most of these techniques regulate access to a single contention point, and hence are directly applicable to output-buffered switches and multiplexors. Such a model does not adequately represent many of the evolved switch architectures, which employ multiple stages such as the ingress port, switch fabric and egress port and have contention points associated with each stage. 
   Feedback control through the mechanism of selective backpressure is well known in the context of multi-stage switches. However, the prior work in this field has concentrated primarily on increasing the throughput of the switching system. In one such instance such as that described by F. M. Chiussi, Y. Xia and V. P. Kumar in an article entitled “Backpressure In Shared-Memory Based ATM Switches Under Multiplexed Bursty Sources”, which apprered in Proc. IEEE INFOCOM&#39; 96—Networking the Next Generation, Volume 2, pp 830–843, San Francisco, Calif., March 1996, whenever congestion occurs at an output link of the second stage, the first stage modules are pre-empted from sending traffic to that link by means of a per-output backpressure signal. Conceptually, it is possible to extend this idea by employing per-flow schedulers in each stage and using per-flow backpressure signals. However, such a replication of functionality in all the stages defeats the purpose of building multi-stage switches, not to mention the increased implementation complexity. Hence, we are motivated to build a system in which we relegate all the fine grain scheduling details to the slower (in terms of aggregate switching capacity) first stage, aggregate a set of flows into a single session in the second stage, and provide an intelligent feedback mechanism that enables to maintain the QoS guarantees at the per-flow level. Some of the recent work done by D. C. Stephens and H. Zhang and presented as a paper entitled “Implementing Distributed Packet Fair Queueing in a Scalable Switch Architecture”, which appeared in Proc. IEEE INFOCOM&#39;98—Gateway to the 21 st  Century, Volume 1, pp. 282–290, San Francisco, Calif., March/April 1998, in multi-stage switches take a similar approach but do not address the issue of flow aggregation. 
   Employing queue length as an indication of congestion with asserting selective feedback with direct or indirect means is a well-known technique both in switching architectures and the Available Bit Rate (ABR) service. Use of the length of a fictitious, or virtual, queue instead of the length of an actual queue for that purpose has also been described before (See, for example, F. M. Chiussi, Y. Xia and V. P. Kumar, “Virtual Queueing Techniques for ABR Service: Improving ABR/VBR Interaction, Proc. IEEE INFOCOM&#39;97—Driving the Information Revolution, Volume 1, pp 406–418, Kobe, Japan, April 1997. The latter method provides a reliable indication of congestion caused by a specific traffic component (ABR type traffic) in the presence of other traffic components (Variable Bit Rate, or VBR, type traffic) when each traffic component is allocated its separate queue and the ABR queue is given strictly lower priority than the VBR queue. The length of a virtual ABR queue is controlled using the reference static ABR service rate and the derived actual arrival rate. 
   SUMMARY OF THE INVENTION 
   The invention presents a method and an apparatus to assert selective feedback from a downstream stage to an upstream stage in a multi-stage switching system carrying traffic flows associated with multiple traffic components each of which is characterized by the specific Quality of Service (QoS) requirements. While the upstream stage of the system implements fine granularity queuing and a sophisticated scheduling discipline, the downstream stage allows the traffic flows of different components to be aggregated within the same queue associated with a specific pair of the input and output ports of the downstream stage. The selective backpressure is asserted for the individual traffic components based on the value of the per-component credit function (credit counter) that is incremented with the service attributed by the downstream scheduler to the specific traffic component and is decremented on arrival of a packet belonging to that component from the upstream stage. If an aggregate downstream stage queue drains completely, the credit functions of all traffic components carried by that queue are reset to their respective maximum values. The per-component backpressure is asserted when the credit function reaches or falls below zero. 
   The disclosed credit-based backpressure mechanism provides a reliable means for per-class congestion indication in a multi-stage switching systems. It distinguishes itself from the prior art in that:
         it uses the concept of service credit rather than actual or virtual queue length;   it provides an indication of congestion for multiple traffic components at the same time;   it controls the per-component credit counter using the actual services attributed to the given component by the downstream stage scheduler and the actual component arrival rate; it contains provisions to guarantee efficient redistribution of resources to the active traffic components in the case when one or more components underutilize their allocated share of resources; and   it employs a single aggregate queue for all traffic components in a downstream stage of the system, while relying on the upstream stage scheduler to control the composition of that aggregate queue.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more readily apparent from the following detailed description of the invention in which: 
       FIG. 1  illustrates the structural components of a representative multi-stage system; 
       FIG. 2  illustrates a scheduling node in isolation; 
       FIG. 3  illustrates a representative backpressure mechanism based on observation of queue occupancy; 
       FIG. 4  illustrates an aggregation of flows in a common queue; 
       FIG. 5  illustrates a mechanism for generating credit-based feedback; 
       FIG. 6  illustrates a possible implementation of the credit-based feedback method in a two-stage switching system with cross-traffic flow aggregation; 
       FIG. 7  illustrates a possible implementation of the credit-based feedback method in a two-stage switching system with in-channel flow aggregation; 
       FIG. 8  illustrates an ingress port scheduling structure in a distributed scheduling architecture; 
       FIG. 9  illustrates a fabric scheduling structure in a distributed scheduling architecture; and 
       FIG. 10  illustrates a credit-based feedback mechanism for control of the Guaranteed bandwidth and Best Effort traffic components within a Non-Guaranteed Delay channel in a distributed scheduling architecture. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference now to  FIG. 1 , there is shown a general view of selected two adjacent stages in a multi-stage switching system. Upstream-stage queuing modules  10  are interconnected by internal links  30  to downstream-stage queuing modules  20 . The system switches flows, i.e., abstract end-to-end traffic streams. Upstream scheduling nodes  15  and the downstream scheduling nodes  25  arbitrate access of the switched flows to the link bandwidth. Each of the upstream stage scheduling nodes  15  services a set of per-flow queues, which may be organized in a hierarchical structure, and feeds a set of the second stage queues and associated scheduling nodes. A spatial channel is defined by a pair of upstream and downstream scheduling nodes. Traffic belonging to a certain flow competes for service bandwidth with other flows of the same spatial channel (in-channel flows) and with traffic of other spatial channels sharing the same upstream and downstream scheduling nodes (cross-traffic flows). For a given spatial channel, the upstream cross-traffic flows and the downstream cross-traffic flows constitute two mutually exclusive sets. Therefore, the contention experienced at the upstream stage by the traffic in the channel is independent of the downstream contention experienced by the flows in the same channel. 
   A more detailed illustration of a scheduling node in isolation is presented in  FIG. 2 . With reference to that FIG, scheduler  40  arbitrates access to the available capacity among a set  50  of sessions  52  . . .  58 , which may correspond to individual traffic flows or their aggregates. Each session  52  . . .  58  is provided an individual queue  62  . . .  68 , which may store the packets before they are scheduled for transmission. Arrival rate of a session is the dynamically varying rate at which its traffic enters the system. The session is backlogged if it has traffic waiting for service in the queue. The portion of available bandwidth instantaneously provided to a session by a scheduler is referred to as granted rate of that session. The session&#39;s traffic leaves the node at the output rate. For an isolated scheduler, the output rate of each backlogged session is equal to its granted rate, unless restricted by an external backpressure signal. The selective backpressure signals  72  . . .  78  from the downstream portion of the system prevent specific session or group of session from being serviced by scheduler  40 . Analogously, the selective backpressure signals  82  . . .  88  are generated for use by the upstream portion of the system. If backpressure is asserted for a specific session, the arrival rate of that session is reduced, most often to zero, within a fixed-length or variable length time interval known as backpressure latency. 
   In any single stage of a multistage system, scheduler  40  may implement an advanced scheduling algorithm, that provides high level Quality of Service (QoS) to its session. The QoS can be expressed in terms of bandwidth and/or delay guarantees as well as fairness. A scheduler in a packet switching system, which necessarily operates on a discrete data units, typically attempts to approximate an idealized fluid scheduling discipline used as a reference. A general scheduling reference is the Decoupled Generalized Processor Sharing (DGPS) approach, introduced in F. Toutain, “Decoupled Generalized Processor Sharing: A Fair Queueing Principle for Adaptive Multimedia Applications, Proc. IEEE INFOCOM&#39;98—Gateway to the 21 st  Century, Vol 1, pp. 291–298, San Francisco, Calif. March/April, 1998 and based on A. K. Parekh and R. G. Gallager, “A Generalized Processor Sharing Approach to Flow Control In Integrated Services Networks: The Single Node Case”, IEEE/ACM Transactions on Networking, 1(3):344–357, June 1993. In DGPS each session is characterized by a reserved, or guaranteed rate g i , the sum of guaranteed rates of all sessions being less than the service capacity, and excess bandwidth weight w i . Let W i (t′, t″) denote the amount of traffic of the i-th session served within time interval (t′, t″), then for any two sessions i and j that are continuously backlogged during the interval (t′, t″), the decoupled fairness criterion requires that: 
                   W   i     ⁡     (       t   ′     ,     t   ″       )       -       g   i     ⁡     (       t   ″     -     t   ′       )           w   i       =             W   j     ⁡     (       t   ′     ,     t   ″       )       -       g   j     ⁡     (       t   ″     -     t   ′       )           w   j       .           
A fluid scheduler implementing DGPS is described by the following set of rules:
     R1. Each backlogged session is granted at least its guaranteed rate g.   R2. For each backlogged session, the granted rate exceeds the guaranteed rate by a value proportional to weight w.   R3. For each non-backlogged session, the granted rate is equal to the lesser of its actual arrival rate and the value determined according to rules R1 and R2.   
   Referring to  FIG. 2 , a DGPS scheduler that provides decoupled fairness criterion can be conveniently represented as containing two components: the Guaranteed Bandwidth Scheduling or GBS component  42  and the Excess Bandwidth Scheduling or EBS component  44 . Both components operate in parallel, the GBS component being a non-work conserving while the EBS component being work-conserving. Whenever the GBS component selects a session, this service receives service. The session selected by the EBS component receives service only if the GBS component does not make a selection. 
   In a two-stage system of  FIG. 1 , a feedback in the form of selective backpressure plays important role, when the traffic is downstream-constrained, i.e., the granted rates at the upstream stage exceed those at the downstream stage. Assuming the stationary behavior of the system which is defined by the unchanged backlog status of all flows and the constant arrival rates of the flows which are not backlogged, the selective backpressure can be modeled using the proportional reduction of rate. The throughput R i  of a backlogged flow i can be found as
 
 R   i =min{β i   r   i   (1)   , r   i   (2) },
 
where r i   (k)  is the flow&#39;s granted rate at the k-th stage and β is the backpressure factor. Ideally, to ensure maximum throughput for the given flow while not imposing restrictions on the cross-traffic flow, backpressure allows to match the output rate of the upstream stage with the granted rate of the downstream stage:
 
   
     
       
         
           
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   The commonly used method of backpressure generation is based on the observation of the queue occupancy, as shown in  FIG. 3 . The feedback control block  90  contains a counter  91  and a comparator  92 . The counter  91  is originally set to zero and subsequently incremented on event of a data unit arrival  95  to the queue  60  and decremented on the event of data unit departure  96  from the queue  60 . The value of the counter  91 , which is thus equal to the queue occupancy, is compared in the comparator  92  with threshold T which may be either or computed dynamically based on the state of the system. If the value of the counter  91  exceeds the threshold, a backpressure signal  97  is asserted towards the upstream stage. The departure from the queue occurs when the queue is not empty, and the stage scheduler selects that queue for service. Referring to  FIG. 2 , to ensure generation of the backpressure signals  82  . . .  88 , a feedback control block  90  has to be replicated and attached to each of the queues  62  . . .  68 . 
   The disclosed invention reduces the number of queues in the downstream queuing node by aggregating queues  62  . . .  68  in a single queue multi-session FIFO queue  69 , as shown in  FIG. 4 , provided that the individual service rates granted to each component session can be accounted for. Regardless of what component sessions within queue  69  are selected by the scheduler  40 , the service is given to the data item at the head of that queue. 
   The method of the invention calls for assigning a value of credit function to one or more component session aggregated within the queue  69 . Whenever the aggregate queue length reaches zero, the credit function of a component is initialized using a pre-computed static or state-dependent dynamic value. Subsequently, the credit function of a component is incremented when the scheduler selects that component for service, regardless of the component to which the data item at the head of the queue  69  actually belongs. The growth of the credit function is subject to truncation. The credit function of a component is decremented when a data item belonging to that component arrives to the queue. For a given session k, the value of the credit function C k (t) is reset to its maximum value C k0  whenever the occupancy of the aggregate FIFO queue reaches and remains at zero. At other times, the credit change is governed by the following mathematical expression: 
                 dC   k     ⁡     (   t   )       =       (         r   k     (   2   )       ⁡     (   t   )       -       a   k     (   2   )       ⁡     (   t   )         )     ·     u   ⁡     (       C   k0     -       C   k     ⁡     (   t   )         )       ·   dt       ,     
     ⁢     where   ⁢     :                     u   ⁡     (   x   )       =     {         1           if   ⁢           ⁢   x     &gt;   0             0           if   ⁢           ⁢   x     ≤   0                   
Here a k   (2) (t) and r k   (2) (t) are the component second stage arrival and granted rates, respectively. The backpressure signal that blocks transmission of the k-th component flow from the upstream stage is asserted whenever C k (t)≦0.
 
   Under the assumptions that the backpressure signals are generated continuously and take effect instantaneously, the aggregate queue length is bounded by the sum of the initial credits, and C k (t) never drops below zero. In practice, with finite backpressure sampling period and finite latency, the queue length remains bounded albeit by a larger value. 
     FIG. 5  shows the schematics of the backpressure generation engine implementing the disclosed feedback control method. For simplicity, it shows two component sessions,  52  and  54 , aggregated in a single downstream queue  69 . Whenever the scheduler  40  selects session  52  (signal  102 ) or session  54  (signal  104 ), a data item at the head of queue  69  departs the queuing stage. The feedback control block  100  contains a queue occupancy counter  105  with a comparator  106  and a set of credit modules  120 ,  140 , one per each component session. The queue occupancy counter  105  is originally set to zero and subsequently incremented on event of a data unit arrival to the queue  69  and decremented on the event of data unit departure from the queue  69 . When the queue occupancy is equal to zero, as determined by the comparator  106 , a reset signal is provided to all credit modules. Each credit module  140 ,  120  contains a credit counter  121 ,  141  with a comparator  123 ,  143  and an initialization unit  122 ,  142 . On receipt of a reset signal from comparator  106 , the corresponding initial credit value provided by initialization units  122 ,  142 , are written to credit counters  121 ,  141 . When the value of the queue occupancy counter  105  differs from zero, the credit counters  121 ,  141  change values with data units arrivals from the upstream stage and services granted by the scheduler  40 . For example, the credit counter  121 , associated with session  52 , is incremented whenever session selection signal  102  is generated by the scheduler  40 , unless it is equal to the initial credit value supplied by initialization unit  122 . The credit counter  121  is decremented when a data unit belonging to session  52  arrives to the queue  69  from the upstream stage. If the value of the credit counter  121  reaches or falls below zero, as determined by comparator  123 , a backpressure signal for session  52  is asserted towards the upstream stage. Credit module  140 , associated with session  54 , operates in a similar fashion. Nothing in this description prevents an implementation following this disclosure from using counters that operate in the reversed direction. 
   The disclosed credit-based feedback method ensures that if all sessions remain backlogged and downstream-constrained, the backpressure mechanism achieves per-flow credit equilibrium near its zero value with the arrival rates matching corresponding granted rates and provides the target throughput of R i =min{r i   (1) , r i   (2) } with the optimal value of the backpressure factor. If some of the flows are not backlogged or are downstream-constrained, then the service rate of the aggregate queue exceeds its stationary arrival rate and, therefore, the credit equilibrium can not be reached. The aggregate queue occupancy oscillates, periodically returning to zero and resetting the credit counters of all flows to their corresponding maximum values. Additionally, if the initial credit values for a set of sessions are statically selected in proportion to the excess bandwidth weight of the corresponding session than a common FIFO queue in a downstream stage that implements the disclosed credit-based feedback mechanism is sufficient to guarantee that the decoupled fairness criterion is met. 
   PREFERRED EMBODIMENTS 
   One embodiment of the disclose invention provides downstream aggregation of the spatial channels traversing different upstream queuing modules, as shown in  FIG. 6 . For simplicity, each spatial channel is assumed to carry a single flow. Flows  211 ,  221 , and  231  traverse upstream queuing modules  210 ,  220 , and  230 , respectively, and share downstream queuing module  250 . Exemplified by upstream queuing module  210 , the traffic of flow  211 , which is configured as a session in the scheduling node  215 , enters queue  212  serviced by that scheduler. Queues  213  and  214 , also served by scheduler  215 , carry cross-traffic flows traversing downstream queuing modules other than  250 . In the downstream queuing module  250 , the traffic flows from the different upstream modules enter the same aggregate queue  252  served by scheduler  255 . The feedback control block  258  monitors the state of the queue along with its arrival and departure events and maintains the credit function in the per-flow basis using the disclosed method. The backpressure signals from the feedback control block  258  are communicated to the upstream modules  210 ,  220 , and  230 , where, when asserted, they prevent schedulers  215 ,  225 , and  235  from selecting the corresponding sessions for service. 
   Alternatively, the disclosed method is used to provide selective feedback for a set of in-channel flows sharing both upstream and downstream queuing modules, as shown in  FIG. 7 . In an upstream queuing module  260 , a set of flows belonging to the same spatial channel are queued individually in queues  261 ,  262 ,  263 . A plurality of queues  264  represents upstream cross-traffic flows. Scheduling node  265  provide high-granularity service its sessions, including the flows of the given spatial channel. In the downstream queuing module  270 , the flows of the given spatial channel are aggregated in a common queue  271 , which is served by the scheduler  275  along with other downstream cross-traffic flows, represented by plurality of queues  274 . The feedback control block  278  monitors the state of the queue  271  together with its arrival and departure events according to the method disclosed in the foregoing presentation, and generates the per-flow backpressure signal to control the behavior of the upstream scheduler  265 . 
   In another embodiment, the feedback method described in this disclosure can be used to provide inter-stage interaction in a distributed scheduling architecture. A reference distributed switch contains ingress port card stage with large amount of buffers, switching fabric with moderate-size buffers, and the bufferless egress port card stage. The per-flow queuing and sophisticated weighted fair scheduling is performed primarily in the ingress stage. For the purpose of QoS provisioning, all traffic flows are classified as either Guaranteed Delay (GD), Guaranteed Bandwidth (GB) or Best Effort (BE). The port card scheduling architecture, which has a hierarchical structure with the top layer represented by the GBS and EBS scheduling pair, is shown in  FIG. 8 . In the switching fabric, the flows of a specific QoS class belonging to a specific spatial channel are aggregated in a common QoS channel queue. The QoS channel queues destined to the same fabric output are served by the same output port EBS/GBS scheduling pair, as shown in  FIG. 9 . The non-hierarchical scheduling structure with fewer number of queues fits the moderate buffering capacity of the switching fabric. The further reduction in the number of fabric queues can be achieved by aggregating the traffic of the GB and BE QoS classes within the same fabric queue, referred to as the Non-Guaranteed Delay (NGD) channel queue. 
   The aggregation technique employs the disclosed method and is based on the observation of the queue length and the EBS granted rates in the port card and the fabric. As there is no direct correspondence between GB and BE QoS channels, on one hand, and the GBS and EBS scheduling components, on the other, a modification of the disclosed method is required. The queue NGD channel feedback is composed of the two signals. A queue-length-based backpressure signal is asserted towards the corresponding port card, when the occupancy of the NGD channel queue exceeds a threshold, which can be either static or dynamic. The queue-length-based backpressure signal prevents the port card scheduler from serving flows of either GB or BE QoS classes. A credit-based backpressure signal is asserted when the EBS credit function in the fabric reaches or falls below zero. The credit function is set to the original value (pre-computed static or state-dependent dynamic) each time the occupancy of the NGD channel fabric queue reaches zero and is incremented, subject to truncation, on each EBS service that this queue receives. The credit increments may optionally be suppressed during the time intervals when the queue-length-based backpressure signal is asserted. The credit function is decremented each time a data unit served port card EBS scheduling component arrives to the fabric NGD channel queue. The arrival events may be optionally filtered (disqualified for the purpose of the credit function decrement) depending on the QoS class of the arriving item.  FIG. 10  depicts the feedback mechanism for a selected spatial channel. Within the port card  290 , the flows belonging to the GB QoS channel are queued in a queue set  291 , whereas the flows belonging to the BE QoS channel, in a queue set  292 . These flows are served by the port card scheduler  295 , along with the cross-traffic flows. The GB flow aggregate of the given spatial channel may be selected by both the GBS component  296  and the EBS component  297  of the scheduler  295 . The BE flow aggregate of the given spatial channel may be selected by the EBS component  297  only. In the switching fabric  300 , both GB and BE flows are aggregated in an NGD queue  302  of the given spatial channel. NGD queue  302  is configured as a session in the fabric scheduler  310 , which contains two components: the GBS component  311  and the EBS component  312 . The feedback control block  320  associated with the NGD queue  302  contains a queue occupancy counter  321  and credit counter  324 . The queue occupancy counter, which is initially set to zero, is incremented on the event of data item arrival into the queue  302 ; and is decremented on the event of data item departure from the queue  302 . The value of the counter  321  is compared in the comparator  323  with a threshold provided by the threshold computation module  322 . If the value of the counter exceeds the threshold, a backpressure signal  331  is asserted. Backpressure signal  331  is used in the port card  290  to prevent the scheduler  295  from selecting both GB and BE flow aggregates of the given spatial channel. Referring again to the feedback control block  320  of the fabric  300 , the value of the counter  321  is compared with zero in comparator  327  to generate a reset signal to the credit counter  324 . On each such reset event, an initial credit value provided by module  328  is written into the credit counter  324 . Subsequently, the value of the credit counter  324  is incremented each time the NGS queue  302  is serviced by the EBS component  312  of the fabric scheduler  310 , and is decremented when a data item serviced by the EBS component  297  of the port card scheduler  295  enters the NGD queue  302 . The credit counter  324  may optionally skip increment events during the time intervals when the backpressure signal  331  is asserted. The credit counter  324  may optionally skip decrement events when the GB flow data item enters the NGD queue. The value of the credit counter  324  is compared with zero in comparator  325 . Whenever the counter value is equal or less than zero, a backpressure signal  332  is generated. Backpressure signal  332  is used in the port card to prevent the scheduler  295  from selecting for service the BE flow aggregate of the given spatial channel. 
   Various additional modifications of this invention will occur to those skilled in the art and all deviations from the specific teachings of this specification that basically rely upon the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.