Abstract:
Multiplexed traffic in a system where the maximum rate of all inputs exceeds the maximum rate of the output is processed via a separate queue for each input, and at any given forwarding clock cycle the earliest enqueued packet is forwarded via the output. In the event of congestion, a proportionally equal number of packets are dropped from each queue, where proportional equality corresponds to the number of packets dropped per number of packets enqueued. One implementation associates a time-stamp with each enqueued packet to indicate the time of enqueing relative to other enqueued packets. At any given forwarding clock cycle, the packet with the earliest time-stamp is forwarded.

Description:
FIELD OF THE INVENTION 
     This invention is generally related to network communications, and more particularly to queue management. 
     BACKGROUND OF THE INVENTION 
     A network node, such as a switch or router, functions to move traffic between different inputs and outputs in order to advance individual units of traffic, such as packets, from source towards destination. Nodes typically includes a plurality of inputs and outputs which are interconnected by a fabric. Inputs may be combined, such as by multiplexing, on line cards in a node to reduce the corresponding complexity and cost of the fabric and other parts of the node. 
     Techniques for providing “fair” forwarding of multiplexed inputs are known. Traffic in a packet-based data network tends to fluctuate over relatively short intervals of time. In order to handle short-term traffic fluctuation it is known to employ input buffering on the line cards. Input buffers are typically organized into queues associated with different inputs, which are multiplexed. Fairness is implemented by managing forwarding of packets from the queues. For example, round-robin scheduling provides equal forwarding fairness to each queue of a group of managed queues by forwarding one packet from each queue in succession. However, round-robin scheduling may result in disproportionate dropping of packets of particular inputs. For example, if in a given time interval one queue in the managed group is full and the other queues are nearly empty then all or nearly all of the packet drop (assuming there is packet drop) may be suffered by the full queue during that time interval. A technique referred to as Weighted Fair Queuing (“WFQ”) provides means for compensating for the disproportionate packet drop of the basic round-robin technique. In WFQ, a priority differentiator, i.e., weight, is assigned to each individual queue. The frequency of forwarding packets from each queue is proportional to the weight of the queue. In practice this may be accomplished by employing different clock rates with different queues. The weights for the different queues may be selected in accordance with anticipated traffic patterns in order to achieve desired form of fairness. However, network traffic rates may be unknown in advance, and may also tend to fluctuate over relatively long intervals of time, thereby causing the predetermined weighting to be less fair. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a queue management fairness procedure drops proportionally equal numbers of data units for each queue of a managed group of queues when congestion occurs, i.e., for each queue that is non-empty during an interval of time, the number of packets dropped divided by the number of packets enqueued is approximately equal for each of those non-empty queues during the time interval. The fairness procedure may include associating a time-stamp with each packet queued in a managed group of queues. The time-stamp is an indicator of the time at which the packet was enqueued relative to other enqueued packets. At any given forwarding clock cycle, the packet with the earliest time-stamp, i.e., the first arrived packet, at the heads of the queues is forwarded. 
     One advantage of the invention is that the fairness policy automatically adjusts to fluctuations in traffic patterns. Time-stamps are a form of packet weighting. However, unlike the predetermined queue weighting of WFQ, time-stamps provide a relative weight that changes with transmission rate because the weight of each time-stamp is dependent on the transmission rates of all of the inputs. 
     Another advantage of the invention is cost savings in implementation. Those skilled in the art will recognize that a single FIFO queue could be employed rather than a group of queues to achieve the intended result of proportional forwarding. The single FIFO queue would queue packets in the order in which they were received and forward the earliest queued packet in a given clock cycle. However, the memory device for implementing such a single FIFO queue would be required to write at a rate at least as great as the sum of the rates of the inputs in order to avoid dropping packets arriving contemporaneously from different inputs. Generally, memory devices capable of functioning at higher read/write rates are more costly than devices which function at lower rates. The present invention enables utilization of less costly memory devices to achieve the same desired result. In particular, because multiple queues are employed the invention enables use of memory devices having a write speed equal to the maximum rate of ingress ports and a read speed equal to the maximum rate of the egress interface. Consequently, cost savings can be realized by utilizing lower read/write rate memory devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only. 
         FIG. 1  is a block diagram of a network node in which time-stamp scheduling is employed. 
         FIG. 2  illustrates the queues and forwarding path of the line card of  FIG. 1  in greater detail. 
         FIG. 3  illustrates processing of packets in accordance with the architecture illustrated in  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring to  FIGS. 1 and 2 , time-stamp scheduled queuing is implemented in a line card associated with a switch/router. The switch/router includes a plurality of line cards  10 - 1  through  10 -N which are interconnected via a fabric or cross-connect  12 . Each line card includes a plurality of ingress ports  1 -N which receive traffic from other network nodes. The line cards are operative to, among other things, multiplex incoming traffic such as packets received on the associated ingress ports for subsequent transmission on an egress interface  14  to the fabric. It should be noted that for each line card  10  the sum of the maximum transmission rates over all ingress ports  1 -N is greater than the maximum rate of the egress interface  14 . Hence, congestion can potentially occur at the egress interface. 
     The line cards each include a separate queue  16  for each ingress port. In particular, queues  16 - 1  through  16 -N are coupled with ingress ports  1 -N on a 1:1 basis. The queues are implemented on one or more memory devices. In each queue, packets are moved from the input end to the head in the order in which they are received, i.e. First In First Out (“FIFO”). Hence, the packets in a queue at any given time are arranged in chronological order with the earliest enqued packet at the head of the queue. 
     A clock circuit  18  is provided to assign time-stamps  20  to packets  22  as the packets are enqueued. In particular, the time stamp is appended to the packet and stored in the queue with the associated packet as the packet is enqueued. The clock may employ various means of time-stamping, but a simple counter may suffice. The counter has a value at any given time which is incremented (or decremented, depending on the convention used) at each forwarding clock cycle, eventually rolling over and restarting. The size of the counter, i.e., the maximum count, is selected such that earlier and later arriving packets will not be assigned the same time-stamp due to counter rollover. 
     A time-stamp scheduler  24  is employed to multiplex the enqueued packets based on their associated time-stamps. In particular, the time stamp scheduler is operative to select the earliest enqueued packet at the heads of the queues  16 - 1  through  16 -N. The earliest enqueued packet may be selected for each forwarding clock cycle by comparing the time-stamps associated with the packets at the heads of the queues to determine which of those packets has the lowest time-stamp counter number, i.e., the earliest time-stamp (or the highest counter number depending on the convention used). The selected packet is then forwarded via the egress interface. 
     The result of the above described technique is that for each non-empty queue during a given time interval I, the number of packets dropped during the time interval PD n (I) divided by the number of packets enqueued during the time interval PE n (I) is at least approximately proportionally equal for each of those non-empty queues, i.e., PD n (I)/PE n (I)≈PPD n (I) for each non-empty queue. This is true for non-empty queues because packets are not dropped for empty queues. Further, while PPD n (I) may be equal for each non-empty queue, PPD n (I) may differ slightly between non-empty queues depending on the duration, start point and stop point of the interval measured relative to queue activity as will be described below. 
       FIG. 3  illustrates processing of packets in accordance with the architecture illustrated in  FIGS. 1 and 2 . The queues  16 - 1  through  16 -N are shown in a simplified form and in practice the number and depth of the queues may differ significantly from the illustrated example. Further, although time-stamps TS 1 -TS 19  are shown in decimal it may be more practical to implement the time-stamps in binary. At a time t=0, queue  16 -N has four queued packets, queue  16 - 3  has five queued packets, queue  16 - 2  has two queued packets, and queue  16 - 1  has four queued packets. Each queued packet has been associated with a time-stamp TSx indicating the time at which the packet was enqueued relative to the other enqueued packets. In particular, a lower numbered time-stamp, e.g., TS 1 , indicates an earlier enqueued packet than a higher numbered time-stamp, e.g., TS 15 . In or before a first forwarding clock cycle the time-stamps of the packets at the heads of the queues are compared to select the earliest enqueued packet. As between the packets at the heads of the queues which have the time-stamps “TS 1 ,” “TS 8 ,” “TS 3 ,” and “TS 4 ,” the packet having the time-stamp of “TS 1 ” has the lowest number and hence is the earliest enqueued packet. Consequently, the packet at the head of queue  16 -N having the timestamp “TS 1 ” is selected and forwarded  30 . The remaining packets in queue  16 -N are then advanced such that the packet having the time-stamp “TS 2 ” is at the head of queue  16 -N. 
     In or before a second forwarding clock cycle the time-stamps of the packets at the heads of the queues are again compared. The packet at the head of queue  16 -N having the time-stamp “TS 2 ” is selected and forwarded  32 . The remaining packets in queue  16 -N are then advanced. In or before a third forwarding clock cycle the time-stamps of the packets at the heads of the queues are again compared. Now, the packet at the head of queue  16 - 2  having the time-stamp “TS 3 ” is selected and forwarded  34 . A newly enqueued packet in queue  16 - 2  is assigned the timestamp “TS 19 ,” indicating the count of the counter when the packet is enqueued. It will be noted that the time-stamps may not necessarily be contiguous in number. Operation in subsequent forwarding clock cycles proceeds in similar a manner. 
     In view of the illustrated example, it will be recognized by those skilled in the art that the described invention will proportionally equally drop packets of each queue if congestion occurs. In particular, the invention will drop a nearly equal percentage of packets from each queue in a given period from when congestion occurs to when congestion ends. Because it is not practical to drop only a portion of a packet or precisely control the duration of congestion, the term “proportionally equal” as used in this application implies equality within a range of tolerance that is mathematically and practically inherent in the inventive concept. Further, it will be recognized that the existence of theoretically extreme cases, such as where one queue is empty for the duration of congestion and hence has zero packets dropped while packets from other queues are dropped, are encompassed within the range of mathematical and practical tolerance inherent in the concept. 
     In view of the description above, it will be understood by those of ordinary skill in the art that modifications and variations of the described and illustrated embodiments may be made within the scope of the inventive concepts. Moreover, while the invention is described in connection with various illustrative structures, those of ordinary skill in the art will recognize that the invention may be employed with other structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.