Abstract:
A network device for dynamically allocating memory locations to plurality of queues. The network device includes means for determining an amount of memory buffers that is associated with a port and means for assigning a fixed allocation of memory buffers to each of a plurality of queues associated with the port. The network device also includes means for sharing remaining memory buffers among the plurality of queues, wherein the remaining memory buffers are used by at least one of the plurality of queues after the fixed allocation of memory buffers assigned to the queue is used by the queue. The network device further includes means for setting a limit threshold for each of the plurality of queues. The limit threshold determines how much of the remaining memory buffer may be used by each of the plurality of queues. When one of the limit threshold is reached for one of the plurality of queues or all of the remaining buffers are used, a request by the one of the plurality of queues is denied.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a network device in a packet switched network and more particularly to a method of dynamically sharing a memory location across all of the port associated with the network device.  
         [0003]     2. Description of the Related Art  
         [0004]     A packet switched network may include one or more network devices, such as a Ethernet switching chip, each of which includes several modules that are used to process information that is transmitted through the device. Specifically, the device includes an ingress module, a Memory Management Unit (MMU) and an egress module. The ingress module includes switching functionality for determining to which destination port a packet should be directed. The MMU is used for storing packet information and performing resource checks. The egress module is used for performing packet modification and for transmitting the packet to at least one appropriate destination port. One of the ports on the device may be a CPU port that enables the device to send and receive information to and from external switching/routing control entities or CPUs.  
         [0005]     As packets enter the device from multiple ports, they are forwarded to the ingress module where switching and other processing are performed on the packets. Thereafter, the packets are transmitted to one or more destination ports through the MMU and the egress module. The MMU enables sharing of packet buffer among different ports while providing resource guarantees for every ingress port, egress port and class of service queue. According to a current switching system architecture, eight class of service queues are associated with each egress port. To ensure bandwidth guarantees across the ports and queues, the device allocates a fixed portion of the memory for the port to each queue. As such, a queue that is associated with a class of service with a high priority may be assigned a greater fixed portion than a queue that is associated with a lower priority class of service. This implementation is inflexible and does not account for dynamic requirements that may be associated with one or more queues.  
       SUMMARY OF THE INVENTION  
       [0006]     According to one aspect of the invention, there is provided a network device for dynamically allocating memory locations to plurality of queues. The network device includes means for determining an amount of memory buffers that is associated with a port and means for assigning a fixed allocation of memory buffers to each of a plurality of queues associated with the port. The network device also includes means for sharing remaining memory buffers among the plurality of queues, wherein the remaining memory buffers are used by at least one of the plurality of queues after the fixed allocation of memory buffers assigned to the queue is used by the queue. The network device further includes means for setting a limit threshold for each of the plurality of queues. The limit threshold determines how much of the remaining memory buffer may be used by each of the plurality of queues. When one of the limit threshold is reached for one of the plurality of queues or all of the remaining buffers are used, a request by the one of the plurality of queues is denied.  
         [0007]     According to another aspect of the invention, there is provided a method for dynamically allocating memory locations to plurality of queues. The method includes the steps of determining an amount of memory buffers that is associated with a port and assigning a fixed allocation of memory buffers to each of a plurality of queues associated with the port. The method also includes the step of sharing remaining memory buffers among the plurality of queues after the fixed allocation of memory buffers assigned to each of the plurality of queues is used by the queue. The method further includes the steps of setting a limit threshold for each of the plurality of queues and denying a request by the one of the plurality of queues when one of the limit threshold is reached for one of the plurality of queues or all of the remaining buffers are used. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention that together with the description serve to explain the principles of the invention, wherein:  
         [0009]      FIG. 1  illustrates a network device in which an embodiment of the present invention may be implemented;  
         [0010]      FIG. 2   a  illustrates the shared memory architecture of the present invention;  
         [0011]      FIG. 2   b  illustrates the Cell Buffer Pool of the shared memory architecture;  
         [0012]      FIG. 3  illustrates buffer management mechanisms that are used by the memory management unit to impose resource allocation limitations and thereby ensure fair access to resource;  
         [0013]      FIG. 4  illustrates a configuration of an egress port arbitration implemented in the present invention;  
         [0014]      FIG. 5  illustrates the implementation of the minimum and maximum bandwidth metering mechanisms; and  
         [0015]      FIG. 6  illustrates an embodiment in which four queues are serviced according to their minimum bandwidth specifications. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]     Reference will now be made to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0017]      FIG. 1  illustrates a network device, such as a switching chip, in which an embodiment the present invention may be implemented. Device  100  includes an ingress module  102 , a MMU  104 , and an egress module  106 . Ingress module  102  is used for performing switching functionality on an incoming packet. The primary function of MMU  104  is to efficiently manage cell buffering and packet pointer resources in a predictable manner even under severe congestion scenarios. Egress module  106  is used for performing packet modification and transmitting the packet to an appropriate destination port.  
         [0018]     Device  100  may also include one internal fabric high speed port, for example a HiGig port,  108 , one or more external Ethernet ports  109   a - 109   x,  and a CPU port  110 . High speed port  108  is used to interconnect various network devices in a system and thus form an internal switching fabric for transporting packets between external source ports and one or more external destination ports. As such, high speed port  108  is not externally visible outside of a system that includes multiple interconnected network devices. CPU port  110  is used to send and receive packets to and from external switching/routing control entities or CPUs. According to an embodiment of the invention, CPU port  110  may be considered as one of external Ethernet ports  109   a - 109   x.  Device  100  interfaces with external/off-chip CPUs through a CPU processing module  111 , such as a CMIC, which interfaces with a PCI bus that connects device  100  to an external CPU.  
         [0019]     Network traffic enters and exits device  400  through external Ethernet ports  109   a - 109   x.  Specifically, traffic in device  100  is routed from an external Ethernet source port to one or more unique destination Ethernet ports. In one embodiment of the invention, device  100  supports twelve physical Ethernet ports  109 , each of which can operate in 10/100/1000 Mbps speed and one high speed port  108  which operates in either 10 Gbps or 12 Gbps speed.  
         [0020]     In an embodiment of the invention, device  100  is built around a shared memory architecture, as shown in  FIGS. 2   a - 2   b  wherein MMU  104  enables sharing of a packet buffer among different ports while providing for resource guarantees for every ingress port, egress port and class of service queue associated with each egress port.  FIG. 2   a  illustrates the shared memory architecture of the present invention. Specifically, the memory resources of device  100  include a Cell Buffer Pool (CBP) memory  202  and a Transaction Queue (XQ) memory  204 . CBP memory  202  is an off chip resource that is made of 4 DRAM chips  206   a - 206   d.  According to an embodiment of the invention, each DRAM chip has a capacity of 288 Mbits, wherein the total capacity of CBP memory  202  is 122 Mbytes of raw storage. As shown in  FIG. 2   b,  CBP memory  202  is divided into 256K 576-byte cells  208   a - 208   x,  each of which includes a 32 byte header buffer  210 , up to 512 bytes for packet data  212  and 32 bytes of reserved space  214 . As such, each incoming packet consumes at least one full 576 byte cell  208 . Therefore in an example where an incoming includes a 64 byte frame, the incoming packet will have 576 bytes reserved for it even though only 64 bytes of the 576 bytes is used by the frame.  
         [0021]     Returning to  FIG. 2   a,  XQ memory  204  includes a list of packet pointers  216   a - 216   x  into CBP memory  202 , wherein different XQ pointers  216  may be associated with each port. A cell count of CBP memory  202  and a packet count of XQ memory  204  is tracked on an ingress port, egress port and class of service basis. As such, device  100  can provide resource guarantees on a cell and/or packet basis.  
         [0022]     Once a packet enters device  100  on a source port  109 , the packet is transmitted to ingress module  102  for processing. During processing, packets on each of the ingress and egress ports share system resources  202  and  204 .  FIG. 3  illustrates buffer management mechanisms that are used by MMU  104  to impose resource allocation limitations and thereby ensure fair access to resources. MMU  104  includes an ingress backpressure mechanism  304 , a head of line mechanism  306  and a weighted random early detection mechanism  308 . Ingress backpressure mechanism  304  supports lossless behaviour and manages buffer resources fairly across ingress ports. Head of line mechanism  306  supports access to buffering resources while optimizing throughput in the system. Weighted random early detection mechanism  308  improves overall network throughput.  
         [0023]     Ingress backpressure mechanism  304  uses packet or cell counters to track the number of packets or cells used on an ingress port basis. Ingress backpressure mechanism  304  includes registers for a set of 8 individually configurable thresholds and registers used to specify which of the 8 thresholds are to be used for every ingress port in the system. The set of thresholds include a limit threshold  312 , a discard limit threshold  314  and a reset limit threshold  316 . If a counter associated with the ingress port packet/cell usage rises above discard limit threshold  314 , packets at the ingress port will be dropped. Based on the counters for tracking the number of cells/packets, a pause flow control is used to stop traffic from arriving on an ingress port that have used more than its fair share of buffering resources, thereby stopping traffic from an offending ingress port and relieving congestion caused by the offending ingress port. Specifically, each ingress port keeps track of whether or not it is in an ingress backpressure state based on ingress backpressure counters relative to the set of thresholds. When the ingress port is in ingress backpressure state, pause flow control frames with a timer value of (0×FFFF) are periodically sent out of that ingress port. When the ingress port is no longer in the ingress backpressure state, the pause flow control frame with a timer value of 0×00 is sent out of the ingress port and traffic is allowed to flow again. If an ingress port is not currently in an ingress backpressure state and the packet counter rises above limit threshold  312 , the status for the ingress port transitions into the ingress backpressure state. If the ingress port is in the ingress backpressure state and the packet counter falls below reset limit threshold  316 , the status for the port will transition out of the backpressure state.  
         [0024]     Head of line mechanism  306  is provided to support fair access to buffering resources while optimizing throughput in the system. Head of line mechanism  306  relies on packet dropping to manage buffering resources and improve the overall system throughput. According to an embodiment of the invention, head of line mechanism  306  uses egress counters and predefined thresholds to track buffer usage on a egress port and class of service basis and thereafter makes decisions to drop any newly arriving packets on the ingress ports destined to a particular oversubscribed egress port/class of service queue. Head of line mechanism  306  supports different thresholds depending on the color of the newly arriving packet. Packets may be colored based on metering and marking operations that take place in the ingress module and the MMU acts on these packets differently depending on the color of the packet.  
         [0025]     According to an embodiment of the invention, head of line mechanism  306  is configurable and operates independently on every class of service queue and across all ports, including the CPU port. Head of line mechanism  306  uses counters that track XQ memory  204  and CBP memory  202  usage and thresholds that are designed to support a static allocation of CBP memory buffers  202  and dynamic allocation of the available XQ memory buffers  204 . A discard threshold  322  is defined for all cells in CBP memory  202 , regardless of color marking. When the cell counter associated with a port reaches discard threshold  322 , the port is transition to a head of line status. Thereafter, the port may transition out of the head of line status if its cell counter falls below a reset limit threshold  324 .  
         [0026]     For the XQ memory  204 , a guaranteed fixed allocation of XQ buffers for each class of service queue is defined by a XQ entry value  330   a - 330   h.  Each of XQ entry value  330   a - 330   h  defines how many buffer entries should be reserved for an associated queue. For example, if 100 bytes of XQ memory are assigned to a port, the first four class of service queues associated with XQ entries  330   a - 330   d  respectively may be assigned the value of 10 bytes and the last four queues associated with XQ entries  330   d - 330   h  respectively may be assigned the value of 5 bytes. According to an embodiment of the invention, even if a queue does not use up all of the buffer entries reserved for it according to the associated XQ entry value, head of line mechanism  306  may not assign the unused buffer to another queue. Nevertheless, the remaining unassigned 40 bytes of XQ buffers for the port may be shared among all of the class of service queues associated with the port. Limits on how much of the shared pool of the XQ buffer may be consumed by a particular class of service queue is set with a XQ set limit threshold  332 . As such, set limit threshold  332  may be used to define the maximum number of buffers that can be used by one queue and to prevent one queue from using all of the available XQ buffers. To ensure that the sum of XQ entry values  330   a - 330   h  do not add up to more than the total number of available XQ buffers for the port and to ensure that each class of service queue has access to its quota of XQ buffers as assigned by its entry value  330 , the available pool of XQ buffer for each port is tracked using a port dynamic count register  334 , wherein dynamic count register  334  keeps track of the number of available shared XQ buffers for the port. The initial value of dynamic count register  334  is the total number of XQ buffers associated with the port minus a sum of the number of XQ entry values  320   a - 320   h.  Dynamic count register  334  is decremented when a class of service queue uses an available XQ buffer after the class of service queue has exceeded its quota as assigned by its XQ entry value  330 . Conversely, dynamic count register  334  is incremented when a class of service queue releases a XQ buffer after the class of service queue has exceeded its quota as assigned by its XQ entry value  330 .  
         [0027]     When a queue requests XQ buffer  204 , head of line mechanism  306  determines if all entries used by the queue is less than the XQ entry value  330  for the queue and grants the buffer request if the used entries are less then the XQ entry value  330 . If however, the used entries are greater than the XQ entry value  330  for the queue, head of line mechanism  306  determines if the amount requested is less than the total available buffer or less then the maximum amount set for the queue by the associated set limit threshold  332 . Set limit threshold  332  is in essence a discard threshold that is associated with the queue, regardless of the color marking of the packet. As such, when the packet count associated with the packet reaches set limit threshold  332 , the queue/port enters into a head of line status. When head of line mechanism  306  detects a head of line condition, it sends an update status so that ingress module  102  can drop packets on the congested port. However, due to latency, there may be packets in transition between ingress module  102  and MMU  104  when the status update is sent by head of line mechanism  306 . In this case, the packet drops may occur at MMU  104  due to the head of line status. In an embodiment of the invention, due to the pipeline of packets between ingress module  102  and MMU  104 , the dynamic pool of XQ pointers is reduced by a predefined amount. As such, when the number of available XQ pointers is equal to or less than the predefined amount, the port is transition to the head of line status and an update status is sent to by MMU  104  to ingress module  102 , thereby reducing the number of packets that may be dropped by MMU  104 . To transition out of the head of line status, the XQ packet count for the queue must fall below a reset limit threshold  336 .  
         [0028]     It is possible for the XQ counter for a particular class of service queue to not reach set limit threshold  332  and still have its packet dropped if the XQ resources for the port are oversubscribed by the other class of service queues. In an embodiment of the invention, intermediate discard thresholds  338  and  339  may also be defined for packets containing specific color markings, wherein each intermediate discard threshold defines when packets of a particular color should be dropped. For example, intermediate discard threshold  338  may be used to define when packets that are colored yellow should be dropped and intermediate discard threshold  339  may be used to define when packets that are colored red should be dropped. According to an embodiment of the invention, packets may be colored one of green, yellow or red depending on the priority level assigned to the packet. To ensure that packets associated with each color are processed in proportion to the color assignment in each queue, one embodiment of the present invention includes a virtual maximum threshold  340 . Virtual maximum threshold  340  is equal to the number of unassigned and available buffers divided by the sum of the number of queues and the number of currently used buffers. Virtual maximum threshold  340  ensures that the packets associated with each color are processed in a relative proportion. Therefore, if the number of available unassigned buffers is less than the set limit threshold  332  for a particular queue and the queue requests access to all of the available unassigned buffers, head of line mechanism  306  calculates the virtual maximum threshold  340  for the queue and processes a proportional amount of packets associated with each color relative to the defined ratios for each color.  
         [0029]     To conserve register space, the XQ thresholds may be expressed in a compressed form, wherein each unit represents a group of XQ entries. The group size is dependent upon the number of XQ buffers that are associated with a particular egress port/class of service queue.  
         [0030]     Weighted random early detection mechanism  308  is a queue management mechanism that pre-emptively drops packets based on a probabilistic algorithm before XQ buffers  204  are exhausted. Weighted random early detection mechanism  308  is therefore used to optimize the overall network throughput. Weighted random early detection mechanism  308  includes an averaging statistic that is used to track each queue length and drop packets based on a drop profile defined for the queue. The drop profile defines a drop probability given a specific average queue size. According to an embodiment of the invention, weighted random early detection mechanism  308  may defined separate profiles on based on a class of service queue and packet.  
         [0031]      FIG. 4  illustrates a configuration of an egress port arbitration implemented in the present invention. According to  FIG. 4 , MMU  104  also includes a scheduler  402  that provides arbitration across the eight class of service queues  404   a - 404   h  associated with each egress port to provide minimum and maximum bandwidth guarantees. Scheduler  402  is integrated with a set of minimum and maximum metering mechanisms  406   a - 406   i  that monitor traffic flows on a class of service basis and an overall egress port basis. Metering mechanisms  406   a - 406   i  support traffic shaping functions and guarantee minimum bandwidth specifications on a class of service queue and/or egress port basis, wherein scheduling decisions by schedule  402  are configured largely via traffic shaping mechanisms  406   a - 406   h  along with a set of control masks that modify how scheduler  402  uses traffic shaping mechanisms  406   a - 406   h.    
         [0032]     As shown in  FIG. 4 , minimum and maximum metering mechanisms  406   a - 406   i  monitor traffic flows on a class of service queue basis and an overall egress port basis. Maximum and minimum bandwidth meters  406   a - 406   h  are used to feed state information to scheduler  402  which responds by modifying its service order across class of service queues  404 . The inventive device  100  therefore enables system vendors to implement a quality of service model by configuring class of service queues  404  to support an explicit minimum and maximum bandwidth guarantee. In an embodiment of the invention, metering mechanisms  406   a - 406   h  monitor traffic flow on a class of service queue basis, provides state information regarding whether or nor a class of service flow is above or below a specified minimum and maximum bandwidth specification, and transmits the information into scheduler  402  which uses the metering information to modify its scheduling decisions. As such, metering mechanisms  406   a - 406   h  aid in partitioning class of service queues  404  into a set of queues that have not met the minimum bandwidth specification, a set that have met its minimum bandwidth but not its maximum bandwidth specification and a set that have exceeded its maximum bandwidth specification. If a queue is in the set that have not met its minimum bandwidth specification and there are packets in the queue, scheduler  402  services the queue according to the configured scheduling discipline. If a queue is in the set that have met its minimum bandwidth specification but has not exceeded it maximum bandwidth specification and there are packets in the queue, scheduler  402  services the queue according to the configured scheduling discipline. If a queue is in the set that have exceeded its maximum bandwidth specification or if the queue is empty, scheduler  402  does not service the queue.  
         [0033]     In an embodiment of the invention, as illustrated in  FIG. 5 , minimum and maximum bandwidth metering mechanisms  406   a - 406   h  are implemented using a simple leaky bucket mechanism which tracks whether or not a class of service queue  404  has consumed its minimum or maximum bandwidth. The range of the minimum and maximum bandwidth setting for each class of service  404  is between 64 kbps to 16 Gbps, in 64 kbps increments. The leaky bucket mechanism has a configurable number of tokens “leaking” out of bucket  502   a - 502   h,  each of which is associated with one of queues  404   a - 404   h,  at a configurable rate. In metering the minimum bandwidth for a class of service queue  404 , as packets enter the class of service queue  404 , a number of tokens in proportion to the size of the packet is added to bucket  502 , with a ceiling of bucket high threshold  504 . The leaky bucket mechanism includes a refresh update interface and a minimum bandwidth  506  which defines how many tokens are to be removed every refresh time unit. A minimum threshold  508  is set to indicate whether a flow has satisfied at least its minimum rate and a fill threshold  510  is set to indicate how many tokens are in leaky bucket  502 . When fill threshold  510  rises above minimum threshold  508 , a flag which indicates that the flow has satisfied its minimum bandwidth specification is set to true. When fill threshold  510  falls below minimum threshold  508 , the flag is set to false.  
         [0034]     Minimum threshold  508  affects what timescale the minimum bandwidth metering mechanism  406  is required to operate. If the minimum threshold  508  is set at a very low level, class of service queue  404  will quickly flag that its minimum bandwidth has been met. This reduces the amount of time queue  404  is classified in the set of queues that have not met the minimum bandwidth requirement and reduces the time period that the queue is given preferential treatment from scheduler  402 . High threshold  504  affects how much credit can be built up after a class of service queue meets it minimum bandwidth  506 . A large high threshold  504  may result in a reduction of time that the queue is classified with the set of queues that have not met the minimum bandwidth requirement and reduces the time period that the queue is given preferential treatment from scheduler  402 .  
         [0035]     After metering mechanisms  406   a - 406   h  indicate that the maximum bandwidth specified has been exceeded high threshold  504 , scheduler  402  ceases to service the queue and the queue is classified as being in the set of queues that have exceeded it maximum bandwidth specification. A flag is then set to indicate that the queue has exceeded its maximum bandwidth. Thereafter, the queue will only receive service from scheduler  402  when its fill threshold  510  falls below high threshold  504  and the flag indicating that it has exceeded its maximum bandwidth is reset. Metering mechanism  406   i  is used to indicate that the maximum bandwidth specified for a port has been exceeded and operates in the same manner as meter mechanisms  406   a - 406   h  when the maximum bandwidth has been exceeded. According to an embodiment of the invention, the maximum metering mechanism on a queue and port basis generally affects whether or not queue  404  or a port is to be included in scheduling arbitration. As such, the maximum metering mechanism only has a traffic limiting effect on scheduler  402 .  
         [0036]     On the other hand, minimum metering on a class of service queue  404  basis has a more complex interaction with scheduler  402 . In one embodiment of the invention, scheduler  402  is configured to support a variety of scheduling disciplines that mimic the bandwidth sharing capabilities of a weighted fair queuing scheme. The weighted fair queue scheme is a weighted version of packet based fair queuing scheme, which is defined as a method for providing “bit-based round robin” scheduling of packets. As such, packets are scheduled for access to an egress port based on their delivery time, which is computed as if the scheduler is capable of providing bit-based round robin service. A relative weight field influences the specifics of how the scheduler makes use of the minimum metering mechanism, wherein the scheduler attempts to provide a minimum bandwidth guarantee. In an embodiment of the invention, the minimum bandwidth guarantee is a relative bandwidth guarantee wherein a relative field determines whether or not scheduler  402  will treat the minimum bandwidth metering settings as a specification for a relative or an absolute bandwidth guarantee. If the relative field is set, the scheduler treats minimum bandwidth  506  setting as a relative bandwidth specification. Scheduler  402  then attempts to provide relative bandwidth sharing across backlogged queues  404 .  
         [0037]      FIG. 6  illustrates an embodiment in which four queues are serviced according to their minimum bandwidth specifications. According to  FIG. 6 , a 1 GE egress port has scheduler  402  configured to be in a weighted fair queue mode and has its relative field set to true, wherein the minimum bandwidth for queue  602   a  and  602   b  is 10 Mbps, for queue  602   c  is 20 Mbps and for queue  602   d  is 40 Mbps. If all queues  602  have packets to be serviced then scheduler  402  will provide relative bandwidth sharing across the active queues according to the predefined minimum bandwidth for each queue. However, since as mentioned above only queues  602   a - 602   d  have packets to be serviced, queue  602   d  will receive twice the bandwidth of queue  602   c  which receives twice the bandwidth that is given to queues  602   a  and  602   b.  If queues  602   a - 602   d  have enough packets to keep the 1 GE link fully utilized, queue  602   d  will be allowed to process 500 Mbps, queue  602   c  will be allowed to process 250 Mbps and queues  602   a  and  602   b  will be allowed to process 125 Mbps. On the other hand, if only queues  602   b - 602   d  are active, the bandwidth distribution will change to appropriately provide relative bandwidth sharing, wherein queue  602   d  will be allowed to process 571.4 Mbps, queue  602   c  will be allowed to process 265.7 Mbps and queue  602   b  will be allowed to process 142.9 Mbps. As such, minimum bandwidth metering mechanisms  406  are constantly being adjusted to achieve the relative bandwidth sharing.  
         [0038]     Returning to  FIG. 5 , according to an embodiment of the invention, in addition to the relative field, a relative threshold  514  is also set in each of queues  404 . Relative threshold  514  is used to indicate that the minimum bandwidth  506  is set too low when fill threshold  510  of all queues have exceeded relative threshold  514 . As such, when fill threshold  510  for each of queues  404   a - 404   h  rises above relative threshold  514 , device  100  calculates a new minimum bandwidth  506 , wherein: 
 
new minimum bandwidth=old minimum bandwidth&lt;&lt;(K−MSB.POS) 
        wherein K is equal to a constant, and     MSB.POS is equal to a position of the Most Significant Bit        
 
         [0041]     The new minimum bandwidth therefore allows device  100  to leak more tokens out of bucket  502  for each of queues  404   a - 404   h,  wherein the new leak is proportional to the old leak. According to another embodiment of the invention, the new minimum bandwidth may be calculated for an individual queue when fill threshold for that queue rises above relative threshold  514  for that queue.  
         [0042]     The foregoing description has been directed to specific embodiments of this invention. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.