Abstract:
A dedicated bandwidth switch backplane has efficient receive processing capable of handling highly parallel traffic. Packets must pass a filtering check and a watermark check before the receive port is allowed to release them to a queue. Highly efficient algorithms are applied to conduct the checks on the packets in a way which expedites receive processing and avoids contention. A hybrid priority/port-based arbitration algorithm is used to sequence filtering checks on pending packets. A watermark comparison algorithm performs preliminary calculations on the current packet using “projected” output queue write addresses for each possible outcome of the queueing decision on the preceding packet and using the actual outcome to select from among preliminary calculations to efficiently address the outcome-dependence of the current packet&#39;s watermark check on the queueing decision made on the preceding packet. Receive ports are operatively divided into full-write receive ports and selective-write receive ports for delivering their packets to the output queue. On the clock cycles where the selective-write receive port is assigned writing privileges, data is read from the queue, unless the selective-write receive port has indicated it wishes to write to the queue, in which case the selective-write receive port writes to the queue. The full-write receive ports always write data, if available, to the queue on the clock cycles where they are assigned writing privileges.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to data communication switching, and more particularly to receive processing in data communication switching architectures of the type which switch packets over dedicated transmission lines between a plurality of switching controllers.  
           [0002]    Local area network (LAN) switches generally perform a switching function on an internal backplane operative between switching controllers supporting external network devices. Such switching has typically been accomplished by configuring the backplane as a shared packet bus and granting the switching controllers having packets for transmission control of the backplane in time-multiplexed fashion. A conventional LAN switch backplane is illustrated in FIG. 1. LAN switch  10  includes switching controllers  110 ,  120 ,  130 ,  140  performing a switching function by transmitting and receiving packets over shared packet bus  150 . Time-multiplexing is known to have been accomplished in such conventional LAN switches several ways. One way is assigning the controllers different clock cycles within a repetitive timing cycle and granting control of the bus to the controllers round-robin in accordance with their assigned clock cycles. Another way involves conducting a priority-based arbitration among the controllers having packets for transmission and granting control of the bus to the controller which “wins” the arbitration. Regardless of which bus control strategy is favored, reliance on a shared packet bus, and its inherent requirement of time-multiplexing packets for release to guarantee contention-free transmission, has led to congestion at the transmit side of the bus and inefficient use of receive processing resources. For example, unicast packets transmitted across the backplane are destined for a network device supported by only one of the controllers. However, where the backplane is a shared packet bus, all controllers must wait for a unicast packet to clear the backplane before a subsequent packet can be transmitted. This often results in clock cycles in which the receive processing resources of many controllers are idle, even while congestion may be developing at the transmit side of the backplane.  
           [0003]    A more efficient approach would obviate the need to time-multiplex data for release across the backplane and, under normal operating conditions, would allow all packets awaiting transmission across the backplane to be forwarded without delay. However, to reap the full benefit of such “on the fly” transmission requires receive processing resources capable of efficiently handling parallel traffic. Otherwise, the conventional problem of underutilization of receive processing resources and transmit side congestion may inadvertently become one of overutilization of receive processing resources and receive side congestion.  
         SUMMARY OF THE INVENTION  
         [0004]    In its most basic feature, the present invention eliminates the inefficiencies of shared bandwidth switch backplanes by implementing a dedicated bandwidth switch backplane having efficient receive processing capable of handling highly parallel traffic. The contemplated switching architecture has a plurality of switching controllers for transmitting and receiving packets across a backplane, with each controller having a transmit interface, a receive interface and filtering logic. The backplane includes a dedicated transmission line for each transmit interface such that all transmit interfaces may simultaneously propagate data bursts to all receive interfaces. Each receive interface includes a dedicated receive port for each transmission line and an output queue. Packets must pass a filtering check and a watermark check before the receive port is allowed to release them to the output queue. Highly efficient algorithms are applied to conduct the checks on the packets in a way which expedites receive processing and avoids contention.  
           [0005]    In one aspect of efficient receive processing, a hybrid priority/port-based arbitration algorithm is used to sequence filtering checks on packets. The hybrid algorithm prioritizes and sequences packets according to how soon their receive port would be able to begin delivering them to the output queue in the event the filtering check were allowed to proceed and the filtering and watermark checks were passed; however, the hybrid algorithm assigns all packets for which delivery could not begin within a threshold number of clock cycles the lowest priority and filtering checks are sequenced on such low priority packets according to their receive port.  
           [0006]    In another aspect of efficient receive processing, the outcome-dependence of the current packet&#39;s watermark check on the queueing decision made on the preceding packet is addressed efficiently by a watermark comparison algorithm which performs preliminary calculations using “projected” output queue write addresses for each possible outcome of the queueing decision on the preceding packet and using the actual outcome, when available, to select from among preliminary calculations.  
           [0007]    In another aspect of efficient receive processing, if a packet passes the filtering check but fails the watermark check, a stall condition is triggered to restrict the transmission of additional packets to the packet&#39;s receive port until the watermark check is passed.  
           [0008]    In another aspect of efficient receive processing, receive ports are operatively divided into one or more full-write receive ports and one or more selective-write receive ports for delivering their packets to the output queue. The full-write receive ports always write data, if available, to the queue on the clock cycles during which they are assigned writing privileges. On the clock cycles during which the selective-write receive ports are assigned writing privileges, data is read from the queue, unless the selective-write receive ports have indicated they wish to write to the queue, in which case the selective-write receive ports write to the queue. By configuring relatively low-traffic ports as selective-write ports, dequeueing may accomplished during unutilized “write” clock cycles, obviating the need to designate “read only” clock cycles.  
           [0009]    These and other aspects of the present invention can be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings which are briefly described below. Of course, the actual scope of the invention is defined by the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagram of a prior art LAN switching backplane;  
         [0011]    [0011]FIG. 2 is a block diagram of a LAN switching backplane with dedicated transmission lines operative;  
         [0012]    [0012]FIG. 3 is a block diagram of a switching controller operative with the LAN switching backplane according to FIG. 2;  
         [0013]    [0013]FIG. 4 is a block diagram of a receive interface operative within such a switching controller;  
         [0014]    [0014]FIG. 5 is a flow diagram illustrating a filtering check arbitration algorithm operative within such a receive interface;  
         [0015]    [0015]FIG. 6 is a block diagram of the interfaces operative between a receive port and an input/output server for exchanging signals within such a receive interface;  
         [0016]    [0016]FIG. 7 is a flow diagram of a watermark comparison algorithm operative within such a receive interface;  
         [0017]    [0017]FIG. 8 is a block diagram of an output queue operative within such a receive interface; and  
         [0018]    [0018]FIG. 9 is a timing diagram illustrating receive processing timing within such a receive interface. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    The present invention applies to a dedicated bandwidth switching architecture such as the LAN switching architecture shown in FIG. 2. The preferred connectivity pattern includes unidirectional data transmission lines  211 - 219  assigned to switching controllers  201 - 209 , respectively. Each of switching controllers  201 - 209  transmits packets on its dedicated one of transmission lines  211 - 219  and receives packets from all controllers on transmission lines  211 - 219 . Thus, on any particular clock cycle, all controllers  201 - 209  may transmit a single data burst and may receive a plurality of data bursts. It will be appreciated that by dedicating a transmission line to each one of controllers  201 - 209 , the need to transfer data across backplane  200  at a reduced rate is obviated and, under normal operating conditions, all packets for transmission across backplane  200  may be forwarded without delay. Of course, the number of controller/transmission line pairs will vary depending on network requirements. However, provided the number is greater than one, efficient receive processing resources must be implemented in the controllers to process the parallel traffic. In a preferred embodiment of the invention, receive processing resources suitable for handling parallel traffic are provided and described hereinafter in greater detail.  
         [0020]    Turning now to FIG. 3, a preferred switching controller  300  is shown. Switching controller  300  includes transmit interface  310  and receive interface  320  sharing filtering logic  330 . Transmit and receive processing are preferably performed using direct memory access (DMA) techniques implemented in integrated circuitry, although processor intervention may be judiciously employed for configuration and relatively high level tasks.  
         [0021]    Transmit interface  310  receives packets from network devices it supports (not shown), performs transmit processing on packets and selectively forwards packets across the backplane on its dedicated transmission line. Transmit interface  310  makes forwarding decisions in accordance with configured MAC bridging rules using the results of associative comparisons performed in filtering logic  330 . By way of example, the following bridging rules may be enforced: Source and destination MAC addresses encoded in a received packet are compared for a match with addresses stored in filtering logic  330  known to be associated with network devices supported by transmit interface  310 . If the filtering results indicate a match for the destination MAC address, filtering is indicated, in which case transmit interface  310  filters the packet. If the filtering results indicate no match for the destination MAC address, forwarding is indicated, in which case transmit interface  310  uses the filtering results as a pointer to a forwarding database, retrieves forwarding instructions for the packet and transmits the packet on the backplane with the forwarding instructions encoded. If the filtering results indicate no match for a source MAC address, transmit interface  310  subjects the address to a source learning process resulting in the address being added to the filtering logic. Of course, additional or different bridging rules may be implemented on transmit interface  310 , such as a rule requiring that the source and destination MAC addresses of a packet share at least one virtual LAN (VLAN) as a condition for forwarding. Transmit interface  310  forwards packets for which forwarding is indicated on the backplane first in, first out (FIFO), in a series of data bursts.  
         [0022]    Receive interface  320  receives packets from the backplane, performs receive processing on packets and selectively queues packets for forwarding to the network devices it supports (not shown). Receive interface  320  makes queueing decisions based on the results of filtering and watermark checks, which both must indicate queueing for queueing clearance to be granted. Filtering checks rely on associative comparisons conducted in filtering logic  330 . The following MAC bridging rules may apply to filtering checks by way of example: Destination MAC addresses encoded in received packets are compared for a match with addresses stored in filtering logic  330  known to be associated with network devices it supports. If the filtering results indicate a match for the destination MAC address, queueing is indicated. If the filtering results indicate no match for the destination MAC address, filtering is indicated if no other switching controller claims the packet and queueing is indicated if another switching controller claims the packet. Naturally, additional or different bridging rules may be implemented on receive interface  320 .  
         [0023]    Filtering logic  330  may be implemented in content-addressable memory (CAM) integrated circuitry which conducts associative comparisons in a CAM on complete addresses, “pseudo CAM” integrated circuitry and software which resolves the most distinctive bits in addresses using a data hashing algorithm and conducts associative comparisons in a random access memory (RAM) on the most distinctive bits, or in conventional processor-driven software.  
         [0024]    Turning now to FIG. 4, in a preferred embodiment of the invention, receive interface  320  is shown in greater detail. The filtering and watermark checks are initiated and conducted on receive interface  320  on a “per packet” basis with the expedient of receive ports  401 - 409 , filtering check arbiter  420  and input/output server  430 . Receive ports  401 - 409  each have a receive buffer fed with data bursts from the backplane on a dedicated transmission line. In this regard, returning momentarily to FIG. 2, it will be recalled that in a preferred embodiment each switching controller has a dedicated transmission line for simultaneously transmitting data bursts to all controllers such that all receive ports  401 - 409  may receive a data burst from all controllers on a particular clock cycle. Receive ports  401 - 409  initiate filtering and watermark checks and are assigned distinct “start release” clock cycles within a repetitive timing cycle such that, if both the filtering and watermark checks are passed, receive ports  401 - 409  initiate the release of packets to output queue  440  round-robin in accordance with their assigned “start release” clock cycles.  
         [0025]    Filtering checks are initiated by engaging filtering check arbiter  420 . In a preferred embodiment, filtering checks proceed as follows. When a sufficient number of data bursts for a packet have been received by one of receive ports  401 - 409 , the receive port sends to arbiter  420  a request for control of bus  430  for the purpose of initiating an associative comparison in filtering logic  330 . If arbiter  420  has only one request pending on a particular clock cycle, arbiter  420  automatically grants the request. If, however, arbiter  420  has multiple pending requests on a particular clock cycle, the competing requests are resolved by granting the pending request made by the one of receive ports  401 - 409  whose assigned “start release” clock cycle would enable it to initiate the release of its packet to output queue  440  the soonest if queueing clearance were granted, provided that a threshold latency would not be exceeded by such packet. If the threshold latency would be exceeded by all packets associated with pending requests, the requests are granted based on port number. It will be appreciated that by implementing the foregoing rules to resolve competing requests to initiate filtering checks, filtering checks are able to proceed contention-free while latency caused by the round-robin packet release initiation protocol is reduced. The filtering check arbitration algorithm implemented by arbiter  420  is illustrated in greater detail in the flow diagram of FIG. 5. When multiple requests are pending a check is made to determine if any of the requests is from a “zero clock latency” receive port, i.e., a receive port which would be able to initiate release of its packet on the first clock cycle after receiving forwarding clearance ( 510 ). If there is a “zero clock latency” request, the request is granted ( 520 ). If not, a check is made to determine if any of the requests is from a “one clock latency” receive port ( 530 ). If there is a “one clock latency” request, the request is granted ( 540 ). If not, arbiter  420  grants the request which is associated with the lowest-numbered receive port ( 550 ,  560 ,  570 ,  580 ). Because clock latency is a determinable constant for each clock cycle and port number pair, latency determinations may be made by consulting a pre-configured “look-up” table in arbiter  420 .  
         [0026]    Filtering checks are conducted by engaging filtering logic  330  over bus  430 . The receive port whose request is granted transmits the destination MAC addresses of its packet to filtering logic  330  on bus  430 . Filtering logic  330  eventually returns filtering results on bus  430  indicating whether a match for the destination MAC address was found. If the filtering results indicate that a match was found, the receive port will eventually queue the packet. Because MAC bridging rules indicate forwarding by all switching controllers if no controller claims a packet transmitted over the backplane, the receive port advises other controllers that it has claimed the packet. For this purpose, receive ports  401 - 409  have claim lines  421 - 429 . The receive port which will eventually queue the packet asserts its claim line after learning that the filtering results indicate that a match was found. If the filtering results indicate that no match was found, the receive port checks whether any other controller has asserted the claim line. If either the filtering results indicate that a match was found, or the claim line has not been asserted, the packet has passed the filtering check and will eventually be queued. Otherwise, the packet has failed the filtering check and will be dropped.  
         [0027]    Watermark checks are initiated by engaging input/output server  430 . The condition for passing a watermark check is:  
           WM&gt; ( WADDR−RADDR )+ SZ    
         [0028]    where  
         [0029]    WM=the output queue watermark;  
         [0030]    WADDR=the output queue write address, adjusted upward to account for queue space definitively allocated to but not yet filled by preceding packets;  
         [0031]    RADDR=the output queue read address; and  
         [0032]    SZ=the size of the current packet.  
         [0033]    Receive ports  401 - 409  are assigned distinct “start check” clock cycles within a repetitive timing cycle for initiating watermark checks. The “start check” clock cycles assigned to a port may differ from its assigned “start release” clock cycle. In a preferred embodiment, watermark checks proceed as follows. Referring to FIG. 6, receive port  401  and input/output server  430  have control lines  601 - 604  running between them, including request line  601 , packet size line  602 , grant line  603  and feedback line  604 , for exchanging necessary signals. In practice, of course, all receive ports  401 - 409  will individually exchange such signals with server  430  in similar fashion over similar lines. Receive port  401  initiates a watermark check by validating request line  601  and conveying on size line  602  the size of its current packet. Server  430  implements the preferred watermark comparison algorithm to determine whether or not allocating output queue space in the requested size amount would violate an established watermark for output queue  440  and either grants or denies the request over grant line  603 . If a grant is issued, the packet has passed the watermark check. Otherwise, the packet has failed the watermark check.  
         [0034]    The watermark check is complicated somewhat by its outcome-dependence on preceding queueing decisions. More particularly, to conduct an accurate watermark check on a current packet, it must be known how much output queue space has been definitively allocated to preceding packets, i.e., it must be known for each preceding packet whether queueing clearance was granted. To enable watermark checks to be conducted efficiently in integrated circuitry despite this outcome-dependence, the preferred watermark comparison algorithm makes preliminary calculations on the current request using “projected” output queue write addresses for each possible outcome of the queueing decision on the preceding packet and applies the actual outcome to select from among the preliminary calculations. The preferred watermark comparison algorithm is illustrated in greater detail by reference to FIG. 7, which may be profitably reviewed in conjunction with FIG. 6. Server  430  obtains the current packet size (SZ) from receive port  401  on size line  602  ( 710 ) and separately adds the packet size to a “minimum projected” write address (WADDRmin) and a “maximum projected” write address (WADDRmax) ( 720 ). Feedback is applied on feedback line  604  to indicate whether the preceding packet received queueing clearance ( 730 ). Naturally, queueing clearance was received if the preceding packet passed both the filtering check and the watermark check, and was otherwise not received. If queueing clearance was not received, the sum calculated using the “minimum projected” write address is compared with the sum of the watermark (WM) and the current read address (RADDR) ( 740 ). If the comparison indicates that the sum calculated using the “minimum projected” write address exceeds WM+RADDR, the request is denied ( 750 ) such that the watermark check is failed. If the comparison indicates that the sum calculated using the “minimum projected” write address does not exceed WM+RADDR, the request is granted ( 760 ) such that the watermark check is passed and the “maximum projected” write address is assigned the value of the sum of the “minimum projected” write address and the current packet size. Turning now to the other branch of FIG. 7, if queueing clearance was received by the preceding packet, the “minimum projected” write address is assigned the value of the “maximum projected” write address ( 770 ) and the sum calculated using the “maximum projected” write address is compared with the sum of the watermark (WM) and the current read address (RADDR) ( 780 ). If the comparison indicates that the sum calculated using the “maximum projected” write address exceeds WM+RADDR, the request is denied ( 750 ) such that the watermark check is failed. If the comparison indicates that the sum calculated using the “maximum projected” write address does not exceed WM +RADDR, the request is granted ( 790 ) such that the watermark check is passed and the “maximum projected” write address is assigned the value of the sum of the “maximum projected” write address and the current packet size. A watermark check may then be accurately performed on the subsequent packet using the newly-calculated values. Server  430  conveys grants to receive port  401  on grant line  603 . The watermark (WM) is a predetermined constant indicating the maximum amount of output queue space which may be allocated and is preferably configurable. The read address (RADDR) is, of course, a variable retained by output queue  440  and incremented as packets are dequeued.  
         [0035]    If a packet passes the filtering check but fails the watermark check, forwarding is indicated but the packet is delayed at its receive port to prevent output queue overflow. To avoid having the delayed packet overwritten by a subsequent packet transmitted to the receive port over the backplane, the receive port imposes a stall condition to suspend the transmission of subsequent packets until release of the current packet to output queue  440  is secured. Referring to FIG. 6, to impose the stall condition, receive port  401  has stall line  605  which runs from receive port  401  to the associated transmit interface, which receive port  401  asserts after it learns that its current packet will be eventually be forwarded but must be held in abeyance. In the stall mode, receive port  401  continues to initiate watermark checks with server  430  on its assigned “start check” clock cycle until a grant is received, at which time the stall condition is lifted by deactivating stall line  605 . Naturally, each of receive ports  401 - 409  has a stall line and stall conditions are imposed on a “per port” basis to avoid unnecessary disruption in the flow of packets over backplane.  
         [0036]    Receive ports  401 - 409  initiate release to output queue  440  of packets for which queueing clearance has been received on their assigned “start release” clock cycles. In a preferred embodiment, receive ports  401 - 408  are operative as full-write ports and receive port  409  is operative as a selective-write port for releasing packets to output queue  440 . Each packet is released in a series of constant-bit words in accordance with a repetitive nine-clock timing cycle. Referring to FIG. 8, output queue  440  is arranged as a rotating crossbar which includes multiplexors  801 - 808  equal to the number of full-write ports  401 - 408  and followed by an equal number of physical memories  811 - 818 . Physical memories  811 - 818  each have the capacity to store numerous constant-bit words delivered from their associated one of multiplexors  801 - 808  and are “stacked” such that they combine to form distinct logical packet queues, with each logical packet queue having the capacity to store up to eight constant-bit words from the same packet. Each constant-bit word released by ports  401 - 409  is made available to all multiplexors  801 - 808 , which each select a different word from among the words made available on each clock and deliver the selected word to their associated one of physical memories  811 - 818 . Multiplexors  801 - 808  select words from ports  401 - 409  round-robin and are staggered such that no two multiplexors ever select the same word. In operation, each of ports  401 - 409  is assigned writing privileges on eight clocks within the nine-clock repetitive timing cycle. The clock assignment of writing privileges is staggered such that eight ports have writing privileges on each clock. On each clock for which it has writing privileges, each full-allocation port  401 - 408  releases to multiplexors  801 - 808  a constant bit word from a packet for which queueing clearance was received and queueing has begun in accordance with the packet release initiation protocol, if available. On each clock for which selective-write port  409  has writing privileges, if port  409  has requested to exercise its writing privileges, port  409  releases to multiplexors  801 - 808  a constant bit word, but if port  409  has not requested to exercise its writing privileges, a constant bit word is read from the one of physical memories  811 - 818  to which port  409  might have otherwise written. Port  409  exercises its writing privileges whenever a packet for which queueing clearance was received and queueing has begun in accordance with the packet release initiation protocol is available. It will be appreciated that by configuring relatively low-traffic receive ports as selective-write ports within the foregoing scheme, dequeueing may be accomplished during unutilized “write” clock cycles.  
         [0037]    Turning now to FIG. 9, a timing diagram illustrates receive processing within receive interface  320  in accordance with a preferred embodiment of the invention. Signal tm_slice indicates the current clock cycle or “time slice” within a repetitive timing cycle. Signals in the form cam_req_n indicate whether receive port n has issued a request to arbiter  420  on the current time slice. Signals cam_grnt_n indicate whether arbiter  420  has granted a request issued by receive port n on the current time slice. Signals cam_mtch_n indicate whether filtering results returned from filtering logic  330  on the current time slice indicate a match. Signals clm_n indicate whether receive port n is asserting its claim line on the current time slice. Signals i/o_req_n indicate whether receive port n has issued a request to server  430  on the current time slice. Signals i/o_grnt_n indicate whether server  430  has granted a request issued by receive port n on the current time slice. Finally, signals q_wrt_n indicate whether receive port n is writing a constant-bit word to output queue  440  on the current time slice. In the illustrated sequence, on time slice eight, cam_req — 0, cam_req — 1 and cam_req — 2 are pulsed high indicating that ports zero, one and two have issued requests to initiate filtering checks. On time slice zero, cam_grnt — 2 is pulsed high, indicating that an arbitration has been conducted and port two&#39;s request has been granted. Naturally, under the preferred filtering check arbitration algorithm, port two&#39;s request was chosen over port zero&#39;s and port one&#39;s requests because port two is a “low clock latency” port on time slice eight. On time slice one, cam_req — 2 is deasserted, while cam_req — 0 and cam_req — 1 continue to be asserted and will be considered by the arbiter  420  in the next arbitration. On time slices one through three, port two sends the destination MAC address of its current packet to filtering logic  330  for associative comparison. On time slice three, cam_grnt — 0 is pulsed high, indicating that a second arbitration has been conducted and port zero&#39;s request has been granted. Naturally, port two&#39;s request was chosen over port one&#39;s request because neither port zero or port one is a “low clock latency” port on time slice two and port zero has a lower port number. On time slice four, cam_req — 0 is deasserted, while cam_req — 1 continues to be asserted and will be considered in the next arbitration. On time slices four through six, port zero sends the destination MAC address of its current packet for associative comparison. On time slice six, cam_grnt — 1 is pulsed high, indicating that a third arbitration has been conducted and port one&#39;s request has been granted. On time slice seven, cam_req — 1 is deasserted. Also on time slice seven, cam_mtch — 2 is pulsed high, indicating that a match for the destination MAC address of port two&#39;s current packet was found. On time slice eight, clm — 2 is pulsed high, indicating that port two has claimed its current packet for eventual queueing. On time slice zero, i/o_req — 2 is pulsed high, indicating that port two has initiated a watermark check. Naturally, port two is assigned time slice zero for initiating watermark checks. On time slice one, i/o_grnt — 2 is pulsed high, indicating that a watermark comparison has been conducted and port two&#39;s request has been granted. Also on time slice one, cam_mtch — 0 is pulsed high, indicating that a match for the destination MAC address of port zero&#39;s current packet was found. On time slice two, q_wrt — 2 become active, indicating that port two is writing output queue  440 . Naturally, port two is assigned time slice two for initiating the release of a packet to output queue  440 . Also on time slice two, clm — 2 is deasserted indicating that queueing of the current packet on port two has begun and clm — 0 is pulsed high, indicating that port zero has claimed its current packet for eventual queueing. On time slice four, cam_mtch — 1 is pulsed high, indicating that a match for the destination MAC address of port one&#39;s current packet was found. On time slice five, clm — 1 is pulsed high, indicating that port one has claimed its current packet for eventual queueing. On time slice seven, i/o_req — 0 is pulsed high, indicating that port zero has initiated a watermark check. Naturally, port one is assigned time slice seven for initiating watermark checks. On time slice eight, i/o_grnt — 0 is pulsed high, indicating that a watermark comparison has been conducted and port zero&#39;s request has been granted. Also on time slice eight, i/o_req — 1 is pulsed high, indicating that port one has initiated a watermark check. On time slice zero, q_wrt — 0 become active, indicating that port zero is writing output queue  440 . Naturally, port zero is assigned time slice zero for initiating the release of a packet to output queue  440 . Also on time slice zero, clm — 0 is deasserted indicating that queueing of the current packet on port zero has begun and i/o_grnt — 1 is pulsed high, indicating that a watermark comparison has been conducted and port one&#39;s request has been granted. On time slice one, q_wrt — 1 become active, indicating that port one is writing output queue  440 . Naturally, port one is assigned time slice zero for initiating the release of a packet to output queue  440 . Also on time slice one, clm — 1 is deasserted indicating that queueing of the current packet on port one has begun and q_wrt — 2 become inactive, indicating that queueing of the current packet on port zero has been completed. q_wrt — 0 and q_wrt — 1 become inactive on subsequent time slices eight and zero, respectively, to complete the illustrated receive processing sequence.  
         [0038]    It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character hereof. The present description is therefore considered in all respects illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.