Abstract:
An improved combined Switching Data Unit (SDU) queuing discipline for unicast and multicast (Protocol Data Unit) PDU forwarding at a switching node is provided. Multicast SDU descriptors are replicated and stored in entries of a First-In/First-Out queue portion of a hybrid output port queue. Unicast SDU descriptors are chained in entries of a linked list queue portion of the hybrid output port queue. Servicing of the hybrid queue uses hybrid queue counters, and inter-departure-counters stored in multicast FIFO queue entries to keep track of the number of unicast SDU linked list entries, to be serviced between the multicast FIFO queue entries. The combined hybrid queue derives storage efficiency benefits from linking unicast PDUs in linked lists and further derives benefits from a simple access to multicast PDU entries.

Description:
FIELD OF THE INVENTION 
     The invention relates to packet-switching data transport technologies, and in particular to methods and apparatus for combined output port queuing of unicast and multicast traffic. 
     BACKGROUND OF THE INVENTION 
     Packet-switching technologies concern the transport of packetized data segments across an interlinked data transport infrastructure including transport nodes and interconnecting links. The term Protocol Data Unit (PDU) will be used throughout the description presented herein, persons of ordinary skill in the art would recognize that the term refers generally to: cells, frames, packets, etc. Once a PDU is received at a switching node, the PDU is inspected, information is extracted from the PDU, and for switch processing purposes the PDU is referred to as a Switching Data Unit (SDU) corresponding to the PDU. 
       FIG. 1  is a schematic diagram showing a generic implementation of a switching node. In general terms the operation of a switching node  100  includes: receiving a PDU from an input port  102 , storing  104  the corresponding SDU while the SDU is pending processing, determining  106  an output port  110  to send the PDU through, and forwarding  108  the PDU via the determined output port  110 . It is understood that the ports of the switching node  100  include bi-directional ports which correspond to pairs of input ports  102  and output ports  110 . 
     Packet-switching technologies from their infancy have largely concentrated on non-deterministic transport of PDUs. Benefits provided by the non-deterministic mode of transport are derived from an ability of transport nodes in a communications network to route PDUs around failed infrastructure. 
     The non-deterministic mode of transport suffers from an inherent inability to guarantee conveyance of PDUs. PDUs may be dropped in accordance with various PDU transport protocol specifications and error/congestion conditions. The inability to guarantee PDU conveyance is referred to as “best-effort transport”. Although packet-switched technologies, such as Asynchronous Transmission Mode (ATM) and MultiProtocol Label Switching (MPLS), providing support for deterministic PDU transport exist, best-effort non-deterministic packet-switched technologies enjoy the widest use. 
     There is an increasing demand for service level guarantees in provisioning services employing best-effort non-deterministic PDU transport technologies. One approach to provide service level guarantees, while still benefiting from non-deterministic PDU transport, is to ascribe traffic class associations to PDUs, and to preferentially process the corresponding SDUs, at network nodes in the transport path, based on the traffic class association. Only low priority SDUs and especially unclassified SDUs remain subject to best-effort processing and transport. Prioritizing SDU processing and transport intends to reduce denial of service instances to a minimum, and for this reason non-deterministic priority based SDU processing is preferred in certain applications. 
     In conveying “unicast” PDU traffic, each PDU traffic stream is generated by a source network node on edge and consumed by a sink network node on edge. “Multicast” traffic is generated by a single source network node on edge, replicated by an undetermined number of transport nodes in the interconnecting network, and aggregate traffic is consumed by multiple sink network nodes on edge. Therefore there is an unbound ability for the network to create traffic during a normal course of operation. Traffic creation in the network has numerous implications which are subject to intense current research and development some of which are addressed herein below. 
     Other service provisioning issues affecting resource utilization in the network relate to traffic patterns. Data traffic is not evenly distributed over the links and therefore processing requirements at network nodes differ. Data traffic created by source network nodes is in itself dynamic, unpredictable, and often bursty. Overall data traffic patterns vary from network to network and may even vary with time. 
     Further, electronic data services are typically provisioned from server network nodes to which a large amount of service request PDUs are directed, and from which a large amount of response PDUs are supplied. 
     In view of the above mentioned issues, the practical operation of a typical switching node must make provisions for queuing SDUs and adhere to queue service disciplines. Methods and apparatus for queuing SDUs pending processing, as well as methods of servicing queues continue to represent areas of intense research. 
     From the point of view of a particular switching node  100 , the switching node  100  has multiple physical ports  102 / 110  via which PDUs are exchanged with the network. As the SDU traffic is conveyed through the switching node  100  in a non-deterministic fashion, multiple SDUs from a variety of input ports  102  may happen to be destined for to the same output port  110  within a short period of time, such is the case if the output port  110  is associated with a server node. Assuming equal port transport capacities and PDUs incoming at high rates close to the full link transport bandwidth, a number of SDUs have to be held in an output port queue  112  for forwarding via the output port  110 . SDUs stored in the output port queue  112  incur a processing latency and depart from the queue  112  when the output port  110  is ready to transmit. 
     Should a multicast PDU be received at the switching node  100 , the replication of the corresponding SDU and the individual forwarding of each replica is yet another factor which may also delay the processing of other SDUs. The processing of SDUs is therefore affected by queuing methods and queue service disciplines employed. 
     A prior art U.S. Pat. No. 6,212,182 entitled “Combined Unicast and Multicast Scheduling” which issued on Apr. 3, 2001, to McKeown describes a complex queuing method for preprocessing received ATM cells. For all cells received via an input port, unicast cells are queued in a group of unicast output port queues associated with the input port, and multicast cells are queued in a multicast queue associated with the input port. The intended ATM application benefits from this complex cell separation. Further McKeown describes an improved request-based scheduling of the separated unicast/multicast cell traffic to effect cell transfer across a switching fabric. Although inventive, the described methods assume reliance on prior reservation of transport and processing bandwidth enforceable in using ATM technologies, addresses global queuing issues only and does not address issues related to output queuing requirements which do not seem to represent a concern. 
     Processing queues may be implemented in a variety of ways between which, the use of physical First-In/First-Out queues (FIFOs) and link list queuing have enjoyed extensive research and implementation. 
     A prior art U.S. Pat. No. 6,349,097 entitled “Multicasting in Switching Apparatus” which issued Feb. 19, 2002 to Smith makes use of global FIFO queues to store unicast and multicast ATM cells so that the received sequence of the cell stream can be retained. The solution is concerned with solving quality-of-service issues related to cell sequencing. Smith also does not address output port queuing resource utilization issues. 
     Physical handling of received SDUs by transferring them between input ports  102 , various buffers  120 , various queues  112 , output port(s)  110 , etc. has drawbacks related to the fact that read and write operations are time consuming. Except for ATM cell processing, where cells have a fixed size, it is hard to contain the amount of time spent on physical SDU transfers within a switching node  110 . 
     As the SDUs have to be stored at switching nodes  100  pending processing, it is best if the physical handling of each SDU was reduced preferably to one write operation when received, and preferably to at least one read operation when the destination port(s)  110  is/are ready to transmit the PDU out. For these and other reasons using centralized SDU storage  120  is preferred. 
     At all other times, structures called “descriptors”, comparatively smaller than SDUs, are used to track SDU attributes. These attributes may include diverse information used to determine an output port  110  to send the SDU through and information to prioritize the forwarding of the SDU. 
     In view of the above, methods of queuing SDUs become in effect methods of queuing SDU descriptors. SDU descriptors may therefore be physically stored in physical FIFO queues  112 . Descriptor FIFO queuing implementations are comparatively simple and more adapted to hardware implementation. Multicast SDUs are simply handled by replicating the multicast SDU descriptor and the resulting multiple descriptors are stored in corresponding output queues  112  associated with corresponding destination ports  110 . 
     Although SDU descriptors are small compared with corresponding SDU sizes, requirements for high integration, high port density per switch, and high throughput capacity, lead to a need to address issues related to SDU descriptor storage in FIFO queues  112 . The processing of multicast SDUs compounds the descriptor storage problem. 
     Consider the above mentioned scenario in which, a switching node  100 , implementing SDU descriptor FIFO output port queuing, provides interconnectivity in a client-server networking environment. Assuming same capacity ports  102 / 110 , the output port  110  associated with the server node, will typically experience congestion as the aggregate traffic from all other input ports  102  is directed to the server node. 
     Worse case scenario switching node design calls for the descriptor FIFO queue  112  associated with the server port  110  to be large (long) enough to accommodate SDU descriptors associated with an expected number of SDUs during a typical burst of server requests. Without knowing which output port  110  will be associated with the server during the manufacturing of the switching node  100 , all output ports  110  need be provided with enough storage resources ( 112 ) in accordance with the worse case scenario. As port ( 102 / 110 ) density per switching node  100  increases, the storage reservation requirements for descriptor FIFO queues  112  increases also. If provisions for prioritization of SDU processing are to be made, the aggregate descriptor storage reservation requirements need be multiplied by the number of priority levels supported per port. 
     Size limitations on FIFO queue implementations exist because exclusive FIFO queue reservations are necessary. The total number of reserved FIFO queue descriptor entry “slots” is usually several times the maximum number of SDUs that can be processed at a switching node. In accordance with a typical implementation, suppose that the switching node having a typical number of 16 ports  102 / 110  has a combined storage capacity to buffer  1000  PDUs. With a single output port queue  112  per output port  110 , typically 500 SDU descriptor slots are reserved for each FIFO queue  112 . Therefore a total amount of storage for 8000 descriptor entries must be provided. 
     Complex switching node implementations accommodating a large number of SDUs in a central storage and/or implementing sophisticated transmit prioritization schemes use linked list queuing in an attempt to alleviate descriptor storage issues. 
     Link list queue implementations include interlinking SDU descriptors stored centrally in shared storage, via SDU descriptor pointers, in link lists defining virtual queues. Storage efficiency is enjoyed in making use of shared storage to hold SDU descriptors. Each SDU descriptor contains at least one next SDU descriptor pointer. SDU descriptor pointers require even less storage space than SDU descriptors. 
     Considering the above mentioned exemplary setup implementing a linked list queuing in a unicast only environment, for 1000 unicast PDUs buffered at the switching node, storage for only 1000 descriptor entries needs to be provided as which is significantly less than 8000. Each of the 1000 descriptors used in linked list queuing have only marginally larger storage requirements to account for the additional descriptor pointer storage. For each of the 16 output ports  110  only a single pointer register is needed to hold the head of line pointer specification for each linked list queue  112 . 
     Although storage resource utilization reductions are achieved by using linked list queuing implementations for unicast traffic, the use of linked lists for queue implementations is cumbersome in handling multicast SDU queuing.  FIG. 2  is exemplary of a typical association between unicast and multicast SDUs pending forwarding via a group of ports. Multicast SDUs cause a multiple-to-one and/or a one-to-multiple fan-out across multiple queues  112 . As shown, each multicast SDU may have a different fan-out diversity across the port queues which makes for a complex combined association structure. 
     An implementation attempting to address this association complexity is illustrated in  FIG. 3  which shows a global linked structure for unicast and multicast SDU queuing at a multi-port switching node. The implementation calls for the use of multiple SDU pointer fields associated with each multicast SDU linked list entry. 
     Each multicast SDU entry has a number of pointer fields equal to the maximum possible fan-out (the total number of ports of the switching node minus one—the port on which the corresponding multicast PDU was received). In accordance with a worst case scenario, for each multicast SDU, N-1 linking operations need to be performed within one SDU processing time interval. Such a task would be hard to achieve for high port density implementations. The processing of multicast SDUs further negatively impacts the operation of a switching node because the processing of multicast SDUs is memory access intensive. With a limited memory access bandwidth, uncontrolled processing multicast SDUs is detrimental to unicast SDU processing. 
     In provisioning support for Virtual Local Area Network (VLAN) or any other multicast domain control provisioning, multicast SDUs have a very diverse output port fan-out, the number of output ports being substantially less than the total number of ports per switching node. The high likelihood of the diversity of the fan-out being substantially less than the total number of ports, enables implementation of high port density designs. The discrepancy between a large number of ports per switching node and the median fan-out diversity leads to a lot of the descriptor pointer fields associated with multicast SDU entries to remain unused (filled with NULL pointers). At high multicast SDU throughput, unused descriptor pointer fields can take up a lot of storage resources and a lot of processing overhead is incurred in scheduling SDU processing based on the complex associativity structure. This represents a major disadvantage in using linked list implementations to queue multicast SDUs leading to a storage space utilization inefficiency wasting SDU processing resources and bandwidth. 
     There therefore is a need to solve the above mentioned issues in providing queuing methods and queue service disciplines for combined unicast and multicast SDU traffic processed at a switching node. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, hybrid unicast and multicast queue for an output port is provided. The hybrid queue includes a unicast linked list queue portion, a multicast FIFO queue portion, and a sequencer. The operation of the sequencer includes inspecting a unicast PDU forwarding request arrival counter, a PDU departure counter, a multicast FIFO state, and a unicast linked list queue status to decide whether to forward a unicast or a multicast PDU over the output port. Each multicast FIFO entry also carries an inter-departure-counter value specifying the number of unicast linked list entries that have to be serviced before servicing the multicast FIFO entry. 
     A combined benefit is derived from unicast SDU descriptor linking to provide flexibility on the size of the hybrid queue, and efficiency in reserving storage resources by employing multicast FIFO queuing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the invention will become more apparent from the following detailed description of the preferred embodiment(s) with reference to the attached diagrams wherein: 
         FIG. 1  is a schematic diagram showing a generic switching node implementation; 
         FIG. 2  is a schematic diagram showing an exemplary unicast and multicast associations between SDUs pending forwarding; 
         FIG. 3  is a schematic diagram showing a prior art unicast and multicast multi-port link list queuing implementation; 
         FIG. 4  is a schematic diagram showing an exemplary combined unicast and multicast SDU processing at a switching node; 
         FIG. 5  is a schematic diagram showing an exemplary implementation of combined priority unicast and multicast queuing for an output port, in accordance with an exemplary embodiment of the invention; 
         FIG. 6  is a schematic diagram showing details of a single hybrid queue implementation, in accordance with the exemplary embodiment of the invention; 
         FIG. 7  is a schematic diagram showing a sequence of queued unicast and multicast SDU entries being serviced, in accordance with exemplary embodiment of the invention; 
         FIG. 8  is a flow diagram showing process steps implementing combined unicast and multicast queuing of PDU forwarding requests, in accordance with the exemplary embodiment of the invention; and 
         FIG. 9  is a flow diagram showing process steps implementing combined unicast and multicast queue servicing, in accordance with the exemplary embodiment of the invention. 
     
    
    
     It will be noted that in the attached diagrams like features bear similar labels. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 4 , a schematic diagram showing an exemplary combined unicast and multicast SDU processing at a switching node is shown. PDUs received via an input port  102  are inspected by a PDU classifier  210  and are retrievably stored  104  in a central store  120 . The PDU classifier  210  generates  212  an SDU descriptor for each received PDU, and stores  214  the SDU descriptor in a central descriptor storage  220 . The SDU descriptors are stored in unicast descriptor entries  222  and multicast descriptor entries  224 . 
     Unicast descriptor entries  222  specify, without limiting the invention, at least: a unicast PDU storage address pointing to a memory storage location in the central shared buffer  120 , and a next descriptor pointer  234 . Upon creating each unicast descriptor entry  222 , the value of the next descriptor pointer  234  is set to NULL. 
     Multicast descriptor entries  224  specify, without limiting the invention, at least: a multicast PDU storage address pointing to a memory storage location in the central shared buffer  120 , and a fan-out diversity value  236 . Upon creating each multicast descriptor entry  224 , the fan-out diversity value  236  is set to zero. 
     Each descriptor entry in the central descriptor storage  220  may include, without limiting the invention, a descriptor entry type specifier identifying the descriptor entity as a unicast or a multicast descriptor entity. Alternatively, the central descriptor storage  220  may be partitioned into unicast descriptor storage and multicast descriptor storage. 
     For each unicast PDU received, a destination network address  230  to forward the unicast PDU to, and a priority class association; and for each multicast PDU received, a group of destination network addresses  232  to forward the multicast PDU to, and a priority class association are extracted by the PDU classifier  210 . Destination network address information and priority class associations are used by the PDU classifier  210  to formulate a switch request  218  for each inspected PDU. The switch request  218  is sent to the switch processor  240  along with an SDU descriptor pointer pointing to the corresponding SDU descriptor entry  222 / 224 . Depending on the implementation, the switch requests  218  may be queued for processing by the switch processor  240 ; the queuing of switch requests  218  may employ multiple priority queues  232  corresponding to PDU priority class associations, and is beyond the scope of the present invention. 
     A switching processor  240  processes switch requests  218  to determine output ports  110  to forward the corresponding SDUs to. In performing the switching function, the switching processor  240  makes use of a switching database  242 . The operation of the switching processor  240  may be subject to a service discipline  244  enabling priority based processing of PDUs. 
     In servicing a unicast SDU switching request  218 , the switching processor  240  makes use of the single destination network address to determine a single output port  110  to forward the corresponding PDU therethrough. Once the destination output port  110  is determined, the switching processor  240  makes use of the priority class association to forward the SDU to an output port hybrid priority queue  250  associated with the determined output port  110 . The output port hybrid priority queue  250  is provided with the corresponding SDU descriptor pointer in a forwarding request  246 . 
     In servicing a multicast SDU switching request  218 , the switching processor  240  makes use of the group of destination network addresses to determine a corresponding group of output ports  110  to forward the corresponding PDU therethrough. Once the group of output ports  110  is determined, the number of unique output ports  110  determined is stored in the fan-out diversity field  236 . The switching processor  240  makes use of the priority class association to queue SDU descriptor replicas in output port hybrid priority queues  250  corresponding to the determined group of unique output ports  110 . Each destination output port hybrid priority queue  250  is provided with an SDU descriptor pointer replica via a forwarding request  248 . 
     In accordance with an exemplary embodiment of the invention, a hybrid output queue implementation used for queuing unicast and multicast SDU pointers at an output port  110  is shown schematically in  FIG. 5 . In particular, for each hybrid priority output queue  250  associated with the output port  110 , the queuing of unicast SDU pointers makes use of link list queuing, and queuing of multicast SDU pointers makes use of FIFO queuing. A sequencer  260  is used in servicing a corresponding the hybrid priority queue  250 . 
     In servicing each hybrid queue  250 , the sequencer  260  must know when to pop multicast SDU FIFO entries and when to follow SDU descriptor pointer links. Each hybrid priority queue  250  maintains a Head-Of-Line (HOIL) pointer specifier  252  for the linked list of unicast SDU entries, and the hybrid priority queue  250  reserves memory storage for a FIFO queue  254 . In accordance with an exemplary implementation of the invention, memory storage space for the FIFO queue  254  can be reserved from a memory block associated with the output port  110 . In accordance with another implementation of the invention, memory storage space for the FIFO queue  254  is reserved from the central shared buffer  120 . In using the central shared buffer  120 , some level of caching may be employed to improve central shared buffer  120  access latency and efficiency. 
       FIG. 6  shows further details of an output hybrid queue  250  of an output port  110 . 
     Unicast SDU descriptors  222  are chained via the next SDU descriptor pointer field  234 . Each multicast SDU descriptor pointer replica will be written to a FIFO queue entry  262  along with extra information enabling the associated sequencer  260  to select between unicast and multicast queue entries to service next. 
     In accordance with the exemplary embodiment of the invention,  FIG. 7  illustrates hybrid queue servicing details. The time line shows a sequence of unicast and multicast PDU forwarding request  246 / 248  arrivals and the sequence these are expected to depart via the output port  110 . 
       FIG. 8  is an exemplary flow diagram showing combined unicast and multicast queuing of PDU forwarding requests, in accordance with the exemplary embodiment of the invention.  FIG. 9  is an exemplary flow diagram showing combined unicast and multicast queue servicing, in accordance with the exemplary embodiment of the invention. 
     Suppose the hybrid queue  250  is empty. The unicast arrival counter  256  holds a value of 0 (zero). The departure counter  258  holds a value of 0 (zero). The HOL pointer specifier  252  points to NULL. The tail pointer specifier  264  also points to NULL. The multicast FIFO queue  254  does not have any entries. The multicast FIFO queue status is “empty” and the unicast linked list status is “empty”. 
     Upon receiving  802  a unicast PDU ( 804 ) forwarding request  246 , the unicast arrival counter  256  will be incremented  806  by 1. The unicast linked list status is set  810  to “not empty”. The HOL pointer specifier value is set  812  to the corresponding unicast SDU descriptor pointer value provided in the unicast PDU forwarding request  246 . Because there are no other unicast SDU entries in the unicast linked list, the tail pointer specifier  264  is also set  814  to point to the same unicast SDU descriptor pointer value. 
     If no subsequent unicast ( 804 ) PDU forwarding request  246  arrives  802  before the first unicast SDU entry is serviced, subsequent to servicing  906  of the unicast SDU entry, because the multicast FIFO queue  254  is empty  912  the arrival counter  256  is decreased  914  by 1. The sequencer  260  determines  902  whether the HOL pointer specifier  252  and the tail pointer specifier  264  hold the same pointer value. If the HOL pointer specifier  252  and the tail pointer specifier  264  hold the same value, then there are no other unicast SDU entries in the unicast linked list and both pointer specifiers  252  and  264  are set to NULL. The sequencer  260  removes  908  the unicast SDU entry from the unicast linked list. The unicast linked list queue status is set to “empty”. The sequencer  260  frees  910  the storage space held by the corresponding PDU. 
     If a subsequent unicast ( 804 ) PDU forwarding request  246  arrives  802  before the first unicast SDU entry is serviced  906 , the unicast arrival counter  256  will be incremented  806  by 1. Because the unicast linked list queue status is “not empty”  808 , the tail pointer specifier  264  is used ( 816 ) to access ( 816 ) the unicast SDU descriptor  222  corresponding to the last queued unicast SDU entry, and store  816  the pointer value to the unicast SDU descriptor  222 , provided in the just received unicast PDU forwarding request  246 , in the next SDU descriptor pointer field  234  of the last queued unicast SDU entry. The tail pointer specifier  264  is then set to point  814  to the subsequent unicast SDU descriptor  222 . 
     As previously mentioned if the first queued unicast SDU entry is serviced  906 , because the multicast FIFO queue  254  is empty  912  the arrival counter  256  is decreased  914  by 1. The output port  110  sets  908  the HOL pointer specifier  252  to the next unicast SDU descriptor pointer value held in the next SDU descriptor field  234  of the serviced SDU descriptor  222 . This results in the HOL pointer pointing to the subsequent SDU. 
     If a multicast ( 804 ) PDU forwarding request  248  is received  802  the multicast PDU descriptor pointer provided is stored in a newly created  818  multicast FIFO queue entry  262 . The multicast FIFO queue status is set  824  to “not empty”. Each multicast FIFO queue entry  262  also has an inter-departure-counter specifier  266 . The value of the arrival counter  256  is stored  820  in the inter-departure-counter specifier  266  and then the arrival counter  256  is reset  822  to 0 (zero). This has the effect of tracking the number of unicast SDU entries which have to be serviced before the multicast SDU entry. 
     As the multicast FIFO queue status is “not empty”, in servicing  906  a unicast SDU entry, the departure counter is increased  916  by 1. With the unicast linked list queue status “not empty”, the sequencer  260  continues to service unicast SDU entries by following unicast SDU descriptor pointer links ( 234 ) until the value of the departure counter  258  equals  918  the value of the inter-departure-counter specifier  266  of the next multicast SDU entry  262 . When the value of the departure counter  258  is equal  918  to the value of the next inter-departure-counter  266 , the sequencer  260  resets  919  the value of the departure counter  258  to 0 (zero) and services  926  the next multicast FIFO entry  262 . 
     In servicing  926  each multicast FIFO entry  262 , the sequencer  260  makes use  924  of the multicast SDU descriptor pointer stored in the multicast FIFO entry  262  to access ( 924 ) the corresponding multicast SDU descriptor  224 . Once a copy of the corresponding PDU is transmitted  926 , the value of the diversity specifier  236  is decreased  928  by 1. If the value of the diversity specifier  236  is 0 (zero) in step  930 , then the storage space held in the central shared buffer  120  is freed  932 . 
     After servicing each multicast FIFO entry  262 , the sequencer  260  pops ( 932 ) the multicast FIFO entry  262  and attempts to service the next multicast FIFO entry  262  if the multicast FIFO queue status is  920  “not empty”. If the inter-departure-counter  266  of the next multicast FIFO entry  262  is 0 (zero) in step  922 , then the sequencer  260  services  926  the multicast FIFO entry  262 . If the inter-departure-counter  266  of the multicast FIFO entry  262  is not 0 (zero), then the sequencer  260  inspects  904  the HOL pointer specifier  252  to find the next unicast SDU entry in the linked list to service  906 . If, in attempting to service a next multicast FIFO entry  262 , a multicast FIFO entry  262  is not found  902 , then the multicast FIFO queue status is set ( 920 ) to “empty”, and the sequencer  260 , if the unicast linked list queue status is “not empty”, inspects  904  the HOL pointer specifier  252  to service the next unicast SDU entry. 
     With an empty hybrid queue  112 , if a multicast PDU forwarding request  248  is received  802  first, the above steps are repeated. A multicast FIFO queue entry  262  is created  818  and populated with the multicast SDU descriptor pointer. The inter-departure-counter  266  is set  820  to the value of the arrival counter  256 , which is 0 (zero) and the multicast FIFO queue status is set  824  to “not empty”. In servicing the hybrid queue  250 , the sequencer  260  finds  902  the link list queue status “empty”, the departure counter value equals (0 zero) the inter-departure-counter value  266  stored in the multicast FIFO queue entry  262  and services  926  the multicast FIFO queue entry  262 . Subsequent to servicing  926  the FIFO queue entry  262 , the sequencer  260  resets the multicast FIFO queue status to “empty” ( 920 ). 
     The HOL pointer specifier  252  holds an SDU descriptor pointer value always points to the next unicast SDU descriptor  222  to be serviced from the hybrid queue  250 , regardless of the number of multicast SDUs pending servicing ahead of the unicast SDU descriptor pointed to. 
     A combined benefit is derived from unicast SDU descriptor linking to provide flexibility on the size of the hybrid queue  250 , and efficiency in reserving storage resources by employing multicast FIFO queuing. 
     Returning to the exemplary 16 port switching node, if the exemplary 16 port switching node is implemented in accordance with the exemplary embodiment of the invention presented above, and assuming one hybrid queue  250  per output port, output queue reservations correspond only to the size of the multicast FIFO queue  254  portion of the hybrid queue  250 . Therefore adjusting the storage reservations for each multicast FIFO queue  254  directly controls the bandwidth of multicast PDU traffic to be forwarded via the corresponding output port  110 . The number of chained unicast SDU entries in the link list can potentially include all unicast PDUs stored in the central shared buffer  120  without requiring output port queuing reservations. Therefore the reservations for output port queuing may be reduced to 100 multicast FIFO queue entries per output port  110  and therefore only a total memory storage space for 2600 SDU descriptors needs to be provided. The presented solution provides the combined storage efficiency of unicast SDU queuing using link lists and easy access to multicast SDU queue entries. 
     The exemplary embodiment of the invention presented herein is well adapted for switch-on-a-chip device implementations. 
     The embodiments presented are exemplary only and persons skilled in the art would appreciate that variations to the above described embodiments may be made without departing from the spirit of the invention. The scope of the invention is solely defined by the appended claims.