Abstract:
A cell relay switch includes an output port for transmitting cells to multiple destinations. The output port further includes a queuing buffer, a controller, and an output processor. The buffer receives a plurality of cells of a packet from a source. The buffer temporarily stores the cells before transmission by the output processor. The controller controls the storing of the received cells in the buffer by determining a total number of the cells in the packet and a number of cells to be received from different packets, deciding whether the buffer contains sufficient space to store the received cells based on the total number of cells in the packet, the number of cells to be received from the different packets, and the rates at which cells are received and drained from the buffer. The buffer is commanded by the controller to store the received cells when the buffer contains sufficient space and discard the received cells when the buffer contains insufficient space.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to cell relay systems and, more particularly, to a system that selectively controls the discarding of information. 
     BACKGROUND OF THE INVENTION 
     Cell relay systems, such as asynchronous transfer mode (ATM) systems, transmit data over a network as a plurality of fixed-length cells. The individual transmissions typically include one or more cells that constitute a portion of variable-length packets used by end systems or applications. Before transmission, a source station segments a packet into one or more cells and then transmits the cells. 
     A destination station, after receiving all of the cells associated with the packet, reassembles the cells and provides them to the end system or application. If a portion of the packet (i.e., one or more cells) becomes corrupt or dropped during transmission, the entire packet becomes corrupt. The end system or application typically has no use for the remaining cells of a corrupt packet. 
     In an ATM system that uses ATM adaption layer 5 (AAL5), the system establishes a particular route or “virtual circuit” over which the cells travel between the source station and the destination station. The source station transmits the cells over the virtual circuit in order and the cells arrive at the destination station in the same order. Sometimes cells from other packets traveling over a different, intersecting virtual circuit interleave with the cells and, thus, alter their time but not their order of arrival at the destination station. The destination station extracts the cells based on virtual circuit information included in the cells before it reassembles them into the associated packets. 
     Problems arise when the network becomes congested and intermediate switches contain insufficient buffer capacity to handle incoming traffic. Conventional switches discard incoming cells when their buffers are full. Then, when sufficient buffer space becomes available, they store incoming cells again. Accordingly, the switches may discard a portion of a packet and retain the preceding and succeeding portions, or fragments of the packet. These essentially useless packet fragments continue to travel over the network, consuming network resources. 
     In addition, because conventional switches store useless packet fragments in their buffers, this valuable buffer space becomes unavailable to the cells of complete or “good” packets. In other words, the switches discard good cells while also storing useless packet fragments. Typically, packets with discarded good cells must be retransmitted, adding to the network congestion. 
     Therefore, a need exists for a discard scheme that improves packet throughput during periods of network congestion and improves resource allocation among source and destination units when network congestion is present on one cell routing path, but less prevalent along another path. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the present invention address this need by providing a cell discard scheme that determines whether to discard a cell of a packet based on the ability of a buffer to store the entire packet. 
     In accordance with the purpose of the invention as embodied and broadly described herein, a system consistent with the present invention includes a cell relay switch having an output port including a queuing buffer, a controller and an output processor. The output port receives a plurality of cells of different packets from multiple sources. The output processor transmits cells to multiple destinations. Upon receiving the cells, the queuing buffer temporarily stores them before transmission by the output processor. 
     The controller controls the storing of the received cells in the queuing buffer by determining a total number of the cells in the packet, a rate at which the packets are received, and a number of cells to be received for other packets. The controller further decides whether the buffer contains sufficient space to store the received cells based on the total number of cells in the packet and the number of cells to be received for the different packets, allows storage of the received cells when the buffer contains sufficient space, and discards the received cells when the buffer contains insufficient space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the principles of the invention. In the drawings, 
     FIG. 1 is a block diagram of an exemplary cell relay network consistent with this invention; 
     FIG. 2 is an exemplary diagram of a packet transmitted in the network of FIG. 1; 
     FIGS. 3A and 3B are exemplary diagrams of components of a cell included in the packet of FIG. 2; 
     FIG. 4 is a block diagram of an output port residing within the exemplary cell relay switch in the network of FIG. 1; 
     FIGS. 5A and 5B are flowcharts of cell discard processing consistent with the present invention; and 
     FIG. 6 is a graph of buffer capacity as a function of time. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Systems and methods consistent with the present invention provide a cell discard scheme for an output port within a cell relay switch. The output port either guarantees delivery of all of the cells of a packet or drops all the cells beginning with the first one. The output port decides whether to store an incoming cell based on whether the buffer has sufficient capacity to store all of the cells of the associated packet. 
     Exemplary Cell Relay System 
     FIG. 1 is a simplified block diagram of a cell relay system  100 . The system  100  includes a number of source stations  110  and  115  and a number of destination stations  120  and  125  connected through switch  130  by virtual circuits  140  and  145 . The source stations  110  and  115  and destination stations  120  and  125  include computers, such as IBM-compatible computers, workstations, or even “dumb” terminals. While only two pairs of source stations  110  and  115  and destination stations  120  and  125  are shown in FIG. 1, those skilled in the art will recognize that systems and method consistent with the present invention may be used in a network with any number of source and destination stations and any number of switches. 
     Switch  130  is a cell relay switch that receives multiple cell streams from any source station  110  or  115  and transmits the cells to any destination station  120  or  125 . Switch  130  receives and transmits the cells via the virtual circuit  140  or  145 . Virtual circuit  140  and  145  are routes established through system  100  by which source stations  110  and  115  transmit the cells to destination stations  120  and  125 . Virtual circuit  140  and  145  transmit data at a peak rate (R). 
     When a source station  110  communicates with a destination station  120  or  125 , for example, source station  110  segments a packet of information into a number of cells. FIG. 2 is a diagram of a packet  200 . Packet  200  is a variable-length packet, such as an Internet Protocol (IP) packet. Source station  110  divides the variable-length packet  200  into several cells  210  of fixed length. 
     FIGS. 3A and 3B are exemplary diagrams of components of a cell  210  of FIG.  2 . The cell  210  includes a payload  310  and a header  320 . Payload  310  includes data of fixed-length. FIG. 3B shows the fields included in header  320  for, an ATM-formatted cell. Header  320  includes a generic flow control (GFC) field  321 , a virtual path identifier (VPI) field  322 , a virtual circuit identifier (VCI) field  323 , a payload type identifier (PTI) field  324 , a cell loss priority (CLP) field  325 , and a checksum (CRC) field  326 . The ATM cell is depicted for exemplary purposes only and the present invention may operate on any type of cell format. 
     Exemplary Output Port 
     FIG. 4 is an exemplary block diagram of an output port  400  contained within cell relay switch  130  shown in FIG.  1 . Output port  400  includes a queuing buffer  410 , a controller  420 , and an output processor  430 . Queuing buffer  410  receives the packets transmitted by various source stations  110 . Output processor  430  relays the packets of cells to a particular destination station  120  via a fiber link  432  or similar communication link. 
     Queuing buffer  410  temporarily stores the cells received by output port  400  before transmission by output processor  430 . Controller  420  includes a standard device, such as a processor, that controls the operation of buffer  410  and output processor  430 . Controller  420  executes software to determine which of the received cells will be discarded and which cells will be stored in queuing buffer  410  for transmission by the output processor  430 . Controller  420  bases its determination on factors that optimize use of buffer  410 . Output processor  430  provides the processing necessary to transmit the selected cells to the destination station  120 . As described in greater detail below, this determination is based upon the present occupancy of buffer  410 , the size of the packet, and the rate at which the packet is received. 
     Exemplary Cell Discard Processing 
     FIGS. 5A and 5B are flowcharts of cell discard processing consistent with the present invention. The processing begins when an output port  400  receives a cell of an incoming packet over a virtual circuit at queuing buffer  410  [step  505 ]. Each packet has an identifier such as a Virtual Circuit Identifier (VCI), for example, to identify the virtual circuit over which it is carried. Controller  420  processes the incoming cell to determine whether it is the first cell of the packet [step  510 ]. Controller  420  might make this determination from information provided in the header  320  (FIG. 3B) of the ATM cell, or from information contained within payload  310 . 
     If controller  420  determines that the cell is the first cell, controller  420  next determines the length of the packet L x  [step  515 ]. This determination may be made from information within cell header  320  or alternatively, from data within the payload of the first cell. Controller  420  also determines the peak arrival rate of the cells R x  of this packet [step  520 ]. The peak rate information is stored within the switch during the initial virtual circuit setup. 
     Next, controller  420  calculates the projected maximum buffer occupancy (B m ) of output port  400  if the packet was to be accepted [step  525 ]. To make this determination, controller  420  uses the current condition of output port  400  and the time needed to complete the receipt of the pending packet (L x , R x ). In accordance with the present invention, the current conditions of output port  400  may be based upon the current buffer occupancy B c , the rate at which output processor  430  processes (outputs) the cells from the buffer, and the number of cells in the current packet that have yet to arrive. 
     To simplify the calculations, the rates at which the cells are received are normalized to correspond to the processing rate of output processor  430 . If the peak arrival rate of a particular virtual circuit is half the rate of output processor  430 , for example, then the normalized rate would be 0.5. 
     Controller  420  determines the number of cells yet to be received from the total length of the packet. To accomplish this determination, controller  420  contains a mechanism to determine the number of remaining cells that have yet to arrive at buffer  410 . For example, the cell may carry a decrementing counter field that explicitly conveys the number of remaining cells. To implement this feature, the system may use the header CRC field  326  (FIG.  3 B), for example, as a counter to convey the length of the packet in terms of cells. This limits the length of the packet, however, to 256 cells or 12,288 bytes. Alternatively, controller  420  might contain a counter that decrements when a cell belonging to the same packet and identified with the same VCI has been stored in buffer  410 . 
     If there are currently a total number of N packets accepted by output port  400 , then the peak arrival rate of these N packets will be R i , where i=1, 2, . . . N. The number of cells of these N packets that have yet to arrive at buffer  410  will be L i , where i=1, 2, . . . N. Given the number of remaining cells and the rate at which cells will be received from a given virtual circuit, controller  420  arranges the packets according to completion time such that 
     
       
           L   1   /R   1   ≦L   2   /R   2   ≦ . . . ≦L   N   /R   N . 
       
     
     Once this ordering is established, controller  420  determine&#39;s the point at which the buffer occupancy reaches its maximum. FIG. 6 is a graph of buffer occupancy (B) as a function of time (T). The graph shows that the buffer occupancy begins as an increasing function with decreased rate. After the maximum point is reached, denoted (T m , B m ), the buffer occupancy becomes a decreasing function with an increasing rate. Initially, the aggregate arrival rate of all the packets is greater than the rate at which the output processor can process the cells in buffer  410 . This aggregate arrival rate decreases when all the cells of a packet have arrived at buffer  410 , and the packet&#39;s contribution to the aggregate arrival rate becomes zero. According to the preferred embodiment, as more and more packets are completed, the aggregate rate will continue to decrease. The maximum point (T m , B m ) is reached when the aggregate arrival rate becomes smaller than the processing rate of output processor  430 . 
     To determine the time at which the maximum buffer occupancy (T m , B m ) is reached, controller  420  must determine the point at which the aggregate arrival rate becomes less than 1. 
     Controller  420  adds the peak rates (R 1 , R 2 , . . . R N ) to determine the point at which the combination of peak rates becomes smaller than 1: 
     
       
         ( R   1   +R   2   + . . . +R   N )&gt;1 
       
     
     
       
         ( R   1   +R   3   + . . . +R   N )&gt;1 
       
     
     
       
         . . . 
       
     
     
       
         ( R   j   +R   j+1   + . . . +R   N )&gt;1 
       
     
     
       
         ( R   j+1   +R   j+2   + . . . +R   N )&gt;1 
       
     
     
       
         . . . 
       
     
     
       
         ( R   N )&gt;1 
       
     
     If the rate R j , for example, increases the aggregate arrival rate from ≦1 to greater than 1, then the time at which buffer  410  reached maximum occupancy is T m =L j /R j  and the maximum occupancy is 
     
       
           B   m   =B   c   +L   1   +L   2   + . . . +L   j +( R   j+1   + . . . +R   N )* T   m −(1 *T   m ) 
       
     
     In this case, B c  refers to the current buffer occupancy; L 1 +L 2 + . . . L j  are the remaining cells of the packets that will be completed by the time T m ; (R j+1 + . . . +R N )*T m  refers to the number cells of packets that will not be completed by time T m ; and (1*T m ) refers to the total number of cells that will be processed by T m . If the aggregate arrival rate is always less than 1, then controller  420  simply uses R N  as R j  in the calculation. 
     After determining the current buffer occupancy, controller  420  uses the information including the current conditions of output port  400  to determine whether to accept the newly arrived packet with length L x  and peak rate R x . Specifically, controller  420  inserts the new packet [L x , R x ] into the collection of accepted packets [L i , R j ], where i=1, 2, . . . , N, and recalculates the new maximum buffer capacity B m  if the new packet were to be accepted. 
     Based upon the new maximum value, B m , controller  420  must determine whether to accept the incoming cell of the packet based on whether buffer  410  contains sufficient remaining capacity to store the new packet [step  530 ]. If the new B m  is greater than the physical capacity of buffer  410 , then the packet must be discarded because buffer  410  will not have the storage capacity to store the entire packet. If, on the other hand, the new B m  is smaller than the physical capacity of buffer  410 , then the cell can be accepted. 
     If the cell is accepted, controller  420  allows it to be stored in buffer  410  [step  535 ]. In accepting the new cell, controller  420  logs the VCI x  of the new packet, such that the length, L x , and the rate, R x  of the new packet may be used in future calculation. If the new B m  is larger than B T  (i.e., the maximum capacity), then the full cell packet cannot be accepted for storage and is therefore discarded [step  540 ]. Upon discarding the cell, controller  420  also logs that the packet identified with VCI x , for example, was discarded and the L x  and R x  of the discarded packet will not be included in future calculations. 
     Returning to step  510 , if the received cell is not the first cell of a packet, then controller  420  determines whether the first cell of the packet was discarded or accepted based upon the VCI of the cell [step  545 ] (FIG.  5 B). Controller  420  might make this determination based on the information stored in the header of the cell, such as the virtual circuit identifier (VCI) field  323  (FIG.  3 B). The decision could also be made from information included within the body or payload of the cell. 
     If controller  420  had accepted the first cell, then it stores the new cell in buffer  410  [step  550 ]. If, on the other hand, controller  420  discarded the first cell, then it also discards this new cell [step  555 ]. Regardless of whether controller  420  stores or discards the cell, it prepares itself for receipt of the next cell by returning to step  505  of FIG.  5 A. If the cell is stored, then all the remaining cells of the accepted packet are subsequently stored. 
     The systems and methods consistent with the present invention optimize packet transmission through a cell relay system by guaranteeing that if one cell of a packet is accepted, then all cells of the packet will also be accepted. 
     The foregoing description of preferred embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The scope of the invention is defined by the claims and their equivalents. 
     For example, the foregoing description assumed that all cells belonging to the same IP packet arrive at the output port at the same maximum peak rate of the corresponding virtual circuit. In some ATM systems, however, the source station enters into a negotiated traffic contract with the system for packets that it transmits. In this case, the arrival rate depends on traffic shaping performed at the source station. 
     In multi-hop relay systems, the system sometimes introduces timing jitter into the cell transmission. The timing jitter prolongs the cell arrival rate.