Abstract:
A switch controller using a congestion control method can relieve congestion in a network. The Ethernet switch controller has the capacity to select a suitable congestion control mode according to each connection port and state of the connected device. If a flow control mode is selected, the switch controller issues a flow control frame and waits for the passing of a guard period roughly equivalent to a period of about inputting three packets. If the connected device continues to transmit network packets after the guard period, the switch controller switches from the flow control mode to a drop control mode. Hence, the received packets are discarded and congestion is prevented. Consequently, the switch controller of this invention can still carry out congestion control operation when the switch controller is connected to a device having no standard flow control capability.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 89109750, filed May 20, 2000. The present application is also related to Taiwan application serial number 89106159 filed Apr. 1, 2000, entitled “METHOD AND SWITCH CONTROLLER FOR EASING FLOW CONGESTION IN NETWORK”, which is also filed on Mar. 29, 2001 in U.S.A. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to an Ethernet switch controller. More particularly, the present invention relates to an Ethernet switch controller capable of relieving flow congestion in a network. 
     2. Description of Related Art 
     Ethernet is a kind of local area network (LAN) standard, most widely used in communication. However, due to a data transmission rate of mere 10 Mbps, a conventional Ethernet can hardly transmit the vast quantity of data flow required in a multi-media system. Consequently, a faster Ethernet system having a data transmission rate of 100 Mbps called Fast Ethernet appears. In the Fast Ethernet design, a physical coding sublayer (PCS) is introduced between a medium access control (MAC) sublayer and a physical medium dependency (PMD) sublayer. To use the Fast Ethernet system, the network interface card in each network workstation has to be replaced by a 100 Mbps fast Ethernet interface card. If one wants to keep the network interface card in each workstation but raise the transmission speed, an Ethernet switch must be employed. In fact, by retaining the original network interface card in each workstation, the 10 Mbps Ethernet equipment formerly invested by a company can be incorporated into the Fast Ethernet network through the Ethernet switch. 
     The conventional Ethernet, using twisted pair (whether the data transmission rate is 10 Mbps or 100 Mbps), is necessary to be connected to a server via an Ethernet hub, so as to share the network information. In general, the bandwidth of an Ethernet hub is normally shared by all the workstations connected to the network. For example, for a 16 port 100 Mbps Ethernet hub, if four workstations are connected to the network, the bandwidth is shared between these four workstations. On the other hand, if each of the 16 ports is connected to a workstation, the bandwidth is shared between all sixteen workstations. As the number of network user increases, the number of collisions in the network increases proportionately. Hence, network bandwidth available for each user accordingly decreases when the number of users increases. In a multimedia-craved world, a conventional Ethernet hub can hardly meet requirement of the information traffic demanded by concurrent users. 
     The Ethernet switch is a new concept for improving data flow so that each of the workstations connected to the device is able to enjoy a faster data transmission. To achieve correct data switching, the switch keeps the records of the various connections between each workstation and each connection port. In other words, the switch must have a module for recording all the addresses similar to a bridging device. When the switch receives a frame, the device will consult a path lookup table to find the destination port with respect to the destination workstation. If the destination workstation is found, a controller will send out a control signal to the switching element redirecting the frame to the destination port. On the contrary, if the destination workstation is not found, the frame is broadcast to all the connection ports just to ensure that the destination workstation is able to receive this frame. 
     The institute of electrical &amp; electronics engineering (IEEE) has provided a standard specification 802.3u for network management 802.3u capable of simplifying network management. The IEEE standard 802.3u introduces an auto-negotiation function, also known as an N-way function. The auto-negotiation function enables the Ethernet switch and the Ethernet interface card of a workstation to have the states of each other. Each device can have a number of states. Table 1 is a listing of various combinations of states between the devices. Through the N-way, the data transmission rate (whether operating at 10 Mbps or 100 Mbps), and multiplexing mode (full duplex or half-duplex) of the link partner can be obtained. Hence, a congestion control method to be employed can be determined. 
     Before auto-negotiation strategies are incorporated into standard specification 802.3u of IEEE, a few manufacturers have already produced Ethernet card is with auto-sensing capability with its own specification. A number of Ethernet switches and Ethernet cards are also shown in Table 1, wherein some of the devices have auto-negotiation functions while some has not. 
     Due to the rapid progress in semiconductor technologies, the difference of the cost of producing an Ethernet switch and an Ethernet hub is getting smaller. Because of the many advantages of an Ethernet switch, Ethernet hubs are gradually replaced by Ethernet switches. Moreover, since an Ethernet switch can perform the functions provided by an Ethernet hub, the combination of devices detailed in Table 1 is equally applicable to Ethernet switches. 
     Furthermore, due to the multiplicity of transmission modes between different Ethernet devices, the automatic-negotiation must rely on an algorithm to determine a priority sequence registered in a table, so as to select the optimal transmission mode between two Ethernet devices. For example, a 10/100 Mbps dual speed network card is capable of operating at 10 Mbps or 100 Mbps. To set the transmission mode, the priority sequence table must opt for working in 100 Mbps unless constrained by other factors. Table 2 is a priority setting for the different transmission modes for the Ethernet devices having auto-negotiation capability. 
     In Table 2, full duplex transmission mode has a higher priority than half-duplex mode because full duplex has a much higher data transmission rate than half-duplex. Transmission mode 10BASE-T has the least priority because it has the slowest data transmission rate. By consulting the priority table 2, the most suitable mode for transmitting data between the Ethernet switch (hub) and the network card can be selected. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Various combination of states between an Ethernet hub (switch) and 
               
               
                 Ethernet card achieved through ‘auto-negotiation’ function or otherwise 
               
             
          
           
               
                   
                   
                   
                   
                 New generation 
               
               
                   
                   
                   
                   
                 of auto- 
               
               
                   
                   
                   
                 New generation 
                 negotiation 
               
               
                   
                 Support only 10 
                 Support only 
                 10/100TX co- 
                 10/100TX co- 
               
               
                   
                 BASE-T hub 
                 100BASE-T 
                 existent network 
                 existent hub 
               
               
                   
                 (switch) 
                 hub (switch) 
                 and hub (switch) 
                 (switch) 
               
               
                   
                   
               
             
          
           
               
                 Support only 
                 10 Mbps 
                 Change to a 100 
                 Manual switching 
                 Automatic 
               
               
                 10BASE-T 
                   
                 Mbps network 
                 of the hub 
                 switching of 
               
               
                 network card 
                   
                 card 
                 (switch) to 10 
                 hub (switch) to 
               
               
                   
                   
                   
                 Mbps 
                 10 Mbps 
               
               
                 Network card 
                 Automatic 
                 Automatic 
                 Automatic 
                 Manual 
               
               
                 with non- 
                 switching of 
                 switching of 
                 switching of 
                 switching of 
               
               
                 standard auto- 
                 network card to 
                 network card to 
                 network card to 
                 hub (switch) 
               
               
                 sensing 
                 10 Mbps 
                 100 Mbps 
                 100 Mbps after 
                 and network 
               
               
                 capability 
                   
                   
                 manual switching 
                 card to 100 
               
               
                   
                   
                   
                 of hub (switch) to 
                 Mbps 
               
               
                   
                   
                   
                 100 Mbps 
               
               
                 10/100TX co- 
                 Automatic 
                 Automatic 
                 Manual switching 
                 Automatic 
               
               
                 existent 
                 switching of 
                 switching of 
                 of hub (switch) 
                 switching of 
               
               
                 network card 
                 network card to 
                 network card to 
                 and network card 
                 hub (switch) 
               
               
                 with new 
                 10 Mbps half- 
                 100 Mbps 
                 to 100 Bbps 
                 and network 
               
               
                 generation auto- 
                 duplex 
                   
                   
                 card to 100 
               
               
                 negotiation 
                   
                   
                   
                 Mbps 
               
               
                 capability 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Priority setting of Ethernet devices with auto-negotiation capability 
               
             
          
           
               
                 Priority 
                 Explanations 
               
               
                   
               
               
                 1 
                 100BASE-T2 full duplex 
               
               
                 2 
                 100BASE-T2 
               
               
                 3 
                 100BASE-TX full duplex 
               
               
                 4 
                 100BASE-T4 
               
               
                 5 
                 100BASE-TX 
               
               
                 6 
                 10BASE-T full duplex 
               
               
                 7 
                 10BASE-T 
               
               
                   
               
             
          
         
       
     
     To increase the overall throughput after the best transmission mode is chosen, the Ethernet switch generally provides a congestion control mechanism for transmitting information packets from the transmission ports fluently. According to the resulting auto-negotiation between the destination device (for example, an network card) and the Ethernet switch, one of the following three congestion control modes are adopted: (1) when the destination device has full-duplex transmission capacity and flow control capability, the Ethernet switch will opt for a flow control mode; (2) when the destination device has full-duplex transmission capacity but no flow control capability, the Ethernet switch will opt for a drop control mode; and (3) when the destination device has neither full-duplex transmission capacity nor flow control capability, the Ethernet switch will opt for a back-pressure control mode. 
     In the aforementioned backpressure control mode, the Ethernet switching controller issues a collision signal to destroy the packet. On detecting the collision, the workstation enters into a binary exponential backoff algorithm to compute a waiting time, and then the packet is re-submitted. In the aforementioned drop control mode, the packet is necessary to be dropped at the source port, so that the packet is not sent to the congested destination port. This is because when the destination port has entered the congestion state, the transmission and receiving paths are different in full duplex mode, such that the collision signal cannot be used to destroy the packet. The packet is dropped at the source port. In the aforementioned flow control mode, the source port enters a flow control mode when the destination port is congested. In subsequent stage, flow control windows (XOFF windows) are triggered. Once XOFF windows are triggered, the Ethernet switching controller controls the flow in/out of packets according to the number of free buffers present. That means, control right is in the receiver. 
     Before IEEE 802.3u standard is made, flow control capability (FC) has no standard. Many network-device manufacturers set up their own flow control standard. Consequently, after the establishment of flow control standard 802.3u by IEEE, a lot of flow control equipment does not operate in accordance with the new standard. For example, an ‘ON’ signal for the flow control capability received by the Ethernet switch may not conform to the standard. Hence, actual flow control does not work. 
     One major drawback of the congestion control method used in the conventional Ethernet switch controller is that the Ethernet switch controller cannot discern whether the connected devices have actual flow control capability or not. As soon as the controller receives a signal from the connected device indicating a flow control capability, the controller issues a flow control frame to the connected device even if the connected device has no standard flow control capability. This fails the flow control. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a switch controller and a method of operating the controller for relieving network congestion substantially. After the switch controller has issued a flow control frame to a connected device, the switch controller waits for the passing of a guard period. The guard period is an interval that permits the input of about three network packets. At the end of the guard period, if there still are new incoming packets from the connected device, the controller switches from a flow control mode of operation to a drop control mode of operation. In other words, the packet is dropped to achieve flow control and avoid the conventional flow control problem. 
     According to this invention, after the switch controller has issued a flow control frame to a connected device, the switch controller waits for the passing of a period defined as a guard period. At the end of the guard period, if the connected device continues to issue packets after the end of the guard period, this implies that the connected device has non-standard-flow control capability. Flow control mode in use by the controller is immediately changed to drop control mode so that packet can be dropped to achieve proper flow control. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
         FIG. 1  is a block diagram showing Ethernet switch controller circuit connections of an Ethernet switch according to one preferred embodiment of this invention; 
         FIG. 2  is a block diagram showing the circuit connections of an Ethernet switch controller according to this invention; 
         FIG. 3  is a block diagram showing the Ethernet port controller shown in  FIG. 2 ; and 
         FIG. 4  is a diagram showing the use of XON-XOFF windows according to the flow control method operated by the Ethernet switch controller of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 1  is a block diagram showing Ethernet switch controller circuit connections of an Ethernet switch according to one preferred embodiment of this invention. As shown in  FIG. 1 , the Ethernet switch  100  includes an Ethernet switch controller  110 , a shared buffer  120 , a plurality of physical layer devices  130 , an electrical erasable programmable read-only-memory (EEPROM) unit and a CPU  150 . Size of the shared buffer  120  is determined by jumpers. Ethernet switch controller  110  connects with the CPU  150  at a CPU port via a medium independent interface (MII). Data can also be transmitted between a CPU port on Ethernet switch controller  110  and the CPU  150  via an ISA/IDE interface. In the meantime, Ethernet switch controller  110  connects with the plurality of physical layer devices  130  via a reduced medium independent interface (RMII). The advantage of using RMII is to save pins so that the number of pins used by MII can be reduced from 14 to 6. Consequently, the total pin out for Ethernet switch controller  110  is also reduced. 
       FIG. 2  is a block diagram showing the circuit connections of an Ethernet switch controller according to an embodiment of this invention. As shown in  FIG. 2 , Ethernet switch controller  110  includes a plurality of Ethernet port controllers  114 , a queue controller  113 , a forwarding device  111  and a buffer controller  112 . The plurality of Ethernet port controllers  114  are connected to the plurality of physical layer devices (PHY)  130  and a plurality of external signal lines (not shown). Through the physical layer devices (PHY)  130 , a plurality of state signals of connected devices can be obtained. These state signals includes duplex mode signals and flow control capability signals. According to external signals, such as flow control enable (Flow_Control_En) signals, drop control enable (Drop_Control_En) signals, or backpressure enable (Backpressure_En) signals, the type of congestion control mode employed by switch controller  110  is determined. The flow control enable signals, the drop control enable signals and the backpressure enable signals can be set by jumpers. Ethernet port controllers  114  generate a plurality of flow control window signals (XOFF_Window[9:0]) for the queue controller  113  according to the plurality of state signals. Moreover, drop signals DROP_On [9:0] are enabled according to the plurality of flow control windows and the plurality of external signals. 
     A forwarding device  111  is coupled to the plurality of Ethernet port controller  114 . The forwarding device  111  looks up a table according to the heading on network packets received from Ethernet port controllers  114  to find out destination ports. The drop control mode is determined by congestion-on (CONGEST_ON[9:0]) signal and drop-on (DROP_ON[9:0]) signal. Buffer controller  112  is coupled to the plurality of Ethernet port controllers  114 . Buffer within the buffer controller  112  is shared by all the Ethernet port controllers  114 . According to the requests submitted by the port controllers  114 , a number of buffers within buffer controller  112  are assigned or released. 
     A queue controller  113  is coupled to the Ethernet port controllers  114 , the buffer controller  112  and the forwarding device  111 . Each Ethernet port controller  114  has a corresponding output queue in the queue controller  113 . According to the request submitted by the port controller  114 , the incoming packet is then queued in the corresponding output queue. Furthermore, according to the plurality of window flow control (XOFF_Window[9:0]) signals and the total output queue length, the congestion control mode of each Ethernet port controller  114  is determined. If any output queue in the queue controller  113  is at a congested state, a congest-on (CONGEST_ON) signal is transmitted to the forwarding device  111 . In addition, flow control window [9:0] signals are activated to request flow control of the source port. 
       FIG. 3  is a block diagram showing the Ethernet port controller shown in  FIG. 2 . As shown in  FIG. 3 , each Ethernet port controller  114  includes a receive media access controller (RMAC)  1142 , an input controller  1141 , an output controller  1143 , a transmit media access controller (TMAC)  1144  and a physical layer controller  1145 . The RMAC  1142  is coupled to one of the physical layer devices  130 . The RMAC  1142  checks the received network packets. A received network packet is accepted if the packet is correct, otherwise the packet is dumped. An input controller  1141  is coupled to the RMAC  1142 , the queue controller  113  and the buffer controller  112 . According to the amount of the buffers requested by received network packet via the buffer controller  112 , the request is sent to the queue controller  113  to request queuing in the output queues. The output controller  1143  is coupled to the queue controller  113  and the buffer controller  112  for outputting packets from the output queue and releasing free buffers to the buffer controller  112 . The TMAC  1144  is coupled to the output controller  1143  and one of the physical layer devices  130 . According to the plurality of window flow control signals and external signals, a drop-on signal to the forwarding device  111  may be issued so that the network packet submitted by the Ethernet port controller  114  may be discarded. The physical layer controller  1145  is coupled to the TMAC  1144  and one of the physical layer devices  130 . According to the plurality of state signals from the physical layer device  130 , a flow control enable (FC_EN) signal is transmitted from the physical layer controller  1145  to the TMAC  1144 . 
       FIG. 4  is a diagram showing the use of XON-XOFF window in the flow control operated by the Ethernet switch controller of an embodiment of this invention. As shown in  FIG. 4 , as long as there is no congestion in the Ethernet switch controller  110 , all connection ports can normally transcieve network packets. When a particular connection port is congested, the Ethernet switch controller  110  initiates a congestion control according to the results of auto-negotiation. If a backpressure control mode is employed, the Ethernet switch controller  110  issues a collision signal to impact the packet. On detecting the collision, the workstation enters into a binary exponential backoff algorithm to compute a waiting period, i.e. a guard period, to retransmit the packet. In the drop control mode, the packet is dropped at the source port instead of sending to the congested destination port. Because the destination device uses full duplex transmission, so different transmission lines are used for sending and receiving data. Since destruction of the packet by collision signal is not allowed, the packet can only be drop at the source port. This ensures that the packet is not sent to the congested destination port. A flow control mode is used when the Ethernet switch controller and the network card of the workstation can operate in full duplex mode and has flow control capability. As shown in  FIG. 4 , when the destination port of a packet is congested, the flow control window (XOFF windows) is triggered. Once the XOFF window is triggered, the source port of the Ethernet switching controller  110  sends out a flow control frame that represents XOFF condition. The XOFF window condition is released only when the number of free buffers is sufficient for the destination ports of the Ethernet switch controller  110  to transmit data normally. Once sufficient free buffers are present, an XON flow control frame is issued to return to XON. 
     In this invention, the flow control is based on XON/XOFF windows. In a XON window, source port is not in a flow control state. Hence, any incoming packets are normally transferred away. If an incoming packet violates congestion control restrictions, flow control state switches from the XON window into the XOFF window after the source port forwards its packet to the output queue of the destination port. In the XOFF window, the source port is in a flow control state. Therefore, any incoming packets can triggers a congestion control operation according to the strategies used. The XON/XOFF windows and DROP enable/disable signal are administrated by TMAC 1144 of Ethernet switch controller  110  according to state of the connection port and the plurality of input signals from queue controller  113 . 
     In addition, related congestion control operations in the XOFF window in  FIG. 4  are as follows: (1) Flow control operation: If a unicast or broadcast packet is sent to the output queue, and some congestion control restrictions are violated, queue controller  113  generates a trigger signal to trigger the TMAC 1144 of the source port to send out a flow control frame having a pause time=FFFFH. After the flow control frame is sent, switch controller  110  will wait for a guard period, preferably equal to a period for accepting three packets. If the connected device continues to send out packets after the expiry of the guard period, the connected device is judged to have no standard flow control capability, and operation mode of switch controller  110  is changed from the flow control to a drop control. (2) Backpressure control for a half-duplex connection port: On receiving a non-local packet, the input controller  1141  generates a non-local signal to inform the TMAC  1144 . If the flow control is in the XOFF window and operates in a half-duplex mode, the TMAC  1144  destroys the packet. (3) Drop control with retained private buffers: When an incoming packet comes from a source port already in the DROP_ON window and the destination port is already in the congestion control window, the packet is detected during a lookup operation in forwarding device  111 , and then the forwarding device  111  removes the destination port mask so that the input controller  1141  can discard the packet. It should be noted that the DROP_ON window informs the drop function of specified connection port to be activated within this time interval, and the DROP_ON window is determined by TMAC  1144 . 
     This invention also provides a network congestion control method. According to a plurality of external signals, a plurality of state signals and flow condition in the Ethernet, an appropriate congestion control mode is selected. The method includes several steps. First, a plurality of packets are transferred to Ethernet port controllers  114 . According to the plurality of external signals and the plurality of state signals, controllers  114  generate a plurality of flow control window signals. Destination ports of various packets are determined through table lookup operations in the forwarding device  111 . If a destination port is in a congested state and the source port of the packet has no support for flow control capability, the packet is discarded. If the destination port is not in the congested state, a buffer request command is sent to the shared buffer  120 . This is followed by another request sent to queue controller  113  for requesting the placement into the output queue corresponding to the destination port. According to the plurality of flow control window signals and the total output queue length, the congestion control mode of each port control device inside the Ethernet port controllers  114  is determined. 
     The plurality of external signals include a flow control enable signal, a drop control enable signal and a backpressure enable signal. The flow control enable signal, the backpressure enable signal and the drop control enable signal can be determined by jumpers. The plurality of state signals includes a duplex mode signal and a flow control capability signal. A flow control frame can be issued in the flow control mode. The flow control frame includes a 16-bit pause time having a value of FFFFH. When the flow control window is closed, the 16-bit pause time value is 0000. After the issue of a flow control frame, the switch controller  110  waits for a guard period preferably equal to a duration for entering of three packets. After the guard period, if the connected device continues to transmit packets, the connected device is judged not to have standard flow control capability. Hence, the switch controller  110  switches from a flow control mode to a drop control mode. Persons skilled in the art may note that length of the guard period can be adjusted according to applications and other factors. 
     Size of shared buffer  120  is determined by size of externally connected static random access memory (SRAM). Size of SRAM can be determined by jumpers, for example, 32 K×32 or 64 K×32 SRAM. Moreover, the number of private buffers in shared buffer  120  can be determined by an externally connected EEPROM  140  or CPU  150 . 
     In summary, the Ethernet switch controller and the network congestion control method used by the controller in the invention has the capability of adaptively adjusting from a flow control mode to a drop control mode. Therefore, congestion control is still possible between the Ethernet switch controller of this invention and a connected device not having a standard flow control capability. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.