Abstract:
The invention provides for a method for running a computer network and such a computer network. The computer network comprises a number of devices being arranged in a stable daisy-chained loop, wherein each device comprises a bridge having at least three ports, whereby during running the computer network each device can take different states to avoid a loop, and whereby in case of rebooting the ports of at least one of the devices keep their current port states.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention provides for a method for running a computer network, especially a method for developing stable daisy-chained Ethernet loops and an arrangement for providing such loops. Furthermore, the invention provides for such a computer network. 
         [0002]    A computer network is a collection of computers and other components interconnected by communication channels. These channels allow for sharing of resources and information. Computer networks can be classified according to a variety of characteristics as the medium used, communication protocols, scale, topology, and organizational scope. 
         [0003]    Document WO 2010/088965 A1 shows a method for using a computer network comprising a number of bridges each having ports for receiving and sending streams. The streams are sent from a router via the ports of bridges that have shown interest to at least one receiver, whereby in case of a link failure at least one receiver sends back a message to the router. The method can be used in a so called Ethernet network. 
         [0004]    Ethernet networks are frame-based computer networks for local area networks. It is to be noted that Ethernet networks cannot handle loops. If a loop is created, packets will continuously be communicated over the loop. Standard protocols are developed to detect loops and isolate both ends of the loops. When the network topology changes, detection is restarted to avoid creation of unconnected parts of the network or new loops. 
         [0005]    Loops are usually created for cable and equipment redundancy using Ethernet bridges. Devices are usually star-wired to the bridges. 
         [0006]    It can take some time before loops are detected. To avoid overload of the network by circling packets, the default in loop detection protocol is that there is no link. This means that there is no communication until the network topology is correctly detected. The best known protocols are Spanning Tree Protocol and Rapid Spanning Tree Protocol. 
         [0007]    The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for bridged Ethernet networks. STP is adapted to prevent bridge loops and broadcast radiation. For this, it creates a spanning tree within a mesh network of connected-2-bridge layers, e.g. Ethernet bridges, and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes. 
         [0008]    The Rapid Spanning Tree Protocol (RSTP) provides for significantly faster spanning tree convergence after a topology change introducing new convergence behaviour and bridge port rules. 
         [0009]    STP takes more than 30 seconds to recreate the network topology, RSTP takes 2 seconds or less in case of physical link failure. 
         [0010]    Audio networks can require a daisy-chained solution for its end devices. This is solved in a standard way by adding an Ethernet bridge with at least three ports to each device. Typically, in audio network the number of devices being daisy-chained is specified large, e.g. 20. Redundancy is an important feature for audio networks. Therefore, it must be possible to create a loop with a chain of 20 devices. Each device in the audio network is an end device, e.g. a loudspeaker. 
         [0011]    Daisy chain is a wiring scheme in which multiple devices are wired together in sequence or in a ring. Each of the network devices includes an upstream port and a downstream port, whereby each port is coupled to a cable. A third port in each device is adapted to receive traffic from and transmit traffic to the network. 
         [0012]    Document EP 1 720 293 A 1 shows a method for providing redundancy in a daisy chain local area network. The method comprises the steps of forming in each network device a passive signal path, connecting pins of an upstream port associated with the normally used twisted pairs of a Cat 5 cable to pins of a downstream port associated with the normally unused twisted pairs of the Cat 5 cable, forming loop back connections at the terminal network devices, and implementing a Spanning Tree Protocol in each of the network devices for defining an active signal path through the local area network. 
       SUMMARY OF THE INVENTION 
       [0013]    According to the invention, a method is for running a computer network, especially an Ethernet network, comprising a number of devices being arranged in a stable daisy-chained loop. Each device comprises a bridge having at least three ports. The third port is adapted to receive traffic from and transmit traffic to the computer network. 
         [0014]    The ports of each device can take different port states during running the computer network, e.g. forwarding and discarding state. It is to be noted that during rebooting the device will not run RSTP for some time. During this period it will keep the port states it had before the reboot. It is assumed that the device was in the correct state before rebooting. The bridge of the device is kept in this state until RSTP state machines are running again. This means that the bridge is neither reset nor reinitialized during rebooting. Therefore, the communication path is not interrupted during rebooting. 
         [0015]    For communication between bridges, the so-called Bridge protocol is used. Packets of the protocol are Bridge Protocol Data Units (BPDU). To ensure that each bridge has enough information, BPDUs are used. A bridge sends a BPDU frame using the unique MAC address of the port itself as a source address. 
         [0016]    RSTP port states are: 
         [0017]    Discarding: No user data is sent and any received data other than a BPDU is discarded. Received BPDUs are processed. 
         [0018]    Learning: Port learns source addresses of any incoming data. 
         [0019]    Forwarding: Port receives and sends data. 
         [0020]    Disabled: Can be done by network administrator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is an Ethernet network comprising a number of bridges with redundant loops. 
           [0022]      FIG. 2  is a number of audio network devices being arranged in a stable daisy-chained loop. 
           [0023]      FIG. 3  is a diagram depicting BPDUs showing hello time timeout. 
       
    
    
       [0024]    It will be understood that the features mentioned above and those described hereinafter can be used not only in the combination specified but also in other combinations or on their own, without departing from the scope of the invention. 
         [0025]    The invention is diagrammatically illustrated in the drawings by means of embodiments by way of example, and is hereinafter explained in detail with reference to the drawings. It is understood that the description is in no way limiting on the scope of the present invention and is nearly an illustration of embodiments of the invention. 
       DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows an Ethernet network  10  comprising a number of bridges with redundant loops. The Ethernet network  10  comprises a bridge A  12 , a bridge B  14 , a bridge C  16 , a bridge D  18 , a bridge E  20 , and a bridge F  22  being connected by communication channels  24 . The network  10  shown also comprises a number of loops. 
         [0027]    It can take some time before loops are detected. To avoid overload of the network by circling packets the default in loop detection protocols is that there is no link. This means there is no communication until the network topology is correctly detected. The most known protocols are Spanning Tree Protocol and Rapid Spanning Tree Protocol. STP takes more than 30 seconds to recreate the network topology where RSTP can do this in less than 2 seconds. 
         [0028]      FIG. 2  shows a computer network  30 , in this case an audio network, comprising a number of devices  32  being arranged in a daisy-chained loop between a first Ethernet bridge  34  and a second Ethernet switch  36  supporting RSTP. Each device  32  comprises a bridge having three ports. 
         [0029]    The bridges within the devices  32  do not have to support RSTP. However, if the devices do not support RSTP it will take dozens of seconds before the network is fully configured. If a cable between any of the devices  32  is interrupted it will again take typically 3×hello time before the network is reconfigured. Audio is very sensitive to communication loss since there is not retry mechanism in place. This means the loss of audio is equal to the network reconfiguration time. 
         [0030]    Bridges  34  and  36  can be root bridges. The bridges  32  in the devices  32  can not. When a root bridge would fail it can take up to  6  seconds before a new root bridge is elected. Therefore, the bridges within the device  32  should not become a root bridge. The root bridge is always the bridge having the lowest priority. The priority of bridges is explicitly set higher than default to prevent it to become a root bridge. 
         [0031]    By running an RSTP algorithm in each device  32 , the network reconfiguration time can be reduced to less than a second. This solves most of the problems but sometimes the device reboots or switches to firmware update mode. In these situations, it will take about 16 seconds before the RSTP algorithm is settled again. 
         [0032]    In the course of this, the network topology is usually not changed and therefore, the state before the reboot is still valid. This avoids a topology change and therefore, no interruption of audio and data. 
         [0033]    To avoid loops the device always starts with the Ethernet ports blocked. Only after communication of the RSTP messages, ports can be placed in the forwarding state. When the device reboots without a power loss, the port state assumably is still correct. Therefore, the state is kept persistently until the RSTP algorithm is full up and running again. The rebooting device will not send BPDUs until the RSTP algorithm is up and running To avoid that the other devices will detect a topology change during this, the default BPDU hello time is increased from the default 2 seconds to x seconds, e.g. 10 seconds. 
         [0034]    If the time it takes to have RSTP up and running is y and the hello time is x, the following contraint must hold: y≦2x if the device is not forced to send a hello BPDU before being reset, and y≦3x if the device is forced to send a hello BPDU before being reset. As it takes about 16 seconds for the device to have RSTP up and running a hello time of 9 seconds is currently used. 
         [0035]    The devices  32  can inform each other of their existence every z seconds, wherein z corresponds to ½ of their boot time. Furthermore, each device  32  can have a mechanism to make sure to react on any converge to avoid loops. 
         [0036]      FIG. 3  shows a diagram for illustrating communication within an Ethernet network. Reference number  50  represents RSTP supporting Ethernet bridges, reference number  52  represents devices supporting the method according to the invention. Thin dash/dotted lines showing the BPDU&#39;s exchange. Between the thick double line and double dotted line the device reboots. The solid lines indicate the BPDUs which are not sent out due to the reboot of the devices. The indication  10  on the left side represents 10 seconds. The double arrow  54  represents 15 seconds. 
         [0037]    The hello time is the time interval between the BPDU&#39;s sent from a bridge. It is used to inform the other bridge “hello I am here”. If you miss 3 BPDUs, this is 30 seconds with 10 seconds hello time, the other bridge will switch to the discarding mode again. The hello time is only used after the network has converged. Ports in forwarding state will then sent a BPDU every hello time seconds, ports in discarding state will expect an incoming BPDU every hello time seconds. It is meant to indicate the network is still converged. 
         [0038]    According to the RSTP specification, a device will issue a topology change when it misses 3 BPDUs. In a worst case scenario, the rebooting device was about to send a BPDU when it reboots. This means that RSTP must be up and running within 2×hello time seconds after reboot. Alternatively, the device could send out a BPDU first before it reboots since this reboot is normally planned. This increases the reboot time to 3×hello time seconds. 
         [0039]    After boot-up of a device the state machines of RSTP are started. Firstly, the state machines will put all ports in discarding state and after this to learning state. If the neighbour devices of the booting device were not reset they will react fast to the BPDU&#39;s sent by the booting device. The ports of the booting device will then move to forwarding state quickly. 
         [0040]    However, if the neighbour devices are also booting as they are reset too, it may take longer before they respond. This may still cause an interruption of the chain as the ports stay in learning state for a longer period and regular traffic is still not forwarded in learning state. In order to solve this, the ports that were in forwarding state are kept in that state, even if the RSTP algorithm indicates that they should go to discarding or learning state. The only exception to this rule is when a topology change BPDU is received on a port which means that the neighbour device has started a new network convergence. In that case the actual RSTP port state is forced onto the port before the acknowledgement of the topology change is sent back. Therefore, loops are prevented. 
         [0041]    In the case that there is a topology change when a device reboots the root bridge will still be able to break the loop. However, this will result in communication loss of dozens of seconds. It is important that the (root) Ethernet bridge is also configured to this x seconds hello time.