Abstract:
In a system in which communication is performed among a plurality of devices having a mechanism of disassembling a packet into ATM cells to send them and a plurality of devices having a mechanism of assembling the received ATM cells into the packet, a frame is prevented from being lost owing to change of virtual connections in the course of transferring the frame, improving reliability of the communication on the packet level.  
     When the switch receives a request for change of switching, it does not processes that request at once, but confirms that the cell located at the boundary of the frame has been processed, before processing the change request, so as to protect the frame.  
     When the switch receives a request for change of switching, it protects a frame by multicasting the cells to both destinations before change and after change, for a given period of time.

Description:
RELATED APPLICATIONS  
       [0001]    This is a U.S. Continuation Application based on U.S. Continuation patent application Ser. No. 09/776,283 filed on Feb. 2, 2001 which claims the priority benefits from U.S. patent application Ser. No. 08/998,382 filed on Dec. 24, 1997 and Japanese Patent Application No. 8-344981 filed Dec. 25, 1996, the entirety of which we incorporated herein by reference.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an IP switch (Internet Protocol switch) for transferring packet data in the lower layer of ATM (Asynchronous Transfer Mode), to an interface circuit and an ATM switch used for that IP switch, and to an IP switch network system.  
           [0004]    2. Related Art Statement  
           [0005]    At present, IP (Internet Protocol) is widely used as a protocol in computer communication in which data is divided into packets on a computer before transmission. According to the 7 layer model of OSI (Open System Interconnection), IP is the protocol which belongs to the network layer, and, to perform communication between different physical networks, communication is realized through routers.  
           [0006]    However, it has been generally pointed out that it is difficult to make the IP router fast and to provide it with mass storage because of the characteristics of IP, and accordingly, even when high-speed lower layers are employed, their capacity can not be utilized sufficiently.  
           [0007]    Recently, as a solution to these problems, there has been noted a technique called “IP Switching” proposed in  Flow Labeled IP: A Connectionless Approach to ATM  (Proc. IEEE Infocom, San Francisco, March 1996). This technique is one that introduces the arrangement of ATM (Asynchronous Transfer Mode) known as a high-speed communication technique into the ordinary IP, in an attempt to speed up the IP router.  
         SUMMARY OF THE INVENTION  
         [0008]    In the above prior art, the process in the network layer, which is handled by software, is simplified as much as possible, to speed up the process, and retransmission by the cell is not taken into consideration. Accordingly, even if only one cell is lost, the whole frame containing that cell is discarded. As a result, when a virtual connection is changed in the middle of transfer of a frame, that frame is lost from the network.  
           [0009]    Conventionally, in ATM, it is provided that a virtual connection is established in advance, and data is transferred on that line, presupposing that the transfer line is not changed in the middle of communication. Thus, there has not been such a problem as a packet being lost owing to change of a transfer line during the course of transfer.  
           [0010]    In the IP switching technique, however, since a virtual connection is changed during the course of data transfer, it is possible that such loss of a packet causes a problem. In the following, such an example will be described.  
           [0011]    Conventionally, such packet loss has been dealt with by retransmission upon timeout in the network layer. However, this processing usually occurs at the time of a fault such as congestion or link breakage, and too frequent occurrence of this processing may give rise to congestion, or a transfer rate may be lowered by awaiting timeout. Thus, from the viewpoint of quality, it is problematic to use the timeout retransmission technique for such a frequent phenomenon as change of a virtual connection in IP switching.  
           [0012]    In particular, in the case that IP switching is used in a public network which employs packet metering as an accounting method, the technique of handling packet loss relying on retransmission in the network layer can not be accepted because double accounting on a user may arise.  
           [0013]    An object of the present invention is to provide an IP switch, interface circuit and an ATM switch used for that IP switch, and an IP switch network system, which can prevent loss of a frame caused by change of a virtual connection in the middle of frame transfer.  
           [0014]    Another object of the present invention is to provide an IP switch, interface circuit and an ATM switch used for that IP switch, and an IP switch network system, which can avoid retransmission owing to the above-described frame loss caused by change of a virtual connection in the middle of frame transfer, and can avoid double accounting on a user due to retransmission of a packet.  
           [0015]    Another object of the present invention is to provide an IP switch, interface circuit and an ATM switch used for that IP switch, and an IP switch network system, which can avoid retransmission owing to the above-described frame loss caused by change of a virtual connection in the middle of frame transfer, and can improve the transfer rate.  
           [0016]    In a first invention, when a virtual connection is to be changed, a PT (Payload Type) field of an ATM cell which is currently in the course of transfer is referred to, and the transfer line is kept from being changed until the boundary of the frame is recognized, so that an AAL (ATM Adaptation Layer) frame is protected and packet loss is prevented.  
           [0017]    In a second invention, when a virtual connection is to be changed, the multicast function of the ATM switch is employed to transfer a sufficient number of cells for transmission of the maximum length of packets through both transfer lines before and after the change, and thereafter the virtual connection before the change is eliminated, so that an AAL5 (ATM Adaptation Layer Type 5) frame is protected and packet loss is prevented. The “sufficient number of cells for transmission of the maximum length of packets” is set in advance, based on the design policy of the network.  
           [0018]    In a third invention, when a virtual connection is to be changed, the multicast function of the ATM switch is employed to transfer cells through both transfer lines before and after the change, for a sufficient period of time for transmission of the maximum length of packets, and thereafter the virtual connection before the change is eliminated, so that an AAL5 frame is protected and packet loss is prevented. The “sufficient period of time for transmission of the maximum length of packets” is set in advance, based on design policy of the network. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a view showing an example of a network to which IP switching is applied;  
         [0020]    [0020]FIG. 2 is a view showing the simplest construction of the IP switching network and internal structures of nodes;  
         [0021]    [0021]FIG. 3 is a view showing how a packet is transferred in a normal state;  
         [0022]    [0022]FIG. 4 is a view showing how a packet is transferred using a new virtual connection;  
         [0023]    [0023]FIG. 5 is a view showing how a packet is transferred using a new virtual connection, when a packet processing in an IP switch is omitted;  
         [0024]    [0024]FIG. 6 is a view showing an example of a virtual connection setting request;  
         [0025]    [0025]FIG. 7 is a view showing an example of a virtual connection change request;  
         [0026]    [0026]FIG. 8 is a sequence diagram showing how information is interchanged in FIGS. 4 and 5;  
         [0027]    [0027]FIG. 9 is a view showing correspondence relations among a packet, an AAL5 frame and ATM cells;  
         [0028]    [0028]FIG. 10 is a sequence diagram showing an example where a packet is lost owing to a virtual connection change request;  
         [0029]    [0029]FIG. 11 is a sequence diagram in the case where a waiting time for a change of virtual connections is set;  
         [0030]    [0030]FIG. 12 is a sequence diagram in the case where multicast period W 2  is set;  
         [0031]    [0031]FIG. 13 is a sequence diagram in the case where multicast period W 3  is set;  
         [0032]    [0032]FIG. 14 is a view showing a basic internal structure of a switch section  24 ;  
         [0033]    [0033]FIG. 15 is a view showing a basic internal structure of an ATM interface section  242 ;  
         [0034]    [0034]FIG. 16 is a flowchart showing a virtual connection change procedure;  
         [0035]    [0035]FIG. 17 is a view showing an internal structure of the ATM interface section  242  provided with a PT field monitoring section;  
         [0036]    [0036]FIG. 18 is a flowchart showing a virtual connection change procedure which includes monitoring of a PT field;  
         [0037]    [0037]FIG. 19 is a view showing an example of rewriting an output-side port identifier, using a header transform table;  
         [0038]    [0038]FIG. 20 is a view showing an internal structure of the ATM interface section  242  provided with a cell counting section;  
         [0039]    [0039]FIG. 21 is a flowchart showing a virtual connection change procedure which includes counting of cells;  
         [0040]    [0040]FIG. 22 is a view showing an example of rewriting a head transform table in the case of using the multicast system;  
         [0041]    [0041]FIG. 23 is a view showing an internal structure of the ATM interface section  242  provided with a timer section;  
         [0042]    [0042]FIG. 24 is a flowchart showing a virtual connection change procedure using the timer section;  
         [0043]    [0043]FIG. 25 is a view showing an internal structure of the switch section provided with the timer section;  
         [0044]    [0044]FIG. 26 is a flowchart showing a virtual connection change procedure in the case where the switch section is provided with the timer section; and  
         [0045]    [0045]FIG. 27 is a view showing an internal structure of the ATM interface section  242 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    [0046]FIG. 1 is a view showing an example of a network to which IP switching is applied. The network comprises nodes connected by communication media. The nodes include IP switches, edge nodes of the IP switching network (terminals or IP gateways), non-IP switching nodes (terminals or IP routers), and the like. An IP switch is a router employing the IP switching technique, and is connected with other IP switches and edge nodes of the IP switching network so as to form the network. Among the edge nodes of the IP switching network, one situated at the border with a non-IP switching network to communicate therewith is called an IP switch (IP gateway).  
         [0047]    Next, the principle of the IP switching will be described. FIG. 2 shows the simplest construction of the IP switching network. The node  1 A and the node  1 B are edge nodes of the IP switching network, and are connected with each other through the node  2  which is an IP switch. The node  1 A is a sending node, and comprises a packet processing section  11 A which performs transfer processing on the IP level, and a packet-to-cell disassembly section  12 A which disassembles a packet into ATM cells and performs transfer processing in ATM. The node  1 B is a receiving node, and comprises a packet processing section  11 B which performs transfer processing on the IP level, and a cell-to-packet assembly section  12 B which assembles cells received in ATM to restore them to a packet. The node  2 , which is an IP switch, comprises a packet processing section  21  which performs transfer processing on the IP level, a cell-to-packet assembly section  22  which assembles cells received in ATM to restore them to a packet, a packet-to-cell disassembly section  23  which disassembles a packet to ATM cells to transfer them in ATM, and a switch section  24  which performs switching on the ATM level. The switch section  24  has ports P 1 -P 4 , which are connected to the cell-to-packet assembly section  22 , the packet-to-cell disassembly section  23 , the node  1 A, and the node  1 B, respectively. A virtual connection VC 51  is established in advance between the node  1 A and the node  2 , and a virtual connection VC 52  is established in advance between the node  2  and the node  1 B. These virtual connections are prepared for ordinary data transfer and control message transfer.  
         [0048]    Here, for the sake of simplicity, the node  1 A is described as having the packet-to-cell disassembly section only, and the node  1 B the cell-to-packet assembly section only. In fact, communication is performed bilaterally, and each node has both the packet-to-cell disassembly section and the cell-to-packet assembly section. Further, here, the packet-to-cell disassembly section and the cell-to-packet assembly section are described as separate functional blocks. However, it is possible for both sections to be implemented in one functional block. In that case, the ports P 1  and P 2  are reduced to one port.  
         [0049]    FIGS.  3 - 5  are views explaining the principle of IP switching. These figures illustrate a state where packets A, B and C are sent from the node  1 A to the node  1 B consecutively.  
         [0050]    At first, FIG. 3 is referred to. Here, a packet A is generated in the node  1 A. Then, in the first place, the packet processing section  11 A judges that the packet A is one destined for the node  1 B, decides to transfer the packet toward the node  2 , the adjacent node in the direction of the node  1 B, and delivers the packet A to the packet-to-cell disassembly section  12 A. The packet-to-cell disassembly section  12 A disassembles the packet A into ATM cells A 1 -An, and transfers them through the virtual connection VC 51 . The switch section  24  receives the ATM cells A 1 -An, and transfers them to the cell-to-packet assembly section  22 . On receiving the ATM cells A 1 -An, the cell-to-packet assembly section  22  assembles them into the packet A and sends it to the packet processing section  21 . The packet processing section  21  judges from the contents of the packet A that the packet A should be sent to the node  1 B, and sends it to the packet-to-cell disassembly section  23 . The packet-to-cell disassembly section  23  disassembles the packet A into the ATM cells A 1 -An again, and transfers them to the node  1 B through the virtual connection VC 52  destined for the node  1 B. Last, the cell-to-packet assembly section  12 B of the node  1 B receives the ATM cells A 1 -An, assembles them into the packet A, and sends it to the packet processing section  11 B, completing the transfer process.  
         [0051]    At this time, the packet processing section  21  of the node  2 , which has processed the packet A, estimates whether another packet is to be received hereafter consecutively, based on the flow rate of cells sent from the node  1  for a given period of time. When it is estimated that another packet is to be received consecutively, the packet processing section  21  sends a request G 1  to the switch section  24  for setting a new virtual connection VC 53  as shown in FIG. 4. At the same time, the packet processing section  21  sends a request I 1  to the node  1 A for sending following packets through the virtual connection VC 53 . In this example, the request I 1  is sent through a control line between the node  1 A and node  2 . It, however, may be sent through the virtual connection VC 51 .  
         [0052]    [0052]FIG. 6 shows an example of the request G 1  for setting a new virtual connection from the packet processing section to the switch section  24 . The reference symbol G 11  refers to a message type (in this case, “Make new connection”), and G 12 -G 15  refer to information elements for setting a virtual connection in the switch section  24 . Namely, G 12  refers to an input-side port identifier, G 13  to an input-side virtual connection identifier x representing the virtual connection VC 53  to be set newly, G 14  to an output-side port identifier, and G 15  to an output-side virtual connection identifier y representing the virtual connection VC 53  to be newly set. Thus, the example shown in FIG. 6 represents a request, “Set a new virtual connection VC 53  which connects the virtual connection identifier x at the input-side port P 3  with the virtual connection identifier y at the output-side port P 1 ”.  
         [0053]    Next, FIG. 4 will be referred to. In FIG. 4, a packet B generated in the node  1 A is transferred to the node  1 B as in FIG. 3. The difference from FIG. 3 lies in that the ATM transfer from the node  1 A to the node  2  is performed through the virtual connection VC 53 .  
         [0054]    In parallel with this, similarly to the node  2  in FIG. 3, the node  1 B estimates from the former packet A that another packet is to be received consecutively hereafter, and sends the node  2  a request I 12  for a new virtual connection for transmission. On receiving the request, instead of making a new virtual connection, the node  2  sends a virtual connection change request G 2  to the switch section  24  for making a virtual connection between the node  1 B and the node  2  corresponding to the virtual connection VC 53 . As a result, the virtual connection VC 53  is changed to a virtual connection VC 53 ′ shown in FIG. 5.  
         [0055]    [0055]FIG. 7 shows an example of the virtual connection change request G 2  sent from the packet processing section  21  to the switch section  24 . Similarly to G 1 , G 21  refers to a message type (in this case, “change connections”), and G 22 -G 27  refer to information elements for setting a virtual connection in the switch section  24 . Namely, G 22  refers to an input-side port identifier, G 23  to an input-side virtual connection identifier x representing the virtual connection VC 53 ′ to be set newly, G 24  to an output-side port identifier before change, G 25  to an output-side virtual connection identifier y before change, G 26  to an output-side port identifier after change, and G 27  to an output-side virtual connection identifier z after change representing the virtual connection VC 53 ′ to be newly set. From G 22 -G 25 , the switch section  24  judges which virtual connection is requested, and replaces only the output-side setting with the information in G 26  and G 27 .  
         [0056]    Thus, the example shown in FIG. 7 represents a request, “Change the virtual connection VC 53 , which is represented by the input-side port P 3 —virtual connection identifier x, and the output-side port P 1 —virtual connection identifier y, to the virtual connection VC 53 ′ which is represented by the input-side port P 3 —virtual connection identifier x, and the output-side port P 4 —virtual connection identifier z”.  
         [0057]    Last, FIG. 5 will be referred to. As shown in FIG. 5, a packet C is transferred from the node  1 A to the node  1 B through the virtual connection VC 53 ′ established by the above process. Here, in the node  2 , the packet processing in the network layer handled by software is dispensed with, and only hardware processing in the ATM layer is performed, and accordingly the packet can be transferred at higher speed.  
         [0058]    [0058]FIG. 8 is a sequence diagram showing how the information in FIGS. 3, 4 and  5  is interchanged. As shown in FIG. 8, the packet processing section  21  sends the switch section  24  the virtual connection change request G 2 . In fact, since the switch section  24  and the packet processing section  21  are not synchronized, there may arise a case that, during the processing in the switch section  24 , a virtual connection change request related to an already-processed packet arrives from the packet processing section  21 .  
         [0059]    For example, a sequence diagram of FIG. 10 shows an example where, while the switch section  24  is processing the cell string C 1 -Cn which is obtained by disassembling the packet C, the switch section  24  receives a virtual connection change request G 2  from the packet processing section  21 . Here, in FIG. 10, after the switch section  24  has completed processing of the cell C 1  disassembled from the packet C, the virtual connection request G 2  arrives at the switch section  24  from the packet processing section  21 . As a result, the virtual connection VC 53  which has been destined for the cell-to-packet transform section  22  of the node  2  is changed to one destined for the cell-to-packet transform section  12 B of the node  1 B. As a result the cell C 1  arrives at the cell-to-packet transform section  22  of the node  2 , and the cells C 2 -Cn arrive at the cell-to-packet transform section  12 B of the node  1 B. In each of the cell-to-packet transform sections  22  and  12 B, the set of cells is not complete, and is discarded in that section. As a result, this packet C is lost on the network, and communication between the node  1 A and the node  1 B is not performed normally.  
         [0060]    As a system for packet communication in ATM, a system called AAL5 (ATM Adaptation Layer Type 5) is generally used, and IP switching also employs this system. Procedures in this system will be described referring to FIG. 9.  
         [0061]    First, on a sending side, a trailer  62  is added to a packet  61  in the network layer to obtain an AAL5 frame. The obtained AAL5 frame is disassembled into pieces of ATM data  65 . Each piece of the ATM data  65  is provided with an ATM header  63  to be an ATM cell, which is sent onto a virtual connection in ATM. An ATM header contains a part called a PT (Payload Type) field  64 . For an ATM header  63  of a cell located at the end of an AAL5 frame, a value “1” is set to its PT field  64 , and for the other cells, a value “0” is set to their respective PT fields  64 , thus indicating a boundary of the frame. On a receiving side, a cell with the value “1” in the PT field  64  is detected to separate a string of cells, and the trailers  62  are checked to confirm that the frame is not broken. Then, after removing the trailers  62 , the frame is delivered in the form of the packet  61  to the network layer.  
         [0062]    [0062]FIG. 11 is a sequence diagram showing basic operation of the present invention. Similarly to FIG. 10, here also, a virtual connection change request G 2  arrives after C 1  has been processed. As shown in FIG. 11 by a waiting time W 1  for a change, the switch section  24  awaits arrival of the cell Cn having the PT field value “1” before changing the virtual connection VC 53 . This waiting time W 1  is set as a sufficient period of time for sending the largest packet allowed by the network. Various values may be set to W 1  depending on the design policy of the network. As a result, no packet is lost in the communication between the node  1 A and the node  1 B, and communication is performed normally. In this FIG. 11, although the order of the packet C and a packet D is reversed, it can be restored by processing in the network layer. Differently from the loss, it is not necessary to await timeout, and hardly any quality problem arises.  
         [0063]    [0063]FIG. 14 shows an internal structure of the switch section  24 . A cell from the outside is received at an ATM interface section  242  on an input side, its header is exchanged, and thereafter, the cell is directed by a switch fabric section  241  toward an objective direction, and outputted through an ATM interface section  242  on an output side. A switch control instruction from the packet processing section  21  is temporarily sent to a switch control section  243  through an ATM interface section  242 . In the switch control section  243 , the switch control instruction is transformed into an interface control instruction, and sent to an ATM interface section  242  through an internal line  244 , to be processed.  
         [0064]    [0064]FIG. 15 shows the structure of an ATM interface section  242 . A cell inputted into the ATM interface section  242 , is temporarily stored in a cell buffer  2421 , and transformed in a header transform section  2422 , and sent to a switch fabric section  2423 . The interface control instruction from the switch control section  243  is received by an interface control section  241  and executed there.  
         [0065]    [0065]FIG. 16 is a flowchart showing an example of a procedure for changing virtual connections. First, when the packet processing section  21  judges that a change of switch setting is necessary, the packet processing section  21  sends a virtual connection change request to switch section  24  (Step S 1 ). This request is received by the switch control section  243  within the switch section  24  (Step S 2 ), and the switch control section  243  transforms the request into an interface control instruction and sends it to the interface section  242  (Step S 3 ). The interface control instruction is received by the interface control section  2423  within the ATM interface section  242  (Step S 4 ), and the interface control section  2423  rewrites a table in the header transform section  2422  (Step S 5 ), completing the change of the virtual connections.  
         [0066]    [0066]FIG. 19 shows an example of rewriting a header transform table. This is an example of a header transform table for the port P 3 , including pairs of an input-side virtual connection identifier and output-side port identifier—virtual connection identifier. In FIG. 19, entries  71  and  72  correspond to the change from the virtual connection VC 53  to the virtual connection VC 53 ′ in FIGS. 4, 5, and  7 . Namely, the entry  71  represents the virtual connection VC 53 , and the entry  72  the virtual connection VC 53 ′.  
         [0067]    [0067]FIG. 17 illustrates an internal structure of the ATM interface section  242  on the input side, according to the present invention. In comparison with FIG. 15, a PT field monitoring section  2424  is newly added.  
         [0068]    [0068]FIG. 18 is a flowchart showing an example of a procedure for changing virtual connections using the PT field monitoring section. Between the step S 4  and the Step S 5  of FIG. 16, Step S 6  and Step S 7  are inserted. In Step  6 , the ATM interface control section  2423  asks the PT field monitoring section  2424  if a cell with PT field value “1” has been processed. In Step  7 , based on the response from the PT field monitoring section  2424 , the interface control section  2423  judges if to proceed to Step  5 .  
         [0069]    Rewriting of the header transform table performed in Step  5  is similar to FIG. 19.  
         [0070]    Further, the PT field monitoring section  2424  may be provided with a function of investigating a cell flow and a timer function, by replacing it with a PT field monitoring section  2424   a  and a cell flow monitoring section  2424   b  (FIG. 27). By this construction, in the case that a virtual connection change request arrives between packets, if the cell counting section  2424   b  does not count a cell flow for a given period of time, it is possible to change virtual connections without awaiting processing of a subsequent packet.  
         [0071]    [0071]FIG. 12 is a sequence diagram showing basic operation of the present invention. The packet D shown in FIG. 12 is a packet sent following the packet C. Here also, similarly to FIG. 10, a virtual connection change request G 2  arrives after a cell C 1  has been processed. The switch section  24  resets the virtual connection VC 53 , which, at this point in time, is destined for the cell-to-packet assembly section  22  of the node  2 , so that the virtual connection VC 53  is destined for the cell-to-packet assembly section  12 B of the node  1 B, in addition to the cell-to-packet assembly section  22 , by a multicast function. The operation can be seen from FIG. 12 by noting a multicast period W 2 . Namely, after completing transmission of a sufficient number (n in this example) of cells for the maximum length of packets, setting of the destination to the cell-to-packet assembly section  22  is eliminated in the virtual connection VC 53 , leaving only the virtual connection VC 53 ′ destined for the cell-to-packet section  12 B. Although the packet D is discarded in the cell-to-packet assembly section  22  once, and the packet C is discarded in the cell-to-packet assembly section  12 B once, both the packet C and packet D arrive at the node  1 B in the result. Thus, the communication is performed normally on the packet level.  
         [0072]    [0072]FIG. 20 is a view illustrating an internal structure of the ATM interface section  242  on the input side, according to the present invention. It differs from FIG. 17 in that the PT field monitoring section  2424  is replaced by a cell counting section  2425 .  
         [0073]    [0073]FIG. 21 is a flowchart showing an example of a procedure for changing virtual connections using the cell counting section. It is the same as FIG. 18 up to Step S 4 . Thereafter, the ATM interface control section  2423 , which has received the virtual connection change request from the switch control section  243 , rewrites the table of the header transform section  2422  so that, while maintaining the virtual connection before the change, the same cell is also multicasted onto the virtual connection after the change (Step S 8 ). Then, the ATM interface control section  2423  inquires of the cell counting section  2425  about the current number of cells (Step S 9 ). Based on a response from the cell counting section  2425 , the interface control section  2423  judges if a given number of cells have been processed (Step S 10 ). Judging that the given number of cells have been processed, the interface control section  2423  eliminates an entry representing the virtual connection before the change from the table of the header transform section  2422  (Step S 11 ), completing the processing of changing the virtual connections.  
         [0074]    [0074]FIG. 22 shows an example of rewriting the head transform table in the multicast system. It differs from FIG. 19 in that there exists a period of multicast processing as shown by an entry  73 . Some cells disassembled from the packet C generated in the sending node  1 A arrive at the receiving node  1 B in duplicate. A larger number of the duplicate cells is preferred for preventing packet loss and retransmission due to packet loss. On the other hand, from the viewpoint of packet processing in the IP switch and a transfer rate of the network, it is more desirable if a smaller number of cells are transferred in duplicate, i.e., multicasted.  
         [0075]    To obtain the above-described given number for cells, the maximum length of packets generally used in an IP network system, i.e., 1500 byte, may be taken into consideration. This number 1500 is divided by 48 bytes, the length of the payload of the ATM cell, raised to a unit, and added with the length of 1 cell of the AAL5 trailer, giving 32, which may be considered an effective value of the given number for cells. Further, when, 65,536 bytes, the maximum length of packets in AAL5 is taken, 1,367 may be considered as the effective value.  
         [0076]    [0076]FIG. 13 is basically similar to FIG. 12 except that a multicast period W 3  is fixed as a given period of time, with its length not being dynamically changed depending on the number of cells.  
         [0077]    [0077]FIG. 23 is a view illustrating an internal structure of the ATM interface section  242  on the input side, according to the present invention. Differently from FIGS. 17 and 20, a timer section  2426  is provided, and the cell buffer is not monitored.  
         [0078]    [0078]FIG. 24 is a flowchart showing an example of a procedure for changing virtual connections in the present embodiment. It is similar to FIG. 21 except that the timer section  2426  is started up, instead of the cell counting section  2425  (Step S 12 ), and that, awaiting the timer expiration, which is notified by a signal from the timer section  2426  (Step  13 ), the virtual connection before the change is eliminated.  
         [0079]    Rewriting of the header transform table in the present embodiment is similar to FIG. 22 which is the example for the embodiment 2.  
         [0080]    [0080]FIG. 25 shows an internal structure of the switch section  24 , according to the present invention. Differently from the embodiments described heretofore, a timer section  245  is connected to the switch control section  243 . By this construction, it is not necessary to add a new structure to the ATM interface section  242 .  
         [0081]    [0081]FIG. 26 is a flowchart showing an example of a procedure for changing virtual connections. It is the same as the above-described flowcharts up to Step S 2 . Thereafter, first, the switch control section  243 , which has received a virtual connection change request, sends a request for setting a virtual connection after the change to the ATM interface section  242  (Step S 14 ). On receiving that request, the interface control section  2423  within the interface section  242  rewrites the table of the header transform section  242  in accordance with the request. As a result, virtual connections are generated for multicasting both to the virtual connection before the change and to the virtual connection after the change (Step S 15 ). Thereafter, the switch control section  243  starts up the timer section  245  (Step S 16 ), and awaits a timer expiration signal (Step S 17 ). When the timer expires, then, the switch control section  243  sends a request for eliminating the virtual connection before the change to the ATM interface section (Step S 18 ). On receiving the request, the interface control section  2423  within the interface section eliminates the entry related to the virtual connection before the change from the table of the header transform section  2422  (Step S 19 ), completing the process of changing the virtual connections.  
         [0082]    Rewriting of the header transform table in the present invention is similar to FIG. 22 which is the example for the embodiments 2 and 3-1.  
         [0083]    According to the first invention, it is possible to prevent packet loss at the time of changing virtual connections. Advantageously, at that time, it is not necessary to transfer useless cells.  
         [0084]    According to the second invention, it is possible to prevent packet loss at the time of changing virtual connections. Advantageously, in the case that the ATM switch supports the multicast function and the ATM interface section is provided with the cell counting section in advance, the invention can be implemented without addition of special hardware.  
         [0085]    According to the third invention, it is possible to prevent packet loss at the time of changing virtual connections. Advantageously, this system does not require any circuit for high speed processing such as searching of the cell buffer&#39;s contents, and has a high degree of freedom in its construction.