Abstract:
A method for transferring a series of layer-3 packets through an ATM network composed of a plurality of ATM switches is disclosed. An ingress gateway determines whether a packet flow has been registered and, when the flow has not been registered, determines a transfer route. A connection setup cell is transmitted to a next-hop ATM switch to ensure a connection dedicated to transfer of tho series of packets. Then, a series of packets is transferred to the next-hop ATM switch through the connection after the connection has been set up.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the method and system to transmit layer 3 packets on an Asynchronous Transfer Mode (ATM) network. 
     2. Description of the Related Art 
     Currently, on the network where Internet Protocol (IP) is widely used on the Internet, each router needs to compare the destination address of all packets sent by the source host with an internal routing table, decide the route to transmit the packets (routing process), and transmit the packets. These conventional routers conduct this routing process for each packet of a flow comprising a series of packets, and as a result, the total quantity of the packets to be processed is accumulated, which causes a decrease in throughput and an increase in transmitting delay time. 
     As an example of solving the above problem, a layer 3 switching method IFMP (Ipsilon Flow Management Protocol in Internet Engineering Taskforce Request for Comment: IETF RFC1953) is used. In the case where a network is composed of a first ATM switch (here, sending) and a second ATM switch (here, receiving), for example, IFMP is implemented as follows: 
     1. Each ATM switch detects a flow by looking at the layer 3 packets which belong to the same flow and are received at the default Virtual Channel (VC); 
     2. The second ATM switch sends a flow redirect message using the default VC to the first ATM switch in order to lot it use a VC allocated for that flow as a dedicated VC; 
     3. The first ATM switch begins sending the packets of the flow using that dedicated VC; and 
     4. After both sides (source and destination) setup the dedicated VCs specially for that flow, the packets are transferred through the dedicated VCs between the source and destination ATM switches 
     However, the above-described IFMP has the following disadvantages. 
     1. The loads on each switch increase because each switch needs to have a flow detection function. 
     2. Redirection of flow on both source and destination ends needs to be completed before the switch can handle the flow. However, each switch will decide and request the flow-redirection to the source switch after it receives the first packet, which results in a long delay from the time when the first packet is received. 
     3. Because VPI and VCI will be allocated for each flow, the number of VPI and VCI Will become huge and the operation and administration of flow-redirection will be overload for switches. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and system which can reduce a processing burden on each switch. 
     Another object of the present invention is to provide a method and system which can achieve the rapid flow switching of each switch. 
     According to the present invention, in a method for transferring a series of layer-3 packets received from a non-ATM network through an ATM network composed of a plurality of ATM switches, at an ingress gateway interfacing between the non-ATM network and the ATM network, the following stops are performed: a) determining whether a flow of the series of packets has been registered; b) determining a transfer route from a first packet of the series of packets according to a layer-3 routing protocol when the flow has not been registered; c) transmitting a connection setup message to a next-hop ATM switch of the ATM network according to the transfer route to ensure a connection dedicated to transfer of the series of packets; and d) transferring the series of packets received from the non-ATM network to the next-hop ATM switch through the connection after the connection has been set up. 
     At each of nodes receiving the connection setup message from an upstream node which is one of the ingress gateway and an adjacent ATM switch, the following steps are performed: determining a transfer route from the connection setup message according to a layer-3 routing protocol; transmitting a connection setup message to a next-hop node of the ATM network according to the transfer route to ensure a connection dedicated to transfer of the series of packets to the next-hop node; and transferring the series of packets received from the upstream node to the next-hop node through the connection after the connection has been set up. 
     At an ATM switch receiving the connection setup message, when the connection to the next-hop node is not set up yet, the series of packets are preferably transferred through a default connection to the next-hop node until the connection to the next-hop node has been set up. 
     Since the connection setup message is transmitted before a packet to be transferred on the layer 3 flow switching, the routing is allowed without assembling packets. The connection setup message is sent before actual packets arrive and the information conveyed by the connection setup message enables the routing of the actual packets. The actual packets will be transmitted on the allocated connection using hardware switching from the very first packet. Therefore, the processing of assembling packets can be removed and thereby the transfer delay is eliminated. 
     Further, there is no need to detect a flow on each switch. The ingress gateway is the only component that needs to detect the flow, and the switches on the way just need to recognize packets sent on the connection specified by the connection setup message as the flow. Therefore there is no need to detect the flow by looking at the port number and the line for each switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram showing a dedicated-path setup operation in a first embodiment of a layer-3 flow switching method according to the present invention; 
     FIG. 1B is a schematic diagram showing a layer-3 flow switching operation using a dedicated path in the first embodiment; 
     FIGS. 2A-2C are diagrams which show a dedicated-path setup operation according to the first embodiment in the case where a connection setup is delayed on a switch; 
     FIG. 2D is a schematic diagram showing a layer-3 flow switching operation using a dedicated path set up as shown in FIGS. 2A-2C; 
     FIG. 3A is a schematic diagram showing a retaining operation of the flow switching path shown in the first embodiment; 
     FIG. 3B is a schematic diagram showing a releasing operation of the flow switching path in the first embodiment; 
     FIG. 4A is diagram showing an example of mail sending sequence which is detected by a flow detector; 
     FIG. 4B is a diagram showing an example of an E-mail message sent by the sequence as shown in the FIG. 4A; 
     FIG. 5A is a schematic diagram showing a retaining operation of two flow switching paths in a second embodiment of a layer-3 flow switching method according to the present invention; 
     FIG. 5B is a schematic diagram showing a flow aggregation operation of the flow switching paths as shown in the FIG. 5A according to the second embodiment; 
     FIG. 6 is a schematic diagram showing a setup operation of a flow switching path with a first type of connection robustness in the first and second embodiments; 
     FIG. 7 is a schematic diagram showing a setup operation of a flow switching path with a second type of connection robustness In the first and second embodiments; 
     FIG. 8 is a schematic diagram showing a setup operation of a flow switching path with a third type of connection robustness in the first and second embodiments; 
     FIG. 9 is a schematic diagram showing the internal circuit of a gateway used in the first or second embodiment as described above; 
     FIG. 10 is a diagram showing one example of a flow table on the gateway as shown in FIG. 9; 
     FIG. 11 is a schematic diagram showing the internal circuit of a ATM switch used in the first or second embodiment as described above; and 
     FIG. 12 is a diagram showing one example of a flow table on the ATM switch as shown in the FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIGS. 1A and 1B, it is assumed for simplicity that an ATM network is composed of an ingress gateway  11  and an ingress gateway  12  from the ATM network to another existing LAN, which are connectable through switches SW 1  and SW 2  The ingress gateway  11  is connected to a non-ATM network such as an existing local-area network (LAN) conforming to IEEE802 or ITU-T recommendations and is an entrance node of the ATM network. The ingress gateway  11  receives a packet  100  from the existing LAN and detects that the packet is a part of a new flow. 
     When detecting a new flow, the ingress gateway  11  sends a connection setup cell (hereafter called BIND cell) BIND 1  to a net-hop switch (here, the switch SW 1 ) through a default virtual channel VC 1  before it sends that flow packet to the next top switch. The BIND cell BIND 1  has the flow identification (layer-3 source and destination address, layer-4 port number, etc.) and the VPI and VCI of a dedicated VC allocated for that flow. 
     When the switch SW 1  has received the BIND cell BIND 1  from the ingress gateway  11  through the default virtual channel VC 1 , the switch SW 1  looks up the flow identification of the BIND cell BIND 1  and performs layer-3 routing process. Then, the switch SW 1  allocates a dedicated virtual channel VC 2  to that flow, and transmits a BIND Cell BIND 2  to the next hop switch (here, the switch SW 2 ). The switch SW 2  sends the BIND cell BIND 3  to the next hop, that is, the egress gateway  12  in the same manner. In this way, a dedicated connection composed of the flow-dedicated virtual channels VC 1 -VC 3  is set up between the ingress gateway  11  and the egress gateway  12 . 
     The ingress gateway  11  transmits the flow packet  102  using the flow-dedicated VC 1  after having sent the BIND cell BIND 1 . When the flow arrives at each switch, the flow-dedicated VC 1 -VC 3  are already allocated, and each switch can use these flow-dedicated VCs for switching process from the very first packet  100 . 
     As shown in FIG. 2A, when detecting a new flow, as described above, the ingress gateway  11  sends the BIND cell BIND 1  to a next-hop switch (here, the switch SW 3 ) through a default virtual channel VC 1  before it sends that flow packet to the next hop switch. 
     Referring to FIG. 2B, however, in the case where the flow-dedicated VC allocation process of the switches can not be completed before the packet  102 ,  103 ,  104  of that flow arrives, the packet will be transmitted using the default VC 2 . Alternatively, the packet will be buffered on the switch SW 3  until the flow-dedicated VC 2  has been allocated. 
     The switch SW 3  performs the routing for the packet received through the flow-dedicated VC 1  and transmits the packet to the next hop switch SW 4  using the default VC 2 . The switch SW 4  performs the routing for the packet received through the default VC 2  and transmits the packet to the next hop using the default VC 3 . 
     As shown In FIGS. 2C and 2D, when the switch SW 3  sends the BIND cell BIND 2  to the switch SW 4  and the switch SW 4  sends the BIND cell BIND 3  to the egress gateway  12 , the flow-dedicated virtual channels VC 1 , VC 2  and VC 3  are set up to allow packets  108 ,  109 ,  110  to be transferred to the egress gateway  12 . 
     Path Retaining 
     Next, retaining and releasing of a flow switching path will be explained referring to FIGS. 3A and 3B. 
     As shown in FIG. 3A, consider that the flow switching path composed of the flow-dedicated virtual channels F-DED-VC 1  through F-DED-VC 3  has been set from the ingress gateway  11  to the egress gateway  12 , as described above. In this state, the egress gateway  12  monitors the traffic of the flow on the flow switching path F-DED-VC 3 . 
     As far as the apparent termination notification of flow like FIN/FIN-ACK of TCP (Transport Control Protocol) is not received, the egress gateway  12  periodically sends a path retaining cell REPRESH 1  to the upstream switch (here, SW 2 ) through the default VC 4  in order to retain the flow switching path F-DED-VC 3 . 
     Similarly, the switch SW 2 , when receiving the cell REFRESH 1  from the egress gateway  12 , sends the path retaining cell REFRESH 2  to the upstream switch SW 1  using the default VC 5  to retain the assigned flow switching path F-DED-VC 2 . Further, in the same manner, the switch SW 1  and the ingress gateway  11  will retain the flow switching path F-DED-VC 1 . 
     Path Releasing 
     Referring to FIG. 3B, when the egress gateway  12  detects that there exists no traffic on the flow, or when it detects the apparent termination notification of the flow like FIN/FIN-ACK, then it stops sending the path retaining cell REFRESH 1  and sends the path releasing cell RECLAIM 1  to the upstream switch SW 2  using the default VC 4  to release the flow switching path F-DED-VC 3 . 
     The switch SW 2 , when receiving the path releasing cell RECLAIM 1  from the egress gateway  12 , sends the path releasing cell RECLAIM 2  to the upstream switch SW 1  to release the assigned flow switching path F-DED-VC 2 . Similarly, the switches SW 2  and SW 1  and the ingress gateway  11  release the flow switching paths F-DED-VC 2  and F-DED-VC 1 . Alternatively, each switch may release the corresponding flow switching path when it did not receive the REFRESH cell during a predetermined time period. 
     Next, one example of the flow detection methods will be explained. 
     To send E-mail, we use the layer-4 protocol TCP. and the TCP header includes the port number  25  indicating that the upper protocol is SMTP (simple rail Transfer Protocol). The SMTP command and the reply will be sent using that port number  25 . 
     Referring to FIG. 4A, the command from the source SMTP client is shown after “S:” while the reply from the destination SMTP server is denoted after “R:”. In this figure, a transmit sequence is shown for the message as shown in FIG. 4B where an E-mail is sent from the sender, whose E-mail address is sender@sendinghost.com, to the receiver, whose E-mail address is receiver@receivinghost.edu. The command from the source SMTP client is first four characters and the reply message from the destination SMTP server is three digit numbers. In this invention, the gateway will detect RCPT command, for example, and if its receiver matches the registered address, then it recognize the new flow and it is possible to setup special connection (e.g. robust connection). Also, when QUIT is detected, any connection for that flow can be released. 
     Flow Integration 
     As shown in FIG. 5A, assuming that two flow-dedicated paths (F-DED-VC 11 , F-DED-VC 21 , F-DED-VC 31 ) and (F-DED-VC 12 , F-DED-VC 22 , F-DED-VC 32 ) have been set up between the ingress gateway  11  and the egress gateway  12 , a flow integration operation will be described as a second embodiment of the present invention with reference to FIGS. 5A and 5B. 
     Referring to FIG. 5A, when the egress gateway  12  transmits the respective path retaining cells REFRESH 31  and REFRESH 32  corresponding to the flow-dedicated virtual channels F-DED-VC 31  and F-DED-VC 32  to the switch SW 2 . Its own ID is added to the path retaining cells REFRESH 31  and REFRESH 32 . As described before, the corresponding path retaining cells are transferred through the switches SW 2  and SW 1  to the ingress gateway  11 . Therefore, by looking at ID information included in the path retaining cells REFRESH 11  and REFRESH 12  received from the switch SW 1 , the ingress gateway  11  can identify an egress node of each flow-dedicated path as the egress gateway  12 . 
     Referring to FIG.  5 B. when detecting the multiple flows going through the egress gateway  12 , the ingress gateway  11  transmits a BIND cell BIND 4  to the destination side switch SW 1  to integrate the flow-dedicated virtual channels F-DED-VC 11  and F-DED-VC 12  into a single flow-dedicated virtual channel F-DED-VC 10 . 
     Similarly, when receiving the BIND cell BIND 4  from the ingress gateway  11 , the switch SW 1  transmits a BIND cell BIND 5  to the destination side switch SW 2  to integrate the flow-dedicated virtual channels F-DED-VC 21  and F-DED-VC 22  into a single flow-dedicated virtual channel F-DED-VC 20 . When receiving the BIND cell BIND 5  from the switch SW 1 , the switch SW 2  transmits a BIND cell BIND 6  to the destination side egress gateway  12  to integrate the flow-dedicated virtual channels F-DED-VC 31  and F-DED-VC 32  Into a single flow-dedicated virtual channel F-DED-VC 30 . 
     In this manner, when the flow-dedicated paths (F-DED-VC 11 , F-DED-VC 21 , F-DED-VC 31 ) and (F-DED-VC 12 , F-DED-VC 22 , F-DED-VC 32 ) are formed between the same nodes, they can be integrated into a single flow-dedicated path (F-DED-VC 10 , F-DED-VC 20 , F-DED-VC 30 ). 
     Connection QOS 
     In the flow switching path setup with connection QOS (Quality of Service) designated, the ingress gateway  11  sends the downstream switch SW 1  the BIND cell BIND 1  including connection QOS information to specify the connection QOS of the flow. When receiving such a BIND cell, each of the switches SW 1  and SW 2  and the egress gateway  12  sets up a flow switching path according to the designated connection QOS. The connection QOS Includes the followings. 
     Connection Priority 
     The connection priority may be set as follows: 
     1) If the line is so crowded that a new path cannot be setup, then a path is certainly allocated to that flow requesting a new path by releasing the path of another flow; 
     2) If there is an available VPI and VCI and a new path can be set up, then the new path is set up to be allocated to that flow; and 
     3) A path is set for the flow, but will be released when the line gets crowded. 
     Connection Robustness 
     Referring to FIG. 6, when the BIND cell BIND 3  arrives at the egress gateway  12  as described before and the connection is successfully set up between the ingress gateway  11  and the egress gateway  12 , the egress gateway  12  sends an acknowledgement message BIND-ACK 1  of the BIND cell BIND 3  to the switch SW 2 . The switch SW 2  sends an acknowledgement message BIND-ACK 2  to the switch SW 1  and the switch SW 1  in turn sends an acknowledgement message BIND-ACK 3  to the ingress gateway  11 . Upon reception of the acknowledgement message BIND-ACK 3 , the ingress gateway  11  begins to transmit the flow through the flow-dedicated virtual channels F-DED-VC 1 , F-DED-VC 2 , and F-DED-VC 3  as described before (see FIG.  1 ). 
     Referring to FIG. 7, each of the switches SW 1  and SW 2  and the egress gateway  12  sends an acknowledgement message back to an upstream node when receiving a BIND cell from the upstream node. More specifically, when receiving the BIND cell BIND 1  from the ingress gateway  11 , the switch SW 1  sends the acknowledgement cell BIND-ACK 1  back to the ingress gateway  11 . It is the same with the switch SW 2  and the egress gateway  12 . At each node, upon reception of the acknowledgement message BIND-ACK, the flow is transmitted through the corresponding flow-dedicated virtual channel. 
     Referring to FIG. 8, just after each node has transmitted the BIND cell to the next hop node, it may begin transmitting the flow. 
     Gateway 
     Referring to FIG. 9, a gateway is connected to a non-ATM network such as LAN at one end and an ATM network at the other end. As will be described hereafter, the gateway works as the ingress gateway  11  and the egress gateway  12  as described above. The gateway is provided with a SAR (Segmentation and Reassembly) and ATM interface  201  to the ATM network and a non-ATM interface  202  to the non-ATM network. A flow detector  203  is connected to the ATM interface  201  and the non-ATM interface  202  to perform the flow detection and control using a routing table  204  and a flow table  205 . 
     Referring to FIG. 10, the flow table  205  has the following fields: flow name, non-ATM interface number, ATM VPI/VCI, QOS, flow switch setup time, refresh timer and flag, and the amount of traffic. 
     When receiving a packet from the non-ATM network, the gateway works as the ingress gateway  11 , and the packet is passed from the non-ATM interface  202  to the flow detector  203 . The flow detector  203  checks the received packet whether it belongs to the registered flow or not by referring to the flow table  205 . 
     If the packet does not belong to any existing flow, the flow detector  203  searches the routing table  204  for the VP/VC of a next-hop node in the ATM network and secures the VP/VC of the ATM network. Thereafter, the searched VP/VC of the ATM network is registered as a new flow onto the flow table  205 . Then, as described before, the BIND cell is transmitted to the next-hop node (here, switch SW 1 ) and the packets are transferred using the secured VP/VC which is the flow-dedicated virtual channel F-DED-VC. 
     On the other hand, if the packet belongs to an existing flow registered in the flow table  205 , the packets are transferred using the registered ATM VP/VC. As needed by the charging system and the like, traffic (the number of packets, the total number of words, the total number of bytes, and the total number of bits of transferred packets, etc.) is measured and the flow table  205  is updated depending on the measurement. 
     When receiving the path retaining cell REFRESH from the ATM network, the flow detector  203  resets the refresh timer of the flow specified by the path retaining cell REFRESH in the flow table  205 . Therefore, the flow-dedicated VC is retained until a timeout occurs at the refresh timer. 
     Contrarily, when the path releasing cell RECLAIM has been received from the ATM network or when the refresh timer of the flow table  205  has been timed out, the flow entry is discarded from the flow table  205 . 
     On the other hand, when receiving a packet from the ATM network, the gateway works as the egress gateway  12 . When receiving the BIND cell from ATM network, the SAR and ATM interface  201  passes the BIND cell to the flow detector  203 . The flow detector  203  searches the routing table  204  for the destination shown by the BIND cell, and adds the flow to the flow table  205 . After this process, the packets of the flow from ATM network will be transmitted to the non-ATM network using the flow table  205 . 
     In the same time, the flow detector  203  measures the traffic of the flow and updates the flow table  205 . For the flow that still has traffic, it sends the path retaining cell REFRESH and resets the refresh timer of the flow table  205  before the timer times out. It there is no traffic of the flow for a predetermined time period and the refresh timer times out, it sends the path releasing cell RECLAIM and discards the entry from the flow table  205 . 
     The functions of the ingress gateway  11  and the egress gateway  12  can co-exist on the same gateway. 
     ATM Switch 
     Referring to the FIG. 11, an ATM switch is provided with a cell switch  301  which performs switching of receive cells from the ATM network The cell switch  301  is connected to a SAR and ATM interface  302 . which is further connected to a flow detector  303 . The flow detector  303  performs the flow detection and control using a routing table  304  and a flow table  305 . 
     Referring to FIG. 12, the flow table  305  has the following fields: flow name, receiving-side VPI/VCI, transmitting-side VPI/VCI, QOS, flow switch setup time, and refresh timer and flag. 
     The BIND cell received from ATM network is switched by the cell switch  301  to the SAR &amp; ATM interface  302 , and the SAR &amp; ATM interface  302  passes the BIND cell to the flow detector  303 . The flow detector  303  searches the routing table  304  for the destination shown by the BIND cell, and allocates the VP/VC to the next-hop node of the ATM network, and sets up the cell switch  301  to switch the VPI/VCI shown by the BIND cell to the VPI/VCI allocated. After this process, the packets of the flow from ATM network will be switched by the cell switch  301  and transmitted to the next-hop node of the ATM network. 
     When receiving the path retaining cell REFRESH from the downstream ATM network. the refresh timer of that flow on the flow table  305  is reset and the path retaining cell REFRESH for the flow is sent to the upstream ATM network. Also, when the path releasing cell RECLAIM has been received from the downstream ATM network or when the refresh timer of that flow on the flow table  305  has been timed out, the entry of that flow is discarded from the flow table  305 , and the path releasing cell RECLAIM for the flow is sent to the upstream ATM network. 
     Charging System 
     In the network system according to the present invention, the following charging methods may be employed. 
     1) Charging by the amount of connection time: 
     On the egress gateway  12 , the length of time is calculated for each flow from the time when the connection is set up and the time when the flow in deleted from the flow table  205  (see FIG.  10 ). The connection service charges by the amount of calculated connection time. 
     2) Charging by the amount of traffic: 
     On the egress gateway  12 , the connection service charges by the amount of traffic for each flow in the flow table  205 . The dimension of the traffic may be the number of passed packets, the total number of words, the total number of bytes and the total number of bits of the passed packets. 
     3) Charging by the amount of traffic modified by lost packets: 
     Based on the above charging method 2), the number of lost packets in the ATM network is obtained by calculating the difference between the amount of traffic on the egress gateway  12  and that on the ingress gateway  11 . The service charges by the amount of the traffic revised by the number of lost packets. 
     4) Charging by QOS: 
     Setting the rate on each connection QOS, and the service charges by the QOS used by the flow. 
     5) Charging by the amount of connection time and QOS 
     The connection service charges by setting the rate for connection time calculated in the charging method 1) on each connection QOS. 
     6) Charging by QOS and the amount of traffic on the egress gateway  12 ; 
     The connection service charges by setting the rate for traffic obtained by the charging method 2) on each connection QOS. 
     7) Charging by QOS and the amount of traffic on the egress gateway  12  and the ingress gateway  11 ; 
     The connection service charges by the amount of the traffic revised by the number of lost packets obtained by the charging method  39 . 
     ADVANTAGES 
     According to the present invention, the following advantages can be obtained by using a BIND cell transmitted before a packet to be transferred on the layer 3 flow switching 
     1) The routing is allowed without assembling packets. The BIND cell is sent before actual packets arrive and the information conveyed by the BIND cell enables the routing of the actual packets. The actual packets will be transmitted on the allocated VC using hardware switching from the very first packet, and this enables the routing without assembling packets. Therefore, the processing of assembling packets can be removed and thereby the transfer delay is eliminated. 
     2) There is no need to detect a flow on each switch. The ingress gateway is the only component that needs to detect the flow, and the switches on the way just need to recognize packets sent on the VP/VC specific in the BIND cell as the flow Therefore, there is no need to detect the flow by looking at the port number and the line for each switch. 
     3) The switching is allowed from the first packet. At the time when the first packet is arrived on each switch, the switching path offered by the VC dedicated to that flow is already setup, and each switch can perform packet switching from the very first packet. 
     4) Connection QOS can be designated. The QOS can be realized on the whole ATM network by adding connection QOS information to the BIND cell going from the ingress gateway  11  to the egress gateway  12 . 
     5) Flexible charging system can be set. With a combination of the connection priority, the connection robustness, the traffic monitoring on the egress gateway  12  and the traffic monitoring on the ingress gateway  11 , the flexible charging system can be set. 
     6) The load of keeping flow switching path on each switch is reduced. Only the egress gateway  12  monitors the traffic of the flow, and decides whether it retains the flow switching path or releases it. Other switches just need to follow the REFRESH or RECLAIM message received from the downstream switch and retain or release the flow switching path. Therefore, each switch is burdened with reduced traffic monitoring. 
     7) A flow switching path is automatically released in case of accident. When the flow does not arrive the egress gateway  12  due to routing loop or line failure, the REFRESH cell to retain the path is not sent out. Therefore, the flow-dedicated path will be automatically released when a timeout occurs. 
     8) A flow integration can be performed using the information on the ingress gateway. By putting the ID of the egress gateway  12  on the REFRESH cell, the ingress gateway  11  can identify the egress gateway, which sent the flow into the network. When the ingress gateway  11  detects the multiple paths are set to the same the egress gateway  12 , it can send these flows through a single VC.