Abstract:
A system for use with communications according to TCP, or a similar protocol, includes a sender host, one or more routers, and a destination host. Each router includes means for generating acknowledgement packets and translating addresses associated with data packets. When a data packet is received by a router, an acknowledgement packet is returned with a source address corresponding to the router and a destination address corresponding to the sender host. The sender host perceives the acknowledgement packet as having been sent by the destination host. As the acknowledgment packet is sent by the router, and not the destination packet is sent by the router, and not the destination host, this system allows acknowledgment packets to be received on a shorter timescale, reducing the effects of a link delay. Although the route between the sender host and the destination host is divided into multiple connections, the hosts perceive a single, conventional connection.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a system for high-speed communication using the Transmission Control Protocol (TCP), or a similar protocol, which requires a host to acknowledge the receipt of data. 
   The TCP protocol, as documented in RFC (Request For Comments) 793, is widely used for high reliability data communication and has functions for error correction, flow control, etc. According to the TCP protocol, when a receiver host receives data, it sends an acknowledgement to a sender host. Where TCP communication is carried out through a link with a large delay, the time interval between the transmission of data by the sender host and its receipt of the acknowledgement may be considerable. This may lead to the TCP window of the sender host becoming full as further data cannot be transmitted until the acknowledgement for the previously transmitted data packets from the receiver host has been received. Thus, in conventional TCP communication through a large-delay link, though the transmission rate of the delay link may be high, the bandwidth may be utilized inefficiently because of this inherent constraint in the TCP protocol. 
   One solution to this problem would be to modify the TCP protocol to reduce the effect of the link delay. It is technically possible to improve the TCP protocol. However, this would require the simultaneous reconfiguration of several thousands or more communication apparatuses, rendering this solution impractical. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a system which, when communication according to TCP or a similar protocol is carried out with use of a high-speed link with a large delay, can reduce the effect of the delay and allow high-speed communication without modifying the existing protocol. 
   In a first aspect of the present invention, a communication system, hereafter referred to as a router, for transmitting information from a source device to a destination device according to a predetermined protocol, the router being associated with an address, comprises a processor configured to receive a data packet from a first device, the data packet comprising a data packet source address and a data packet destination address, send the data packet to a second device, send an acknowledgement packet to the first device in response to the reception of a data packet, the acknowledgement packet comprising an acknowledgement packet source address and translate addresses associated with a data or acknowledgement packet, wherein the address translator is configured to change the acknowledgement packet source address from the router address to the data packet destination address. 
   One or more routers according to this aspect of the invention may be included in a communication system. The communication system may be arranged to allow bi-directional communication or uni-directional communication 
   The router may comprise means for terminating connections between the hosts on its path and means for causing the connections to simulate a single connection. The address translating means translates the destination address of a packet sent from the network of the local system into the address of the router and with respect to a packet sent from the network system to the router and forwarded to a host, its source address from the address of the router to its original source address. 
   As a result, a user can transmit data through the series of connections while perceiving a conventional single connection. As the connection is terminated at the router, an acknowledgement can be returned to the sender host quickly. Even when a delay link is used as the communication route, high-speed communication can be realized without modifying the existing protocol. 
   In conventional systems, a router does not perform an address translation, but simply analyses the packet contents and sends acknowledgement packets. On the other hand, the present invention applies the method of translating addresses of packets and terminating connections. The application of this system does not require any changes to the protocol program itself, facilitating system integration. 
   The invention further provides a method for managing a communication packet in a communication apparatus, which is coupled between a first computer and a second computer where the first computer and the second computer communicate with each other, said method executed by said communication apparatus comprising the steps of receiving a communication packet sent by said first computer, said communication packet having an address of said first computer as a source address and an address of said second computer as a destination address, generating an acknowledgement packet corresponding to said communication packet and having the address of said second computer as a source address and the address of the first computer as a destination address, sending said acknowledgement packet to said first computer whereby a connection is established between said first computer and said communication apparatus and forwarding a communication packet, which includes at least a part of the data included in said communication packet sent by said first computer, to said second computer in order to establish a connection between said communication apparatus and said second computer. 
   A communication system according to the present invention, wherein TCP communication is carried out between hosts via a network system, and a time until a window size of a source host reaches its upper limit is longer than a time until reception of a TCP acknowledgement, comprises said network system and a relay device including one edge router and another edge router connected to a local system to which each host for communication belongs and provided between nodes, wherein said one edge router is provided at a boundary between the local system to which a source host belongs and said network system and said another edge router provided at a boundary between the local system to which a destination host belongs and said network system terminates a TCP connection on its own edge router, and divides the TCP connection into three TCP connections between the source host and edge router, between the edge routers and between edge router and destination host to cause the edge routers to perform a TCP acknowledgement operation. 
   Embodiments of the present invention will now be described with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a TCP performance enhancement system according to the present invention; 
       FIG. 2  shows the hardware configuration of a router according to the present invention; 
       FIG. 3  shows the data structure of an address translation information table; 
       FIG. 4  is a diagram depicting the principle of the first embodiment of the present invention; 
       FIG. 5  shows the attributes of a TCP packet at a number of stages during its transmission; 
       FIG. 6  is a flowchart of a dazzler program; 
       FIG. 7  shows the data structure of a virtual connection information table; 
       FIG. 8  is a flowchart of a mediator program; 
       FIG. 9  is a flowchart of a parent coupler program; 
       FIG. 10  is a flowchart of a child coupler program; 
       FIG. 11  is a diagram showing an arrangement of the system according to a second embodiment of the present invention, 
       FIG. 12  is a block diagram of the second embodiment of the communication system; 
       FIG. 13  shows the attributes of a TCP packet at a number of stages during its transmission according to the second embodiment; 
       FIG. 14   a  is a diagram showing the communication architecture of a conventional system; 
       FIG. 14   b  depicts the structure of the communication architecture shown in  FIG. 14   a , and subsequent  FIGS. 15 and 16 ; 
       FIG. 15  shows the communication structure of the first embodiment of the communication system; 
       FIG. 16  depicts the communication structure of the second embodiment of the communication system. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The disclosures of all articles and references, including patent documents, mentioned in this application are incorporated herein by reference for all purposes. 
     FIG. 1  shows an enhanced TCP communication system according to the present invention, which comprises a receive gateway  10  (RGW), an integrated receiver decoder  11  (IRD), a send gateway  20  (SGW), an uplink station  21 , a satellite  30 , an Internet  40 , a source host, e.g., a server  50 , a destination host, e.g., a personal computer (PC)  60 , and routers  70   a, b.    
   A transmitter antenna and a receiver antenna can communicate uni-directionally via the satellite  30 . The antennae can also communicate bi-directionally over the Internet  40 . A system in which the RGW  10  and SGW  20  can communicate through both a satellite link and the Internet allows asymmetric routing of data packets. In the present embodiment, a packet to be sent from the PC  60  to server  50  is transmitted via a first local network, an edge router ‘b’  70   b , the RGW  10 , the Internet  40 , SGW  20 , an edge router ‘a’  70   a  and a second local network. A packet to be sent from the server  50  to the PC  60  is transmitted via the second local network, edge router ‘a’  70   a , SGW  20 , uplink station  21 , satellite  30 , IRD  11 , RGW  10 , router ‘b’  70   b , and the first local network to PC  60 . For details of the asymmetric routing using the RGW  10  and SGW  20 , refer to JP-A-10-346455, entitled “Communication equipment, Asymmetric Link Communication System And Asymmetric Link Communication Method”. 
   Each of the routers  70  forms the terminal of the TCP connections between the hosts, which simulate a single unbroken TCP connection. Since the routers  70  form the ends of the TCP connections, an acknowledgement can be sent over a network other than the delay link, so that high-speed communication can be realized without causing the TCP window to fill up. In previous systems, the time interval between the transmission of data from a sender host and its reception of an acknowledgement could be considerable and the effect of this delay was reduced by increasing the window size of the router  70 . Furthermore, since the user perceives a conventional TCP connection between the PC  60  and the server  50 , the user can conduct the TCP communication in the same manner as in a conventional system. This “virtual” TCP connection is simulated using address translation means provided in the router  70 . 
   The SGW  20  and edge router  70   a  or the RGW  10  and edge router  70   a  of the TCP performance enhancement system may, in some cases, be formed as identical communication apparatuses. The communication system may be used when one or more PC&#39;s  60  are connected to the local network and communication between the server  50  and PC  60  and is not limited to a one-to-one relationship as simultaneous transmission from a single server  50  to a plurality of PC&#39;s  60  can be implemented. In the communication system, a communication route other than the Internet  40  and satellite  30  can be employed. Even when there is a plurality of different communication routes, a communication system according to the present invention can be used. 
   The router  70  will now be described in greater detail. The function of the router  70  is identical for both edge routers  70   a ,  70   b . Thus the following description does not make a distinction between edge router ‘a’  70   a  and edge router ‘b’  70   b.    
   A processing program to be operated in the router may include three software modules: a dazzler (also called a virtualizing module), a coupler (also called a connection module) and a mediator (also called an establishing module). 
   The dazzler facilitates the simulation of a single continuous TCP connection by means of address translation, which is performed according to an address translation information table. When a packet is received by the router with address information that does not correspond to any of the entries in the address translation table, the router issues a new registration request (active nominate (ANOM) command) to the mediator, and receives new address translation information from the mediator. When receiving a command from the mediator for deleting address translation information, the router deletes the relevant information from the table. The word ‘active’ as used herein refers to a host that sent the connection establishment command. Thus the active- and passive-side hosts dynamically vary, but, in this embodiment, the router function is identical, whether the router is located on the active or passive side of the network. 
   The mediator is a software module that instructs another module to establish or release a TCP connection. The mediator has a virtual connection information table. When an active nominate command is received from the dazzler, the mediator sends a passive nominate (PNOM) command to the opposing mediator, sends an active couple (ACOU) command to the coupler, and sends an address translation information update (ATIU) command to the dazzler. When receiving a passive nominate command from the mediator of the opposing edge router  70 , the mediator sends a passive couple command to the coupler and sends an address translation information register (ATIR) command to the dazzler. When receiving a release (RELE) command from the coupler, the mediator sends the release command to the opposing mediator, sends the release command to the coupler, and further sends an address translation information delete (ATID) command to the dazzler. 
   The coupler performs a process that establishes a connection with the opposing coupler, copying packet data sent from the host and sending it to the opposing coupler. The coupler operates as a separate process for each connection. A parent process, when receiving an active couple command or passive couple (PCOU) command from the mediator, connects to the opposing coupler and generates a child process. The child process performs copying operations between TCP connections terminating on the router  70 . When completing the copying operation, the child process sends a release command to the mediator. The parent process terminates the child process when a release command is received from the mediator. 
     FIG. 2  shows the hardware configuration of the router  70 . As shown in  FIG. 2 , the router includes a CPU  1001 , a memory  1002 , a disk controller  1003 , a hard disk  1100 , a console controller  1004 , a console  1200 , and a network controller  1005 . 
   The CPU  1001  controls the entire operation of the router  70 . The memory  1002  stores a program and data therein. The disk controller  1003  controls the hard disk  1100 . The hard disk  1100  stores a program and data therein. The console controller  1004  controls the console  1200 . The console  1200  performs input/output operation between users. A network controller ‘a’  1005   a  communicates with the SGW  20  or RGW  10 . A network controller ‘b’  1005   b  communicates with the associated local network. 
   The edge router  70  is controlled by the CPU  1001 , which executes a processing program to be explained below. The program includes three software modules: the dazzler (which will be referred to as virtual module), the coupler (which will be referred to as connection module), and the mediator (which will be referred to as establishment module). 
     FIG. 3  shows a data structure of an address translation information table  1500 , which is used by the dazzler program in its operation. The address translation information table  1500  has an outer IP address field  1501 , an outer port number field  1502 , an inner IP address field  1503 , an inner port number field  1504 , an edge IP address field  1505 , and an edge port number field  1506 . 
   The IP address and port number of an outside host when viewed from the edge router  70  are stored in the outer IP address field  1501  and outer port number field  1502  respectively. The inner IP address field  1503  and inner port number field  1504  contain the IP address and port number of a host provided inside when viewed from the edge router  70 . Also stored in the edge IP address field  1505  and edge port number field  1506  are the IP address and port number of the network controller ‘b’  1005   b  installed in the edge router  70 . 
   The word ‘outer’ as used herein refers to a network of the edge router  70  connected to the local network, whereas, the word ‘inner’ refers to a network of the edge router  70  connected on its side opposite to the local network. 
   When a data packet is received by the edge router  70 , the dazzler performs address translation in one of two ways according to the transmission direction of the packet. With respect to a packet sent from the outer side to the inner, when the destination address of the packet coincides with an inner address and the source address thereof coincides with an outer address, the dazzler translates the destination address into the edge router address (hereafter referred to as an edge address). With respect to a packet sent from the inner side to the outer, where the destination address of the packet coincides with an outer address and the source address thereof coincides with the edge address, the dazzler translates the source address into an inner address. 
     FIG. 4  depicts the transmission of a TCP packet through a system according to the present invention and  FIG. 5  shows the properties of a TCP packet transmitted between a source host  80 , which is the sender of a packet, and a destination host  90 , which is the destination of the packet. A first edge router  70   d  and a second edge router  70   e  are provided between the source host  80  and the destination host  90 . The first edge router  70   d  includes a first dazzler  101  and a first coupler  201 . Similarly, the second edge router  70   e  includes a second coupler  202  and a second dazzler  102 . 
   The source host  80  sends a TCP packet to the first edge router  70   d  (step  2901 ), where the source address of the packet is the address of the source host  80  and the destination address of the packet corresponds to the address of the destination host  90 . In  FIG. 5 , IP-PC 1  and P-PC 1  are IP address and port number of the source host  80 , and IP-PC 2  and P-PC 2  are IP address and port number of the destination host  90 . 
   When the TCP packet reaches the first edge router  70   d , it is accepted by the first dazzler  101 , which conducts address translation according to rules which are registered in the address translation information table  1500 . The first dazzler translates the destination address of the packet to the edge address of the first coupler  201 . Then the first dazzler  101  sends a translated packet to the first coupler  201  (step  2902 ). In  FIG. 5 , IP-C 1 E and P-C 1 E show edge IP address and edge port number of the first coupler  201 . 
   The first coupler  201  terminates the TCP connection from the source host  80  because the destination address of the TCP packet corresponds to the edge address of the first coupler  201 . Therefore an acknowledgement packet is sent to the source host  80  by the first coupler  201 . That is, the first coupler  201  sends an acknowledgement packet, the source address of which is the edge address of the first coupler  201  and destination address of which is an address of the source host  80 , to the first dazzler  101  (step  2903 ). This packet contains ACK flag in TCP header. 
   The first dazzler  101  translates an address in the acknowledgement packet in accordance with the address translation information table  1500 . More precisely, the first dazzler  101  translates source address of the packet to the address of the destination host  90 . 
   The translated acknowledgement packet is sent from the first dazzler  101  to the source host  80  (step  2904 ). The source host  80 , which receives the acknowledgement packet, recognizes the packet as one sent from the destination host  90 . 
   On the other hand, the TCP packet, which has been sent to the first coupler  201 , is forwarded to the destination host  90 . In order to achieve this, the first coupler  201  copies the data and sends it to the second coupler  202 . Any types of protocols (TCP protocol, etc.) can be used for communication between these two couplers. In this embodiment, it is assumed that the TCP protocol is used. 
   The first coupler  201  sends a TCP packet (the source address of which is an internal address of first coupler  201 , the destination address of which is an internal address of the second coupler  202 ) to the second coupler  202  (step  2905 ). In  FIG. 5 , IP-C 1 I and P-C 1 I correspond to the internal IP address and port number of the first coupler  201 , and IP-C 2 I and P-C 2 I are the internal IP address and port number of the second coupler  202 . 
   When the second coupler  202  receives the packet, the second coupler  202  sends an acknowledgement packet to the first coupler  201  (step  2906 ). Communication between these two couplers is done in this way. 
   Then the second coupler  202  sends a TCP packet (the source address of which is the edge address of 2nd coupler  202 , the destination address of which is the address of the destination host  90 ) to the second dazzler  102  (step  2907 ). In  FIG. 5 , IP-C 2 E and P-C 2 E corresponds to the edge IP address and edge port number of the second coupler  202 . 
   The second dazzler  102  conducts address translation of the TCP packet according to address translation information table  1500 . More precisely, the second dazzler  102  translates source address of the packet to the address of the source host  80 . 
   After that, a translated TCP packet is sent from the second dazzler  102  to the destination host  90  (step  2908 ). The destination host  90 , which receives this packet, recognizes this packet as one sent from the source host  80 . 
   Then the destination host  90  sends an acknowledgement packet (the source address of which is the address of the destination host  90 , the destination address of which is the address of the source host  80 ) to the second edge router  70   e  (step  2909 ). 
   When the second edge router  70   e  receives the acknowledgement packet from the destination host  90 , the packet is accepted by the second dazzler  102 . The second dazzler  102  conducts address translation according to the translation rules registered in the address translation information table  1500 , i.e., the second dazzler  102  translates destination address of the packet to the edge address of the second coupler  202 . Then the second dazzler  102  sends a translated packet to the second coupler  202  (step  2910 ). 
   The second coupler  202  terminates TCP connection from the destination host  90  because the destination address of this packet corresponds to the edge address of the second coupler  202 . 
   Since the first dazzler  101 , first coupler  201 , second dazzler  102 , and second coupler  202  operate in this way, the TCP connection between the source host  80  and the destination host  90  is divided into separate TCP connections as follows: “source host  80 —first coupler  201 ”, “first coupler  201 —second coupler  202 ”, “second coupler  202 —destination host  90 ”. In addition, the packets which the source host  80  sends/receives coincide with the packets which the destination host  90  receives/sends. Therefore, each host perceives a single TCP connection “source host  80 —destination host  90 ” and does not acknowledge the presence of edge routers  70   d  and  70   e . Furthermore, the performance of TCP communication will be improved because edge routers send acknowledgement packets and each host can receive these packets immediately as the acknowledgement packets do not pass through a delay link. 
   In this embodiment, the enhancement of the performance of a TCP communication system will be greatest when there is a delay link between the first coupler  201  and the second coupler  202  or the second coupler  202  and destination host  90 . The improvement will not be so marked when a delay link is between the source host  80  and the first coupler  201 . 
   As mentioned above, since the first and second edge routers  70   d  and  70   e  perform these operations, the original TCP connection ‘between the source host  80  and source host  90 ’ is divided into discrete TCP connections ‘between the source host  80  and first edge router  70   d ’, ‘between the first edge router  70   d  and second edge router  70   e ’ and ‘between the second edge router  70   e  and destination host  90 ’. However, when viewed from the source host  80  and destination host  90 , it looks as if a TCP connection were established ‘between the source host  80  and destination host  90 ’. This is a virtual TCP connection. Each host can perform conventional TCP communication while the presence of the edge router  70  is imperceptible to each host. 
   The procedure for sending a data packet from the destination host  90  to the source host  80  follows a series of steps similar to that described above and thus its explanation will be omitted. 
     FIG. 6  is a flowchart for explaining the operation of a dazzler program. Firstly, the dazzler program determines whether a command from the mediator has been received (step  1601 ). If so, then the program examines whether or not the command indicates an address translation information registration command (step  1602 ). Where the received command is an ATIR command, the program registers the address translation information passed to the command in the address translation information table  1500  (step  1603 ). If not, then the program determines whether the command is an address translation information delete command (step  1604 ) and, if so, then the program deletes the address translation information specified by the command from the address translation information table  1500  (step  1605 ). If not, then the program examines whether or not the command is an address translation information update command (step  1606 ). If so, then the program updates the contents of the address translation information table  1500  specified by the command to new information (step  1607 ). 
   Next, the program examines whether a TCP packet has been received from either the network controller ‘a’  1005   a  and network controller ‘b’  1005   b  (step  1608 ). If so, then the program determines whether the IP address and port number of the packet have already been registered in the address translation information table  1500  (step  1609 ). If so, then the program performs address translation over the address according to the information of the address translation information table  1500  and outputs the address-translated packet (step  1610 ). If not, then the program examines whether or not the packet was received from the outer side and indicates a connection establishment command (step  1611 ). In this case, the TCP packet is determined to be a connection establishment command when a SYN bit is set in a TCP header and an ACK bit is cleared. For details thereof, refer to the RFC793. When the above conditions coincide with the conditions of the step  1611 , the program sends an active nominate (ANOM) command to the mediator (step  1612 ). 
   The processing carried out by the dazzler program has been explained above. 
   The operation of the mediator will now be described.  FIG. 7  shows the data structure of a virtual connection information table used by the mediator program in its operation. A virtual connection information table  2500  includes an active host IP address field  2501 , an active host port number field  2502 , an active edge IP address field  2503 , an active edge port number field  2504 , an active internal IP address field  2505 , an active internal port number field  2506 , a passive host IP address field  2507 , a passive host port number field  2508 , a passive edge IP address field  2509 , a passive edge port number field  2510 , a passive internal IP address field  2511 , and a passive internal port number field  2512 . Stored in the active host IP address field  2501  and active host port number field  2502  are the IP address and port number of the actively connected host respectively. Stored in the active edge IP address field  2503  and active edge port number field  2504  are the IP address and port number of the network controller ‘b’  1005   b  installed in the edge router  70  on its actively connected host side. Also stored in the active internal IP address field  2505  and active internal port number field  2506  are the IP address and port number of the network controller ‘a’  1005   a  installed in the edge router  70  on its actively connected host side. Stored in the passive host IP address field  2507  and passive host port number field  2508  are the IP address and port number of the passively connected host. Further stored in the passive edge IP address field  2509  and passive edge port number field  2510  are the IP address and port number of the network controller ‘b’  1005   b  installed in the edge router  70  on its passively connected host side. Also stored in the passive internal IP address field  2511  and passive internal port number field  2512  are the IP address and port number of the network controller ‘a’  1005   a  installed in the edge router  70  on its passively connected host side. 
   The operation of the mediator program is shown in  FIG. 8 . When a command is received (step  2601 ), the mediator program examines whether or not the command is an active nominate command (step  2602 ). If so, then the program sends the active nominate command to the mediator of the opposing edge router  70 , sends an active couple command to the coupler, sends an address translation information update command to the dazzler, and further registers the connection information in the virtual connection information table  2500  (step  2603 ). If not, then the program examines whether or not the command is a passive nominate command (step  2604 ). If so, then the program sends a passive couple command to the coupler, sends an address translation information register command to the dazzler, and registers the connection information in the virtual connection information table  2500  (step  2605 ). If not, then the program examines whether or not the command is a release command (step  2606 ). If so, then the program sends a release command to the opposing mediator, sends a release command to the coupler, sends an address translation information delete command to the dazzler, and further deletes connection information specified by the command from the virtual connection information table  2500  (step  2607 ). 
   The operation of the coupler will now be described. As mentioned above, there are two types of couplers, i.e., a parent process (parent coupler) and a child process (child coupler). 
     FIG. 9  is a flowchart for explaining the parent coupler. When a command is received from a mediator (step  3601 ), the parent coupler examines whether or not the command is an active couple (ACOU) command (step  3602 ). If so, then the parent coupler generates a socket on the edge side of the edge router to passively connect it to the passive side host. Thereafter, the parent coupler generates a socket inside of the edge router to actively connect it to the internal address of the opposing edge router  70 . And the parent coupler generates a child couple (step  3603 ). If not, then the parent coupler examines whether or not the command is a passive couple (PCOU) command (step  3604 ). If so, then the parent coupler generates a socket inside of the edge router to passively connect it to the internal address of the opposing edge router  70 . Thereafter, the parent coupler generates a socket on the edge side of the edge router to passively connect it to the passive host address. And the parent coupler generates a child coupler (step  3605 ). If not, then the parent coupler examines whether or not the command is a release (RELE) command (step  3606 ). If so, then the parent coupler terminates the corresponding child coupler (step  3607 ). In this connection, the word ‘socket’ as used herein refers to a pair of an IP address and a port number for identification of connection end point (end point). For details, refer to “UNIX Network Programming-Second Edition”, written by W. Richard Stephenson, 1999, published by K. K. Piason Education, ISBN4-89471-205-9. 
     FIG. 10  is a flowchart for explaining the child coupler. When the parent coupler generates a child coupler by the active or passive couple command (step  4601 ), the child coupler examines whether or not there is a received data in the edge socket (step  4602 ). If so, then the child coupler receives the data and sends it to the inner socket (step  4603 ). If not, then the child coupler examines whether or not there is received data in the inner socket (step  4604 ). If so, then the child coupler receives the data and sends it to the edge side socket (step  4605 ). If not, then the child coupler examines whether or not the edge side socket is closed (step  4606 ). If so, then the child coupler closes the inner socket and sends a release command to the mediator (step  4607 ). If not, then the child coupler examines whether or not the inner socket is closed (step  4608 ). If so, then the child coupler closes the edge side socket and sends the release command to the mediator (step  4609 ). 
     FIG. 11  shows an arrangement of the system in accordance with another embodiment of the present invention. 
   The exemplary arrangement of  FIG. 11  includes a receive gateway (RGW)  10 , an integrated receiver decoder (IRD)  11 , a send gateway (SGW)  20 , an uplink station  21 , a satellite  30 , Internet  40 , a server  50 , a personal computer (PC)  60  and an edge router  70   c . Although the edge routers ‘a’  70   a  and edge router ‘b’  70   b  have been installed in satellite send and receive station sides, respectively in the embodiment of  FIG. 1 , the edge router  70   c  is installed in only the satellite send station side in  FIG. 11 . However, in place of installing the edge router  70   c  only in the satellite send station side, it is also possible to install the edge router  70   c  only in the satellite receive station side, or to install a router  70   c  in the satellite  30  itself. 
   The edge router  70   c  in the present embodiment includes a single communication apparatus corresponding to a combination of the first and second edge routers  70   d  and  70   e  in  FIG. 4 . The edge router  70   c  performs the role of the first and second edge routers  70   d  and  70   e  in the single communication apparatus, and performs, in principle, exactly the same operations as shown in  FIG. 4 . Thus, the edge router  70   c  performs address translation over the TCP packet, divides the TCP connection and performs acknowledgement operations, thereby simulating a single TCP connection. 
     FIG. 12  shows the communication system of the second embodiment of the invention and  FIG. 13  shows the attributes of the TCP packets that are sent using the system. 
   In this embodiment an edge router  70  is provided between a source host  80  and a destination host  90 . The edge router  70  includes a first dazzler  101 , a coupler  200 , and a second dazzler  102 . 
   Firstly, the source host  80  sends a TCP packet (the source address of which is the address of the source host  80 , and the destination address is the address of the destination host  90 ) to the edge router  70  (step  2801 ). In  FIG. 13 , IP-PC 1  and P-PC 1  are the IP address and port number of the source host  80 , and IP-PC 2  and P-PC 2  are the IP address and port number of the destination host  90 . 
   The edge router  70  receives the TCP packet, which is accepted by the first dazzler  101 . The first dazzler  101  conducts address translation according to translation rules which are registered in an address translation information table  1500  in advance. More precisely, the first dazzler  101  translates the destination address of the packet to the source-side edge address of the coupler  200 . Then the first dazzler  101  sends a translated packet to the coupler  200  (step  2802 ). In  FIG. 13 , IP-CA and P-CA show the source-side edge IP address and port number of the coupler  200 . 
   The coupler  200  terminates the TCP connection from the source host  80  because the destination address of the TCP packet corresponds to the source-side edge address of the coupler. Therefore, an acknowledgement packet is sent to the source host  80  by the coupler  200 . That is, the coupler  200  sends an acknowledgement packet (the source address of which is the source-side edge address of coupler and the destination address of which is the address of the source host  80 ) to the first dazzler  101  (step  2803 ). This packet contains ACK flag in TCP header. 
   The first dazzler  101  performs an address translation on the acknowledgement packet according to the translation rules in the address translation information table  1500 , wherein the first dazzler  101  translates source address of this packet from the source-side edge address of the coupler  200  to the address of the destination host  90 . 
   After that, the translated acknowledgement packet is sent from the first dazzler  101  to the source host  80  (step  2804 ). The source host  80 , which receives the acknowledgement packet, recognizes the packet as one sent from the destination host  90 . 
   On the other hand, the TCP packet, which has been sent to the coupler  200 , must be sent to the destination host  90 . In order to achieve this, the coupler  200  sends the TCP packet (the source address of which is the destination-side edge address of coupler, the destination address of which is the address of the destination host  90 ) to the second dazzler  102  (step  2805 ). In  FIG. 13 , IP-CB and P-CB correspond to the destination-side edge IP address and port number of the coupler  200 . 
   The second dazzler  102  conducts address translation of the TCP packet according to the address translation information table  1500 , i.e., the second dazzler  102  translates source address of this packet to the address of the source host  80 . 
   After that, the translated TCP packet is sent from the second dazzler  102  to the destination host  90  (step  2806 ). The destination host  90 , which receives the packet, recognizes that this packet is sent from the source host  80 . 
   The destination host  90  then sends an acknowledgement packet (the source address of which is the address of the destination host  90 , the destination address of which is the address of the source host  80 ) to the edge router  70  (step  2807 ). 
   The edge router  70  receives the acknowledgement packet, which is accepted by the second dazzler  102 . The second dazzler  102  conducts address translation according to translation rules registered in the address translation information table  1500 , wherein the second dazzler  102  translates destination address of the packet to the destination-side edge address of the coupler  200 . Then the second dazzler  102  sends a translated packet to the coupler  200  (step  2808 ). 
   The coupler  200  terminates the TCP connection from the destination host  90  because the destination address of the TCP packet corresponds to the destination-side edge address of the coupler  200 . 
   Since the first dazzler  101 , the coupler  200 , and the second dazzler  102  execute in this way, TCP connection between the source host  80  and the destination host  90  is divided into “source host  80 —coupler  200 ” and “coupler  200 —destination host  90 ”. In addition, packets being sent/received by the source host  80  coincide with packets being received/sent by the destination host  90 . Therefore, each host perceives a single TCP connection “source host  80 —destination host  90 ” and does not acknowledge of the presence of edge router  70 . Furthermore, the performance of TCP communication link will be improved because edge router sends acknowledgement packets and each host can receives these packets immediately without them passing through a delay link. 
   In this embodiment, the performance enhancement of TCP communication will be greatest when there is a delay link in “the coupler  200 —destination host  90 ”. The improvement in TCP performance will not be so marked when a delay link is in “source host  80 —the coupler  200 ”. 
     FIG. 14   a  shows TCP connection between source host  80  and destination host  90  through a conventional router, and  FIG. 14   b  shows the communication architecture of communication equipments thereof. 
   In  FIG. 14   a , the TCP connection between the source host  80  and destination host  90  is indicated by a broken line. 
   As shown in  FIG. 14   b , the communication architecture consists of layers, “data link, physical layer”, “network layer”, “transport layer” and “application layer” from the bottom to top. Each layer provides its own function. The TCP protocol belongs to transport layer and IP protocol belongs to network layer. 
   Regarding TCP connection between the source host  80  and the destination host  90 , the conventional router forwards packets at the network layer according to the destination address (IP address) of packets. The TCP connection is not divided at the conventional router. 
     FIG. 15  shows communication architecture of the first embodiment. 
   As shown in  FIG. 15 , in the first edge router  70   d , the dazzler conducts address translation of TCP packets from the source host  80  at the data link, in physical layer. The translated TCP packet is sent to the transport layer, and the TCP connection is terminated at this layer. 
   The coupler of the first edge router  70   d  is coupled to the coupler of the second edge router  70   e  via a connection according to any protocol. In this case, this connection is a TCP connection. 
   The coupler of the second edge router  70   e  is coupled to the destination host by a TCP connection. The dazzler of the second edge router conducts address translation of packets and forwards the TCP packets to the destination host  90 . 
   In this case, 3 TCP connections “the source host  80 —the 1st edge router  70   d ”, “the 1st edge router  70   d —the 2nd edge router  70   e ”, and “the 2nd edge router  70   e —the destination host  90 ” are established. 
     FIG. 16  shows the communication architecture of the second embodiment. 
   As shown in  FIG. 16 , in the edge router  70 , dazzler conducts address translation of TCP packets from the source host  80  at the data link, in physical layer. Translated TCP packets are sent to the transport layer, and TCP connection is terminated at this layer. 
   The coupler of the edge router  70  is coupled to the destination host  90  by a connection using TCP. The dazzler of the edge router  70  conducts address translation again and then the TCP packets are sent to the destination host  90 . 
   In this case, 2 TCP connections “the source host  80 —the edge router  70 ” and “the edge router  70 —the destination host  90 ” are established. 
   As has been mentioned above, in accordance with the present invention, there can be formed a system which, when communication is carried out in accordance with TCP, or another protocol which requires a destination host to return an acknowledgement for data received from a source host and limits the amount of data that can be sent in a manner similar to the restricted TCP window size, using a high-speed but large-delay link, can suppress delay influences and realize high-speed communication without modifying the existing protocol. 
   Furthermore, while the described embodiments comprise edge routers ( 70 ), the router, or routers, according to the present invention may be situated anywhere along the route between the source host ( 80 ) and the destination host ( 90 ), including a location aboard a satellite ( 30 ). 
   The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims.