Abstract:
An internetwork device comprises a receiving module, an inter-device packet transfer interface, an inter-device packet transfer controller, an address translation module, and a transmission module. The receiving module receives a packet from a first network. The inter-device packet transfer interface carries out inter-device packet transfer involving transfer of a packet to or from another internetwork device. The inter-device packet transfer controller controls the inter-device packet transfers such that multiple fragmented packets created from a same original packet are collected by the same internetwork device, the fragmented packets being packets created by dividing a single original packet into multiple parts. The address translation module translates between a local address used within a specific network and a global address used across multiple networks, for at least one of a source address and a destination address of a packet. The transmission module transmits an address-translated packet to a second network.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority based on a Japanese Patent Applications No. 2009-144073 filed on Jun. 17, 2009, and No. 2009-219903 filed on Sep. 25, 2009, the disclosures of which are hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an internetwork device adapted to translate between local addresses used within a specific network and global addresses used across several networks. 
     2. Description of Related Art 
     Communications between a local network that use local addresses, and a global network that connect several networks and uses global addresses (herein termed “the internet” by way of example) take place through address translation between local addresses and global addresses, carried out in a router located at the boundary of the local network and the internet. Such address translation is known as Network Address Translation. Also, in order to effectively utilize a limited number of global addresses, a translation scheme that also utilizes TCP/UDP communication port numbers in addition to local addresses and global addresses (known as Network Address and Port Translation) is commonly used as well. Herein, “Address Translation” (also called “NAT”) is used to mean “Network Address and Port Translation”. Such address translation is employed in routers intended to connect small scale networks in household or businesses to the internet. 
     More recently, for reasons such as a shortage of global addresses available with IPv4, even among Internet Service Providers (hereinafter also called “ISP”) that serve large numbers of users there is an increasing need to accommodate users by allocating local addresses to internal network routers. In such instances, NAT functionality is required at the connection point of the ISP to the internet (the edge of the carrier network). NAT at this location differs significantly both in terms of role and required capabilities from NAT used to connect enterprise users and individual users to the ISP, and is called Large Scale NAT (LSN) or Carrier Grade NAT (CGN). Routers having LSN functionality are required to have so-called “carrier grade” performance and reliability. Routers having LSN functionality are also called “internetwork devices”. 
     In most instances higher reliability is required of LSN than of conventional NAT. Accordingly, LSN is sometimes implemented through a configuration in which two redundant internetwork devices operate simultaneously (called “Double ACT configuration”). Where LSN is implemented by internetwork devices in Double ACT configuration, the problem arises that fragmented packets, which are packets created by dividing a single packet (original packet) into multiple parts, may not be processed correctly. Specifically, if network load dispersion is carried out at random by a communication device upstream from the internetwork devices, it is possible that a set of fragmented packets created from a given original packet (hereafter called a “same-source fragmented packet group”) may not be collected by the same internetwork device. If a same-source fragmented packet group is not collected by the same internetwork device, it becomes difficult for the internetwork device to carry out identical address translation on each fragmented packet contained in the same-source fragmented packet group. Thus, the terminal that is the recipient of the fragmented packets may not recognize that the received fragmented packets belong to a same-source fragmented packet group, and may not be able to correctly reassemble the original packet. Also, depending on the load on each internetwork device, differences may arise in terms of latency from input to output of fragmented packets, leading to the possibility that fragmented packets are not forwarded in the correct sequence number to a device downstream from the internetwork devices. 
     Such problems are not limited to configurations with two internetwork devices, but are common to instances in which address translation is carried out in a configuration with multiple internetwork devices operating simultaneously. 
     SUMMARY 
     An object of the present invention is to provide technology whereby fragmented packets may be processed correctly in instances where address translation is carried out in a configuration with multiple internetwork devices operating simultaneously. 
     In one aspect of the present invention, there is provided an internetwork device. The internetwork device comprises a receiving module, an inter-device packet transfer interface, an inter-device packet transfer controller, an address translation module, and a transmission module. The receiving module receives a packet from a first network. The inter-device packet transfer interface carries out inter-device packet transfer involving transfer of a packet to or from another internetwork device. The inter-device packet transfer controller controls the inter-device packet transfers such that multiple fragmented packets created from a same original packet are collected by the same internetwork device, the fragmented packets being packets created by dividing a single original packet into multiple parts. The address translation module translates between a local address used within a specific network and a global address used across multiple networks, for at least one of a source address and a destination address of a packet. The transmission module transmits an address-translated packet to a second network. 
     According to this internetwork device, received packets are forwarded to and from the other internetwork device in such a way that multiple fragmented packets created from the same original packet are collected by the same internetwork device, thereby making it possible for the fragmented packets to be processed correctly in instances where address translation is carried out in a configuration with multiple internetwork devices operating simultaneously. 
     The present invention can be realized in various aspects. For example, the present invention can be realized in aspects such as an internetwork device, method of address translation, a network system having multiple internetwork devices, an integrated circuit or a computer program that execute the functions of these devices, methods and systems, a recording medium on which such computer program is recorded, or a computer program product that includes this recording medium. 
     These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration schematically depicting the configuration of a network  10  according to an Embodiment 1 of the present invention; 
         FIG. 2  is an illustration schematically depicting the configuration of internetwork devices  100 ; 
         FIG. 3  is a flowchart depicting the flow of the outbound packet forwarding process; 
         FIG. 4  is an illustration of packet forwarding routes in the outbound packet forwarding process; 
         FIG. 5  is a flowchart depicting the flow of the inter-device transfer outbound packet forwarding process; 
         FIG. 6  is a flowchart depicting the flow of the inbound packet forwarding process; 
         FIG. 7  is an illustration of packet forwarding routes in the inbound packet forwarding process; 
         FIG. 8  is a flowchart depicting the flow of the inter-device transfer inbound packet forwarding process; 
         FIG. 9  is a flowchart depicting the flow of a startup process of an internetwork device  100 ; 
         FIG. 10  is a flowchart depicting the flow of an outbound packet forwarding process; 
         FIG. 11  is a flowchart depicting the flow of a startup process of an internetwork device; 
         FIG. 12  is a flowchart depicting the flow of a startup process of an internetwork device; 
         FIG. 13  is an illustration schematically depicting the control packet  400  for requesting transfer of address/port translation information; 
         FIG. 14  is an illustration schematically depicting the control packet  500  for transferring address/port translation information; and 
         FIG. 15  is an illustration schematically depicting the control packet  600  for providing notification of NAT-enabled status. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiments of the present invention are described below in the following order. 
     A. Embodiment 1 
     A-1. System and Device Configuration 
     A-2. Outbound Packet Forwarding Process 
     A-3. Inter-device Transfer Outbound Packet Forwarding Process 
     A-4. Inbound Packet Forwarding Process 
     A-5. Inter-device Transfer Inbound Packet Forwarding Process 
     C. Embodiment 2 
     C-1. Internetwork Device Startup Process (Prevention of Discarding of Inter-device Packets) 
     C-2. Internetwork Device Startup Process (Prevention of Discarding of Packets from Network by Bringing Down the Link) 
     C-3. Internetwork Device Startup Process (Prevention of Discarding of Packets from Network by Route Propagation) 
     B. Modified Embodiments 
     A. Embodiment 1 
     A-1. System and Device Configuration 
       FIG. 1  is an illustration schematically depicting the configuration of a network  10  according to an Embodiment 1 of the present invention. The network  10  includes three ISPs (Internet Service Providers)  200 . Each ISP  200  accommodates a number of individual users  220  via a home router  210  having NAT functionality, as well as a number of enterprise users  250  via an enterprise network (enterprise internal networks)  240  and an enterprise router  230  having NAT functionality. 
     ISP-A ( 200 A) is an ISP having a global address, and is able to communicate with the internet  300  through a direct connection with a carrier edge router  310 . An HTTP server  320  is provided on the internet  300 , at a location accessible by individual user  220  and enterprise user  250  clients. 
     ISP-B ( 200 B) and ISP-C ( 200 C) are ISPs that lack a global address, and are able to communicate with the internet  300  by connecting to the carrier edge router  310  via internetwork devices  100  with LSN (Large Scale NAT) functionality. The present embodiment employs a redundant configuration in which two internetwork devices  100  (internetwork device A ( 100 A) and internetwork device B ( 100 B)) having identical functionality and configuration operate simultaneously (Double ACT configuration). Specifically, ISP-B ( 200 B) and ISP-C ( 200 C) are each connected to the two internetwork devices  100  and are able thereby to connect to the carrier edge router  310  through either internetwork device  100 . The two internetwork devices  100  synchronize with one another information needed for address translation (session information). ISP-B ( 200 B) and ISP-C ( 200 C) respectively have routers (not shown) connected to the two internetwork devices  100 . These routers direct packets in communications bound from the ISP  200  (local network) to the internet  300  (hereinafter termed “outbound communications”) to either of the two internetwork devices  100 . Similarly, the carrier edge router  310  directs packets in communications bound from the internet  300  to the ISP  200  (hereinafter termed “inbound communications”) to either of the two internetwork devices  100 . Load dispersion between the two internetwork devices  100  is accomplished thereby. If either of the two internetwork devices  100  encounters a problem, it is possible for address translation to continue by directing all packets to the other internetwork device  100 . The functions of the internetwork devices  100  may be viewed as part of the functionality of the carrier edge router  310 . 
       FIG. 2  is an illustration schematically depicting the configuration of internetwork devices  100 . The configurations of two internetwork devices  100  (internetwork device A ( 100 A) and internetwork device B ( 100 B)) are shown in  FIG. 2 . Because the two internetwork devices have identical configurations, in the following description, the identifiers for distinguishing between devices (i.e. “A” and “B”) are not appended to names or drawing symbols except where necessary to do so to distinguish the two internetwork devices  100  and their constituent elements from one another. 
     The internetwork devices  100  are devices for performing address translation (address and port translation) so that communication can take place between a local network LNET and a global network GNET. Each internetwork device  100  includes an inside packet interface (packet I/F (I))  101  for connecting to the local network LNET; an outside packet interface (packet I/F (O))  103  for connecting to the global network GNET; a routing controller  102  for controlling packet routing; an inside address translation module (address translation module (I))  104  for performing address translation of outbound communication packets (hereinafter termed “outbound packets OBP”); an outside address translation module (address translation module (O))  105  for performing address translation of inbound communication packets (hereinafter termed “inbound packets IBP”); an inside inter-device packet transfer interface (inter-device packet forwarding I/F (I))  106  for forwarding outbound packets OBP to the other internetwork device  100 ; and an outside inter-device packet transfer interface (inter-device packet forwarding I/F (O))  107  for forwarding inbound packets IBP to the other internetwork device  100 . 
     The inside packet interface  101  is connected to the routing controller  102 , and to a router (not shown) located on the local network LNET and that carries out load dispersion. The inside packet interface  101  receives outbound packets OBP sent from the router and forwards these to the routing controller  102 , and also receives inbound packets IBP forwarded from the routing controller  102  and forwards these to the router. The outside packet interface  103  is connected to the routing controller  102  and to the carrier edge router  310  ( FIG. 1 ). The outside packet interface  103  receives inbound packets IBP sent from the carrier edge router  310  and forwards these to the routing controller  102 , and also receives outbound packets OBP forwarded from the routing controller  102  and forwards these to the carrier edge router  310 . The inside packet interface  101  and the outside packet interface  103  function as the transmission module and the receiving module in the present invention. 
     The routing controller  102  is connected to constituent elements of the internetwork device  100 . The routing controller  102  carries out forwarding of packets received from the various interfaces, as well as a process to determine whether received packets are fragmented packets. The packet forwarding process of the routing controller  102  will be discussed in detail later. 
     The inside redundant inter-device packet transfer interface  106  is an interface adapted for direct connection to another internetwork device  100 . Specifically, for operation in Double ACT configuration, the inside redundant inter-device packet transfer interface  106  is connected via a dedicated line  112  to the inside redundant inter-device packet transfer interface  106  of the other internetwork device  100 . The inside redundant inter-device packet transfer interface  106  is adapted to receive outbound packets OBP forwarded from the routing controller  102  in its home device and to transmit these to the inside redundant inter-device packet transfer interface  106  of the other internetwork device  100 ; as well as to receive outbound packets OBP forwarded from the other internetwork device  100  (termed “inter-device transfer outbound packets TOBP”) and to forward these to the routing controller  102  in its home device. In the present embodiment, the communication band (line speed) of the inside redundant inter-device packet transfer interface  106  is the same as the communication band (line speed) of the inside packet interface  101 , so as to be able to handle instances in which all outbound packets OBP received by the inside packet interface  101  are forwarded to the other internetwork device  100  as inter-device transfer outbound packets TOBP. 
     The outside redundant inter-device packet transfer interface  107  is an interface adapted for direct connection to another internetwork device  100 . Specifically, for operation in Double ACT configuration, the outside redundant inter-device packet transfer interface  107  is connected via a dedicated line  114  to the outside redundant inter-device packet transfer interface  107  of the other internetwork device  100 . The outside redundant inter-device packet transfer interface  107  is adapted to receive inbound packets IBP forwarded from the routing controller  102  in its home device and to transmit these to the outside redundant inter-device packet transfer interface  107  of the other internetwork device  100 ; as well as to receive inbound packets IBP forwarded from the other internetwork device  100  (termed “inter-device transferred inbound packets TIBP”) and to forward these to the routing controller  102  in its home device. In the present embodiment, the communication band (line speed) of the outside redundant inter-device packet transfer interface  107  is the same as the communication band (line speed) of the outside packet interface  103 , so as to be able to handle instances in which all inbound packets IBP received by the outside packet interface  103  are forwarded to the other internetwork device  100  as inter-device transferred inbound packets TIBP. 
     The inside redundant inter-device packet transfer interface  106  and the outside redundant inter-device packet transfer interface  107  function as the inter-device packet forwarding interfaces in the present invention. The routing controller  102  functions as the inter-device packet forwarding controller in the present invention. 
     The inside address translation module  104  is adapted to maintain address and port translation information (session information); and on the basis of the address and port translation information, to carry out translation of the source address (sending address) and communication port number (address translation) of outbound packets OBP forwarded from the routing controller  102 , then re-forward the address-translated outbound packets OBP back to the routing controller  102 . 
     The outside address translation module  105  is adapted to maintain address and port translation information (session information); and on the basis of the address and port translation information, to carry out translation of the destination address and communication port number (address translation) of inbound packets IBP forwarded from the routing controller  102 , then re-forward the address-translated inbound packets IBP back to the routing controller  102 . 
     The inside address translation module  104  and the outside address translation module  105  are connected to one another via a line  126 , and are also connected to the inside address translation module  104  and the outside address translation module  105  of the other internetwork device  100  via lines  122  and  124 . The inside address translation modules  104  and the outside address translation modules  105  of the internetwork devices  100  carry out synchronization of address and port translation information via these lines. Address and port translation information includes an address translation rule table indicating mappings between local addresses and global addresses; a free port management table indicating In Use/Not In Use status of communication ports in global addresses; and an address/port translation table indicating corresponding relations between address/communication port combinations on the local network side and address/communication port combinations on the global network side. The inside address translation module  104  and the outside address translation module  105  function as the address translation modules in the present invention. 
     A-2. Outbound Packet Forwarding Process 
       FIG. 3  is a flowchart depicting the flow of the outbound packet forwarding process.  FIG. 4  is an illustration of packet forwarding routes in the outbound packet forwarding process. The outbound packet forwarding process is a process whereby an internetwork device  100  that has received outbound packets OBP from the local network LNET forwards the received packets to the global network GNET. The following description assumes that the two internetwork devices  100  are operating simultaneously in Double ACT redundant configuration, and describes the outbound packet forwarding process that takes place when outbound packets OBP sent from the local network LNET are received by the internetwork device  100 A (see  FIG. 4 ). 
     In Step S 110  ( FIG. 3 ), the inside packet interface  101  of the internetwork device  100  ( 100 A) receives outbound packets OBP from the local network LNET. The inside packet interface  101  then forwards the received outbound packets OBP to the routing controller  102  (Step S 120 ). 
     In Step S 130  ( FIG. 3 ), the routing controller  102  determines whether the received outbound packets OBP meet either a first condition or a second condition. Here, the first condition and the second condition are set up so that any outbound packet OBP meets either the first condition or the second condition. Also, the first condition and the second condition are set up so that all fragmented packets contained in a same-source fragmented packet group meet the same condition. Herein, a same-source fragmented packet group means a set of multiple fragmented packets created from the same original packet. The first condition and the second condition in one internetwork device  100  are set up to be the opposite of the first condition and the second condition in the other internetwork device  100 . In the present embodiment, in the internetwork device  100 A, the first condition is that “the source address is an even number” and the second condition is that “the source address is not an even number (is an odd number)”. In the internetwork device  100 B on the other hand, in order to make the relationship of the first condition and the second condition the reverse of their relationship in the internetwork device  100 A, the first condition is that “the source address is not an even number (is an odd number)” and the second condition is that “the source address is an even number”. Where the first and second conditions are set up in this way, any outbound packet OBP meets either the first condition or the second condition. Also, because fragmented packets contained in a given same-source fragmented packet group share the same source address, they all meet the same condition. 
     In Step S 130  ( FIG. 3 ), the routing controller  102  of the internetwork device  100 A determines whether the outbound packet OBP source address is an even number (i.e. whether it meets the first condition) or an odd number (i.e. whether it meets the second condition). If it is determined that the outbound packet OBP meets the second condition and not the first condition (Step S 130 : NO), the routing controller  102  forwards the outbound packet OBP to the other internetwork device  100  ( 100 B) via the inside redundant inter-device packet transfer interface  106  (Step S 140 ) (see path P 2  in  FIG. 4 ). The process that takes place when an internetwork device  100  receives an outbound packet OBP transferred from the other internetwork device  100  (inter-device transfer outbound packet TOBP) will be discussed later. 
     If on the other hand it is determined that the outbound packet OBP meets the first condition (Step S 130 : YES), the routing controller  102  then determines whether the outbound packet OBP is a fragmented packet (Step S 160 ). If determined that the outbound packet OBP is not a fragmented packet (Step S 160 : NO), the routing controller  102  forwards the outbound packet OBP to the inside address translation module  104  (Step S 190 ) (see path P 1  in  FIG. 4 ). The inside address translation module  104  having received the outbound packet OBP references the address/port translation information and carries out address translation (translation of the source address and communication port number) (Step S 200 ). 
     If on the other hand it is determined that the outbound packet OBP is a fragmented packet, (Step S 160 : YES), the routing controller  102  waits to receive all of the packets that make up the same-source fragmented packet group to which the received fragmented packet belongs (Step S 170 ). Through monitoring of packets forwarded from the local network LNET via the inside packet interface  101  and of packets transferred from the other internetwork device  100  via the inside redundant inter-device packet transfer interface  106 , the routing controller  102  determines whether all of the packets that make up the same-source fragmented packet group have been received. 
     Once all of the packets that make up the same-source fragmented packet group are received, the routing controller  102  sorts all the received fragmented packets in order of sequence number (Step S 180 ). The routing controller  102  then forwards the fragmented packets (outbound packets OBP) in order of sequence number to the inside address translation module  104  (Step S 190 ) (see path P 1  in  FIG. 4 ). Once the inside address translation module  104  has received the outbound packets OBP, it references the address/port translation information and carries out address translation (translation of the source address and communication port number) (Step S 200 ). 
     The inside address translation module  104  forwards the address-translated outbound packets OBP to the routing controller  102 , whereupon the routing controller  102  carries out routing according to the destination address, and forwards the outbound packets OBP to the global network GNET via the outside packet interface  103  (Step S 210 ) (see path P 1  in  FIG. 4 ). 
     The outbound packet forwarding process when outbound packets OBP are received by the internetwork device  100 B takes place in the same manner as above. In this case, in Step S 130  ( FIG. 3 ) it is determined whether the outbound packet OBP source address is an odd number (i.e. meets condition  1 ) or an even number (i.e. meets condition  2 ), and according to the outcome of the decision either a process to forward the outbound packet OBP to the other internetwork device  100  (Step S 140 ), or to process it internally (Steps S 160 - 210 ) is carried out. 
     A-3. Inter-Device Transfer Outbound Packet Forwarding Process 
       FIG. 5  is a flowchart depicting the flow of the inter-device transfer outbound packet forwarding process. The inter-device transfer outbound packet forwarding process is a process by which an internetwork device  100  having received outbound packets OBP (inter-device transfer outbound packets TOBP) from the other internetwork device  100  forwards the received packets to the global network GNET. The following description assumes that the two internetwork devices  100  are operating simultaneously in Double ACT redundant configuration, and describes the inter-device transfer outbound packet forwarding process that takes place when inter-device transfer outbound packets TOBP transferred from the internetwork device  100 A are received by the internetwork device  100 B (see path P 2  in  FIG. 4 ). The specifics of the inter-device transfer outbound packet forwarding process that takes place when inter-device transfer outbound packets TOBP transferred from the internetwork device  100 B are received by the internetwork device  100 A would be the same. 
     In Step S 310  ( FIG. 5 ), the inside redundant inter-device packet transfer interface  106  of the internetwork device  100  ( 100 B) receives an outbound packet OBP (inter-device transfer outbound packet TOBP) from the other internetwork device  100  ( 100 A). The inside redundant inter-device packet transfer interface  106  forwards the received outbound packet OBP to the routing controller  102  (Step S 320 ). 
     In Step S 330  ( FIG. 5 ), the routing controller  102  determines whether the outbound packet OBP is a fragmented packet (Step S 330 ). If determined that the outbound packet OBP is not a fragmented packet (Step S 330 : NO), the routing controller  102  forwards the outbound packet OBP to the inside address translation module  104  (Step S 360 ) (see path P 2  in  FIG. 4 ). The inside address translation module  104  having received the outbound packet OBP references the address/port translation information and carries out address translation (translation of the source address and communication port number) (Step S 370 ). 
     If on the other hand it is determined that the outbound packet OBP is a fragmented packet, (Step S 330 : YES), the routing controller  102  waits to receive all of the packets that make up the same-source fragmented packet group to which the received fragmented packet belongs (Step S 340 ). Through monitoring of packets forwarded from the local network LNET via the inside packet interface  101  and of packets transferred from the other internetwork device  100  via the inside redundant inter-device packet transfer interface  106 , the routing controller  102  determines whether all of the packets that make up the same-source fragmented packet group have been received. 
     Once all of the packets that make up the same-source fragmented packet group are received, the routing controller  102  sorts all the received fragmented packets in order of sequence number (Step S 350 ). The routing controller  102  then forwards the fragmented packets (outbound packets OBP) in order of sequence number to the inside address translation module  104  (Step S 360 ). Once the inside address translation module  104  has received the outbound packets OBP, it references the address/port translation information and carries out address translation (translation of the source address and communication port number) (Step S 370 ). 
     The inside address translation module  104  forwards the address-translated outbound packets OBP to the routing controller  102 , whereupon the routing controller  102  carries out routing according to the destination address, and forwards the outbound packets OBP to the global network GNET via the outside packet interface  103  (Step S 380 ) (see path P 2  in  FIG. 4 ). 
     A-4. Inbound Packet Forwarding Process 
       FIG. 6  is a flowchart depicting the flow of the inbound packet forwarding process.  FIG. 7  is an illustration of packet forwarding routes in the inbound packet forwarding process. The inbound packet forwarding process is a process whereby an internetwork device  100  that has received inbound packets IBP from the global network GNET forwards the received packets to the local network LNET. The following description assumes that the two internetwork devices  100  are operating simultaneously in Double ACT redundant configuration, and describes the inbound packet forwarding process that takes place when inbound packets IBP sent from the global network GNET are received by the internetwork device  100 A (see  FIG. 7 ). 
     In Step S 410  ( FIG. 6 ), the outside packet interface  103  of the internetwork device  100  ( 100 A) receives inbound packets IBP from the global network GNET. The outside packet interface  103  then forwards the received inbound packets IBP to the routing controller  102  (Step S 420 ). 
     In Step S 430  ( FIG. 6 ), the routing controller  102  determines whether the received inbound packets IBP meet either a first condition or a second condition. Here, the first condition and the second condition are set up so that any inbound packet IBP meets either the first condition or the second condition. Also, the first condition and the second condition are set up so that all fragmented packets contained in a same-source fragmented packet group meet the same condition. The first condition and the second condition in one internetwork device  100  are set up to be the opposite of the first condition and the second condition in the other internetwork device  100 . In the present embodiment, in the internetwork device  100 A, the first condition is that “the source address is an even number” and the second condition is that “the source address is not an even number (is an odd number)”. In the internetwork device  100 B on the other hand, in order to make the relationship of the first condition and the second condition the reverse of their relationship in the internetwork device  100 A, the first condition is that “the source address is not an even number (is an odd number)” and the second condition is that “the source address is an even number”. Where the first and second conditions are set up in this way, any inbound packet IBP meets either the first condition or the second condition. Also, because fragmented packets contained in a given same-source fragmented packet group share the same source address, they all meet the same condition. 
     In Step S 430  ( FIG. 6 ), the routing controller  102  of the internetwork device  100 A determines whether the inbound packet IBP source address is an even number (i.e. whether it meets the first condition) or an odd number (i.e. whether it meets the second condition). If it is determined that the inbound packet IBP meets the second condition and not the first condition (Step S 430 : NO), the routing controller  102  forwards the inbound packet IBP to the other internetwork device  100  ( 100 B) via the outside redundant inter-device packet transfer interface  107  (Step S 440 ) (see path P 2  in  FIG. 7 ). The process that takes place when an internetwork device  100  receives an inbound packet IBP transferred from the other internetwork device  100  (inter-device transfer inbound packet TIBP) will be discussed later. 
     If on the other hand it is determined that the inbound packet IBP meets the first condition (Step S 430 : YES), the routing controller  102  then determines whether the inbound packet IBP is a fragmented packet (Step S 460 ). If determined that the inbound packet IBP is not a fragmented packet (Step S 460 : NO), the routing controller  102  forwards the inbound packet IBP to the outside address translation module  105  (Step S 490 ) (see path P 1  in  FIG. 7 ). The outside address translation module  105  having received the inbound packet IBP references the address/port translation information and carries out address translation (translation of the destination address and communication port number) (Step S 500 ). 
     If on the other hand it is determined that the inbound packet IBP is a fragmented packet, (Step S 460 : YES), the routing controller  102  waits to receive all of the packets that make up the same-source fragmented packet group to which the received fragmented packet belongs (Step S 470 ). Through monitoring of packets forwarded from the global network GNET via the outside packet interface  103  and of packets transferred from the other internetwork device  100  via the outside redundant inter-device packet transfer interface  107 , the routing controller  102  determines whether all of the packets that make up the same-source fragmented packet group have been received. 
     Once all of the packets that make up the same-source fragmented packet group are received, the routing controller  102  sorts all the received fragmented packets in order of sequence number (Step S 480 ). The routing controller  102  then forwards the fragmented packets (inbound packets IBP) in order of sequence number to the outside address translation module  105  (Step S 490 ) (see path P 1  in  FIG. 7 ). Once the outside address translation module  105  has received the inbound packets IBP, it references the address/port translation information and carries out address translation (translation of the destination address and communication port number) (Step S 500 ). 
     The outside address translation module  105  forwards the address-translated inbound packets IBP to the routing controller  102 , whereupon the routing controller  102  carries out routing according to the destination address, and forwards the inbound packets IBP to the local network LNET via the inside packet interface  101  (Step S 510 ) (see path P 1  in  FIG. 7 ). 
     The inbound packet forwarding process when inbound packets IBP are received by the internetwork device  100 B takes place in the same manner as above. In this case, in Step S 430  ( FIG. 6 ) it is determined whether the inbound packet IBP source address is an odd number (i.e. meets condition  1 ) or an even number (i.e. meets condition  2 ), and according to the outcome of the decision either a process to forward the inbound packet IBP to the other internetwork device  100  (Step S 440 ), or to process it internally (Steps S 460 - 510 ) is carried out. 
     A-5. Inter-Device Transfer Inbound Packet Forwarding Process 
       FIG. 8  is a flowchart depicting the flow of the inter-device transfer inbound packet forwarding process. The inter-device transfer inbound packet forwarding process is a process by which an internetwork device  100  having received inbound packets IBP (inter-device transfer inbound packets TIBP) from the other internetwork device  100  forwards the received packets to the local network LNET. The following description assumes that the two internetwork devices  100  are operating simultaneously in Double ACT redundant configuration, and describes the inter-device transfer inbound packet forwarding process that takes place when inter-device transfer inbound packets TIBP transferred from the internetwork device  100 A are received by the internetwork device  100 B (see path P 2  in  FIG. 7 ). The specifics of the inter-device transfer inbound packet forwarding process that takes place when inter-device transfer inbound packets TIBP transferred from the internetwork device  100 B are received by the internetwork device  100 A would be the same. 
     In Step S 610  ( FIG. 8 ), the outside redundant inter-device packet transfer interface  107  of the internetwork device  100  ( 100 B) receives an inbound packet IBP (inter-device transfer inbound packet TIBP) from the other internetwork device  100  ( 100 A). The outside redundant inter-device packet transfer interface  107  forwards the received inbound packet IBP to the routing controller  102  (Step S 620 ). 
     In Step S 630  ( FIG. 8 ), the routing controller  102  determines whether the inbound packet IBP is a fragmented packet (Step S 630 ). If determined that the inbound packet IBP is not a fragmented packet (Step S 630 : NO), the routing controller  102  forwards the inbound packet IBP to the outside address translation module  105  (Step S 660 ) (see path P 2  in  FIG. 7 ). The outside address translation module  105  having received the inbound packet IBP references the address/port translation information and carries out address translation (translation of the destination address and communication port number) (Step S 670 ). 
     If on the other hand it is determined that the inbound packet IBP is a fragmented packet, (Step S 630 : YES), the routing controller  102  waits to receive all of the packets that make up the same-source fragmented packet group to which the received fragmented packet belongs (Step S 640 ). Through monitoring of packets forwarded from the global network GNET via the outside packet interface  103  and of packets transferred from the other internetwork device  100  via the outside redundant inter-device packet transfer interface  107 , the routing controller  102  determines whether all of the packets that make up the same-source fragmented packet group have been received. 
     Once all of the packets that make up the same-source fragmented packet group are received, the routing controller  102  sorts all the received fragmented packets in order of sequence number (Step S 650 ). The routing controller  102  then forwards the fragmented packets (inbound packets IBP) in order of sequence number to the outside address translation module  105  (Step S 660 ). Once the outside address translation module  105  has received the inbound packets IBP, it references the address/port translation information and carries out address translation (translation of the destination address and communication port number) (Step S 670 ). 
     The outside address translation module  105  forwards the address-translated inbound packets IBP to the routing controller  102 , whereupon the routing controller  102  carries out routing according to the destination address, and forwards the inbound packets IBP to the local network LNET via the inside packet interface  101  (Step S 680 ) (see path P 2  in  FIG. 7 ). 
     As described above, according to the present embodiment, when an internetwork device  100  receives packets from either the local network LNET or the global network GNET, after first determining whether the packets meet either the first condition or the second condition (Step S 130  in  FIG. 3  and Step S 430  in  FIG. 6 ), the received packets are directed to either the home internetwork device  100  or the other internetwork device  100 . Here, because the first condition and the second condition are set up such that all fragmented packets included in a same-source fragmented packet group meet the same condition, the entire same-source fragmented packet group is collected by the same internetwork device  100 . Typically, the initial fragmented packet in a same-source fragmented packet group contains in its data portion information identifying the communication port number, whereas the second and subsequent fragmented packets do not contain information identifying the communication port number. In the present embodiment however, because the entire same-source fragmented packet group is collected by the same internetwork device  100 , address translation inclusive of communication port number (address and port translation) can be accomplished on such fragmented packets as well. Thus, according to the present embodiment, fragmented packets contained in a same-source fragmented packet group can undergo identical address translation, and the terminal for which the fragmented packets are destined will be able to correctly reassemble the original packet on the basis of the received fragmented packets. Thus, where the present embodiment is employed for address translation in a configuration with multiple internetwork devices  100  operating simultaneously, address translation of fragmented packets can be carried out correctly. 
     Moreover, in the present embodiment, if a received packet is a fragmented packet, the device waits until all of the fragmented packets that make up the same-source fragmented packet group are received, and having received these then sorts all of the fragmented packets in order of sequence number, and forwards them in order of sequence number. Thus, according to the present embodiment, fragmented packets can be forwarded in order of sequence number to devices downstream from the internetwork devices  100 . 
     According to the present embodiment, conditions that pertain to information originally included in packets, i.e. the source address, are employed as the first and second conditions, so same-source fragmented packet groups can be collected by the same internetwork device  100  without any need to append to the packets additional information for the purpose of packet collection. 
     C. Embodiment 2 
     In LSN, address and port translation information (session information) that indicates currently corresponding relations of address/communication port combinations on the local network side and address/communication port combinations on the global network side is determined either dynamically or statically in the respective internetwork devices  100  according to the IP address and port number of the original packet that was initially received. In configurations where packets are transferred among multiple internetwork devices  100  operating simultaneously and address translation frequently takes place in a different internetwork device, because it is necessary for the respective internetwork devices to carry out address translation based on the same session information regardless of which internetwork device receives the original packet, the multiple internetwork devices  100  must synchronize session information with one another in order to maintain the same session information. In the event that, for example, a stopped internetwork device  100  is restarted, another operational internetwork device  100  must transfer the session information it maintains to the internetwork device  100  that has started up, so that the two internetwork devices  100  can maintain the same session information. 
     Under these circumstances, if for example when a first internetwork device  100  is started up, packets are transferred between the two internetwork devices  100  prior to synchronization of session information between the devices, address translation cannot take place because the first internetwork device  100  lacks session information, so the packets are discarded. 
     The present embodiment describes an embodiment of the present invention whereby in a configuration in which address translation is carried out on packets transferred between multiple internetwork devices  100  operating simultaneously, if an internetwork device  100  is started up, discarding of packets by the started up internetwork device  100  is prevented so that correct address translation can take place on the basis of session information. 
     C-1. Internetwork Device Startup Process (Prevention of Discarding of Inter-Device Packets) 
       FIG. 9  is a flowchart depicting the flow of a startup process of an internetwork device  100 .  FIG. 10  is a flowchart depicting the flow of an outbound packet forwarding process. The following description of the outbound packet forwarding process assumes that, in a redundant configuration with the internetwork device  100 A already operating, when the internetwork device  100 B is started up, outbound packets OBP sent from the local network LNET are received by the internetwork device  100 A, transferred between the devices, and then sent from the internetwork device  100 B to the global network GNET (see path P 2  in  FIG. 4 ). 
     In Step S 910  ( FIG. 9 ), functioning of the address translation module (I)  104 B of the internetwork device  100 B is started up. 
     In Step S 920  ( FIG. 9 ), upon startup, the address translation module (I)  104 B of the internetwork device  100 B, via the line  122 , sends the address translation module (I)  104 A of the other internetwork device  100 A a control packet  400  requesting it to transfer address/port translation information. 
       FIG. 13  is an illustration schematically depicting the control packet  400  for requesting transfer of address/port translation information. The control packet  400  includes a MAC header  410  and a command  420  requesting transfer of address/port translation information. 
     In Step S 930  ( FIG. 9 ), the address translation module (I)  104 A of the other internetwork device  100 A that received the control packet  400  now sends the address translation module (I)  104 B a control packet  500  that contains the address/port translation information currently maintained by itself, in order to transfer the address/port translation information to the address translation module (I)  104 B of the internetwork device  100 B via the line  122 . 
       FIG. 14  is an illustration schematically depicting the control packet  500  for transferring address/port translation information. The control packet  500  includes a MAC header  510 , a command  520  for transferring address/port translation information, a maintained address/port translation information count  530 , and address/port translation information  540 . 
     In Step S 940  ( FIG. 9 ), once the address translation module (I)  104 B of the internetwork device  100 B has acquired the address/port translation information contained in the received control packet  500 , it now sends a control packet  600  providing notification of NAT-enabled status to the routing controller  102 A of the other internetwork device  100 A via the line  122 . 
       FIG. 15  is an illustration schematically depicting the control packet  600  for providing notification of NAT-enabled status. The control packet  600  includes a MAC header  610  and a command  620  for notification of NAT-enabled status. 
     Through notification of NAT-enabled status by the control packet  600 , the internetwork device  100 B can notify the other internetwork device  100 A that synchronization of address/port translation information is complete and that the address translation process is enabled. 
     Next, the outbound packet forwarding process in the other internetwork device  100 A is described with reference to  FIG. 10 . The flowchart of  FIG. 10  is substantially identical to the flowchart of  FIG. 3  except for an additional Step S 1040  between Step S 130  and Step S 140 ; parts that are not appreciably different from  FIG. 3  are not described. 
     In Step S 1010  ( FIG. 10 ), the inside packet interface  101  of the internetwork device  100  ( 100 A) receives an outbound packet OBP from the local network LNET. The inside packet interface  101  forwards the received outbound packet OBP to the routing controller  102  (Step S 1020 ). 
     In Step S 1030  ( FIG. 10 ), the routing controller  102  determines if the received outbound packet OBP meets either the first condition or the second condition. If it is determined that the outbound packet OBP meets the second condition and not the first condition (Step S 1030 : NO), in Step S 1040 , the routing controller  102  of the internetwork device  100 A determines whether the address translation module (I)  104 B of the internetwork device  100 B has NAT-enabled status. If determined to have NAT-enabled status (Step S 1040 : YES), the routing controller  102  transfers the outbound packet OBP to the other internetwork device  100 B via the inside inter-device packet transfer interface  106  (Step S 1050 ) (see path P 2  in  FIG. 4 ). 
     On the other hand, if determined that the outbound packet OBP meets the first condition (Step S 1030 : YES) or that transfer of address/port translation information is not complete and NAT is not enabled (Step S 1040 : NO), the routing controller  102  then determines whether the outbound packet OBP is a fragmented packet (Step S 1060 ). If determined that the outbound packet OBP is not a fragmented packet (Step S 1060 : NO), the routing controller  102  forwards the outbound packet OBP to the inside address translation module  104  (Step S 1090 ) (see path P 1  in  FIG. 4 ). The inside address translation module  104  having received the outbound packet OBP then refers to the address/port translation information and carries out address translation (translation of the source address and communication port number) (Step S 1100 ). 
     If on the other hand it is determined that the outbound packet OBP is a fragmented packet, (Step S 1060 : YES), the routing controller  102  waits to receive all of the packets that make up the same-source fragmented packet group to which the received fragmented packet belongs (Step S 1070 ). Through monitoring of packets forwarded from the local network LNET via the inside packet interface  101  and of packets transferred from the other internetwork device  100  via the inside redundant inter-device packet transfer interface  106 , the routing controller  102  determines whether all of the packets that make up the same-source fragmented packet group have been received. 
     Once all of the packets that make up the same-source fragmented packet group are received, the routing controller  102  sorts all the received fragmented packets in order of sequence number (Step S 1080 ). The routing controller  102  then forwards the fragmented packets (outbound packets OBP) in order of sequence number to the inside address translation module  104  (Step S 1090 ) (see path P 1  in  FIG. 4 ). Once the inside address translation module  104  has received the outbound packets OBP, it references the address/port information and carries out address translation (translation of the source address and communication port number) (Step S 1100 ). 
     The inside address translation module  104  forwards the address-translated outbound packets OBP to the routing controller  102 , whereupon the routing controller  102  carries out routing according to the destination address, and forwards the outbound packets OBP to the global network GNET via the outside packet interface  103  (Step S 1110 ) (see path P 1  in  FIG. 4 ). 
     According to the present embodiment, because the internetwork device  100 A performs inter-device packet transfer only if the address translation module (I)  104 B of the internetwork device  100 B has NAT-enabled status, discarding of packets due to inability for address translation to take place on the internetwork device  100 B can be prevented. 
     While the present embodiment described the outbound packet forwarding process, in the inbound packet forwarding process as well, discarding of inbound packets IBP can be prevented analogously through an additional step equivalent to Step S 1040  of  FIG. 10  coming between Step S 430  and Step S 440  of the flowchart of  FIG. 6 . 
     C-2. Internetwork Device Startup Process (Prevention of Discarding of Packets from Network by Bringing Down the Link) 
       FIG. 11  is a flowchart depicting the flow of a startup process of an internetwork device. The following description of the outbound packet forwarding process assumes that, in a redundant configuration with the internetwork device  100 A already operating, when the internetwork device  100 B is started up, outbound packets OBP sent from the local network LNET are received by the internetwork device  100 B and sent to the global network GNET (see  FIG. 4 ). 
     In Step S 1210  ( FIG. 11 ), functioning of the address translation module (I)  104 B of the internetwork device  100 B is started up. 
     In Step S 1220  ( FIG. 11 ), upon startup, the address translation module (I)  104 B of the internetwork device  100 B, via the line  122 , sends the address translation module (I)  104 A of the other internetwork device  100 A a control packet  400  requesting it to transfer address/port translation information. 
     In Step S 1230  ( FIG. 11 ), the address translation module (I)  104 A of the other internetwork device  100 A that received the control packet  400  now sends the address translation module (I)  104 B a control packet  500  that contains the address/port translation information currently maintained by itself, in order to transfer the address/port translation information to the address translation module (I)  104 B of the internetwork device  100 B via the line  122 . 
     In Step S 1240  ( FIG. 11 ), once the address translation module (I)  104 B of the internetwork device  100 B has acquired the address/port translation information contained in the received control packet  500 , it notifies its own routing controller  102 B of NAT-enabled status. 
     In Step S 1250  ( FIG. 11 ), the routing controller  102 B of the internetwork device  100 B having received notification of NAT-enabled status uplinks to the packet I/F (I)  101 B. 
     When the link from internetwork device  100 B to the packet I/F (I)  101 B is up, outbound packets OBP sent from the internetwork local network LNET are received from the internetwork device  100 B through routing control in the local network LNET. 
     On the other hand, when the link from the internetwork device  100 B to the packet I/F (I)  101 B is down, outbound packets OBP sent from the local network LNET are received from the internetwork device  100 A through routing control in the local network LNET. 
     According to the present embodiment, the internetwork device  100 B upon startup uplinks to the packet I/F (I)  101 B only if the address translation module (I)  104 B has NAT-enabled status, thereby preventing packets from being discarded due to inability to carry out address translation of packets received from the packet I/F (I)  101 B. 
     Additionally, while the present embodiment described the outbound packet forwarding process, in the inbound packet forwarding process as well, discarding of inbound packets IBP can be prevented analogously by uplinking to the packet I/F (O) only after the address translation module (O)  105  has finished synchronizing address/port translation information according to the flowchart of  FIG. 11 . 
     C-3. Internetwork Device Startup Process (Prevention of Discarding of Packets from Network by Route Propagation) 
       FIG. 12  is a flowchart depicting the flow of a startup process of an internetwork device. The following description of the outbound packet forwarding process assumes that, in a redundant configuration with the internetwork device  100 A already operating, when the internetwork device  100 B is started up, outbound packets OBP sent from the local network LNET are received by the internetwork device  100 B and sent to the global network GNET (see  FIG. 4 ). 
     In Step S 1310  ( FIG. 12 ), functioning of the address translation module (I)  104 B of the internetwork device  100 B is started up. 
     In Step S 1320  ( FIG. 12 ), upon startup, the address translation module (I)  104 B of the internetwork device  100 B, via the line  122 , sends the address translation module (I)  104 A of the other internetwork device  100 A a control packet  400  requesting it to transfer address/port translation information. 
     In Step S 1330  ( FIG. 12 ), the address translation module (I)  104 A of the other internetwork device  100 A that received the control packet  400  now sends the address translation module (I)  104 B a control packet  500  that contains the address/port translation information currently maintained by itself, in order to transfer the address/port translation information to the address translation module (I)  104 B of the internetwork device  100 B via the line  122 . 
     In Step S 1340  ( FIG. 12 ), once the address translation module (I)  104 B of the internetwork device  100 B has acquired the address/port translation information contained in the received control packet  500 , it notifies its own routing controller  102 B of NAT-enabled status. 
     In Step S 1350  ( FIG. 12 ), the routing controller  102 B of the internetwork device  100 B having received notification of NAT-enabled status now advertises to the local network LNET the route to the destination address via the packet interface I/F (I)  101 B. 
     With the route from the routing controller  102 B of the internetwork device  100 B advertised in this way, outbound packets OBP sent from the internetwork local network LNET are received from the internetwork device  100 B through routing control in the local network LNET. 
     On the other hand, if the route from the routing controller  102 B of the internetwork device  100 B is not advertised, outbound packets OBP sent from the internetwork local network LNET are received from the internetwork device  100 A through routing control in the local network LNET. 
     According to the present embodiment, upon startup, the routing controller of the internetwork device  100 B advertises to the local network LNET the route via the packet I/F (I)  100 B only if NAT is enabled, thereby preventing packets from being discarded due to inability to carry out address translation of packets received from the packet I/F (I)  101 B. 
     Additionally, while the present embodiment described the outbound packet forwarding process, in the inbound packet forwarding process as well, discarding of inbound packets IBP can be prevented analogously by having the routing controller advertise to the global network GNET the route via the packet I/F (O) after the address translation module (O)  105  has finished synchronizing the address/port translation information according to the flowchart of  FIG. 12 . 
     B. Modified Embodiments 
     The present invention is not limited to the embodiments and aspects described above. The present invention may be worked in various aspects within limits that involve no departure from the spirit of the invention; for example, the following modifications are possible. 
     B1. Modified Embodiment 1 
     In the preceding embodiment, a combination of the conditions that “the source address is an even number” and “the source address is an odd number” are employed as the combination of the first condition and the second condition; however, other combinations are possible. For example, a combination of conditions that “the ones position of the source address is any digit from 0 to 4” and “the ones position of the source address is any digit from 5 to 9” may be employed as the combination of the first condition and the second condition. Alternatively, a combination of conditions that “the ones position of the source address is either 0 or 1” and “the ones position of the source address is any digit from 2 to 9” may be employed. It is preferable to monitor the load on each internetwork device  100  and to establish a combination of conditions such the load assigned to each internetwork device  100  may be adjusted to the desired value (e.g. so that their respective loads are equal). The combination of conditions may be reset at periodic intervals or on an as needed basis. 
     As yet another alternative, a combination of the conditions that “the destination address is an even number” and “the destination address is an odd number” may be employed as the combination of the first condition and the second condition. Because all of the fragmented packets contained in a same-source fragmented packet group share the same destination address, this combination of conditions also results in the packets meeting the same condition. Other combinations of a first condition and a second condition that relate to information shared in common by all fragmented packets of a same-source fragmented packet group could be adopted as well. However, where a combination of conditions relating to the source address are employed as the first and second conditions as taught in the preceding embodiment, all packets sent from a given terminal, not just fragmented packets, are collected by the same internetwork device  100 , which is preferable in terms of packet management. 
     B2. Modified Embodiment 2 
     While the preceding embodiment described an example of a redundant configuration in which two internetwork devices  100  with identical functions and features operate simultaneously (Double ACT configuration), the present invention may be implemented in any redundant configuration in which N (N is an integer equal to 3 or greater) internetwork devices  100  operate simultaneously. In this case, a number N of conditions ranging from a first condition to an N-th condition would be established in manner analogous to the preceding embodiment, so that any packet meets one of the N conditions, and all of the fragmented packets included in a same-source fragmented packet group meet the same condition. Packets received by any one of the internetwork devices  100  are then distributed to any of the N internetwork devices  100  based on the determination as to which of the N conditions they meet. With such an arrangement, fragmented packets can be processed correctly in instances where address translation takes place in a configuration with N internetwork devices operating simultaneously. 
     B3. Modified Embodiment 3 
     Some of the features implemented through hardware in the preceding embodiment may be replaced by software, and conversely some of the features implemented through software may be replaced by hardware.