Abstract:
Provided is a communication apparatus that is capable of enhancing fault resistance of a network. A communication apparatus includes, in a network where messages are exchanged among a plurality of communication apparatuses and route control is performed, a route summary processing means that creates route summary information obtained by summarizing route information of a local apparatus from a route table holding route information used for route control and traffic information between communication apparatuses, the route summary processing means further exchanging the route summary information that is created with other communication apparatuses; and a fault influence degree information processing means that calculates fault influence degree information indicating a degree of influences on traffic and the other communication apparatuses when the local apparatus is in a fault state, the fault influence degree information processing means further exchanging the fault influence degree information that is calculated with the other communication apparatuses; and a route adjustment processing means that adjusts the route table of the local apparatus using the route summary information and the fault influence degree information that are obtained by the local apparatus.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a communication apparatus, a network, and a route control method used therefor, and more particularly, to a route control method that reduces influences on data transfer upon occurrence of faults in a router and a link. 
       BACKGROUND ART 
       [0002]    Dynamic route control (routing) in the IP (Internet Protocol) network is carried out by routing protocols such as OSPF (Open Shortest Path First) disclosed in a non-patent literature 1, RIP (Routing information Protocol) disclosed in a non-patent literature 2 and the like. 
         [0003]    Upon occurrence of a fault in a communication apparatus (router) or a line (link), these routing protocols calculate a new route by exchanging route information to recover the communication path. The communication path is basically selected that minimizes the total link cost defined for each link or minimizes the number of routers (number of hops) that are passed through. 
         [0004]    One of defects of the dynamic route control is, since recalculation of the route requires a certain period of time, the communication that passes through the router or link with fault is kept interrupted for a few seconds to a several tens of seconds from the occurrence of the fault to the completion of the re-calculation of the route. 
         [0005]    One technique for avoiding such a long-term interruption is an IP-FRR (IP Fast Reroute) technique, which is disclosed in a non-patent literature 3. In the IP-FRR technique, an alternative transfer destination (alternative next hop) that is not looped is determined in advance. Accordingly, it is possible to quickly restart packet transfer without waiting for the re-calculation of the route of the routing protocol upon occurrence of a fault in a router or a link. 
         [0006]    Another defect is that there is a possibility that the load is concentrated (congested) in a specific router or link for the purpose of selecting the route with minimum cost. A traffic distribution system is one technique for avoiding concentration of traffic. In the OSPF-OMP (OSPF-Optimized Miltipath) disclosed in a non-patent literature 4, each router advertises the link utilization rate or the like to the whale network with the route information, and when there are a plurality of equal-cost routes to the same destination, the traffic is dispersed according to the link utilization rate among the equal-cost routes. 
         [0007]    The technique disclosed in the patent literature 1 avoids congestion by switching routes upon detection of congestion. The technique disclosed in the patent literature 2 avoids congestion by specifying the route by a transmission source so as to avoid the congestion area. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [Patent literature 1] Japanese Unexamined Patent Application Publication No. 2003-092593 
         [Patent literature 2] Japanese Unexamined Patent Application Publication No. 2002-368787 
       
     
       Non Patent Literature 
       [0010]    [Non-patent literature 1]
   J. Moy, “OSPF version 2”, IETF (Internet Engineering Task Force) RFC (Request For Comments) 2328, April 1998
 
[Non-patent literature 2]
   G. Malkin, “RIP version 2”, IETF RFC2453, 1998
 
[Non-patent literature 3]
   A. Atlas et al., “Basic Specification for IP Fast-Reroute: Loop-free Alternates”, IETF Internet draft draft-ietf-rtgwg-ipfrr-spec-base-12.txt, March 2008
 
[Non-patent literature 4]
   C. Villamizar, “OSPF Optimized Multipath”, IETF Internet draft draft-ietf-ospf-omp-02.txt, 1999   
 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0015]    As described above, by using the IP-FRR as a technique for enhancing the fault tolerance of the IP network, the packet transfer can be restarted without waiting for the route calculation of the routing protocol upon occurrence of a fault in a link or a router. 
         [0016]    However, since not all the destinations have alternative next hops, the packet to the destination that has no alternative next hop is not transferred until the completion of the re-calculation of the route. Hence, there are caused influences such as packet loss due to overflow of transmission queue, or an increase in the delay. 
         [0017]    Further, when each router switches to the alternative next hop due to the occurrence of the fault, the bypassed traffic may concentrate on the alternative next hop, which may cause congestion. This congestion causes occurrence of the packet loss, or in some cases down of a router due to overload, for example. In the worst case, the cycle of fault, down of router→traffic bypass to alternative next hop→down of alternative next hop, is repeated, which may result in the whole network being down. 
         [0018]    An object of the present invention is to solve the problem stated above, and to provide a communication apparatus, a network, and a route control method used therefor that make it possible to increase fault resistance of the network. 
       Solution to Problem 
       [0019]    One aspect of a communication apparatus according to the present invention includes, in a network where messages are exchanged among a plurality of communication apparatuses and route control is performed, a route summary processing means that creates route summary information obtained by summarizing route information of a local apparatus from a route table holding route information used for route control and traffic information between communication apparatuses, the route summary processing means further exchanging the route summary information that is created with other communication apparatuses; and a fault influence degree information processing means that calculates fault influence degree information indicating a degree of influences on traffic and the other communication apparatuses when the local apparatus is in a fault state, the fault influence degree information processing means further exchanging the fault influence degree information that is calculated with the other communication apparatuses; and a route adjustment processing means that adjusts the route table of the local apparatus using the route summary information and the fault influence degree information that are obtained by the local apparatus. 
         [0020]    A network according to the present invention includes the communication apparatus described above. 
         [0021]    A route control method according to the present invention is a route control method used for a network where messages are exchanged among a plurality of communication apparatuses and route control is performed, the method executing the following processing of: route summary processing that creates route summary information obtained by summarizing route information of a local apparatus from a route table holding route information used for route control and traffic information between the communication apparatuses, and exchanges the route summary information that is created with other communication apparatuses; fault influence degree information processing that calculates fault influence degree information indicating a degree of influences on traffic and the other communication apparatuses when the local apparatus is in a fault state, and exchanges the fault influence degree information that is calculated with the other communication apparatuses; and route adjustment processing that adjusts the route table of the local apparatus using the route summary information and the fault influence degree information that are obtained by the local apparatus. 
       Advantageous Effects of Invention 
       [0022]    The present invention employs the configuration and the operation as described above, thereby making it possible to enhance fault resistance of a network. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  is a block diagram showing a configuration example of a communication apparatus (router) according to an embodiment of the present invention; 
           [0024]      FIG. 2  is a diagram showing a part of entry constitution of a route table shown in  FIG. 1 ; 
           [0025]      FIG. 3  is a diagram showing a configuration example of a network according to the embodiment of the present invention; 
           [0026]      FIG. 4A  is a diagram showing one example of a route table of each of routers calculated by an existing routing protocol according to the embodiment of the present invention; 
           [0027]      FIG. 4B  is a diagram showing one example of a route table of each of routers calculated by an existing routing protocol according to the embodiment of the present invention; 
           [0028]      FIG. 4C  is a diagram showing one example of a route table of each of routers calculated by an existing routing protocol according to the embodiment of the present invention; 
           [0029]      FIG. 5A  is a diagram showing one example of traffic information of each of routers shown in  FIG. 3 ; 
           [0030]      FIG. 5B  is a diagram showing one example of traffic information of each of routers shown in  FIG. 3 ; 
           [0031]      FIG. 6A  is a diagram showing route summary information of each of routers shown in  FIG. 3 ; 
           [0032]      FIG. 6B  is a diagram showing route summary information of each of routers shown in  FIG. 3 ; 
           [0033]      FIG. 7  is a diagram showing fault influence degree information of each of routers  201  to  204  shown in  FIG. 3 ; 
           [0034]      FIG. 8  is a diagram showing adjacent node information extracted by a router  101  shown in  FIG. 3 ; 
           [0035]      FIG. 9  is a diagram showing adjacent node information extracted by a router  102  shown in  FIG. 3 ; 
           [0036]      FIG. 10  is a diagram showing information of routers adjacent to the router  101  after Step 1 of the route adjustment is completed in the embodiment of the present invention; 
           [0037]      FIG. 11  is a diagram showing information of routers adjacent to the router  102  after Step 1 of the route adjustment is completed in the embodiment of the present invention; 
           [0038]      FIG. 12  is a diagram showing information of routers adjacent to the router  101  after Step 2 of the route adjustment is completed in the embodiment of the present invention; 
           [0039]      FIG. 13  is a diagram showing information of routers adjacent to the router  102  after Step 3 of the route adjustment is completed in the embodiment of the present invention; and 
           [0040]      FIG. 14  is a diagram showing route tables of the routers  101  and  102  after route adjustment processing is completed in the embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0041]    Next, an embodiment of the present invention will be described with reference to the accompanying drawings. First, description will be made of the outline of a network of the present invention. The network according to the present invention includes, for the purpose of accomplishing the object described above, a route summary processing unit that creates a summary (aggregation) of route information of a local apparatus in each communication apparatus (router) to notify another router of the summary, and receives route summary information from another router to hold the received information; a fault influence degree information processing unit that calculates a fault influence degree of the local apparatus based on the route summary information of another router to notify another router of the fault influence degree, and receives the fault influence degree information from another router to hold the fault influence degree information; and a route adjustment processing unit that adjusts the route of the local apparatus based on the fault influence degree information of another router. 
         [0042]    In a communication apparatus according to the present invention, a route summary processing unit creates a summary of route information of a local apparatus to notify another router of the summary, and receives route summary information from another router to hold the route summary information, a fault influence degree information processing unit calculates a fault influence degree of the local apparatus based on the route summary information of another router to notify another router of the fault influence degree, and receives the fault influence degree information from another router to hold the fault influence degree information, and a route adjustment processing unit adjusts the route of the local apparatus based on the fault influence degree information of another router, so as to mitigate the influence degree upon occurrence of a fault in each router in a network and to achieve route control to enhance fault resistance of the network. 
         [0043]    As described above, the present invention minimizes the influence given on a user or a network upon occurrence of a fault, thereby being capable of enhancing fault resistance of the network. Here, the influence given on the user or the network in case of a fault means occurrence of transfer interruption of traffic due to a fault of a router or a link, or congestion caused by traffic bypassed to an alternative next hop upon occurrence of a fault. 
         [0044]      FIG. 1  is a block diagram showing a configuration example of a communication apparatus (router) according to an embodiment of the present invention. In  FIG. 1 , a router  11  includes a packet transfer processing unit  20 , a route control processing unit  21 , a route table  22 , a route summary processing unit  23 , a fault influence degree information processing unit  24 , a route adjustment processing unit  25 , and a traffic statistics processing unit  26 . Further, the router  11  exchanges a data packet  30 , route control information  32 , route summary information  33 , and fault influence degree information  34  with a router  12 . 
         [0045]    In the route control processing unit  21 , route control is performed by the OSPF (Open Shortest Path First) or the RIP (Routing Information Protocol) that are existing routing protocols. Further, the route control processing unit  21  is based on a router in which an alternative next hop upon occurrence of a fault in a current next hop is calculated using the IP-FRR (IP Fast Reroute) or the ECMP (Equal Cost Multi Path). It is assumed that the current next hop and the alternative next hop (route information  31 ) calculated in the route control processing unit  21  are held in the route table  22 . 
         [0046]      FIG. 2  is a diagram showing a part of entry constitution of the route table  22  shown in  FIG. 1 . In  FIG. 2 , the entry of the route table  22  includes a destination prefix, a current next hop, alternative next hops (a plurality of candidates for alternative next hop that are ranked by priority) and the like. 
         [0047]      FIG. 3  is a diagram showing a configuration example of the network according to the embodiment of the present invention. In  FIG. 3 , the network according to the embodiment of the present invention includes five subnets N 0  to N 4 , and  11  routers  101 ,  102 ,  201  to  204 , and  301  to  305 . Each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  has a configuration similar to that of the router  11  shown in  FIG. 1 . 
         [0048]    The route control processing unit  21  in each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  calculates the route to each of the subnets N 0  to N 4  using the existing routing protocol. 
         [0049]      FIGS. 4A to 4C  are diagrams showing one example of the route table  22  of each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  calculated by the existing routing protocol according to the embodiment of the present invention. The alternative next hop is determined by the method disclosed in non-patent literature 3 and the like stated above. 
         [0050]    Further, the traffic statistics processing unit  26  in each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  receives the traffic information  35  relayed by the local apparatus. 
         [0051]      FIGS. 5A and 5B  are diagrams showing one example of the traffic information of each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  shown in  FIG. 3 . In this specification, for the sake of simplicity of description, all the traffic are generated from the subnet N 0 , and the destinations of the traffic are the subnets N 1  to N 4 . 
         [0052]    The route summary processing unit  23  of each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  creates, from the route table  22  of the current local apparatus and the traffic information calculated by the traffic statistics processing unit  26 , the route summary information  33 , regularly or when there is a change in the route information  31  or the traffic information  35 . 
         [0053]    The route summary information  33  includes the information of: 
         [0000]    (1) the number of paths (number of destination subnets) which is the current next hop for each adjacent router and the traffic amount for the destination subnet;
 
(2) the number of paths which is the alternative next hop upon occurrence of fault in the current next hop for each adjacent router and the traffic amount for the destination subnet;
 
(3) a list of the destination subnet with no alternative next hop; and
 
(4) transfer capacity of the local apparatus.
 
         [0054]      FIGS. 6A and 6B  are diagrams showing the route summary information in each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  shown in  FIG. 3 . The route summary information  33  created in each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  is transmitted to the adjacent routers. 
         [0055]    The route summary information  33  that is received by the adjacent router is recorded in the route summary processing unit  23  of each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305 . The notation of 1/100 or the like shown in (1) or (2) in  FIGS. 6A and 6B  means that the total number of paths is 1, and the total traffic amount is 100 (Mbps). The same thing is also applied to the following description. 
         [0056]    The notation of router  201 →router  202  shown in (2) means that the route is bypassed to the router  202  as the alternative next hop when the router  201  is in the fault state. 
         [0057]    The fault influence degree information processing unit  24  of each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  calculates the fault influence degree information  34  of the local apparatus regularly or when there is a change in the route summary information  33  based on the route summary information  33  received from each of the adjacent routers. The fault influence degree information  34  is the indicator indicating how much influence is given on the adjacent router and the traffic upon occurrence of the fault in the local apparatus. 
         [0058]    The fault influence degree information  34  includes the following information of: 
         [0000]    (a) the number of destination subnets and the traffic amount in which the local apparatus is the next hop;
 
(b) the number of destination subnets and the traffic amount bypassed to another node when the local apparatus is in the fault state; and
 
(c) the number of destination subnets and traffic amount having no alternative next hop when the local apparatus is in the fault state.
 
         [0059]      FIG. 7  is a diagram showing the fault influence degree information  34  of each of the routers  201  to  204  shown in  FIG. 3 . Although other routers  101 ,  102 ,  301  to  305  create the fault influence degree information  34  as is similar to the routers  201  to  204 , the description will be omitted here. The fault influence degree information  34  that is created is transmitted to the adjacent routers. 
         [0060]    The fault influence degree information  34  received from the adjacent router is recorded in the fault influence degree information processing unit  24  of each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305 . 
         [0061]    For example, the fault influence degree information  34  of the router  201  shown in  FIG. 7  is calculated as follows using the route summary information  33  of the routers  101 ,  301 , and  302  that are adjacent routers: 
         [0000]    (a)=estimating the number of paths and the traffic amount in which the local apparatus is the current next hop in (1) of the route summary information  33  of each adjacent router;
 
(b)=estimating the number of paths and the traffic amount for each alternative router of the local apparatus in (2) of the route summary information  33  of each adjacent router; and
 
(c)=estimating the difference between the number of paths in which the local apparatus is the current next hop in (1) of the route summary information  33  of each adjacent router and the number of paths including the alternative next hop of the local apparatus in (2) of the route summary information  33  of each adjacent router.
 
         [0062]    The route adjustment processing unit  25  of each of the routers  101 ,  102 ,  201  to  204 , and  301  to  305  performs the route adjustment processing regularly or when there is a change in the fault influence degree information  34  or the route summary information  33 . The router  101  performs the route adjustment based on the fault influence degree information  34  and the route summary information  33  obtained from the routers  201 ,  203 , and  204 . The router  102  performs the route adjustment based on the fault influence degree information  34  and the route summary information  33  obtained from the routers  202 ,  203 , and  204 . Although other routers  201  to  204 , and  301  to  305  also perform adjustment as is similar to the routers  101  and  102  described above, the description is omitted here. 
         [0063]      FIG. 8  is a diagram showing adjacent node information extracted by the router  101  shown in  FIG. 3 , and  FIG. 9  is a diagram showing adjacent node information extracted by the router  102  shown in  FIG. 3 . 
         [0064]    The router  101  extracts the adjacent node information as shown in  FIG. 8  from the fault influence degree information  34  and the route summary information  33  of the routers  201  to  204 , the traffic statistical information, and the route table  22  of the local apparatus. The router  102  also extracts the adjacent node information as shown in  FIG. 9  as is similar to the router  101 . Now, the process of adjusting the router  101  and the router  102  will be described below. 
         [0000]    Step 1: change to next hop having alternative route 
         [0065]    The route adjustment processing unit  25  of the router  101  extracts, from the adjacent node information shown in  FIG. 8 , the destination subnet which is the current next hop target and the destination without alternative. In the example shown in  FIG. 8 , the subnet N 2  of the router  201  and the subnet N 4  of the router  203  correspond to the destination subnet which is the destination without alternative. The route adjustment processing unit  25  judges whether the subnet N 2  and the subnet N 3  are able to be switched to another next hop as follows. 
         [0066]    In the Case of Subnet N 2   
         [0067]    The adjacent node which is the alternative next hop target of the subnet N 2  is searched. In this case, the router  203  corresponds to the adjacent node. 
         [0068]    It is checked whether the subnet N 2  is included in the destination without alternative of the router  203 . 
         [0069]    Since the subnet N 2  is the destination without alternative in the router  203 , the current next hop of the subnet N 2  is not changed. 
         [0070]    In the Case of Subnet N 4   
         [0071]    The adjacent node which is the alternative next hop target of the subnet N 4  is searched. In this case, the router  204  corresponds to the adjacent node. 
         [0072]    It is checked whether the subnet N 4  is included in the destination without alternative of the router  204 . 
         [0073]    In the router  204 , the subnet N 4  is not the destination without alternative (there is an alternative next hop). 
         [0074]    Next, the route adjustment processing unit  25  judges whether the current load can be stored. 
         [0075]    Since 600 (traffic amount of the subnet N 4 )&lt;1500 (capacity of the router  204 )−0 (current load), the route adjustment processing unit  25  judges that the current load can be stored. Thus, the route adjustment processing unit  25  changes the current next hop of the traffic for the subnet N 4  to the router  204 . 
         [0076]    Note that, when there are a plurality of candidates for alternative next hop of a change destination, the route adjustment processing unit  25  selects an alternative next hop according to the standard such as the smallest rate of the load after change with respect to the capacity (current load+traffic amount of the destination which is to be changed). 
         [0077]    The route adjustment processing unit  25  changes the next hop of the subnet N 4  to the router  204 , so as to calculate changes of the current load, alternative load, current next hop target, alternative next hop target of the routers  203  and  204 , and to change the adjacent node information. The adjacent node information after change will be shown in  FIG. 10 . 
         [0078]    The route adjustment processing unit  25  of the router  102  also performs route adjustment as is similar to the router  101  described above.  FIG. 9  shows information of routers adjacent to the router  102  before adjustment. First, the route adjustment processing unit  25  extracts a destination subnet, which is the current next hop target and the destination without alternative. The subnet N 1  and the subnet N 4  of the router  201  correspond to it. 
         [0079]    Since there is no adjacent node which is the alternative next hop target of the subnet N 1 , the route adjustment processing unit  25  does not perform the processing regarding the subnet N 1 . Since the subnet N 4  includes the router  204  as the alternative next hop target, the route adjustment processing unit  25  checks the destination without alternative of the router  204 . 
         [0080]    Since the route adjustment processing unit  25  does not include the subnet N 4  as the destination without alternative of the router  204 , and there is no problem also in terms of the load, the current next hop of the subnet N 4  is changed from the router  201  to the router  204 . According to this change, the router  102  updates the adjacent node information as is similar to the router  101 . The adjacent node information after change will be shown in  FIG. 11 . 
       Step 2: Adjustment of Current Traffic 
       [0081]    The router  101  starts calculation from the state shown in  FIG. 9 , which is the state in which the Step 1 is completed. The rate of the current load to the capacity after change is obtained for each adjacent node. 
         [0082]    In the case of  FIG. 9 , 
         [0000]    800/1500=0.53 in the router  201 ;
 
500/2000=0.25 in the router  203 ; and
 
600/1500=0.4 in the router  204 .
 
Since the rate of the router  201  is the highest and the rate of the router  203  is the lowest, the traffic movement from the router  201  to the router  203  is considered first.
 
         [0083]    Only the traffic that can be moved from the router  201  to the router  203  is the subnet N 2  (subnet N 1  is impossible) including the router  203  as the alternative next hop target. Since neither the router  201  nor the router  203  include the alternative route to the subnet N 2 , it can be judged that the reliability of the traffic to the subnet N 2  does not change no matter to which direction the movement is made. 
         [0084]    When the subnet N 2  is moved to the router  203 , 
         [0000]    500/1500=0.33 in the router  201 ; and
 
800/2000=0.4 in the router  203 .
 
         [0085]    Now, the standard deviation of the load rate is as follows: 
         [0000]    before change: √(0.53−0.39)̂2+(0.25−0.39)̂2+(0.4−0.39)̂2=0.198; and
 
after change: √(0.33−0.376)̂2+(0.4−0.376)̂2+(0.4−0.376)̂2=0.0571.
 
Thus, the standard deviation of the load rate to the adjacent router decreases. Thus, it is judged in the router  101  that the change should be made, so as to perform change.
 
         [0086]    According to this change, the router  101  calculates changes of the current load, alternative load, current next hop target, alternative next hop target, so as to change the adjacent node information. The adjacent node information after change is shown in  FIG. 12 . 
         [0087]    The router  102  also starts adjustment from the state of the adjacent router information shown in  FIG. 11  as is similar to the router  101  described above. 
         [0088]    In the router  102 , the load rate of each adjacent router is: 
         [0000]    400/2000=0.2 in the router  202 ;
 
1100/2000=0.55 in the router  203 ; and
 
100/1500=0.0666 in the router  204 .
 
         [0089]    Since the router  203  does not include a current next hop target destination, no adjustment is possible. The only current next hop target destination that can be changed from the router  201  to the router  204  is the subnet N 3  which is the alternative next hop target destination of the router  204 . However, since the router  204  does not have the alternative route to the subnet N 3 , no change is performed. Since there is no other destination that can be changed, the router  102  completes the route adjustment processing. 
       Step 3: Adjustment of Alternative Traffic 
       [0090]    Since the router  101  has no other alternative next hop candidate as shown in the route table  22  of  FIG. 4A , the router  101  cannot switch the alternative next hop and thus does not adjust the alternative traffic. 
         [0091]    The router  102  performs adjustment calculation since there are alternative next hop candidates for the subnet N 3  and the subnet N 4  that are destinations and thus there is room for adjustment. In the state shown in  FIG. 11 , the combination to change the alternative next hop in the router  102  is as follows. That is, 
         [0000]    (1) changing the alternative next hop of the subnet N 4  when the router  204  is in the fault state from the router  202  to the router  203 ; and
 
(2) changing the alternative next hop of the subnet N 3  when the router  202  is in the fault state from the router  203  to the router  204 .
 
         [0092]    First, the router  102  calculates (1). The router  102  adds the current load and the traffic bypassed to the routers  202  and  203  when the router  204  is in the fault state, so as to calculate the rate with the capacity. 
         [0093]    The traffic load ratio at the time of substitution of the router  202  before change is (400+100)/2000=0.25, and the traffic load ratio at the time of substitution of the router  203  is (1100+0)/2000=0.55. 
         [0094]    The traffic load ratio at the time of substitution of the router  202  after change is (400+0)/2000=0.2, and the traffic load ratio at the time of substitution of the router  203  is (1100+100)/2000=0.6. Since the standard deviation of the load ratio clearly increases, the change is not performed. 
         [0095]    Next, the router  102  calculates (2). The router  102  adds the current load and the traffic bypassed to the routers  203  and  204  when the router  202  is in the fault state, so as to calculate the rate with the capacity. 
         [0096]    The traffic load ratio at the time of substitution of the router  203  before change is (1100+300)/2000=0.7, and the traffic load ratio at the time of substitution of the router  204  is (100+0)/1500=0.0666 . . . . 
         [0097]    The traffic load ratio at the time of substitution of the router  203  after change is (1100+100)/2000=0.6, and the traffic load ratio at the time of substitution of the router  204  is (100+200)/1500=0.2. Since the deviation of the load ratio is changed to be smaller, it is judged that this change is effective, and the router  102  performs the change to the alternative next hop. 
         [0098]    According to this change, the router  102  calculates changes of the current load, alternative load, current next hop target, and alternative next hop target, so as to change the adjacent node information. The adjacent node information after change is shown in  FIG. 13 . 
         [0099]    As a result of the adjustment shown in the Steps 1 to 3 above, the route table  22  of the router  101  and the route table  22  of the router  102  are changed as shown in  FIG. 14 . The adjustment is also performed in other routers  201  to  204 , and  301  to  305 , as is similar to the routers  101  and  102 . This adjustment is asynchronously performed in the routers  101 ,  102 ,  201  to  204 , and  301  to  305 . The route summary information that is updated is advertised after adjustment, and the operation and the adjustment stated above are repeated at regular time intervals. 
         [0100]    As described above, according to the embodiment, it is possible to mitigate the traffic in which transfer is interrupted by a fault of a router or a link. Further, according to the embodiment, it is possible to mitigate the occurrence of the congestion due to the traffic bypassed to the alternative next hop upon occurrence of the fault. 
         [0101]    According to the present invention, when the deviation cannot be made smaller even when 
         [0000]    the current next hop or the alternative next hop of any destination subnet is changed in the Step 2 and the Step 3 of the route adjustment processing described above, the destination subnet can be divided and assigned to another next hop. 
         [0102]    For example, in the route adjustment processing of the router  101  of the Step 2 described above, the subnet N 2  is divided into two subnets N 2 - 1  and N 2 - 2  so that the traffic amount is substantially halved, so as to change the current next hop of only the subnet N 2 - 2  to the router  203 . 
         [0103]    Further, the present invention is capable of notifying the route summary information  33  and the fault influence degree information  34  not only to the adjacent routers but also to routers up to N hops ahead. In this case, the present invention increases the value of N, so as to be able to share the information in the whole network to perform route adjustment. Each router performs route adjustment based on the route summary information  33  and the fault influence degree information  34  up to N hops ahead. 
         [0104]    Furthermore, the present invention is capable of including in the fault influence degree information  34  the influence degree when each link included in the router is in the fault state. Each router performs route adjustment for the purpose of mitigating the influence degree upon occurrence of fault in the adjacent router and mitigating the influence degree upon occurrence of fault in each link. 
         [0105]    Still further, the present invention is capable of performing calculation by weighting the fault influence degree by the fault occurrence probability of each router when performing the route adjustment. The fault occurrence probability can be calculated based on the operating time of the router. 
         [0106]    On the other hand, the present invention may employ the conception of the Class of Service as disclosed in the documents 1 and 2, which is a method for priority control to achieve the communication quality or secure bandwidth by dividing a service into a plurality of Classes, to enhance the fault tolerance for each Class even in the environment in which traffic having different delay or bandwidth request is mixed.
   Document 1: J. Heinanen et al., Assured Forwarding PHB Group, IETF RFC2597, June, 1999   Document 2: V. Jacobson et al., An Expedited Forwarding PHB, IETF RFC2598, June, 1999   
 
         [0109]    Each router manages the traffic amount for each Class of priority control to achieve at least the communication quality and secure bandwidth, and creates and advertises the current load and the capacity for each Class in the route summary information. Each router executes the route adjustment as is similar to the one described above for each Class, so as to determine the appropriate current next hop and the alternative next hop for each Class. 
         [0110]    Although the present invention has been described as the configuration of hardware in the embodiment described above, the present invention is not limited to this. The present invention is able to achieve any processing by causing a CPU (Central Processing Unit) to execute a computer program. In this case, the computer program may be provided by being recorded to a recording medium or by being transmitted by way of the Internet or other communication media. Further, the storage medium includes, e.g., a flexible disk, a hard disk, a magnetic disk, an optical magnetic disk, CD-ROM, DVD, a ROM cartridge, a RAM memory cartridge with battery backup, a flash memory cartridge, and a non-volatile RAM cartridge. Further, the communication media include a wired communication medium (e.g., telephone line) or a wireless communication medium such as a microwave line. 
         [0111]    Although the present invention has been described with reference to the embodiment, the present invention is not limited by the above description. The configuration and the detail of the present invention may be variously changed as long as a person skilled in the art can understand within the scope of the present invention. 
         [0112]    This application claims the benefit of priority, and incorporates herein by reference in its entirety, the following Japanese Patent Application No. 2008-283808 filed on Nov. 5, 2008. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           11 ,  12 ,  101 ,  102 ,  201  to  204 ,  301  to  305  ROUTER 
           20  PACKET TRANSFER PROCESSING UNIT 
           21  ROUTE CONTROL PROCESSING UNIT 
           22  ROUTE TABLE 
           23  ROUTE SUMMARY PROCESSING UNIT 
           24  FAULT INFLUENCE DEGREE INFORMATION PROCESSING UNIT 
           25  ROUTE ADJUSTMENT PROCESSING UNIT 
           26  TRAFFIC STATISTICS PROCESSING UNIT 
           30  DATA PACKET 
           31  ROUTE INFORMATION 
           32  ROUTE CONTROL INFORMATION 
           33  ROUTE SUMMARY INFORMATION 
           34  FAULT INFLUENCE DEGREE INFORMATION 
           35  TRAFFIC INFORMATION 
         N 0  to N 4  SUBNET