Abstract:
A caching method for caching network contents of a packet transfer network including a packet relay for relaying accesses of clients to servers supplying contents, and a network contents cache device are disclosed. Accesses from the clients to the servers are monitored by the said packet relay to determine a cache priority on the basis of a product of relaying path lengths from the packet relay to the server and frequency of access from the client to the servers and the network contents cache device preferentially caches a response to the access to the server having higher cache priority.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an access to content servers in a packet transfer network and, particularly, to a caching of network contents. 
     2. Description of the Related Art 
     In a known caching method of network contents, in which a server and clients are arranged through a network and a cache device is provided in the network, the cache device monitors accesses of the clients to the server and directly returns responses corresponding to accesses similar to the monitored accesses to the clients. 
     In each of Japanese Patent Application Laid-open Nos. Hei 9-251414 and Hei 10-31615 which relate to such network caching method discloses a construction in which a cache device caches for all accesses to a server. Further, in Japanese Patent Application Laid-open No. Hei 9-330265, respective clients have their own cache devices and use a cache information commonly. 
     Japanese Patent Application Laid-open No. Hei 10-49422 discloses a cache device, which caches an access to a server having high access frequency, preferentially, and, when a finite caching capacity of the cache device is going to overflow, deletes the caching content to hold an available cache capacity. FIG. 8 shows a flowchart of an operation of the above mentioned cache device. As shown in FIG. 8, a packet is received (step S 31 ) and it is determined whether or not the received packet is an HTTP (Hypertext Transfer Protocol) packet (step S 32 ). If the packet is the HTTP packet (Yes in step S 32 ), then it is determined whether or not the packet is a request packet from a server (step S 33 ). If the packet is a response packet to a request from a server (No instep S 33 ), the cache device checks a cache capacity as to whether or not it is short (step S 38 ). If the cache capacity is enough (No in step S 38 ), the cache device caches the received response corresponding to the request having a certain access frequency (step S 39 ). If the cache capacity is short (Yes in step S 38 ), the cache device searches any packet having lower access frequency (step S 41 ). If there is such packet therein (Yes in step S 41 ), the cache device deletes such packet (step S 42 ) and caches the received response (step S 39 ). 
     Further, Japanese Patent Application Laid-open No. Hei 9-282289 discloses a cache method in which an arrangement of a cache device in a network is automatically determined on the basis of the access frequency to a server and the route length to the server. 
     In the network system disclosed in either Japanese Patent Application Laid-open No. Hei 9-251414 or Japanese Patent Application Laid-open No. Hei 10-31615, the cache device performs the caching for all accesses to the server. Therefore, when there are accesses beyond the finite cache capacity, it is impossible to perform an efficient caching with which the coefficient of use of network resources becomes optimal. Further, in Japanese Patent Application Laid-open No. Hei 9-330265, in which the clients have the cache devices, respectively, to use the cache information commonly, it is impossible to perform a caching by taking the coefficient of use of network resources into consideration. 
     In the cache device disclosed in Japanese Patent Application Laid-open No. Hei 10-49422, the efficient use of the network resources is taken into consideration and the caching for the access to the server, whose access frequency is high, is performed preferentially as shown in FIG.  8 . However, such cache device can not perform efficient caching in view of the coefficient of use of the network since, if the length of route to the server is large, more resources of the network are consumed for each access even when the frequency of the latter is low. 
     Further, in the network system disclosed in Japanese Patent Application Laid-open No. Hei 9-282289, the arrangement of cache devices in the network is determined on the basis of the frequency of access to the server and the route length to the server. However, such information is not considered in determining a cache in a cache site and, therefore, it is impossible to perform a caching by taking the coefficient of use of the network into consideration. 
     In these prior arts mentioned above, it is difficult to realize an efficient caching in view of the coefficient of use of the network. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a caching method of network contents and an apparatus for performing the same method, which enable a caching of the network contents with minimum use of network resources. 
     According to a first aspect of the present invention, a caching method of network contents of a packet transfer network including a packet relay for relaying accesses of clients to servers supplying the contents is featured by that the packet relay monitors accesses from the clients to the servers, determines a cache priority on the basis of a product of relaying path lengths between the packet relay and the servers and the frequency of access from the clients to the servers and preferentially caches responses to the accesses to servers, which have high cache priority. 
     According to a second aspect of the present invention, a caching method of network contents of a packet transfer network including packet relays for relaying accesses of clients to servers supplying the contents is featured by that, after a first packet relay caches a response to an access to a specific server, a second packet relay compares the length of relaying path between the second packet relay and the server with the length of relaying path between the second packet relay and the first packet relay, for the same access to the server in the second packet relay, and transfers the access to the first packet relay when the route length between the second packet relay and the first packet relay is smaller than compares the length of relaying path between the second packet relay and the server. 
     According to a third aspect of the present invention, a cache device for network contents, which includes a first and second termination devices connected to a client side network and a server side network, respectively, a switch device connected between the first and second termination devices and a cache server connected to the switch device, is featured by that the switch device transfers a request packet from the client to either the server side network and the cache server or the cache server, depending upon the state of cache of a response of the cache server, transfers the response packet from the server to the network and the cache server on the client side and transfers the response packet from the cache server to the network on the client side and that the cache server determines a cache priority of the response from the server on the basis of a product of relaying path lengths and the access frequency to the server, which is calculated on the basis of the received request packet, preferentially caches the response corresponding to the access to the server, which has a high cache priority, on the basis of the response packet from the server and, after the response is cached, returns the response packet having the cached response corresponding to the access packet to the switch device. 
     According to the present invention, in determining the priority of cache of the contents, the length of relaying path up to the server is used in addition to the frequency of access to the server. Therefore, a server access through a long route up to the server and consuming a large network resources is cached in access unit (from a client access to a server to a response thereto) even when the frequency of access to the server is low to some extent. On the contrary, an access through a short route to the server and consuming small network resources is not cached in access unit even if the access frequency is high to some extent. In this manner, the caching is realized by a finite cache capacity with minimum consumption of network resources. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a network construction for realizing a network cache method according to the present invention; 
     FIG. 2 shows another network construction for realizing a network cache method according to the present invention; 
     FIG. 3 is a block diagram showing a construction of a network contents cache device according to the present invention; 
     FIG. 4 shows an example of a routing table used in the present invention; 
     FIG. 5 shows a flowchart of an operation of a destination sorter of the network contents cache device according to the present invention; 
     FIG. 6 shows a flowchart of an operation of a cache server of the network contents cache device according to the present invention; 
     FIG. 7 shows block diagrams of the network contents cache device according to the present invention, showing an operation thereof; and 
     FIG. 8 shows a flowchart of an operation of a cache server of the conventional network contents cache device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of a network in which the network contents cache method of the present invention is performed. In FIG. 1, WWW (World Wide Web) servers  14   a  and  14   b  and a client  15  are connected each other through a network  11 . The network  11  is constructed with routers  12   a,    12   b,    12   c  and  12   d  and a network contents cache device (referred to as a “cache device”, hereinafter)  13  is mounted on the router  12   a.    
     FIG. 2 shows a second embodiment of the network in which the network contents cache method of the present invention is performed. The second embodiment shown in FIG. 2 is constructed with WWW servers  24   a  and  24   b  and clients  25   a  and  25   b  through a network  21  which is constructed with routers  22   a,    22   b,    22   c,    22   d  and  22   e.  A cache device  23  is mounted on the router  22   a.    
     FIG. 3 shows an example of the cache device  13  ( 23 ) for executing the network contents cache method of the present invention. The cache device is connected to other network devices through physical termination devices  31   a  and  31   b.  Packets received by the physical termination devices  31   a  and  31   b  are terminated by layer-2 termination devices  32   a  and  32   b  and, then, transmitted to destination sorters  33   a  and  33   b  which determine destinations of packet transfer on the basis of IP addresses and layer-4 port numbers of the packets, respectively. The packets are transferred to the respective destinations determined by the destination sorters  33   a  and  33   b,  by a switch  34 . The transfer operation realized by the destination sorters  33   a  and  33   b  and the A switch  34  makes a multi-cast for transferring one packet to a plurality of destinations possible. A cache server  35  caches a response to a server access and sets destinations of the destination sorters  33   a  and  33   b  for same sever accesses as the cached previous server access such that the same server accesses are transferred to only the cache server  35 . 
     &lt;First Embodiment&gt; 
     An operation of the first embodiment of the network contents cache method and apparatus of the present invention will be described with reference to FIGS. 1 and 3. 
     In a case where the client  15  accesses the WWW server  14   a,  an HTTP request packet having destination to the WWW server  14   a  is transmitted from the client  15  to the router  12   a.  Then, the router  12   a  transfers the request packet to the router  12   b  on the basis of a routing table. Then, the request packet is transferred to the router  12   c  and, then, to router  12   d  in a similar manner and finally reaches the WWW server  14   a.    
     The WWW server  14   a  transmits a response packet containing contents related to the request and having a destination to the client  15  to the router  12   d.  The response packet is then transferred to the router  12   c,  the router  12   b  and then the router  12   a  in the opposite direction to the route of the request packet and finally reaches the client  15 . 
     In this case, the cache device  13  monitors accesses of the client  15  to the WWW server  14   a  to record an information of the frequency of access to the WWW server  14   a  and the relaying path length from the client  15  up to the WWW server  14   a  and determines the cache priority of the server access on the basis of the information of the access frequency and the relaying path length. Then, the caching is performed for the access having the highest priority, first, and, then for other accesses having lower priorities in the priority order. In this embodiment, metrics in the routing table shown in FIG. 4 is used as the information related to the route length. As shown in FIG. 4, the routing table is written with destination addresses, addresses of next routers, port numbers of an interface and metrics up to the destination of transfer, which can be acquired by masks obtained from the IP address. 
     The router  12   a  calculates metrics up to the destination network according to the routing protocol to determine the route of transfer. The metrics of the routing table is expressed by the number (hop number) of routers passed through by the packet until the latter reaches the destination network, although it depends upon the routing protocol to be used. 
     Therefore, the router  12   a  can utilize the metrics of the routing table as an amount of the network resources used during a time from the one access of the client to the WWW server  14   a  up to the response thereto (access unit). 
     For example, the metrics of the access of client to the WWW server  14   a  is larger than that to the WWW server  14   b.  Therefore, when the access frequencies thereof are the same, the cache priority of the server access to the WWW server  14   a  is set to a value larger than that of the access to the WWW server  14   b  such that the access to the WWW server  14   a  is cached preferentially. 
     When the router  12   a  receives a next request packet from the client  15  to the WWW server  14   a  after the response to the previous server access to the WWW server  14   a  is cached by the cache device  13 , the router  12   a  causes the cache device  13  to process the next request packet without transferring it to the WWW server  14   a  and returns the response of the WWW server  14   a,  which is cached in the cache device  13 , to the client  15 . 
     An operation of the cache device shown in FIG. 3 will be described in detail. 
     FIG. 5 shows a flowchart of the destination sorter shown in FIG. 3, which performs a packet transfer, and FIG. 6 shows a flowchart of the operation of the cache server also shown in FIG.  3 . As shown in FIG. 5, when the destination sorters  33   a  and  33   b  receive packets (step S 1 ), it is determined whether or not the received packets are HTTP packets (step S 2 ). If they are HTTP packets (Yes in step S 2 ), then the destination sorters transfer the packets to the cache server  35  (step S 3 ). In the cache server, it is determined whether or not the received packets are response packets and cached (step S 4 ). If the received packets are not the response packets, which are cached (No in step S 4 ), the destination sorters control a transfer to a next hop (step S 5 ) The cache server  35  performs the setting of this transfer control. 
     Further, as shown in FIG. 6, the cache server  35  uses the product of the access frequency and the relaying path length as the cache priority and, when the received HTTP packet (step S 11 ) is a response packet from the server to the request (No in step S 12 ), checks the cache capacity (step S 17 ). When the available cache capacity is enough, the cache server caches the received response corresponding to the request having high cache priority (step S 18 ). If the available cache capacity is short (Yes in step S 17 ), the cache server  35  searches a cache having a lower cache priority (step S 19 ) and, after its cache is deleted (step S 20 ), controls the caching of the response (step S 18 ). 
     FIGS.  7 ( a ) to  7 ( c ) show a detailed operation of the cache device of the present invention. 
     The IP destination address of a request packet  36  from the client  15  to the WWW server  14   a,  which is terminated by the physical termination device  31   a  and the layer-2 termination device  32   a,  is referenced by the destination sorter  33   a  to determine a transfer route according to the routing table. Simultaneously, the destination sorter  33   a  references the layer-4 port number of the request packet  36  and, when the packet  36  is identified as an HTTP packet, it is transferred to the cache server  35 . The switch  34  transfers the packet  36  to the network on the side of the WWW server  14   a  and to the cache server  35  according to the determination made by the transfer destination distributor (FIG.  7 ( a )). 
     The cache server  35  calculates the access frequency to the server  14   a  on the basis of a count of the request packets  36  received by the cache server  35  and searches the routing table shown in FIG. 3 by using the destination IP address of the packet  36  to obtain the metrics up to the server  14   a.  The cache server  35  calculates a product of the access frequency and the metrics as the cache priority. 
     Similarly, a response packet  37  transferred from the WWW server  14   a  is transferred to the network on the client side, which is the destination, and, simultaneously, to the cache server  35 . The cache server  35  caches the received response packet  37 . When the available cache capacity is short, the cache server  35  deletes a cache of an access having a cache priority lower than that of the response packet  37  and then caches the response packet  37 . If there is no cache having access priority lower than that of the response packet  37 , the cache server  35  does not cache the response packet  37  (FIG.  7 ( b )). 
     After the response packet is cached, the cache server  35  sets the destination sorters  33   a  and  33   b  such that the cache server  35  does transfer request packets corresponding to the cached response not to the network on the server side but to only the cache server  35 . Thereafter, a request packet  38  to the WWW server  14   a  is transferred to only the cache server  35  and the cache server  35  returns a response packet  39  corresponding to the request packet  38  cached by the cache server  35  (FIG.  7 ( c )). 
     When the cached response is deleted or the caching time thereof is over, the cache server  35  initializes the setting of the destination sorters  33   a  and  33   b  such that a response corresponding thereto is transferred to the network on the server side. 
     In this embodiment, the contents of the WWW server  14   a  are unity and the request/response can be made correspondent to the contents by only the IP address of the WWW server  14   a.  However, when there are a plurality of contents in the WWW server  14   a,  the correspondence between the request/response and the contents is provided by using identifiers (such as URL (Uniform Resource Locator)) for identifying the contents, in addition to the IP addresses of the WWW server  14   a.    
     &lt;Second Embodiment&gt; 
     An operation of the second embodiment of the present invention will be described with reference to FIG.  2 . 
     The embodiment shown in FIG. 2 is constructed with the WWW servers  24   a  and  24   b  and the clients  25   a  and  25   b,  which are connected each other through the network  21  which is constructed with the routers  22   a,    22   b,    22   c,    22   d  and  22   e.  The cache device  23  is mounted on the router  22   a,  as mentioned previously. 
     The operation of the second embodiment is similar to that of the first embodiment and the cache operation is performed in the cache device  23 . In this case, the router  22   a  notifies the other routers  22   b,    22   c,    22   d  and  22   e  of an address of the router  22   a  and an identifier of a cached access (such as an address of the server  24   a,  URL, etc.). Upon reception of the address of the router  22   a  and the identifier of the cached access, the routers  22   b,    22   c,    22   d  and  22   e  compare the metrics up to the server  24   a  with the metrics up to the router  22   a  according to the routing table within the routers. When the metrics up to the router  22   a  is smaller, the destination sorter is set such that the request packet corresponding to the cached access is transferred to not the server  24   a  but the router  22   a.    
     Thereafter, the router  22   e,  which receives the same request packet sent from the client  25   b  toward the server  24   a,  transfers the request to the router  22   a  and the router  22   a  returns the cached response corresponding to that request to the client  25   b  as in the case of the first embodiment. 
     When the caching time of the response cached in the cache device  23  is over or the cached response is deleted, the router  22   a  notifies the other routers  22   b,    22   c,    22   d  and  22   e  of the address of the router  22   a  and the identifier of the access which was deleted or whose caching time was over. The routers  22   a,    22   b,    22   c,    22   d  and  22   e  set their destination sorters such that the request packet of the corresponding server access is transferred again to the WWW server  24   a,  and are returned to the initial states. 
     The first and second embodiments, which use the metrics in the routing table as the information related to the relaying path length up to the server, have been described. However, in a case where an IP over ATM is used as the packet transfer network, it is possible to use a geographic distance up to the server, etc., which is obtainable from the ATM address (E164 address) corresponding to the IP address of the server, etc., as the relaying path length. 
     As described hereinbefore, according to the network contents cache method of the present invention, since the relaying path length up to the server, etc., is taken into consideration in determining the cache priority, the cache is performed on the basis of the amount of consumption of the network resources even when the amount of the network resources used by in access unit varies dependent on the position of server. Therefore, it becomes possible to perform the network cache with a finite cache capacity and with minimum network resource consumption. 
     Further, according to the cache device of the present invention, the transfer is performed to the cache server and the designated network simultaneously by the multi-cast. Therefore, the cache processing does not influence on the transfer delay between the client and the server, so that the network cache becomes possible without delay in access to the server.