Abstract:
A user terminal connection control method to limit the number of sessions simultaneously connectable to the Internet, wherein an access server or authentication server has a user management table for specifying the maximum number of connections and a preferential terminal identifier indicating a user terminal to be preferentially connected to the Internet for each group comprising a plurality of user identifiers. The access server or authentication server determines, during a communication procedure for authenticating a user who requests to access to the Internet, whether the current number of connections for a group to which the user belongs has reached the maximum number of connections, and if so, determines whether to allow the user terminal to connect to the Internet depending on whether the user terminal identifier matches the preferential terminal identifier.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application serial No. 2004-337273, filed on Nov. 22, 2004, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a user terminal connection control method and apparatus for carrying out control of connecting user terminals to the Internet. More particularly, the invention relates to a packet forwarding apparatus for controlling the connection of a user terminal to an IP network after establishing a session with the user terminal in accordance with a Point to Point Protocol (PPP), and an authentication server for restricting user terminal access to the Internet. 
     (2) Description of Prior Art 
     When connecting a user terminal to the Internet via an Internet Service Provider (ISP) network managed by an Internet Connection Service Provider, an Internet Protocol (IP) address required for communication on the Internet is delivered from the ISP. In the network configuration including such an ISP network for Internet connection, an access network lies between user terminals and the ISP network. 
     In the access network, an access server for controlling connection/disconnection of each user terminal to the Internet or a packet forwarding apparatus called a Broadband Access Server (BAS) is located. In most cases, each user terminal is connected to the BAS by PPP over Ethernet (PPPoE), which is a technique for PPP-based connection on Ethernet. In standard Internet connection services, one IP address is delivered to every user. In this case, each user is allowed to connect only one terminal to the Internet at a time. 
     To enable a user to connect multiple terminals to the Internet using one IP address delivered to the user, it is needed to make the multiple terminals share the IP address by applying an address translation technique such as Network Address Translation (NAT) or IP Masquerade, using a device such as, for example, a broadband router. However, when the NAT or IP Masquerade is applied, a specific communication procedure may be required or types of usable applications may be restricted. Some ISPs offer an additional service of delivering multiple IP addresses to one user so that the user can connect multiple terminals to the Internet at the same time. 
     Since an access network is generally built as an Ethernet network which is a broadcast type network, it is relatively easy to attach user terminals to the network. Therefore, the ISP usually limits the number of connections to the Internet for each user in order to prevent unauthorized user access and to make effective use of network resources such as IP addresses and communication bandwidths. 
     Limitation of the number of connections for each user in PPPoE-based Internet connection environment is traditionally performed within an access network, in particular, at a Layer 2 (L2) switch located between user terminals and the BAS. The L2 switch is able to limit the number of connections of terminals per user by referring to a Media Access Control (MAC) address of each communication packet, for example, as described in Japanese Unexamined Patent Publication No. 2000-112852 (patent document 1). 
     The L2 switch described in the patent document 1 registers, for all packets it received, entry information indicating the correspondence of a source MAC address to a receiving port into a MAC address table and determines, by referring to this table, a forwarding destination port for each packet that will be received subsequently. According to the patent document 1, the L2 switch limits the number of user terminals connectable to the Internet at the same time, by deleting entry information for which no new packet having the registered MAC address has been received within a pre-specified period of aging time, from the MAC address table, and by limiting the number of entries (MAC addresses) allowed to be registered into the table for each port. 
     More specifically, the L2 switch limits the number of connections (MAC addresses) enabling communications at the same time for each port by stopping, when the number of entries for a specific port registered in the MAC address table has reached a predetermined upper limit, registering a new entry to the MAC address table (learning function) with respect to a packet having a new MAC address which will be received thereafter through the specific port and by discarding all subsequent incoming packets each having an unlearned MAC address for which the MAC address table has no entry. 
     As a prior art technique for limiting the number of connection terminals, for example, Japanese Unexamined Patent Publication No. 2003-16031 (patent document 2) proposes a connection control scheme for limiting the number of clients simultaneously connectable to a server in a client-server system using a TCP/IP protocol. According to the scheme described in the patent document 2, a priority table indicating the relation between the IP address of each client and connection priority level is equipped in a server to which clients may connect. When a connection request from a new client occurs in the state where the number of clients connected to the server has reached the upper limit, the server compares the connection priority level of the requester client to the lowest priority level among the clients already being connected to the server in the priority table. If the connection priority level of the requester client is higher, the server disconnects a session for one of the clients having the lowest priority level and accepts the new connection request. Otherwise, the server rejects the new connection request. 
     However, in the case of limiting the number of connections based on the MAC address, since the L2 switch regards multiple PPP sessions connected to the same user terminal as one connection, the number of PPP sessions cannot be limited. Further, this control scheme cannot limit such a terminal that is connected to the Internet via another L2 switch by a roaming user. In the control scheme that deletes a registered MAC address according to the aging time as described in the patent document 1, a user terminal that requests an urgent connection is unable to get immediate access to the Internet when the MAC address table has no space for new entries. 
     On the other hand, in the control scheme in which connection priority levels are assigned to all IP addresses and a lowest priority session being active is deleted when a new connection request occurs in the state where the number of connections has reached the upper limit as described in the patent document 2, such problems would occur that time is required to make a priority decision between the new connection request and existing connections and steady communication services cannot be provided to users because lowest priority sessions are often forcibly disconnected. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a user terminal connection control method and apparatus capable of limiting the number of sessions simultaneously connectable to the Internet for each user and reducing the frequency of forcible disconnection of sessions. 
     To achieve the above object, the present invention is characterized in that, during a communication procedure for authenticating a user who requests to access to the Internet, either a packet forwarding apparatus operating as an access sever or an authentication server connected to an ISP network determines, using a user management table in which the maximum number of connections simultaneously connectable to the Internet and a preferential terminal identifier indicating a user terminal to be preferentially connected to the Internet are specified for each group comprising a plurality of user identifiers, whether the current number of connections for a group to which the requester user belongs has reached the maximum number of connections, and if so, determines whether to allow the user terminal to connect to the Internet depending on whether the user terminal identifier matches the preferential terminal identifier. 
     Even if the user has been authenticated successfully, unless the user terminal identifier is registered as the preferential terminal identifier, the access to the Internet is rejected when the current number of connections has reached the maximum number of connections within the group. When the user terminal identifier is registered as the preferential terminal identifier, the access to the Internet is allowed after disconnecting a session with one of access allowed user terminals within the same group. 
     In the case where the user management table is equipped in the authentication server, a response message transmitted from the authentication server to the access server includes, besides a result of normal user authentication based on a password, a result of comparison between the current number of connections and the maximum number of connections for the group to which the user belongs, and a result of determination as to whether the user terminal is the preferential terminal. If the authentication server allows the user terminal to access to the Internet, an IP address to be used by the user terminal is specified in the response message. 
     If the user management table is equipped in the access server (packet forwarding apparatus), the packet forwarding apparatus decides a response to be returned to the user terminal by comparing the current number of connections and the maximum number of connections within the group to which the user belongs and determining whether the user terminal is the preferential terminal when a response message indicating successful authentication is received from the authentication server. 
     According to the present invention, since the access server or the authentication server limits the number of connections to the Internet for each group comprising a plurality of user identifiers, limitation of the number of connections can be implemented consistently with access in static situations even if a roaming user requests to access to the Internet via another L2 switch. By registering a user terminal identifier that is preferentially allowed to connect to the Internet in each group, the registered user terminal in each group can be allowed to connect to the Internet immediately even if the current number of connections has reached the maximum number of connections for the group. In that event, any session having been established is disconnected, but this disconnection occurs with one of the terminals belonging to the same group including the preferential user terminal and, therefore, this disconnection has no effect on the user terminals belonging to other groups. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a configuration of a communication network system to which the present invention is applied. 
         FIG. 2  shows an example of a structure of a packet forwarding apparatus (BAS  10  in  FIG. 1 ) of the present invention. 
         FIG. 3  shows an example of a routing table to be equipped in the packet forwarding apparatus of the present invention. 
         FIG. 4  is a block diagram showing an embodiment of an authentication server  40 . 
         FIG. 5  shows an example of a user management table to be equipped in the authentication server. 
         FIG. 6  shows a first embodiment of a communication sequence that is initiated when a request for connection to the Internet is issued from a user terminal to the BAS. 
         FIG. 7  is a flowchart illustrating an embodiment of a processing for confirming the number of connections  54 , which is performed by the authentication server. 
         FIG. 8  shows a format of an Access-Accept message to be transmitted from the authentication server to the BAS. 
         FIG. 9  shows a first embodiment of a communication sequence that is initiated when a disconnection request is issued from the user terminal to the BAS. 
         FIG. 10  shows a second embodiment of a communication sequence that is initiated when a request for connection to the Internet is issued from the user terminal to the BAS. 
         FIG. 11  is a flowchart illustrating an embodiment of processing for confirming the number of connections  34 , which is performed by the BAS. 
         FIG. 12  shows a second embodiment of a communication sequence that is initiated when a disconnection request is issued from the user terminal to the BAS. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  shows an example of a configuration of a communication network system to which the present invention is applied. 
     The communication network system shown here is comprised of an access network NW 1  including a packet forwarding apparatus (hereinafter referred to as a broadband access server: BAS)  10  and a plurality of L2 switches  60  ( 60 - 1 ,  60 - 2 ), an ISP network NW 2  managed by an ISP, and the Internet NW 3 . 
     Each L2 switch accommodates a plurality of LANs (LAN- 1  to LAN- 4 ) and one or a plurality of user terminals H (H 1 - 1  to H 1 -L, H 2 - 1 , H 3 - 1  to H 3 -M, and H 4 - 1  to H 4 -N) are connected to each LAN. These user terminals H are connected to the broadband access server (BAS)  10  through PPP over Ethernet (PPPoE) via the L2 switch  60  and get access to any server  90  ( 90 - 1  to  90 - m ) on the Internet NW 3  via the BAS  10  and the ISP network  2 . 
     To the ISP network NW 2 , a subscriber authentication server (hereinafter simply referred to as an authentication server) is connected. The authentication server manages user information such as user ID, password, address, name, and account number for each user registered beforehand. In the present embodiment, user terminals H are grouped by LAN and a plurality of user terminals belonging to the same group use a common password for user authentication. 
     When one of user terminals H establishes a PPP session with the BAS  10  via an L2 switch  60 , the BAS  10  requests the user terminal to send authentication data. Upon receiving authentication data including a user ID and a password from the user terminal, the BAS  10  transmits an authentication request message including the user ID and the password to the authentication server  40 . 
     Upon receiving the authentication request from the BAS  10 , the authentication server  40  refers to user information relevant to the user ID received, checks validity of the password submitted by the user (user authentication), and returns an authentication result to the BAS  10 . According to the response result from the authentication server  40 , the BAS  10  determines whether the user terminal should be allowed to connect to the Internet. 
     For communication between the BAS,  10  and the authentication sever  40 , for example, a Remote Authentication Dial In User Service (RADIUS) protocol is used. The authentication server that operates according to the RADIUS protocol is generally called a RADIUS server. In the embodiment, as will be described hereinafter, the authentication server  40  is provided with the function of managing accounting information for each Internet user. 
       FIG. 2  is a block diagram showing an embodiment of the broadband access server (BAS)  10 . 
     The BAS  10  is comprised of a plurality of line interfaces  11  ( 11 - 1  to  11 - n ) connected to input and output lines LI-i, LO-i (where i=1 to n and n is a positive integer) for communicating with the L2 switches or the ISP network NW 2  and a protocol processing unit  12  and a control unit  15  connected to these input/output interfaces. 
     The protocol processing unit  12  takes in a packet (or frame) that a line interface  11 - i  received from an input line LI-i, refers to a routing table  13  based on the destination address extracted from the header of the received packet, and transfers the received packet to one of line interfaces or the control unit  15  corresponding to the destination address. The protocol processing unit  12  collects statistics information required for accounting for each PPP connection accessing to the Internet and stores the information into a statistics table  14 . Accounting information stored in the statistics table  14  is reported to the authentication server  40  periodically and when a PPP connection is disconnected. 
     The control unit  15  is comprised of a processor  20 , a memory  21  for storing various kinds of programs to be executed by the processor  20 , a memory  22  for storing data and tables, and a terminal interface  23  for connection to a control terminal  100 . 
     In the memory  21 , a connection control routine  30 , PPP LCP/NCP communication control routine  31 , and a RADIUS communication control routine  32  for communication with the authentication server  40  are stored as programs related to the present invention. The connection control routine  30  performs overall control of a communication sequence between a user terminal and the authentication server, which will be described later by referring to  FIG. 6 , instruction to the protocol processing unit  12  to start and stop collecting statistics information, periodical reporting of statistics information to the authentication server, and other control operations. A user management table  33  shown in a dotted block in the memory  22  and a routine for managing the number of connections  34  shown in a dotted block in the memory  21  relate to only a second embodiment of the present invention which will be described later and, therefore, their explanation is omitted here. 
       FIG. 3  shows an embodiment of a routing table  13  to be updated by the control unit  15  and referred to by the protocol processing unit  12 . 
     The routing table  13  stores a plurality of table entries each indicating the relation between a destination IP address  141  and an output port number  142 . Each entry, in which an IP address assigned from the authentication server  40  to a user terminal having been authenticated successfully is registered as the destination IP address  141 , further includes output link information  143  and a destination MAC address  144 . Each entry is registered into the routing table  14  when the BAS  10  delivers the IP address to the user terminal having been authenticated successfully and deleted from the routing table  14  when the PPP session of the user terminal is released. 
       FIG. 4  is a block diagram showing an embodiment of the authentication server  40 . 
     The authentication server  40  is comprised of a processor  41 , a line interface  42  for connection to the ISP network NW 2 , a memory  43  for storing various kinds of programs to be executed by the processor  41 , a memory  44  for storing tables and data that the processor  41  uses, and an input device  45  and a display unit  46  as interfaces between the authentication server  40  and an administrator. 
     In the memory  43 , a RADIUS communication control routine for communicating with BAS  10  in accordance with the RADIUS protocol, an authentication routine  51  for authenticating users, an IP address management routine  52  for retrieving an IP address to be assigned to a user terminal from an address pool and storing a released IP address back to the address pool as an idle address, an accounting routine  53 , and a routine for managing the number of connections  54 , which will be detailed by referring to  FIG. 7  as processing for confirming the number of connections, are stored as programs related to the present invention. In the memory  44 , an IP address pool  55 , a user management table  56 , and an accounting information file  57  are formed. 
       FIG. 5  shows an embodiment of the user management table  56  to be equipped in the authentication server  40 . 
     The user management table  56  is comprised of a plurality of subtables  560  ( 560 - 1 ,  560 - 2 , . . . ) having group numbers  561 . Each subtable  560  includes, as user information, user IDs  562 , a password  563 , IP addresses  564 , and other personal information (not shown) and, besides, includes a preferential MAC address  565 , maximum number of connections  567 , and current number of connections  568 . 
     Here, the user ID  562  indicates an identifier of user who belongs to a group having the group number  561  and the password  563  indicates a password to be used in common for the multiple users in the group when user authentication is performed. The IP address  564  indicates an IP address assigned to the user by the authentication server  40 . The IP address is stored in the entry of a user who is connecting a PPP session. The preferential MAC address  565  indicates the MAC address of a user terminal that should be preferentially connected to the Internet NW 3 . The maximum number of connections  567  indicates the number of sessions allowed to simultaneously connect to the Internet NW 3 , which is predetermined by contract for each group. The current number of connections  568  indicates the number of sessions being connected to the Internet NW 3  in each group. 
     Although the upper limit of the number of addresses that can be registered as preferential MAC addresses  565  is equal to the maximum number of connections  567 , it is limited to a smaller number in practical application. As other personal information, for example, information about time when the last communication took place between a user and a server  90  being connected with a PPP session and information such as the total number of packets that the user terminal has transmitted to and received from the server  90  is stored in the table. These information items are obtained from statistics information that the authentication server  40  periodically receives from the BAS  10 . 
     Next, by referring to  FIG. 6 , a communication sequence will described in the case where one of user terminals H shown in  FIG. 1 , for example, the user terminal H 1 - 2  having user ID “user1-2@isp1” and MAC address “00:11:22:33:44:55” issues a request for connection to the Internet to the BAS  10 , using Point to Point Protocol (PPP). 
     The user terminal H 1 - 2  establishes a PPP LCP session with the BAS  10  in accordance with a normal connection sequence of PPP (SQ 1 ). The PPP LCP session is established by transmitting a PPP LCP Configure-Request from the user terminal H 1 - 2  to the BAS  10  and by returning a PPP LCP Configure-Response from the BAS  10  to the user terminal H 1 - 2  in response to the request. 
     Having established the PPP LCP session, the BAS  10  requests the user terminal H 1  to send a user ID and password as authentication data in order to determine whether the user terminal H 1 - 2  is authorized to connect to the Internet (SQ 2 ). When the user terminal H 1 - 2  transmits an authentication response message including the user ID and password (SQ 3 ) in response to the request, the BAS  10  transmits an access request message “Access-Request” including the user ID “user1-2@isp1” and password PW “user1” received from the user terminal H 1 - 2  to the authentication server  40  managed by the ISP. In the present embodiment, the access request message includes the MAC address “00:11:22:33:44:55” of the user terminal H 1 - 2  as additional information. 
     Upon receiving the access request message “Access-Request”, the authentication server  40  searches the user management table  56  for an entry corresponding to the user ID “user1-2@isp1” indicated in the received message and compares the password indicated in the received message with the password  563  previously registered in the entry retrieved from the table (user authentication JB 1 ). A feature of the present embodiment resides in that, if having verified the validity of the user terminal H 1 - 2 , the authentication server  40  executes processing for confirming the number of connections (JB 2 ) in order to compare the maximum number of connections and the current number of connections for a group to which the user ID belongs and determine whether an additional connection is allowed. 
     In the processing for confirming the number of connections (JB 2 ), as shown in detail in  FIG. 7 , the processor  41  refers to the user management table  56  using the user ID indicated in the access request message “Access-Request” as a search key and specifies a subtable (a subtable  560 - 1  in this example) to which the user ID belongs (step  541 ). The processor  41  compares the value of the current number of connections  568  and the value Max(1) of the maximum number of connections  567  indicated in the subtable (step  542 ). 
     If the value of the current number of connections  568  is smaller than the value Max(1) of the maximum number of connections  567 , the processor  41  increments the current number of connections  568  by one (step  543 ). Then, the processor  41  retrieves an idle IP address to be assigned to the user terminal H 1 - 2  from the IP address pool  55  (step  544 ) and transmits an access request response message “Access-Response” including the IP address to the BAS  10  (step  545 , SQ 5  in  FIG. 6 ). 
     If the current number of connections  568  has reached the maximum number of connections  567 , the processor  41  checks whether the MAC address of the user terminal indicated in the access request message “Access-Request” matches the preferential MAC address  565  registered in the above subtable (step  546 ). If the MAC address of the user terminal is not registered as the preferential MAC address, the processor  41  generates a response message “Access-Reject” indicating authentication failure and transmits the response message to the BAS  10  (step  549 , SQ 5  in  FIG. 6 ). 
     If the MAC address of the user terminal is registered as the preferential MAC address, the processor  41  selects a user ID whose session should be disconnected forcibly from among the user IDs associated with PPP sessions established and belonging to the same group (step  547 ). The processor  41  requests the BAS  10  to disconnect the session for the selected user ID (step  548 ) and executes steps  544  and  545 . The selection of PPP session to be disconnected is not restrictive. For example, a session having the longest intermission in communication with a server  90  at the time or a session smallest in the total number of communication packets may be disconnected. 
     The response message “Access-Accept” that the authentication server  40  sends to the BAS  10  upon successful user authentication is transmitted in the form of an IP packet in which an IP header  101  and a UDP header  102  are added to the message, for example, as shown in  FIG. 8 . The response message includes, as RADIUS attributes following RADIUS Code  103  indicating the message type “Access-Accept”, a Framed-IP-Address attribute  104 , Vendor-Specific attribute  105 , and other attributes  106 . In the Framed-IP-Address attribute  104 , the value of the IP address (“192.168.1.2” in this example) that the authentication server  40  assigned to the user terminal H 1 - 2  is specified. 
     Returning to  FIG. 6 , when the response message “Access-Accept” is received from the authentication server  40 , the BAS  10  transmits an authentication complete notification message to the user terminal H 1 - 2  (SQ 6 ). If a response message “Access-Reject” indicating authentication failure is returned from the authentication server  40 , the BAS  10  transmits an authentication failure notification message to the user terminal H 1 - 2  and disconnects the PPP session with that terminal. 
     Upon receiving the authentication complete notification message indicating access request permission from the BAS  10 , the user terminal H 1 - 2  transmits a request “IPCP Configure-Request” for IP address assignment to the BAS  10  in accordance with an Internet Protocol Control Protocol (IPCP) (SQ 7 ). In response to the request for IP address assignment, the BAS  10  generates an IP address delivery message “IPCP Configure-Acknowledgement” including the IP address which has already been received from the authentication server  40  and returns the message to the user terminal H 1 - 2  (SQ 8 ). 
     After that, the BAS  10  transmits an accounting start request message “Accounting-Request (start)” to the authentication server  40  (SQ 9 ). Upon receiving an accounting start response message “Accounting-Response (start)” from the authentication server (SQ 10 ), the BAS  10  adds a new table entry for a destination IP address “192.168.1.2” into the routing table  13  (JB 11 ) and the protocol processing unit  12  starts the collection of statistics data for accounting for packets with the source ID address “192.168.1.2”. Then, the BAS starts forwarding the packets for accessing the Internet NW 3  from the user terminal H (SQ 11 ). 
       FIG. 9  shows a PPP session disconnection sequence. 
     When the user issues a command to disconnect an Internet connection, the user terminal H 1 - 2  transmits a PPP LCP session disconnection request message “PPP LCP Terminate-Request” to the BAS  10  (SQ 21 ). Upon receiving the disconnection request message, the BAS  10  returns a response message “PPP LCP Terminate-Response” to the user terminal H 1 - 2  (SQ 22 ) and transmits an accounting stop request message “Accounting-Request (stop)” for the user ID “user1-2@isp1” to the authentication server  40  (SQ 23 ). The accounting stop request message includes the statistics data on the user terminal H 1 - 2  collected by the protocol processing unit  12 . 
     Having received the accounting stop request message, the authentication server  40  refers to the user management table  56 , decrements by one the value of the current number of connections  568  in the subtable  560 - 1  to which the specified user ID “user1-2@isp1” belongs (JB 3 ), and releases the IP address having become useless back to the address pool  44  (JB 4 ). Then, the authentication server  40  transmits an accounting stop response message “Accounting-Response (step)” to the BAS  10  (SQ 24 ). Upon receiving the accounting stop response message from the authentication server  40 , the BAS  10  deletes the information relevant to the disconnected PPP session from the routing table (JB 12 ). 
     Second Embodiment 
     While the limitation of the number of connections by group is performed on the authentication server in the first embodiment, this limitation can be performed on the broadband access server (BAS)  10 . The second embodiment of the invention is characterized in that a user management table  33  is formed in the memory  22  of the BAS  10  and the processor  20  executes a routine for managing the number of connections  34  prepared in the memory  21 , as indicated by the dotted blocks in  FIG. 2 . 
     The user management table  33  equipped in the BAS  10  is the same as the user management table  56  shown in  FIG. 5 , with the exception of that the password  563  and IP address  564  columns are eliminated from the table. In the second embodiment, the user management table  56  equipped in the authentication server  40  does not need to have the columns of IP address  564 , preferential MAC address  565 , maximum number of connections  567 , and current number of connections  568 . 
       FIG. 10  shows a connection sequence of the second embodiment. 
     Since the communication procedures SQ 1  to SQ 10  performed between the user terminal H 1 - 2  and the BAS  10  or between the BAS  10  and the authentication server  40  are the same as the sequence for the first embodiment shown in  FIG. 6 , their detailed explanation will be omitted here. 
     Upon receiving an access request message “Access-Request” including, for example, the user ID “user1-2@isp1” and password PW “user1” from the BAS  10 , the authentication server searches the user management table  56  for a table entry corresponding to the user ID “user1-2@isp1” indicated in the received message and compares the password given in the received message with the password  563  previously registered in the table entry (user authentication JB 1 ). When a password match occurs, the authentication server  40  retrieves an idle IP address to be assigned to the user terminal H 1 - 2  from the IP address pool  55  and transmits an access request response message “Access-Response” including the retrieved IP address to the BAS  10  (SQ 5 ). 
     In the second embodiment, the BAS  10  having received the access request response message “Access-Response” from the authentication server  40 , executes the processing for confirming the number of connections (JB 10 ) by referring to the user management table  33 . If the current number of connections is less than the maximum number of connections for the group to which the user ID belongs, or if the user terminal is registered as a preferential terminal, the BAS  10  transmits an authentication complete message to the requester user terminal H 1 - 2 . Otherwise, the BAS  10  transmits an authentication failure message to the user terminal (SQ 6 ). 
       FIG. 11  shows a flowchart of the processing for confirming the number of connections (JB 10 ) to be performed by the processor  20  of the BAS  10 . 
     In similar to the case for the first embodiment, the processor  20  refers to the user management table  33  using the user ID of the access-requester as a search key and specifies a subtable (a subtable  560 - 1  in this example) to which the user ID belongs (step  341 ). The processor  20  compares the value of the current number of connections  568  and the value Max(1) of the maximum number of connections  567  indicated in the subtable (step  342 ). 
     If the value of the current number of connections  568  is smaller than the value Max(1) of the maximum number of connections  567 , the processor  20  increments the current number of connections  568  by one (step  343 ) and transmits an authentication complete message to the user terminal H 1 - 2  (step  345 , SQ 6  in  FIG. 11 ). 
     If the current number of connections  568  has reached the maximum number of connections  567 , the processor  20  checks whether the MAC address of the user terminal matches the preferential MAC address  565  registered in the above subtable (step  346 ). If the MAC address of the user terminal is not registered as the preferential MAC address, the processor  20  generates a response message “Access-Reject” indicating authentication failure and transmits the response message to the user terminal H 1 - 2  (step  349 , SQ 6  in  FIG. 11 ). 
     If the MAC address of the user terminal is registered as the preferential MAC address, the processor  41  selects a user ID whose session should be disconnected forcibly from among the user IDs associated with PPP sessions having been established and belonging to the same group (step  347 ). The processor  20  disconnects the session for the selected user ID (step  348 ) and executes step  345 . In similar to the case for the first embodiment, the selection of PPP session to be disconnected is not restrictive. For example, a session having the longest intermissions in communication with a server  90  at the time or a session smallest in the total number of communication packets may be disconnected. 
       FIG. 12  shows a PPP session disconnection process in the second embodiment. 
     Since the communication procedures SQ 21  to SQ 24  performed between the user terminal H 1 - 2  and the BAS  10  or between the BAS  10  and the authentication server  40  are the same as the sequence for the first embodiment shown in  FIG. 9 , their detailed explanation will be omitted here. 
     Upon receiving an accounting stop request message “Accounting-Request (stop)” from the BAS  10  (SQ 23 ), the authentication server  40  releases the IP address registered in association with the specified user ID in the user management table  56  back to the address pool  44  (JB 4 ) and transmits an accounting stop response message “Accounting-Response (stop)” to the BAS  10  (SQ 24 ). 
     Upon receiving the accounting stop response message from the authentication server  40 , the BAS  10  deletes the information relevant to the disconnected PPP session from the routing table (JB 12 ). The BAS  10  refers to the user management table  33  and decrements by one the value of the current number of connections  568  in the subtable to which the specified user ID belongs (JB 13 ).