Abstract:
An internet protocol (IP) network device is provided including a detector adapted to detect an event in the internet protocol (IP) network device. The network device also includes a generator adapted to generate a trap message including event information having information about the detected event. The network device also includes a transmitter adapted to transmit, to an internet protocol (IP) network management device, the trap message including the event information and a first sequence indicator. The network device also includes a timer adapted to measure a time period from a time at which the transmitter transmits the trap information. If the measured time period is equal to or longer than a predetermined time, the generator generates an empty trap message and the transmitter transmits, to the network management device, the empty trap message and a second sequence indicator.

Description:
BACKGROUND 
     Exemplary embodiments of the present invention relate to an Internet Protocol (IP) network, in particular, to a Network Management System (NMS) for an IP network based on a Simple Network Management Protocol (SNMP). 
     DESCRIPTION OF THE RELATED ART 
     In an IP network composed of a plurality of Network Elements (NEs) and an NMS that remotely monitors and controls individual NEs based on the SNMP, which is an Open Systems Interconnection (OSI) standard management protocol, if an event such as a failure detected by each NE occurs, the NMS is notified of a message called a trap that represents the event. 
     However, it is not certain that a trap (message) will reach the NMS. For example, if a communication failure occurs over a communication path between an NE and the NMS, while such a communication failure is occurring, a trap generated by the NE does not reach the NMS. Thus, there may be a trap that is lost in those that the NMS side receives. 
     To solve such a problem, a technique that detects a lost trap has been proposed as described in Japanese Patent Laid-Open No. 2003-244142 (Patent Literature 1). 
       FIG. 1  shows a procedure for a process of the lost trap detection technique. In this example, SNMP manager  100 , which is an NMS, monitors SNMP agent  101 , which is an NE. 
     Whenever an event occurs, SNMP agent  101  generates a trap, assigns a sequence number to the trap, and transmits the trap with the assigned sequence number to SNMP manager  100 . 
     SNMP manager  100  receives traps from SNMP agent  101  and detects a lost trap based on the continuity of sequence numbers assigned to the traps. If there is a lost trap, SNMP manager  100  transmits a retransmission request for information about the lost trap to SNMP agent  101 . 
     SNMP agent  101  retransmits information about the lost trap to SNMP manager  100  corresponding to the retransmission request received from SNMP manager  100 . Thus, SNMP manager  100  can acquire information about the trap that SNMP manager  100  has not received due to a communication failure or the like. 
     However, according to the foregoing lost trap detection technique, a lost trap that has occurred is detected once SNMP manager  100  receives traps. Thus, if SNMP manager  100  has not received traps, it cannot determine whether it has not received a trap because the next event has not occurred in SNMP agent  101  or because a communication failure has occurred. 
     In addition, events infrequently occur in SNMP agent  101 , so it infrequently transmits traps to SNMP manager  100 . If the period between the time when an event occurs in SNMP agent  101  until the time when the next event occurs in SNMP agent  101  is long, SNMP manager  100  does not receive the next trap for a long time. In this case, SNMP manager  100  cannot recognize that a communication failure has occurred or that an event has occurred in SNMP agent  101  for a long time. 
     To solve the foregoing problem, according to the technique described in Patent Literature 1, SNMP manager  100  periodically checks whether or not a predetermined time has elapsed after it has received the last trap. If the predetermined period has elapsed, SNMP manager  100  asks SNMP agent  101  about the sequence number assigned to the last trap that SNMP agent  101  has transmitted. 
     In reply, SNMP agent  101  transmits the sequence number assigned to the last trap that SNMP agent  101  has transmitted to SNMP manager  100 . 
     SNMP manager  100  checks whether or not the sequence number, in the reply from SNMP agent  101 , matches the sequence number assigned to the last trap that SNMP manager  100  received. If they match each other, SNMP manager  100  determines that there is no lost trap. If they do not match each other, SNMP manager  100  determines that there is a lost trap due to a communication failure or the like. 
     According to Japanese Patent Laid-Open No. 2001-202305 (Patent Literature 2), if SNMP manager  100  has not received traps for a predetermined time or longer, SNMP manager  100  performs a heart-beat check for SNMP agent  101  in such a manner that SNMP manager  100  causes SNMP agent  101  to transmit traps to SNMP manager  100  at every predetermined period. 
     Specifically, SNMP manager  100  uses a timer that is reset whenever SNMP manager  100  receives a trap from SNMP agent  101  and determines that a heart-beat check has been successfully performed only when SNMP manager  100  has received a predetermined number of traps while the timer has not timed out. SNMP manager  100  can recognize a communication state based on the result of the heart-beat check. 
     SUMMARY 
     In the technique described in Patent Literature 1, a process composed of three steps is periodically executed in which SNMP manager  100  asks SNMP agent  101  about a sequence number, SNMP agent  101  transmits the sequence number to SNMP manager  100 , and SNMP manager  100  determines whether there is a lost trap based on the sequence number. Thus, there is a problem in which the number of steps performed to detect a trap that has been lost after the last trap has been received increases. 
     According to the technique described in Patent Literature 2, if SNMP manager  100  has not received a trap, a process is executed that is composed of three steps in which SNMP manager  100  performs a heart-beat check for SNMP agent  101 , SNMP agent  101  periodically transmits traps to SNMP manager  100 , and SNMP manager  100  determines whether it has received traps in the foregoing manner. Thus, according to the technique described in Patent Literature 2, there is a problem in which the number of steps performed to detect a lost trap that has been lost after the last trap that has been received increases. 
     An object of certain exemplary embodiments of the invention is to solve the foregoing problem and to provide a network device, a network management device, a network management system, a trap transmission method, and a network management method that avoids increasing the number of processing steps to detect a lost trap. 
     To accomplish the foregoing object, an aspect of certain exemplary embodiments of the invention provides a network device, comprising: 
     an event detection section that detects an event that occurs in the own device; a trap generation/transmission section that generates a trap containing information about the event detected by said event detection section, assigns a sequence number to the trap, and transmits the trap with the assigned sequence number to an external network management device; and 
     a trap information retransmission section that transmits information about a requested trap to said external network management device corresponding to a retransmission request received from said external network management device for information about a trap, 
     wherein if the period after said trap is transmitted until the next trap is transmitted is equal to or longer than a predetermined time, said trap generation/transmission section periodically generates empty traps at least in the period, and assigns sequence numbers to the empty traps, and transmits the empty traps with the assigned sequence numbers to said external network management device. 
     Another aspect of certain exemplary embodiments of the invention provides a network management device, comprising: 
     a trap reception section that receives a trap containing information about an event from an external network device whenever it occurs therein and, if the period after an event occurs until the next event occurs is equal to or longer than a predetermined time, receives an empty trap at least in the period at every said predetermined time from said external network device; 
     a lost trap detection section that detects a lost trap based on the continuity of sequence numbers assigned to said traps and empty traps received by said trap reception section; and 
     a trap information acquisition section that acquires information about a lost trap detected by said lost trap detection section. 
     A further aspect of certain exemplary embodiments of the invention provides a network management device, comprising: 
     a trap reception section that receives a trap containing information about an event from an external network device whenever it occurs therein and, if the period after an event occurs until the next event occurs is equal to or longer than a predetermined time, receives an empty trap at least in the period at every said predetermined time from said external network device; 
     a lost trap detection section that detects whether or not said traps and empty traps received by said trap reception section have been lost; and 
     a trap information acquisition section that acquires information about a lost trap detected by said lost trap detection section. 
     wherein said lost trap detection section has a timer that measures a time longer than said predetermined time as a measured value in such a manner that the measured value is cleared at the time of the reception of each of said traps and empty traps, and 
     wherein if the timer times out, said lost trap detection section determines that a lost trap has occurred. 
     A further aspect of certain exemplary embodiments of the invention provides a network management system, comprising: 
     a network device; and 
     a network management device connected to the network device through a network, 
     wherein said network device includes: 
     an event detection section that detects an event that occurs in the own device; 
     a trap generation/transmission section that generates a trap containing information about the event detected by said event detection section, assigns a sequence number to the trap, and transmits the trap with the assigned sequence number to said network management device; and 
     a trap information retransmission section that transmits information about a requested trap to said network management device corresponding to a retransmission request received from said network management device for information about a trap, and 
     wherein if the period after said trap is transmitted until the next trap is transmitted is equal to or longer than a predetermined time, said trap generation/transmission section periodically generates empty traps at least in the period, and assigns sequence numbers to the empty traps, and transmits the empty traps with the assigned sequence numbers to said network management device, 
     wherein said network management device includes: 
     a trap reception section that receives said traps and empty traps from said network device; 
     a lost trap detection section that detects a lost trap based on the continuity of sequence numbers assigned to said traps and empty traps received by said trap reception section; and 
     a trap information acquisition section that acquires information about a lost trap detected by said lost trap detection section. 
     A further aspect of certain exemplary embodiments of the invention provides a network management system, comprising: 
     a network device; and 
     a network management device connected to the network device through a network, 
     wherein said network device includes: 
     an event detection section that detects an event that occurs in the own device; 
     a trap generation/transmission section that generates a trap containing information about the event detected by said event detection section, assigns a sequence number to the trap, and transmits the trap with the assigned sequence number to said network management device; and 
     a trap information retransmission section that transmits information about a requested trap to said network management device corresponding to a retransmission request received from said network management device for information about a trap, 
     wherein if the period after said trap is transmitted until the next trap is transmitted is equal to or longer than a predetermined time, said trap generation/transmission section periodically generates empty traps at least in the period, and assigns sequence numbers to the empty traps, and transmits the empty traps with the assigned sequence numbers to said network management device, 
     wherein said network management device includes: 
     a trap reception section that receives said traps and empty traps from said network device; 
     a lost trap detection section that detects whether or not said traps and empty traps received by said trap reception section have been lost; and 
     a trap information acquisition section that acquires information about a lost trap detected by said lost trap detection section, 
     wherein said lost trap detection section has a timer that measures a time longer than said predetermined time as a measured value in such a manner that the measured value is cleared at the time of the reception of each of said traps and empty traps, and 
     wherein if the timer times out, said lost trap detection section determines that a lost trap has occurred. 
     A further aspect of certain exemplary embodiments of the invention provides a trap transmission method, comprising: 
     detecting an event that occurs in an own device; 
     generating a trap containing information about said event, assigning a sequence number to the trap, and transmitting the trap with the assigned sequence number to a network management device; and 
     if the period after said trap is transmitted until the next trap is transmitted is equal to or longer than a predetermined time, periodically generating empty traps at least in the period, assigning sequence numbers to the empty traps, and transmitting the empty traps with the assigned sequence numbers to said network management device. 
     A further aspect of certain exemplary embodiments of the invention provides a network management method, comprising: 
     causing a network device to detect an event that occurs in the own device, to generate a trap containing information about said event, to assign a sequence number to the trap, to transmit the trap with the assigned sequence number to a network management device, if the period after said trap is transmitted until the next trap is transmitted is equal to or longer than a predetermined time, to periodically generate empty traps at least in the period, to assign sequence numbers to the empty traps, and to transmit the empty traps with the assigned sequence numbers to said network management device, and 
     causing said network management device to receive said traps and empty traps from said network device, to detect a lost trap based on the continuity of the sequence numbers assigned to said traps and empty traps received from said network device, and if a lost trap is detected, to acquire information about the lost trap from said network device. 
     A further aspect of certain exemplary embodiments of the invention provides a network management method, comprising: 
     causing a network device to detect an event that occurs in the own device, to generate a trap containing information about said event, to assign a sequence number to the trap, to transmit the trap with the assigned sequence number to a network management device, if the period after said trap is transmitted until the next trap is transmitted is equal to or longer than a predetermined time, to periodically generate empty traps at least in the period, to assign sequence numbers to the empty traps, and to transmit the empty traps with the assigned sequence numbers to said network management device, and 
     causing said network management device to receive said traps and empty traps from said network device, to determine that a lost trap has occurred if a timer that measures a time longer than said predetermined time as a measured value in such a manner that the measured value is cleared at the time of the reception of each of said traps and empty traps times out, and to acquire information about the lost trap from said network device. 
     One or more exemplary embodiments of the invention may avoid increasing of the number of processing steps. Also, one or more exemplary embodiments of the invention will detect a lost trap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram describing an example of a lost trap detection technique. 
         FIG. 2  is a block diagram showing the structure of a network management system according to a first embodiment. 
         FIG. 3  is a flow chart showing a procedure for a trap transmission process that an SNMP agent of the network management system shown in  FIG. 1  performs. 
         FIG. 4  is a flow chart showing a procedure for a lost trap detection process that an SNMP manager of the network management system shown in  FIG. 1  performs. 
         FIG. 5  is a schematic diagram describing a network management method that the network management system shown in  FIG. 1  performs. 
         FIG. 6  is a flow chart showing a procedure for a lost trap detection process that an SNMP manager of a network management system according to a second embodiment. 
         FIG. 7  is a schematic diagram describing a network management method that the network management system according to the second embodiment performs. 
         FIG. 8  is a flow chart showing a procedure for a trap transmission process that an agent of a network management system according to a third embodiment performs. 
         FIG. 9  is a schematic diagram describing a network management method that the network management system according to the third embodiment performs. 
         FIG. 10  is a schematic diagram describing a network management method that a network management system according to a fourth embodiment performs. 
         FIG. 11  is a block diagram showing the structure of a network management system according to another embodiment performs. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the invention will be described below with reference to the drawings. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
     First Exemplary Embodiment 
       FIG. 2  is a block diagram showing the structure of a network management system according to a first exemplary embodiment. 
     Referring to  FIG. 2 , the network management system is a system that manages an IP network based on the SNMP. The network management system has SNMP agent  11  that is a network device (also called NE) and SNMP manager  10  that is a network management device (also called NMS) and that remotely monitors and controls SNMP agent  11  based on the SNMP. 
     SNMP agent  11  and SNMP manager  10  can mutually communicate with each other through the IP network.  FIG. 2  shows only one SNMP agent  11 . It will be appreciated that the network management system may have a plurality of SNMP agents  11 . 
     SNMP agent  11  has event detection section  11 - 1 , trap generation section  11 - 2 , periodic trap generation section  11 - 3 , sequence number management section  11 - 4 , trap transmission section  11 - 5 , polling transmission and reception section  11 - 6 , and trap information storage section  11 - 7 . 
     Event detection section  11 - 1  detects an event that occurs in its own device. Trap generation section  11 - 2  generates a trap containing information about an event detected by event detection section  11 - 1 . 
     If the period from just after one trap is transmitted until the next trap is transmitted (or the period from just after one event is detected until the next event is detected) is equal to or longer than a predetermined time, periodic trap generation section  11 - 3  generates an empty trap at every predetermined time at least in the period such that SNMP manager  10  can monitor SNMP agent  11 . 
     Specifically, periodic trap generation section  11 - 3  has timer T 1  that measures time t 1  in such a manner that the measured value is reset at the time of the transmission of a trap. In this context, time t 1  corresponds to the foregoing predetermined time. Periodic trap generation section  11 - 3  can determine whether or not the period from just after one trap is transmitted until the next trap is transmitted is equal to or longer than the predetermined time using timer T 1 . 
     Timer T 1  may be cleared at the time of the detection of an event. In this case, periodic trap generation section  11 - 3  can determine whether or not the period from just after one event is detected until the next event is detected is equal to or longer than the predetermined time using timer T 1 . 
     Sequence number management section  11 - 4  assigns sequence numbers to traps generated by trap generation section  11 - 2  and to empty traps generated by periodic trap generation section  11 - 3  in the order that they are generated. These sequence numbers are indicators of elements in a sequence, in which a first sequence indicator immediately precedes a second sequence indicator in the sequence. Trap transmission section  11 - 5  transmits the traps and empty traps with assigned sequence indicators, in this case sequence numbers, to SNMP manager  10 . 
     Trap information storage section  11 - 7  and polling transmission and reception section  11 - 6  constitute a trap information retransmission section that transmits information about a requested trap to SNMP manager  10 . The transmitted information corresponds to a retransmission request received from SNMP manager  10  for information about a trap. 
     Specifically, trap information storage section  11 - 7  stores information about a trap (including sequence number and event information) generated by trap generation section  11 - 2 . Polling transmission and reception section  11 - 6  receives a retransmission request for information about the trap from SNMP manager  10 , acquires the information about the requested trap from trap information storage section  11 - 7  corresponding to the retransmission request, and transmits a reply containing the acquired information to SNMP manager  10 . 
     Since information about empty traps is not stored in trap information storage section  11 - 7 , if trap information storage section  11 - 7  has not stored information about the requested trap, polling transmission and reception section  11 - 6  transmits to SNMP manager  10   a  reply that contains information that denotes that a lost trap is an empty trap. 
     SNMP manager  10  has trap reception section  10 - 1 , lost trap detection section  10 - 2 , and polling transmission and reception section  10 - 3 . 
     Trap reception section  10 - 1  receives a trap that contains information about an event and an empty trap from SNMP agent  11 . Lost trap detection section  10 - 2  detects a lost trap based on the continuity of sequence numbers assigned to traps containing events and empty traps received by trap reception section  10 - 1 . 
     When lost trap detection section  10 - 2  detects a lost trap, polling transmission and reception section  10 - 3 , a trap information acquisition section, transmits a retransmission request for information about the lost trap to SNMP agent  11 . In addition, polling transmission and reception section  10 - 3  receives information about the lost trap from SNMP agent  11  corresponding to the retransmission request. 
     Next, the operation of the network management system according to this embodiment will be described. 
       FIG. 3  is a flow chart showing a procedure for a trap transmission process that SNMP agent  11  performs. 
     At step S 10 , periodic trap generation section  11 - 3  activates timer T 1 . At step S 11 , periodic trap generation section  11 - 3  determines whether event detection section  11 - 1  has detected an event before timer T 1  has timed out. 
     If the determined result at step S 11  is “YES,” the control advances to steps S 12  to S 17  of the process. 
     At step S 12 , event detection section  11 - 1  detects an event. At step S 13 , trap generation section  11 - 2  generates a trap containing information about the trap detected by event detection section  11 - 1 . 
     At step S 14 , sequence number management section  11 - 4  assigns a sequence number to the trap generated by trap generation section  11 - 2 . At step S 15 , trap information storage section  11 - 7  stores information about the trap (sequence number and event information. 
     At step S 16 , trap transmission section  11 - 5  transmits the trap with the assigned sequence number to SNMP manager  10 . At step S 17 , periodic trap generation section  11 - 3  clears the counted value of timer T 1  at the time of the transmission of the trap. 
     If the determined result at step S 11  is “No,” the control advances to step S 18  to S 20  of the process. 
     At step S 18 , periodic trap generation section  11 - 3  generates an empty trap. At step S 19 , sequence number management section  11 - 4  assigns a sequence number to the empty trap generated by periodic trap generation section  11 - 3 . 
     At step S 20 , trap transmission section  11 - 5  transmits the empty trap with the assigned sequence number to SNMP manager  10 . Thereafter, the control returns to step S 10  of the process. When timer T 1  times out, it returns to the initial state (namely, the counted value of timer T 1  is cleared). 
       FIG. 4  is a flow chart showing a procedure for a lost trap detection process that SNMP manager  10  performs. 
     At step S 30 , SNMP manager  10  determines whether or not trap reception section  10 - 1  has detected a trap. 
     If trap reception section  10 - 1  has detected a trap, the control advances to step S 31  of the process. At step S 31 , SNMP manager  10  determines whether or not the sequence number assigned to the trap that has been detected this time and the sequence number assigned to the trap that has been received last time are successive. 
     If the sequence numbers are not successive, the control advances to step S 32  of the process. At step S 32 , polling transmission and reception section  10 - 3  acquires information about a lost trap from SNMP agent  11 . 
     Next, a trap transmission/lost trap detection process that the network management system performs will be described with a specific example. 
       FIG. 5  shows a procedure for a process of a network management method (trap transmission/lost trap detection process). This procedure is performed if events E 1  and E 2  occur before timer T 1  has timed out, a trap containing information about event E 2  is lost, and then timer T 1  times out. In  FIG. 5 , SNMP agent  11  performs steps S 40  to S 43  of the process, whereas SNMP manager  10  performs steps S 50  to S 52  of the process. 
     If event E 1  occurs in SNMP agent  11 , the control advances to step S 40  of the process. 
     At step S 40 , event detection section  11 - 1  detects event E 1 . Trap generation section  11 - 2  generates a trap containing information about event E 1 . Sequence number management section  11 - 4  assigns sequence number # 1  to the trap. Trap transmission section  11 - 5  transmits the trap with assigned sequence number # 1  to SNMP manager  10 . Trap information storage section  11 - 7  stores information about the trap with assigned sequence number # 1 . Periodic trap generation section  11 - 3  activates timer T 1  at the time of the transmission of the trap with assigned sequence number # 1 . 
     If event E 2  occurs in SNMP agent  11  before timer T 1  has timed out, the control advances to step S 41  of the process. 
     At step S 41 , event detection section  11 - 1  detects event E 2 . Trap generation section  11 - 2  generates a trap containing information about event E 2 . Sequence number management section  11 - 4  assigns sequence number # 2  to the trap. Trap transmission section  11 - 5  transmits the trap with assigned sequence number # 2  to SNMP manager  10 . Trap information storage section  11 - 7  stores information about the trap with assigned sequence number # 2 . Periodic trap generation section  11 - 3  clears the measured value of timer T 1  and activates timer T 1  at the time of the transmission of the trap with assigned sequence number # 2 . 
     If timer T 1  times out before the next event E 3  has occurred, the control advances to step S 42  of the process. 
     At step S 42 , periodic trap generation section  11 - 3  detects that timer T 1  has timed out and generates an empty trap. Sequence number management section  11 - 4  assigns sequence number # 3  to the empty trap. Trap transmission section  11 - 5  transmits the empty trap with assigned sequence number # 3  to SNMP manager  10 . Periodic trap generation section  11 - 3  activates timer T 1  at the time of the transmission of the empty trap with assigned sequence number # 3 . 
     In the example shown in  FIG. 5 , the trap with assigned sequence number # 2  has been lost due to a communication failure or the like. Thus, in SNMP manager  10 , trap reception section  10 - 1  receives the trap with assigned sequence number # 1  and the empty trap with assigned sequence number # 3 . 
     At step S 50 , lost trap detection section  10 - 2  determines that a trap with assigned sequence number # 2  has been lost based on the continuity of the sequence numbers (# 1  and # 3 ) assigned to the traps received by trap reception section  10 - 1 . 
     At step S 51 , polling transmission and reception section  10 - 3  transmits a retransmission request for information about the trap with assigned sequence number # 2  to SNMP agent  11 . 
     At step S 43 , polling transmission and reception section  11 - 6  receives the retransmission request from SNMP manager  10 , acquires information about the requested trap with assigned sequence number # 2  from trap information storage section  11 - 7  corresponding to the retransmission request, and transmits a reply containing the acquired information to SNMP manager  10 . 
     At step S 52 , polling transmission and reception section  10 - 3  receives information about the retransmitted trap with assigned sequence number # 2  from SNMP agent  11 . As a result, the lost trap can be restored. 
     According to the foregoing network management method, if the period after one transmission is transmitted until the next trap is transmitted is equal to or longer than the predetermined time, SNMP agent  11  transmits an empty trap with an assigned sequence number to SNMP manager  10  at every predetermined time. Thus, if SNMP manager  10  has not received a trap containing information about an event, SNMP manager  10  checks the continuity of sequence numbers assigned to empty traps so as to determine whether SNMP manager  10  has not received the next trap because it has not occurred or because a communication failure has occurred. 
     According to this embodiment, since it is not necessary to ask for and reply to sequence numbers as described in Patent Literature 1 and to perform heart-beat check as described in Patent Literature 2, the number of processing steps can be decreased compared with those described in Patent Literatures 1 and 2. 
     Second Exemplary Embodiment 
     Although a network management system according to a second exemplary embodiment has the structure shown in  FIG. 2 , the network management system according to the second embodiment is different from the first embodiment in that lost trap detection section  10 - 2  has timer T 2  that measures time t 2  (&gt;t 1 ) in such a manner that the measured value is cleared at the time of the reception of a trap and lost trap detection section  10 - 2  detects a lost trap depending on whether or not timer T 2  has timed out. Like timer T 1 , if timer T 2  times out, it returns to the initial state (namely, the measured value is cleared). 
     Since a trap transmission process that SNMP agent  11  according to the second embodiment performs is the same as that according to the first embodiment, its description will be omitted. 
       FIG. 6  is a flow chart showing a procedure for a lost trap detection process that SNMP manager  10  performs. 
     At step S 60 , SNMP manager  10  determines whether or not trap reception section  10 - 1  has received a trap. 
     If trap reception section  10 - 1  has received a trap, the control advances to step S 61  of the process. At step S 61 , lost trap detection section  10 - 2  activates timer T 2 . 
     At step S 62 , lost trap detection section  10 - 2  determines whether or not timer T 2  has timed out before trap reception section  10 - 1  has received the next trap. 
     If timer T 2  has timed out before trap reception section  10 - 1  has received the next trap, the control advances to step S 63  of the process. At step S 63 , lost trap detection section  10 - 2  determines that a lost trap has occurred. Polling transmission and reception section  10 - 3  acquires information about the lost trap from SNMP agent  11 . 
     If the determined result at step S 60  is “No” or the determined result at step S 62  is “No,” the control returns to step S 60  of the process 
     Since it is likely that a plurality of traps have been lost, at step S 63 , polling transmission and reception section  10 - 3  acquires information about all traps transmitted from SNMP agent  11  after polling transmission and reception section  10 - 3  has received the last trap from SNMP agent  11 . 
     Next, a process of a network management method (trap transmission/lost trap detection process) that the network management system performs will be described with a specific example. 
       FIG. 7  shows a procedure for the process of the network management method. This procedure is performed if events E 1  and E 2  occur before timer T 1  has timed out and then a trap containing information about event E 2  is lost. In  FIG. 7 , SNMP agent  11  performs steps S 70  to S 72  of the process, whereas SNMP manager  10  performs steps S 80  to S 83  of the process. 
     If event E 1  occurs in SNMP agent  11 , the control advances to step S 70  of the process. 
     At step S 70 , event detection section  11 - 1  detects event E 1 . Trap generation section  11 - 2  generates a trap containing information about event E 1 . Sequence number management section  11 - 4  assigns sequence number # 1  to the trap. Trap transmission section  11 - 5  transmits the trap with assigned sequence number # 1  to SNMP manager  10 . Trap information storage section  11 - 7  stores information about the trap with assigned sequence number # 1 . Periodic trap generation section  11 - 3  activates timer T 1  at the time of the transmission of the trap with assigned sequence number # 1 . 
     If event E 2  occurs in SNMP agent  11  before timer T 1  has timed out, the control advances to step S 71  of the process. 
     At step S 71 , event detection section  11 - 1  detects event E 2 . Trap generation section  11 - 2  generates a trap containing information about event E 2 . Sequence number management section  11 - 4  assigns sequence number # 2  to the trap. Trap transmission section  11 - 5  transmits the trap with assigned sequence number # 2  to SNMP manager  10 . Trap information storage section  11 - 7  stores information about the trap with assigned sequence number # 2 . Periodic trap generation section  11 - 3  resets the measured value and activates timer T 1  at the time of the transmission of the trap with assigned sequence number # 2 . 
     When trap reception section  10 - 1  receives the trap with assigned sequence number # 2 , the control advances to step S 80  of the process. At step S 80 , lost trap detection section  10 - 2  activates timer T 2  at the time of the reception of the trap with assigned sequence number # 1 . 
     In the example shown in  FIG. 7 , a trap with assigned sequence number # 2  has been lost due to a communication failure or the like. Thus, in SNMP manager  10 , trap reception section  10 - 1  cannot receive the trap with assigned sequence number # 2 . As a result, timer T 2  times out. Thereafter, the control advances to step S 81  of the process. 
     At step S 81 , lost trap detection section  10 - 2  detects that timer T 2  has timed out and determines that the trap with assigned sequence number # 2  has been lost. 
     At step S 82 , polling transmission and reception section  10 - 3  transmits a retransmission request for information about the trap with assigned sequence number # 2  to SNMP agent  11 . 
     At step S 72 , polling transmission and reception section  11 - 6  receives the retransmission request from SNMP manager  10 , acquires information about the trap with assigned sequence number # 2  from trap information storage section  11 - 7 , and transmits a reply containing the acquired information to SNMP manager  10 . 
     At step S 83 , polling transmission and reception section  10 - 3  receives information about the trap with assigned sequence number # 2  from SNMP agent  11 . Thus, the lost trap can be restored. 
     According to the foregoing network management method, SNMP agent  11  transmits a trap containing information about an event with an assigned sequence number to SNMP manager  10 . In addition, if the period from just after one trap is transmitted until the next trap is transmitted is equal to or longer than the predetermined time, SNMP agent  11  transmits an empty trap with an assigned sequence number to SNMP manager  10  at every predetermined time. Unless a communication failure has occurred, SNMP manager  10  has received a trap containing information about an event with an assigned sequence number or an empty trap with an assigned sequence number in the predetermined time. Thus, if SNMP manager  10  has received neither a trap containing information about an event nor an empty trap for the predetermined time, it is likely that a trap has been lost due to a communication failure or the like. If SNMP manager  10  has received neither a trap containing information about an event nor an empty trap and timer T 2  has timed out, SNMP manager  10  can determine that a trap has been lost due to a communication failure or the like based on such a theory. According to the lost trap detection process using timer T 2 , if SNMP manager  10  has not received a trap containing information about an event, SNMP manager  10  can determine whether it has not received the next trap because it has not occurred or because a communication failure or the like has occurred. 
     According to this embodiment, since it is not necessary to ask for and reply to sequence numbers as described in Patent Literature 1 and to perform a heart-beat check as described in Patent Literature 2, the number of processing steps can be decreased compared with those described in Patent Literatures 1 and 2. 
     Third Exemplary Embodiment 
     Although a network management system according to a third exemplary embodiment has the structure shown in  FIG. 2 , the network management system according to the third embodiment is different from that according to the first embodiment in that periodic trap generation section  11 - 3  generates an empty trap at every time t using timer T 1  regardless of occurrence of an event. 
     Since a lost trap detection process that SNMP manager  10  according to the third embodiment performs is the same as that according to the first embodiment, its description will be omitted. 
       FIG. 8  is a flow chart showing a procedure for a trap transmission process that SNMP agent  11  performs. 
     At step S 90 , periodic trap generation section  11 - 3  activates timer T 1 . At step S 91 , periodic trap generation section  11 - 3  determines whether or not timer T 1  has timed out. 
     If the determined result at step S 91  is “Yes,” the control advances to steps S 92  to S 94  of the process. 
     At step S 92 , periodic trap generation section  11 - 3  generates an empty trap. At step S 93 , sequence number management section  11 - 4  assigns a sequence number to the empty trap generated by periodic trap generation section  11 - 3 . 
     At step S 94 , trap transmission section  11 - 5  transmits the empty trap with the assigned sequence number to SNMP manager  10 . 
     After step S 94 , the control returns to step S 90  of the process. 
     If the determined result at step S 91  is “No,” the control advances to steps S 95  to S 100  of the process. 
     At step S 95 , event detection section  11 - 1  determines whether an event has occurred. If an event has not occurred, the control returns to step S 91  of the process. 
     If an event has occurred, the control advances to step S 96  of the process. At step S 96 , event detection section  11 - 1  detects an event. At step S 97 , trap generation section  11 - 2  generates a trap containing information about the event detected by event detection section  11 - 1 . 
     At step S 98 , sequence number management section  11 - 4  assigns a sequence number to the trap generated by trap generation section  11 - 2 . At step S 99 , trap information storage section  11 - 7  stores information about the trap (sequence number and information about the event). 
     At step S 100 , trap transmission section  11 - 5  transmits the trap with the assigned sequence number to SNMP manager  10 . 
     After step S 100 , the control returns to step S 91  of the process. 
     Next, a process of a network management method that the network management system according to this embodiment performs will be described with a specific example. 
       FIG. 9  shows the procedure for the process of the network management method. This procedure is performed if an empty trap is generated, event E 1  occurs before timer T 1  has timed out, a trap containing information about event E 1  is lost, and then timer T 1  times out. In  FIG. 9 , SNMP agent  11  performs steps S 110  to S 113  of the process, whereas SNMP manager  10  performs steps S 120  to S 122  of the process. 
     At step S 110 , periodic trap generation section  11 - 3  generates an empty trap. Sequence number management section  11 - 4  assigns sequence number # 1  to the empty trap. Trap Transmission section  11 - 5  transmits the empty trap with assigned sequence number # 1  to SNMP manager  10 . Periodic trap generation section  11 - 3  activates timer T 1  at the time of the transmission of the empty trap with assigned sequence number # 1 . 
     If event E 1  occurs in SNMP agent  11  before timer T 1  has timed out, the control advances to step S 111  of the process. 
     At step S 111 , event detection section  11 - 1  detects event E 1 . Trap generation section  11 - 2  generates a trap containing information about event E 1 . Sequence number management section  11 - 4  assigns sequence number # 2  to the trap. Trap information storage section  11 - 7  stores information about the trap with assigned sequence number # 2 . Trap transmission section  11 - 5  transmits the trap with assigned sequence number # 2  to SNMP manager  10 . 
     If timer T 1  times out before the next event E 2  has occurred, the control advances to step S 112  of the process. 
     At step S 112 , periodic trap generation section  11 - 3  detects that timer T 1  has timed out and generates an empty trap. Sequence number management section  11 - 4  assigns sequence number # 3  to the empty trap. Trap transmission section  11 - 5  transmits the empty trap with assigned sequence number # 3  to SNMP manager  10 . Periodic trap generation section  11 - 3  activates timer T 1  at the time of the transmission of the empty trap with assigned sequence number # 3 . 
     In the example shown in  FIG. 9 , the trap with sequence number # 2  has been lost due to communication failure or the like. Thus, in SNMP manager  10 , trap reception section  10 - 1  receives the empty trap with assigned sequence number # 1  and the empty trap with assigned sequence number # 3 . 
     At step S 120 , lost trap detection section  10 - 2  determines that the trap with assigned sequence number # 2  has been lost based on the continuity of the sequence numbers (# 1  and # 3 ) assigned to the traps received by trap reception section  10 - 1 . 
     At step S 121 , polling transmission and reception section  10 - 3  transmits a retransmission request for information about the trap with assigned sequence number # 2  to SNMP agent  11 . 
     At step S 113 , polling transmission and reception section  11 - 6  receives the retransmission request from SNMP manager  10 , acquires information about the requested trap with assigned sequence number # 2  from trap information storage section  11 - 7  corresponding to the retransmission request, and transmits a reply containing the acquired information to SNMP manager  10 . 
     At step S 122 , polling transmission and reception section  10 - 3  receives the retransmitted information about the trap with assigned sequence number # 2  from SNMP agent  11 . As a result, the lost trap can be restored. 
     According to the foregoing network management method, SNMP agent  11  transmits a trap containing information about an event with an assigned sequence number to SNMP manager  10 . In addition, SNMP agent  11  transmits an empty trap with an assigned sequence number to SNMP manager  10  at every predetermined time. Unless a communication failure has occurred, SNMP manager  10  has received not only a trap containing information about an event, but also an empty trap with an assigned sequence number at every predetermined time. Thus, if SNMP manager  10  has not received a trap containing information about an event, SNMP manager  10  can determine whether it has not received the next trap because it has not occurred or because a communication failure or the like has occurred. 
     According to this embodiment, since it is not necessary to ask for and reply to sequence numbers as described in Patent Literature 1 and to perform a heart-beat check as described in Patent Literature 2, the number of processing steps can be decreased compared with those described in Patent Literatures 1 and 2. 
     Fourth Exemplary Embodiment 
     Although a network management system according to a fourth exemplary embodiment has the structure shown in  FIG. 2 , the network management system according to the fourth embodiment is different from that according to the third embodiment in that lost trap detection section  10 - 2  has timer T 2  that measures time t 2  (&gt;t 1 ) in such a manner that the counted value is cleared at the time of the reception of a trap and lost trap detection section  10 - 2  detects a lost trap depending on whether or not timer T 2  has timed out. 
     A trap transmission process that SNMP agent  11  performs according to this embodiment is the same as that according to the third embodiment. A lost trap detection process that SNMP manager  10  according to the fourth embodiment performs is the same as that according to the second embodiment. 
     Next, a process of a network management method (trap transmission/lost trap detection process) that the network management system according to this embodiment performs will be described with a specific example. 
       FIG. 10  shows the procedure for the process of the network management method. This procedure is performed if an empty trap is generated, event E 1  occurs before timer T 1  has timed out, both a trap containing information about event E 1  and an empty trap that has been transmitted are lost, and then timer T 1  times out. In  FIG. 10 , SNMP agent  11  performs steps S 130  to S 133  of the process, whereas SNMP manager  10  performs steps S 140  to S 143  of the process. 
     At step S 130 , periodic trap generation section  11 - 3  generates an empty trap. Sequence number management section  11 - 4  assigns sequence number # 1  to the empty trap. Trap Transmission section  11 - 5  transmits the empty trap with assigned sequence number # 1  to SNMP manager  10 . Periodic trap generation section  11 - 3  activates timer T 1  at the time of the transmission of the empty trap with assigned sequence number # 1 . 
     If event E 1  occurs in SNMP agent  11  before timer T 1  has timed out, the control advances to step S 131  of the process. 
     At step S 131 , event detection section  11 - 1  detects event E 1 . Trap generation section  11 - 2  generates a trap containing information about event E 1 . Sequence number management section  11 - 4  assigns sequence number # 2  to the trap. Trap information storage section  11 - 7  stores information about the trap with assigned sequence number # 2 . Trap transmission section  11 - 5  transmits the trap with assigned sequence number # 2  to SNMP manager  10 . 
     If timer T 1  times out before the next event E 2  has occurred, the control advances to step S 132  of the process. 
     At step S 132 , periodic trap generation section  11 - 3  detects that timer T 1  has timed out and generates an empty trap. Sequence number management section  11 - 4  assigns sequence number # 3  to the empty trap. Trap transmission section  11 - 5  transmits the empty trap with assigned sequence number # 3  to SNMP manager  10 . Periodic trap generation section  11 - 3  activates timer T 1  at the time of the transmission of the empty trap with assigned sequence number # 3 . 
     When trap reception section  10 - 1  receives the empty trap with assigned sequence number # 1 , the control advances to step S 140  of the process. At step S 140 , lost trap detection section  10 - 2  activates timer T 2  at the time of the reception of the empty trap with assigned sequence number # 1 . 
     In the example shown in  FIG. 10 , since the trap with assigned sequence number # 2  and the empty trap with assigned sequence number # 3  have been lost due to a communication failure or the like, in SNMP manager  10 , trap reception section  10 - 1  cannot receive the trap with assigned sequence number # 2  and the empty trap with assigned sequence number # 1 . As a result, timer T 2  times out. 
     At step S 141 , lost trap detection section  10 - 2  detects that timer T 2  has timed out and determines that traps with sequence number # 1  and later sequence numbers have been lost. 
     At step S 142 , polling transmission and reception section  10 - 3  transmits a retransmission request for information about the traps with assigned sequence number # 1  and later sequence numbers to SNMP agent  11 . 
     At step S 133 , polling transmission and reception section  11 - 6  receives the retransmission request from SNMP manager  10 , acquires information about the requested traps with assigned sequence number # 1  and later sequence numbers from trap information storage section  11 - 7  corresponding to the retransmission request, and transmits a reply containing the acquired information to SNMP manager  10 . 
     At step S 143 , polling transmission and reception section  10 - 3  receives the retransmitted information about the traps with assigned sequence number # 1  and later sequence numbers from SNMP agent  11 . As a result, the lost traps can be restored. 
     According to the foregoing network management method, SNMP agent  11  transmits a trap containing information about an event with an assigned sequence number to SNMP manager  10 . SNMP agent  11  transmits an empty trap with an assigned sequence number to SNMP manager  10  at every predetermined time. Unless a communication failure has occurred, SNMP manager  10  has received a trap containing information about an event with an assigned sequence number or an empty trap with an assigned sequence number in the predetermined time. Thus, if SNMP manager  10  has not received a trap containing information about an event or an empty trap for the predetermined time or longer, it is likely that a trap has been lost due to a communication failure or the like. If SNMP manager  10  has not received a trap containing information about an event or an empty trap and timer T 2  has timed out, SNMP manager  10  can determine that a trap has been lost due to a communication failure or the like based on such a theory. According to the lost trap detection process using timer T 2 , if SNMP manager  10  has not received a trap containing information about an event, SNMP manager  10  can determine that the reason why it is not received the next trap is because it is has not occurred, or the reason why it has not received the next trap is because a communication failure or the like has occurred. 
     According to this embodiment, since it is not necessary to ask for and reply to sequence numbers as described in Patent Literature 1 and to perform a heart-beat check as described in Patent Literature 2, the number of processing steps can be decreased compared with those described in Patent Literatures 1 and 2. 
     The foregoing embodiments are exemplary embodiments of the invention. However, it should be understood by those skilled in the art that the structure and details of the exemplary embodiments may be changed in various manners without departing from the scope of the present invention. 
     For example, in the structure shown in  FIG. 2 , trap transmission section  11 - 5  may have the function of trap generation section  11 - 2 . 
     Alternatively, trap transmission section  11 - 5  may have the functions of trap generation section  11 - 2  and periodic trap generation section  11 - 3 . 
     As another alternative, trap transmission section  11 - 5  may have the functions of trap generation section  11 - 2 , periodic trap generation section  11 - 3 , and sequence number management section  11 - 4 . 
     Another Exemplary Embodiment 
       FIG. 11  is a block diagram showing the structure of a network management system according to another exemplary embodiment. 
     Referring to  FIG. 11 , the network management system is a system that manages an IP network based on the SNMP. The network management system has network device  21  that is an NE and network management device  20  that is an NMS that remotely monitors and controls network device  21  based on the SNMP.  FIG. 11  shows only one network device  21 . However, it should be appreciated that network device  21  may have a plurality of network devices  21 . 
     Network device  21  has event detection section  21 - 1  that detects an event that occurs in the own device; trap generation/transmission section  21 - 2  that generates a trap containing information about an event detected by event detection section  21 - 1 , assigns a sequence number to the trap, and transmits the trap with the assigned sequence number to network management device  20 ; and trap information retransmission section  21 - 3  that transmits information about a requested trap to network management device  20  corresponding to a retransmission request received from network management device  20  for the information about the trap. 
     If the period from just after one trap is transmitted until the next trap is transmitted is equal to or longer than a predetermined time, trap generation/transmission section  21 - 2  periodically generates empty traps at least in the period, assigns sequence numbers to the empty traps, and transmits the empty traps with the assigned sequence numbers to network management device  20 . 
     Network management device  20  has trap reception section  20 - 1  that receives traps containing information about events and empty traps from network device  21 ; lost trap detection section  20 - 2  that detects a lost trap based on the continuity of sequence numbers assigned to traps containing information about events and empty traps received by trap reception section  20 - 1 ; and trap information acquisition section  20 - 3  that transmits a retransmission request for information about the lost trap detected by lost trap detection section  20 - 2  to network device  21  and acquires the information about the lost trap from network device  21 . 
     In the foregoing structure, trap generation/transmission section  21 - 2  has a timer that measures a predetermined time in such a manner that the measured value is cleared at the time of the transmission of a trap. If the timer times out before event detection section  21 - 1  detects an event, trap generation/transmission section  21 - 2  may generate empty traps. 
     Alternatively, trap generation/transmission section  21 - 2  may have a timer that measures a predetermined time. If the timer times out, trap generation/transmission section  21 - 2  may generate empty traps. 
     As another alternative, lost trap detection section  20 - 2  may have a timer that measures a time longer than the predetermined time in such a manner that the measured value is cleared at the time of the reception of a trap containing information about an event or an empty trap. If the timer times out, lost trap detection section  20 - 2  may determine that a lost trap has occurred. 
     The present application claims a priority based on Japanese Patent Application JP 2012-123134 filed on May 30, 2012, the entire contents of which being incorporated herein by reference in its entirety. 
     It should be noted that the present inventive concept is not limited to the above exemplary embodiments but modification can be made as needed without deviating from the spirit and scope as defined by the claims.