Abstract:
Provided are a handover method and apparatus in a WLAN environment and an ICMP message used in such a WLAN environment. The handover method involves a) issuing a request for distributed information table (DIT) information, which is network information on access routers, to an access router of a Basic Service Set that a mobile node belongs to when booted; b) receiving DIT information from the access router and storing the received DIT information; and c) performing binding update using the stored DIT information.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims the priority of Korean Patent Application No. 2002-87938, filed on Dec. 31, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
           [0002]    1. Field of the Invention  
           [0003]    Apparatuses and methods consistent with the present invention relate to a handover method and apparatus in a wireless local area network (WLAN) environment and an Internet control message protocol (ICMP) message used in a WLAN environment.  
           [0004]    2. Description of the Related Art  
           [0005]    The Internet has become one of the most important information infrastructures of an information society that we are living in. In accordance with the advent of the Internet, the numbers of Internet users and Internet-associated equipment have been rapidly increasing. Against this backdrop, Internet Protocol Version 6 (IPv6), which aims at solving IP address shortages and guaranteeing routing efficiency, mobility, and quality-of-service (QoS), has been developed and is now expected to be commercialized in the near future.  
           [0006]    In addition, mobile IPv6, which can more reliably and more efficiently guarantee mobility while providing typical IPv6 functions, has been developed. However, in order to enable seamless mobile communications, a handover process on Layer  3  (IP layer) is essential.  
           [0007]    Recently, an Internet-Draft of the Internet Engineering Task Force (IETF) entitled, “Fast Handovers for Mobile IPv6,” has been suggested. The Internet-Draft presents an algorithm capable of carrying out a quick handover with the help of Layer  2  (link layer).  
           [0008]    A mobile IP presents an explanation of necessary protocol operations for maintaining a mobile node&#39;s connection to the Internet during a handover process in which the mobile node is disconnected from one access router and then is handed over to another access router. The necessary protocol operations include detecting movements, creating an IP address, and updating a location.  
           [0009]    In the meantime, a wireless local area network (WLAN) has a structure in which small networks called Basic Service Sets (BSSs) exist in a larger network called an extended service set (ESS). In each of the BSSs, access points exist for enabling interconnections among the BSSs. These access points share BSS identification (ID), which is a media access control (MAC) layer identifier, with one another. Currently, a mobile IP is only enabled within the ESS. Therefore, given the current level of WLAN technology, it is safe to say that communications between ESSs are impossible.  
           [0010]    Hereinafter, a conventional handover process in a mobile IP will be described in greater detail with reference to FIG. 1.  
           [0011]    An access router periodically generates a beacon signal for broadcasting its own ID, i.e., BSSID, over a mobile network. In other words, referring to FIG. 1, a previous access router (PAR)  120  and a new access router (NAR)  130  periodically generate a beacon signal. The PAR  120  is a default router of a mobile node  110  before a handover, and the NAR  130  is a default router of the mobile node  110  after the handover.  
           [0012]    Let us assume that the PAR  120  and the NAR  130  create Basic Service Sets BSS1 and BSS2, respectively. Then, the mobile node  110  belongs to BSS1 until it moves. On the other hand, the mobile node  110  belongs to BSS2 after it moves. Even when the mobile node  110  belongs to BSS1, it can receive a beacon signal from the NAR  130 . However, the received beacon signal is very weak. Likewise, when the mobile node  110  belongs to BSS2, it can receive a beacon signal from the PAR  120 . However, the received beacon signal is very weak.  
           [0013]    While the mobile node  110  moves from the BSS1 to BSS2, as shown in FIG. 1, there is a moment when the intensity of the beacon signal received from the NAR  130  becomes higher than the intensity of the beacon signal received from the PAR  120 . At this moment, an L2 trigger occurs on an L2 layer. When the L2 trigger occurs, the mobile node  110  can identify BSSID of the NAR  130  based on the beacon signal received from the NAR  130 .  
           [0014]    Finally, the mobile node  110  performs a handover by issuing a request for information on the NAR  130  to the PAR  120  using the BSSID of the NAR  130 , carrying out binding update, and enabling a handover initiation message and an ACK message to be transmitted between the PAR  120  and the NAR  130 .  
           [0015]    [0015]FIG. 2 is a flowchart of a conventional handover operation.  
           [0016]    Referring to FIG. 2, an L2 trigger occurs in step S 210 . The occurrence of the L2 trigger means that a mobile node has just entered a new network. The mobile node can identify BSSID of an NAR.  
           [0017]    In step S 220 , the mobile node issues a request for information on the NAR corresponding to the identified BSSID to a PAR by sending a Router Solicitation for Proxy (RtSolPr) message to the PAR.  
           [0018]    In step S 230 , the PAR searches a distributed information table containing network information regarding all access routers in a current ESS. In step S 240 , the PAR transmits the requested information on the NAR, for example, prefix information, a link layer address, and an IP address, to the mobile node by sending a Proxy Router Advertisement (PrRtAdv) message to the mobile node.  
           [0019]    In step S 250 , the mobile node creates a care-of-address (CoA) using its own MAC address and the prefix information of the NAR. In step S 260 , the mobile node sends a fast binding update message (FBU message) to the PAR.  
           [0020]    In step S 270 , the PAR sends an acknowledgement (ACK) message to the mobile node in response to the reception of the quick binding update message.  
           [0021]    In step S 280 , the PAR sends a handover initiation message (HI message) to the NAR. In step S 290 , the NAR sends an ACK message to the PAR in response to the reception of the handover initiation message.  
           [0022]    Then, the entire handover operation is completed. If there is a packet to be transmitted to the mobile node during the handover operation, the packet is transmitted to the mobile node via the NAR. After the handover operation, the mobile node uses the NAR rather than the PAR.  
           [0023]    However, the conventional handover method generally results in handover latency because it usually takes a while to carry out a handover operation in the prior art. Here, a period of time during which a mobile node is unable to receive or transmit packets over an IP layer due to a handover operation is referred to as the handover latency. Therefore, it is almost impossible to apply the conventional handover method to various fields, such as VoIP-based video telecommunications, video conferences, or moving picture transmissions, in which real-time communications are required.  
           [0024]    In a small-sized WLAN environment where a network, comprised of BSSs, includes access routers and a handover operation is frequently carried out by many mobile nodes accessing the network, the conventional handover method may bring about a waste of time and resources.  
           [0025]    In the meantime, Korean Patent Laid-open Publication No. 2001-87890, entitled “Fast Handover Method Using Beacon Message in WLAN,” discloses a handover technique in which an access router, to which a mobile node is to be handed over, periodically sends out a beacon message containing power intensity measurement information and prefix information and a user terminal that receives the beacon message creates an address using the prefix information and carries out a handover operation using the created address. In this technique, an NAR&#39;s prefix information is contained in a beacon message, and then the beacon message is transmitted to a user terminal. Therefore, the beacon message needs to be large enough to contain the NAR&#39;s prefix information.  
         SUMMARY OF THE INVENTION  
         [0026]    The present invention provides a handover method and apparatus, which are capable of handover latency in a small-sized WLAN environment, and an Internet control message protocol (ICMP) message used in such a WLAN environment.  
           [0027]    According to an aspect of the present invention, there is provided a handover method in a wireless local area network (WLAN) environment. The handover method involves a) issuing a request for a distributed information table (DIT) information, which is network information on access routers, to an access router of a Basic Service Set that a mobile node belongs to when booted; b) receiving DIT information from the access router and storing the received DIT information; and c) performing binding update using the stored DIT information.  
           [0028]    According to another aspect of the present invention, a handover apparatus in a WLAN environment is provided. The handover apparatus includes a DIT information requesting unit which issues a request for DIT information, which is network information on access routers, to an access router of a Basic Service Set that a mobile node belongs to when booted; a DIT information storage unit which stores DIT information received from the access router; and a binding update processing unit which carries out binding update using the stored DIT information.  
           [0029]    According to another aspect of the present invention, there is provided an ICMP message used in a WLAN environment. The ICMP message includes a type field which indicates a request for DIT information on access routers of a Basic Service Set that the mobile node belongs to when booted.  
           [0030]    According to another aspect of the present invention, there is provided an ICMP message used in a WLAN environment. The ICMP message includes a type field which indicates a response to a request for the DIT information; a number field which indicates the number of pieces of access router information to be transmitted; and an access router information field which indicates information on access routers.  
           [0031]    In the present invention, some steps of a handover operation are carried out before rather than after an L2 trigger occurs on a layer  2 . By doing so, the number of steps of the handover operation that need to be carried out after the L2 trigger occurs can be reduced.  
           [0032]    In other words, information on an access router of a network that a mobile node accesses after being booted is obtained in advance by using an ICMP message. Then, a step of obtaining information on a next access router when a mobile node accesses a new network can be omitted, and thus handover latency can be reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0034]    [0034]FIG. 1 is a diagram illustrating a conventional handover method in a wireless local area network (WLAN) environment;  
         [0035]    [0035]FIG. 2 is a flowchart of a conventional handover method in a WLAN environment;  
         [0036]    [0036]FIG. 3 is a diagram illustrating a fast handover method in a WLAN environment according to an exemplary embodiment of the present invention;  
         [0037]    [0037]FIG. 4 is a block diagram of a handover apparatus according to an exemplary embodiment of the present invention that carries out a handover for a mobile node in a WLAN environment;  
         [0038]    [0038]FIG. 5 is a detailed block diagram of a DIT information storage unit of FIG. 4;  
         [0039]    [0039]FIG. 6 is a flowchart of a handover method in a WLAN environment according to another exemplary embodiment of the present invention;  
         [0040]    [0040]FIG. 7 is a diagram illustrating an example of an ICMP message according to an exemplary embodiment of the present invention, which is used by a mobile node to issue a request for DIT information; and  
         [0041]    [0041]FIG. 8 is a diagram illustrating an example of an ICMP message according to another exemplary embodiment of the present invention, which is a response message created by an access router in response to a request for DIT information issued by a mobile node. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    [0042]FIG. 3 is a diagram illustrating a fast handover method in a wireless local area network (WLAN) environment, according to an exemplary embodiment of the present invention.  
         [0043]    Referring to FIG. 3, when a mobile node  310  moves from BSS1 to BSS2, an L2 trigger occurs in BSS2. According to the present invention, the mobile node  310  multicasts a DIT information requesting message to an access router (PAR)  320  of BSS1 when it is booted in BSS1. Then, the PAR  320  transmits a DIT information response message to the mobile node  310  in response to the reception of the DIT information requesting message, and the mobile node  310  stores DIT information received from the PAR  320 . Then, when the L2 trigger occurs, the mobile node  310  can identify BSSID of a new access router (NAR)  330 . Thereafter, the mobile node  310  obtains prefix information of the NAR  330  by searching the DIT information for the identified BSSID. The mobile node  310  creates a care-of-address using the prefix information of the NAR  330  and carries out a binding update using the created care-of-address. Transmissions of a handover initiation message and an ACK message between the PAR  320  and the NAR  330  are the same as their counterparts in the prior art.  
         [0044]    [0044]FIG. 4 is a block diagram of a mobile node  400  according to an exemplary embodiment of the present invention. The mobile node  400  includes a DIT information requesting unit  410 , a beacon scanning unit  420 , a DIT information storage unit  440 , and a binding update processing unit  430 . The DIT information requesting unit  410  issues a request for DIT information to an access router of a BSS that the mobile node  400  currently belongs to. The beacon scanning unit  420  scans a beacon signal transmitted from the access router. The DIT information storage unit  440  stores the DIT information input thereinto. When an L2 trigger occurs, the binding update processing unit  430  creates a care-of-address using prefix information of a new access router, stored in the DIT information storage unit  440  and sends a binding update message to a previous access router using the created care-of-address.  
         [0045]    A data table stored in the DIT information storage unit  440  of FIG. 4 is illustrated in FIG. 5. DIT information includes BSSID information  441 , prefix information  442 , and address information  443 . The address information  443  includes a link layer address and an IP address.  
         [0046]    [0046]FIG. 6 is a flowchart of a handover method in a WLAN environment according to an exemplary embodiment of the present invention.  
         [0047]    Referring to FIG. 6, when the mobile node  400  of FIG. 4 is booted, the DIT information requesting unit  410  of the mobile node  400  multicasts a message requesting DIT information to an access router of a BSS that the mobile node  400  currently belongs to, in step S 610 .  
         [0048]    An example of an Internet Control Message Protocol (ICMP) message containing a request for DIT information is illustrated in FIG. 7.  
         [0049]    ICMP, which is part of an IP layer, presents an error message and information on a predetermined state that needs to be watched carefully. In general, an ICMP message is used by the IP layer or a higher protocol.  
         [0050]    A DIT information requesting message  700  includes a type field  710  which indicates a certain type of ICMP message, a code field  720  which may have different values for certain types of status, a checksum field  730 , an identifier field  740  which is provided for differentiating the DIT information requesting message  700  from other messages, and a reservation field  750 .  
         [0051]    The standards regarding the ICMP messages define  15  different types of ICMP messages using the type field  710 . In the present invention, the type field  710  is set to an arbitrary value that is currently not used in the standards in order to differentiate the DIT information requesting message  700  from other ICMP messages.  
         [0052]    An access router which receives the DIT information requesting message  700  searches its DIT information and sends requested DIT information to the mobile node  400  in step S 620 .  
         [0053]    An example of a DIT information response message created by the access router to send the requested DIT information to the mobile node  400  is illustrated in FIG. 8.  
         [0054]    A DIT information response message can be an IPv6 ICMP message. As shown in FIG. 8, a DIT information response message  800  includes a type field  805 , a code field  810 , a checksum field  815 , an identifier field  820 , a number field  825  which indicates the number of pieces of access router information to be contained in the DIT information response message  800 , and a reservation field  830 . In addition, the DIT information response message  800  contains a BSSID field  835 , a prefix length field  840 , a reservation field  845 , and a first access router&#39;s global address field  850  as pieces of information on a first access router. The DIT information response message  800  also contains a BSSID field  855 , a prefix length field  860 , a reservation field  865 , and an n-th access router&#39;s global address field  870  as pieces of information on an n-th access router. The type field  805  is set to an arbitrary value that is currently not used in the standards in order to differentiate the DIT information response message  800  from other ICMP messages.  
         [0055]    Thereafter, in step S 630 , the mobile node  400  stores DIT information, prefix information, and address information received from an access router in the DIT information storage unit  440 .  
         [0056]    Thereafter, if an L2 trigger occurs in the mobile node  400 , the beacon scanning unit  420  receives a beacon message from a new access router (NAR) and identifies BSSID information of the NAR in step S 640 .  
         [0057]    In step S 650 , the mobile node  400  obtains prefix information ( 442 ) of the NAR by searching the DIT information stored in the DIT information storage unit  440  with reference to the identified BSSID information of the NAR.  
         [0058]    In step S 660 , the binding update processing unit  430  creates a care-of-address using the prefix information of the NAR. In other words, the mobile node  400  creates the care-of-address using its own MAC address and the prefix information of the NAR among pieces of the DIT information that have already been stored in the DIT information storage unit  440  before the L2 trigger occurs.  
         [0059]    In step S 670 , the mobile node  400  transmits a binding update message to the PAR using the created care-of-address.  
         [0060]    Then, the PAR responds to the reception of the binding update message by sending an ACK message to the mobile node  400  and transmits a handover initiation message to the NAR. In response to the reception of the handover initiation message, the NAR transmits an ACK message to the PAR.  
         [0061]    According to the present invention, it is possible to reduce handover latency by obtaining information on a new access router when a mobile node is booted.  
         [0062]    Although the exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.