Abstract:
In a Multi-Protocol Label Switching (MPLS) network and a method of applying a mobile IP to the MPLS network, the method includes: performing label assignment using a label-tunnel configured by doubly stacking a label between a first edge router and a second edge router upon a Mobile Node (MN) moving from a position of the first edge router to a position of the second edge router; and including label mapping information based on the assigned label in a registration request message in the second edge router, and transmitting the included label mapping information from the second edge router to the first edge router.

Description:
CLAIM OF PRIORITY  
       [0001]     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for MPLS NETWORK AND METHOD FOR APPLYING MOBILE IP TO MPLS NETWORK earlier filed in the Korean Intellectual Property Office on the 14 of Sep. 2005 and there duly assigned Ser. No. 10-2005-0085922.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a multi-protocol label switching (MPLS) network and a method for applying mobile Internet Protocol (IP) to the MPLS network, and more specifically, to an MPLS network and a method for applying mobile IP to the MPLS network that reduce a number of entries of a label forwarding information base (LFIB) and a routing look-up process using a label stack in an MPLS-based network.  
         [0004]     2. Description of the Related Art  
         [0005]     In a communication mode of an Internet protocol (IP)-based network, routing for transmitting a packet with reference to a destination address included in the packet is performed. A host address is assigned to a fixed point in the network. If a destination of the packet is a Mobile Node (MN), a new IP address should be assigned every time at each connection point changing upon movement. In a transmission control protocol (TCP) hierarchy, a connection set with the new IP address assigned means a new connection, and therefore mobility is not supported.  
         [0006]     Mobile IP is a protocol for solving the drawback of not supporting an Internet service in a mobile host, and is a technology for, when a user moves from one network to another, maintaining a connection of his/her assigned IP address with an IP network as is. For this, when the mobile IP is used, a MN has two IP addresses. One of the two IP addresses is a home address, which is a unique inherent identification address for identifying the MN. The other address is a Care-of-Address (CoA), which is an IP address used as a forwarding address when the MN accesses an external network, and changes at each new connection point.  
         [0007]     An exemplary wireless network using mobile IP includes a Mobile Node (MN), a Correspondent Node (CN), a Home Agent (HA), and a Foreign Agent (FA).  
         [0008]     The MN is a host, which moves by changing an accessed network, and the CN is for communicating with the MN. The HA accesses a home network of the MN, and the FA is connected to a network to which the MN is currently accessing outside of the home network.  
         [0009]     For example, a home address assigned to the MN is 10.10.10.2, and a CoA is 20.20.20.1 which is identical to an IP address of the FA. Assume that the MN is moving from an area of the HA to an area of the FA. The HA registers and manages the home address and the CoA of the MN in a format of the mobility binding table.  
         [0010]     In communication of the MN with the CN, the CN, aware of the home address of the MN, encapsulates data to be transmitted to the MN using home address information of the MN. The encapsulated data is transmitted to the HA of the MN depending on the home address information. The HA once again encapsulates reception data using the stored CoA of the MN, and then transmits the encapsulated data to the MN.  
         [0011]     The operation flow of the mobile IP when a terminal moves is as follows. The FA, positioned in an area where the MN moves to, performs agent advertisement and leads registration of the MN. If it is determined that the MN receiving the agent advertisement is in an external network, the MN transmits a registration request to the HA through the FA. The HA receives the registration request and stores a position of the MN, and then transmits a registration reply to the FA. After the FA receives the registration reply from the HA and creates a visitor list, the FA transmits the registration reply to the MN.  
         [0012]     When transmitting the packet to the MN, the CN transmits the packet to the HA along a general routing path. The HA receives the packet from the CN and tunnels the packet to the CoA of the MN with reference to a binding list including a position of the MN. The FA receives the tunneled packet, de-tunnels a corresponding packet, and transmits the corresponding packet to the MN with reference to the visitor list.  
         [0013]     The above routing of the Internet network is based on checking a header of each IP packet, determining a next hop, and transmitting the packet to the next hop. In this method, a header must be checked at each packet as well as in all routers within a routing path. Therefore, traffic processing is inefficient. Unlike a transmission type router, in MPLS, the packet is transmitted using a short and fixed-length label, without passing through three hierarchies.  
         [0014]     The MPLS mainly relating to packet forwarding is a combination of simplicity of an IP routing and capability of high-speed switching of an Asynchronous Transfer Mode (ATM). In an MPLS network, a packet having a short-length label is transmitted through a path generally called a Label Switching Path (LSP), thereby simplifying packet transmission and making it possible to control traffic flow through traffic engineering.  
         [0015]     In the MPLS, a Forwarding Equivalence Class (FEC) is classified with a key of the same destination IP address on the basis of a forwarding table created by a routing protocol, and the same label is assigned to a routing entry belonging to the same FEC so that packets having the same destination can have the same label and be transmitted to a destination at a high speed using label exchange.  
         [0016]     A connection structure of the MPLS network is comprised of an end system performing the function of a router and a Label Switching Router (LSR) and can be classified as an edge LSR (that is, a Label Edge Router (LER)) positioned at a contact point with a given network and a center LSR positioned within a corresponding MPLS network. The LSP is set in an edge LSR of a corresponding LSP. IP tunneling puts an IP datagram inside an IP datagram and can surround and redirect a datagram previously forwarded to one IP address, to another IP address.  
         [0017]     In such a network, one packet is mapped to one FEC at each router, whereas in the MPLS, this mapping operation is performed only in an ingress router of an MPLS domain. In the FEC for setting the LSP in the MPLS for the IP network, there is a method of determining all IP prefixes with the same egress router which is the destination by one FEC, and a method of determining each IP destination address field of a routing table by the FEC.  
         [0018]     A general flow of setting the LSP in the MPLS network is as follows. When a plurality of MNs are positioned in an area of the HA, an LSP from an LERI, which is the edge router using the host FEC, to an LER 2  is set and a Label Forwarding Information Base (LFIB) is set in each router. The HA and the FA are operated as edge routers. The MNs , having addresses of 1.1.1.2, 1.1.1.3, and 1.1.1.4, are positioned in the LER 2  which is the HA.  
         [0019]     In each router, the LSP is set as an MN having a prefix of 32 bits. This is to manage an LFIB entry for each MN when the MN moves. The LFIB is a table of which method a frame is to be transmitted for a label value created by the LSR performing a function of label switching.  
         [0020]     When the three MNs are registered in the LER 2 , three labels are required in each router. This means that, when there are N MNs, N labels are required in each router.  
         [0021]     A packet flow is as follows. When the MN is not out of the HA area, the label is no longer assigned for the address of 1.1.1.3 in the LFIB table of the LER 2 . In contrast, when the MN moves from the HA to FA, a label (L 8 ) is again assigned for the address of 1.1.1.3 (address of MN), and the packet is transmitted to a LER 3  through a middle router.  
         [0022]     The middle router positioned between the LER and the LER 3  attaches a label (L 7 ) to the packet input with the label (L 8 ) attached, and transmits the packet to the LER 3 . The LER 3  receives the packet, pops out the label (L 8 ) from the received packet, performs IP routing, and transmits the packet to the MN.  
         [0023]     An operation flow for setting the LSP between agents when the MN moves is as follows. The MN moves from an area of the LER 2  to an area of the LER 3 , receives the agent advertisement from the FA, and transmits a registration request message to the HA through the FA, thereby informing a new FA of its movement. The HA receives the registration request message, and requests the FA to set the LSP to a Label Distribution Protocol (LDP). The FEC is a CoA IP address of the MN, that is, the FA.  
         [0024]     When the LDP of the LER 3  receives the label request message, the LDP transmits a label mapping message to the LER 2 . The LER 2  pops out an out label for FEC 1.1.1.3 of its own LFIB table, and changes the out label to have a number of the assigned label. The assigned label will be the label (L 8 ). In this manner, the LER 2  transmits the packet to the LER 3  through MPLS packet forwarding for FEC 1.1.1.3.  
         [0025]     Summarizing the procedure above, it can be appreciated that four steps of registration request, label request, label mapping, and registration reply are performed.  
         [0026]     When the LSP is set using the host FEC and the MN moves, only the table of the LFIB changes, thereby making it possible to transmit the packet from the HA to the FA.  
         [0027]     However, if the LSP is set using the host FEC (32 bits of prefix length), the LFIB table entry is increased in the middle routers positioned between the edge routers, thereby causing a drawback in extension. In order to overcome this drawback, a method of setting the LSP using a prefix FEC, not the host FEC, is used. However, even in this method, the edge router requires a procedure of referring to the LFIB table and a Routing Information Base (RIB), and therefore there is a drawback of increased time taken for packet forwarding.  
       SUMMARY OF THE INVENTION  
       [0028]     It is, therefore, an objective of the present invention to provide a Multi-Protocol Label Switching (MPLS) network and a method of applying a mobile Internet Protocol (IP) in the MPLS network that sets an additional label-tunnel in addition to an in label between edge routers, thereby performing packet transmission between an MN and a correspondent node.  
         [0029]     According to one aspect of the present invention, a method of applying a mobile IP to an MPLS network is provided, the method including: performing label assignment using a label-tunnel configured by doubly stacking a label between a first edge router and a second edge router upon a Mobile Node (MN) moving from a position of the first edge router to a position of the second edge router; and including label mapping information based on the assigned label in a registration request message in the second edge router, and transmitting the included label mapping information from the second edge router to the first edge router.  
         [0030]     The label-tunnel is preferably configured by doubly stacking an in label assigned at each host Forwarding Equivalence Class (FEC) and a tunnel label assigned for a prefix FEC.  
         [0031]     The label mapping information preferably includes at least one of a prefix length, information on a FEC for assigning the label, and information on the label assigned to the FEC.  
         [0032]     The label assignment preferably includes: transmitting the registration request message to the second edge router from the MN; and receiving the registration request message and performing a Label Distribution Protocol (LDP) label assignment in the second edge router.  
         [0033]     The method preferably further includes: receiving the label mapping information and configuring a label forwarding table in the first edge router; normally completing configuration of the label forwarding table in the first edge router, and transmitting a registration reply message from the first edge router to the second edge router; and transmitting the registration reply message from the second edge router to the MN.  
         [0034]     The label forwarding table is preferably configured using at least one of an IP address of the MN, an IP address of the second edge router, and the label mapping information.  
         [0035]     The method preferably further includes: performing the label assignment based on the label-tunnel and setting a label switching path between the first edge router and a third edge router where a correspondent node communicating with the MN is positioned; and transmitting a packet from the correspondent node to the MN through the label switching path set between the first and second edge routers, and between the second and third edge routers.  
         [0036]     Transmitting a packet from the correspondent node to the MN preferably includes: receiving the packet from the first edge router in a middle router positioned one hop before the second edge router, popping out a tunnel label of a corresponding packet, and transmitting the corresponding packet to the second edge router; and referring to an in label included in the received packet in the second edge router and transmitting the packet from the second edge router to the MN.  
         [0037]     According to another aspect of the present invention, a method of applying mobile Internet Protocol (IP) to a Multi-Protocol Label Switching (MPLS) network is provided, the method including: performing label assignment using a label-tunnel configured by doubly stacking a label between a first edge router and a second edge router upon a Mobile Node (MN) moving from a position of the first edge router to a position of the second edge router; including label mapping information based on the assigned label in a registration request message in the second edge router, and transmitting the included label mapping information from the second edge router to the first edge router; receiving the label mapping information and configuring a label forwarding table in the first edge router; normally completing configuration of the label forwarding table in the first edge router and transmitting a registration reply message from the first edge router to the second edge router; and transmitting the registration reply message from the second edge router to the MN.  
         [0038]     The label-tunnel is preferably configured by doubly stacking an in label assigned at each host Forwarding Equivalence Class (FEC) and a tunnel label assigned for a prefix FEC.  
         [0039]     The label mapping information preferably includes at least one of a prefix length, FEC information for assigning the label, and information on the label assigned to the FEC.  
         [0040]     The label forwarding table is preferably configured using at least one of an IP address of the MN, an IP address of the second edge router, and the label mapping information.  
         [0041]     The method preferably further includes: performing the label assignment based on the label-tunnel and setting a label switching path between the first edge router and a third edge router where a correspondent node communicating with the MN is positioned; receiving the packet from the first edge router in a middle router positioned one hop before the second edge router, popping out a tunnel label of a corresponding packet, and transmitting the corresponding packet to the second edge router; and referring to an in label included in the received packet in the second edge router and transmitting the packet from the second edge router to the MN.  
         [0042]     According to still another aspect of the present invention, a Multi-Protocol Label Switching (MPLS) network is provided including: a Mobile Node (MN); a first edge router adapted to receive a registration request message from the MN, to perform label assignment using a label-tunnel configured by doubly stacking a label, and to include and transmit label mapping information based on the assigned label in the registration request message upon the MN moving within its own service area; and a second edge router adapted to receive the registration request message from the first edge router, to configure a label forwarding table based on the label mapping information included in the received registration request message, and to transmit a registration reply message to the first edge router upon the MN moving out of its own service area.  
         [0043]     The label-tunnel is preferably configured by doubly stacking an in label assigned at each host Forwarding Equivalence Class (FEC) and a tunnel label assigned for a prefix FEC.  
         [0044]     The label mapping information preferably includes at least one of a prefix length, FEC information for assigning the label, and information on the label assigned to the FEC.  
         [0045]     The label forwarding table is preferably configured using at least one of an IP address of the MN, an IP address of the second edge router, and the label mapping information. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]     A more complete appreciation of the present invention and many of the attendant advantages thereof, will be readily apparent as the present invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:  
         [0047]      FIG. 1  is an example of a wireless network using mobile Internet Protocol (IP);  
         [0048]      FIG. 2  is an operation flow of mobile IP when a terminal moves;  
         [0049]      FIG. 3  is a general flow of setting a Label Switching Path (LSP) in a multi-protocol label switching (MPLS) network;  
         [0050]      FIG. 4  is a packet flow when a Mobile Node (MN) moves in an MPLS network;  
         [0051]      FIG. 5  is an operation flow of setting an LSP between agents when an MN moves;  
         [0052]      FIG. 6  is a flow of setting an LSP using a prefix Forwarding Equivalence Class (FEC) according to the present invention;  
         [0053]      FIG. 7  is a procedure of referring to a routing table in setting an LSP using a prefix FEC;  
         [0054]      FIG. 8  is the structure of label mapping information according to an embodiment of the present invention;  
         [0055]      FIG. 9  is an LSP structure for an MPLS-based mobile IP according to an embodiment of the present invention;  
         [0056]      FIG. 10  is a procedure of setting an LSP for an MPLS-based mobile IP according to an embodiment of the present invention;  
         [0057]      FIG. 11  is an LSP structure according to an embodiment of the present invention when an MN moves; and  
         [0058]      FIG. 12  is a procedure of distributing a label between a home agent (HA) and a Foreign Agent (FA) when an MN moves. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0059]      FIG. 1  is an example of a wireless network using mobile Internet Protocol (IP). The wireless network of  FIG. 1  includes a Mobile Node (MN)  10 , a Correspondent Node (CN)  20 , a Home Agent (HA)  31 , and a Foreign Agent (FA)  41 .  
         [0060]     The MN  10  is a host, which moves by changing an accessed network, and the CN  20  is for communicating with the MN. The HA  30  accesses a home network of the MN  10 , and the FA  40  is connected to a network to which the MN  10  is currently accessing outside of the home network.  
         [0061]     In  FIG. 1 , an HA assigned to the MN is 10.10.10.2, and a Care-of-Address (CoA) is 20.20.20.1 which is identical to an IP address of the FA  41 . In  FIG. 1 , the MN  10  is moving from an area of the HA  31  to an area of the FA  41 . The HA  31  registers and manages the home address and the CoA of the MN  10  in a format of the mobility binding table.  
         [0062]     In communication of the MN  10  with the CN  20 , the CN  20 , aware of the home address of the MN  10 , encapsulates data to be transmitted to the MN  10  using home address information of the MN  10 . The encapsulated data is transmitted to the HA  31  of the MN  10  depending on the home address information. The HA  31  once again encapsulates reception data using the stored CoA of the MN  10 , and then transmits the encapsulated data to the MN  10 .  
         [0063]      FIG. 2  is an operation flow of the mobile IP when a terminal moves. The FA  41 , positioned in an area where the MN moves to, performs agent advertisement and leads registration of the MN  10  (Step  201 ). If it is determined that the MN  10  receiving the agent advertisement is in an external network, the MN  10  transmits a registration request to the HA  31  through the FA  41  (Steps  202  and  203 ). The HA  31  receives the registration request and stores a position of the MN  10 , and then transmits a registration reply to the FA  41  (Step  204 ). After the FA  41  receives the registration reply from the HA  31  and creates a visitor list, the FA  41  transmits the registration reply to the MN  10  (Step  205 ).  
         [0064]     When transmitting the packet to the MN  10 , the CN  20  transmits the packet to the HA  31  along a general routing path. The HA  31  receives the packet from the CN  20  and tunnels the packet to the CoA of the MN  10  with reference to a binding list including a position of the MN  10 . The FA  41  receives the tunneled packet, de-tunnels a corresponding packet, and transmits the corresponding packet to the MN  10  with reference to the visitor list.  
         [0065]     The above routing of the Internet network is based on checking a header of each IP packet, determining a next hop, and transmitting the packet to the next hop. In this method, a header must be checked at each packet as well as in all routers within a routing path. Therefore, traffic processing is inefficient. Unlike a transmission type router, in MPLS, the packet is transmitted using a short and fixed-length label, without passing through three hierarchies.  
         [0066]     The MPLS mainly relating to packet forwarding is a combination of simplicity of an IP routing and capability of high-speed switching of an Asynchronous Transfer Mode (ATM). In an MPLS network, a packet having a short-length label is transmitted through a path generally called a Label Switching Path (LSP), thereby simplifying packet transmission and making it possible to control traffic flow through traffic engineering.  
         [0067]     In the MPLS, a Forwarding Equivalence Class (FEC) is classified with a key of the same destination IP address on the basis of a forwarding table created by a routing protocol, and the same label is assigned to a routing entry belonging to the same FEC so that packets having the same destination can have the same label and be transmitted to a destination at a high speed using label exchange.  
         [0068]     A connection structure of the MPLS network is comprised of an end system performing the function of a router and a Label Switching Router (LSR), and can be classified as an edge LSR (that is, a Label Edge Router (LER)) positioned at a contact point with a given network and a center LSR positioned within a corresponding MPLS network. The LSP is set in an edge LSR of a corresponding LSP. IP tunneling puts an IP datagram inside an IP datagram and can surround and redirect a datagram previously forwarded to one IP address, to another IP address.  
         [0069]     In such a network, one packet is mapped to one FEC at each router, whereas in the MPLS, this mapping operation is performed only in an ingress router of an MPLS domain. In the FEC for setting the LSP in the MPLS for the IP network, there is a method of determining all IP prefixes with the same egress router which is the destination by one FEC, and a method of determining each IP destination address field of a routing table by the FEC.  
         [0070]      FIG. 3  is a general flow of setting the LSP in the MPLS network.  FIG. 3  shows that, when a plurality of MNs are positioned in an area of the HA  10 , an LSP from an LERI  50 , which is the edge router using the host FEC, to an LER 2   30  is set and a Label Forwarding Information Base (LFIB) is set in each router. The HA and the FA are operated as edge routers. The MNs  10 - 1 ,  10 - 2 , and  10 - 3 , having addresses of 1.1.1.2, 1.1.1.3, and 1.1.1.4, are positioned in the LER 2   30  which is the HA.  
         [0071]     In each router, the LSP is set as an MN having a prefix of 32 bits. This is to manage an LFIB entry for each MN when the MN moves. The LFIB is a table of which method a frame is to be transmitted for a label value created by the LSR performing a function of label switching.  
         [0072]     As confirmed in  FIG. 3 , when the three MNs  10 - 1 ,  10 - 2 , and  10 - 3  are registered in the LER 2   30 , three labels are required in each router. This means that, when there are N MNs, N labels are required in each router.  
         [0073]      FIG. 4  is a packet flow when the MN moves in the MPLS network. That is,  FIG. 4  shows the procedure of transmitting the LFIB and the packet in each LSR when the MN  10  having the address of 1.1.1.3 described in  FIG. 3  moves from the HA area to the FA area.  
         [0074]     In  FIG. 3  when the MN  10  is not out of the HA area, the label is no longer assigned for the address of 1.1.1.3 in the LFIB table of the LER 2 . In contrast, in  FIG. 4 , it can be confirmed that a label (L 8 ) is again assigned for the address of 1.1.1.3 (address of MN), and the packet is transmitted to a LER 3   40  through a middle router.  
         [0075]     The middle router positioned between the LER 2   30  and the LER 3   40  attaches a label (L 7 ) to the packet input with the label (L 8 ) attached, and transmits the packet to the LER 3   40 . The LER 3   40  receives the packet, pops out the label (L 8 ) from the received packet, performs IP routing, and transmits the packet to the MN  10 .  
         [0076]      FIG. 5  is an operation flow for setting the LSP between agents when the MN moves. The MN  10  moves from an area of the LER 2   30  to an area of the LER 3   40 , receives the agent advertisement from the FA  30  (Step  501 ), and transmits a registration request message to the HA  31  through the FA  41  (Step  502 ), thereby informing a new FA of its movement (Step  503 ). The HA  31  receives the registration request message, and requests the FA  41  to set the LSP to a Label Distribution Protocol (LDP)  32  (Step  504 ). The FEC is a CoA IP address of the MN, that is, the FA  41 .  
         [0077]     When the LDP  42  of the LER 3   40  receives the label request message (Step  505 ), the LDP  42  transmits a label mapping message to the LER 2   30  (Step  506 ). The LER 2   30  pops out an out label for FEC 1.1.1.3 of its own LFIB table, and changes the out label to have a number of the assigned label. In matching with  FIG. 4 , the assigned label will be the label (L 8 ). In this manner, the LER 2   30  transmits the packet to the LER 3   40  through MPLS packet forwarding for FEC 1.1.1.3.  
         [0078]     As described above,  FIG. 5  shows the procedure of setting the LSP from the HA to the FA when the MN moves. Summarizing the procedure of  FIG. 5 , it can be appreciated that four steps of registration request, label request, label mapping, and registration reply are performed.  
         [0079]     As described in  FIGS. 4 and 5 , when the LSP is set using the host FEC and the MN moves, only the table of the LFIB changes, thereby making it possible to transmit the packet from the HA to the FA.  
         [0080]     However, if the LSP is set using the host FEC (32 bits of prefix length), the LFIB table entry is increased in the middle routers positioned between the edge routers, thereby causing a drawback in extension. In order to overcome this drawback, a method of setting the LSP using a prefix FEC, not the host FEC, is used. However, even in this method, the edge router requires a procedure of referring to the LFIB table and a Routing Information Base (RIB), and therefore there is a drawback of increased time taken for packet forwarding.  
         [0081]     The present invention is described below with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown.  
         [0082]     When Multi-Protocol Label Switching (MPLS) is embodied using a host Forwarding Equivalence Class (FEC), there is a drawback in that a Label Forwarding Information Base (LFIB) table entry increases in middle routers. In order to overcome this defect, a method using a prefix FEC is described as follows.  
         [0083]      FIG. 6  is a flow of setting a Label Switching Path (LSP) using a prefix FEC according to the present invention.  
         [0084]     In  FIG. 6 , each label switching router (LSR) (LER 1 , LER 2 , LER 3 , and middle router) creates the LFIB necessary for label switching, through a Label Distribution Protocol (LDP) operation. An input interface, an output interface, an in label, and an out label corresponding to each FEC constitute the LFIB.  
         [0085]     In a description where the FEC is 1.1.1.0 in an LFIB table of the LERI  60 , it can be appreciated that a prefix FEC is used. In other words, all Internet Protocols (IPs) corresponding to 1.1.1.x are assigned to one label (L 1 ). The middle router receives a packet with the label (L 1 ) attached, assigns a label (L 2 ) as the out label to the packet, and transmits the packet. If the LSP from the LER 1   60  to the LER 2   70  is set to 1.1.1.0/24 as described above, the LSPs on a path through which the LSP passes maintain one LFIB entry.  
         [0086]     When an MN  10  moves from an area of the LER 2   70  to an area of the LER 3   80 , the LER 2   70  refers to the LFIB table, pops out the label for the FEC having a destination address of 1.1.1.3, and transmits the FEC to an IP forwarding engine.  
         [0087]      FIG. 7  is the procedure of referring to the routing table in setting the LSP using the prefix FEC. That is,  FIG. 7  shows the details of the procedure of referring to the routing table in the LER 2   70  which is in charge of the function of the HA when the MN  10  moves as in  FIG. 6 .  
         [0088]     It is assumed that the MN  10  having the address of 1.1.1.3 moves from the area of the LER 2   70  to the area of the LER 3   80 . Referring to the LFIB table of  FIG. 7  managed by the LER  2   70 , it can be appreciated that the packet transmitted from a correspondent node  20  to the MN  10  is attached to the label  2  and input to the LER 2   70 . The LER 2   70  refers to the LFIB table, pops out the label for the FEC having the destination address of 1.1.1.3, and transmits the FEC to the IP forwarding engine. The IP forwarding engine recognizes that a next hop for FEC 1.1.1.3 is the LER 3   80  having an address of 2.2.2.1 through a Routing Information Base (RIB), and transmits corresponding information to an MPLS forwarding engine. The MPLS forwarding engine searches for the LFIB entry being forwarded to the address of 2.2.2.1, recognizes that the out label is a label (L 4 ), attaches the packet being forwarded to the address of 1.1.1.3 to the label (L 4 ) and transmits the packet with the label attached.  
         [0089]     When the LSP is set to the prefix FEC as mentioned above, there is an advantage in that the LSPs on the LSP maintain only one LFIB entry, whereas there is a disadvantage of performing a process of referring to the table several times, thereby delaying packet forwarding as long as a time taken to refer to the table.  
         [0090]     An MPLS domain can form a hierarchical structure according to need. For this, each packet can include two or more labels using a structure called a label stack. Accordingly, when the label is encoded, a newly encoded label is inserted using a push function into an uppermost position of the label stack. When out of a corresponding hierarchy, one label is eliminated using a pop function from the uppermost position of the label stack. The corresponding packet is transmitted on the basis of the label at the uppermost position of the label stack. Each label is encoded by a total of 32 bits of which 20 bits substantially become the label. In the present invention, the LSP is set through the label-tunnel using the above label stack.  
         [0091]      FIG. 8  is the structure of label mapping information according to the present invention. The label mapping information of  FIG. 8  is added to a registration request message of an extended type.  
         [0092]     A “Type” field represents the extension type of the registration request message. Here, the “type” field has a value of “35” representing the label mapping information. A “Prefix Length” field represents a length of a prefix, and a “Prefix” field represents information on the FEC for assigning the label. Furthermore, a “Label” field includes information on the label assigned to the FEC.  
         [0093]      FIG. 9  is an LSP structure for MPLS-based mobile IP according to the present invention. According to the present invention, as in  FIG. 9 , label-tunnel LSPs are set between the LER 1   60  and the LER 2   70 , and between the LER 2   70  and the LER 3   80 .  
         [0094]     In a description of the LFIB of the LER 1   60 , it can be appreciated that two labels are used. At each FEC, the in labels (IL 1 , IL 2 , and IL 3 ) are set, and another tunnel-label (T-label) is separately set. These can be regarded as a kind of label stacking. In the present invention, two label stacks are used. The separate in label is set in each FEC of the 1.1.1.2, 1.1.1.3, and 1.1.1.4, but the tunnel labels are all set to be the same. This means that the packets having the three different FECs are all transmitted through the same tunnel.  
         [0095]     End points of each set tunnel respectively become the IP address of the LER 2   70  and the IP address of the LER 3   80 . The LER 2   70  and the LER 3   80  transmit the label mapping information of “implicit NULL” to the router positioned at its front, for “Penultimate Hop Popping”, which is a method for popping out the label in the router just before of an egress edge router and is intended to reduce a loop-up work for the packet.  
         [0096]     For this, “implicit NULL” is distributed by an LDP of the egress edge router. The middle router to which the label “implicit NULL” is assigned by the egress edge router pops out the label directly itself and transmits the packet to a next-positioned edge router.  
         [0097]     In  FIG. 9 , it can be confirmed that the out label of the middle router positioned between the LER 1   60  and the LER 2   70  is “IMP_NULL”. The LER 2   70  receives the packet from the middle router, pops out the corresponding label from the packet, and performs IP forwarding through the look-up process.  
         [0098]      FIG. 10  is the procedure of setting the LSP for the MPLS-based mobile IP according to the present invention. That is,  FIG. 10  shows the procedure of setting the LSP between the LER 1   60  which is the ingress router and the LER 2   70  which is the egress router when the MN  10  is positioned in the area of the LER 2 . 70 .  
         [0099]     The MN  10  receives the agent advertisement from the HA  71  (Step  1001 ), and transmits a valid registration request message to the HA  71  (Step  1002 ). In response, the HA  71  transmits a label mapping request message to the LDP  72  (Step  1003 ). The registration request message has the mapping information added as in  FIG. 8 .  
         [0100]     The LDP  72  receives the label mapping request message from the HA  71 , and requests to transmit the label mapping message for the host FEC of the MN  10  to the LER 1   60  where the correspondent node  20  is positioned (Step  1004 ). In this step, an LDP session is set using a target peer and the label is distributed between the LER 1   60  and the LER 2   70 . The LDP  72  receives label information assigned to the corresponding MN  10  for the mapping message transmitted with the target peer, and generates an entry of the LFIB table of the LER 2   70 . Upon completion of setting the LSP, the HA  71  transmits a registration reply message to the MN  10  (Step  1005 ).  
         [0101]      FIG. 11  is an LSP structure according to the present invention when the MN moves. That is,  FIG. 11  shows how the LSP structure of  FIG. 9  is changed when the MN  10  moves from the area of the LER 2   70  to the area of the LER 3   80 .  
         [0102]     When the MN  10  moves, the LFIB of the LER 2   70  performing the function of HA changes. As shown in  FIG. 11 , it can be confirmed that, for the packet attached to the in label (IL 3 ) assigned to the MN  10  and input, the out label is set as IL 4 , and an out T-label is set as L 4 . In other words, a tunnel is set between the LER 2   70  and the LER 3   80 .  
         [0103]     In a description of the LFIB of the LER 2   70 , it can be appreciated that the packet attached to the label IL 3  and input is attached to IL 4  as the out label, with L 4  as the out T-label, and is output through the output interface  2 . The LER 3   80  receives the packet with a destination being the MN  10  having the address of 1.1.1.3, through the tunnel set between the LER 2   70  and the LER 3   80 , and transmits the corresponding packet to the MN  10  through the IP routing for the received packet.  
         [0104]      FIG. 12  is the procedure of distributing the label between the HA  71  and the FA  81  when the MN  10  moves. The MN  10  moved to the area of the LER 3   80  receives the agent advertisement from the FA  81  (Step  1201 ), and transmits the registration request message (S 1202 ) to the FA  81 . The FA  81  receives the registration request message and transmits a label assignment request message to the LDP  82  (Step  1203 ). The LDP  82  is requested for label assignment from the FA  81 , and assigns the label and transmits a label assignment message to the FA  81  (Step  1204 ).  
         [0105]     The FA  81  receives the label assignment message and transmits the registration request message to the HA  71 . The registration request message is transmitted with the label mapping information assigned from the LDP  82  and added (Step  1205 ). The registration request message includes a Care-of-Address (CoA). This should be identical with the end point of the set tunnel LSP from the LER 2   70  to the LER 3   80 , that is, with the IP address of the FA  81 . The label mapping information has the format shown in  FIG. 8 .  
         [0106]     The HA  71  receives the registration request message including the label mapping information, and sets the LFIB of the LER 2   80 , using the IP address, the CoA, and the label mapping information of the MN  10 . The setting of the LFIB is performed with the HA  71  providing the label mapping information to the LDP  72  (Step  1206 ). The LDP  72  sets the in label and the out label of the LFIB, using the FEC, the label information, and the CoA provided from the HA  71 .  
         [0107]     Upon setting of the LFIB, the HA  71  transmits the registration reply message to the FA  81  (Step  1207 ), and the FA  81  receives the registration reply message, transmits a label reply message to the LDP  72  informing that the label is normally mapped (Step  1208 ), and transmits the registration reply message to the MN  10  (Step  1209 ).  
         [0108]     Comparing the procedure of  FIG. 12  with the procedure of  FIG. 5 , it can be appreciated that the greatest difference is in a registration request process to a registration reply process from the FA to the HA. In  FIG. 5 , the registration request, label request, label mapping, and registration reply processes are performed, whereas in  FIG. 12 , only the registration request and registration reply processes including the label mapping information are performed. This means that, according to the present invention, the time taken to set the LSP can be reduced considerably.  
         [0109]     According to the present invention, through the setting of the LSP using the label-tunnel between the edge routers, the number of entries and the routing look-up process are reduced. Also, the steps of transmitting and receiving the message for registration and label assignment are reduced so that network overhead is reduced and packet transmission efficiency increased.  
         [0110]     While the present invention has been described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various modifications in form and detail can be made therein without departing from the spirit and scope of the present invention as defined by the following claims.