Abstract:
Disclosed is a method for controlling a routing path for a voice-over-internet protocol service based on a mobile internet protocol and a session initiation protocol, the method including the steps of sensing a mobile terminals entry into a different network and performing a mobile internet protocol re-registration; the mobile terminal transmitting a register message, which includes a Foreign Agent Care-of Address and a home address of the mobile terminal, to a session initiation protocol server after performing the mobile internet protocol re-registration through a foreign agent, thereby carrying out a session initiation protocol registration; the session initiation protocol server creating a tunnel between the session initiation protocol server and the foreign agent with reference to the register message when receiving an invite message from a correspondent terminal after the session initiation protocol registration is carried out, and transmitting the invite message to the mobile terminal through the tunnel.

Description:
PRIORITY  
       [0001]     This application claims priority to an application entitled “Mobile Terminal, Session Initiation Protocol Server, And Method Of Controlling Routing Path For Voice-Over-Internet Protocol Service, Based On Mobile Internet Protocol, Voice-Over-Internet Protocol, And Session Initiation Protocol” filed in the Korean Intellectual Property Office on Jun. 10, 2004 and assigned Serial No. 2004-42451, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a mobile communication system, and more particularly to a mobile terminal, a session initiation protocol server, and a method for controlling a routing path, which are based on a Mobile Internet protocol (hereinafter, referred to as a “mobile IP”), a Voice-over-Internet Protocol (hereinafter, referred to as a “VoIP”), and a Session Initiation Protocol (hereinafter, referred to as an “SIP”).  
         [0004]     2. Description of the Related Art  
         [0005]     With the development of mobile communication technologies, techniques have been proposed for assigning IP addresses to personal mobile terminals, so that their users can use the Internet while moving.  
         [0006]     A mobile IP is a technique for indicating mobility in Internet Protocol layers. The mobile IP enables users of mobile terminals to use the Internet without disruption of service while moving, by using protocols of various subscriber networks. The protocols of subscriber networks include, for example, xDSLs (Digital Subscriber Line and its variations), a WLAN (Wireless Local Area Network), a CDMA (Code-Division Multiple Access) 2000-1x, GPRS (General Packet Radio Services), and a WCDMA (wideband CDMA). The mobile IP combined with a VoIP enables mobile terminals to communicate through voice phones with subscribers of a public switched telephone network (PSTN), such as a local/toll telephone network. Also, the mobile IP combined with an H.323 video communication protocol enables mobile terminals to receive a mobile video phone service.  
         [0007]     The SIP is a control protocol of an application layer used for establishing, modifying, and terminating a session or a call for multimedia communication, including voice and video.  
         [0008]     An SIP user agent represents an SIP-based mobile terminal or an SIP-based mobile node (MN). An SIP server includes a proxy server, a location server, a redirect server, and a registra server, because these servers are physically located on the same platform.  
         [0009]     The SIP user agent registers an IP address of a current location together with its own specific URI (address-of-record) in the registra server at an initiating step, and a correspondent terminal, or a correspondent node (CN), tries to establish a session connection with reference to an IP address of a corresponding mobile terminal registered in the registra server. Providing mobility by the SIP as described above enables a mobile terminal to perform communication with another mobile terminal or a new access point without changing its own SIP URI.  
         [0010]     However, when a mobile terminal moves into a different network in a call-connected state, the mobile terminal has to receive a new IP so as to receive a voice packet in a new area, which causes a call to be disconnected. That is, the SIP supports mobility in an application level, but does not support mobility in a network level. However, when a mobile terminal uses the mobile IP together with SIP, the mobile terminal can maintain a call without disconnection even while moving. In other words, when a VoIP service using the SIP is performed, if the mobile IP is associated therewith, it is possible to maintain an SIP session without disconnection while a mobile terminal moves between different networks.  
         [0011]     However, in the case of IPv4, when a mobile IP is used, a packet received by a mobile terminal has to pass through a home agent (HA), so that a triangular routing path is established. As a result, since a first path, of packets received by the mobile terminal, and a second path, of packets transmitted from the mobile terminal, are different from each other, callers may notice a difference in voice quality. Moreover, as the first path of packets received via the home agent lengthens, there is a commensurate deterioration of the quality of service (QoS) caused by such factors as packet loss, time delay, jitter, etc. Additionally, these and other factors cause an increase in the home agent&#39;s load. Meanwhile, before voice packets are exchanged between mobile terminals, a session through the SIP is established. In this case, when the SIP server operates as a proxy mode (a proxy server), the triangular routing problem is also caused between the mobile terminal and the SIP proxy server, as between the mobile terminal and the correspondent terminal.  FIG. 1  is a diagram illustrating a triangular routing problem in a VoIP service in a conventional mobile communication system based on a mobile IP, an SIP, and a VoIP. Reference numeral  123  indicates a triangular routing form for voice traffic, which is connected from a correspondent terminal (CN)  14  to a home agent  10 , from the home agent  10  to a foreign agent  12 , and from the foreign agent  12  to the correspondent terminal  14 .  
         [0012]     Recently, routers have used an ingress-filtering function when a source address is not correct from the viewpoint of a network topology. For example, in a network environment such as IPv4, there are some cases in which a reverse tunneling mode must be used. In this case, a path for an up-link and a path for a down-link are identical to each other, but data transmitted between two callers must pass through the home agent, so that their transmission pathways are unnecessarily lengthened. In addition, since traffic is concentrated at the home agent, the quality of communication deteriorates, and what is worse, the system may crash. Herein, the ingress-filtering is a function which is used when a source address of an IP packet is not correct from the viewpoint of a network topology. That is, according to the ingress-filtering function, a source address field of an IP header is checked, and a packet is removed if the packet is received from an address which is inappropriate for a corresponding link.  
         [0013]     A solution to the triangular-path routing problem discussed above is a routing optimization method that has been proposed for use with mobile IP. According to this method, the correspondent terminal  14  binds and stores a home address and a current location (care-of address; hereinafter, referred to as “CoA”) of the mobile terminal  16  (i.e., a mobile node) in a cache, so that the correspondent terminal  14  can perform the tunneling of a packet directly to the CoA of the mobile terminal, without passing through the home agent  10 . Herein, tunneling is a technique used to encapsulate a packet, which will be transmitted, with a header capable of being routed, and then to transmit the packet. The encapsulation is necessary because routing is impossible when a receiver address of the packet is not identical to a domain address format of an area in which the receiver is located. However, in this case, there is an additional burden in that a routing-optimized home agent  10  or a routing-optimized foreign agent  12  must be established and newly installed.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an apparatus and method for improving the communication quality of VoIP and for efficiently managing a network using an SIP server and a mobile terminal, by minimizing a load on the network and optimizing a routing path of traffic when mobile IP is used, as well as for achieving a VoIP service capable of maintaining a session during movement.  
         [0015]     To accomplish this object, in accordance with one aspect of the present invention, there is provided a method for controlling a routing path for a voice-over-internet protocol service based on a mobile internet protocol and a session initiation protocol, the method including the steps of a mobile terminal sensing movement of the mobile terminal into a different network and performing a mobile internet protocol re-registration; the mobile terminal transmitting a register message, which includes an FACoA (Foreign Agent Care-of Address) and a home address of the mobile terminal, to a session initiation protocol server after performing the mobile internet protocol re-registration through a foreign agent, thereby carrying out a session initiation protocol registration; the session initiation protocol server creating a first tunnel between the session initiation protocol server and the foreign agent with reference to the register message when receiving an invite message from a correspondent terminal after the session initiation protocol registration is carried out, and transmitting the invite message to the mobile terminal through the first tunnel; and the foreign agent creating a second tunnel between the foreign agent and the correspondent terminal when receiving a response from the mobile terminal having received the invite message, and the mobile terminal and the correspondent terminal exchanging voice packets with each other through the second tunnel.  
         [0016]     In accordance with another aspect of the present invention, there is provided a mobile terminal based on a mobile internet protocol, a voice-over-internet protocol, and a session initiation protocol, the mobile terminal including a sensing unit for sensing movement of the mobile terminal into a different network; a register-message generating unit for generating a register message including a home address and a CoA (Care-of-Address) when location movement of the mobile terminal is sensed; and a transmitting unit for transmitting the register message to a session initiation protocol server.  
         [0017]     In accordance with still another aspect of the present invention, there is provided a session initiation protocol server including a storage unit for storing a register message which includes a CoA and a home address transmitted from a first mobile terminal; a detecting unit for detecting the CoA of the first mobile terminal in the storage unit, in response to an invite message for the first mobile terminal transmitted from a second mobile terminal; and a transmitting unit for transmitting the invite message to the first mobile terminal according to the CoA of the first mobile terminal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0019]      FIG. 1  is a diagram which illustrates a triangular routing problem in a VoIP service in the conventional mobile communication system based on a mobile IP, an SIP, and a VoIP;  
         [0020]      FIG. 2  is a table illustrating a construction of an SIP register message in a visitor network according to a first embodiment of the present invention;  
         [0021]      FIG. 3  is a flow diagram for explaining a method of controlling a routing path for a VoIP service according to a second embodiment of the present invention, with respect to an example using an FACoA;  
         [0022]      FIG. 4  is a flow diagram for explaining a method of controlling a routing path for a VoIP service according to a third embodiment of the present invention, with respect to an example using a Collocated CoA (C-CoA);  
         [0023]      FIG. 5  is a flow diagram showing a signaling sequence in which the mobile terminal notifies the correspondent terminal of a changed FACoA of the mobile terminal, when the mobile terminal moves from a network to another network during a session, according to a fourth embodiment of the present invention;  
         [0024]      FIG. 6  is a flow diagram showing a signaling sequence, in which the mobile terminal notifies the correspondent terminal of a changed C-CoA of the mobile terminal, when the mobile terminal moves from a network to another network during a session, according to a fifth embodiment of the present invention;  
         [0025]      FIG. 7  is a detailed view for explaining the mobile IP re-registration and SIP registration process shown in  FIG. 3 ; and  
         [0026]      FIG. 8  is a detailed view for explaining a routing-path optimizing process of a VoIP traffic using the SIP shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]     Hereinafter, preferred embodiments according to the present invention will be described with reference to the accompanying drawings. Although a number of specific features, such as a domain name, an address, etc., are given below, they are presented for a better understanding of the present invention only. Also, it will be clear to those skilled in the art that the present invention can be practiced without such specific features. In the following description of the present invention, a detailed description (for example, operation of a mobile IP) of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.  
         [0028]     In the present invention, it is assumed that a mobile terminal (MN) having a mobile IP function is using a VoIP service utilizing an SIP in a different (visitor) network. Also, in order to achieve the methods and constructions of the present invention, the MN is equipped with a mobile IP and an SIP user agent, a home agent, a foreign agent, an SIP server (a proxy server, a registra server, and a location server), and a correspondent terminal CN for communicating with the MN are included.  
         [0029]      FIG. 2  is a table which shows a construction of an SIP register message in a visitor network according to a first embodiment of the present invention.  
         [0030]     The register message according to the present invention uses new parameters together with a contact header. In order to distinguish whether an address indicated by a contact header is a home address or a CoA (Care of Address), a new parameter “type” is defined. For example, “type=hoa” represents that an address indicated by a contact header is a home address, and “type=coa” represents that an address indicated by a contact header is a CoA. A parameter “expires” represents an available period of the register message, which is expressed in terms of minutes. For example, as shown in  FIG. 2 , “expires=30” indicates that the available registration period is 30 minutes. If the parameter “expires” exists in only a specific contact header, only information of the specific contact header expires when its available period is over. Therefore, it is necessary to send another register message for re-registration before expiration.  
         [0031]     As shown in  FIG. 2 , an address-of-record of a mobile terminal is “sip:foo@samsung.com”. Also, a home address is “sip:foo@165.213.230.200”, and a CoA is “sip:foo@165.213.137.195”. Therefore, the location server stores the home address and the CoA in the address-of-record of the mobile terminal, mapping the home address and the CoA.  
         [0032]     The location server, which is a type of database server, functions to reply to a registration or a query of an address-of-record, which is requested from the registra server or the proxy server. When the mobile terminal loads its own CoA (IP address) in a register message and sends it, the registra server receives the CoA and stores the CoA, together with the specific URI of the mobile terminal (which is called an address-of-record that means an URI registered in the location server), in the location server. When the proxy server receives an invite message from a first mobile terminal, the proxy server queries a location of a second mobile terminal, which is indicated by an URI included in the invite message, to the location server. Next, after establishing the location of the second mobile terminal, the proxy server transmits the invite message to the second mobile terminal. The above-mentioned servers are shown as “SIPs (SIP server)” in FIGS.  3  to  6 , without distinction of the respective servers for the sake of convenience. However, in the following description, each of the servers will be differentially represented. But, it should be understood that the servers may be represented as an SIP server when it is unnecessary to distinguish between the respective servers or when it is not necessary to name all of the servers.  
         [0033]     Meanwhile, when a mobile terminal is located in a home network, only a home address is registered in accordance with the conventional method, without using the parameter “type” as described above.  
         [0034]     In the following description of FIGS.  3  to  6 , it is assumed that a mobile terminal registers its home address as a current location through a register message in a home network before mobility detection and registration (as indicated by steps  331 ,  441 ,  551 , or  661 , in  FIG. 3, 4 ,  5 , or  6 , respectively) are performed. Additionally, it should be understood that the operation of the mobility detection and registration (as indicated by steps  331 ,  441 ,  551 , or  661 , in  FIG. 3, 4 ,  5  or  6 , respectively) is performed in a mobile IP layer, and the following operations are performed in an SIP layer.  
         [0035]     An application of a mobile terminal operating as shown in  FIGS. 3 through 6  is called a user application, which includes a UAC (User Agent Client) and a UAS (User Agent Server). The UAC is a client application for creating and transmitting an SIP request message. The UAS is a server application for creating and transmitting a response message, which includes a meaning of acceptance, rejection, or redirection with a change of a receiving address and retransmission. For convenience, these will be described as applied to a mobile terminal in the following description.  
         [0036]      FIG. 3  is a flow diagram for explaining a method of controlling a routing path for a VoIP service according to a second embodiment of the present invention, with respect to an example of using a foreign agent CoA.  
         [0037]     In step  331 , when a mobile terminal  30  in a standby state having been located in a home network moves into a different network, mobility detection is performed by a mobile IP mechanism of the mobile terminal  30 , and then the mobile terminal  30  requests registration to the home agent  34 .  
         [0038]     In step  332 , the mobile terminal  30  transmits a register message, that includes a contact header into which its own home address and a new CoA—a foreign agent CoA (hereinafter, referred to as “FACoA”) are recorded, to the foreign agent  32 . In this case, in order to discriminate between the two addresses, the parameter “type” is used as described in the above-mentioned  FIG. 2 .  
         [0039]     The foreign agent  32  transmits the register message to the SIP server  36 , that is, to the registra server, via the home agent  34 . Then, the registra server receives and transfers the new FACoA and the home address of an address-of-record of the mobile terminal  30  to the location server, and the location server stores the information transferred from the registra server.  
         [0040]     Meanwhile, the foreign agent  32  understands the message transmitted from the mobile terminal  30  to be a register message by parsing the message, and checks information of a contact header and the like. Additionally, during this time period, the foreign agent  32  can obtain desired information which is needed for tunneling between the foreign agent  32  and the proxy server, that is, the FACoA and a server IP address.  
         [0041]     Herein, the register message may be directly transmitted from the foreign agent  32  to the registra server, or may be transmitted from the foreign agent  32  to the registra server via the home agent  34  as shown in  FIG. 3 , according to a reverse-tunneling setup state of a mobile IP.  
         [0042]     In step  333 , a “200 OK” message, which is a response message of the registra server with respect to the register message, is transmitted to the mobile terminal  30  through a tunnel (not shown) established between the registra server and the foreign agent  32  via the home agent  34 . The foreign agent  32  parses the “200 OK” message and confirms that a response has been successfully received. After confirming that a response has been successfully received, “tunneling” between the foreign agent  32  and the proxy server can be created.  FIG. 3 , a first tunnel  11  represents a tunnel created between the foreign agent  32  and the proxy server  36 .  
         [0043]     In step  334 , a correspondent terminal  38 , of a party who wants to perform VoIP communication with the user of the mobile terminal  30 , transmits an invite message to the SIP server  36 , that is, the proxy server. In step  335 , the proxy server extracts the home address and the FACoA of the corresponding mobile terminal  30  from the location server using the address-of-record as a key. The proxy server transmits the invite message to a current location of the mobile terminal  30  through the first tunnel  11  with reference to the extracted information, and then the invite message is transmitted to the mobile terminal  30  after being decapsulated in the foreign agent  32 . During this time period, the foreign agent  32  can obtain, from the invite message, information which is necessary for creating a second tunnel  21  between the foreign agent  32  and the correspondent terminal  38 , in which the necessary information is an IP address of the correspondent terminal  38 , because an end point of a second tunnel  21  corresponds to the correspondent terminal  38  from the viewpoint of the foreign agent  32 .  
         [0044]     In step  336 , when the mobile terminal  30  generates and transmits a “180 Ring” message, the foreign agent  32  receives and transmits the “180 Ring” message to the proxy server through the first tunnel  11  which has been established between the foreign agent  32  and the proxy server. In step  337 , the proxy server receives and transmits the “180 Ring” message to-the correspondent terminal  38 .  
         [0045]     In step  338 , a “send” button of the mobile terminal  30  is pressed by a user, and the mobile terminal  30  records its own home address and the FACoA in the contact header of the “200 OK” message, and transmits the “200 OK” message to the foreign agent  32 . The foreign agent  32  transmits the “200 OK” message to the proxy server through the first tunnel  11 . In step  339 , the “200 OK” message is transmitted to the correspondent terminal  38  by the proxy server  36 . Then, the correspondent terminal CN parses the “200 OK” message, and can obtain information needed for creating a second tunnel  21  between the correspondent terminal  38  and the foreign agent  32 , which contains an IP address of the foreign agent  32 , that is, the FACoA, because an end point of a second tunnel  21  corresponds to the foreign agent  32  from the viewpoint of the correspondent terminal  38 . In step  340 , the correspondent terminal  38  transmits an ACK message to the mobile terminal  30  through the second tunnel  21 , so that the mobile terminal  30  and the correspondent terminal  38  can exchange voice packets with each other through the second tunnel  21  as shown in (step  341 ).  
         [0046]     According to the method of the present invention described above, it can be understood that transmitted messages do not pass through the home agent  34  from the time that an invite is initiated to the time that terminals exchange voice packets with each other.  
         [0047]     IPinIP (IP Encapsulation within IP) (RFC 2003), Minimal Encapsulation (RFC 2004), or GRE (General Routing Encapsulation) (RFC 1701) may be used as a protocol for the tunneling. In addition, it is possible to refer to IETF ‘RFC 2003’ and ‘RFC 2004’ for specifications relating to the tunneling.  
         [0048]     A description will be given in detail for a method for creating the first tunnel  11  between the foreign agent  32  and the SIP server  36 .  
         [0049]     From the viewpoint of the foreign agent  32 , the foreign agent  32  receives a register message from the mobile terminal  30 , transmits the register message to the SIP server  36 , and then enters a standby state, waiting for a response from the SIP server  36 . After this, when receiving a response message from the SIP server  36 , the foreign agent  32  parses the response message, that is, an SIP message. As a result of the parsing, when the response message is a “200 OK” message, the foreign agent  32  creates the first tunnel  11 , end points of which are its own address (FACoA) and a source IP address (an SIP network server address).  
         [0050]     In contrast, from the viewpoint of the SIP server  36 , the registra server receives a register message, registers a location of a corresponding mobile terminal  30  in the location server (that is, stores a home address, an FACoA, and an expiration value), and creates the first tunnel  11  which uses its own address and the FACoA of the mobile terminal  30  understood through the register message.  
         [0051]     Herein, the “tunnel” means a virtual link between two nodes, for example, between the foreign agent  32  and the SIP server  36 , and “creating a tunnel” means that a corresponding node recognizes where both end points of the tunnel exist. An expiration period of the first tunnel  11  is established as a value corresponding to the parameter “expires” of a CoA in a register message. In order to maintain the first tunnel  11 , it is necessary to update the first tunnel  11  with a new register message within an established expiration period. If the foreign agent FA does not receive the “200 OK” message but instead receives a different response, or does not receive any response until a predetermined period of time lapses, the foreign agent  32  determines that the creation of the first tunnel  11  has ended in a failure. Also, when an expiration period has lapsed, the first tunnel  11  is automatically eliminated. Therefore, it receives an invite message from the correspondent terminal  38  after the first tunnel  11  is eliminated, the SIP server SIPs transmits the invite message with the home address of a corresponding mobile terminal MN as a destination because the first tunnel  11  connected to the foreign agent FA does not exist. In this case, the invite message is transmitted in sequence as follows: from the correspondent terminal  38  to the SIP server  36  to the home agent  34  to the foreign agent  32  and then, to the mobile terminal  30 .  
         [0052]     A description will be given in detail for a method for creating the second tunnel  21  between the foreign agent  32  and the correspondent terminal  38 .  
         [0053]     The foreign agent  32  parses the invite message received from the correspondent terminal  38 , reads an IP address of the correspondent terminal  38  which is included in a contact header, and waits for the “200 OK” message to be transmitted from the mobile terminal  30 . When it receives the “200 OK” message, the foreign agent  32  creates the second tunnel  21 , both end points of which are its own address and the correspondent terminal  38 , and then transmits an ACK message, which is received from the correspondent terminal  38  through the second tunnel  21 , to the mobile terminal  30 . The second tunnel  21  has no connection with the parameter “expires”, and expires when a “200 OK” message is received in response to a BYE message. Herein, the BYE message is a request message, which is transmitted by an UAC to an UAS when the end of a call is desired during a media session.  
         [0054]     In contrast, from the viewpoint of the correspondent terminal  38 , the correspondent terminal  38  creates the second tunnel  21 , both end points of which are its own address and an FACoA of a mobile terminal  30  understood by a received “200 OK” message.  
         [0055]      FIG. 4  is a flow diagram for explaining a method of controlling a routing path for a VoIP service according to a third embodiment of the present invention, with respect to a case of using a Collocated CoA (hereinafter referred to as a “C-CoA”).  
         [0056]     The FACoA is an IP address of a foreign agent, which interfaces a mobile terminal to a foreign network where a mobile terminal  40  is visiting. In contrast, the C-CoA (Collocated CoA) is an IP address which is temporarily assigned for interfacing with a mobile terminal itself. The C-CoA represents a current location of a mobile terminal which is located in a foreign network. When an end point of a tunnel, which is established by a home agent for transmitting a packet, is a mobile terminal, the mobile terminal has to have the capability of decapsulating encapsulated packets, and in this case, the CoA used by the mobile terminal is the C-CoA.  
         [0057]     As compared with the embodiment shown in  FIG. 3 , the embodiment shown in  FIG. 4  uses the mobile terminal  40  as an end point of a tunnel, while the embodiment shown in  FIG. 3  uses a foreign agent  32  as an end point of a tunnel. In the following description, an SIP signaling sequence will be explained in detail with respect to an example in which a mobile terminal  40  uses the C-CoA with reference to  FIG. 4 .  
         [0058]     In step  441 , when the mobile terminal  40  in a standby state having been located in a home network moves into a different network, mobility detection is performed by a mobile IP mechanism of the mobile terminal  40 , and the mobile terminal MN transmits a registration request message to the home agent  42 .  
         [0059]     In step  442 , the mobile terminal  40  transmits a register message, that includes a contact header into which its own home address and a new C-CoA are recorded, directly to the SIP server  44 , that is, the registra server. In this case, in order to detect a difference between the two addresses, the parameter “type” is used (as shown in  FIG. 2  and described in the corresponding text).  
         [0060]     In step  443 , the registra server generates a “200 OK” message in response to the register message, and transmits the “200 OK” message to the mobile terminal  40  through the home agent  42 .  
         [0061]     In step  444 , a correspondent terminal  46 , of a party who wants to perform VoIP communication with the user of the mobile terminal  40 , transmits an invite message to the SIP server  44 , that is, the proxy server. The proxy server extracts the home address and the C-CoA of the mobile terminal  40  from the location server by using the address-of-record as a key. The proxy server establishes a third tunnel  31  between the proxy server and the mobile terminal  40  with reference to the extracted information, and transmits an invite message to the mobile terminal  40  through the third tunnel  31 .  
         [0062]     In step  445 , the invite message is received by the mobile terminal  40 , and is decapsulated.  
         [0063]     In step  446 , the mobile terminal  40  transmits a “180 Ring” message to the proxy server through the third tunnel  31 .  
         [0064]     In step  447 , the proxy server transmits the “180 Ring” message to the correspondent terminal  46 .  
         [0065]     In step  448 , a “send” button of the mobile terminal  40  is pressed by a user, and the mobile terminal  40  records its own home address and the C-CoA in the contact header of the “200 OK” message, and transmits the “200 OK” message to the proxy server.  
         [0066]     In step  449 , the proxy server transmits the “200 OK” message to the correspondent terminal  46 .  
         [0067]     The mobile terminal  40  and the correspondent terminal  46  can obtain information needed for tunneling in steps  445  and  449  respectively. That is, the mobile terminal  40  can obtain the IP address of the correspondent terminal  46  from the received invite message in step  445 , and the correspondent terminal  46  can obtain the C-CoA of the mobile terminal  40  from the received “200 OK” message. A fourth tunnel  1  represents a tunnel both end points of which are the mobile terminal  40  and the correspondent terminal  46 .  
         [0068]     In step  450 , the correspondent terminal  46  transmits an ACK message to the mobile terminal  40 , not through the home agent  42 , but through the fourth tunnel  41 , which has been established between the correspondent terminal  46  and the mobile terminal  40 .  
         [0069]     In step  451 , the mobile terminal  40  and the correspondent terminal  46  exchange voice packets with each other through the fourth tunnel  41 , and thus bypass the home agent  42  as shown.  
         [0070]     IPinIP (RFC  2003 ), Minimal Encapsulation (RFC 2004), or GRE (RFC 1701) may be used as a protocol for the tunneling according to the embodiments of the present invention.  
         [0071]      FIG. 5  is a flow diagram showing a signaling sequence in which a mobile terminal  50  notifies a correspondent terminal  50  of a changed CoA of the mobile terminal  50 , when the mobile terminal  50  moves from a network to another network during a session, according to a fourth embodiment of the present invention. Herein, it is assumed that FACoA is used as described in the embodiment which is shown in  FIG. 3 .  
         [0072]     The description of the operations of steps  551  to  553  will be omitted as they are identical to the operations of steps  331  to  333  as shown in  FIG. 3  respectively.  
         [0073]     The SIP server  56 , that is, the registra server, and the foreign agent  52  obtain an FACoA and a server IP address, which are included in a register message transmitted from the mobile terminal  50  in step  552 , so that a fifth tunnel  51  can be created.  
         [0074]     In step  554 , the mobile terminal  50  transmits an information message (hereinafter referred to as an “INFO message”) to the SIP server  56 , that is, the proxy server, through the foreign agent  52 . In this case, the INFO message is transmitted through the fifth tunnel  51  created between the proxy server and the foreign agent  52 . In step  555 , the proxy server transmits the INFO message to the correspondent terminal  58 .  
         [0075]     In step  556 , the correspondent terminal  58 , having received the INFO message, replaces an end point of the fifth tunnel  51  with a newly changed FACoA. In addition, the correspondent terminal  58  generates a “200 OK” message in response to the INFO message, and transmits the “200 OK” message to the proxy server. In step  557 , the proxy server transmits the “200 OK” message to the foreign agent  52  through the fifth tunnel  51 , and then, the foreign agent  52  transmits the “200 OK” message to the mobile terminal  50 .  
         [0076]     A sixth tunnel  61  represents a tunnel created between the correspondent terminal  58  and a foreign agent  52  corresponding to the newly changed FACoA. In step  558 , an ACK message, which is transmitted from the correspondent terminal  58  to the mobile terminal  50  through the foreign agent  52 , passes through the sixth tunnel  61 . In step  559 , voice packets, which are exchanged between the correspondent terminal  58  and the mobile terminal  50  via the foreign agent  52 , pass through the sixth tunnel  61 .  
         [0077]      FIG. 6  is a flow diagram showing a signaling sequence, in which a mobile terminal  60  notifies the correspondent terminal  66  of a changed CoA of the mobile terminal  60 , when the mobile terminal  60  moves from a network to another network during a session, according to a fifth embodiment of the present invention. Herein, it is assumed that the C-CoA is used as described in the embodiment shown in  FIG. 4 .  
         [0078]     The description of the operations of steps  661  to  663  will be omitted as they are identical to the operations of steps  441  to  443 , shown in  FIG. 4  respectively.  
         [0079]     The SIP server  64 , that is, the registra server, and the foreign agent (not shown) obtain a C-CoA and a server IP address, which are included in a register message transmitted from the mobile terminal  60  in step  662 , so that a seventh tunnel  71  shown is created.  
         [0080]     In step  664 , the mobile terminal  60  transmits an INFO message to the SIP server  64 , that is, the proxy server. In this case, the INFO message is transmitted through the seventh tunnel  71  established between the proxy server  64  and the mobile terminal  60 . In step  665 , the proxy server transmits the INFO message to the correspondent terminal  66 .  
         [0081]     In step  666 , the correspondent terminal  66 , having received the INFO message, replaces an end point of the seventh tunnel  71  with a newly changed C-CoA, generates a “200 OK” message in response to the INFO message, and transmits the “200 OK” message to the proxy server  64 . In step  667 , the proxy server  64  transmits the “200 OK” message directly to the mobile terminal  60  through the seventh tunnel  71 .  
         [0082]     In  FIG. 6 , an eighth tunnel  81  represents a tunnel created between the correspondent terminal  66  and the mobile terminal  60  corresponding to the newly changed C-CoA. In step  668 , an ACK message, which is transmitted from the correspondent terminal  66  to the mobile terminal  60 , passes through the eighth tunnel  81 . In step  669 , voice packets, which are exchanged between the correspondent terminal  66  and the mobile terminal  60 , pass through the eighth tunnel  81 .  
         [0083]     The above-mentioned operations are described on the assumption that the SIP server operates as a proxy server. If the SIP server is used as a redirect server, the sequence will be further simplified. That is, when the redirect server transmits a current C-CoA of the mobile terminal by means of a “200 OK” message to the a calling terminal, the calling terminal can establish a direct tunneling between the calling terminal and a called terminal, so that packets can be exchanged between the calling terminal and the called terminal. In this case, a tunnel between the SIP server SIPs and the called terminal is not necessary.  
         [0084]     As compared with the embodiment of  FIG. 5 , the embodiment of  FIG. 6  enables messages, such as a register message, a “200 OK” message, an INFO message, and an ACK message, to be exchanged between the mobile terminal and the home agent without passing through the foreign agent.  
         [0085]     Hereinafter, the embodiment of  FIG. 3  will be described in detail with reference to  FIGS. 7 and 8 .  
         [0086]      FIG. 7  is a detailed view for explaining a mobile IP re-registration and SIP registration process in  FIG. 3 , and  FIG. 8  is a detailed view for explaining a routing-path optimizing process of a VoIP traffic using the SIP which is shown in  FIG. 3 .  
         [0087]     Referring to  FIG. 7 , steps  770  to  774  correspond to step  331  of  FIG. 3 . As shown in  FIG. 7 , a mobile terminal MN 1  located in an area DM 1  of domain “sbs.com” and a mobile terminal MN 2  located in another area DM 2  of domain “samsung.com” are physically the same terminal, but will be represented by reference numbers which are different from each other for convenient recognition with respect to the before and after location movement of the mobile terminal.  
         [0088]     When the mobile terminal MN 2  detects that the mobile terminal MN 2  has moved from the area DM 2  of domain “samsung.com” to the area DM 1  of domain “sbs.com” (step  780 ), the mobile terminal requests a mobile IP registration from the foreign agent FA 7  (step  771 ). The foreign agent FA 7  having received the request transmits the mobile IP registration request to the home agent HA 7  (step  772 ). The home agent HA 7  receives the mobile IP registration request, and transmits a mobile IP registration response to the foreign agent FA 7  (step  773 ). The foreign agent FA having reoceived the response transmits the mobile IP registration response to the mobile terminal MN 1  (step  774 ).  
         [0089]     Steps  775  to  779  in  FIG. 7  correspond to steps  332  and  333  in  FIG. 3 . The mobile terminal MN 1  transmits a register message, which includes a contact header into which its own home address and a new CoA (FACoA) are recorded, to the foreign agent FA 7  (step  775 ). The foreign agent FA 7  transmits the register message to the SIP server, that is, to the registra server RS 7  (step  776 ). Then, the registra server RS 7  receives and transfers the register message to the location server LS 7  so that the location server LS 7  may store the new CoA and the home address of an address-of-record of the mobile terminal MN 1 . The registra server RS 7  transmits a “200 OK” message to the home agent HA 7  as a response message with respect to the register message (step  777 ). The home agent HA 7  transmits the “200 OK” message to the foreign agent FA 7  (step  778 ). The foreign agent FA 7  transmits the “200 OK” message to the mobile terminal MN 1  (step  779 ).  
         [0090]     While  FIG. 7  is explained with reference to  FIG. 3 , it should be understood that  FIG. 7  can be used as a view for explaining a mobile IP re-registration and SIP registration process in each of FIGS.  4  to  6  in detail. That is, steps  770  to  774  may correspond to step  441  in  FIG. 4 , step  551  in  FIG. 5 , or step  661  in  FIG. 6 . Also, steps  775  to  779  may correspond to steps  442  and  443  in  FIG. 4 , steps  552  and  553  in  FIG. 5 , or steps  662  and  663  in  FIG. 6 .  
         [0091]     Referring to  FIG. 8 , steps  881 ,  884 , and  885  correspond to steps  334  and  335  in  FIG. 3 . Steps  891 ,  892  and  893  correspond to steps  336  and  337  in  FIG. 3 . Steps  886 ,  887 , and  888  correspond to steps  338  and  339  in  FIG. 3 . Also, step  889  corresponds to step  340  as shown in  FIG. 3 .  
         [0092]     In  FIG. 8 , a first tunnel  11  and a second tunnel  21  are represented by an identical reference numeral  91 .  
         [0093]     Unlike the voice traffic  123  of the prior art shown in  FIG. 1  which is formed is in a triangular shape, voice traffic  894  according to the present invention is formed in a straight line as shown in  FIG. 8 .  
         [0094]     In order to optimize a routing path of VoIP traffic, the SIP server functions as a location server LS 8  and a proxy server PS 8 . The location server LS 8  and the proxy server PS 8  transfer and accept a query or a reply to or from each other. A query transferred from the proxy server PS 8  to the location server LS 8  is used to understand the CoA of a terminal invited by a correspondent terminal CN 8  (step  882 ). A reply transferred from the location server LS 8  to the proxy server PS 8  includes the CoA of the terminal intuited by the correspondent terminal CN 8  (step  883 ).  
         [0095]     According to the present invention as describe above, it is possible to solve the triangular routing problem which occurs when VoIP is used with a mobile terminal employing mobile IP. In addition, according to the present invention, owing to a shortened routing path, the delays which are often important factors in voice communication quality can be significantly reduced, and the load of the whole network is reduced, thus decreasing the loss of data, so that high-quality voice communication are achieved.  
         [0096]     While the present invention has been shown and described With reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Accordingly, the scope of the invention is not to be limited by the above embodiments but by the claims and the equivalents thereof.