Patent Publication Number: US-9906603-B2

Title: System and method for transferring a session between multiple clients

Description:
PRIORITY 
     This continuation application claims priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 12/990,749 (now in condition for allowance), which was a national stage application under 35 U.S.C. 371 of Int&#39;l Stage PCT Application Ser. No. PCT/KR2009/002212, filed on Apr. 28, 2009, and which claimed priority under 35 U.S.C. 119(a) to Korean Patent Application Ser. No. 10-2008-0041340, filed on May 2, 2008, the entire disclosures of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a service providing method and system implemented through a Session Initiation Protocol/Internet Protocol (SIP/IP) core network, and in particular, to a method and system for transferring a session between multiple clients. 
     2. Description of the Related Art 
     In an existing mobile environment, terminals transmit sporadic messages of Short Message Service (SMS), Multimedia Messaging Service (MMS), etc. On the other hand, users, may need a messaging service to easily exchange a conversation using an MSN messenger in a wired environment. Thus, an instant messaging service has been introduced into terminals and networks on the basis of an SIP/IP core network. According to the demand of customers and enterprises for Push To Talk (PTT) (that is, walkie-talkie), a PTT over Cellular (PoC) service and system based on the SIP/IP core network have been developed. According to rapid variation of markets for enterprises, communication businesses, etc., the desire of users to integrate and process various types of messages is increasing. In consideration of this point, a standard technology is being developed for service Converged IP Messaging (CPM) implemented through the SIP/IP core network in the standard organization called Open Mobile Alliance (OMA). The CPM is intended to allow a user to experience a seamless messaging service environment by integrating IP-based messaging services. Accordingly, the implementation of interworking between a CPM service and a non-CPM service is needed such that a service disconnection is not perceived in any messaging service. One of many features of the above-described CPM service is that multi-client environments can be provided to users. 
     There is the case where the user needs to transfer a session to another client while the session is performed through one client even when one user accesses a service by simultaneously using multiple clients through the above-described method. In this case, a proper session transfer method and system are needed. 
     Accordingly, a method and system that can transfer a session in a messaging service providing system implemented through an SIP/IP core network are needed. 
     SUMMARY OF THE INVENTION 
     The present invention addresses at least the problems and disadvantages described above and provides at least the advantages described below. 
     An aspect of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method and system for transferring a session from one client to another client when one user uses multiple clients. 
     According to an aspect of the present invention, there is provided a converged messaging service providing system for transferring a session between multiple clients, including: a first client that requests session transfer to a second client; a message &amp; media storage that stores a session history; and a conversation server that temporarily stores a message from a third client of another user when the session transfer request exists, acquires a session history from the message &amp; media storage, generates a session with the second client, and transmits the temporarily stored message and the session history to the second client when the session is generated completely. 
     According to an aspect of the present invention, there is provided a converged messaging service providing system for transferring a session between multiple clients, including: a first client that requests session transfer to a second client; a message &amp; media storage that stores a session history; a conversation server that temporarily stores a message from a third client of another user when the session transfer request exists, generates a session with the second client, and commands to generate a session between the second client and the message &amp; media storage after transmitting the temporarily stored message to the second client when the session is generated completely; and the second client that generates a session with the message &amp; media storage in response to the session generation command and acquires a session history from the message &amp; media storage. 
     According to an aspect of the present invention, there is provided a method for transferring a session between multiple clients in a converged messaging service providing system, including: requesting session transfer from a first client to a second client; temporarily storing, by a converged server, a message from a third client when the session transfer request exists; acquiring, by the converged server, a session history from a message &amp; media storage; generating, by the converged server, a session with the second client; and transmitting, by the converged server, the temporarily stored message and the session history to the second client when the session is generated completely. 
     According to an aspect of the present invention, there is provided a method for transferring a session between multiple clients in a converged messaging service providing system, including: requesting session transfer from a first client to a second client; temporarily storing, by a converged server, a message from a third client when the session transfer request exists; generating, by the converged server, a session with the second client; transmitting, by the converged server, the temporarily stored message to the second client when the session is generated completely; acquiring, by the converged server, a session history from a message &amp; media storage; and transmitting, by the converged server, the session history to the second client. 
     According to an aspect of the present invention, there is provided a method for transferring a session between multiple clients in a converged messaging service providing system, including: requesting session transfer from a first client to a second client; temporarily storing, by a converged server, a message from a third client when the session transfer request exists; generating, by the converged server, a session with the second client and transmitting the temporarily stored message to the second client when the session is generated completely; commanding, by the converged server, to generate a session between the second client and a message &amp; media storage; generating, by the second client, a session with the message &amp; media storage in response to the session generation command; and acquiring, by the second client, a session history from the message &amp; media storage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a configuration diagram of a CPM system according to an embodiment of the present invention; 
         FIG. 2  illustrates an operation relation between CPM system elements for transferring a session according to an embodiment of the present invention; 
         FIG. 3  illustrates a session transfer flow according to a first embodiment of the present invention; 
         FIG. 4  is a state diagram of client #A- 1  according to the first embodiment of the present invention; 
         FIG. 5  is a state diagram of client #A- 2  according to the first embodiment of the present invention; 
         FIG. 6  is a state diagram of a conversation server according to the first embodiment of the present invention; 
         FIG. 7  is a state diagram of a message &amp; media storage according to the first embodiment of the present invention; 
         FIG. 8  illustrates a session transfer flow according to a second embodiment of the present invention; 
         FIG. 9  is a state diagram of client #A- 2  according to the second embodiment of the present invention; 
         FIG. 10  is a state diagram of a conversation server according to the second embodiment of the present invention; 
         FIG. 11  illustrates a session transfer flow according to a third embodiment of the present invention; 
         FIG. 12  is a state diagram of a client #A- 2  according to the third embodiment of the present invention; 
         FIG. 13  is a state diagram of a conversation server according to the third embodiment of the present invention; 
         FIG. 14  is a state diagram of a message &amp; media storage according to the third embodiment of the present invention; and 
         FIG. 15  illustrates an SIP REFER message format according to an embodiment of the present invention. 
     
    
    
     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features and structures. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION 
     Embodiments of the present invention will now be described in detail with reference to the drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness. 
     According to embodiments of the present invention, session transfer is naturally performed without lost content by temporarily storing and transmitting a session history and a message of a counterpart user to a second client during the session transfer. In a target client to which the session has been transferred, a session history of an existing client is output to a screen, such that a user can continue a conversation while referring to existing conversation content. The user does not need to read conversation content from the screen of the previous client. User convenience can be improved using a CPM service. 
     In the present invention, an example of CPM messaging among messaging services implemented through an SIP/IP core network will be described. The present invention provides a method for transferring a session between clients when one user uses multiple clients on a service (for example, PoC, Instant Messaging (IM), or CPM) providing system implemented through an SIP/IP core network. In particular, the present invention provides a session transfer method when the user needs to transfer a session to another client while the session is performed through one client. The present invention uses a temporary storing technique in which a conversation server or client as one of elements of a seamless CPM system for providing a seamless user environment temporarily stores data transmitted from another user while the session is transferred. 
     Hereinafter, the configuration and operation of the CPM system according to an embodiment of the present invention will be described with reference to  FIG. 1 . In the embodiment of the present invention, an example of the CPM will be described, but the present invention is applicable to all services implemented through the SIP/IP core network such as PoC, IM, etc. For the session transfer according to the embodiment of the present invention, the CPM system includes a conversation server and a message &amp; media storage. Specifically, referring to  FIG. 1 , the CPM system includes a CPM client  200 , a CPM conversation server  210 , a message &amp; media storage  220 , a CPM user preference entity  230 , an interworking function entity  240 , and a notification entity  250 . 
     First, the CPM client  200  is a service requestor to generate a session with another CPM client through the CPM conversation server  210  and receive a service. The message &amp; media storage  220  stores data to be exchanged in a session between the CPM clients. An operation for storing CPM session data is determined by user preferences stored in the CPM user preference entity  230 . The constituent elements are mutually connected through interfaces. Among the interfaces, an interface between the CPM conversation server  210  and the CPM client  200  uses SIP, Message Session Relay Protocol (MSRP), and Real-time Transfer Protocol (RTP) as CPM-CONV. 
     The operation relation between the CPM system elements will be described with reference to  FIG. 2 . One user can use multiple clients. In  FIG. 2 , when a user A uses first and second clients and a user B uses a third client, a session transfer method between the first and second clients is proposed. Specifically,  FIG. 2  schematically illustrates a process in which the user A attempts a client change to a client #A- 2   1100  while a client #A- 1   1000  exchanges data with a client #B  2000 . In the present invention, the session transfer method is implemented such that the user B does not know the fact that the user A has made the client change. In the present invention, the session transfer is that in a messaging session and an additionally continuous media session. Here, the message session is the session for exchanging a message, file, photo, etc. between users. That is, it can be defined as a discontinuous media session. A concept as opposed thereto is a continuous media session like a video conferencing session or a voice chatting session for a voice conversation. 
     In the following description, it is assumed that the user A owns the client #A- 1   1000  and the client #A- 2   1100  and the user B owns the client #B  2000 . Here, the client #A- 1   1000  and the client #B  2000  are connected to a conversation server  3000  by the SIP session. The conversation server  3000  and a message &amp; media storage  4000  use Internet Message Access Protocol 4 (IMAP4)/Lemonade protocol, such that the IMAP4/Lemonade protocol is used for message exchange with the message &amp; media storage  4000 . In the present invention, the message &amp; media storage  4000  is additionally required to store a session history. 
     On the other hand, when the user A desires to attempt the client change to the client #A- 2   1100 , the client #A- 1   1000  as a source device requests the conversation server  3000  to transfer the session. At this time, the client #A- 1   1000  has its own client list and selects one of clients belonging to the user. For example, the client #A- 1   1000  must request the conversation server  3000  to transfer the session after selecting the client #A- 2   1100 . 
     Upon receipt of a session transfer command, the conversation server  3000  temporarily stores data transmitted from the client #B  2000  and transmits the stored data to the client #A- 2   1100  when the session transfer process is completed. At this time, a session history is stored in the message &amp; media storage  4000 . The client #A- 2   1100  acquires the session history stored in the message &amp; media storage  4000  through the conversation server  3000  or by directly accessing the message &amp; media storage  4000 . A session history transmission time and method are determined by the CPM user preference and service policy. Here, the session history is defined as a set of all types of data exchanged from a session start time to a current time. This session history can be managed by the message &amp; media storage  4000  according to the CPM user preference and service policy or directly managed by the client #A- 1   1000  as the originator device. 
     The embodiments herein are divided as follows. 
     A first embodiment of the present invention is a process in which the conversation server  3000  receives and temporarily stores a session history from the message &amp; media storage  4000  when a session transfer command is generated and transmits the session history and a temporarily stored message to the client #A- 2   1100  after the session with the client #A- 2   1000  is generated, and the client #A- 2   1100  outputs a transmission completion notification to a screen when the transmission is completed and resumes a conversation with the client #B  2000 . 
     A second embodiment of the present invention is a process in which the conversation server  3000  temporarily stores a message to be transmitted to the client #A- 2   1100  while a session with the client #A- 2   1100  is generated, the temporarily stored message is transmitted to the client #A- 2   1100  when the session is generated, the conversation is resumed between the client #A- 2   1100  and the client #B  2000  when the transmission is completed, the conversation server  3000  requests the message &amp; media storage  4000  to transmit a session history and receives and temporarily stores the session history, the session history is transmitted to the client #A- 2   1100 , and the session history is output to the screen of the client #A- 2   1100 . 
     A third embodiment of the present invention is the same as the second embodiment in terms of the process until a conversation is resumed between the client #A- 2   1100  and the client #B  2000 . In a subsequent process of the third embodiment, the conversation server  3000  commands the client #A- 2   1100  to generate a session with the message &amp; media storage  4000 , and the client #A- 2   1100  directly receives a session history through the generated session and outputs the received session history to the screen. 
     First, a session transfer flow according to the first embodiment of the present invention will be described with reference to  FIG. 3 . 
     Referring to  FIG. 3 , the client #A- 1   1000  transmits a request for transferring a session to the client #A- 2   1100  to the conversation server  3000  using an SIP REFER message as a control message (S 101 ). The format of the SIP REFER message is illustrated in  FIG. 15 . Referring to  FIG. 15 , a Refer-To header field P 1  indicates destination address information. To transfer the session from the client #A- 1   1000  to the client #A- 2   1100 , an SIP URI address of the client #A- 2   1100  is indicated. The REFER message includes attribute information indicating that the REFER message to be currently transmitted is used for the session transfer. Accordingly, SESSION TRANSFER P 2  is indicated by a Session Description Protocol (SDP) attribute parameter. Upon acquisition of the REFER message, the conversation server  3000  uses an ACCEPTED message to indicate whether the REFER message has been received (S 102 ). To notify that a REFER event occurs in the conversation server  3000 , the conversation server  3000  transmits a NOTIFY message to the client #A- 1   1000  (S 103 ). The client #A- 1   1000  returns an OK message in response to the NOTIFY message (S 104 ). The above-described steps S 101  to S 104  indicated by reference numeral  300  are a process in which the client #A- 1   1000  requests the conversation server  3000  to transfer the session. 
     Then, the conversation server  3000  performs the following session transfer procedure. Specifically, when the session transfer is performed, the conversation server  3000  temporarily stores data, for example, messages  305 , to be transmitted from the user B to the user A. In other words, during the session transfer, a temporary storing operation  310  is started such that data from the user B to the user A is transmitted to the client #A- 2   1100  of the user A without loss. 
     The conversation server  3000  selects a storage space of the user A within the message &amp; media storage  4000  using an IMAP SELECT message (S 105 ). At this time, the conversation server  3000  uses an IMAP protocol to acquire a session history stored in the message &amp; media storage  4000 . Then, the message &amp; media storage  4000  returns an OK FETCH message in response to the storage space selection by the conversation server  3000  (S 106 ). Subsequently, the conversation server  3000  transmits an IMAP FETCH message to retrieve the session history stored in the message &amp; media storage  4000  (S 107 ). The message &amp; media storage  4000  transmits the session history to the conversation server  3000  and the conversation server  3000  temporarily stores the session history in its own storage space (S 108 ). The above-described steps S 105  to S 108  indicated by reference numeral  315  are a process in which the conversation server  3000  retrieves and receives the session history from the message &amp; media storage  4000 . 
     Subsequently, the conversation server  3000  makes a notification of a newly generated session with the client #A- 2   1100  using an INVITE message (S 109 ). The client #A- 2   1100  returns an OK message in response to the INVITE message (S 110 ). Then, the conversation server  3000  returns an ACK message in response to the OK message (S 111 ). The above-described steps S 109  to S 111  indicated by reference numeral  320  are a new SIP session generation process. 
     When the session with the client #A- 2   1100  is generated completely, a routing change  325  is made such that the conversation server  3000  connects the conversation with the user B to the client #A- 2   1100 . That is, routing setup is changed to transmit data from the client #B  2000  to the client #A- 2   1100 . At a routing change point, the conversation server  3000  terminates an operation for temporarily storing data received from the client #B  2000  as indicated by reference numeral  330 . 
     Then, the conversation server  3000  notifies the client #A- 1   1100  of REFER event termination using a NOTIFY message (S 112 ). The client #A- 1   1000  returns an OK message to the conversation server  3000  in response to the NOTIFY message (S 113 ). The client #A- 1   1000  notifies the conversation server  3000  of existing session termination (S 114 ). The conversation server  3000  transmits the OK message to confirm the session termination (S 115 ). Accordingly, the session with the client #A- 1   1100  is terminated through steps S 112  to S 115  indicated by reference numeral  335 . 
     Then, the conversation server  3000  transmits a session history and a temporarily stored message to the client #A- 2   1100  using the MSRP protocol (S 116  and S 117 ). In response thereto, the client #A- 2   1100  returns an OK message to the conversation server  3000  (S 118  and S 120 ). When the session history and the temporarily stored message are transmitted completely, the client #A- 2   1100  notifies the conversation server  3000  of transmission completion using an MSRP REPORT message (S 119  and S 121 ). Through the above-described steps S 116  to S 121 , the client #A- 2   1100  outputs the temporarily stored message and the session history received from the conversation server  3000  to the screen. Accordingly, the conversation server  3000  receives data, for example, messages, from the user B to the user A, such that the conversation with the client #B  2000  is continued. 
     Hereinafter, the operation of constituent elements of the CPM system operating as illustrated in  FIG. 3  will be described with reference to  FIGS. 4 to 7 . 
       FIG. 4  illustrates the state variation of the client #A- 1   1000  according to the first embodiment of the present invention. When the client #A- 1   1000  of an initial state in step  400  requests the conversation server  3000  to transfer a session in step  402 , the transition to an idle state is made in step  404 . Then, when a termination command to terminate an existing session is received from the conversation server  3000  in step  406 , the transition to a termination state is made in step  408 . 
       FIG. 5  illustrates the state variation of the client #A- 2   1100 . When the client #A- 2   1100  of an initial state in step  410  receives a temporarily stored message and a session history from the conversation server  3000  in step  412 , the session history and the temporarily stored message are rendered in steps  414  and  416 . Subsequently, when the conversation with the client #B  2000  is resumed in step  418 , the transition to a conversation resumption state is made in step  420 . 
       FIG. 6  illustrates the state variation of the conversation server  3000 . When the conversation server  3000  of an initial state in step  422  receives a session transfer command from the client #A- 1   1000  in step  424 , a temporary storing operation is started in step  426 . Subsequently, when a storage space of the message &amp; media storage  4000  is selected in step  428 , an operation is performed in a selection state in step  430 . When a session history retrieval operation must be performed in the message &amp; media storage  4000  in step  432 , a session history is retrieved in step  434 . Accordingly, when the session history within the message &amp; media storage  4000  is received in step  436 , an operation is performed in a session history reception state to receive the session history in step  438 . Then, when the session with the client #A- 2   1100  is generated in step  440 , an operation for inviting the client #A- 2   1100  is performed in step  442 . Then, when a routing change to the client #A- 2   1100  is made in step  444 , the conversation server  300  performs a temporary storing operation in step  446 . Thereafter, when the temporarily stored message and the session history must be transmitted to the client #A- 2   1100  in step  448 , an operation is performed in a state for transmitting the temporarily stored message and the session history in step  450 . Then, when the transmission is completed in step  452 , the conversation server  3000  operates in an idle state in step  454 . 
       FIG. 7  illustrates the state variation of the message &amp; media storage  4000 . When the message &amp; media storage  4000  is in an initial state in step  456  and a storage space within the message &amp; media storage  4000  is selected in step  458 , it proceeds to step  460  to perform an operation in a selection state. Subsequently, when a session history is retrieved from the message &amp; media storage  4000  in step  462 , an operation is performed in a session history retrieval state in step  464 . Then, when the message &amp; media storage  4000  must transmit the retrieved session history to the conversation server  3000  in step  466 , an operation for transmitting the session history is performed in step  468 . When the transmission is completed in step  470 , an operation is performed in an idle state in step  472 . 
     In the case of source device initiated session transfer in which a session history is received in a foreground manner according to the first embodiment of the present invention as described above, a process for outputting a session history and a temporarily stored message to the screen of the client #A- 2   1100  and resuming the conversation with the client #B  2000  is performed. Accordingly, the present invention can minimize the inconvenience due to the session transfer since the user A continues the conversation through the client #A- 2   1100 , and can provide user convenience since a waiting time of the user B due to the session transfer of the user A is minimized. 
     Hereinafter, a session transfer flow according to the second embodiment of the present invention will be described with reference to  FIG. 8 . The operation of steps S 201  to S 204  of  FIG. 8  as indicated by reference numeral  500  is the same as that of steps S 101  to S 104  of  FIG. 3 . An operation  515  for temporarily storing a message  510  received from the client #B  2000  is started. During this operation, the conversation server  3000  performs an operation for generating a session with the client #A- 2   1100 . The operation of steps S 205  to S 207  as indicated by reference numeral  520  is the same as that of steps S 109  to S 111  of  FIG. 3  as indicated by reference numeral  320 . Since a routing change operation  525  and a temporary storage termination operation  530  are the same as those as indicated by reference numerals  325  and  330  of  FIG. 3 , their description is omitted. Steps S 208  to S 211  as indicated by reference numeral  535  are the same as steps S 112  to S 115  of  FIG. 3  as indicated by reference numeral  335 . 
     In this regard, the process of  FIG. 3  in which the conversation server  3000  receives a session history from the message &amp; media storage  4000  and then transmits both a temporarily stored message and the session history to the client #A- 2   1100  is performed, while the process of  FIG. 8  in which the conversation server  3000  first transmits the temporarily stored message to the client #A- 2   1100  and then transmits the session history from the message &amp; media storage  4000  to the client #A- 2   1100  is performed. As described above, when the client #A- 2   1100  receives the temporarily stored message from the conversation server  3000 , the message can be output to the screen, such that the conversation with the client #B  2000  is continued. 
     Accordingly, when the conversation server  3000  transmits the temporarily stored message in steps S 212  to S 214  as indicated by reference numeral  545 , the session history is received from the message &amp; media storage  4000  in steps S 215  to S 218  as indicated by reference numeral  550 . Here, the operation of steps S 212  to S 214  as indicated by reference numeral  545  is the same as that of steps S 116 , S 118 , and S 119  of  FIG. 3 , and the operation of steps S 215  to S 218  as indicated by reference numeral  550  is the same as that of steps S 105  to S 108  of  FIG. 3  as indicated by reference numeral  315 . An operation in which the conversation server  3000  transmits the session history to the client #A- 2   1100  in steps S 219  to S 221  as indicated by reference numeral  555  is the same as that of steps S 117 , S 120 , and S 121  of  FIG. 3 . When the session history is received, the client #A- 2   1100  outputs the received session history to the screen. 
     The operation of constituent elements of the CPM system operating as illustrated in  FIG. 8  will be described with reference to  FIGS. 9 and 10 . Since the state variation of the client #A- 1   1000  requesting the session transfer and the state variation of the message &amp; media storage  4000  are the same as those of  FIGS. 4 and 7 , their description is omitted. 
       FIG. 9  illustrates the state variation of the client #A- 2   1100 . When the client #A- 2   1100  of an initial state in step  600  receives a temporarily stored message from the conversation server  3000  in step  602 , the temporarily stored message is rendered in step  604 . Subsequently, when the conversation with the client #B  2000  is resumed in step  606 , the transition to a conversation resumption state is made in step  608 . Then, when the client #A- 2   1100  receives a session history from the conversation server  3000  in step  610 , the session history is rendered in step  612 . 
       FIG. 10  illustrates the state variation of the conversation server  3000 . When the conversation server  3000  of an initial state in step  614  receives a session transfer command from the client #A- 1   1000  in step  616 , a temporary storing operation is started in step  618 . After pre-requesting the message &amp; media storage  4000  to provide the session history in step  620 , the conversation server  3000  invites the client #A- 2   1100  in step  622 . Then, when a routing change to the client #A- 2   1100  is made in step  624 , the conversation server  3000  performs a temporary storing operation in step  626 . Subsequently, when a storage space of the message &amp; media storage  4000  is selected in step  628 , the conversation server  3000  operates in a selection state in step  630 . When a session history retrieval operation must be performed in the message &amp; media storage  4000  in step  632 , a session history is retrieved in step  634 . Accordingly, when the session history within the message &amp; media storage  4000  is received in step  636 , an operation is performed in a session history reception state to receive the session history in step  638 . Then, when the temporarily stored message and the session history must be transmitted to the client #A- 2   1100  in step  640 , the session history is transmitted in step  642 . When the transmission is completed in step  644 , an operation is performed in an idle state in step  646 . 
     Hereinafter, a session transfer flow according to the third embodiment of the present invention will be described with reference to  FIG. 11 . An operation for transmitting the temporarily stored message in steps S 301  to S 314  of  FIG. 11  is the same as that of steps S 201  to S 214  of  FIG. 8 . In this regard, in  FIG. 11 , a process in which the client #A- 2  directly accesses the message &amp; media storage  4000  and directly acquires a session history without passing through the conversation server  3000  is performed. 
     Accordingly, the client #A- 2   1100  must newly generate a session with the message &amp; media storage  4000 . The conversation server  3000  makes a command for generating a session between the client #A- 2   1100  and the message &amp; media storage  4000  using a REFER message (S 315 ). Upon acquisition of the REFER message, the client #A- 2   1100  uses an ACCEPTED message to indicate whether the REFER message has been received (S 316 ). To notify that a REFER event operates in the client #A- 2   1100 , the client #A- 2   1100  transmits a NOTIFY message to the conversation server  3000  (S 317 ). The conversation server  3000  returns an OK message in response to the NOTIFY message (S 318 ). 
     Then, the client #A- 2   1100  transmits an INVITE message to generate a session with the message &amp; media storage  4000  (S 319 ). The message &amp; media storage  4000  returns an OK message in response to the INVITE message (S 320 ). In response to the OK message, an ACK message is returned to the message &amp; media storage  4000  (S 321 ). Accordingly, a session between the client #A- 2   1100  and the message &amp; media storage  4000  is generated completely and the client #A- 2   1100  can receive a session history through the session. 
     On the other hand, the client #A- 2   1100  uses an IMAP protocol to acquire a session history stored in the message &amp; media storage  4000 . The client #A- 2   1100  uses an IMAP LOGIN message for user login in the message &amp; media storage  4000  (S 322 ). The client #A- 2   1100  receives an OK LOGIN message from the message &amp; media storage  4000  (S 323 ). The client #A- 2   1100  selects a storage space of the user A of the message &amp; media storage  4000  using an IMAP SELECT message (S 324 ). The message &amp; media storage  4000  returns a response to the selection of the storage space of the client #A- 2   1100  (S 325 ). The client #A- 2   1100  transmits an IMAP FETCH message to retrieve the session history stored in the message &amp; media storage  4000  (S 326 ). The message &amp; media storage  4000  transmits the session history to the client #A- 2   1100  (S 327 ). 
     The client #A- 2   1100  notifies the conversation server  3000  of REFER EVENT termination using a NOTIFY message (S 328 ). The conversation server  3000  returns a response to the NOTIFY message to the client #A- 2   1100  (S 329 ). 
     The operation of constituent elements of the CPM system operating as illustrated in  FIG. 11  will be described with reference to  FIGS. 12 to 14 . Here, since the state variation of the client #A- 1   1000  requesting the session transfer is the same as that of  FIG. 4 , its description is omitted. 
       FIG. 12  illustrates the state variation of the client #A- 2   1100 . When the client #A- 2   1100  of an initial state in step  800  receives a temporarily stored message from the conversation server  3000  in step  802 , the temporarily stored message is rendered in step  804 . Subsequently, when the conversation with the client #B  2000  is resumed in step  806 , the transition to a conversation resumption state is made in step  808 . Then, when a session with the message &amp; media storage  4000  is generated in step  810 , an operation is performed in a session generation state in step  812 , such that the session generation operation is completed. Subsequently, when the client #A- 2   1100  logs in the message &amp; media storage  4000  in step  814 , the transition to a login state is made in step  816 . When a storage space of the message &amp; media storage  4000  is selected in step  818 , an operation is performed in a selection state in step  820 . When a session history retrieval operation must be performed in the message &amp; media storage  4000  in step  822 , the client #A- 2   1100  retrieves a session history in step  824 . Accordingly, when the session history is transmitted from the message &amp; media storage  4000  in step  826 , the session history is received in step  828 . When a screen output operation is performed in step  830 , the received session history is output on the screen of the client #A- 2   1100 . Accordingly, the session history is rendered in step  832 . 
       FIG. 13  illustrates the state variation of the conversation server  3000 . When the conversation server  3000  of an initial state in step  834  receives a session transfer command from the client #A- 1   1000  in step  836 , a temporary storing operation is started in step  838 . After pre-requesting the message &amp; media storage  4000  to provide the session history in step  840 , the conversation server  3000  invites the client #A- 2   1100  in step  842 . Then, when a routing change to the client #A- 2   1100  is made in step  844 , the conversation server  3000  performs a temporary storing operation in step  846 . Subsequently, when the conversation server  3000  must transmit the temporarily stored message to the client #A- 2   1100  in step  848 , the temporarily stored message is transmitted to the client #A- 2   1100  in step  850 . Subsequently, when the message transmission is completed, the conversation server  3000  determines that a session generation command is needed in step  852  and operates in an idle state in step  856  after commanding the client #A- 2   1100  to generate a session with the message &amp; media storage  4000  in step  854 . 
       FIG. 14  illustrates the state variation of the message &amp; media storage  4000 . When the message &amp; media storage  4000  is in an initial state in step  858  and the client #A- 2   1100  performs a login operation in step  860 , the message &amp; media storage  4000  operates in a login state in step  862 . Subsequently, when a storage space of the message &amp; media storage  4000  is selected in step  864 , the message &amp; media storage  4000  operates in a selection state in step  866 . Subsequently, when a session history is retrieved from the message &amp; media storage  4000  in step  868 , the message &amp; media storage  4000  operates in a session history retrieval state in step  870 . Then, when the session history must be transmitted to the client #A- 2   1100  in step  872 , the message &amp; media storage  4000  performs an operation for transmitting the session history to the client #A- 2   1100  in step  874 . When the transmission is completed in step  876 , the message &amp; media storage  4000  operates in an idle state in step  878 . 
     In the case of a source device initiated session transfer flow model in which a session history is transmitted in a background manner according to the second embodiment of the present invention and a source device initiated session transfer flow model in which the client directly receives a session history from the message &amp; media storage  4000  according to the third embodiment of the present invention, a waiting time of the user B can be minimized by minimizing a session transfer time with only a session generation time in the conversation server  3000 . Accordingly, after session history transmission is completed through session media transmission immediately after the user A performs the session transfer, the session history can be referred to, such that the user A can refer to the session history during the conversation. 
     While the session transfer between multiple clients is in progress, for example, while the session transfer is in progress when the user A makes a request for transferring the session to another client of the user A, the user B holding a conversation with the user A does not know this situation. Thus, the session transfer according to the embodiment of the present invention can be referred to as transparency session transfer. For the transparency session transfer, it must be ensured that data from the user B to the user A safely arrives at the client #A- 2   1100  without data loss. In the present invention, the transparency session transfer is ensured through the temporary storing technique. 
     Specifically, from the moment when the user A requests the session transfer and sends its notification to the conversation server  3000 , the conversation server  3000  intercepts and temporarily stores data, for example, a message, transmitted from the user B. Subsequently, the conversation server  3000  must generate a session with the client #A- 2   1100  and make a routing change. The temporary storing operation is terminated and the temporarily stored message is transmitted to the client #A- 2   1100 . Thus, the message transmitted by the user B is transmitted to the user A in the temporarily stored message form without omission. 
     As is apparent from the foregoing description, according to embodiments of the present invention, session transfer is naturally performed without lost content by temporarily storing and transmitting a session history and a message of a counterpart user to a second client during the session transfer. In a target client to which the session has been transferred, a session history of an existing client is output to a screen, such that a user can continue a conversation while referring to existing conversation content. The user does not need to read conversation content from the screen of the previous client. User convenience can be improved using a CPM service. 
     While the invention has been shown and described with reference to a certain preferred embodiment 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.