Abstract:
A method and device of session control are used to resolve the problem that when there is a failure in the session control function bound with the user, the UE can not re-login in time or the session can not be continued. In the flow of the user initial login, an address information of P-CSCF bound with the user in HSS is saved, so that when the user is called, an available S-CSCF can get the address information of P-CSCF bound with the called user from the HSS, and inform the user to do re-initial login according to the address information of P-CSCF or complete the session continuation; further, the address information of the user in D-HSS is saved, so that when the user is called, an available A-CSCF can get the address information of the called user from the D-HSS, and inform the user to do re-initial login according to the address information of the called user or complete the session continuation.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of IP Multimedia Subsystem (IMS) and particularly to a disaster recovery in a session control function. 
       BACKGROUND OF THE INVENTION 
       [0002]    An IMS is a network architecture defined by the Third Generation Partnership Project (3GPP) and overlying a network in the circuit domain and the packet domain to provide a User Equipment (UE) with various services over an IP network. 
         [0003]    The IMS involves primary function entities including a Call Session Control Function (CSCF) capable of UE registration control, session control, etc., and a Home Subscriber Server (HSS) for centralized management of UE subscription data. A user equipment accesses the IMS through a proxy node Proxy-Call Session Control Function (P-CSCF) in the domain where the UE is currently located and performs session and service trigger control through a Service-Call Session Control Function (S-CSCF) in the home domain. 
         [0004]    Each UE who has subscribed for an IMS service is assigned with one or more IMS Private User Identities (IMPI) by a home network operator to be used for registration, authorization, management and charging and typically not to be open to the other UEs and is also provided with one or more IMS Public User Identities (IMPU) to be open to the other UEs and to be used for sessions of various services to identify himself or herself and another UE with whom he or she communicates. 
         [0005]    Access flows of an IMS UE may include initial UE registration, renewed UE registration, UE de-registration, and network initiated renewed registration. 
         [0006]    A UE initiated initial registration flow is as illustrated in  FIG. 1 : 
         [0007]    a UE composes a REGISTER request message in the Session Initiation Protocol (SIP) from an IMPU, an IMPI, a Contact address and a home domain name stored in its own IMS Subscriber Identity Module (ISIM), where the register request message further includes the type and identifier information of an access network of the UE, information on an option of supported encryption and an integrity algorithm, and information on a port and a time-out period required to set up Security Association (SA) with a P-CSCF, and then transmits the register request message to a default address of the P-CSCF found by the UE during a finding process of P-CSCF; 
         [0008]    upon reception of the register request message, the P-CSCF stores the identifier of the UE and other necessary information, makes a query about an address of an Interrogating-Call Session Control Function (I-CSCF) in the home domain of the UE according to the domain name of the home domain of the UE, and composes and then transmits a new REGISTER request message (including information on a visited network) to the address of the I-CSCF obtained from the query; 
         [0009]    the I-CSCF inquires an HSS about the registration status of the UE according to the IMPI of the UE, and if the UE has not been registered, the I-CSCF may select an S-CSCF for processing the register request of the UE and transmit the register request message to the selected S-CSCF for further processing after selecting the S-CSCF; 
         [0010]    the S-CSCF determines initial registration of the UE upon reception of the register request message and requests the HSS for assigning the UE with authentication vectors, which are the same as authentication vectors of a 3G UE in terms their compositions and components, including a five-element vector of a Random Number (RAND), an Expected Response (XRES), an Authentication Token (AUTN), an Integrity Key (IK) and a Ciphering Key (CK). Upon reception of an assignment result from the HSS, the S-CSCF selects one of the authentication vectors to be included in a  401  message of the S-CSCF, removes the XRES item from the authentication vector and transmits the  401  message to the P-CSCF through the I-CSCF, where the  401  message typically includes the RAND, the AUTN, the IK and the CK; 
         [0011]    the P-CSCF stores the CK and the IK and thereafter removes the CK and the IK from the authentication vector, and includes the remaining information into the  401  message and transmits it to the UE, where the  401  message primarily includes the RAND and the AUTH; 
         [0012]    the UE calculates the CK, the IK and a Response (RES) from an authentication key shared with the network and the received RAND, and also may further process the CK in a selected encryption algorithm. After calculating the RES required for the network, the UE composes, encrypts, integrity-protects and then transmits a new REGISTER request message to the P-CSCF over a secure channel of the P-CSCF. The REGISTER request message includes the RAND, the AUTN and the UE-selected encryption algorithm used for the UE to perform calculation and the calculated authentication result RES; 
         [0013]    the P-CSCF performs corresponding decryption on the REGISTER request message upon reception thereof, and if the register request can result from resolution, it indicates that the network and the UE have performed encryption and integrity-protection, and then the I-CSCF transmits the REGISTER request message including the authentication result to the S-CSCF, and upon reception of the REGISTER request message, the S-CSCF compares the RES therein and the previously stored XRES for consistency, and if they are consistent, it indicates that authentication is passed. The S-CSCF transmits to the HSS a Server-Assignment-Request (SAR) for the service registration information on an Initial Filter Criteria (iFC) of the UE, and the HSS updates the UE registration information with “Registered”, stores the domain name information of the S-CSCF and also returns the iFC in a Server-Assignment-Answer (SAA) to the S-CSCF. Then, the S-CSCF transmits a  200  OK message to the UE to indicate successful registration, where the  200  OK message includes a network-designated period of valid registration in second (600000 seconds in the standard of 3GPP 24.229); 
         [0014]    the P-CSCF initiates a flow of subscribing a registration period packet of the UE to the S-CSCF upon reception of the  200  OK response to the register request, and upon successful subscription, the S-CSCF returns  200  OK indicating successful subscription. Thus, the UE completes initial registration. 
         [0015]    Assumed both a UE 1  and a UE 2  belong to the same IMS network and both of them support 100 rel and Precondition, they can perform a session after performing initial registration described above, and as illustrated in  FIG. 2 , a session flow is as follows: 
         [0016]    the UE 1  transmits an INVITE session request to an S-CSCF 1  through a P-CSCF 1 ; 
         [0017]    the S-CSCF 1  transmits a domain name resolution request to a Domain Name Server (DNS) for the IP address of an I-CSCF 2 , and the S-CSCF 1  transmits the INVITE session request to the I-CSCF 2  according to the IP address of the I-CSCF 2  returned from the DNS; 
         [0018]    the I-CSCF 2  transmits a Location-Info-Request (LIR) to the HSS through a reference point Cx for the domain name information of an S-CSCF 2  bound with the UE 2 , and the HSS returns a Location-Info-Answer (LIA) including the domain name information of the S-CSCF 2  bound with the UE 2  through the reference point Cx; 
         [0019]    the I-CSCF 2  transmits a domain name resolution request to the DNS according to the domain name information of the S-CSCF 2  returned from the HSS, and the DNS returns the IP address of the S-CSCF 2  to the I-CSCF 2 ; 
         [0020]    the I-CSCF 2  transmits the INVITE session request to the S-CSCF 2  according to the IP address of the S-CSCF 2  returned from the DNS, the S-CSCF 2  forwards the INVITE session request to the UE 2  through a P-CSCF 2 , and the UE 2  transmits a message indicating an ongoing session to the UE 1  over a path shown in the steps  10 - 14  in  FIG. 2  to indicate that the UE 1  and the UE 2  may set up a session flow. 
         [0021]    However, if a failure occurs with the I-CSCF 2  and/or S-CSCF 2  to which the UE 2  belongs during the session, as illustrated in  FIG. 3 , the INVITE session request transmitted from the UE 1  can not be transmitted to the UE 2 , thus failing to switch the session; and also due to a long period of time for renewed registration of the UE, approximately seven days (600000 seconds in the standard of 3GPP 24.229), timely renewed registration of the UE to an available call session control function can not be performed. If the UE 2  does not initiates actively any call prior to renewed registration, the core network can not find the failure with the I-CSCF 2  and/or S-CSCF 2  to which the UE 2  belongs, and consequently the UE 2  can not be called successfully from any other UE. 
       SUMMARY OF THE INVENTION 
       [0022]    Embodiments of the invention provide a session control method by which it is possible to notify a called UE of renewing initialized registration in time or switch a session to the called UE in the event that a relevant session control function controlling switching for the called UE fails. 
         [0023]    An embodiment of the invention provides a session control method in an IMS, which includes:
       requesting, by a third Interrogation-Call Session Control Function, I-CSCF, a Domain Name Server, DNS, for address information of a second Service-Call Session Control Function, S-CSCF, bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE;   requesting, by the third I-CSCF, again the DNS for address information of an available third S-CSCF upon failing to transmit the session request of the first UE to the second S-CSCF according to the address information of the second S-CSCF;   forwarding, by the third I-CSCF, the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the DNS; and   instructing, by the third S-CSCF, the second UE to renew initialized registration through a second Proxy-Call Session Control Function, P-CSCF, bound with the second UE according to address information of the second P-CSCF, or obtaining service registration information iFC of the second UE from an HSS and switching the current session to the second UE according to the address information of the second P-CSCF bound with the second UE and the iFC.       
 
         [0028]    Preferably, the third I-CSCF obtains the address information of the second P-CSCF from the HSS and carries the address information of the second P-CSCF in the session request forwarded to the third S-CSCF; or the third S-CSCF obtains the address information of the second P-CSCF together with the service registration information iFC of the second UE from the HSS. 
         [0029]    Preferably, before the third I-CSCF receives the session request of the first UE transmitted from the first S-CSCF bound with the first UE, the method further includes:
       obtaining, by the first S-CSCF, address information of a second I-CSCF to which the second UE belongs from the DNS upon reception of the session request of the first UE, and forwarding the session request of the first UE to the second I-CSCF; and   requesting again the DNS for address information of the available third I-CSCF upon failing to forward the session request of the first UE, and transmitting the session request of the first UE to the third I-CSCF according to the address information of the third I-CSCF.       
 
         [0032]    Preferably, the third S-CSCF further notifies the first UE of a failure of switching the session through the second P-CSCF bound with the second UE after instructing the second UE to renew initialized registration through the second P-CSCF according to the address information of the second P-CSCF. 
         [0033]    An embodiment of the invention provides a session control device in an IMS, which includes:
       a unit configured to obtain, from an HSS, domain name information of a second S-CSCF bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE;   a unit configured to request again a DNS for address information of an available third S-CSCF upon failing to transmit the session request of the first UE to the second S-CSCF according to the domain name information of the second S-CSCF;   a unit configured to forward the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the DNS; and   a unit configured to instruct the second UE to renew initialized registration through a second P-CSCF bound with the second UE according to address information of the second P-CSCF, or a unit configured to obtain, from the HSS, service registration information iFC of the second UE and switch the current session to the second UE according to the address information of the second P-CSCF bound with the second UE and the iFC.       
 
         [0038]    An embodiment of the invention further provides a session control method in an IMS, includes:
       requesting, by a third I-CSCF, an Enhanced-Domain Name Server, E-DNS, for address information of a second S-CSCF bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE;   transmitting, by the E-DNS, address information of an available third S-CSCF to the third I-CSCF upon determining, from results of monitoring S-CSCFs, that the second S-CSCF fails;   forwarding, by the third I-CSCF, the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the E-DNS; and   instructing, by the third S-CSCF, the second UE to renew initialized registration through a second P-CSCF bound with the second UE according to address information of the second P-CSCF, or obtaining service registration information iFC of the second UE from an HSS and switching the current session to the second UE according to the address information of the second P-CSCF bound with the second UE and the iFC.       
 
         [0043]    An embodiment of the invention further provides a session control device in an IMS, which includes:
       a unit configured to request an E-DNS for address information of a second S-CSCF bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE;   a unit configured to transmit address information of an available third S-CSCF to a third I-CSCF upon determining, from results of monitoring S-CSCFs, that the second S-CSCF fails;   a unit configured to forward the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the E-DNS; and   a unit configured to instruct the second UE to renew initialized registration through a second P-CSCF bound with the second UE according to the address information of the second P-CSCF, or a unit configured to obtain, from an HSS, service registration information iFC of the second UE and switch the current session to the second UE according to the address information of the second P-CSCF bound with the second UE and the iFC.       
 
         [0048]    An embodiment of the invention provides a session control method in the next generation IMS, which includes:
       obtaining, by a first Access-Control Session Control Function, A-CSCF, bound with a first UE, domain name information of a second A-CSCF bound with a second UE involved in a current session from a Distributed-Home Subscriber Server, D-HSS, upon reception of a session request of the first UE;   requesting, by the first A-CSCF, again a Domain Name Server, DNS, for address information of an available third A-CSCF upon failing to forward the session request of the first UE to the second UE through the second A-CSCF according to the domain name information of the second A-CSCF;   forwarding, by the first A-CSCF, the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF returned from the DNS; and   instructing, by the third A-CSCF, the second UE to renew initialized registration according to address information of the second UE, or obtaining service registration information iFC of the second UE from the D-HSS and switching the current session to the second UE according to the address information and iFC of the second UE.       
 
         [0053]    An embodiment of the invention provides a session control device in the next generation IMS, which includes:
       a unit configured to obtain from a D-HSS domain name information of a second A-CSCF bound with a second UE involved in a current session when a first A-CSCF bound with a first UE receives a session request of the first UE;   a unit configured to request again a DNS for address information of an available third A-CSCF upon failing to forward the session request of the first UE to the second UE through the second A-CSCF according to the domain name information of the second A-CSCF;   a unit configured to forward the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF returned from the DNS; and   a unit configured to instruct the second UE to renew initialized registration according to address information of the second UE, or a unit configured to obtain the service registration information iFC of the second UE from the D-HSS and switch the current session to the second UE according to the address information and iFC of the second UE.       
 
         [0058]    An embodiment of the invention further provides a session control method in the next generation IMS, which includes:
       obtaining, by a first A-CSCF bound with a first UE, domain name information of a second A-CSCF bound with a second UE involved in a current session from a D-HSS upon reception of a session request of the first UE, and transmitting to an E-DNS a request for resolving the domain name information of the second A-CSCF;   transmitting, by the E-DNS, address information of an available third A-CSCF to the first A-CSCF upon determining, from results of monitoring A-CSCFs, that the second A-CSCF fails;   forwarding, by the first A-CSCF, the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF; and   instructing, by the third A-CSCF, the second UE to renew initialized registration according to address information of the second UE, or obtaining service registration information iFC of the second UE from the D-HSS and switching the current session to the second UE according to the address information and iFC of the second UE.       
 
         [0063]    An embodiment of the invention further provides a session control device in the next generation IMS, which includes:
       a unit configured to obtain from a D-HSS domain name information of a second A-CSCF bound with a second UE involved in a current session when a first A-CSCF bound with a first UE receives a session request of the first UE and transmit to an E-DNS a request for resolving the domain name information of the second A-CSCF;   a unit configured to transmit address information of an available third A-CSCF to the first A-CSCF upon determining, from results of monitoring A-CSCFs, that the second A-CSCF fails;   a unit configured to forward the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF; and   a unit configured to instruct the second UE to renew initialized registration according to address information of the second UE, or a unit configured to obtain service registration information iFC of the second UE from the D-HSS and switch the current session to the second UE according to the address information and iFC of the second UE.       
 
         [0068]    With the session control in an IMS according to the embodiments of the invention, in an initial registration flow of the UE, the address information of a P-CSCF bound with the UE is stored into an HSS so that when the UE is called, an available S-CSCF can obtain the address information of the P-CSCF bound with the called UE from the HSS and instruct the UE to renew initialized registration or switch a session according to the address information of the P-CSCF. 
         [0069]    Furthermore, with the session control in the next generation IMS according to the embodiments of the invention, the address information of the UE is stored into a D-HSS so that when the UE is called, an available A-CSCF can obtain the address information of the called UE from the D-HSS and instruct the UE to renew initialized registration or switch a session according to the address information of the called UE. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0070]      FIG. 1  illustrates a flow chart of initial registration of a UE in the prior art; 
           [0071]      FIG. 2  illustrates a flow chart of a session between two UEs in the prior art; 
           [0072]      FIG. 3  illustrates a schematic flow chart of a failing session control function in the prior art; 
           [0073]      FIGS. 4A and 4B  illustrate schematic flow charts of two session control methods according to a first embodiment of the invention respectively; 
           [0074]      FIG. 5  illustrates a schematic flow chart of a session control method according to a second embodiment of the invention; 
           [0075]      FIG. 6  illustrates a schematic flow chart of a session control method according to a third embodiment of the invention; 
           [0076]      FIG. 7  illustrates a schematic flow chart of a session control method according to a fourth embodiment of the invention; 
           [0077]      FIG. 8  illustrates a schematic flow chart of a session control method according to a fifth embodiment of the invention; 
           [0078]      FIG. 9  illustrates a schematic flow chart of a session control method according to a sixth embodiment of the invention; 
           [0079]      FIG. 10  illustrates a schematic flow chart of a session control method according to a seventh embodiment of the invention; and 
           [0080]      FIG. 11  illustrates a schematic flow chart of a session control method according to an eighth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0081]    In order to address the problem in an IMS that if an S-CSCF selected by a domain name server for a called UE fails during a session, the called UE can not be called prior to renewed initialized registration thereof, address information of a P-CSCF bound with a UE is stored into an HSS of an access network system to thereby ensure that an available S-CSCF requested again for can instruct the called UE to renew initialized registration or switch the session according to the address information of the P-CSCF bound with the UE according to embodiments of the invention. 
         [0082]    In embodiments of the invention, a P-CSCF, an S-CSCF and an I-CSCF can be integrated into a session control function referred to as an A-CSCF in the next generation IMS. In order to address the problem that if an A-CSCF bound with a called UE fails during a session, the called UE can not be called according to the A-CSCF bounded therewith, address information, IK and CK of the UE are stored upon registration of the UE into a D-HSS adopted for an access network system to thereby ensure that an available A-CSCF requested again for can instruct the called UE to renew initialized registration or switch the session according to the address information, IK and CK of the UE. 
         [0083]    Various embodiments of the invention will be detailed below with reference to the drawings. 
       THE FIRST EMBODIMENT 
       [0084]    As illustrated in  FIG. 4A , a first session control method according to the first embodiment of the invention includes the following steps: 
         [0085]    in steps S 4 A 01  and S 4 A 02 , a UE 1  transmits a session request to an S-CSCF 1  through a P-CSCF 1 ; 
         [0086]    where the session request transmitted from the UE 1 , e.g., an INVITE session request, includes an IMPU of an UE 2 , e.g., Bob@home.net; 
         [0087]    in step S 4 A 03 , the S-CSCF 1  triggers a service request of the UE 1  according to the service registration information iFC of the UE 1  which is obtained during registration of the UE 1 ; 
         [0088]    in steps S 4 A 04  and S 4 A 05 , the S-CSCF 1  transmits a domain name resolution request to an DNS, and the DNS returns to the S-CSCF 1  an IP address assigned to an I-CSCF 2 ; 
         [0089]    where the S-CSCF 1  transmits the domain name resolution request to the DNS according to the IMPU of the UE 2  in the session request, and the DNS returns to the S-CSCF the IP address assigned to the I-CSCF 2 ; 
         [0090]    in steps S 4 A 06  and S 4 A 07 , the S-CSCF 1  transmits the INVITE session request to the I-CSCF 2 , and if the transmission fails, the I-CSCF 2  returns information indicating the failure of the I-CSCF 2  to the S-CSCF 1 ; 
         [0091]    where the S-CSCF 1  transmits the INVITE session request to the I-CSCF 2  according to the IP address of the I-CSCF 2  returned from the DNS, and if the S-CSCF 1  fails to transmit the session request to the I-CSCF 2 , the I-CSCF 2  returns the information indicating the failure of the I-CSCF 2  to the S-CSCF 1 ; 
         [0092]    in steps S 4 A 08  and S 4 A 09 , the S-CSCF 1  retransmits a domain name resolution request including the information indicating the failure of the I-CSCF 2  to the DNS, and according to the information indicating the failure of the I-CSCF 2 , the DNS reassigns an available I-CSCF 3  and returns an IP address of the I-CSCF 3  to the S-CSCF 1 ; 
         [0093]    in step S 4 A 10 , the S-CSCF 1  transmits the INVITE session request to the reselected I-CSCF 3  according to the IP address of the I-CSCF 3  returned from the DNS; 
         [0094]    in step S 4 A 11 , the I-CSCF 3  transmits an LIR to an HSS through a reference point Cx; 
         [0095]    where during initial registration of a UE, the HSS stores the address information of P-CSCFs bound with the UEs, which primarily includes the IP addresses and port number of the P-CSCFs. The IP addresses and port number of the P-CSCFs will be taken as an example throughout the following description. Therefore, the I-CSCF 3  may obtain the address information of the P-CSCF 2  upon transmission of the LIR to the HSS. 
         [0096]    in step S 4 A 12 , the HSS returns an LIA including the address information of the P-CSCF 2  to the I-CSCF 3  through the reference point Cx; 
         [0097]    in steps S 4 A 13  and S 4 A 14 , the I-CSCF 3  transmits a domain name resolution request to the DNS, and the DNS returns an IP address of an S-CSCF 2  to the I-CSCF 3 ; 
         [0098]    in steps S 4 A 15  and S 4 A 16 , the I-CSCF 3  transmits the INVITE session request to the S-CSCF 2  according to the IP address of the S-CSCF 2  returned from the DNS, and if the I-CSCF 3  fails to transmit the session request to the S-CSCF 2 , the S-CSCF 2  returns information indicating the failure of the S-CSCF 2  to the I-CSCF 3 ; 
         [0099]    in steps S 4 A 17  and S 4 A 18 , the I-CSCF 3  retransmits to the DNS a domain name resolution request including the information indicating the failure of the S-CSCF 2 , and the DNS reselects an available S-CSCF 3  according to the information indicating the failure of the S-CSCF 2  and returns the IP address of the S-CSCF 3  to the I-CSCF 3 ; 
         [0100]    in step S 4 A 19 , the I-CSCF 3  transmits the INVITE session request to the S-CSCF 3  including the IP address and port number of the P-CSCF 2  according to the IP address of the S-CSCF 3  returned from the DNS; 
         [0101]    in steps S 4 A 20  and S 4 A 21 , the S-CSCF 3  transmits a Notify notice to the UE 2  through the P-CSCF 2 ; 
         [0102]    where upon reception of the INVITE session request from the I-CSCF 3 , the S-CSCF 3  determines, from a correspondence relationship between an S-CSCF, a P-CSCF and a registered UE resulting from registration of the UE, that the UE 2  has not been registered with the S-CSCF 3 , and then transmits the Notify notice to the P-CSCF 2  according to the address information of the P-CSCF 2  in the INVITE session request, and the P-CSCF 2  forwards the Notify notice to the UE 2  to trigger a flow of renewing initialized registration of the UE 2 ; 
         [0103]    in steps S 4 A 22  and S 4 A 23 , the UE 2  transmits  200  OK to the S-CSCF 3  through the P-CSCF 2  to indicate successful reception of the notice to renew initialized registration; 
         [0104]    in steps S 4 A 24  and S 4 A 25 , the S-CSCF 3  transmits  503  Service Unavailable to the UE 1  through the S-CSCF 1  to indicate that the UE 2  has not been registered with the S-CSCF 3 , so no session can be set up with the UE 2 , thus failing to switch a session; and 
         [0105]    in step S 4 A 26 , the UE 2  requests the DNS again to obtain address information of another S-CSCF from resolution and then initiates a flow of renewing initialized registration to thereby ensure that the UE 2  can be called successfully from another UE. 
         [0106]    The flow of renewing initialized registration is the same as a renewed registration process in the prior art and a repeated description thereof is omitted here. 
         [0107]    As illustrated in  FIG. 4B , a second session control method according to the first embodiment of the invention includes the following steps: 
         [0108]    where steps S 4 B 01 -S 4 B 10  are the same as the corresponding steps S 4 A 01 -S 4 A 10  in 
         [0109]      FIG. 4A  and steps S 4 B 20 -S 4 B 26  are the same as the corresponding steps S 4 A 20 -S 4 A 26  in  FIG. 4A , so a repeated description thereof is omitted here and only the differences will be detailed below: 
         [0110]    S 4 B 11 . the I-CSCF 3  transmits to the DNS a domain name resolution request for the address information of the S-CSCF 2 ; 
         [0111]    S 4 B 12 . the DNS returns the address information of the S-CSCF 2  to the I-CSCF 3 ; 
         [0112]    S 4 B 13  and S 4 B 14 . the I-CSCF 3  forwards the session request to the S-CSCF 2  according to the address information of the S-CSCF 2  returned from the DNS, and the S-CSCF 2  returns the failure indication information to the I-CSCF 3 ; 
         [0113]    S 4 B 15  and S 4 B 16 . the I-CSCF 3  retransmits a domain name resolution request carrying the information indicating the failure of the S-CSCF 2  to the DNS, and the DNS selects and returns to the I-CSCF 3  the address information of the available S-CSCF 3  according to the failure indication information carried in the domain name resolution request; 
         [0114]    S 4 B 17 . the I-CSCF 3  forwards the session request of the UE 1  to the S-CSCF 3 ; 
         [0115]    S 4 B 18 . the S-CSCF 3  transmits through the reference point to the HSS the LIR for the IP address and port number of the P-CSCF 2 ; and 
         [0116]    S 4 B 19 . the HSS returns the address information of the P-CSCF 2  to the S-CSCF 3 . 
         [0117]    The first embodiment of the invention further provides a session control device in an IMS, which includes: 
         [0118]    a unit configured to obtain, from an HSS, domain name information of a second S-CSCF bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE; 
         [0119]    a unit configured to request again a DNS for address information of an available third S-CSCF upon failing to transmit the session request of the first UE to the second S-CSCF according to the domain name information of the second S-CSCF; 
         [0120]    a unit configured to forward the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the DNS; and 
         [0121]    a unit configured to instruct the second UE to renew initialized registration through a second P-CSCF bound with the second UE according to address information of the second P-CSCF. 
       THE SECOND EMBODIMENT 
       [0122]    As illustrated in  FIG. 5 , a session control method according to the second embodiment of the invention includes the following steps: 
         [0123]    where steps S 501 -S 519  are the same as the corresponding steps S 4 A 01 -S 4 A 19  in  FIG. 4A , so a repeated description thereof is omitted here and only the differences will be detailed below: 
         [0124]    in step S 520 , the S-CSCF 3  transmits the SAR to the HSS through the reference point Cx; 
         [0125]    where the SAR transmitted from the S-CSCF 3  includes UE 2  SIP URI and S-CSCF 3  SIP, and the service registration information iFC of the UE 2  is obtained from the HSS; 
         [0126]    where the S-CSCF 3  may obtain the iFC in various ways, for example, by copying, downloading, etc., from the HSS; 
         [0127]    in step S 521 , the HSS returns the SAA to the S-CSCF 3  through the reference point Cx; 
         [0128]    where the HSS detects, from the received SAR, whether the registration status of the UE 2  is “Registered”, and if so, the HSS has stored the information on UE 2  SIP URI and S-CSCF 3  SIP and returns the SAA including the iFC of the UE 2  to the S-CSCF 3  through the reference point Cx; 
         [0129]    in step S 522 , the S-CSCF 3  triggers the session request of the UE 2  according to the obtained iFC of the UE 2 , and since the S-CSCF 3  has obtained the iFC of the UE 2 , the S-CSCF 3  may forward the INVITE session request of the UE 1  to the UE 2 ; 
         [0130]    in steps S 523  and S 524 , the S-CSCF 3  transmits the INVITE session request to the UE 2  through the P-CSCF 2  according to the iFC of the UE 2  and the obtained address information of the P-CSCF 2  bound with the UE 2 ; 
         [0131]    where it shall be noted that alike the two embodiments in  FIGS. 4A and 4B , the S-CSCF 3  may obtain the address information of the P-CSCF 2  from the received session request or download the IP address and port number of the P-CSCF 2  from the HSS upon reception of the session request to thereby obtain the address information of the P-CSCF 2  in the second embodiment of the invention; 
         [0132]    in steps S 525 -S 527 , the UE 2  returns  200  OK to the S-CSCF 1  through the P-CSCF 2  to indicate successful switching of the session upon reception of the INVITE session request, and then the UE 1  and the UE 2  may perform the current session. A session flow thereof is the same as that in the prior art and a repeated description thereof is omitted here. 
         [0133]    The second embodiment of the invention further provides a session control device in an IMS, which includes: 
         [0134]    a unit configured to obtain, from a HSS, domain name information of a second S-CSCF bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE; 
         [0135]    a unit configured to request again a DNS for address information of an available third S-CSCF upon failing to transmit the session request of the first UE to the second S-CSCF according to the domain name information of the second S-CSCF; 
         [0136]    a unit configured to forward the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the DNS; and 
         [0137]    a unit configured to obtain, from the HSS, service registration information iFC of the second UE and switch the current session to the second UE according to address information of a second P-CSCF bound with the second UE and the iFC of the second UE. 
       THE THIRD EMBODIMENT 
       [0138]    In the present embodiment, a traditional DNS is replaced with an Enhanced Domain 
         [0139]    Name Server (E-DNS), which may perform the following functions as compared with the traditional DNS. 
         [0140]    The E-DNS may monitor in real time the statuses of I-CSCFs and S-CSCFs, i.e., set up connections with the I-CSCFs and S-CSCFs in a specific region, possibly in a heartbeat monitoring method. Typically, an application layer based heartbeat monitoring method is based upon the User Datagram Protocol (UDP), and due to a low overhead of the UDP, real time monitoring may be performed, for example, in a period of 1 s. Alternatively, another monitoring method may be used, for example, a monitoring mechanism performed in a protocol at the transmission layer, and this monitoring method is similar to the UDP-based heartbeat monitoring method, for example, a Keep-alive monitoring method in the Transmission Control 
         [0141]    Protocol (TCP) with a typical monitoring period of 60 s, a heartbeat monitoring method in the Streaming Control Transmission Protocol (SCTP) with a typical monitoring period of 30 s. When a UE initiates a initial session request, if it is found that an I-CSCF and/or an S-CSCF fails, an available I-CSCF and/or S-CSCF may be selected in a load-balance policy as a session control function. Thus as illustrated in  FIG. 6 , a session control method according to the third embodiment of the invention includes the following steps: 
         [0142]    where steps S 601 - 5603  are the same as the corresponding steps S 4 A 01 -S 4 A 03  in  FIG. 4A  and steps S 611 - 5618  are the same as the corresponding steps S 4 A 19 -S 4 A 26  in  FIG. 4A , so a repeated description thereof is omitted here and only the differences will be detailed below: 
         [0143]    in steps S 604  and S 605 , the S-CSCF 1  transmits to the E-DNS a domain name resolution request for the address information of the I-CSCF 3 ; 
         [0144]    where the S-CSCF 1  requests the E-DNS for resolving the domain name of the I-CSCF 2 , and the E-DNS monitors and determines that the I-CSCF 2  fails and returns the IP address of the available I-CSCF 3  to the S-CSCF 1 ; 
         [0145]    where the E-DNS may monitor the statuses of the I-CSCFs and the S-CSCFs in a specific region in a heartbeat monitoring method or a Keep-alive monitoring method. That is, the E-DNS may set up a heartbeat or keep-alive connection with the I-CSCF and the S-CSCF of a called UE to have a knowledge of the load conditions of the I-CSCF and the S-CSCF, and if the I-CSCF or the S-CSCF fails, the E-DNS identifies the I-CSCF or the S-CSCF as “Unavailable” and selects duly an available I-CSCF or an available S-CSCF as a session control function for the UE; 
         [0146]    in step S 606 , the S-CSCF 1  forwards the INVITE session request to the available I-CSCF 3  according to the IP address of the I-CSCF 3  returned from the E-DNS; 
         [0147]    in step S 607 , the I-CSCF 3  transmits through the reference point Cx to the HSS the LIR for the address information of the S-CSCF 2  bound with the UE 2  from the HSS, the address information includes the IP address and the port number. 
         [0148]    in step S 608 , the HSS returns the LIA to the I-CSCF 3  through the reference point Cx; 
         [0149]    where the returned LIA carries the address information of the S-CSCF 2 ; 
         [0150]    in steps S 609  and S 610 , the I-CSCF 3  requests the E-DNS for the address information of the S-CSCF 2 ; and 
         [0151]    the E-DNS monitors and detects that the S-CSCF 2  corresponding to registration of the UE fails and returns the IP address of the available S-CSCF 3  to the I-CSCF 3 . 
         [0152]    It shall be noted that the S-CSCF 3  may obtain the address information of the P-CSCF 2  from the received session request or download the IP address and port number of the P-CSCF 2  from the HSS upon reception of the session request to obtain the address information of the P-CSCF 2 . 
         [0153]    The third embodiment of the invention further provides a session control device in an IMS, which includes: 
         [0154]    a unit configured to request an E-DNS for address information of a second S-CSCF bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE; 
         [0155]    a unit configured to transmit address information of an available third S-CSCF to a third I-CSCF upon determining, from results of monitoring S-CSCFs, that the second S-CSCF fails; 
         [0156]    a unit configured to forward the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the E-DNS; and 
         [0157]    a unit configured to instruct the second UE to renew initialized registration through a second P-CSCF bound with the second UE according to address information of the second P-CSCF. 
       THE FOURTH EMBODIMENT 
       [0158]    As illustrated in  FIG. 7 , the fourth embodiment of the invention provides a session control method, and steps S 701 -S 711  are the same as the corresponding steps S 601 -S 611  in the third embodiment and steps S 712 -S 719  are the same as the corresponding steps S 520 -S 527  in the second embodiment, so a repeated description thereof is omitted here. 
         [0159]    It shall be noted that the S-CSCF 3  may obtain the address information of the P-CSCF 2  from the received session request or from the HSS. 
         [0160]    The fourth embodiment of the invention further provides a session control device in an IMS, which includes: 
         [0161]    a unit configured to request an E-DNS for address information of a second S-CSCF bound with a second UE involved in a current session upon reception of a session request of a first UE transmitted from a first S-CSCF bound with the first UE; 
         [0162]    a unit configured to transmit address information of an available third S-CSCF to a third I-CSCF upon determining, from results of monitoring S-CSCFs, that the second S-CSCF fails; 
         [0163]    a unit configured to forward the session request of the first UE to the third S-CSCF according to the address information of the third S-CSCF returned from the E-DNS; and 
         [0164]    a unit configured to obtain, from an HSS, service registration information iFC of the second UE and switch the session to the second UE according to address information of a second P-CSCF bound with the second UE and the iFC. 
         [0165]    With the session control in an IMS according to the embodiments of the invention, the address information of a P-CSCF bound with a UE is stored into an HSS in an initial registration flow of the UE so that when the UE is called, an available S-CSCF may obtain the address information of the P-CSCF bound with the called UE from the HSS and instruct the called UE to renew initialized registration or switch a session according to the obtained address information of the P-CSCF. 
       THE FIFTH EMBODIMENT 
       [0166]    Based upon the methods according to the foregoing embodiments, the inventors identify that the session flow based upon an IMS logic architecture is rather complex and then propose a Next Generation IP Multimedia Subsystem (NGIMS) logic architecture and also identify that the foregoing disaster discovery of a session control device is equally applicable to the NGIMS. 
         [0167]    In the NGIMS, there are primarily two control functions, an Access-Call Session 
         [0168]    Control Function (A-CSCF) and a Distributed Home Subscriber Server (D-HSS), both of which exist at the session control layer. 
         [0169]    The A-CSCF in the NGIMS includes the functions of an I-CSCF, an S-CSCF and a P-CSCF in the IMS, that is, a control function A-CSCF is integrated with the functions of the three control functions, an I-CSCF, an S-CSCF and a P-CSCF, to thereby simply the logic architecture of the system. 
         [0170]    The D-HSS stores therein both service registration information iFC of a UE, domain name information of an A-CSCF bound with a UE when the UE is registered, and address information and cipher information of the UE. The address information of the UE primarily includes an IP address and port number of the UE. The IP address and port number of the UE will be taken as an example throughout the following description. Typically, the cipher information includes a pair of IK and CK. 
         [0171]    As illustrated in  FIG. 8 , a flow of a session control method according to the fifth embodiment of the invention includes the following steps: 
         [0172]    in step S 801 , a UE 1  transmits an INVITE session request (including To: Bob@home.net) to an A-CSCF 1  bound with the UE 1 ; 
         [0173]    in step S 802 , the A-CSCF 1  triggers a session request of the UE 1  to a UE 2  according to the iFC of the UE 1  upon reception of the INVITE session request; 
         [0174]    in step S 803 , the A-CSCF 1  transmits through a reference point Cx to a D-HSS an LIR (including Bob@home.net) for address information and cipher information of the UE 2  and domain name information of an A-CSCF 2  from the D-HSS; 
         [0175]    in steps S 804  and S 805 , the D-HSS returns the LIA to the A-CSCF 1  through the reference point Cx; 
         [0176]    where the D-HSS searches data stored throughout the network for the domain name information of the A-CSCF 2  bound with the UE 2  and the address information and cipher information of the UE 2  according to the received LIR; 
         [0177]    in step S 806 , the A-CSCF 1  transmits to the DNS a request for resolving the domain name information of the A-CSCF 2  according to the domain name information of the A-CSCF 2  returned from the D-HSS; 
         [0178]    in step S 807 , the DNS returns the IP address of the A-CSCF 2  to the A-CSCF 1 ; 
         [0179]    in steps S 808  and S 809 , the A-CSCF 1  forwards the INVITE session request including the address information and cipher information of the UE 2  to the A-CSCF 2  according to the IP address of the A-CSCF 2  returned from the DNS, and if the A-CSCF 1  fails to forward the INVITE session request to the A-CSCF 2 , the A-CSCF 2  returns information indicating the failure of the A-CSCF 2  to the A-CSCF 1 ; 
         [0180]    in step S 810 , the A-CSCF 1  retransmits a domain name resolution request to the DNS according to the failure indication information; 
         [0181]    in step S 811 , the DNS returns address information of an available A-CSCF 3  to the A-CSCF  1  according to the information indicating the failure of the A-CSCF 2 ; 
         [0182]    in step S 812 , the A-CSCF 1  forwards the INVITE session request including the address information and cipher information of the UE 2  to the A-CSCF 3  upon reception of the address information of the A-CSCF 3  returned from the DNS; 
         [0183]    in step S 813 , upon reception of the INVITE session request, the A-CSCF 3  determines, from a correspondence relationship between an A-CSCF and a registered UE resulting from registration of the UE, that the UE 2  has not been registered with the A-CSCF 3 , and then transmits a Notify notice to the UE 2  to trigger the UE 2  to renew initialized registration; 
         [0184]    where since the session request received by the A-CSCF 3  carries the address information of the UE 2 , the A-CSCF 3  may transmit the Notify notice to the UE 2  in time to thereby ensure that the UE 2  may be called successfully from another UE after timely renewing of initialized registration; 
         [0185]    in step S 814 , the UE 2  returns  200  OK to the A-CSCF 3  to indicate successful reception of the notice to renew initialized registration upon reception of the Notify notice transmitted from the A-CSCF 3 ; 
         [0186]    in steps S 815  and S 816 , the A-CSCF 3  transmits  503  Service Unavailable to the UE 1  through the A-CSCF 1  to indicate that the UE 2  has not been registered with the A-CSCF 3 , so no session can be set up with the UE 2 , thus failing to switch a session; and 
         [0187]    in step S 817 , the UE 2  requests the DNS again to obtain address information of another A-CSCF from resolution and then initiates a flow of renewing initialized registration to thereby ensure that the UE 2  may be called successfully from another UE after renewing initialized registration. A flow of renewing initialized registration initiated by the UE 2  is the same as that in the prior art and a repeated description thereof is omitted here. 
         [0188]    It shall be noted that in the fifth embodiment of the invention, the A-CSCF 3  may obtain the address information and cipher information of the UE 2  from the received session request or download the IP address, port number, IK and CK of the UE 2  from the D-HSS to obtain the address information and cipher information of the UE 2  upon reception of the session request. 
         [0189]    The fifth embodiment of the invention further provides a session control device in the next generation IMS, which includes: 
         [0190]    a unit configured to obtain from a D-HSS domain name information of a second A-CSCF bound with a second UE involved in a current session when a first A-CSCF bound with a first UE receives a session request of the first UE; 
         [0191]    a unit configured to request again a DNS for address information of an available third A-CSCF upon failing to forward the session request of the first UE to the second UE through the second A-CSCF according to the domain name information of the second A-CSCF; 
         [0192]    a unit configured to forward the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF returned from the DNS; and 
         [0193]    a unit configured to instruct the second UE to renew initialized registration according to address information of the second UE. 
       THE SIXTH EMBODIMENT 
       [0194]    As illustrated in  FIG. 9 , a session control method according to the sixth embodiment of the invention includes the following steps: 
         [0195]    where steps S 901 -S 912  are the same as the corresponding steps S 801 -S 812  in the fifth embodiment, so a repeated description thereof is omitted here and only the differences will be detailed below: 
         [0196]    in step S 913 , the A-CSCF 3  transmits the SAR to the D-HSS through the reference point Cx; 
         [0197]    where upon reception of the INVITE session request, the A-CSCF 3  determines, from a correspondence relationship between an A-CSCF and a registered UE resulting from registration of the UE, that the UE 2  has not been registered with the A-CSCF 3 , and then transmits through the reference point Cx the SAR to the D-HSS for the iFC of the UE 2 ; 
         [0198]    in step S 914 , the D-HSS returns the SAA to the A-CSCF 3  through the reference point Cx; 
         [0199]    where the D-HSS detects whether the registration status of the UE 2  is “Registered” upon reception of the SAR transmitted from the A-CSCF 3 , and if so, the D-HSS stores the domain name information of the available A-CSCF 3  (A-CSCF 3  SIP URL) and returns the SAA including the iFC of the UE 2  to the A-CSCF 3  through the reference point Cx; 
         [0200]    in step S 915 , the A-CSCF 3  triggers a called session request of the UE 2  according to the iFC of the UE 2  obtained from the D-HSS; 
         [0201]    where since the A-CSCF 3  obtains the iFC of the UE 2 , the A-CSCF 3  may forward to the UE 2  the INVITE session request of the UE 1  including the address information and cipher information of the UE 2 ; 
         [0202]    in step S 916 , the A-CSCF 3  forwards the INVITE session request to the UE 2 , so the 
         [0203]    UE 1  and the UE 2  set up a session flow to thereby switch the current session; and 
         [0204]    in steps S 917 -S 919 , the UE 2  returns the  200  OK message to the UE 1  through the A-CSCF 3  to indicate that the UE 1  and the UE 2  may perform the current session upon reception of the INVITE session request. A session flow thereof is the same as that in the prior art and a repeated description thereof is omitted here. 
         [0205]    It shall be noted that in the sixth embodiment of the invention, the A-CSCF 3  may obtain the address information and cipher information of the UE 2  from the received session request or download the IP address, port number, IK and CK of the UE 2  from the D-HSS to obtain the address information and cipher information of the UE 2  upon reception of the session request. 
         [0206]    The sixth embodiment of the invention further provides a session control device in the next generation IMS, which includes: 
         [0207]    a unit configured to obtain from a D-HSS domain name information of a second A-CSCF bound with a second UE involved in a current session when a first A-CSCF bound with a first UE receives a session request of the first UE; 
         [0208]    a unit configured to request again a DNS for address information of an available third A-CSCF upon failing to forward the session request of the first UE to the second UE through the second A-CSCF according to the domain name information of the second A-CSCF; 
         [0209]    a unit configured to forward the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF returned from the DNS; and 
         [0210]    a unit configured to obtain service registration information iFC of the second UE from the D-HSS and switch the current session to the second UE according to address information and iFC of the second UE. 
       THE SEVENTH EMBODIMENT 
       [0211]    In the seventh embodiment, a traditional DNS is replaced with an E-DNS, which may perform the following functions as compared with the traditional DNS. 
         [0212]    The E-DNS may monitor in real time the statuses of A-CSCFs, for example, set up heartbeat connections with A-CSCFs in a specific region. When a UE initiates a session request, if it is found that the A-CSCF bounded with the UE fails, an available A-CSCF may be selected in a load-balance policy as a session control function. Thus as illustrated in  FIG. 10 , a session control method according to the seventh embodiment of the invention includes the following steps: 
         [0213]    where steps S 1001 -S 1005  are the same as the corresponding steps S 801 -S 805  in the fifth embodiment and steps S 1008 -S 1013  are the same as the corresponding steps S 812 -S 817  in the fifth embodiment, so a repeated description thereof is omitted here and only the differences will be detailed below: 
         [0214]    in step S 1006 , the A-CSCF 1  transmits to the E-DNS a request for resolving the domain name information of the A-CSCF 2  according to the LIA returned from the D-HSS; and 
         [0215]    in step S 1007 , the E-DNS detects that the A-CSCF 2  fails and returns the address information of the available A-CSCF 3 , i.e., the IP address of the A-CSCF 3 , to the A-CSCF 1 . 
         [0216]    The E-DNS may monitor the A-CSCF 2  and the A-CSCF 3  in a monitoring method, possibly a heartbeat monitoring method, that is, the E-DNS sets up heartbeat connections with the A-CSCF 2  and the A-CSCF 3 , detects that the A-CSCF 2  fails and returns the IP address of the available A-CSCF 3  to the A-CSCF 1 . 
         [0217]    It shall be noted that in the seventh embodiment of the invention, the A-CSCF 3  may obtain the address information and cipher information of the UE 2  from the received session request or download the IP address, port number, IK and CK of the UE 2  from the D-HSS to obtain the address information and cipher information of the UE 2  upon reception of the session request. 
         [0218]    The seventh embodiment of the invention further provides a session control device in the next generation IMS, which includes: 
         [0219]    a unit configured to obtain from a D-HSS domain name information of a second A-CSCF bound with a second UE involved in a current session when a first A-CSCF bound with a first UE receives a session request of the first UE and transmit to an E-DNS a request for resolving the domain name information of the second A-CSCF; 
         [0220]    a unit configured to transmit address information of an available third A-CSCF to the first A-CSCF upon determining, from results of monitoring A-CSCFs, that the second A-CSCF fails; 
         [0221]    a unit configured to forward the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF; and 
         [0222]    a unit configured to instruct the second UE to renew initialized registration according to address information of the second UE. 
       THE EIGHTH EMBODIMENT 
       [0223]    As illustrated in  FIG. 11 , a session control method according to the eighth embodiment of the invention includes the following steps: 
         [0224]    where steps S 1101 -S 1108  are the same as the corresponding steps S 1001 -S 1008  in the seventh embodiment and steps S 1109 -S 1115  are the same as the corresponding steps S 913 -S 919  in the sixth embodiment, so a repeated description thereof is omitted here. 
         [0225]    It shall be noted that in the eighth embodiment of the invention, the A-CSCF 3  may obtain the address information and cipher information of the UE 2  from the received session request or download the IP address, port number, IK and CK of the UE 2  from the D-HSS to obtain the address information and cipher information of the UE 2  upon reception of the session request. 
         [0226]    The eighth embodiment of the invention further provides a session control device in the next generation IMS, which includes: 
         [0227]    a unit configured to obtain from a D-HSS domain name information of a second A-CSCF bound with a second UE involved in a current session when a first A-CSCF bound with a first UE receives a session request of the first UE and transmit to an E-DNS a request for resolving the domain name information of the second A-CSCF; 
         [0228]    a unit configured to transmit address information of an available third A-CSCF to the first A-CSCF upon determining, from results of monitoring A-CSCFs, that the second A-CSCF fails; 
         [0229]    a unit configured to forward the session request of the first UE to the third A-CSCF according to the address information of the third A-CSCF; and 
         [0230]    a unit configured to obtain service registration information iFC of the second UE from the D-HSS and switch the current session to the second UE according to address information and iFC of the second UE. 
         [0231]    With the session control in the next IMS according to the embodiments of the invention, address information of a UE is stored into a D-HSS so that when the UE is called, an available A-CSCF may obtain the address information of the called UE from the D-HSS and instruct the UE to renew initialized registration or switch a session according to the address information of the called UE. 
         [0232]    It will be appreciated that one skilled in the art may make various modifications and alterations to the present invention without departing from the scope of the present invention. Accordingly, if these modifications and alterations to the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention intends to include all these modifications and alterations.