Patent Publication Number: US-2016241601-A1

Title: Technique for restoring a service in a network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a Section 371 National Stage Application of International Application No. PCT/FR2014/052406, filed Sep. 25, 2014, the content of which is incorporated herein by reference in its entirety, and published as WO 2015/044596 on Apr. 2, 2015, not in English. 
    
    
     FIELD OF THE DISCLOSURE 
     The invention lies in the field of Internet protocol (IP) type communications networks, and more particularly it relates to a technique of restoring a service for a user in an IP multimedia system (IMS) type network. 
     BACKGROUND OF THE DISCLOSURE 
     An IMS network architecture, as defined by the third generation partnership project (3GPP) standards group, enables multimedia sessions to be set up and controlled between two user equipments and also enables resources to be reserved for the multimedia streams in the transport network. By means of this architecture, network operators can conveniently control the quality of service they offer. At present, IMS architecture makes it possible to offer telephone, videophone, presence, and instant messaging type services and it also manages interactions between these services. In general, it makes use of session initiation protocol (SIP) as defined by the Internet engineering task force (IETF) in Document RFC 3261 as the protocol for managing sessions, thus making it possible to set up, modify, and terminate multimedia sessions in a network using IP protocol. 
     Such an IMS network architecture comprises in particular:
         one or more home subscriber servers (HSSs) each having a user database. Each subscriber server contains the “profile” of some number of user equipments in the network, where the profile contains the registration status, authentication and location data, and the services to which those pieces of equipments have subscribed;   one or more registration servers known as serving-call server control function (S-CSCF) servers serving in particular to manage the procedure of registering user equipments connected to the network;   one or more interrogation servers referred to as interrogating-call server control function (I-CSCF) servers serving to interrogate a subscriber server when registering user equipment in order to obtain an identifier of a registration server having the requisite characteristics for reaching the level of service to which the user has subscribed, where such a server is referred to below as the “nominal registration server”; and   one or more proxy servers known as proxy-call server control function (P-CSCF) servers serving as connection entities between the IMS core network and the access network used by the user equipments, and suitable for relaying all of the signaling messages between user equipments on one side and S-CSCF or I-CSCF servers on the other.       

     In order to ensure continuity of service in an IMS network, the 3GPP standards group has published a technical specification Document TS-23.380 version 11.1.0 entitled “IMS restoration procedures” defining procedures relating to restoring service within an IMS network when a nominal registration service cannot be reached (e.g. a network failure, or a failure of the registration server itself). In particular, two restoration procedures are described, the first taking place when registering user equipment with the IMS network (TS 23.380, §4.2.2), and the second taking place at the end of a session with user equipment (TS 23.830, §4.3.3). Both of those procedures provide for an I-CSCF interrogation server to send a capabilities request message to an HSS subscriber server so that the I-CSCF interrogation server can select a replacement S-CSCF registration server. These “capabilities” are defined in chapter 6.7 of the 3GPP technical specifications TS 29.228. That makes it possible to ensure continuity of service in the IMS network in a manner that is transparent to the user. 
     A drawback of the service restoration procedures as specified in specification document TS 23.380 is that while processing a service request relating to a user, the request is always transferred to the nominal registration server. In the event of the nominal registration server remaining unreachable, the wait for a reply from that server and the retransmissions of the service request to that nominal registration server consume a particularly large amount of network resources, even though the network capabilities are reduced, in particular as a result of the nominal registration server being unavailable. There therefore exists a need to improve the processing of a service request relating to a user while restoring service in an IMS network. 
     SUMMARY 
     In a first aspect, the invention provides a processing method to be performed in a communications network to process a service request relating to a user, said method comprising the following steps performed by an interrogation server during an optimization stage:
         a/ receiving said service request relating to a user and coming from a user equipment;   b/ interrogating the subscriber server in order to obtain information enabling the service request to be processed;   c/ receiving capabilities that a registration server needs to possess in order to be capable of delivering the services authorized for said user, the capabilities being received from a subscriber server; and   d/ determining a replacement registration server on the basis of the received capabilities, and transferring the service request to the replacement registration server as determined;   wherein, during a detection stage during which the registration server is considered to be reachable, said nominal registration server being associated with said user, and said method further comprises the following steps prior to the step c/ of receiving capabilities:   b1/ receiving an identifier of a nominal registration server in response to the interrogation of step b/;   b2/ transferring the service request to the nominal registration server; and in the event of a failure of making this transfer to the nominal registration server:   b3/ interrogating the subscriber server in order to obtain said capabilities;   and once the nominal registration server is considered to be unreachable at the end of the detection stage, starting the optimization stage, during which the transfer of step d/ is the sole transfer of the service request that is made to registration servers.       

     The detection stage performs the steps specified in the restoration procedures described in technical specification TS 22.380. The processing method thus provides an optimization stage that is started when the nominal registration server is considered to be unreachable. By means of this processing method, the service request relating to the user is transferred only once to a registration server during the optimization stage. It is thus possible to avoid the waiting and retransmission time required following the transfer of the service request to a registration server that is not reachable. The service restoration procedures as defined in technical specification TS 23.380 are implemented only once it has been observed that the nominal registration server is unavailable after making a first transfer of the service request to the nominal registration server. A single transfer of the service request during the optimization stage makes it possible to economize the use of network resources and to optimize the processing of service requests relating to a user in the event of a nominal registration server being unavailable. Traffic within the network is also reduced. Continuity of service is thus reinforced, thereby contributing to improving the quality of service as perceived by the user. 
     According to a particular characteristic, the processing method further comprises a step of determining that the nominal registration server should be considered as being unreachable, which step is performed by the interrogation server. 
     The step of determining that the nominal registration server should be considered as being unreachable serves to start the optimization stage. The management of service restoration is local to each interrogation server of the network. It is possible to implement service restoration strategies that are specific to each interrogation server of the network, e.g. as a function of the number of transfer failures needed to start the optimization stage. 
     According to another particular characteristic, the processing method comprises a step of determining that the nominal registration server should be considered as being unreachable, which step is performed by the subscriber server. 
     The subscriber server starting the optimization stage enables service restoration management to be centralized. The subscriber server can keep track of information about the unavailability of a registration server for each of the registration servers in the network. When the subscriber server is interrogated by an interrogation server in order to obtain an identifier for a nominal registration server associated with a user, the subscriber server returns capabilities directly in response to the interrogation on identifying the registration server as being unreachable or unavailable. This avoids any need for a second interrogation of the subscriber server by the interrogation server in order to obtain capabilities and potentially suffer a new failure to transfer the service request, thereby having the effect of reducing network traffic. It should also be observed that since the management is centralized, the number of subscriber server interrogations is independent of the number of interrogation servers in the network. 
     According to a particular characteristic, the unreachable status of the nominal registration server is determined using at least one counter associated with said nominal registration server that is updated during the detection stage in the event of a failure to transfer to the nominal registration server. 
     Using a counter makes it possible to have direct information about the number of transfer failures for a given registration server. In association with an unreachability status of a registration server, it makes it possible to define various strategies about starting the optimization stage. By way of example, this stage may be started as a function of the number of failures to transfer the service request. By way of example, an operator may decide to define a small number of transfer failures prior to starting the optimization stage for interrogation servers that require better continuity of service, and a higher number for environments that are tolerant of service interruptions. It is thus possible for the operator to propose different levels of quality of service to a user. 
     Since the counter is also associated with a registration server independently of the type of service request, it is simple for the counter to manage values to starting the optimization stage. Only one counter needs to be updated for each registration server known to an interrogation server. 
     According to a particular characteristic, the unreachable status of the nominal registration server is determined on the basis of a plurality of counters respectively associated with a plurality of types of service request. 
     Counters that are specialized by type of service request make it possible to implement fine management of strategies for restoring service in an IMS network. By way of example, an operator may give precedence to continuity of service for requests relating to registration (e.g. user authorization requests (UARs)) for a user equipment on the IMS network, and may adopt a different service restoration policy for location info requests (LIRs), for example. 
     In a second aspect, the invention provides an interrogation server arranged to process a service request relating to a user, the server comprising:
         a first transceiver module arranged to receive said service request relating to a user from a user equipment and to transfer it to a registration server;   a second transceiver module arranged to interrogate a subscriber server in order to obtain in return an identifier of a registration server associated with the user or to receive capabilities that a registration server needs to possess in order to be capable of delivering the services authorized for a user;   a selector module arranged to determine a replacement registration server on the basis of the received capabilities; and   a control module arranged to cause a subscriber server to be interrogated so that, during a first stage, an identifier is obtained for a nominal registration server associated with the user, a transfer is made to the nominal registration server, and in the event of a failure to transfer the service request to said nominal registration server, capabilities are obtained; and also arranged, during a second stage, to act when the nominal registration server is considered to be unreachable so as to make a single transfer of the service request relating to a user to a registration server.       

     According to a particular characteristic, the interrogation server also comprises a calculation module arranged to update at least one counter relating to an unreachability status of a registration server. 
     The advantages set out for the processing method in accordance with any of the characteristics of the first aspect are directly transposable to the interrogation server of the second aspect. 
     In a third aspect, the invention provides a subscriber server arranged to process a service request relating to a user, the server comprising:
         a transceiver module arranged to respond to an interrogation from an interrogation server, in particular to supply it with an identifier of a nominal registration server associated with the user or to supply it with capabilities that a registration server needs to possess in order to be able to deliver the services authorized for a user; and   a control module arranged to act during an optimization stage during which the nominal registration server is considered to be unreachable, to cause capabilities to be supplied directly in response to a request for the identifier of said nominal registration server.       

     According to a particular characteristic, the subscriber server further comprises a calculation module arranged to update at least one counter relating to an unreachability status of a registration server. 
     In a fourth aspect, the invention provides a system for processing a service request in a communications network, said system comprising:
         an interrogation server of the second aspect;   a calculation module arranged to update at least one counter relating to an unreachability status of a registration server; and   a subscriber server comprising:
           a transceiver module arranged to respond to an interrogation from an interrogation server, in particular to supply it with an identifier of a registration server associated with the user or to supply it with capabilities that a registration server needs to possess in order to be capable of delivering services authorized for a user; and   a control module arranged to make the response to the interrogation server.   
               

     According to a particular characteristic, the control module of the subscriber server is also arranged, during an optimization stage in which the nominal registration server is considered to be unreachable, to supply capabilities directly in response to a request for an identifier of said nominal registration server. 
     In a fifth aspect, the invention provides a program for an interrogation server comprising program code instructions for controlling the execution of steps of the above-described method when said program is executed by said server, and a data medium readable by an interrogation server containing a program for an interrogation server. 
     In a sixth aspect, the invention provides a program for an subscriber server comprising program code instructions for controlling the execution of steps of the above-described method when said program is executed by said server, and a data medium readable by a subscriber server containing a program for an subscriber server. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood from the following description of particular implementations given with reference to the accompanying drawings, in which: 
         FIG. 1  shows a system for processing a service request relating to a user in a communications network; 
         FIG. 2  shows an interrogation server performing a method of processing a service request in a particular implementation; 
         FIG. 3  shows a subscriber server in a particular embodiment; and 
         FIGS. 4 a  and 4 b    show steps of a method of processing a service request relating to a user in two particular implementations. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  shows a system  2  for processing a service request in a communications network  1 . The multimedia services offered by the network  1  may comprise telephone, videophone, content sharing, presence, instant messaging, or television services. The system  2  serves to transfer a registration request from a user equipment (UE)  10  over the network  1  to a registration server and to benefit from services made available by the network. 
     The user equipment  10  may for example be a fixed or mobile terminal, or a home or business gateway. The system  2  comprises an interrogation server  20 , a subscriber server  30 , and a calculation module (not shown) serving to determine whether a registration server is reachable. By way of example, in a first embodiment described below the calculation module may be incorporated in the interrogation server  20 , or else, in a second embodiment also described below, it may be incorporated in the subscriber server  30 . The subscriber and interrogation servers  30  and  20  also communicate with each other. The interrogation server  30  also communicates with registration servers  40 - 41 . 
     It should also be observed that  FIG. 1  is shown in simplified manner in order to make it easier to understand. Nevertheless, there is no limit concerning the number of subscriber servers, of registration servers, or of interrogation servers belonging to the communications network. Likewise, there is no limit on the number of user equipments. 
     In the particular situation where the communications network  1  presents IMS type architecture, the interrogation server  20  is an I-CSCF server, the registration servers  40 - 41  are S-CSCF servers, and the subscriber server  30  is an HSS server, it being possible for some of these various servers to be combined within a single entity, when appropriate. With such an IMS architecture, the SIP signaling protocol is used for exchanges with the user equipment  10 , and also between the interrogation and registration servers  20 ,  40 - 41 . The exchanges between the subscriber server  30  on one side and the interrogation and registration servers  20 ,  40 - 41  on the other are conveyed by the Diameter protocol. 
     The implementations described below with reference to  FIGS. 4 a  and 4 b    show a method of processing a service request relating to a user. They rely on two stages: a first stage P 1  of detection and a second stage P 2  of optimization. During the detection stage P 1 , in the event of a failure to transfer the service request relating to a user to a nominal registration server  40 , the service restoration procedures as defined in the technical specification TS 23.380 are performed. Then on detecting that the registration server is not reachable, the optimization, second stage P 2  is started. 
       FIG. 4 a    describes the steps of the method of processing a service request relating to a user in a first implementation. 
     In this first implementation, detecting that the nominal registration server  40  is not reachable is performed in the IMS communications network  1  by the interrogation server  20 . 
     In a step E 1 , the interrogation server  20  receives a service request M 1  relating to a user and coming from a user equipment  10 . The service request M 1  may for example be an SIP REGISTER request for registration with a registration server. 
     In a step E 2 , the interrogation server  20  interrogates the subscriber server  30  in order to obtain an identifier for the nominal registration server  40  that possesses the characteristics required to achieve the level of service to which the user has subscribed. This interrogation is performed more particularly by sending a user-authorization-request (UAR) message M 2  in the Diameter protocol as defined in RFC 4740 and in compliance with the 3GPP specifications TS 24.229 and TS 29.228. 
     During a step E 3 , the interrogation server  20  receives a user-authorization-answer (UAA) message M 3  from the subscriber server  30  in response to the UAR request M 2 , which UAA message specifies an identifier Id S-CSCF1  of the nominal registration server  40  with which the user is registered. By way of example, this identifier is a uniform resource identifier (URI) as defined by RFC 3986. 
     In a step E 4 , the interrogation server  20  uses a counter CPT to verify whether a threshold S 1  has been reached for passing to a second stage P 2  referred to as an “optimization” stage. The threshold S 1  is defined by the operator of the communications network. Reaching this threshold S 1  terminates a first stage P 1  referred to as the “detection” stage, after which the nominal registration server  40  is considered to be unreachable. The second stage P 2  is then started. Before the second stage P 2  has been started, the interrogation server  20  acts in a step E 5  to transfer the SIP REGISTER request (message M 4 ) to the nominal registration server  40  identified in step E 3 . 
     In step E 6 , an absence of response to the SIP REGISTER request is observed by the interrogation server  20 . The SIP protocol makes provision for mechanizing the retransmission of messages, at the end of which, after several transmission attempts without response, the absence of response is considered as being identical to a  408  Request Timeout response code, indicating that no response has been received from the nominal registration server  40  within the time allowed for processing the message. By way of example, the absence of response from the nominal registration server  40  may be the result of it suffering a failure, of disturbed network traffic, or of any other cause that prevents it being reached or that prevents it from responding to a request it has received. 
     In a step E 7 , in compliance with the IMS service restoration procedures as defined in the 3GPP technical specifications TS 23.380, a new UAR message M 5  including a request for capabilities is sent to the subscriber server  30 . The request for capabilities is constructed by putting the REGISTRATION AND CAPABILITIES value in the user-authorization-type field of the UAR message M 5 . 
     Thereafter, the interrogation server  20  increments the counter CPT associated with the registration server  40  in a step E 8 . The counter CPT is incremented each time a UAR request is transmitted including a request for capabilities during the detection stage. In the presently-described implementation, a counter is associated with each registration server. In a step E 9 , the interrogation server  20  receives a response from the subscriber server  30  in the form of a UAA message M 6  including the requested capabilities. 
     The interrogation server  20  then acts during a step E 10  on the basis of the received capabilities to determine a new registration server  41  capable of providing the service requested by the user equipment  10 . This registration server  41  that has been determined in this way, referred to as the replacement registration server, thus takes the place of the nominal registration server  40  with which an attempt at transferring a SIP REGISTER request has previously failed. 
     In a step E 11 , the interrogation server  20  transfers the SIP REGISTER request M 7  to the registration server  41  as determined in step E 10 . 
     Steps E 1  to E 11  are performed once more each time a service request is received relating to a user so long as the detection stage has not come to an end. I.e., so long as the counter associated with the registration server  40 , as incremented in step E 8 , does not exceed the threshold S 1 . 
     When the threshold S 1  is reached, the nominal registration server  40  is considered to be unreachable and the optimization second stage P 2  is triggered. This second stage P 2  differs from the first stage P 1  by the absence of steps E 5  and E 6 . Only steps E 1  to E 4  and E 7  to E 11  are performed for processing a service request relating to a user. 
     During this second stage P 2 , the interrogation server  20  uses the counter CPT associated with the nominal registration server  40  to verify whether the threshold S 1  has been reached for passing to the optimization second stage P 2 . Reaching this threshold S 1  brings the detection first stage P 1  to an end, at the end of which the nominal registration server  40  is considered to be unreachable. Transfer of the service request relating to a user to the nominal registration server  40  is then inhibited during the optimization stage. 
     Steps E 7  to E 11  are then performed as described above with reference to the first stage P 1  in order to transfer the service request relating to a user to the replacement registration server  41 . A single service request transfer is thus performed during the second stage P 2 . The method advantageously makes it possible to avoid the waiting time and the retransmissions generated (during step E 5 ) by the absence of response from the nominal registration server  40 . This makes it possible in particular to optimize the use of the resources of the network. 
     In a particular implementation, the interrogation server  20  continues during the second stage P 2  to increment that counter CPT associated with the nominal registration server  40  during a step E 8 . During this second stage P 2 , this step makes it possible, by way of example, to define a second threshold S 2  associated with the nominal registration server  40  that, on being exceeded, makes it possible to return to nominal processing of the service request. This second threshold makes it possible, by way of example, to detect that a certain number of UAR requests including a request for capabilities have been sent. It is then probable that the nominal registration server  40  is once more in operation, e.g. if it had suffered a failure. 
     The steps E 1  to E 4  and E 7  to D 11  of this second stage P 2  are performed once more on each occasion a service request is received relating to a user, e.g. until the second threshold S 2  is reached enabling a return to nominal processing of service requests relating to a user or enabling the network operator to take action to replace the nominal registration server, thereby terminating the second stage P 2 . 
     In another implementation, on reaching the second threshold S 2 , the interrogation server ( 20 ) transfers the service request relating to the user to the nominal registration server ( 40 ). In the event that this nominal registration server is reachable once again, the counter CPT is reset to zero, thus enabling service requests to return to nominal processing. By way of example, the nominal registration server ( 40 ) remains unreachable, a new attempt is made to transfer the service request relating to the user to that server may be performed on reaching a new threshold value S n  that is a function of the threshold S 2 . By way of non-limiting example, a transfer attempt may be made for each threshold value S n =2 n ×S2. 
       FIG. 4 b    describes the steps of the method of processing a service request relating to a user in a second implementation. 
     In this second implementation, detection that the nominal registration server  40  cannot be reached is performed in the IMS communications network  1  by the subscriber server  30 . 
     In this second implementation, the counter associated with the nominal registration server  40  is no longer managed by the interrogation server  20 , but rather by the subscriber server  30 . The subscriber server  30 , and more precisely the calculation module, then performs a step E 8   b  that is similar to step E 8 . The counter associated with the nominal registration server  40  is then incremented by the subscriber server  30  each time a UAR message M 5  is received that includes a request for capabilities. Likewise, the determination that the nominal registration server  40  is to be considered as being unreachable, thus enabling the second stage P 2  to be performed, is undertaken by the subscriber server  30  during a step E 4   b,  similar to the step E 4 . In this second implementation, during the second stage P 2 , the interrogation server  20  performs the steps E 1  and E 2  as described above with reference to the first implementation and it transmits a nominal UAR request M 2 . On receiving this nominal UAR request, the subscriber server  30  responds in the second stage P 2  directly with a UAA message containing capabilities as received by the interrogation server  20  in the step E 9 . The subscriber server  30  thus responds as though it had received a UAR request including a request for capabilities from the interrogation server  20 . In this second implementation, the interrogation server  20  is thus arranged to receive capability information without previously requesting it. 
     It is emphasized at this point that, since the steps E 3  to E 7  described above with reference to  FIG. 4 a    are not performed in this second implementation during the second stage P 2 , the processing method serves to avoid exchanging the UAR message (step E 7 ) and the UAA message (step E 3 ) between the interrogation server  20  and the subscriber server  30 . Sending an SIP REGISTER request to the nominal registration server  40  is also avoided (step E 5 ). This makes it possible in particular to optimize the use of the resources of the network, most particularly by reducing signaling message traffic due to the redundancy of the UAR and UAA messages. 
     Steps E 10  and E 1  are then performed in the manner described above with reference to  FIG. 4   a.    
     The management of the counter used for determining whether a registration server is unreachable is centralized in the subscriber server  30 , and as a result it is likewise easier to implement. This second implementation makes it possible in particular for the subscriber server  30  to respond to an interrogation server for a service request relating to the user without the interrogation server needing to request capabilities explicitly. 
     This second implementation also makes it possible for the advantages of performing the processing method to benefit an interrogation server receiving a service request relating to the user and presenting, by way of example, little traffic, given that account is taken of UAR messages received from other interrogation servers during the step E 8   b  of updating the counter. 
     The above implementations are described for processing an SIP REGISTER service request. Nevertheless, there is no limitation on the type of the service request that can be processed. By way of example, the method applies equally well to an SIP INVITE service request. Under such circumstances, the processing method performed is equivalent to that performed for processing an SIP REGISTER request, with only the types of messages exchanged between the interrogation server  20  and the subscriber server  30  differing. Instead of using UAR and UAA messages, the servers exchange respectively location info request (LIR) and location info answer (LIA) messages. A counter for these subscriber server interrogations is then incremented in the first implementation, during step E 8  each time an LIR request is sent including a request for capabilities, or in the second implementation during the step E 8   b  each time an LIR request is received. 
     In another implementation, instead of a counter associated with a particular type of request (UAR or LIR in the above-described implementations), it may be advantageous to use a global counter that is incremented both when sending a UAR request or an LIR request, each of which includes a request for capabilities, with the counter being managed by the interrogation server  20  as described above for the first implementation, and also on receiving these requests without any explicit request for capabilities, with the counter being managed by the subscriber server  30 , as described above for the second implementation. Taking account of all of the requests that give rise to a UAR or an LIR message being sent makes it possible in particular to start the optimization stage P 2  more quickly. 
     Likewise, in another implementation, a plurality of thresholds are used for starting the second stage P 2 . By way of example, a threshold is associated with each counter defined by type of request. This makes it possible to adopt service restoration strategies of greater fineness, e.g. with a plurality of threshold values as a function of incoming traffic or indeed as a function of times of day. By way of example, the network operator may choose to perform the second stage P 2  more quickly during times when network traffic is high. 
       FIG. 2  shows an interrogation server  20  arranged to process a service request relating to a user. It comprises in particular:
         a first transceiver module  200  arranged to receive a service request relating to a user and to transfer it to a registration server;   a second transceiver module  202  arranged to interrogate a subscriber server in order to obtain in response an identifier of a registration server associated with the user or the capabilities that a registration server needs to possess in order to be capable of delivering the services that are authorized for a user;   a selector module  204  arranged to determine a replacement registration server on the basis of the received capabilities; and   a control module  206  arranged to control interrogation of the subscriber server.       

     In a first stage, the control module  206  is arranged to interrogate the subscriber server in order to obtain an identifier of a nominal registration server associated with the user, and after a failure to transfer the service request to the identified nominal registration server, to interrogate the subscriber server in order to obtain capabilities. In a second stage, as started once the nominal registration server is considered to be unreachable, the control module  206  is arranged to control a single transfer of the service request relating to a user to a registration server. 
     In the first implementation, the interrogation server  20  also includes a calculation module  208  arranged to update at least one counter relating to a reachability status of a registration server. The calculation module  208  determines that the nominal registration server is considered to be unreachable and starts the second stage P 2 . 
       FIG. 3  shows a subscriber server  40  arranged to process a service request relating to a user. It comprises in particular:
         a transceiver module  300  arranged to respond to an interrogation by an interrogation server, in particular in order to supply it with an identifier of a registration server associated with the user or in order to provide it with the capabilities that a registration server needs to possess in order to be capable of delivering the services that are authorized for a user; and   a control module  302  arranged to control the response to the interrogation server.       

     In the second embodiment, the subscriber server  30  further includes a calculation module  304  arranged to update at least one counter relating to a reachability status of a registration server. 
     In the second stage, started on the calculation module  304  detecting that the nominal registration server should be considered as being unreachable, the control module  302  is arranged to cause capabilities to be supplied directly in response to a request for a nominal registration server identifier, corresponding to the registration server that is considered to be unreachable. 
     The invention is performed by software and/or hardware components. In this context, the term “module” may correspond in this document equally well to a software component or to a hardware component or to a set of hardware and/or software components, suitable for performing a function or a set of functions, in the manner described above for the module in question. 
     A software component corresponds to one or more computer programs, one or more subprograms of a program, or in more general manner any element of a program or of software. Such a software component is stored in memory and then loaded and executed by a data processor of a physical entity and it is capable of accessing the hardware resources of that physical entity (memories, storage media, communication bus, electronic input/output cards, user interfaces, etc.). 
     In the same manner, a hardware component comprises any element of a set of hardware. It may comprise an optionally programmable hardware component, with or without an incorporated processor for executing the software. By way of example, it may be an integrated circuit, a smart card, an electronic card for executing firmware, etc. 
     In a particular embodiment, the modules  200 ,  202 ,  204 ,  206 , and  208  are arranged to perform the above-described processing method. They are preferably software modules comprising software instructions for executing the steps of the above-described processing method as performed by an interrogation server. The invention thus also provides:
         a program for an interrogation server, comprising program code instructions for controlling the execution of steps of the above-described processing method when said program is executed by said interrogation server; and   a data medium that is readable by an interrogation server and that stores the program for an interrogation server.       

     Likewise, the modules  300 ,  302 , and  304  are arranged to perform the above-described processing methods. They are preferably software modules comprising software instructions for executing the steps of the above-described processing method when performed by a subscriber server. The invention thus also provides:
         a program for a subscriber server, comprising program code instructions for controlling the execution of steps of the above-described processing method when said program is executed by said subscriber server; and   a data medium that is readable by a subscriber server and that stores the program for a subscriber server.       

     The software modules may be stored in or transmitted by a data medium. The data medium may be a hardware storage medium, e.g. a compact disk read only memory (CD ROM), a magnetic floppy disk or a hard disk, or indeed a transmission medium such as an electrical, optical, or radio signal, or a telecommunications network. 
     Although the present disclosure has been described with reference to one or more examples, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the disclosure and/or the appended claims.