WO2013001213A1

WO2013001213A1 - Method for filtering early media flows in an ims network and server implementing said method

Info

Publication number: WO2013001213A1
Application number: PCT/FR2012/051409
Authority: WO
Inventors: Bertrand Bouvet
Original assignee: France Telecom
Priority date: 2011-06-30
Filing date: 2012-06-21
Publication date: 2013-01-03
Also published as: FR2977430A1

Abstract

The invention relates to a filtering method implemented by a server (TERM) in an IMS network, during a phase of establishing a call between a calling terminal (UA) and a called terminal (UB). Said method comprises: a step (G20) of receiving an SIP invitation message emitted by an upstream entity (UA, ORIG) in a flow interruption between the calling terminal (UA) and said server (TERM); a step (G40) of sending, to the S-CSCF server managing the signalling of the called terminal (UB), a SIP REGISTER message comprising the IMPU public identity of the called party in order to determine the contact addresses (UBl, UB2) recorded in the heart of the IMS network with said IMPU identity in addition to the priority of said contact addresses; a step (G80) of determining an order of priority between said contact addresses and optionally other terminals of the called party, from said priorities and, if necessary, from priorities of said terminals; a step (G90) of extending said SIP invitation message towards a CSCF server processing said at least one contact address (UBl, UB2) or towards one of the terminals according to said order of priority; a step (G120) of selecting a media flow to be restored to said calling terminal according to the priority of the contact addresses and the terminals that have responded (G100) to the SIP invitation message; and a step (G140) of sending, to said upstream entity, a single SIP response of type lxx, selected according to said media flow in order to enable the restitution thereof to said calling terminal.

Description

Early Media flow filtering method in an IMS network and server implementing this method.

Background of the invention

The present invention is in the field of telecommunications and more specifically in that of IMS networks (IP Multimedia Subsystem).

In an IMS network, the services are provided by application servers AS (for Application Server).

Among these application servers, it is known to distinguish the servers that provide a triggered service to the transmission of the call, otherwise known to those skilled in the art under the name "service originating". A server offering an originating service is usually referred to as an "originating server".

In the same way, the term "terminating server" is commonly called an AS application server offering a service triggered upon receipt of the call, such service being referred to as "service terminating".

Some originating and terminating services known to date generate media streams to the calling terminal during the call setup phase. These streams are often referred to as "early media feeds".

It is recalled that the call set-up phase designates the entire phase comprised between the sending of the invitation SIP INVITE message (SIP for Session Initiation Protocol) by the calling terminal and the reception by this calling terminal of a SIP response. type 2xx, 3xx, 4xx, 5xx, 6xx for example a positive acknowledgment response 200 OK or a negative acknowledgment response of type 403 Forbidden when the call is not authorized.

As an example of early media flow known to those skilled in the art, mention may in particular be made of the voice announcement sent by a media server controlled by an originating server when the caller uses the incoming call consultation service that has not been used. no outlet with possibility of callback or the personalized callback (Colored Ring Back Tone) returned by a media server controlled by the terminating server of the called terminal or by the called terminal.

With the deployment of increasingly complex services, it frequently happens that SIP equipment has to deal with several competing SIP dialogues during the call establishment phase, each of these dialogues being associated or not with an early media flow. To date, the standardization bodies (IETF, 3GPP, ...) do not provide a solution to deterministically restore a plurality of early media streams intended for the same calling terminal.

Object and summary of the invention

The invention relates to a filtering method implemented by a server in a first IMS network, during a phase of establishment of a call between a calling terminal and a called terminal. This process comprises:

a step of receiving a SIP invitation message sent by an upstream entity in a flow failure between the calling terminal and this server;

a step of sending to the server S-CSCF managing the signaling of the called terminal, of a SIP REGISTER message comprising the public identity IMPU of the called party in order to determine the contact addresses registered at the heart of the network IMS with this identity IMPU as well as the priority of these contact addresses;

a step of determining an order of priority between these contact addresses and possibly other terminals of the called party, based on said priorities and, if appropriate, priorities of these terminals;

a step of extending the SIP invitation message to an S-CSCF server processing at least one of these contact addresses;

a step of selecting a media stream to be restored to the calling terminal according to the priority of the contact addresses and the terminals having responded to this SIP invitation message; and

a step of sending, to the upstream entity, a single SIP response of type lxx, chosen as a function of this media stream to enable its return to the calling terminal.

Correlatively, the invention relates to a server belonging to a first IMS network this server comprising, means for implementing, during a phase of establishment of a call between a calling terminal and a called terminal: - means receiving a SIP invitation message sent by an upstream entity in a flow failure between the calling terminal and the server;

means for sending, to the S-CSCF server managing the signaling of the called terminal, a SIP REGISTER message comprising the public identity IMPU of the called terminal in order to determine the contact addresses registered at the core of the IMS network with this identity. IMPU as well as the priority of these contact addresses; means for determining an order of priority between these contact addresses and possibly other terminals of the called party, based on these priorities and, where appropriate, priorities of said terminals;

means for extending the SIP invitation message to an S-CSCF server processing at least one of these contact addresses or to one of the terminals, according to the order of priority;

means for selecting a media stream to be restored to the calling terminal according to the priority of the contact addresses and the terminals having responded to the SIP invitation message; and

means for sending, to the upstream entity, a single SIP response of type lxx, chosen according to said media stream to enable it to be restored to the calling terminal.

In this document, the term "upstream entity", the calling terminal, or any entity of the IMS network placed in flux between the calling terminal and the server according to the invention and "downstream entity", the called terminal, or any entity of the IMS network placed in a power failure between the server according to the invention and the called terminal.

Those skilled in the art will understand that the originating servers of the calling party and the terminating server of the called party respectively constitute upstream and downstream entities within the meaning of the invention. When the calling terminal belongs to a PSTN / GSM circuit network, an upstream entity is for example an MGCF server. When the calling terminal belongs to another IMS network than the called party, an upstream entity may be an IBCF interconnection server.

The notation "SIP response type lxx" refers to any SIP response whose type begins with the number "1" and in particular the SIP 180 Ringing and 183 In Progress responses.

Thus, and very advantageously, the media streams sent to the upstream entity (for example to the calling terminal), during the call establishment phase, these media streams being known to those skilled in the art under the the name of "early media flow", are systematically controlled by the server according to the invention, a single lxx type SIP response going back to the calling terminal.

This characteristic advantageously makes it possible to define, in a perfectly predictable manner, the scenario and the scheduling of restitution of the early media flows by the calling terminal.

In a particular embodiment of the invention, the server according to the invention is a terminating server. In a particular embodiment, the filtering method according to the invention comprises:

a step of reserving a first resource with an MRF (Multimedia Resource Function) server of said network;

a step of reserving at least one second resource with the MRF server, said extended SIP invitation message to said S-CSCF server comprising a SDP (Session Description Protocol) field identifying said second resource;

a step of controlling said MRF server to restore, in the first resource, said selected media stream.

This characteristic advantageously makes it possible to define, in a perfectly predictable manner, the scenario and the scheduling of rendering of several early media streams by the calling terminal while optimizing the resources of the IMS network through which these early media flows transit.

In a particular embodiment, the filtering method according to the invention comprises:

a step of sending, to said upstream entity, a 183 In Progress SIP response comprising an SDP field identifying said first resource;

a step of controlling said MRF server to restore, in this first resource, a first media stream generated by the server.

This feature is particularly advantageous because it makes it possible to restore the different early media streams to the caller in the same resource of an MRF server controlled by the server according to the invention.

This characteristic advantageously makes it possible to define, in a perfectly predictable manner, the scenario and the scheduling of rendering of several early media streams by the calling terminal.

In one embodiment of the invention, the SIP response message sent to the upstream entity may be:

a 180 Ringing type message, without SDP, if no lxx type response with an SDP field is received; or

- the message of type lxx with SDP received.

In one embodiment of the invention, the method according to the invention comprises, when the media stream to be restored is received on the second resource, a control step of the MRF server for it to filter the packets received on the second resource to replicate on the first resource, only those of the media stream to render. In one embodiment of the invention, the server according to the invention comprises means for triggering a ringback return media stream on the first resource when the selected message is of type lxx without SDP.

A complete scenario for selecting the media stream to be returned to the caller will be detailed below.

In a particular embodiment, the various steps of the filtering method according to the invention are determined by instructions of computer programs.

Consequently, the invention also relates to a computer program on an information medium, this program being capable of being implemented by a server, this program comprising instructions adapted to the implementation of the steps of a filtering process as mentioned above.

This program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any other form desirable shape.

The invention also relates to a computer-readable information medium, comprising instructions of a computer program as mentioned above.

The information carrier may be any entity or device capable of storing the program. For example, the medium may comprise storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a magnetic recording medium, for example a floppy disk or a disk. hard.

On the other hand, the information medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can be downloaded in particular on an Internet type network.

Alternatively, the information carrier may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method in question.

Brief description of the drawings

Other features and advantages of the present invention will emerge from the description given below with reference to the drawings which illustrate an embodiment having no limiting character. In the figures: FIGS. 1 and 2 represent, in the form of a chronogram and a flowchart, a filtering method according to a particular embodiment of the invention;

- Figure 3 shows the hardware architecture of a server according to a particular embodiment of the invention.

Detailed description of the invention

Referring to Figure 1, we will now describe a filtering method according to a particular embodiment of the invention. In order not to overload the figure, the SIP 100 TRYING responses are not represented.

In the exemplary embodiment described here, the server according to the invention is an AS terminating application server (referenced as TERM in the figures).

In this first exemplary embodiment, this Terminating server does not generate an early media stream.

In the exemplary embodiment described here, one places oneself in the context in which a user wishes to be put in contact with a SIP subscriber of the same IMS network. For this, the user dials with his terminal UA the number of the called subscriber UB. We will assume here that two SIP terminals with the same public identity IMPU of the called subscriber are registered at the heart of the IMS network.

When the calling terminal UA dials the number of the called subscriber, the terminal UA sends, to the CSCF server, an SIP message of INVITE type whose session description SDP field includes the information related to the codecs supported by the terminal UA, and the IP address and port on which the UA terminal expects to receive the negotiated media stream.

When the CSCF server receives the SIP INVITE message from the terminal UA, the latter controls the SPDF so as to reserve a resource at the CBGF thereby forcing the media streams sent and received by the calling terminal UA to pass through the CBGF equipment and modifies the SDP of the INVITE message to replace the IP address and port of the UA terminal by the IP address and port on which the CBGF server used by the UA terminal expects to receive the negotiated media stream.

Thus the SIP INVITE message sent by the CSCF to the AS originating ORIG server contains the SDP = CBGF-UA field. On receipt of the INVITE message, the originating ORIG server triggers the originating services of the caller, for example the calling number masking positioning service or the verification that the caller's number the compound call is allowed. In the embodiment described here, these services do not generate an early media stream to be transmitted to the caller.

The INVITE message is then forwarded to the CSCF of the calling party who routes it according to the standardized IMS method to the CSCF processing the called SIP. The called party's CSCF triggers the AS TERM server in a known manner in step G20 by sending an INVITE invitation message.

According to another embodiment of the invention, the caller may belong to a PSTN / GSM circuit network, in which case the INVITE message received by the AS TERM server originates from an MGCF server.

According to another embodiment of the invention, the caller may belong to an IMS network different from the called party's IMS network, in which case the INVITE message received by the AS TERM server originates from an IBCF server.

According to the invention, the terminating server TERM seeks to anticipate the forking environment managed by the called CSCF server to several terminals called UB, within the meaning of the aforementioned RFC 3261 standard. For this purpose, it sends during a step G40 to the S-CSCF server managing the signaling of the terminal or terminals called

UB, a SIP REGISTER message including the public identity IMPU of the called party, the contact address field AoC being empty.

In accordance with the RFC3261 standard, the S-CSCF server responds in a G60 step to this message with a 200 OK response including the list of contact addresses registered at the heart of the IMS network with this public identity IMPU, and for each of them. between them the remaining recording time as well as the priority q defined by this standard, knowing that if the parameter q is not provided, this equates to a lowest implicit priority, that is to say q = 0 .

According to the invention, the terminating server TERM determines, during a step G80, an order of priority between these contact addresses, from said priorities q.

In practice, this step consists in determining if one is in a case of forking (more than one contact address in the SIP 200 OK response of the SIP REGISTER message) and if it is, if it is is sequential, parallel or mixed forking.

The terminating server TERM then extends, during a step G90, the SIP invitation message to the S-CSCF server of the called party which splits it to each of the terminals respectively associated with a contact address in accordance with the forking mechanism. described in RFC 3261. The called party's P-CSCF server modifies the SDP field by replacing the IP address and port number corresponding to the CBGF-UA by the IP address and port number corresponding to the CBGF-UB so as to force the transmitted and received media flows to pass through the CBGF called UB.

In the exemplary embodiment described here, this INVITE message is extended by the P-CSCF of the called party to the terminals UB1 and UB2.

According to the invention, the terminating server TERM selects a media stream to be restored to the calling terminal UA as a function of the priority q of the contact addresses that have responded to the SIP invitation message, these responses being received during a step G100 .

We describe below a possible strategy of the terminating TERM server:

In the absence of forking

In the embodiment described here, in the absence of forking, when a single terminal UB1 is registered at the heart of the IMS network, the AS terminating server TERM transmits the SIP INVITE message (SDP = CBGF-UA) initially received at the step G20 to S-CSCF, possibly after applying terminating services. The call is therefore presented to the terminal called unique UB1.

If this called terminal does not generate an early media stream, it responds 180 Ringing without SDP and this response is forwarded by the AS terminating upstream. The calling terminal then plays a RBT ringback ring locally. If there was a call originating service generating an early media flow, an MRF server controlled by the originating ORIG server AS would play the ringing tone to the calling terminal UA.

If the called terminal generates an early media stream, it responds 180 Ringing (SDP = UB1) and the called party's P-CSCF server pilots the SPDF which reserves a resource on the CBGF and modifies the SDP field of the 180 Ringing response. The codec selected by the terminal UB1 is kept in the SDP but the IP address and port number of the terminal UB1 are replaced by the IP address and port number of the called CBGF so as to force the media streams transmitted and received by the terminal called B to transit in the CBGF. The response is transmitted by the AS Terminating upstream and the calling terminal UA plays this early media flow.

In the presence of simultaneous forking only

In the exemplary embodiment described here, the AS Terminating waits on the order of one to a few seconds (for example five) to collect the maximum of responses and choose the most appropriate to go back:

- if no response has an SDP field (no early media), the AS Terminating TERM server sends a single 180 Ringing response without SDP to the upstream Originating AS server, which itself sends it back to the calling terminal UA. Since the 180 Ringing response does not include an SDP field, the calling terminal UA locally plays the RBT callback; if at least one response includes an SDP field (presence of an Early Media), the AS terminating TERM server selects the one that was received first and sends a 180 Ringing message (SDP = CBGF-B1 / B2) to the AS server Originating upstream which itself returns it to the calling terminal UA. As there is a SDP, the caller restores the Early Media feed played by the terminal of the called party who answered first.

- In both cases the other lxx responses received and not selected are filtered by the AS TERM.

In the presence of sequential forklna only

In this embodiment, the AS terminating TERM restores the response of the highest priority terminal upstream. The calling terminal UA locally plays RBT call back if the highest priority contact address does not generate an Early Media feed, or the Early Media feed played by the highest priority contact address of the called party. In the presence of a mixed forklna

In the embodiment described here, the terminating server TERM considers the very first phase of call presentation to the terminal or terminals having the highest priority. It then applies the above-described policy of "simultaneous forking only" or "sequential forking only" depending on the simultaneous / sequential type of the first call presentation phase.

Returning to FIG. 1, it will be assumed that the two terminals UB1, UB2 generate early media and respond (step G100) by a 180 Ringing message with SDP UB1 / UB2. The SDP of these 2 responses are modified by the P-CSCF which controls the SPDF which itself controls the CBGF so as to transit the media streams transmitted and received by the CBGF equipment. As a result, the AS TERM receives 180 Ringing responses with SDP CBGF-U 1 and SDP CBGF-U2.

It is assumed that these two terminals have the same priority parameter q.

In step G120, the terminating server TERM selects the media stream of the terminal UB1, the answer of the latter having been received first. The terminating server TERM sends, during a step G140, to the originating upstream server ORIG a SIP response 180 RINGING SDP = CBGF-UB1. This response is extended to the terminal UA, as in a known manner and the terminal UA receives the early media stream generated by the terminal UB1.

In accordance with the invention, only one answer lxx is transmitted upstream and the server S-CSCF of the calling party UA does not receive the answer 180 RINGING SDP = CBGF-UB2. As a result, the early media stream generated by UB2 is blocked by the calling party's CBGF and is not returned to the calling terminal UA.

Referring to Figure 2, we will now describe a filtering method according to another particular embodiment of the invention. The steps already described with reference to FIG. 1 will not be detailed.

In this second exemplary embodiment, the Terminating TERM server first reserves a first resource for transmitting the Early Media feeds upstream and a second resource to receive the possible Early Media feeds from the downstream. This second embodiment has the advantage of optimizing the resources of the IMS network by returning to the caller's CBGF only the early media flow corresponding to the selected response.

In this second exemplary embodiment, the Terminating TERM server may offer the terminating service Terminating to n destinations in sequential or simultaneous mode known to those skilled in the art. This service, hereinafter referred to as FLS, is an application forking service that consists of forwarding a call intended for a called subscriber of the IMS network to one or more secondary destinations defined beforehand by the called subscriber. To provide the FLS service, the terminating TERM server requests the AS TERM-driven MRF entity to generate a systematic early-stream flow to make the calling terminal user wait for the time to join the serial terminals until that one of them answers.

In the exemplary embodiment described here, after receiving the INVITE message (step G20), the terminating server TERM establishes a SIP dialogue with the called party's MRF server to reserve a first resource MRF1 (step G200).

The terminating TERM server then determines during a G21 test whether the called party is subscribed to the FLS forwarding service.

Of course, terminating TERM server could also offer services without application forking or not generating early media flow.

Scenario without application forking In this case, the called party is not subscribed to the FLS forwarding service, the terminating TERM server does not implement application forking. The result of the G21 test is negative. This test is then followed by the sequence of steps G40-G80 during which the terminating TERM server prioritizes between the contact addresses registered in the IMS network with the public identity of the called party (forking network in the sense of the RFC 3261). In this embodiment of the invention, when there is no application forking, the AS terminating server TERM establishes a SIP dialogue with the MRF server to reserve a single resource MRF2, during a step G81.

Remarkably, the SIP INVITE invitation message sent during this step G90 to the S-CSCF includes an SDP field which identifies this second resource.

In the example described here, the S-CSCF server extends the INVITE message to each of these two terminals UB1, UB2 and the P-CSCF replaces the IP address and port number of the second resource with the IP address and number. of the CBGF called in order to transit the media streams transmitted and received by this equipment.

We will assume, in this example, that the two terminals UB1, UB2 generate early media and respond (step G100) by a 180 Ringing message with SDP UB1 / UB2, the SDP field being modified by the P-CSCF with the value CBGF-UB1 / UB2 to force the streams to pass through the CBGF of the called party.

These two early media streams are therefore received in the same MRF2 resource.

It is assumed that the two terminals UB1, UB2 have the same priority parameter q and that the terminating server TERM selects (G120), by application of the selection policy described above, the media stream of the terminal UB1.

The terminating TERM server controls the MRF server to request the resource MRF2 to filter the early media stream received and sent by the terminal UB2 by providing it with the IP address and port number of the sender of this early media stream. (On the basis of the information contained in the SDP of the lxx received and blocked by the AS TERM) and replicates in the first resource MRF1, the packets received in the second resource MRF2, only those sent by the terminal UB1 being retained.

The terminating server TERM sends, during a step G140, to the upstream orig originating server a SIP response 180 RINGING SDP = MRF1. This response is extended to the terminal UA, as in a known manner and the terminal UA receives the early media stream generated by the terminal UB1.

Thus, advantageously, a single media stream is transmitted in the network to the caller's CBGF. In the event that the terminals UB1 and UB2 respond with a 180 Ringing without SDP without generating an early media stream, the TERM server would drive the MRF to play the ringback on the resource MRF1. 2. Scenario with application forking

In this scenario, the called party is subscribed to the FLS service. The result of the G21 test is positive, the terminating TERM server must then drive an MRF server to send the caller an early media flow corresponding to the waiting message of the referral service.

During a step G210, the AS terminating server TERM sends a 183 In Progress SIP response upstream, in response to the invitation message received in step G20, the SDP field of this response including the identifier MRF1 of the resource.

During a step G220, the AS terminating server TERM controls the MRF server to restore, in the first resource MRF1, the media stream associated with the FLS service, intended to make the caller wait.

In the exemplary embodiment described here, the terminating server TERM must set up, as part of the FLS service, an application forking. This application forking can be sequential or simultaneous depending on the service defined by the called subscriber. Depending on the type of application forking, the terminating TERM server must extend simultaneously or sequentially the INVITE invitation SIP request received in step G20 to several downstream entities. These downstream entities may belong to the same IMS network as the public identity of the called party, or to a PSTN / GSM circuit network or to another IMS network.

The terminating TERM server determines in this example, the public identity of the highest priority destination (step G150), and treats this public identity as previously described by applying steps G40 to G90.

In the embodiment described here, for each public identity of a less priority destination, the terminating server TERM reserves a second specific resource MRFi with the MRF server (step G155) and extends the invitation message INVITE to this destination, the SDP field with the identifier MRFi of the resource reserved for this destination.

Each destination receiving the INVITE invitation message is likely to send a response to the terminating TERM server in the form of an lxx message with or without SDP, via an MGCF or IBCF server depending on the type of network to which it belongs. the destination. If the terminating TERM server decides, by applying the aforementioned policy, to restore the early media flow of this less priority destination to the calling terminal UA, it is sufficient for it to replicate on the first resource MRF1, the packets received on the resource MRFi, without particular filtering at the packet level.

When there is no priority destination, for example if all the terminals have the same priority, or if the priority destination does not return a response, the selection of the media stream to replicate is made according to a priority order. priority associated with the different lxx responses received. Such an order of priority can for example be determined according to the type lxx and the presence or absence of an SDP field in the response. An order of priority of the responses received can be defined such that the highest priority response is a 183 In Progress response, then 180 Ringing with SDP and finally 180 Ringing without SDP.

As a variant, it is also possible to share the same resource between several public identities and, when the media stream to be restored has been selected, to control the MRF server to ask the resource concerned to filter the received early media stream and issued by the terminals at the origin of the unretained early media flows by providing the IP address and port number of the sender of these flows.

Thus, in this scenario with application forking, the calling terminal UA receives the early media stream generated by one of the called subscriber terminals from the same MRFI resource used for the early media stream generated by the call forwarding service. sequential FLS.

FIG. 3 represents the architecture of an AS application server according to the invention.

In the embodiment described here, this server has the hardware architecture of a computer. It comprises a processor 10, a read-only memory ROM 11, a random access memory RAM 12 and communication means 14 able to implement the SIP protocol to communicate with other entities implementing the same protocol in a IMS network.

ROM type ROM 11 is a medium in the sense of the invention. More specifically, this read-only memory comprises a computer program PGT whose instructions, when executed by the processor 10, implement the filtering method whose main steps have been described with reference to FIG. 2.

The communication means 14 of the server AS are able to receive, transmit and interpret a SIP invitation message, to create a SIP response with or without a request. an SDP field, to send such a response on the IMS network, to issue a SIP Register message to an S-CSCF server and to interpret its response, in particular to determine the priority q of terminals having the same identity IMPU according to the RFC standard 3261.

The processor 10, when it executes the computer program PGT, is able to select a media stream to be delivered to a terminal SIP, according to these priorities and to filter the different SIP responses lxx.

The processor 10, when executing the PGT computer program, allows the AS TERM server to reserve one or more resources with an MRF server and to control an MRF server to make it play a RBT call return or to restore an media stream on a given resource and replicate a first resource on a second resource, and then control the MRF to filter certain received media streams.

Claims

A filtering method implemented by a server (TERM) in an IMS network, during a phase of establishing a call between a calling terminal (UA) and a called terminal (UB), this method comprising :

a step (G20) of receiving a SIP invitation message sent by an upstream entity (UA, ORIG) in a flow cutoff between the calling terminal (UA) and said server (TERM); a step (G40) of sending to the server S-CSCF managing the signaling of the called terminal (UB), of a SIP REGISTER message comprising the public identity IMPU of the called party in order to determine the contact addresses (UB1, UB2) registered at the core of the IMS network with said identity IMPU as well as the priority of these contact addresses;

a step (G80) of determining an order of priority between said contact addresses and possibly other terminals of the called party, from said priorities and, if appropriate, priorities of said terminals;

a step (G90) of extending said SIP invitation message to a CSCF server processing said at least one contact address (UB1, UB2) or to one of the terminals according to said priority order;

a step (G120) for selecting a media stream to be restored to said calling terminal as a function of the priority of the contact addresses and the terminals that responded (G100) to said SIP invitation message; and

a step (G140) of sending, to said upstream entity, a single SIP response of type lxx, chosen as a function of said media stream to enable it to be restored to said calling terminal.

2. Filtering method according to claim 1 characterized in that it comprises: - a step (G200) for reserving a first resource (MRF1) with a server

MRF of said network;

a step (G81) of reservation of at least one second resource to the MRF server, said extended SIP invitation message (G90) to said CSCF server comprising an SDP field identifying this second resource;

3. Filtering method according to claim 2, characterized in that it comprises: a step (G210) of sending, to said upstream entity, a 183 In Progress SIP response comprising an SDP field identifying said first resource; (MRF1); a step (G220) of controlling said MRF server to restore, in said first resource (MRF1), a first media stream generated by said server (TERM).

4. Method according to claim 1, characterized in that said selected lxx message is:

- the message of type lxx with SDP received.

5. Method according to claim 2, characterized in that it comprises, said media stream to be restored being received on said second resource, a step of controlling said MRF server for it to filter received packets on the second resource for replicate to the first resource, only those of said media stream to be rendered.

6. Server (TERM) belonging to a first IMS network, characterized in that it comprises means for implementing, during a phase of establishment of a call between a calling terminal (UA) and a terminal called (UB):

means (14) for receiving a SIP invitation message sent by an upstream entity (UA, ORIG) in a flow cutoff between the calling terminal (UA) and said server (TERM); means (14) for sending, to the server S-CSCF managing the signaling of the called terminal (UB), a SIP REGISTER message comprising the public identity IMPU of the called party in order to determine the addresses of the contacts (UBl , UB2) registered at the core of the IMS network with said identity IMPU as well as the priority of these contact addresses;

- means (11, PGT) for determining an order of priority between said contact addresses and possibly other terminals of the called party, from said priorities, and if necessary, the final deduced priorities;

means (14) for extending said SIP invitation message to a CSCF server processing said at least one contact address (UB1, UB2) or to one of said terminals according to said priority order;

means (11, GPT) for selecting a media stream to be restored to said calling terminal as a function of the priority of said contact addresses and the terminals that responded (G100) to said SIP invitation message; and

means (14) for sending, to said upstream entity, a single SIP response of type lxx, chosen according to said media stream to enable it to be restored to said calling terminal.

7 server according to claim 6, characterized in that it comprises means for triggering a ringing return media stream on the first resource when the selected message is type lxx without SDP.

A computer program (PGT) comprising instructions for executing the steps of the filtering method according to claim 1 when said program is executed by a computer (AS).

Computer-readable recording medium (11) on which is recorded a computer program comprising instructions for performing the steps of the filtering method according to claim 1.