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4.22 Support of Explicit Congestion Notification (ECN)
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4.22.1 General
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The ECN profile used to trigger codec rate adaptation for Multimedia Telephony is defined in TS 26.114 [76] and affects the following IMS entities: UE, MGCF/IM-MGW, IBCF/TrGW, IMS-ALG/ IMS-AGW, MRFP/MRFC and the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW.
As specified in TS 26.114 [76]:
- an MGCF/IM-MGW can be used for inter-working between an ECN-capable client in a 3GPP network that properly handles ECN-marked packets and a CS network;
- an IBCF/TrGW supporting Multimedia Telephony can be used for interworking between an ECN-capable entity in a 3GPP network that properly handles ECN-marked packets and:
- a remote entity that does not use ECN;
- a remote entity that supports ECN in different way than what is specified for Multimedia Telephony clients;
- a network which does not handle ECN-marked packets properly.
A UE supporting Multimedia Telephony and ECN shall support the procedures described in TS 26.114 [76].
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4.22.2 CS GERAN/UTRAN Interworking at MGCF/IM-MGW
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If MGCF/IM-MGW supports Multimedia Telephony compliant ECN, it shall:
- support ECN Multimedia Telephony client procedures as described in TS 26.114 [76], except that the MGCF and IM-MGW do not determine whether ECN can be used based on the Radio Access Technology that is used towards the client; the MGCF/ IM-MGW act as an ECN endpoint towards the Multimedia Telephony terminal;
- support SDP ability to negotiate to ECN according to TS 26.114 [76];
- be capable of enabling end-to-end rate adaptation between Multimedia Telephony terminal and the CS terminal or IM-MGW by performing the following:
- negotiate the use of ECN with the Multimedia Telephony terminal, if it can be confirmed that the network used towards the Multimedia Telephony terminal properly handles ECN-marked packets;
NOTE 1: An operator can ensure that the network used towards the Multimedia Telephony terminal properly handles ECN-marked packets by setting corresponding requirements on network equipment and verifying that those requirements are met. No active signalling ("probing") during normal operation is required to ensure this.
- trigger rate adaptation request towards the Multimedia Telephony terminal when receiving in the incoming IMS media flow IP packets marked with ECN-CE, regardless of whether the IM-MGW applies or does not apply transcoding;
- inter-work adaptation requests between the Multimedia Telephony terminal and the CS GERAN/UTRAN when the IM-MGW bridges compatible codec configurations between the interfaces without applying a transcoding function; if the IM-MGW prefers to receive a lower codec mode rate from the Multimedia Telephony terminal than what the CS network indicates, e.g. after having received IP packets with ECN-CE, the IM-MGW may replace the codec mode requested from the CS side with the codec mode that the IM-MGW prefers;
- perform media adaptation (e.g. reduce media bit-rate) towards the Multimedia Telephony terminal when receiving from the latter an adaptation request and the IM-MGW applies transcoding.
NOTE 2: For CS interworking the MGCF/MGW does not have to perform transcoding if the same codec is selected on the CS and PS network.
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4.22.3 Interworking with non-ECN IP network and/or terminal at IBCF/TrGW
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An IBCF/TrGW may support Multimedia Telephony using ECN and can be used to enable ECN within the local network when either the remote network cannot be confirmed to properly handle ECN-marked packets or the remote entity does not support or use ECN.
In order to support Multimedia Telephony using ECN when interworking with a remote network that cannot be confirmed to properly handle ECN-marked packets and/or with a remote terminal does not support or use ECN, the IBCF/TrGW shall:
- determine from local configuration if the remote network properly handles ECN-marked packets;
- determine with SDP offer/answer procedures if the remote entity supports ECN;
- support SDP ability to negotiate to ECN according to TS 26.114 [76];
- be capable of enabling end-to-end rate adaptation between the local Multimedia Telephony terminal and the remote entity or TrGW by performing the following towards the local Multimedia Telephony terminal:
- negotiate the use of ECN;
- support ECN Multimedia Telephony client procedures as described in TS 26.114 [76], except that the IBCF/TrGW does not determine whether ECN can be used based on the Radio Access Technology; the IBCF/TrGW acts as an ECN endpoint towards the ECN capable Multimedia Telephony terminal;
- trigger rate adaptation request towards the Multimedia Telephony terminal when receiving in the incoming IMS media flow IP packets marked with ECN-CE, regardless of whether the TrGW applies or does not apply transcoding; this requires that the IBCF provides the TrGW with the media configuration, even if transcoding is not supported or applied, when the IP termination is configured with ECN;
- forward adaptation requests between the Multimedia Telephony terminal and the remote entity when the TrGW bridges compatible codec configurations between the interfaces without applying a transcoding function; if the TrGW prefers to receive a lower codec mode rate from the Multimedia Telephony terminal than what the other SIP network indicates, e.g. after having received IP packets with ECN-CE, the TrGW may replace the codec mode requested from the other SIP network with the codec mode that the TrGW prefers;
- perform media adaptation (e.g. reduce media bit-rate) towards the Multimedia Telephony terminal when receiving from the latter an adaptation request and the TrGW applies transcoding.
NOTE: For this interworking the IBCF/TrGW does not have to perform transcoding if the same codec is selected on both sides of the TrGW.
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4.22.4 Interworking with non-3GPP ECN IP terminal at IBCF/TrGW
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An IBCF/TrGW supporting Multimedia Telephony compliant ECN can also be used to enable ECN end-to-end if the remote entity uses ECN in a different way than what is described in TS 26.114 [76], e.g. if the remote entity only supports probing for the ECN initiation phase or it needs the ECN feedback.
NOTE: For this interworking the IBCF/TrGW does not have to perform transcoding if the same codec is selected between both terminals.
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4.22.5 ECN support at IMS-ALG/IMS-AGW
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The P-CSCF shall be able to disallow the negotiation of ECN during SDP offer/answer exchanges if the IMS-AGW does not support transparent forwarding of the ECN bits or if the IMS core network or the access network used towards the Multimedia Telephony terminal do not properly handle ECN-marked packets.
An IMS-AGW supporting ECN shall be able to forward the ECN bits as instructed by the IMS-ALG.
The IMS-ALG/IMS-AGW may act as an ECN endpoint towards the access network and/or the IMS Core Network to enable ECN when the ECN connection cannot be established or maintained transparently (e.g. after PS-CS Access Transfer).
When acting as an ECN endpoint the IMS-AGW shall support end-to-end rate adaptation between the local terminal and the remote entity by performing the following:
- trigger rate adaptation request towards the ECN-capable peer when receiving in the incoming IMS media flow IP packets marked with ECN-CE, regardless of whether the IMS-AGW applies or does not apply transcoding;
- forward adaptation requests between the local and the remote peer when the IMS-AGW bridges compatible codec configurations between the interfaces without applying a transcoding function;
- perform media adaptation (e.g. reduce media bit-rate) towards the ECN-capable peer when receiving from the latter an adaptation request and the IMS-AGW applies transcoding.
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4.22.6 ECN support at MRFC/MRFP
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An MRFC/MRFP may support Multimedia Telephony using ECN and may act as an ECN endpoint to enable ECN with a local ECN-capable terminal within a local network that properly handles ECN-marked packets (see TS 23.333 [73]).
This requires that the MRFC/MRFP performs the following:
- support SDP ability to negotiate to ECN according to TS 26.114 [76];
- be capable of enabling end-to-end rate adaptation between the local Multimedia Telephony terminal and the MRFP by performing the following towards the local Multimedia Telephony terminal:
- negotiate the use of ECN;
- support ECN Multimedia Telephony client procedures as described in TS 26.114 [76], except that the MRFC/MRFP does not determine whether ECN can be used based on the Radio Access Technology; the MRFC/MRFP acts as an ECN endpoint towards the ECN capable Multimedia Telephony terminal;
- trigger rate adaptation request towards the Multimedia Telephony terminal when receiving in the incoming IMS media flow IP packets marked with ECN-CE;
- perform media adaptation (e.g. reduce media bit-rate) towards the Multimedia Telephony terminal when receiving from the latter an adaptation request.
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4.22.7 CS GERAN/UTRAN Interworking at the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW
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If the MSC Server enhanced for ICS/MSC Server enhanced for SRVCC with SIP/CS-MGW supports Multimedia Telephony compliant ECN, it shall support the procedures specified in clause 4.22.2 respectively for the MGCF and IM-MGW.
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4.23 Support of Load Balancing
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4.23.1 General
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An IMS network may implement functionality for IMS serving network nodes (S‑CSCF, Transit Function) to handle load balancing, i.e. the technique to distribute workload evenly across two or more network nodes implementing the same functions, in order to get optimal resource utilization.
For S‑CSCFs, load balancing may be applied for received initial registration requests (see clause 4.23.2).
For Transit Functions, load balancing may be applied for received service requests, i.e. initial SIP requests requesting a service (see clause 4.23.3).
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4.23.2 Registration-based load balancing of S-CSCFs
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Load balancing of S‑CSCFs for received initial registration requests may be based on load balancing functionality performed by an I‑CSCF or it may be based on mechanisms outside IMS functional entities (such as DNS). An example of the DNS-based load balancing approach is the use of functionality that collects load information of S‑CSCFs, applies a load balancing algorithm and provides the outcome to a (Dynamic) DNS, which subsequently is used implicitly by I‑CSCFs.
An I‑CSCF that performs load balancing of S‑CSCFs for initial registration requests shall assign a received registration request to one of the available and suitable S‑CSCFs using a load balancing algorithm. Such load balancing algorithm may take load information of the S‑CSCFs into consideration if available. Load information may be obtained via management interfaces, via proprietary interfaces, or via other mechanisms.
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4.23.3 Registration independent load balancing of Transit Functions
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Load balancing of Transit Functions for received service requests may be based on load balancing functionality performed by an IMS functional entity that is the immediate source of the service request to the Transit Function (e.g. an IBCF or an I‑CSCF), or it may be based on mechanisms outside IMS functional entities (such as DNS). An example of the DNS-based load balancing approach is the use of functionality that collects load information of Transit Functions, applies a load balancing algorithm and provides the outcome to a (Dynamic) DNS, which subsequently is used implicitly by IMS functional entities that require the IP-address of a Transit Function for routing a service request.
An IMS functional entity that performs load balancing of Transit Functions for service requests shall assign a received service request to one of the available and suitable Transit Functions using a load balancing algorithm. Such load balancing algorithm may take load information of the Transit Functions into consideration if available. Load information may be obtained via management interfaces, via proprietary interfaces, or via other mechanisms.
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4.24 Support of Restoration Procedures
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An I‑CSCF that performs recovery of S‑CSCFs for registration requests shall determine failure of an S‑CSCF implicitly (i.e. via a timeout) or explicitly (i.e. via a failure message) and shall reroute the registration request to another S‑CSCF based on the restoration procedures as defined in TS 23.380 [80].
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4.25 Support of Overload Control
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4.25.1 General
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Network elements within the IP Multimedia CN subsystem may have to cope with high volume of signalling traffic with significant traffic peaks. An IMS network may therefore implement overload control functionality in IMS network elements to prevent overload situations.
For that purpose, two different mechanisms are provided:
- A mechanism based on next-hop monitoring of overload, where an IMS node acting as a SIP server may provide overload control feedback to its neighbours. This mechanism is described in clause 4.25.2.
NOTE 1: This mechanism is well suited for preventing overload of core network servers (CSCF) where overload is not due to calls to a specific application/destination.
- A filter-based mechanism where an IMS node acting as SIP server, may send load control filters to another IMS node that has subscribed for receiving this information. This mechanism is described in clause 4.25.3.
NOTE 2: This mechanism is particularly well suited for application servers when the source of overload is due to calls to a specific destination (e.g. a 800 application overloaded by mass calling to a particular destination).
Emergency calls shall not be affected by the overload traffic restrictions due to overload control and MPS priority information shall be taken into account when applying by the overload traffic restrictions based on received indications.
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4.25.2 Next-hop monitoring of overload
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For IMS entities supporting next-hop monitoring of overload, these IMS entities shall support a mechanism for monitoring overload of neighbour nodes with minimal overhead, to enable scenarios where the overload status needs to be adjusted frequently.
IMS entities supporting SIP, e.g. S-CSCF, shall be able to provide overload control feedback to their neighbours by providing load reduction directives within SIP responses. The neighbour nodes shall be able to adapt the traffic sent to the overloaded node by restricting the traffic offered to the overloaded neighbour node in accordance with the overload status information received.
It is recommended that when next-hop monitoring of overload is deployed for a specific function, all neighbour nodes also support the feedback of overload control status and support the overload traffic restriction procedures towards the specific function being monitored.
The next hop monitoring of overload procedures should at least be possible to use for the following scenarios:
- Network internal overload control between core functions, e.g. between different CSCFs.
- Application Server overload control (i.e. between CSCF and AS).
- Roaming and Interconnect (e.g. between IBCFs of two different networks).
- Transit scenarios (e.g. between IBCF and Transit function).
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4.25.3 Filter based Overload Control
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When filter based Overload Control is deployed, an IMS entity supporting SIP overload control (e.g. originating AS or S-CSCF) may subscribe to traffic filter information of specific IMS SIP server destination which may be subject to overload (e.g. an 800 application overloaded by mass calling to a specific number) and perform overload traffic restriction according to the information received. Alternatively, when the destination is not in IMS, the IMS entity performing the overload traffic restrictions may subscribe to an IMS SIP server providing traffic filter information related to the destination, or can have configured overload traffic filter information for the specific destination.
IMS entities supporting this mechanism shall match all SIP requests they send against received traffic filter information.
If such a filter based mechanism is used, it is recommended that the function performing the overload traffic restrictions be as close to originating party as possible, while still having sufficiently aggregated traffic to perform restrictions on, e.g. by allowing the originating AS/S-CSCF to subscribe to the traffic filter information from a terminating AS.
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4.26 Support for Business Trunking
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An IMS network may support business trunking for IP‑PBXs in two different modes.
- Registration mode, or
- Static mode.
In both modes, the IP‑PBX can be provisioned as a subscriber in the HSS.
In registration mode, the IP‑PBX registers to and receives service from the IMS network in same manner as an ordinary subscriber.
In static mode, the IP‑PBX does not perform any registration procedures.
Description on the support for IP‑PBX business trunking is provided in Annex S and TS 24.525 [81]. The support for business trunking in static mode is provided by either an IBCF or a P‑CSCF and clarified in Annex S.
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5 IP multimedia subsystem procedures
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5.0 General
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This clause documents the main procedures that are used for the provision of services in the IP multimedia subsystem. These procedures are described using text description as well as information flow diagrams. The procedures described in this document are meant to provide a high level description and are not intended to be exhaustive.
In the following clauses, user roaming procedures apply to cases where P‑CSCF is located in the visited network. Procedures for cases where the user is roaming and the P‑CSCF is located in the home network are similar to procedures for a non-roaming user.
5.0a Session-unrelated procedures
The IM CN Subsystem provides means to conduct session-unrelated interactions between users, e.g. OPTIONS query, outband REFER. These interactions are described in IETF RFC 3261 [12] and other possible IETF RFCs. The generic capability exchange mechanism is defined in TS 23.279 [52].
These interactions shall use and fully comply with the basic mechanisms described for session-related procedures of the IM CN Subsystem. These mechanisms include e.g. routing, security, service control, network hiding as described in other clauses and specifications.
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5.1 CSCF related procedures
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5.1.0 Establishing IP-Connectivity Access Network bearer for IM CN Subsystem Related Signalling
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Before the UE can request IM services, an appropriate IP‑CAN bearer must be available to carry IM Subsystem related signalling.
For a UE using the IMS Local Breakout procedure as shown in Annex M, the IP address of the UE obtained from the local Gateway (i.e. single IP address) is used for both IM Subsystem related signalling and media.
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5.1.1 Procedures related to Proxy‑CSCF discovery
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5.1.1.0 General
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The Proxy‑CSCF discovery shall be performed using one of the following mechanisms:
- As part of the establishment of connectivity towards the IP-Connectivity Access Network, if the IP-Connectivity Access Network provides such means.
- Alternatively, the P‑CSCF discovery may be performed after the IP connectivity has been established. To enable P‑CSCF discovery after the establishment of IP connectivity, the IP-Connectivity Access Network shall provide the following P‑CSCF discovery option to the UE:
- Use of DHCP to provide the UE with the domain name and/or IP address of a Proxy‑CSCF and the address of a Domain Name Server (DNS) that is capable of resolving the Proxy‑CSCF name, as described below in clause 5.1.1.1.
- The UE may be configured (e.g. during initial provisioning or via a 3GPP IMS Management Object (MO), TS 24.167 [64] or in the ISIM, TS 31.103 [69]) to know the fully qualified domain name (FQDN) of the P‑CSCF or its IP address. If the domain name is known, DNS resolution is used to obtain the IP address.
If DNS is used to obtain the IP address of the P-CSCF, the name-address resolution mechanism is allowed to take the load information of the P-CSCFs (e.g. obtained using network management procedures) into consideration when deciding the address of the P-CSCF for the UE.
In the case where UE is aware of more than one P‑CSCF address, the selection shall be based on home operator configured policy to select the P‑CSCF.
NOTE: Subject to home operator policy, the UE selects the Home P‑CSCF to be used by either using a pre-configured Home P‑CSCF FQDN or according to TS 24.167 [64]. This can be done without the UE first performing the local P‑CSCF discovery (e.g. DHCP).
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5.1.1.1 DHCP/DNS procedure for P‑CSCF discovery
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The DHCP relay agent within the IP-Connectivity Access Network relays DHCP messages between UE and the DHCP server.
Figure 5.0a: P‑CSCF discovery using DHCP and DNS
1. Establish an IP-Connectivity Access Network bearer if not already available by using the procedures available in the IP-Connectivity Access Network.
2. The UE requests a DHCP server and additionally requests the domain name and/or IP address of the P‑CSCF and IP addresses of DNS servers. It may require a multiple DHCP Query/Response message exchange to retrieve the requested information.
3. The UE performs a DNS query to retrieve a list of P‑CSCF(s) IP addresses from which one is selected. If the response does not contain the IP addresses, an additional DNS query is needed to resolve a Fully Qualified Domain Name (FQDN) to an IP address.
After reception of domain name and IP address of a P‑CSCF the UE may initiate communication towards the IM subsystem.
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5.1.1.2 Void
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5.1.2 Procedures related to Serving‑CSCF assignment
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5.1.2.1 Assigning a Serving‑CSCF for a user
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When a UE attaches and makes itself available for access to IMS services by explicitly registering in the IMS, a S‑CSCF shall be assigned to serve the UE.
The assignment of an S‑CSCF is performed in the I‑CSCF. The following information is needed in the selection of the S‑CSCF:
1. Required capabilities for user services
This information is provided by the HSS.
2. Operator preference on a per-user basis
This information is provided by the HSS.
3. Capabilities of individual S‑CSCFs in the home network
This is internal information within the operator's network. This information may be used in the S‑CSCF selection. This information is obtained by the I‑CSCF by methods not standardised in this release.
4. Topological (i.e. P‑CSCF) information of where the user is located
This is internal information within the operator's network. This information may be used in the S‑CSCF selection. The P‑CSCF name is received in the registration request. The topological information of the P‑CSCF is obtained by the I‑CSCF by methods not standardised in this Release.
5. Topological information of where the S‑CSCF is located
This is internal information within the operator's network. This information may be used in the S‑CSCF selection. This information is obtained by the I‑CSCF by methods not standardised in this release.
6. Availability of S‑CSCFs
This is internal information within the operator's network. This information may be used in the S‑CSCF selection. This information is obtained by the I‑CSCF by methods not standardised in this release.
In order to support the S‑CSCF selection described above and to allow the S‑CSCF to perform its tasks, it is required that the following types of information be transferred between the CSCF and the HSS:
1 The Cx reference point shall support the transfer of CSCF-UE security parameters from HSS to CSCF.
- This allows the CSCF and the UE to communicate in a trusted and secure way (there is no à priori trust relationship between a UE and a CSCF)
- The security parameters can be for example pre-calculated challenge-response pairs, or keys for an authentication algorithm, etc.
2 The Cx reference point shall support the transfer of service parameters of the subscriber from HSS to CSCF.
- This may include e.g. service parameters, Application Server address, triggers, information on subscribed media etc. The information on subscribed media is provided in the form of a profile identifier; details of the allowed media parameters associated with the profile identifier are configured in the S‑CSCF.
3 The Cx reference point shall support the transfer of CSCF capability information from HSS to CSCF.
- This may include e.g. supported service set, protocol version numbers etc.
4 The Cx reference point shall support the transfer of session signalling transport parameters from CSCF to HSS. The HSS stores the signalling transport parameters and they are used for routing mobile terminated sessions to the Serving‑CSCF.
- The parameters may include e.g. IP-address and port number of CSCF, transport protocol etc.
The information mentioned in items 1 – 4 above shall be transferred before the CSCF is able to serve the user. It shall also be possible to update this information while the CSCF is serving the user, for example if new services are activated for the user.
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5.1.2.2 Cancelling the Serving‑CSCF assignment
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Cancellation of the assigned Serving CSCF is either:
- Initiated from the Serving CSCF itself, e.g. due to timeout of the registration
- Performed as a result of an explicit deactivation/de-registration from the IMS. This is triggered by the UE.
- Performed due to a request from the HSS over the Cx interface, e.g. due to changes in the subscription.
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5.1.2.3 Void
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5.1.3 Procedures related to Interrogating‑CSCF
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The architecture shall support multiple I‑CSCFs for each operator. A DNS-based mechanism for selecting the I‑CSCF shall be used to allow requests to be forwarded to an I‑CSCF based, for example, on the location or identity of the forwarding node.
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5.1.4 Procedures related to Proxy‑CSCF
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The routing of the SIP registration information flows shall not take into account previous registrations (i.e. registration state). The routing of the session information flows (e.g. INVITE) shall take into account the information received during the registration process.
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5.1.5 Subscription Updating Procedures
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5.1.5.0 General
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Whenever a modification has occurred in the subscription data that constitutes the data used by the S‑CSCF, the complete subscription data set shall be sent to the S‑CSCF by the HSS. HSS shall use the Push model for downloading the subscription data to the S‑CSCF.
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5.1.5.1 Subscription updating information flow
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This clause provides the information flows for subscription data updating procedure.
Figure 5.0b: Subscription data updating
1. The HSS sends the Cx-Update_Subscr_Data with the subscription data to the S‑CSCF.
2. The S‑CSCF sends Cx-Update_Subscr_Data Resp to the HSS to acknowledge the sending of Cx-Update_Subscr_Data
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5.2 Application level registration procedures
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5.2.0 General
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The following clauses address requirements and information flows related to registration in the IP multimedia subsystem. Assumptions that apply to the various information flows are listed as appropriate.
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5.2.1 Requirements considered for registration
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The following points are considered as requirements for the purpose of the registration procedures.
1. The architecture shall allow for the Serving‑CSCFs to have different capabilities or access to different capabilities. E.g. a VPN CSCF or CSCFs in different stages of network upgrade.
2. The network operator shall not be required to reveal the internal network structure to another network. Association of the node names of the same type of entity and their capabilities and the number of nodes will be kept within an operator's network. However disclosure of the internal architecture shall not be prevented on a per agreement basis.
3. A network shall not be required to expose the explicit IP addresses of the nodes within the network (excluding firewalls and border gateways).
4. It is desirable that the UE will use the same registration procedure(s) within its home and visited networks.
5. It is desirable that the procedures within the network(s) are transparent to the UE, when it registers with the IM CN subsystem.
6. The Serving‑CSCF is able to retrieve a service profile of the user who has IMS subscription. The S‑CSCF shall check the registration request against the filter information and if necessary inform Application Servers about the registration of the user; it shall be possible for the filter information to allow either just the initial registrations of the user or also subsequent re-registrations to be communicated to the Application Servers. The Serving‑CSCF knows how to reach the Proxy‑CSCF currently serving the user who is registered.
7. The HSS shall support the possibility to bar a Public User Identity from being used for IMS non-registration procedures. The S‑CSCF shall enforce these barring rules for IMS. Examples of use for the barring function are as follows:
- Currently it is required that at least one Public User Identity shall be stored in the ISIM application or, for UEs supporting only non-3GPP accesses or UEs accessing IMS via SNPN, in the IMC, if IMC is present. If the user/operator wants to prevent this Public User Identity from being used for IMS communications, it shall be possible to do so in the network without affecting the ISIM application or IMC directly.
8. The HSS shall support the possibility to restrict a user from getting access to IM CN Subsystem from unauthorized visited networks.
9. It shall be possible to register multiple public identities via single IMS registration procedure from the UE. See clause 5.2.1a for details.
10. It shall be possible to register a Public User Identity that is simultaneously shared across multiple contact addresses (at the same or via separate UEs) via IMS registration procedures. However, each registration and each de-registration process always relates to a particular contact address and a particular Private User Identity.
The number of allowed simultaneous registrations is defined by home operator policy, e.g. locally configured at the S-CSCF or, when registration is requested via separate UEs, per subscribed value if received from the HSS.
10a. It shall be possible for the UE to indicate to the network whether the registration adds a new contact to an existing registration from the same UE.
11. Registration of a Public User Identity shall not affect the status of already registered Public User Identity(s), unless due to requirements by Implicit Registration set defined in clause 5.2.1a.
12. When multiple UEs share the same public identity (es), each UE shall be able to register its contact address(es) with IMS.
13. The UE may indicate its capabilities and characteristics in terms of SIP User Agent capabilities and characteristics described in IETF RFC 3840 [38] during IMS registration. The UE may also update its capabilities by initiating a re-registration when the capabilities are changed on the UE.
14. If a UE supports GRUU, the UE shall indicate its support for GRUUs and obtain a P‑GRUU and a T‑GRUU for each registered Public User Identity during IMS registration as described in RFC 5627 [49].
15. The P‑CSCF may subscribe to notifications of the status of the IMS Signalling connectivity after successful initial user IMS Registration.
16. When the access network type information is available from the access network, the P‑CSCF shall ensure that the IMS registration request received from the UE to the SIP server (e.g. S‑CSCF) contains the correct information. The P‑CSCF may subscribe to notification of changes in the type of access network.
17. The P‑CSCF shall cancel any active subscription e.g. to notifications of the status of the IMS Signalling connectivity and/or of the change of access network type when the user is de-Registered from the IM CN subsystem.
18. When the UE determines that the radio conditions are suitable for IMS PS voice/video services (e.g. UE is in normal coverage or in CE mode A as defined in Annex E, clause E.1.2) then UE may register for IMS voice/video services.
5.2.1a Implicit Registration
5.2.1a.0 General
When an user has a set of Public User Identities defined to be implicitly registered via single IMS registration of one of the Public User Identity's in that set, it is considered to be an Implicit Registration. No single public identity shall be considered as a master to the other Public User Identities. Figure 5.0c shows a simple diagram of implicit registration and Public User Identities. Figure 5.0d shows a similar diagram when multiple Private User Identities are involved. In order to support this function, it is required that:
- HSS has the set of Public User Identities that are part of implicit registration.
- Cx reference point between S‑CSCF and HSS shall support download of all Public User Identities associated with the implicit registration, during registration of any of the single Public User Identities within the set.
- All Public User Identities of an Implicit Registration set must be associated to the same Private User Identities. See figure 5.0d for the detailed relationship between the public and private user entities within an Implicit Registration set.
- When one of the Public User Identities within the set is registered, all Public User Identities associated with the implicit registration set are registered at the same time.
- When one of the Public User Identities within the set is de-registered, all Public User Identities that have been implicitly registered are de-registered at the same time.
- Registration and de-registration always relates to a particular contact address and a particular Private User Identity. A Public User Identity that has been registered (including when implicitly registered) with different contact addresses remains registered in relation to those contact addresses that have not been de-registered.
- Public User Identities belonging to an implicit registration set may point to different service profiles; or some of these Public User Identities may point to the same service profile.
- When a Public User Identity belongs to an implicit registration set, it cannot be registered or de-registered individually without the Public User Identity being removed from the implicit registration list.
- All IMS related registration timers should apply to the set of implicitly registered Public User Identities
- S‑CSCF, P‑CSCF and UE shall be notified of the set of Public User Identities belonging to the implicitly registered function. Session set up shall not be allowed for the implicitly registered Public User Identities until the entities are updated, except for the explicitly registered Public User Identity.
- The S‑CSCF shall store during registration all the Service profiles corresponding to the Public User Identities being registered.
- When a Public User Identity is barred from IMS communications, only the HSS and S‑CSCF shall have access to this Public User Identity.
Figure 5.0c: Relationship of Public User Identities when implicitly registered
Figure 5.0d: The relation of two shared Public User Identities (Public-ID-3 and 4) and Private User Identities
5.2.1a.1 Implicit Registration for UE without ISIM or IMC
If an UE is registering in the IMS without ISIM or, for UEs supporting only non-3GPP accesses or UEs accessing IMS via SNPN, without IMC, it shall require the network's assistance to register at least one Public User Identity, which is used for session establishment & IMS signalling. Implicit registration shall be used as part of a mandatory function for these ISIM-less or IMC-less UEs to register the Public User Identity(s). In addition to the functions defined in clause 5.2.1a, the following additional functions are required for this scenario.
- The Temporary public identity shall be used for initial registration process
- It shall be defined in HSS that if the user does not have implicit registration activated then the user shall not be allowed to register in the IMS using the Temporary Public User Identity.
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5.2.2 Registration flows
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5.2.2.1 Requirements to consider for registration
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The additional requirement for the registration information flow for this clause is:
1. A Serving‑CSCF is assigned at registration, this does not preclude additional Serving‑CSCFs or change of CSCF at a later date. Procedures for use of additional CSCFs are not standardised in this release.
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5.2.2.2 Assumptions
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The following are considered as assumptions for the registration procedures as described in clause 5.3.2.3:
1. IP‑CAN bearer is already established for signalling and a mechanism exists for the first REGISTER message to be forwarded to the proxy.
2. The I‑CSCF shall use a mechanism for determining the Serving‑CSCF address based on the required capabilities. The I‑CSCF obtains the name of the S‑CSCF from its role as an S‑CSCF selector (Figure 5.1) for the determination and allocation of the Serving‑CSCF during registration.
3. The decision for selecting the S‑CSCF for the user in the network is made in the I‑CSCF.
4. A role of the I‑CSCF is the S‑CSCF selection.
In the information flows described in clauses 5.2.2.3 and 5.2.2.4, there is a mechanism to resolve a name and address. The text in the information flows indicates when the name-address resolution mechanism is utilised. These flows do not take into account security features such as user authentication. The description of the impact of IMS security features is done in TS 33.203 [19].
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5.2.2.3 Registration information flow – User not registered
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The application level registration can be initiated after the registration to the access is performed and after IP connectivity for the signalling has been gained from the access network. For the purpose of the registration information flows, the user is considered to be always roaming. For user roaming in their home network, the home network shall perform the role of the visited network elements and the home network elements.
Figure 5.1: Registration – User not registered
1. After the UE has obtained IP connectivity, it can perform the IM registration. To do so, the UE sends the Register information flow to the proxy (Public User Identity, Private User Identity, home network domain name, UE IP address, Instance Identifier, GRUU Support Indication).
2. Upon receipt of the register information flow, the P‑CSCF shall examine the "home domain name" to discover the entry point to the home network (i.e. the I‑CSCF). The proxy shall send the Register information flow to the I‑CSCF (P‑CSCF address/name, Public User Identity, Private User Identity, P‑CSCF network identifier, UE IP address). A name-address resolution mechanism is utilised in order to determine the address of the home network from the home domain name. The P‑CSCF network identifier is a string that identifies at the home network, the network where the P‑CSCF is located (e.g. the P‑CSCF network identifier may be the domain name of the P‑CSCF network).
3. The I‑CSCF shall send the Cx-Query/Cx-Select-Pull information flow to the HSS (Public User Identity, Private User Identity, P‑CSCF network identifier).
The HSS shall check whether the user is registered already. The HSS shall indicate whether the user is allowed to register in that P‑CSCF network (identified by the P‑CSCF network identifier) according to the User subscription and operator limitations/restrictions if any.
4. Cx-Query Resp/Cx-Select-Pull Resp is sent from the HSS to the I‑CSCF. It shall contain the S‑CSCF name, if it is known by the HSS, or the S‑CSCF capabilities, if it is necessary to select a new S‑CSCF. When capabilities are returned, the I‑CSCF shall construct a name from the capabilities returned.
If the checking in HSS was not successful the Cx-Query Resp shall reject the registration attempt.
5. The I‑CSCF, using the name of the S‑CSCF, shall determine the address of the S‑CSCF through a name-address resolution mechanism. The name-address resolution mechanism is allowed to take the load information of the S‑CSCFs (e.g. obtained using network management procedures) into consideration when deciding the address of the S-CSCF. The I‑CSCF also determines the name of a suitable home network contact point, possibly based on information received from the HSS. I‑CSCF shall then send the register information flow (P‑CSCF address/name, Public User Identity, Private User Identity, P‑CSCF network identifier, UE IP address to the selected S‑CSCF. The home network contact point will be used by the P‑CSCF to forward session initiation signalling to the home network.
The S‑CSCF shall reject the registration if the number of registered contact addresses for a Public User Identity from the same UE exceeds the limit of simultaneous registrations configured at the S‑CSCF. The S-CSCF shall also reject the registration from separate UEs if the allowed number of simultaneous registrations according to the S-CSCF configuration or per subscribed value for a Public User Identity received from the HSS exceeds the limit of simultaneous registrations. The S‑CSCF shall store the P‑CSCF address/name, as supplied by the visited network. This represents the address/name that the home network forwards the subsequent terminating session signalling to the UE. The S‑CSCF shall store the P‑CSCF Network ID information.
6. The S‑CSCF shall send Cx-Put/Cx-Pull (Public User Identity, Private User Identity, S‑CSCF name) to the HSS.
7. The HSS shall store the S‑CSCF name for that user and return the information flow Cx-Put Resp/Cx-Pull Resp (user information) to the S‑CSCF. The user information passed from the HSS to the S‑CSCF shall include one or more names/addresses information which can be used to access the platform(s) used for service control while the user is registered at this S‑CSCF. The S‑CSCF shall store the information for the indicated user. In addition to the names/addresses information, security information and the MPS for Messaging indication may also be sent for use within the S‑CSCF.
8. Based on the filter criteria, the S‑CSCF shall send register information to the service control platform and perform whatever service control procedures are appropriate.
9. The S‑CSCF shall return the 200 OK information flow (home network contact information, a GRUU set) to the I‑CSCF.
10. The I‑CSCF shall send information flow 200 OK (home network contact information, a GRUU set) to the P‑CSCF. The I‑CSCF shall release all registration information after sending information flow 200 OK.
11. The P‑CSCF shall store the home network contact information and shall send information flow 200 OK (a GRUU set) to the UE. The P‑CSCF may subscribe to notifications of the status of the IMS Signalling connectivity from PCRF/PCF (see TS 23.203 [54] and TS 23.503 [95] for more details).
If the S-CSCF receives the priority information of the MPS subscribed-UE, including an MPS for Messaging indication, as a part of user profile from the HSS, the S-CSCF provides the priority information to the P-CSCF and the P-CSCF stores this information for the MPS-subscribed UE.
NOTE: After the UE is registered, any changes in the user profile in the HSS, including changes in the MPS subscription and/or MPS for Messaging indication, are propagated to the S-CSCF and the P-CSCF.
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5.2.2.4 Re-Registration information flow – User currently registered
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Periodic application level re-registration is initiated by the UE either to refresh an existing registration or in response to a change in the registration status of the UE. A re-registration procedure can also be initiated when the capabilities of the UE have changed or the IP‑CAN has changed. The UE should perform IMS re-registration when the IP-CAN used by the UE changes between 3GPP access and WLAN access.
Re-registration follows the same process as defined in clause 5.2.2.3 "Registration Information Flow – User not registered". When initiated by the UE, based on the registration time established during the previous registration, the UE shall keep a timer shorter than the registration related timer in the network.
NOTE 1: if the UE does not re-register, any active sessions may be deactivated.
Figure 5.2: Re-registration - user currently registered
1. The UE initiates a re-registration. For periodic registration, the UE initiates a re-registration prior to expiry of the agreed registration timer. To re-register, the UE sends a new REGISTER request. The UE sends the REGISTER information flow to the proxy (Public User Identity, Private User Identity, home network domain name, UE IP address, capability information, Instance Identifier, GRUU Support Indication).
2. Upon receipt of the register information flow, the P‑CSCF shall examine the "home domain name" to discover the entry point to the home network (i.e. the I‑CSCF). The proxy does not use the entry point cached from prior registrations. The proxy shall send the Register information flow to the I‑CSCF (P‑CSCF address/name, Public User Identity, Private User Identity, P‑CSCF network identifier, UE IP address). A name-address resolution mechanism is utilised in order to determine the address of the home network from the home domain name. The P‑CSCF network identifier is a string that identifies at the home network, the network where the P‑CSCF is located (e.g. the P‑CSCF network identifier may be the domain name of the P‑CSCF network).
NOTE 2: The P-CSCF may force the UE to attempt initial registration with another P-CSCF, instead of forwarding its re-registration request. This is useful e.g. to move users from a P-CSCF to another P-CSCF without interrupting the service to these users.
3. The I‑CSCF shall send the Cx-Query information flow to the HSS (Public User Identity, Private User Identity and P‑CSCF network identifier).
4. The HSS shall check whether the user is registered already and return an indication indicating that an S‑CSCF is assigned. The Cx-Query Resp (indication of entry contact point, e.g. S‑CSCF) is sent from the HSS to the I‑CSCF.
5. The I‑CSCF, using the name of the S‑CSCF, shall determine the address of the S‑CSCF through a name-address resolution mechanism. The I‑CSCF also determines the name of a suitable home network contact point, possibly based on information received from the HSS. I‑CSCF shall then send the register information flow (P‑CSCF address/name, Public User Identity, Private User Identity, P‑CSCF network identifier, UE IP address to the selected S‑CSCF. The home network contact point will be used by the P‑CSCF to forward session initiation signalling to the home network.
The S‑CSCF shall store the P‑CSCF address/name, as supplied by the visited network. This represents the address/name that the home network forwards the subsequent terminating session signalling to the UE.
6. The S‑CSCF shall send Cx-Put/Cx-Pull (Public User Identity, Private User Identity, S‑CSCF name) to the HSS. Note: Optionally as an optimisation, the S‑CSCF can detect that this is a re-registration and omit the Cx-Put/Cx-Pull request.
7. The HSS shall stores the S‑CSCF name for that user and return the information flow Cx-Put Resp/Cx-Pull-Resp (user information) to the S‑CSCF. The S‑CSCF shall store the user information for that indicated user.
8. Based on the filter criteria, the S‑CSCF shall send re-registration information to the service control platform and perform whatever service control procedures are appropriate.
NOTE 3: The service control environment can be notified of the current IP-CAN type serving the UE via this procedure.
9. The S‑CSCF shall return the 200 OK information flow (home network contact information, a GRUU set) to the I‑CSCF.
10. The I‑CSCF shall send information flow 200 OK (home network contact information, a GRUU set) to the P‑CSCF. The I‑CSCF shall release all registration information after sending information flow 200 OK.
11. The P‑CSCF shall store the home network contact information and shall send information flow 200 OK (a GRUU set) to the UE.
If the S-CSCF receives the priority information of the MPS subscribed-UE as a part of user profile from the HSS, the S-CSCF provides the priority information to the P-CSCF and the P-CSCF stores this information for the MPS-subscribed UE.
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5.2.2.5 Stored information.
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Table 5.1 provides an indication of some of the information stored in the indicated nodes during and after the registration process. Note that Table 5.1 is not an exhaustive list of stored information, i.e. there can be additional information stored due to registration.
Table 5.1 Information Storage before, during and after the registration process
Node
Before Registration
During Registration
After Registration
UE - in local network
Credentials
Home Domain
Proxy Name/Address
Same as before registration
Credentials
Home Domain
Proxy Name/Address
UE P‑GRUU
At least one T‑GRUU
Proxy‑CSCF
- in Home or Visited network
Routing Function
Initial Network Entry point
UE Address
Public and Private User IDs
Access Network Type
Final Network Entry point
UE Address
Public and Private User IDs
Access Network Type
Interrogating‑CSCF - in Home network
HSS or SLF Address
Serving‑CSCF address/name
P‑CSCF Network ID
Home Network contact Information
No State Information
HSS
User Service Profile
P‑CSCF Network ID
Serving‑CSCF address/name\
Serving‑CSCF (Home)
No state information
HSS Address/name
User profile (limited – as per network scenario)
Proxy address/name
P‑CSCF Network ID
Public/Private User ID
UE IP Address
UE P‑GRUU
UE T‑GRUU
May have session state Information
Same as during registration
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5.3 Application level de-registration procedures
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5.3.1 Mobile initiated de-registration
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When the UE wants to de-register from the IMS then the UE shall perform application level de-registration. De-registration is accomplished by a registration with an expiration time of zero seconds. De-registration follows the same path as defined in clause 5.2.2.3 "Registration Information Flow – User not registered".
Figure 5.3: De-registration - user currently registered
1. The UE decides to initiate de-registration. To de-register, the UE sends a new REGISTER request with an expiration value of zero seconds. The UE sends the REGISTER information flow to the proxy (Public User Identity, Private User Identity, home network domain name, UE IP address).
2. Upon receipt of the register information flow, it shall examine the "home domain name" to discover the entry point to the home network (i.e. the I‑CSCF). The proxy does not use the entry point cached from prior registrations. The proxy shall send the Register information flow to the I‑CSCF (P‑CSCF address/name, Public User Identity, Private User Identity, P‑CSCF network identifier, UE IP address). A name-address resolution mechanism is utilised in order to determine the address of the home network from the home domain name. The P‑CSCF network identifier is a string that identifies at the home network, the network where the P‑CSCF is located (e.g. the P‑CSCF network identifier may be the domain name of the P‑CSCF network).
3. The I‑CSCF shall send the Cx-Query information flow to the HSS (Public User Identity, Private User Identity, P‑CSCF network identifier).
4. The HSS shall determine that the Public User Identity is currently registered. The Cx-Query Resp (indication of entry point, e.g. S‑CSCF) is sent from the HSS to the I‑CSCF.
5. The I‑CSCF, using the name of the S‑CSCF, shall determine the address of the S‑CSCF through a name-address resolution mechanism and then shall send the de-register information flow (P‑CSCF address/name, Public User Identity, Private User Identity, UE IP address) to the S‑CSCF.
6. Based on the filter criteria, the S‑CSCF shall send de-registration information to the service control platform and perform whatever service control procedures are appropriate. Service control platform removes all subscription information related to this specific Public User Identity.
7. Based on operator choice the S‑CSCF can send either Cx-Put (Public User Identity, Private User Identity, clear S‑CSCF name) or Cx-Put (Public User Identity, Private User Identity, keep S‑CSCF name) and the Public User Identity is no longer considered registered in the S‑CSCF. If the user has (originating – see 5.6.5, or terminating – see 5.12) services related to unregistered state, the S‑CSCF sends Cx-Put (Public User Identity, Private User Identity, keep S‑CSCF name) in order to keep the S‑CSCF name in the HSS for these services.
The HSS then either clears or keeps the S‑CSCF name for that Public User Identity according to the Cx-Put request. If the S‑CSCF name is kept, then the HSS shall be able to clear the serving S‑CSCF name at any time.
8. The HSS shall send Cx-Put Resp to the S‑CSCF to acknowledge the sending of Cx-Put.
9. The S‑CSCF shall return the 200 OK information flow to the I‑CSCF. The S‑CSCF may release all registration information regarding this specific registration of the Public User Identity after sending information flow 200 OK.
10. The I‑CSCF shall send information flow 200 OK to the P‑CSCF.
11. The P‑CSCF shall send information flow 200 OK to the UE. The P‑CSCF releases all registration information regarding this specific registration of the Public User Identity after sending information flow 200 OK. If the P‑CSCF has an active subscription to notifications of the status of the IMS Signalling connectivity, the P‑CSCF shall cancel the subscription (see TS 23.203 [54] and TS 23.503 [95] for more details).
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5.3.2 Network initiated de-registration
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5.3.2.0 General
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If an ungraceful session termination occurs (e.g. flat battery or mobile leaves coverage), when a stateful proxy server (such as the S‑CSCF) is involved in a session, memory leaks and eventually server failure can occur due to hanging state machines. To ensure stable S‑CSCF operation and carrier grade service, a mechanism to handle the ungraceful session termination issue is required. This mechanism should be at the SIP protocol level in order to guarantee access independence for the IM CN subsystem.
The IM CN subsystem can initiate a Network Initiated De-Registration procedures for the following reasons:
- Network Maintenance.
Forced re-registrations from users, e.g. in the case of data inconsistency at node failure, in the case of UICC lost, etc. Cancelling the current contexts of the user spread among the IM CN Subsystem network nodes at registration and imposing a new IM registration solves this condition.
- Network/traffic determined.
The IM CN subsystem must support a mechanism to avoid duplicate registrations or inconsistent information storage. This case will occur when a user roams to a different network without de-registering the previous one. This case may occur at the change of the roaming agreement parameters between two operators, imposing new service conditions to roamers.
- Application Layer determined.
The service capability offered by the IM CN Subsystem to the Application Layers may have parameters specifying whether all IM CN subsystem registrations are to be removed, or only those from one or a group of terminals from the user, etc.
- Subscription Management
The operator must be able to restrict user access to the IM CN subsystem upon detection of contract expiration, removal of IM subscription, fraud detection, etc. In the case of changes in service profile of the user, e.g. the user subscribes to new services, it may possible that new S‑CSCF capabilities, which are required from the S‑CSCF, are not supported by the current S‑CSCF which has been assigned to the user. In this case, it shall be possible to actively change the S‑CSCF by using the network initiated de-registration by HSS procedure.
The following clauses provide scenarios showing SIP application de-registration. Note that these flows have avoided the strict use of specific SIP protocol message names. This is in an attempt to focus on the architectural aspects rather than the protocol.
Two types of network-initiated de-registration procedures are required:
- To deal with registrations expirations.
- To allow the network to force de-registrations following any of the approved possible causes for this to occur.
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5.3.2.1 Network Initiated Application (SIP) De-registration, Registration Timeout
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The following flow shows a network initiated IM CN subsystem terminal application (SIP) de-registration based on a registration timeout. A timer value is provided at initial registration and is refreshed by subsequent re-registrations. The flow assumes that the timer has expired. The locations (home or visited network) of the P‑CSCF and S‑CSCF are not indicated as the scenario remains the same for all cases.
Figure 5.4: Network initiated application de-registration, registration timeout
1. The registration timers in the P‑CSCF and in the S‑CSCF expire. The timers are assumed to be close enough that no external synchronisation is required. The P‑CSCF updates its internal databases to remove the Public User Identity from being registered. It is assumed that any cleanup of IP-Connectivity Access Network resources will be handled by independent means. If the P‑CSCF has an active subscription to notifications of the status of the IMS Signalling connectivity, the P‑CSCF shall cancel the subscription (see TS 23.203 [54] and TS 23.503 [95] for more details).
2. Based on the filter criteria, the S‑CSCF shall send de-registration information to the service control platform and perform whatever service control procedures are appropriate. Service control platform removes all subscription information related to this specific Public User Identity.
3. Based on operator choice the S‑CSCF can send either Cx-Put (Public User Identity, Private User Identity, clear S‑CSCF name) or Cx-Put (Public User Identity, Private User Identity, keep S‑CSCF name) and the Public User Identity is no longer considered registered in the S‑CSCF. If the user has (originating – see 5.6.5, or terminating – see 5.12) services related to unregistered state, the S‑CSCF sends Cx-Put (Public User Identity, Private User Identity, keep S‑CSCF name) in order to keep the S‑CSCF name in the HSS for these services.
The HSS then either clears or keeps S‑CSCF name for that Public User Identity according to Cx-Put the request. If the S‑CSCF name is kept, then the HSS shall be able to clear the serving S‑CSCF name at any time.
4. The HSS shall send Cx-Put Resp to the S‑CSCF to acknowledge the sending of Cx-Put.
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5.3.2.2 Network Initiated Application (SIP) De-registration, Administrative
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5.3.2.2.0 General
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For different reasons (e.g. subscription termination, lost terminal, etc.) a home network administrative function may determine a need to clear a user's SIP registration. This function initiates the de-registration procedure and may reside in various elements depending on the exact reason for initiating the de-registration.
One such home network element is the HSS, which already knows the S‑CSCF serving the user and that for this purpose makes use of the Cx-Deregister. Another home network element that could initiate the de-registration is the S‑CSCF, in which case it makes use of the Cx-Put to inform the HSS. Other trusted/secured parties may also initiate de-registration to the S‑CSCF.
The following flow shows a network initiated IM CN subsystem terminal application (SIP) de-registration based on an administrative action for example. The IP transport infrastructure is not notified. If complete packet access is to be denied, a transport layer administrative mechanism would be used. This scenario does not address the administrative mechanisms used for updating any subscriber records, EIR records, access authorization, etc. This scenario only addresses the specific action of clearing the SIP application registration that is currently in effect.
As determined by the operator, on-going sessions may be released by using network initiated session release procedures in clause 5.10.3.
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5.3.2.2.1 Network Initiated De-registration by HSS, administrative
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Figure 5.5: Network initiated application de-registration by HSS, administrative
1. HSS initiates the de-registration, sending a Cx-Deregister (user identity) which may include the reason for the de-registration.
2. Based on the filter criteria, the S‑CSCF shall send de-registration information to the service control platform and perform whatever service control procedures are appropriate.
3. The S‑CSCF issues a de-registration towards the P‑CSCF for this user and updates its internal database to remove the user from being registered. The reason for the de-registration received from the HSS shall be included if available.
4. The P‑CSCF informs the UE of the de-registration and without modification forwards the reason for the de-registration, if available. Due to loss of contact with the mobile, it might be possible that the UE does not receive the information of the de-registration.
5. The P‑CSCF sends a response to the S‑CSCF and updates its internal database to remove the user from being registered. If the P‑CSCF has an active subscription to notifications of the status of the IMS Signalling connectivity, the P‑CSCF shall cancel the subscription (see TS 23.203 [54] for more details).
6. When possible, the UE sends a response to the P‑CSCF to acknowledge the de-registration. A misbehaving UE or a UE that is out of P‑CSCF coverage could not answer properly to the de-registration request. The P‑CSCF should perform the de-registration in any case, e.g. after the timer for this request expires.
If the UE does not perform automatic re-registration due to the de-registration the user shall be informed about the de-registration and of the reason, if available.
NOTE 1: Steps 4 and 5 may be done in parallel: the P‑CSCF does not wait for an answer from the UE before answering to the S‑CSCF
7. The S‑CSCF returns a response to the entity that initiated the process.
NOTE 2: Another trusted/secured party may also request for de-registration via HSS through administrative mechanisms provided by the operator.
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5.3.2.2.2 Network Initiated De-registration by Service Platform
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A service platform may determine a need to clear a user's SIP registration. This function initiates the de-registration procedure and resides in a service platform.
The following flow shows a service control initiated IMS terminal application (SIP) de-registration.
Figure 5.5a: Network initiated application de-registration, service platform
1. The S‑CSCF receives de-registration information from the service platform and invokes whatever service logic procedures are appropriate. This information may include the reason for the de-registration.
2. The S‑CSCF issues a de-registration towards the P‑CSCF for this user and updates its internal database to remove the user from being registered. The reason for the de-registration shall be included, if available.
3. The P‑CSCF informs the UE of the de-registration and without modification forwards the reason for the de-registration, if available. Due to loss of contact with the mobile, it might be possible that the UE does not receive the information of the de registration.
4. The P‑CSCF sends a response to the S‑CSCF and updates its internal database to remove the user from being registered. If the P‑CSCF has an active subscription to notifications of the status of the IMS Signalling connectivity, the P‑CSCF shall cancel the subscription (see TS 23.203 [54] for more details).
5. When possible, the UE sends a response to the P‑CSCF to acknowledge the de-registration. A misbehaving UE or a UE that is out of P‑CSCF coverage could not answer properly to the de-registration request. The P‑CSCF should perform the de-registration in any case, e.g. after the timer for this request expires.
If the UE does not perform automatic re-registration due to the de-registration the user shall be informed about the de-registration and of the reason, if available.
NOTE 1: Steps 4 and 5 may be done in parallel: the P‑CSCF does not wait for an answer from the UE before answering to the S‑CSCF
6. Based on operator choice the S‑CSCF can send either Cx-Put (Public User Identity, Private User Identity, clear S‑CSCF name) or Cx-Put (Public User Identity, Private User Identity, keep S‑CSCF name). In both cases the Public User Identity is no longer considered registered in the S‑CSCF. If the user has (originating - see 5.6.5, or terminating - see 5.12) services related to unregistered state, the S‑CSCF may send Cx-Put (Public User Identity, Private User Identity, keep S‑CSCF name) in order to keep the S‑CSCF name in the HSS for these services.
The HSS then either clears or keeps S‑CSCF name for that Public User Identity according to Cx-Put the request.
7. The HSS shall send Cx-Put Resp to the S‑CSCF to acknowledge the sending of Cx-Put.
NOTE 2: Another trusted/secured party may also initiate the de-registration, for example, by issuing a third party SIP registration with timer set to 0 via S‑CSCF.
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5.4 Procedures for IP multi-media sessions
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5.4.0 General
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Basic IMS sessions between users will always involve two S‑CSCFs (one S‑CSCF for each). The session flow is decomposed into two parts: an origination part between the UE & the S‑CSCF and termination part between the S‑CSCF and the UE, including all network elements in the path.
A basic session between a user and a PSTN endpoint involves an S‑CSCF for the UE, a BGCF to select the PSTN gateway and an MGCF for the PSTN.
The session flow is decomposed into three parts – an origination part, an inter-Serving‑CSCF/ MGCF part and a termination part. The origination part covers all network elements between the UE (or PSTN) and the S‑CSCF for that UE (or MGCF serving the MGW). The termination part covers all network elements between the S‑CSCF for the UE (or MGCF serving the MGW) and the UE (or PSTN).
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5.4.1 Bearer interworking concepts
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Voice bearers from the IM CN subsystem need to be connected with the voice bearers of other networks. Elements such as Media Gateway Functions (MGW) are provided to support such bearer interworking. One of the functions of the MGW may be to support transcoding between a codec used by the UE in the IM CN subsystem and the codec being used in the network of the other party.
Default codecs to be supported within the UE are IP‑CAN dependent and hence are defined in the respective IP‑CAN specific documents. The use of default codecs within the UE enables the IM CN subsystem to interwork with other networks on an end to end basis or through transcoding.
The IM CN subsystem is also able to interwork with the CS networks (e.g. PSTN, ISDN, CS domain of some PLMN) by supporting transcoding in the IMS MGW element. Furthermore to allow interworking between users of the IM CN subsystem and IP multimedia fixed terminals and other codecs may (this is implementation dependent) be supported by the MGW.
In order to support existing network capabilities, it is required that IMS supports endpoints (e.g. UE, MRFP, MGCF for interworking with the PSTN) able to send or receive DTMF tone indications using the bearer, i.e. inband signalling. An additional element for bearer interworking is the interworking of these DTMF tones and out-of-band signalling between one network and another. In such a case, the MGW shall provide tone generation and may provide detection under the control of the MGCF.
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5.4.2 Interworking with Internet
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Depending on operator policy, the S‑CSCF may forward the SIP request or response to another SIP server located within an ISP domain outside of the IM CN subsystem.
It is possible that the external SIP client does not support one or more of the SIP extensions required for IMS end points to set up IMS sessions (e.g. Preconditions, Update, 100Rel) as described in TS 24.229 [10a], then the UE or other SIP user agents within the IMS should be able to fall back to SIP procedures which allow interworking towards the external client. Depending on the home network operator policy, the network may restrict session initiation requests towards and from external SIP clients without the support of SIP extensions defined for IMS sessions.
5.4.2a IP version interworking
Following interworking scenarios exist:
Application Level Interworking
It should be possible for users connected to an IMS network to communicate with users that are connected to SIP based networks that use a different IP version via interworking or that are in a separate addressing range (e.g. NA(P)T functionality is set at the border of the IMS). Annex I describes in more detail how such interworking is performed for IMS.
Transport Level Interworking
Inter-working also includes tunnelling level interconnection of IMS networks via transit networks that use a different IP version using for example, configured tunnels as described in TS 23.221 [7]. Figure 5.5b below shows an example configuration scenario where two IPv6 IMS networks are connected via an IPv4 network.
Figure 5.5b: Example tunnelling of IPv6 traffic over IPv4 networks
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5.4.3 Interworking with PSTN
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The S‑CSCF, possibly in conjunction with an Application Server, shall determine that the session should be forwarded to the PSTN. The S‑CSCF will forward the Invite information flow to the BGCF in the same network.
The BGCF selects the network in which the interworking should occur and the selection of the interworking network is based on local policy.
If the BGCF determines that the interworking should occur in the same network, then the BGCF selects the MGCF which will perform the interworking, otherwise the BGCF forward the invite information flow to the BGCF in the selected network.
The MGCF will perform the interworking to the PSTN and control the MG for the media conversions.
The high level overview of the network initiated PSTN interworking process is shown in figure 5.6.
Figure 5.6: Network based PSTN interworking breakout process
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5.4.4 Requirements for IP multi-media session control
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In order for operators to be able to offer a "carrier-grade" IP multimedia service and to require bearers whose features (e.g. Bandwidth) are coherent with the media components negotiated through CSCFs, the following features shall be offered:
1. Both end points of the session shall be able to negotiate (according to service /UE settings,) which resources (i.e. which media components) need to be established before the destination party is alerted. The session signalling shall ensure that these resources (including IP-Connectivity Access Network resources and IP multimedia backbone resources) are made available or reserved before the destination UE rings.
This should nevertheless not prevent the UE from offering to the end-user the choice of accepting or rejecting the components of the session before establishing the bearers.
2. Depending on regulatory requirements, the IP multimedia service shall be able to charge the originating party for the IP-Connectivity Access Network service of both originating and destination side or when reverse charging applies to charge the terminating party for the IP-Connectivity Access Network service of both originating and terminating side. This implies that it should be easy to correlate CDR held by the IP-Connectivity Access Network service with a session.
3. The session control function of IP multimedia network of an operator (CSCF) shall be able (according to operator choice) to have a strict control (e.g. on source /destination IP address, QoS) on the flows associated with session established through SIP entering the IP multimedia bearer network from IP-Connectivity Access Network service. This does not mean that CSCF is the enforcement point (which actually is the Gateway between the IP-Connectivity Access Network and the IP multimedia network) but that the CSCF may be the final decision point for this control.
4. The session control and bearer control mechanisms shall allow the session control to decide when user plane traffic between end-points of a SIP session may start/shall stop. This allows this traffic to start/stop in synchronisation with the start/stop of charging for a session.
5. The IP-Connectivity Access Network service shall be able to notify the IP multimedia session control when the IP-Connectivity Access Network service has either modified or suspended or released the bearer(s) of a user associated with a session (because e.g. the user is no longer reachable).
6. The solution shall comply with the architectural rules relating to separation of bearer level, session control level and service level.
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5.4.5 Session Path Information
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5.4.5.1 Session Path Information during Registration and Session Initiation
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During registration and session initiation there are SIP mechanisms, which provide the means to determine the session path.
After registration the P‑CSCF stores the S‑CSCF name, possibly IBCF names and the S‑CSCF stores the P‑CSCF name and possibly IBCF names (see 4.3.4) as part of the UE related information.
There is a need to store the session path that is determined during the session initiation request in order to route the subsequent session requests through this determined path. This is needed in order to route these session requests through certain nodes, e.g. the ones performing Service Control, or interconnect functions. CSCFs are assumed to perform certain actions:
1. CSCFs (Proxy and Serving) store a certain part of the session path determined during session initiation. This allows CSCFs to generate requests that traverse all elements on a Route path.
2. The P‑CSCF shall check correct usage of the header values. Should an UE build inaccurate header(s) in a SIP request, the P‑CSCF may reject the request. If an operator policy requires enforcing the routes stored in P‑CSCF, the P‑CSCF shall overwrite the header(s) provided by the UE with the appropriate values.
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5.4.5.2 P‑CSCF in the Session Path
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All SIP signalling to or from the UE traverses the P‑CSCF.
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5.4.5.3 S‑CSCF in the Session Path
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All initial requests to or from the UE traverse the S‑CSCF assigned to the UE. The S‑CSCF uses the "Record-Route" mechanism defined in IETF RFC 3261 [12] to remain in the signalling path for subsequent requests too; in short terms: the S‑CSCF "record-routes". This is considered the default behaviour for all IMS communication. However, if Application Servers under operator control guarantee the home control of the session, then it may not be required that all subsequent requests traverse the S‑CSCF. In such cases the operator may choose that the S‑CSCF does not "record-route". The detailed record-route behaviour is configured in the S‑CSCF, e.g. on a per-service basis. The S‑CSCF decides whether it performs record-routing or not based on operator configuration in the S‑CSCF.
See also Annex F for background information.
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5.4.6 End-user preferences and terminal capabilities
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5.4.6.0 General
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Due to different capabilities of the originating and terminating terminals, it might not be possible to establish all the media suggested by the originator for a particular session. In addition, the destination user may have different preferences of type of media depending on who is originating and on the situation e.g. being in a meeting or driving the car, etc.
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5.4.6.1 Objectives
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The general objectives concerning terminal capabilities and end-user behaviour are listed below.
- The capabilities of the terminal have impact on the SDP description in the SIP session flows, since different terminals may support different media types (such as video, audio, application or data) and may have implemented different set of codecs for audio and video. Note that the capabilities of the terminal may change when an external device, such as a video camera is attached to the terminal.
- The configuration of the terminal changes the capabilities of the terminal. This can be done by attaching external devices or possibly by a user setting of certain parameters or profiles in the terminal.
- The preferences of the destination user may depend on who is originating the session and on the situation. Cost, associated with the session, may also be another factor, i.e. depending on time of the day or day of the week etc. Due to this reason the user may want to accept or reject certain media components.
- The available resources in the network play an important role, as certain media streams, consuming high bandwidth, may be denied. Therefore, before the user is alerted that the session set up is successful, it is assumed that the network has guaranteed and has reserved the needed resources for one or several media streams of the session. This does not preclude the possibility for the user to indicate his/her preferences regarding the session also after the alerting, in which case the initial resource reservations may have to be modified.
- End-to-end quality of service may be provided by using a variety of mechanisms, including guaranteed end-to-end QoS and best effort. The network may not be able to guarantee the requested end-to-end QoS. This may be the case when the user is establishing sessions through the public Internet. On the other hand, certain sessions, with the agreement of the initiating and terminating endpoints, should have the right to go through even without having the requested QoS guarantee.
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5.4.6.2 End-user expectations
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From the end-user point of view the following user interactions can be listed:
- For outgoing sessions, it is assumed that the user would like to select certain parameters that define the proposed session. This can be pre-configured as preferences or defined on a per session basis.
- For incoming sessions, it is assumed that the terminal will establish a dialogue with the user. Such dialogue allows the user to manually accept some of the proposed parameters by the originator. This is typically media type (audio, video, whiteboard) and different quality parameters per media type. As an alternative, the user preferences may be pre-configured.
- Before establishing or accepting a new session, the user may define or agree on the following parameters. Some of these parameters may be pre-configured and others are defined on a per session basis.
1. Type of media, i.e. audio, video, whiteboard, etc. This represents the user preferences of media types.
2. Combination of QoS attributes and selection of codec. This represents the quality of the media component, the cost and the probability of availability of resources both in the access network and in the core network.
3. Subset of capabilities used in the terminal. Terminals can have different set of capabilities. However, the user may or may not want to use the maximum set of capabilities. For instance, a user might want to establish a low cost video session with a small window on the screen.
4. End-to-end quality of service. For certain media streams, the user may want assured end-to-end QoS while for other streams the QoS may be optional or even not desired at all (best effort).
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5.4.6.3 Mechanism for bearer establishment
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In order to fulfil the above requirements, it is needed that the destination user can be pre-alerted before the bearer establishment and negotiation and IP-Connectivity Access Network bearer activation has taken place. This gives room for the destination user to choose the media streams and codecs required before an expensive resource (as the air interface is) is established.
Figure 5.7 shows the mechanism for the bearer establishment in which the pre-alerting occurs before the initial bearer creation procedures are performed. Furthermore, a user interaction may also occur after the initial bearers are created as shown in figure 5.7. If the session originator receives multiple provisional responses for the same session indicating that the session has been forked in the network, the UE may choose to process a pre-configured number of responses. In the case of multiple responses, the resources requested by the UE shall be the "logical OR" (i.e. least upper bound) of the resources indicated in the multiple responses to avoid allocation of unnecessary resources. The UE shall never request more resources then was originally proposed in the Original INVITE.
The "Other x‑CSCFs" entity in figure 5.7 comprises several CSCFs: I‑CSCF and S‑CSCFs. For the sake of simplicity only the IP-Connectivity Access Network is shown and the Policy Decision Functions have been omitted from the diagram.
Figure 5.7: Bearer establishment showing optional pre-alerting
1. UE(A) starts a Session Initiation procedure to UE(B) that includes an SDP proposal.
The steps 2-4 are optional and may depend on terminal implementation and/or terminal pre-configured settings.
2. The user at UE(B) is pre-alerted.
3. An indication of the pre-alerting may be sent towards UE(A).
4. User at UE(B) will then interact and express his/her wishes regarding the actual session.
5. UE(B) generates accepted SDP based on terminal settings, terminal pre-configured profiles and optionally the user's wishes.
6. The accepted SDP is forwarded to UE(A) in the payload of a reliable SIP response.
7. Initial bearer creation procedure is performed. During this bearer creation step the resources in the UE(A)'s and UE(B)'s IP‑CANs are reserved. Bearer resources in external networks may also be reserved at this point.
The steps 8-10 are also optional and may be skipped.
8. Terminal at UE(B) starts ringing.
9. The alerting indication is sent towards UE(A).
10. User at UE(B) may interact and express his/her wishes regarding the actual session.
11. UE(A) and UE(B) may perform bearer modification procedure at this point, if the initial bearers reserved in step 7 and the wishes of user at UE(B) are different. During this bearer modification step the resources in the IP‑CANs of UE(A) and UE(B) may be modified and the resource reservation in the external network may also be modified.
12. Session initiation procedure is acknowledged.
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5.4.6.4 Session progress indication to the originating UE
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The pre-alerting or alerting indications returned to the originating UE shall enable the originating UE to inform the calling user of the session progress prior to the arrival of the incoming media (for example the originating UE may synthesise ringing locally).
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5.4.7 Interaction between QoS and session signalling
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5.4.7.0 General
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At IP‑CAN bearer activation the user shall have access to either IP‑CAN services without Policy and Charging Control, or IP‑CAN services with Policy and Charging Control. It is operator choice whether to offer both or only one of these alternatives for accessing the IM Subsystem.
When using IP‑CAN without Policy and Charging Control, IP-CAN bearers are established according to the user's subscription, local operator's IP bearer resource based policy, local operator's admission control function and roaming agreements.
When using IP‑CAN with Policy and Charging Control, PCC decisions (e.g. authorization and control) are also applied to the IP-CAN bearer.
The description in this clause and the following clauses (clauses 5.4.7.1 - 5.4.7.7) is applicable for the case when Policy and Charging Control is employed.
The IP-Connectivity Access Network contains a Policy and Charging Enforcement Function (PCEF, in 5GS corresponding to the combination of SMF and UPF) that has the capability of policing packet flow into the IP network and restricting the set of IP destinations that may be reached from/through an IP‑CAN bearer according to a packet classifier.
NOTE: How PCEF is distributed in SMF and UPF in 5GS is specified in TS 23.501 [93] and TS 23.203 [54].
This policy 'gate' function has an external control interface that allows it to be selectively 'opened' or 'closed' on the basis of IP destination address and port. When open, the gate allows packets to pass through (to the destination specified in the classifier) and when closed, no packets are allowed to pass through. The control is performed by a PCRF/PCF (the interface between the PCRF/PCF and the P‑CSCF is the Rx interface standardised in TS 23.203 [54] or the N5 interface (using the Npcf_PolicyAutorization service), standardised in TS 23.503 [95] and TS 29.514 [96]).
There are eight interactions defined for Policy and Charging Control:
1. Authorize QoS Resources.
2. Resource Reservation.
3. Enabling of media flows authorized in (1), e.g. 'open' the 'gate'.
4. Disabling of media flows authorized in (1), e.g. 'close' the 'gate'.
5. Revoke Authorization for IP‑CAN and IP resources.
6. Indication of IP‑CAN bearer release from the PCEF in the IP-Connectivity Access Network to the PCRF/PCF.
7. Authorization of IP‑CAN bearer modification.
8. Indication of IP‑CAN bearer modification from the PCEF in the IP-Connectivity Access Network to the PCRF/PCF.
These requirements and functional description of these interactions are explained further in the following clauses. The complete specification of the interface between the PCRF/PCF and the PCEF is contained in TS 23.203 [54] and TS 23.503 [95].
The Policy and Charging Control can also be used to enable the P-CSCF to retrieve the user location and/or UE Time Zone information from the access network as specified in TS 23.203 [54] and TS 23.503 [95].
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5.4.7.1 Authorize QoS Resources
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The Authorize QoS Resources procedure is used during an establishment and a modification of a SIP session. The P‑CSCF shall use the SDP contained in the SIP signalling to derive the session information that is relevant for Policy and Charging Control and forwards it to the PCRF/PCF. The PCRF/PCF shall use the received information to calculate the proper authorization. This enables the PCRF/PCF to authorize the required QoS resources.
NOTE: Although session information is incomplete in the terminating side P‑CSCF at the reception of the SDP offer, it can be sent to PCRF/PCF whenever the SDP offer (contained in the session establishment request or session modification request) indicates no requirements for resource reservation or that the required resources are already available on the originating side, as in such cases no SDP answer is received before the PCRF/PCF is requested to authorize the required QoS resources.
The authorization shall be expressed in terms of the IP resources to be authorized and shall include limits on media flows and may include restrictions on IP destination address and port. The PCC shall authorize each SIP session independently (including additional parallel sessions, e.g. Call Waiting) and shall take into consideration the amount of IP resources the user's subscription allows.
5.4.7.1a Resource Reservation with Policy and Charging Control
The IP‑CAN provides the Policy and Charging Enforcement Point that implements the policy decisions for performing admission control and authorising the IP‑CAN and IP BS QoS Resource request and policing media flows entering the external IP network.
Authorization of IP‑CAN and IP QoS Resources shall be required for access to the IP Multimedia Subsystem. The IP‑CAN shall determine the need for authorization, possibly based on provisioning and/or based on requested parameters, which may be IP‑CAN specific.
Resource Reservation is initiated either by the UE or the IP‑CAN depending on the bearer establishment mode selected for the IP‑CAN session, see TS 23.203 [54] and TS 23.503 [95]:
- Resource reservation requests initiated from the UE shall (if possible for the used IP‑CAN) contain the traffic mapping information which enables the IP‑CAN to correctly match the reservation request to the corresponding authorization. The authorization is normally 'Pulled' from the PCRF/PCF by the PCEF within the IP‑CAN when the reservation request is received from the UE.
NOTE: When a UE combines multiple media flows onto a single IP‑CAN bearer, all the traffic mapping information related to those media flows are provided in the resource reservation request.
With a request for IP‑CAN QoS resources, the PCEF within the IP‑CAN shall verify the request is less than the sum of the authorized IP resources (within the error tolerance of the conversion mechanism) for all of the combined media flows.
- Resource reservation requests initiated by the IP‑CAN take place after successful authorization of QoS resources. The PCRF/PCF "Pushes" the authorization for IP‑CAN bearer resources to the PCEF within the IP‑CAN, which then enforces the authorization by either modifying the characteristics of one existing IP‑CAN bearer or requesting the establishment of a new one.
- Resource reservation requests initiated by the IP‑CAN shall (if possible for the used IP‑CAN) contain the traffic mapping information which enables the UE to correctly match the reservation request to the corresponding media of the SIP session.
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5.4.7.2 Enabling of Media Flows
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The PCRF/PCF makes policy decisions and provides an indication to the PCEF within the IP‑CAN that the user is now allowed to use the allocated QoS resources for per-session authorizations unless this was done based on Policy and Charging Control at the time of the Resource Reservation procedure. If there is more than one response for the same session, indicating that the session has been forked in the network, the PCRF/PCF may authorize the "logical OR" of the resources requested in the responses. When the session established indication has been received, if the PCRF/PCF earlier have authorized the "logical OR" of the resources then the PCRF/PCF will modify the authorization and enable the corresponding media flows according to the session established indication.
The PCEF within the IP‑CAN enforces the policy decisions. The IP‑CAN shall restrict any use of the IP resources prior to this indication from the PCRF/PCF, e.g. by keeping the gate closed and disabling the use of resources for the media flow. Based on local policy, IP‑CAN and/or IP resources may be allowed to be used by the user at the time they are authorized by the PCRF/PCF.
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5.4.7.3 Disabling of Media Flows
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The PCRF/PCF makes policy decisions and provides an indication to the PCEF within the IP‑CAN about revoking the user's capacity to use the allocated QoS resources for per-session authorizations. The indication for disabling media flows shall be sent as a separate decision to the PCEF within the IP‑CAN corresponding to the previous request to enable media flows.
The PCEF within the IP‑CAN enforces the policy decisions. The IP‑CAN shall restrict any use of the IP resources after this indication from the PCRF/PCF, e.g. by closing the gate and blocking the media flow.
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5.4.7.4 Revoke Authorization for IP-Connectivity Access Network and IP Resources
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At IP multimedia session release, the UE should deactivate the IP‑CAN bearer(s) used for the IP multimedia session. In various cases the UE will be unable to perform this release itself. The PCRF/PCF provides indication to the PCEF within the IP‑CAN when the resources previous authorized and possibly allocated by the UE, are to be released. The IP‑CAN shall deactivate the IP‑CAN bearer used for the IP multimedia session.
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5.4.7.5 Indication of IP-Connectivity Access Network bearer release
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Any release of IP‑CAN bearer(s) that were established based on authorization from the PCRF/PCF shall be reported to the PCRF/PCF by the PCEF within the IP‑CAN.
This indication is forwarded to the P‑CSCF and may be used by the P‑CSCF to initiate a session release towards the remote endpoint e.g. if all IP-CAN bearer(s) associated with the session were released, the procedures in clause 5.10.3.1 can be executed.
NOTE: If only a subset of IP-CAN bearer(s) were released, then the UE can update the ongoing session with the remainder of allowed media flows or a subset of allowed media flows.
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5.4.7.6 Authorization of IP-Connectivity Access Network bearer modification
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When an IP‑CAN bearer is modified by the UE, such that the requested QoS falls outside of the limits that were authorized at IP‑CAN bearer activation (or last modification) or such that new binding information is received, then the PCEF within the IP‑CAN shall verify the authorization of this IP‑CAN bearer modification.
If the PCEF within the IP‑CAN does not have sufficient information to authorize the IP‑CAN bearer modification request, the PCEF within the IP‑CAN shall send an authorization request to the PCRF. The PCRF authorizes the modified IP‑CAN bearer based on the current session information. Note that the P‑CSCF sends an update of the session information in the case of a modification of a SIP session which results in an update of the authorization as described in clause 5.4.7.1.
When the P‑CSCF sends an update of the session information and the bearer establishment is controlled by the IP‑CAN, the PCRF/PCF shall send an updated authorization to the PCEF. The PCEF within the IP‑CAN enforces the policy decision accordingly (e.g. by requesting the reservation of new IP‑CAN bearer resources in the case of the addition of a new media component to the session or release of previously reserved resources if a media component has been removed from the IP Multimedia session).
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5.4.7.7 Indication of IP-Connectivity Access Network bearer modification
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When an IP‑CAN bearer is modified such that the maximum bit rate (downlink and uplink) is downgraded to 0 kbit/s or changed from 0 kbit/s to a value that falls within the limits that were authorized at IP‑CAN bearer activation (or last modification) then the PCEF within the IP‑CAN shall report this to the PCRF/PCF.
This indication is forwarded to the P‑CSCF and may be used by the P‑CSCF to initiate a session release towards the remote endpoint.
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5.4.7.8 Sharing of Resources for Network Detected Concurrent Sessions
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5.4.7.8.1 Network Detected Concurrent Sessions
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The following scenarios for concurrent sessions are subject to resource sharing:
- The UE is engaged in a session, puts the session on hold, then initiates a new session to a new UE.
- The UE is engaged in a session and receives an incoming session, puts the ongoing session on hold to accept the incoming session.
- The UE is engaged in multiple sessions, puts all sessions on hold and creates a conferencing session.
For a UE engaged in multiple sessions, with only one active session at any time, the network may be able to share resources for media components of the same type and that are common to these multiple sessions.
Resource sharing shall only be indicated for concurrent sessions that employ resource reservation based on TS 23.203 [54] or TS 23.503 [95].
When a UE puts a session on hold, it may put all or a subset of the media components of the session on hold. In such a case, the resource sharing applies only to the media components that are on hold and not to the rest of media components belonging to this IMS session
An emergency session shall not share resources with any other session.
NOTE: Resource sharing to be shared for any media component can be allowed for one direction as well as both directions. According to TS 24.229 [10a] and TS 24.610 [91] it is not prohibited that a UE may send media towards a held UE, i.e. uplink media can occur even when a UE puts a remote UE on hold. The P-CSCF or SIP AS can therefore be locally configured to allow resource sharing in both directions if the P-CSCF or SIP AS knows that the UE will not send media to a remote UE on hold.
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5.4.7.8.2 Initiating Resource Sharing for Network Detected Concurrent Sessions
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If the P-CSCF is configured to apply resource sharing, it may at establishment of a new Rx session or a new Npcf_PolicyAutorization application session context with the PCRF/PCF, indicate that resources may be shared in uplink and/or downlink direction by assigning an uplink and/or downlink tag to each media component of an IMS session unless it is an IMS emergency session.
NOTE: The assignment of tags to media components is regardless of any future sharing decision.
If this is the first IMS session of a UE, the P-CSCF shall assign new tags. Upon detection by the P-CSCF that a UE is engaged in multiple sessions, it shall determine if these sessions fulfil the criteria specified in clause 5.4.7.8.1 and have common media components that can share resources. If so, the P-CSCF may activate resource sharing by assigning the same existing tag to each media component whose resources can be shared amongst the IMS sessions. Otherwise, the P-CSCF shall assign new tags that are different from any tag previously assigned for this UE. The PCRF/PCF may then authorize resource sharing based on these tags. For further information, see TS 23.203 [54].
Whenever resource sharing is active, the P-CSCF shall ensure that the UE can only receive the media flow for one session at a time by closing the gates for all other media flows that can share the same resource, i.e. having the same tag.
SIP AS may indicate to a supporting P-CSCF to apply resource sharing to each media lines included in SDP of an IMS session. When SIP AS is used, P-CSCF, based on the local policy, may follow the resource sharing policy from SIP AS. The interaction between P-CSCF and SIP AS for handling Resource sharing procedure is defined in TS 24.229 [10a].
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5.4.7.9 Priority sharing for concurrent sessions
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The P-CSCF may indicate to the PCRF/PCF that the resource allocation for a media flow is allowed to use the same priority as other media flows of the same media type for the UE engaged in multiple sessions by providing a priority sharing indicator and an optional pre-emption control information in addition to the application identifier and the service priority. For MCPTT, the service priority and the priority sharing indicator are defined in TS 23.179 [92]. The pre-emption control information is used to indicate to the PCRF/PCF how to perform pre-emption in accordance to TS 23.203 [54] and TS 23.503 [95].
The following scenario is subject to priority sharing:
-- The P-CSCF receives a priority sharing indicator associated with the media flow from the Application Server.
Upon detection by the P-CSCF that a session includes a priority sharing indicator for a media flow and may optionally include pre-emption control information, the P-CSCF shall convey to the PCRF/PCF the priority sharing indicator and, when available, the pre-emption control information as described in TS 23.203 [54] and TS 23.503 [95].
NOTE 1: The Application Server is not supposed to include the priority sharing indicator for an emergency session.
NOTE 2: The priority sharing enables the usage of one bearer per QCI/5QI for a UE having multiple sessions; otherwise sessions with different service priorities would end up in different bearers.
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5.4.8 QoS-Assured Preconditions
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This clause contains concepts for the relation between the resource reservation procedure and the procedure for end-to-end sessions.
A precondition is a set of constraints about the session, which are introduced during the session initiation. The recipient of the session generates an answer, but does not alert the user or otherwise proceed with session establishment until the preconditions are met. This can be known through a local event (such as a confirmation of a resource reservation), or through a new set of constraints sent by the caller.
The set-up of a "QoS-Assured" session will not complete until required resources have been allocated to the session. In a QoS-Assured session, the QoS bearer for the media stream shall be successfully established according to the QoS preconditions defined at the session level before the UE may indicate a successful response to complete the session and alert the other end point. The principles for when a UE shall regard QoS preconditions to be met are:
- A minimum requirement to meet the QoS preconditions defined for a media stream in a certain direction, is that an appropriate IP‑CAN bearer is established at the local access for that direction.
- Segmented resource reservation is performed since the end points are responsible to make access network resource reservations via local mechanisms.
- The end points shall offer the resources it may want to support for the session and negotiate to an agreed set. Multiple negotiation steps may be needed in order to agree on a set of media for the session. The final agreed set is then updated between the end points.
- The action to take if a UE fails to fulfil the pre-conditions (e.g. failure in establishment of an RSVP session) depends on the reason for failure. If the reason is lack of resources in the network (e.g. an admission control function in the network rejects the request for resources), the UE shall fail to complete the session. For other reasons (e.g. lack of RSVP host or proxy along the path) the action to take is local decision within the UE. It may for example 1) choose to fail to complete the session, 2) attempt to complete the session by no longer requiring some of the additional actions.
NOTE 1: To avoid unwanted session setup delay and IP‑CAN signalling load, QoS-Assured sessions needs to be used with care and it is decided per application which media that require resource reservation.
The following cases exist in the context of using "QoS-Assured" preconditions for IMS:
a. The IMS session requires the reservation of additional bearer resources and the UE requires confirmation from the other endpoint of the fulfilment of the pre-conditions related to this resource reservation. An endpoint may not require the reservation of bearer resources and may therefore immediately indicate the local fulfilment of the pre-conditions. One example of such SIP endpoint is the MGCF used for PSTN interworking. In these cases, one or both of the reservation confirmation messages may not be sent.
b. The IMS session does not require the reservation of additional bearer resources and both endpoints indicate in their initial session setup message that the pre-conditions are fulfilled.
c. The IMS session does not require the reservation of additional bearer resources and the endpoints do not use the mechanism to indicate "QoS-Assured" pre-conditions.
NOTE 2: The flows of clauses 5.5, 5.6 and 5.7 depict the case where both UEs require confirmation from each other of the fulfilment of the pre-conditions. The flow in clause 5.7a depicts the case where the IMS session does not require the reservation of additional bearer resources and the endpoints do not use pre-conditions.
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5.4.9 Event and information distribution
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5.4.9.0 General
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The S‑CSCF and Application Servers (SIP‑AS, IM-SSF, OSA-SCS) shall be able to send service information messages to endpoints. This shall be done based on a SIP Request/Response information exchange containing the service information and/or a list of URI(s) pointing to the location of information represented in other media formats. The stimulus for initiating the service event related information message may come from e.g. a service logic residing in an Application Server.
In addition, the end points shall also be able to send information to each other. This information shall be delivered using SIP based messages. The corresponding SIP messages shall be forwarded along the IMS SIP signalling path. This includes the S‑CSCF but may also include SIP Application Servers. The information may be related or unrelated to any ongoing session and/or may be independent of any session. Applicable mechanisms (for e.g. routing, security, charging, etc) defined for IMS SIP sessions shall also be applied for the SIP based messages delivering the end-point information. The length of the information transferred is restricted by the message size (e.g. the MTU), so fragmentation and re-assembly of the information is not required to be supported in the UE. This information may include e.g. text message, http url, etc.
This mechanism considers the following issues:
- The IMS has the capability to handle different kinds of media. That is, it is possible to provide information contained within several different media formats e.g. text, pictures or video.
- The UE's level of supporting service event related information and its exchange may depend on the UE's capabilities and configuration.
- A UE not participating in the service related information exchange shall not be effected by a service related information exchange possibly being performed with another UE of the session.
NOTE: The service event related information exchange may either take place in the context of a session, or independently outside the context of any existing session.
Figure 5.8: Providing service event related information to related endpoint
1. When a service event occurs that the S‑CSCF or the Application Server wishes to inform an endpoint about, the S‑CSCF or the Application Server generates a message request containing information to be presented to the user. The contents may include text describing the service event, a list of URI(s) or other service modification information.
2. P‑CSCF forwards the message request.
3. UE presents the service-related information, to the extent that it conforms to its capabilities and configuration, to the user.
4. Possibly after interaction with the user, the UE will be able to include information in the response to the S‑CSCF.
5. P‑CSCF forwards the response.
NOTE 1: The UE may retrieve service event related information using IP‑CAN or IMS procedures.
NOTE 2: transport aspects of the information transfer described above may require further considerations.
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5.4.9.1 Subscription to event notifications
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The SIP‑event notification mechanism allows a SIP entity to request notification from remote nodes indicating that certain standardised events have occurred. Examples of such of events are changes in presence states, changes in registration states, changes in Subscription authorization policies (see TS 23.141 [36]) and other events that are caused by information changes in e.g. Application Servers or S‑CSCF.
It shall be possible to either fetch relevant information once or monitor changes over a defined time. It shall be possible for a user to subscribe to events related to his/her own subscription (e.g. when the user subscribes to his own registration state) or to events related to other users' subscription (an example is when a watcher subscribes to presence information of a presentity, see TS 23.141 [36]).
The S‑CSCF is not mandated to stay in the path after the initial SubscribeEvent request and ACK has been exchanged, if the S‑CSCF does not execute any functions for the subsequent requests and responses of the dialog. The example, in figure 5.8a below, assumes that the S‑CSCF does not want to execute any functions for the subsequent requests.
Figure 5.8a: Subscription to event in AS
1. The UE initiates a subscription to an AS requesting notification of any changes in specified information stored in the control of the AS
2. The P‑CSCF remembers (from the registration process) the next hop CSCF for this UE, i.e. the SubscribeEvent is forwarded to the S‑CSCF in the home network.
3. The S‑CSCF invokes whatever service logic procedures are appropriate for this request.
4. The S‑CSCF applies regular routing procedures and forwards the request to the next hop.
5. The AS acknowledges the SubscribeEvent request.
6. The S‑CSCF forwards the acknowledgement to the P‑CSCF.
7. The P‑CSCF forwards the acknowledgement to the UE.
8. As soon as the AS sends an acknowledgement to accept the subscription, the AS sends an EventNotification message with the current information the UE subscribed to. The EventNotification is sent along the path set-up by the SubscribeEvent dialog to the P‑CSCF allocated to the UE. Further notifications, if monitor of changes was requested, sent by the AS is sent along the same path.
9. The P‑CSCF forwards the EventNotification to the UE.
10. The UE acknowledges the EventNotification.
11. The P‑CSCF forwards the acknowledgement to the AS.
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5.4.10 Void
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5.4.11 Signalling Transport Interworking
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A Signalling gateway function (S‑GW) is used to interconnect different signalling networks i.e. SCTP/IP based signalling networks and SS7 signalling networks. The signalling gateway function may be implemented as a stand alone entity or inside another entity (see TS 23.002 [1]). The session flows in this specification do not show the S‑GW, but when interworking with PSTN/CS domain, it is assumed that there is a S‑GW for signalling transport conversion.
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5.4.12 Configuration and Routing principles for Public Service Identities
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5.4.12.0 General
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Depending on the service nature, different mechanisms may be used for configuration and routing of PSIs according to operator preference.
When PSIs are created, the uniqueness of a PSI shall be ensured. Note that only the username part of a PSI is definable within a predefined hostname(s).
Whenever possible, routing to/from a Public Service Identity (PSI) should be provided using basic principles used for IMS routing.
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5.4.12.1 PSIs on the originating side
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The Application Server hosting the PSI may be invoked as an originating Application Server. This can be achieved by modifying the filter information within the subscription information of the users intending to use the service identified by the PSI. The PSI is then made available to these users.
The SIP requests are directed to the corresponding Application Server hosting the service according to the originating filtering rules in the S‑CSCF of the user who is using the service.
Such statically pre-configured PSIs are only accessible internally from within the IMS of the operator's domain where the PSI is configured.
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5.4.12.2 PSIs on the terminating side
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The Application Server hosting the PSI may be invoked as a terminating Application Server via information stored in the HSS. Such PSIs are globally routable and can be made available to users within and outside the operator domain and can take the following form:
- Distinct PSIs are defined in TS 23.003 [24]. Distinct PSIs can be created, modified and deleted in the HSS by the operator via O&M mechanisms. Distinct PSIs can also be created and deleted by users using the Ut interface using the means described in clause 5.4.12.3 for subdomain-based PSIs.
- The distinct PSI may be activated in the HSS by the AS using the Sh interface.
- Wildcarded PSIs are defined in TS 23.003 [24]. Wildcarded PSI ranges can be created, modified and deleted in the HSS by the operator via O&M mechanisms. Specific PSIs within a wildcarded range can be created and deleted by users using the Ut interface to the AS hosting the wildcarded range, or by the operator via O&M mechanisms.
For both the distinct PSIs and wildcarded PSIs, there are two ways to route towards the AS hosting the PSI:
a) The HSS maintains the assigned S‑CSCF information and ISC Filter Criteria information for the "PSI user" to route to the AS hosting the PSI according to IMS routing principles. In this case, the I‑CSCF receives SIP requests at the terminating side, queries the HSS and directs the request to the S‑CSCF assigned to the "PSI user". The S‑CSCF forwards the session to the Application Server hosting the PSI according to the terminating ISC Filter Criteria.
b) The HSS maintains the address information of the AS hosting the PSI for the "PSI user". In this case, the AS address information for the PSI is returned to the I‑CSCF in the location query response, in which case the I‑CSCF will forward the request directly to the AS hosting the PSI.
The AS hosting the PSI in combination with its entry in the HSS is referred to as "PSI user".
Figure 5. 19d depicts a routing example for incoming session where the session request is routed directly to the AS hosting the PSI.
Figure 5.19e depicts an example routing scenario where the basic IMS routing via S‑CSCF is used to route the session.
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5.4.12.3 Subdomain based PSIs
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Subdomains defined for PSIs allow both operators and users to define specific PSIs within subdomains for specific applications. For this purpose, subdomains can be defined by the operator in the DNS infrastructure. Specific PSIs within a subdomain can be created and deleted by users using the Ut interface to the AS hosting the subdomain, or by the operator via O&M mechanisms.
Subdomain based PSIs are globally routable and can be made available to users within and outside the operator domain.
In this case, there are two ways to route towards the AS hosting the PSI:
a) When the subdomain name is defined in the global DNS, then the originating S‑CSCF or a Transit Function receives the IP address of the AS hosting the PSI, when it queries DNS. The principles defined in IETF RFC 3263 [44] may be used. For example, a NAPTR query and then a SRV query may be used to get the IP address of the AS.
b) The PSI is resolved by the global DNS to an I‑CSCF address in the domain where the AS hosting the PSI is located. The I‑CSCF recognises the subdomain (and thus does not query the HSS). It resolves the same PSI to the address of the actual destination AS hosting the PSI using an internal DNS mechanism and forwards the requests directly to the AS.
Figure 5.19f shows an example of DNS based routing of an incoming session from an external network. The routing from the external network leads to the entry point of the IMS subsystem hosting the subdomain of the PSI.
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5.4.12.4 PSI configuration in the HSS
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In order to support configuration of an AS hosting a PSI, the distinct PSIs and/or wildcarded PSI ranges hosted in the AS need to be configured in the HSS. The configuration shall include procedures to allow:
- Distinct PSIs and wildcarded PSI ranges to be configured in the HSS via operation and maintenance procedures,
- Authorization and verification of access as "PSI user" with the Public Service Identity hosted by the AS, e.g. for AS-originating requests,
- Access to "PSI user" information (e.g. the S‑CSCF assigned) over the Cx reference point from the CSCF nodes,
- Defining the "PSI user" similar to the principle of IMS user, without requiring any subscription/access information (e.g. CS/PS domain data) that are required for IMS user.
Note that the PSI configuration in the HSS does not affect the filter criteria based access to an AS as defined in the user profiles.
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5.4.12.5 Requests originated by the AS hosting the PSI
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The AS hosting the PSI may originate requests with the PSI as the originating party. For such originating requests, the home IMS network shall be capable to perform the following functions:
- Network Domain Security, TS 33.210 [20], shall be used where applicable.
- Charging requirements such as providing appropriate accounting and charging functions via the charging entities shall be supported according to TS 32.240 [25].
- If the target identity is a Tel URI (as specified in IETF RFC 3966 [15]), ENUM translation needs to be performed as described in clause 4.3.5.2 and the request shall be routed based on the translation result appropriately.
Routing from the Originating AS hosting the PSI can be performed as follows:
a) If the AS supports routing capabilities (e.g. ENUM support, etc), the AS may forward the originating request to the destination network without involving a S‑CSCF. If this option is applied where the target identity is a Tel URI, the AS shall perform an ENUM query and route the request based on the translation result.
b) If the AS does not support routing capabilities, the AS may forward the originating request to the IMS Transit Functions. The IMS Transit Functions will then route the session initiation request to the destination.
c) If the session requires the use of a S‑CSCF: either the PSI has an S‑CSCF assigned, in which case the AS forwards the originating request to this S‑CSCF, which then processes the request as per regular originating S‑CSCF procedures, or the PSI has no S‑CSCF assigned, in which case the AS sends the session initiation request to an I‑CSCF that will allocate an S‑CSCF to the PSI.
To prevent fraudulent or unsecure IMS traffic possibly caused by AS originated requests, security and authentication procedures may be performed towards the AS.
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5.4.13 Transcoding concepts
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IMS control plane entities, including the P‑CSCF, Application Servers or (for inter-domain sessions) the IBCF, may check the SDP offer/answer information associated with session requests and responses, to determine the need for transcoding. If such a need is determined to exist, media transcoding resources are reserved from the MRFP (via the MRFC), the IMS-AGW, or the TrGW.
Transcoding requires knowledge of the codecs supported by the end points and may be invoked at the originating or terminating network based on interworking agreements (e.g. local policy) or service level agreement (SLA).
For more details concerning transcoding involving MRFC/MRFP interworking see clause 5.14, Annex P and TS 23.218 [71] and for the IBCF/TrGW implementation consult clause 4.14 and Annex I, clause I.3.3.
5.4a Overview of session flow procedures
5.4a.1 End-to-End session flow procedures
This clause contains the overview description and list of individual procedures for the end-to-end session flows.
For an IP Multi-Media Subsystem session, the session flow procedures are shown in the following diagram.
Figure 5.9: Overview of Session Flow Sections
The following procedures are defined:
For the origination sequences:
- (MO#1) Mobile origination, roaming , see clause 5.6.1;
- (MO#2) Mobile origination, home, see clause 5.6.2;
- (PSTN-O) PSTN origination, see clause 5.6.3;
- (NI-O) Non-IMS network origination (external SIP client), see clause 5.6.4;
- (AS-O) Application Server origination, see clause 5.6.5.
For the termination sequences:
- (MT#1) Mobile termination, roaming, see clause 5.7.1;
- (MT#2) Mobile termination, home, see clause 5.7.2;
- (MT#3) Mobile termination, CS Domain roaming, see clause 5.7.2a;
- (PSTN-T) PSTN termination, see clause 5.7.3;
- (NI-T) Non-IMS network termination (external SIP client), see clause 5.7.4;
- (AS-T#1) PSI based Application Server termination, direct, see clause 5.7.5;
- (AS-T#2) PSI based Application Server termination, indirect, see clause 5.7.6;
- (AS-T#3) PSI based Application Server termination, direct, using DNS, see clause 5.7.7;
- (AS-T#4) PUI based Application Server termination, indirect, see clause 5.7.8.
For Serving‑CSCF/MGCF-to-Serving‑CSCF/MGCF sequences:
- (S-S#1) Session origination and termination are served by different network operators, see clause 5.5.1;
- (S-S#2) Session origination and termination are served by the same operator, see clause 5.5.2;
- (S-S#3) Session origination with PSTN termination in the same network as the S‑CSCF, see clause 5.5.3;
- (S-S#4) Session origination with PSTN termination in a different network to the S‑CSCF, see clause 5.5.4.
The media being offered and acknowledged to can take multiple negotiation steps or only one negotiation may be used. In these flows, a minimum of two negotiations has been shown. But the subsequent responses may not carry any media information and just confirm the initial media set agreement.
For example, for a non-roaming user initiating a session to another non-roaming user, each a subscriber of the same network operator, it is possible to construct a complete end-to-end session flow from the following procedures:
- (MO#2) Mobile origination, home,
- (S-S#2) Single network operator,
- (MT#2) Mobile termination, home.
There are a large number of end-to-end session flows defined by these procedures. They are built from combinations of origination, serving to serving and termination procedures, as determined from the following table. For each row of the table, any one of the listed origination procedures can be combined with any one of the serving-serving procedures, which can be combined with any one of the termination procedures.
Service control can occur at any point during a session, based on the filter criteria.
Note that the flows show service control only for the initial INVITE for originating and terminating party as an example.
The flows assume precondition mechanism is used, but as shown in clause 5.7a, a UE may originate a session without using preconditions.
Table 5.2: Combinations of session procedures
Origination Procedure
(pick one)
Serving‑CSCF-to-Serving‑CSCF Procedure (pick one)
Termination Procedure (pick one)
MO#1 Mobile origination,
roaming, home control of
services (2).
MO#2 Mobile origination,
located in home service
area.
PSTN-O PSTN origination.
AS-O Application Server
origination
NI-O Non-IMS network
origination
S-S#1 Different network operators
performing origination and
termination, with home control of
termination (2).
MT#1 Mobile termination,
roaming, home control of
services(2).
MT#2 Mobile termination,
located in home service
area.
MT#3 Mobile termination, CS
Domain roaming.
AS-T#1,2,3,4 Application
Server terminations
NI-T Non-IMS network
termination
MO#1 Mobile origination,
roaming, home control of
services (2).
MO#2 Mobile origination,
located in home service
area.
AS-O Application Server
origination
S-S#2 Single network operator
performing origination and
termination, with home control of
termination.
MT#1 Mobile termination,
roaming, home control of
services(2).
MT#2 Mobile termination,
located in home service
area.
MT#3 Mobile termination, CS
Domain roaming.
AS-T#1,2,3,4 Application
Server terminations
MO#1 Mobile origination,
roaming, home control of
services (2).
MO#2 Mobile origination,
located in home service
area.
AS-O Application Server
origination
S-S#3 PSTN termination in the
same network as the S‑CSCF.
PSTN-T PSTN termination.
MO#1 Mobile origination,
roaming, home control of services (2).
MO#2 Mobile origination,
located in home service
area.
AS-O Application Server
origination
S-S#4 PSTN termination in
different network than the
S‑CSCF
PSTN-T PSTN termination.
5.4a.2 Transit network session flow procedures
In addition to the combinations of flows constructed from the above scenarios, elements of an IMS network may be used by an operator in support of transit network scenarios. Figure 5.9a shows session flow combinations for transit network scenarios.
Figure 5.9a: Overview of Session Flow Sections for transit scenarios
Table 5.2a: Combinations of IMS transit network procedures
Origination Procedure
(pick one)
IMS Transit Network Procedure
Termination Procedure
(pick one)
MO#1 Mobile origination,
roaming, home control of
services (2).
MO#2 Mobile origination,
located in home service
area.
PSTN-O PSTN origination.
NI-O Non-IMS network
origination
I-T IMS Transit Network
MT#1 Mobile termination,
roaming, home control of
services(2).
MT#2 Mobile termination,
located in home service
area.
MT#3 Mobile termination, CS
Domain roaming.
PSTN-T PSTN termination.
NI-T Non-IMS network
termination
The following procedures are defined:
For the origination sequences:
- (MO#1) Mobile origination, roaming , see clause 5.6.1;
- (MO#2) Mobile origination, home, see clause 5.6.2;
- (PSTN-O) PSTN origination, see clause 5.6.3;
- (NI-O) Non-IMS network origination (external SIP client), see clause 5.6.4;
For the termination sequences:
- (MT#1) Mobile termination, roaming, see clause 5.7.1;
- (MT#2) Mobile termination, home, see clause 5.7.2;
- (MT#3) Mobile termination, CS Domain roaming, see clause 5.7.2a;
- (PSTN-T) PSTN termination, see clause 5.7.3;
- (NI-T) Non-IMS network termination (external SIP client), see clause 5.7.4;
For the IMS transit network aspects see clause 5.19.
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5.5 Serving‑CSCF/MGCF to serving‑CSCF/MGCF procedures
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5.5.0 General
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This clause presents the detailed application level flows to define the procedures for Serving‑CSCF/MGCF to Serving‑CSCF/MGCF.
In the IM CN subsystem the MGCF is considered as a SIP endpoint. It translates ISUP/BICC messages of the PSTN side to SIP signalling of the IM CN subsystem side and vice-versa. It should also be noted that the MGCF does not invoke Service Control.
This clause contains four session flow procedures, showing variations on the signalling path between the Serving‑CSCF that handles session origination and the Serving‑CSCF that handles session termination. This signalling path depends on:
- whether the originator and destination are served by the same network operator,
- whether the network operators have chosen to hide their internal configuration.
The Serving‑CSCF handling session origination performs an analysis of the destination address and determines whether it is a subscriber of the same network operator or a different operator.
If the analysis of the destination address determined that it belongs to a subscriber of a different operator, the request is forwarded) to a well-known entry point in the destination operator's network, the I‑CSCF. The I‑CSCF queries the HSS for current location information. The I‑CSCF then forwards the request to the S‑CSCF. If the analysis of the destination address determines that it belongs to a subscriber of the same operator, the S‑CSCF passes the request to a local I‑CSCF, who queries the HSS for current location information. The I‑CSCF then forwards the request to the S‑CSCF.
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5.5.1 (S-S#1) Different network operators performing origination and termination
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The Serving‑CSCF handling session origination performs an analysis of the destination address and determines that it belongs to a subscriber of a different operator. The request is therefore forwarded to a well-known entry point in the destination operator's network, the I‑CSCF. The I‑CSCF queries the HSS for current location information and finds the user either located in the home service area, or roaming. The I‑CSCF therefore forwards the request to the S‑CSCF serving the destination user.
Refer to table 5.2 in clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Originating Network".
MO#1 Mobile origination, roaming. The "Originating Network" of S-S#1 is therefore a visited network.
MO#2 Mobile origination, home. The "Originating Network" of S-S#1 is therefore the home network.
PSTN-O PSTN origination. The "Originating Network" of S-S#1 is the PSTN network. The elements of figure 5.16 replace all elements of the Originating network and Originating Home Network in figure 5.10.
AS-O Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S‑CSCF#1 corresponds to the S-SCSF in figure 5.16b.
NI-O Non-IMS network origination. The external SIP client of figure 5. 16b replaces all elements of the Originating network and Originating Home Network in figure 5.10. There may be other non-IMS SIP servers on the path that are not shown.
Refer to table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
MT#1 Mobile termination, roaming. The "Terminating Network" of S-S#1 is a visited network.
MT#2 Mobile termination, located in home service area. The "Terminating Network" of S-S#1 is the home network.
MT#3 Mobile termination, CS Domain roaming. The "Terminating Network" of S-S#1 is a CS domain network.
AS-T#1,2,3,4 Application Server termination. The elements of the corresponding AS-T termination figure (5.7.5, 5.7.6, 5.7.7 and 5.7.8) replace all elements of the Terminating Home Network and Terminating Network off figure 5.10.
NI-T Non-IMS network terminations. The external SIP client of figure5.19a replaces all elements of the Terminating Home Network and Terminating Network in figure 5.10. There may be other non-IMS SIP servers on the path that are not shown.
Figure 5.10: Serving to serving procedure - different operators
Procedure S-S#1 is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session attempt.
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the subscriber belongs. For S-S#1, this flow is an inter-operator message to the I‑CSCF entry point for the terminating user. S‑CSCF#1 forwards the INVITE request directly to I‑CSCF#2, the well-known entry point into the terminating user's network
4. I‑CSCF#2 (at the border of the terminating user's network) shall query the HSS for current location information.
5. HSS responds with the address of the current Serving‑CSCF for the terminating user.
6. I‑CSCF#2 forwards the INVITE request to the S‑CSCF (S‑CSCF#2) that will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt
8. The sequence continues with the message flows determined by the termination procedure.
9. The media stream capabilities of the destination are returned along the signalling path, as per the termination procedure.
10. S‑CSCF#2 forwards the SDP to I‑CSCF#2
11. I‑CSCF#2 forwards the SDP to S‑CSCF#1.
12. S‑CSCF#1 forwards the SDP to the originator, as per the originating procedure.
13. The originator decides on the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation and forwards this information to S‑CSCF#1 by the origination procedures. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
14-15. S‑CSCF#1 forwards the offered SDP to S‑CSCF#2.
16. S‑CSCF#2 forwards the offered SDP to the terminating endpoint, as per the termination procedure
17-20. The terminating end point acknowledges the offer with answered SDP and passes through the session path to the originating end point.
21-24. Originating end point acknowledges successful resource reservation and the message is forwarded to the terminating end point.
25-28. Terminating end point acknowledges the response and this message is sent to the originating end point through the established session path.
29-32. Terminating end point then generates ringing and this message is sent to the originating end point through the established session path.
33-36. Terminating end point then sends 200 OK via the established session path to the originating end point.
37-40. Originating end point acknowledges the establishment of the session and sends to the terminating end point via the established session path.
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