5 NBIFOM High level functions

23.1613GPPNetwork-Based IP Flow Mobility (NBIFOM)Release 17Stage 2TS

5.1 NBIFOM capability discovery/negotiation and change detection

During initial PDN connection establishment over the first access (or in the corresponding EAP-AKA’ signalling in S2a/SCM case), the UE includes a NBIFOM request. The intermediate nodes (e.g. MME, SGSN, TWAG, ePDG, etc.) indicate to the PDN GW whether they support NBIFOM in the corresponding Create Session Request. When PCC is deployed, the PDN GW communicates to the PCRF whether the UE and network support NBIFOM. The PCRF then takes the decision on whether NBIFOM may apply to a PDN connection and communicates this decision to the PDN GW. If the network (i.e. all the EPC elements in the signalling chain from UE to PDN GW) supports NBIFOM and is willing to accept NBIFOM for this PDN connection, the PDN GW confirms the NBIFOM support in a "NBIFOM accepted" indication sent back to the UE in the PDN connection establishment accept message (or in the corresponding EAP-AKA’ signalling in S2a/SCM case).

NOTE: It is assumed that intermediate nodes (e.g. MME, SGSN, TWAG, ePDG, etc.) do not need to know whether NBIFOM finally applies to the PDN connection.

The UE shall enforce that NBIFOM applies either to none or to all PDN connections set by an UE on a given APN; this e.g. applies when the UE decides whether to require NBIFOM at the activation of a PDN connection, or when the UE decides whether adding or removing an access to or from a PDN connection.

During the inter PLMN mobility, the new MME, SGSN and S-GW also indicate to the P-GW whether they support the NBIFOM. If the PDN GW detects a change in the support of NBIFOM, the P-GW shall initiate the procedure to remove the 3GPP access from the multi-access PDN connection.

The UE sends also to the network the NBIFOM mode it has selected (as described in clause 5.4) as part of the initial PDN connection establishment over the first access.

5.2 Routing rules

A Routing Rule contains the following parameters:

– Routing Filter.

– Routing Access Information.

– Routing Rule Priority.

– Routing Rule identifier.

A Routing Filter consists of IP header parameter values/ranges used to identify one or more IP flows as defined in clause 3.1.

The Routing Access Information identifies the access type where the IP flow shall be routed.

For the purpose of matching user traffic against routing rules, the filters are applied in the order of the Routing Rule Priority.

The Routing Rule identifier uniquely identifies a Routing Rule for one PDN Connection. The Routing Rule identifier is allocated by the entity creating the routing rule, i.e. by the UE in UE-initiated NBIFOM mode and by the PDN GW in Network-initiated NBIFOM mode.

When NBIFOM applies to a PDN Connection, between UE and the PDN GW there is always a default access via which packets not matching any specific routing filter (of any Routing Rule) are routed. In the case of 3GPP access, MCM/S2a and S2b, the UE and the network determine as follows the default access for a PDN Connection during the procedure of addition of one access to a PDN connection:

– In UE-initiated NBIFOM mode, the UE proposes a default access to the network. The network may accept the default access if it complies with the subscription. The default access replied by the network may be different than the default access requested by the UE.

– In Network-initiated NBIFOM mode, the network decides the default access and indicates it to the UE.

In the case of SCM/S2a, the UE and the network determine as follows the default access for a PDN Connection during the first procedure of IP flow mobility within a PDN connection:

– In UE-initiated NBIFOM mode, the UE proposes a default access to the network during the first UE-initiated IP flow mobility procedure via 3GPP. The network may accept the default access if it complies with the subscription. The default access replied by the network may be different than the default access requested by the UE.

– In Network-initiated NBIFOM mode, the network decides the default access and indicates it to the UE during the first Network-initiated IP flow mobility procedure via 3GPP.

NOTE: Whether the default access corresponds to an explicit parameter or to a default Routing Rule is to be defined as part of Stage 3.

The default access for a PDN Connection only applies when it is active over both 3GPP and WLAN access networks. When the connectivity over one access is not active, the UE and the network shall ignore the default access.

The PCRF may learn about the accesses that support a PDN connection via event triggers it sets on the PDN GW.

5.3 Delivery of routing rules and IP Flow mapping

Routing rules are exchanged between the UE and the PDN GW via control plane signalling.

IP Flow mapping is used in Network-initiated NBIFOM mode when the UE wants to request the network to apply specific IP mappings to IP flows to 3GPP/WLAN access. The network provides Routing Rules to the UE for these IP flows unless the request from the UE is not allowed by the subscription. IP Flow mapping is sent via control plane signalling.

In the case of 3GPP access, MCM/S2a and S2b, a Routing Rule and IP Flow mapping are sent on the destination access as indicated by the Routing Access information in the Routing Rule or IP Flow mapping. In the case of SCM/S2a, Routing Rules and IP Flow mapping are only sent on the 3GPP access.

For the UE-initiated NBIFOM procedure, the routing rules are provided from the UE to the network as follows:

– For untrusted WLAN access, via IKEv2 procedure from the UE to the ePDG.

– For 3GPP access, via 3GPP NAS signalling from the UE to the MME/SGSN and then over GTP-c to S-GW.

– For trusted WLAN access in MCM mode, via WLCP procedure from the UE to the TWAG.

And then:

– via GTP-c signalling from the TWAG (MCM) / ePDG / SGW to the PDN GW.

– via Gx procedure from the PDN GW to the PCRF.

The UE does not apply new routing rules until the PDN GW has acknowledged them.

For the network-initiated NBIFOM procedure, the PCRF makes the decision on when and what flows should be moved to which access. The PCRF learns about the multiple accesses supporting a PDN connection via event triggers from the PDN GW.

– The PCRF triggers an update of routing rules by sending PCC rules to the PDN GW via an IP-CAN Session Modification procedure. The PDN GW builds Routing Rules corresponding to PCC rules received from the PCRF. The PDN GW does not apply the new routing rules until the UE has acknowledged them.

– The routing rules are sent by the PDN GW to S-GW / TWAG (for MCM mode) / ePDG via GTP-c signalling.

– The routing rules are then relayed:

– from SGW to the MME/SGSN via GTP-C and further to the UE via the 3GPP access specific signalling (i.e. NAS) defined in TS 23.401 [4].

– from ePDG to the UE via IKE signalling over SWu.

– from TWAG to the UE via WLCP signalling over trusted TWAN defined in TS 23.402 [2] clause 16.

If updated routing rules result in that no IP flow is left for a particular access, the PDN GW should not release the PDN connection for that particular access unless the PDN connection is explicitly torn down by the UE or by the network as described in clause 6.5.

5.4 NBIFOM Modes of Operations

5.4.1 General

A multi-access PDN connection can operate either in UE-initiated NBIFOM mode or in Network-initiated NBIFOM mode. This NBIFOM mode is selected when the PDN connection is established and remains the same as long as the PDN connection is active. The following clauses specify (a) what the functional characteristics of each mode are and (b) how the operation mode is selected.

5.4.2 UE-initiated NBIFOM Mode

In the UE-initiated NBIFOM mode only the UE controls the traffic routing within the multi-access PDN connection by applying its ANDSF for IFOM rules and/or the user-configured routing rules. When ANDSF for IFOM rules are used, the UE decides the Routing Rules for downlink traffic.

NOTE: If the UE has valid ANDSF rules for NSWO, for IARP and/or MAPCON, then the UE can also control traffic routing outside the multi-access PDN connection.

The UE can request to move selected IP flows from an old access to a new access within the PDN connection by sending one or more routing rules to the network. These routing rules specify the selected IP flows and the new access.

The network may reject IP flow mobility requests from UE due to subscription (or quota) limitations. When the network rejects an IP flow mobility request, the network shall provide to UE a certain cause value indicating why the request was rejected. This cause value may be used by UE to determine when/if this IP flow mobility operation can be requested again.

The network cannot request IP flow mobility. The IP flow mobility procedures used in UE-initiated NBIFOM mode are described in clause 6.

5.4.3 Network-initiated NBIFOM mode

In the Network-initiated NBIFOM mode, the network controls the traffic routing within the multi-access PDN connection.

NOTE: However, if the UE has valid ANDSF rules for NSWO, for IARP and/or MAPCON, then the UE can still control traffic routing outside the multi-access PDN connection.

The network can request to move selected IP flows from an old access to a new access within the multi-access PDN connection by sending one or more routing rules to the UE. These routing rules specify the selected IP flows and the new access.

The UE may reject (e.g. due to Local Operating Environment information) an IP flow mobility request from the network. When the UE rejects an IP flow mobility request from the network, the UE shall provide a cause value indicating why the request was rejected. This cause value may be used by the network to determine when/if this IP flow mobility operation can be requested again.

If the UE wants to request a mapping of IP flows to an access type, the UE provides the requested mapping from to the network. This is however not considered a routing rule provision as the network is the decision point for Routing Rules in Network-initiated NBIFOM mode. The network then provides new or updated routing rules to the UE based on the request received from the UE, unless the request is against what is allowed by the subscription. IP Flow mapping information contains the same parameters as the Routing Rule. When the UE uses the IP flow mapping procedure to add a new Routing Filter, the Routing Rule identifier is assigned by the network. When the UE uses the IP flow mapping procedure to request the network to modify or delete an existing Routing Rule, the UE includes the same Routing Rule identifier as the one previously assigned by the network. The UE is not allowed to modify/remove a Routing Filter that the network has previously added using network-initiated IP flow mobility procedure. The corresponding call flows are provided in clause 6.4.

The UE can indicate to the network when an access of a multi-access PDN connection can or cannot be used for traffic routing. For example:

– When the UE loses the WLAN signal and the UE has active IP flows over WLAN, then the UE indicates to the network that WLAN cannot be used for traffic routing. This triggers the network to stop sending downlink traffic for these IP flows over WLAN access and provide updated routing rules to the UE. The UE and the network re-route one or more of these IP flows to 3GPP access, if the existing routing rules do not prevent them from using 3GPP access.

– When the UE recovers the WLAN signal and the existing routing rules in the UE require some active IP flows to be routed over WLAN, the UE indicates to the network that WLAN can again be used for traffic routing within the PDN connection. The network provides updated routing rules to the UE for the IP flows that are moved to WLAN access. The UE and the network re-route these active IP flows to WLAN access.

– The UE uses its Local Operating Environment (as defined in TS 23.261 [7]) to decide when the above indications should be sent to the network.

The network may provide to UE a "RAN rules / RCLWI handling" parameter during the PDN connection establishment procedure via 3GPP if the Network-initiated mode is selected (as specified in clause 5.4.4). When this parameter is set, it indicates that the UE shall use the "move traffic to/from WLAN" indications from Access Stratum under the conditions specified in bullet (a) below. When this parameter is not set or is not provided, it indicates that the UE shall not use the "move traffic to/from WLAN" indications. The UE uses the "RAN rules / RCLWI handling" parameter as specified below:

(a) If the "RAN rules / RCLWI handling" is set, the UE has no ISRP rule valid in the registered PLMN and the UE supports RAN-assisted WLAN interworking and/or and RCLWI, then the UE shall use the "move traffic to/from WLAN" indications from Access Stratum as follows:

– When Access Stratum indicates that traffic should be moved to 3GPP access, the UE shall send a move-from-WLAN indication to the network. The network then shall provide updated routing rules to the UE for traffic routing within the multi-access PDN connection.

– When Access Stratum indicates that traffic should be moved to WLAN access and the Local Operating Environment Information in the UE does not contain non-radio-related conditions that prohibit the use of WLAN access, the UE shall send a move-to-WLAN indication to the network. The network then shall provide updated routing rules to the UE for traffic routing within the multi-access PDN connection.

– The UE shall ignore the WLAN offloadability indication sent by MME.

(b) In all other cases, the UE shall not use the "move traffic to/from WLAN" indications from the Access Stratum. The UE shall perform traffic routing within a multi-access PDN connection by using the NBIFOM routing rules provided by the network.

5.4.4 Mode Selection

A multi-access PDN connection can operate in one of the following modes:

– UE-initiated mode;

– Network-initiated mode.

The details of UE-initiated mode are specified in clause 5.4.2 and the details of Network-initiated mode are specified in clause 5.4.3.

The NBIFOM mode is selected by the network (i.e. the PCRF, if PCC is deployed; or the PGW, otherwise) with the following procedure:

When an NBIFOM-capable UE supports ANDSF and has an ISRP rule valid in the registered PLMN then:

– If the ISRP rule includes at least one "ISRP for IFOM" rule (irrespectively of its validity), the UE requests UE-initiated mode;

– in non-roaming scenario, the network selects the UE-initiated mode;

– in roaming scenario:

– If the visited network selects the NBIFOM mode according to description in clause 5.5, the visited network shall select UE-initiated mode; otherwise

– If the home PLMN has not provided the UE with "ISRP for IFOM" rules, the home network makes the final decision of the NBIFOM mode, i.e., the home network may select either UE-initiated or NW-initiated mode according to home operator’s policy;

– otherwise, the network selects UE-initiated mode.

– Otherwise, the UE requests Network-initiated mode and the network selects Network-initiated mode.

When an NBIFOM-capable UE does not support ANDSF or supports ANDSF but does not have an ISRP rule valid in the registered PLMN then:

– The UE requests Network-initiated mode and the network selects Network-initiated mode.

Figure 5.4.4-1 provides a schematic illustration of the NBIFOM mode selection procedure as specified above. This figure assumes that the UE supports both ANDSF procedures and RAN-assisted WLAN interworking.

Figure 5.4.4-1: NBIFOM Mode Selection

5.5 PCC Interactions

When PCC applies for NBIFOM support on a PDN connection:

– The PCRF controls whether NBIFOM may apply to a PDN connection.

– The PCRF selects the NBIFOM mode.

– The PDN GW shall notify the PCRF when an access is added or removed for the PDN connection.

– The PDN GW shall notify the PCRF when an access becomes no more Usable or Usable again. This allows the PCRF to be aware when the UE losses or recovers connectivity on an access and to react accordingly in network-initiated NBIFOM mode.

– Multiple IP-CAN types (3GPP EPS and Non 3GPP EPS) may be simultaneously associated with the same IP-CAN session.

– The PCRF can send PCC rules including access type routing information per PCC rule to the PDN GW.

– For UE-initiated NBIFOM mode, the PDN GW may notify the PCRF about the Routing Rules received from the UE. When the Routing Rules received from the UE comply with the user subscription the PCRF installs / modifies PCC rules based on these notifications.

– For network-initiated NBIFOM mode, the PCRF determines the desired access type routing information for a PCC rule and accordingly installs / modifies PCC rules to the PDN GW. The PCRF request may be rejected by the UE due to local radio conditions. In that case the PCRF gets notified that the PCC rule cannot be fulfilled.

In following conditions the PCRF mentioned above is the Home PCRF:

– The UE is served by its HPLMN, or

– The PDN connection is served by a PGW in the Home PLMN (Home Routed configuration), or

– The PDN connection is served by a PGW in the V-PLMN (LBO configuration), and S9 is deployed and the V-PCRF supports NBIFOM.

The PCRF mentioned above is the Visited PCRF in the case when, through roaming agreement, the HPLMN operator allows the VPLMN operator to operate the V‑PCRF without S9; this includes authorization of roamers to use NBIFOM. In that case, policy control based on subscription policy information from the HPLMN cannot apply.

NOTE: If the Home operator wants to enforce control by its own PCRF of the NBIFOM functionality on a PDN connection, the Home operator should ensure that the Home Routed roaming configuration applies to this PDN connection.

PCC impacts of NBIFOM are further described in TS 23.203 [3].

5.6 Addition of one access to a Multi-access PDN connection

During PDN connection establishment over an access, the UE may provide to the network a Handover Indication and a NBIFOM indication (in the PCO in the case of 3GPP access or TWAN access) in order to request that the PDN connection is to be used for NBIFOM: in this case the PDN connection establishment corresponds to the addition of an access to an already established PDN connection.

NOTE 1: In case a Handover Indication and no NBIFOM indication in the PCO are provided by the UE the PDN connection is handed over to the new access per the procedures described in TS 23.402 [2]

NOTE 2: In case No Handover Indication is provided by the UE, the PDN connection establishment corresponds to the set-up of a new PDN connection per the procedures described in TS 23.402 [2]

Intermediate node (e.g. MME/SGSN, SGW, TWAG or ePDG) indicate in the Create Session Request whether they support NBIFOM regardless of whether the Create Session Request set-up carries a Handover Indication. In the case of 3GPP access and WLAN access with S2a, NBIFOM is not supported by an intermediate node (i.e. MME/SGSN, SGW, TWAG) while the UE requests to add an additional access to a PDN connection, the network should reject the PDN connectivity request with the appropriate cause value to indicate the failure of adding an additional access for NBIFOM. However, the previously established PDN connection shall be maintained on the first access.

NOTE 3: In the case of WLAN access with S2b, when NBIFOM is not supported by an ePDG, the addition of WLAN access is treated by the PGW as handover request. If the UE successfully connects with the ePDG but does not receive an NBIFOM indication in IKEv2 signalling from the network, the UE determines that its PDN connection over 3GPP access was handed over to WLAN.

When PCC applies, the PDN GW initiates an IP-CAN Modification Procedure to inform the PCRF about the UE request to add an access to an already established IP-CAN session and about the added RAT type and IP-CAN type. The PDN GW confirms the NBIFOM support in a "NBIFOM accepted" indication sent back to the UE in the PDN connection establishment accept message (or in the corresponding EAP-AKA’ signalling in S2a/SCM case)

The Addition of one access to a Multi-access PDN connection is further described in clause 6.2

5.7 IP flow mobility within a Multi-access PDN connection

In UE-initiated NBIFOM mode, the IP flow mobility within a Multi-access PDN connection relies on the UE request to move selected IP flows from an old access to a new access by sending one or more routing rules to the network. This is further described in clauses 5.4.2 and 6.3.3

In Network-initiated NBIFOM mode, the IP flow mobility within a Multi-access PDN connection relies on the network request to move selected IP flows from an old access to a new access by sending one or more routing rules to the UE. The procedures IP flow mobility in Network-initiated NBIFOM mode are further described in clauses 5.4.3 and 6.3.2. The network decision for IP Flow Mobility may be based on notifications received from the UE that an access has become Unusable or has become Usable again as described in clauses 5.4.3, 6.6 and 6.7.

5.8 Removal of an access from a PDN connection

Both the UE and the network may initiate removal of an access procedure in both UE-initiated NBIFOM mode and network-initiated NBIFOM mode. When an access is removed from a multi-access PDN connection,

– In the UE-initiated removal of an access procedure, the PCEF (in the PDN GW) notifies the PCRF about the removal of an access. The PCRF determines the affected PCC rules and replies with updated PCC Rules or informs about the PCC Rules that are to be removed. The UE shall delete the Routing Rules locally

– In the Network-initiated removal of an access procedure, the PCRF should indicate the removed access to the PCEF by updating or removing all PCC rules that are bound to this access. In UE-initiated NBIFOM mode, the UE shall delete the Routing Rules locally.

The Removal of an access from a PDN connection is further described in clause 6.5.

5.8a Network-initiated release of NBIFOM PDN connection

The network may need to release a NB-IFOM PDN connection over both 3GPP and WLAN accesses, e.g. for SIPTO as defined in TS 23.401 [4] or for P-CSCF restoration as defined in TS 23.380 [8].

In this procedure, the network, after initiating the release of the PDN connection over one access, also initiates the release of the PDN connection over the other access thus avoiding the dependence on the UE-initiated PDN connection disconnect procedure.

The network-initiated release of a NBIFOM PDN connection is further described in clause 6.8.

5.9 Void

5.10 Functional entities

5.10.1 UE

To support NBIFOM the UE needs to be capable to:

– Indicate support and request for NBIFOM for a PDN connection as well as to request NBIFOM mode to the network.

– Send a default access for NBIFOM to the network in UE-initiated NBIFOM mode.

– Receive from the network decisions about whether NBIFOM applies to a PDN connection, about the NBIFOM mode and about the default access.

– Provide handover indication and NBIFOM indication when an access is added for a PDN connection.

– Exchange routing rules with the network over control plane protocols.

– Route IP flow(s) by using the default access for NBIFOM and routing access information which is part of the routing rules.

– Notify the network that an access becomes "usable" or "unusable" in Network-initiated NBIFOM mode.

5.10.2 PDN GW

To support NBIFOM the PDN GW needs to be capable to:

– Negotiate the support of NBIFOM and confirm the NBIFOM mode when the UE requests PDN connectivity over the first access. This negotiation includes receiving UE request for NBIFOM as well as receiving indication of support from entities in the network (MME, ePDG, etc.)

– Negotiate the support of NBIFOM and of the default access for NBIFOM when the UE requests PDN connectivity over an additional access. This negotiation includes receiving UE request for NBIFOM as well as indication of support from entities in the network (MME, ePDG, etc.)

– Support a PDN connection using multiple accesses.

– Provide the Charging Id for the PDN Connection to the S-GW/TWAN/ePDG in a separate information element.

– Receive UE request on a default access for NBIFOM in UE-initiated NBIFOM mode.

– Route the IP flow(s) by using the default access and the routing access information which is part of the routing rules.

– Exchange routing rules with the UE over control plane protocols.

– Receive UE notification that an access has become "usable" or "unusable" in Network-initiated NBIFOM mode.

– Support PCC interactions described in clause 5.5 and TS 23.203 [3] when PCC is enabled for the PDN connection.

5.10.3 MME

To support NBIFOM, the MME needs to be capable to:

– Provide an indication to the PDN GW for its support for NBIFOM during the PDN connection establishment over the first access and over an additional access.

– Forward the routing rules between the network (S-GW) and the UE via NAS and S11.

– Forward over S11 notifications received over NAS from the UE that an access has become "usable" or "unusable".

5.10.4 S-GW

To support NBIFOM, the S-GW needs to be capable to:

– Provide an indication to the PDN GW for its support for NBIFOM and relays the support indication from the MME/SGSN during the PDN connection establishment (over the first access and over an additional access).

– Forward the routing rules between the PDN GW and the MME/SGSN via S4/S11 and S5/S8.

– Forward over S5/S8 notifications received over S4/S11 from the SGSN/MME that an access has become "usable" or "unusable".

– Maintain the Charging Id for the PDN Connection as an information element independent from the Charging Id for a bearer.

5.10.5 ePDG

To support NBIFOM, the ePDG needs to be capable to:

– Receive the UE indication to negotiate NBIFOM support and NBIFOM mode in SWu during the initial and additional access and forward it over S2b;

– Receive over S2b the negotiation result of NBIFOM support from the PDN GW and forward it to the UE via SWu signalling;

– Provide an indication to the PDN GW for its support for NBIFOM during the PDN connection establishment (over the first and over an additional access);

– Forward the routing rules between the PDN GW and the UE via SWu and S2b;

– Forward over S2b notifications received over SWu from the UE that an access has become "usable" or "unusable".

– Handle the Charging Id for the PDN Connection as an information element independent from the Charging Id for a bearer.

5.10.6 TWAN

To support NBIFOM, the TWAN needs to be capable to:

– Provide an indication to the PDN GW for its support for NBIFOM, during the PDN connection establishment over the first access and over an additional access,

– Provide indication to the PDN GW whether the PDN connection is being established as SCM or MCM.

– Handle the Charging Id for the PDN Connection as an information element independent from the Charging Id for a bearer.

In the case of MCM, the TWAG:

– Forwards the routing rules between the PDN GW and the UE via WLCP and S2a,

– Forwards over S2a notifications received over WLCP from the UE that an access has become "usable" or "unusable".

5.10.7 SGSN

To support NBIFOM, the SGSN needs to be capable to:

– Provide an indication to the PDN GW for its support for NBIFOM, during the PDN connection establishment over the first access and over an additional access.

– Forward the routing rules between the network (S-GW) and the UE via NAS and S4.

– Forwards over S4 notifications received over NAS from the UE that an access has become "usable" or "unusable".

5.10.8 PCRF

PCC Enhancements are described in clause 5.5 and TS 23.203 [3].