Section: 8.17.3.2 IAB inter-CU topology adaptation procedure with descendant IAB-node
Content: Figure 8.17.3.2-1 shows an example of the topology adaptation procedure where the migrating IAB-MT is migrated from a source IAB-donor-CU to a target IAB-donor-CU, and where the migrating IAB-node has a descendant IAB-node which retains both its RRC connection and F1 connection with the source IAB-donor-CU. Figure 8.17.3.2-1: IAB inter-CU topology adaptation procedure with descendant IAB-node 0. The topology adaptation procedure of clause 8.17.3.1 is performed for the migrating IAB-node. 1. The source IAB-donor-CU sends an IAB TRANSPORT MIGRATION MANAGEMENT REQUEST message to the target IAB-donor-CU to provide the context of the descendant IAB-node’s traffic to be offloaded. The message may include a request for new TNL address(es) for the descendant IAB-node(s), anchored at a target IAB-donor-DU. The source IAB-donor-CU includes an identifier of the migrating IAB-node in the request message. This could be performed in parallel with step 0 after the source IAB-donor-CU receives HANDOVER REQUEST ACKNOWLEDGE message, e.g., the context of the traffic to be offloaded for the migrating/descendant nodes and IP address request information could be contained in the same IAB TRANSPORT MIGRATION MANAGEMENT REQUEST message. 2. The target IAB-donor-CU determines the target IAB-donor-DU, based on the identifier of the migrating IAB-node. The target IAB-donor-CU may configure or modify BH RLC channels and BAP-sublayer routing entries on the target path between the boundary IAB-node and target IAB-donor-DU, as well as DL mappings on the target IAB-donor-DU for the migrating IAB-node’s target path. These configurations may support the transport of UP and non-UP traffic on the target path. 3. The target IAB-donor-CU may obtain new TNL address(es) from the target IAB-donor-DU, based on the request for TNL address(es) received in step 1. 4. The target IAB-donor-CU responds with an IAB TRANSPORT MIGRATION MANAGEMENT RESPONSE message to the source IAB-donor-CU, to provide the mapping information for the traffic to be offloaded. The message includes the L2 info from the target IAB-donor-CU topology that is necessary to configure the migrating IAB-node with the BAP-sublayer routing, header-rewriting and BH RLC CH mapping entries of traffic indicated in step 1. The message includes the DSCP/IPv6 Flow Label values to be used for the DL traffic to be offloaded as indicated in step 1. The message may include the new TNL address(es) obtained in step 3, if any. NOTE 1: The target IAB-donor-CU should select the same IAB-donor-DU in its IAB topology for all to-be-offloaded traffic, whose UL BH mappings received from the source IAB-donor-CU in step 1 share the same BAP address. NOTE 2: The target IAB-donor-CU should provide the same Egress BAP Routing ID for all to-be-offloaded uplink traffic, whose UL BH mappings received from the source IAB-donor-CU in step 1 share the same Ingress BAP Routing ID. 5. The source IAB-donor-CU configures the migrating IAB-node’s IAB-DU with the BAP-sublayer routing, header-rewriting and BH RLC CH mapping entries of the migrating IAB-node. 6. The source IAB-donor-CU sends a DL RRC MESSAGE TRANSFER message to the descendant IAB-node’s parent IAB-DU, which includes an RRCReconfiguration message for the descendant IAB-MT. The RRC configuration may include the new TNL addresses received in step 4. If needed, the source IAB-donor-CU may also provide a new default UL mapping which includes a default BH RLC channel and a default BAP Routing ID on the target path, to the descendant nodes via RRCReconfiguration message. 7. The descendant IAB-node’s parent IAB-DU forwards the received RRCReconfiguration message to the descendant IAB-MT. 8. The descendant IAB-MT responds to the migrating IAB-node’s IAB-DU with an RRCReconfigurationComplete message. 9. The migrating IAB-node’s IAB-DU sends an UL RRC MESSAGE TRANSFER message to the source IAB-donor-CU, to convey the received RRCReconfigurationComplete message. 10. If needed, the source IAB-donor-CU configures UL BH mappings on the descendant node and BAP-sublayer routing entries between the descendant node and the migrating IAB-node. This step may be performed at an earlier stage, e.g., immediately after step 4. 11. The F1-C connections and F1-U tunnels are switched to use the descendant IAB-node’s new TNL address(es), if any, as described in Steps 15 and 19 of the inter-CU topology adaptation procedure in clause 8.17.3.1. NOTE: The IP address selected by the descendant IAB-node for an offloaded traffic needs to be anchored at the IAB-donor-DU whose BAP address is contained in the BAP routing ID of the UL mapping for this traffic, and obtained in step 7. 12. The source IAB-donor-CU sends an IAB TRANSPORT MIGRATION MANAGEMENT REQUEST message to the target IAB-donor-CU, to modify the context of the descendant IAB-node’s offloaded traffic. The message may include the DL TNL address information received in step 11 that is necessary for the target IAB-donor-CU to configure or modify DL mappings on the target IAB-donor-DU. 13. The target IAB-donor-CU responds with an IAB TRANSPORT MIGRATION MANAGEMENT RESPONSE message to the source IAB-donor-CU. 14. The of steps above may be repeated, if needed, for the source IAB-donor-CU to request addition, modification or release of the offloaded traffic pertaining to the descendant IAB-node. The target IAB-donor-CU can fully or partially reject addition or modification requests by the source IAB-donor-CU. The target IAB-donor-CU may trigger the modification of the L2 transport of the offloaded traffic in the target IAB-donor-CU’s topology using the IAB TRANSPORT MIGRATION MODIFICATION REQUEST message. Based on this message, the source IAB-donor-CU may reconfigure the UL BH mappings on the descendant nodes, the routing entries and BH RLC channel mappings on the migrating node and the descendant nodes, and the BAP header rewriting entries on the migrating node, and acknowledges the modification via the IAB TRANSPORT MIGRATION MODIFICATION RESPONSE message. The target IAB-donor-CU may further provide updated TNL address information for the descendant IAB-node to the source IAB-donor-CU. The full or partial release or revoking of traffic offload pertaining to the descendant IAB-nodes and their served UEs follows the same procedure as defined for the partial migration in clause 8.17.3.1.