5 Procedures

38.3403GPPBackhaul Adaptation Protocol (BAP) specificationNRRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.1 BAP entity handling

5.1.1 BAP entity establishment

When upper layers request establishment of a BAP entity, the node shall:

– establish a BAP entity;

– follow the procedures in clause 5.

5.1.2 BAP entity release

When upper layers request release of a BAP entity, the node shall:

– release the BAP entity and the related BAP configurations.

5.2 Data transfer

5.2.1 Transmitting operation

5.2.1.1 General

The transmitting part of the BAP entity on the IAB-MT can receive BAP SDUs from upper layers and BAP Data Packets from the receiving part of the BAP entity on the IAB-DU of the same IAB-node, and construct BAP Data PDUs as needed (see clause 4.2.2). The transmitting part of the BAP entity on the IAB-DU can receive BAP Data Packets from the receiving part of the BAP entity on the IAB-MT of the same IAB-node, and construct BAP Data PDUs as needed (see clause 4.2.2). The transmitting part of the BAP entity on the IAB-donor-DU can receive BAP SDUs from upper layers, and construct BAP Data PDUs as needed (see clause 4.2.2).

Upon receiving a BAP SDU from upper layers, the transmitting part of the BAP entity shall:

– select a BAP address and a BAP path identity for this BAP SDU in accordance with clause 5.2.1.2;

– construct a BAP Data PDU by adding a BAP header to the BAP SDU, where the DESTINATION field is set to the selected BAP address and the PATH field is set to the selected BAP path identity, in accordance with clause 6.2.2;

When the BAP entity has a BAP Data PDU to transmit, the transmitting part of the BAP entity shall:

– for the boundary IAB-node, if the BAP Data PDU is received from the collocated BAP entity, perform the BAP header rewriting operation (if configured) in accordance with clause 5.2.1.5;

– perform routing to determine the egress link in accordance with clause 5.2.1.3;

– determine the egress BH RLC channel in accordance with clause 5.2.1.4;

– submit this BAP Data PDU to the selected egress BH RLC channel of the selected egress link.

NOTE: Data buffering on the transmitting part of the BAP entity, e.g., until RLC-AM entity has received an acknowledgement, is up to implementation. In case of BH RLF, the transmitting part of the BAP entity may reroute the BAP Data PDUs, which has not been acknowledged by lower layer before the BH RLF, to an alternative path in accordance with clause 5.2.1.3.

5.2.1.2 BAP routing ID selection

5.2.1.2.1 BAP routing ID selection at IAB-node

At an IAB-node, for a BAP SDU received from upper layers and to be transmitted in upstream direction, the BAP entity performs mapping to a BAP address and BAP path identity based on:

– Uplink Traffic to Routing ID Mapping Configuration, which is derived from F1AP on the IAB-node in TS 38.473 [5],

– defaultUL-BAP-RoutingID, which is configured by RRC on the IAB-node in TS 38.331[3].

Each entry of the Uplink Traffic to Routing ID Mapping Configuration contains:

– a traffic type specifier, which is indicated by UL UP TNL Information IE for F1-U packets and Non-UP Traffic Type IE for non-F1-U packets in TS 38.473 [5],

– a BAP routing ID, which includes a BAP address and a BAP path identity, indicated by BAP Routing ID IE in BH Information IE in TS 38.473 [5], and

– a Topology indicator, indicating whether the BAP routing ID belongs to the non-F1-terminating donor topology, which is optionally indicated by Non-F1-terminating IAB-donor Topology Indicator IE.

At the IAB-node, for a BAP SDU received from upper layers and to be transmitted in upstream direction, the BAP entity shall:

– if the Uplink Traffic to Routing ID Mapping Configuration has not been (re)configured by F1AP after the last (re)configuration of defaultUL-BAP-RoutingID by RRC:

– select the BAP address and the BAP path identity as configured by defaultUL-BAP-RoutingID in TS 38.331 [3] for non-F1-U packets;

– else:

– for the BAP SDU encapsulating an F1-U packet:

– select an entry from the Uplink Traffic to Routing ID Mapping Configuration with its traffic type specifier corresponds to the destination IP address and TEID of this BAP SDU;

– for the BAP SDU encapsulating a non-F1-U packet:

– select an entry from the Uplink Traffic to Routing ID Mapping Configuration with its traffic type specifier corresponds to the traffic type of this BAP SDU;

– select the BAP address and the BAP path identity from the BAP routing ID in the entry selected above;

– if the selected entry is configured with Non-F1-terminating IAB-donor Topology Indicator IE:

– consider this BAP Data PDU as data to be routed to non-F1-terminating donor topology.

NOTE: Uplink Traffic to Routing ID Mapping Configuration may contain multiple entries for F1-C traffic. It is up to IAB node’s implementation to decide which entry is selected.

5.2.1.2.2 BAP routing ID selection at IAB-donor-DU

For a BAP SDU received from upper layer at the IAB-donor-DU, the BAP entity performs mapping to a BAP address and a BAP Path identity based on:

– Downlink Traffic to Routing ID Mapping Configuration, which is derived from IP-to-layer-2 traffic mapping Information List IE configured on the IAB-donor-DU in TS 38.473 [5].

Each entry of the Downlink Traffic to Routing ID Mapping Configuration contains:

– a destination IP address, which is indicated by Destination IAB TNL Address IE in IP header information IE, including an IPv4 address or IPv6 address or an IPv6 address prefix,

– an IPv6 flow label, if configured, which is indicated by IPv6 Flow Label IE in IP header information IE,

– a DSCP, if configured, which is indicated by DSCP IE in DS Information List IE in IP header information IE, and

– a BAP routing ID, which is indicated by BAP Routing ID IE in BH Information IE in TS 38.473 [5].

At the IAB-donor-DU, for a BAP SDU received from upper layers and to be transmitted in downstream direction, the BAP entity shall:

– for the BAP SDU encapsulating an IPv6 packet:

– select an entry from the Downlink Traffic to Routing ID Mapping Configuration which fulfils the following conditions:

– the Destination IP address of this BAP SDU matches the destination IP address in this entry; and

– the IPv6 Flow Label of this BAP SDU matches IPv6 flow label in this entry if configured; and

– the DSCP of this BAP SDU matches DSCP in this entry if configured;

– for the BAP SDU encapsulating an IPv4 packet:

– select an entry from the Downlink Traffic to Routing ID Mapping Configuration which fulfils the following conditions:

– the Destination IP address of this BAP SDU matches the destination IP address in this entry; and

– the DSCP of this BAP SDU matches DSCP in this entry if configured;

– select the BAP address and the BAP path identity from the BAP routing ID in the entry selected above;

5.2.1.3 Routing

The BAP entity performs routing based on:

– the BH Routing Configuration derived from an F1AP message as specified in TS 38.473 [5].

Each entry of the BH Routing Configuration contains:

– a BAP Routing ID consisting of a BAP address and a BAP path identity, which is indicated by BAP Routing ID IE,

– a Next Hop BAP Address which is indicated by Next-Hop BAP Address IE, and

– a Topology indicator, indicating whether the entry belongs to the non-F1-terminating donor topology, which is optionally indicated by Non-F1-terminating IAB-donor Topology Indicator IE.

In the BH Routing Configuration, the entry configured with Non-F1-terminating IAB-donor Topology Indicator IE applies to the BAP Data PDU considered as data to be routed to non-F1-terminating donor topology, and the entry not configured with Non-F1-terminating IAB-donor Topology Indicator IE only applies to the BAP Data PDU not considered as data to be routed to non-F1-terminating donor topology.

For a BAP Data PDU to be transmitted, BAP entity shall:

– if the BAP Data PDU corresponds to a BAP SDU received from the upper layer, and

– if the BH Routing Configuration has not been (re)configured by F1AP after the last (re)configuration of defaultUL-BH-RLC-Channel by RRC:

– select the egress link on which the egress BH RLC channel corresponding to defaultUL-BH-RLC-Channel is configured as specified in TS 38.331 [3] for non-F1-U packets;

– else if there is an entry in the BH Routing Configuration whose BAP address matches the DESTINATION field, whose BAP path identity is the same as the PATH field, and whose egress link corresponding to the Next Hop BAP Address is available:

– select the egress link corresponding to the Next Hop BAP Address of the entry;

NOTE 1: An egress link is not considered to be available if the link is in BH RLF.

NOTE 2: For each combination of a BAP address and a BAP path identity, there should be at most one entry with the same Topology indicator in the BH Routing Configuration. There could be multiple entries of the same BAP address with the same Topology indicator in the BH Routing Configuration.

NOTE 3: An egress link may not be considered available for a BH Routing Configuration entry with a given BAP routing ID, if it is determined as congested, e.g. based on the received flow control feedback, as defined in clause 5.3.1.

– else if there is at least one entry in the BH Routing Configuration whose BAP address matches the DESTINATION field, and whose egress link corresponding to the Next Hop BAP Address is available:

– select an entry from the BH Routing Configuration whose BAP address is the same as the DESTINATION field, and whose egress link corresponding to the Next Hop BAP Address is available;

– select the egress link corresponding to the Next Hop BAP Address of the entry selected above;

– else if, for the transmitting part of IAB-MT, at least one egress link is available, and if Re-routing Enable Indicator IE is set to true by F1AP:

– if this egress link belongs to F1-terminating donor topology of the boundary IAB-node or the transmitting part of IAB-MT is not at the boundary IAB-node, and there is an entry in the BH Routing Configuration not configured with Non-F1-terminating IAB-donor Topology Indicator IE whose Next Hop BAP Address corresponds to this egress link, or

– if this egress link belongs to non-F1-terminating donor topology, and there is an entry in the BH Routing Configuration configured with Non-F1-terminating IAB-donor Topology Indicator IE whose Next Hop BAP Address corresponds to this egress link:

– select the egress link;

– rewrite the BAP header of this BAP Data PDU, where the DESTINATION field is set to the leftmost 10 bits of BAP Routing ID of the entry in the BH Routing Configuration (i.e. BAP address), and the PATH field is set to the rightmost 10 bits of BAP Routing ID of the entry (i.e. BAP path identity).

5.2.1.4 Mapping to BH RLC Channel

5.2.1.4.1 Mapping to BH RLC Channel for BAP Data Packets from collocated BAP entity at IAB-node

For a BAP Data Packet received from the collocated BAP entity, the transmitting part of the BAP entity performs mapping to an egress BH RLC channel based on:

– BH RLC Channel Mapping Configuration, which is derived from BAP layer BH RLC channel mapping Information List IE, and optionally together with the Configured BAP address IE and the BH RLC Channel to be Setup/Modified List IE, as configured on the IAB-node in TS 38.473 [5],

Each entry of the BH RLC Channel Mapping Configuration contains:

– an ingress link ID, which is indicated by Prior-Hop BAP Address IE, or by the Configured BAP address IE in UE-associated F1AP message for upstream, belonging to topology indicated by Ingress Non-F1-terminating IAB-donor Topology Indicator IE in F1AP if configured,

– an egress link ID, which is indicated by Next-Hop BAP Address IE, or by the Configured BAP address IE in UE-associated F1AP message for downstream, belonging to topology indicated by Egress Non-F1-terminating IAB-donor Topology Indicator IE in F1AP if configured,

– an ingress BH RLC channel ID, which is indicated by Ingress BH RLC CH ID IE, or by the BH RLC CH ID IE in UE-associated F1AP message for upstream, and,

– an egress BH RLC channel ID, which is indicated by Egress BH RLC CH ID IE, or by the BH RLC CH ID IE in UE-associated F1AP message for downstream.

For a BAP Data PDU received from an ingress BH RLC channel of an ingress link and for which the egress link has been selected as specified in clause 5.2.1.3:

– if there is an entry in the BH RLC Channel Mapping Configuration, whose ingress BH RLC channel ID matches the BAP Data PDU’s ingress BH RLC channel, whose ingress link ID matches the BAP Data PDU’s ingress link, and whose egress link ID corresponds to the selected egress link;

– select the egress BH RLC channel corresponding to egress BH RLC channel ID of this entry;

– else:

– select any egress BH RLC channel on the selected egress link;

5.2.1.4.2 Mapping to BH RLC Channel for BAP SDUs from upper layers at IAB-node

For a BAP SDU received from upper layers at the IAB-node, the BAP entity performs mapping to an egress BH RLC channel based on:

– Uplink Traffic to BH RLC Channel Mapping Configuration, which is derived from F1AP message, configured on the IAB-node in TS 38.473 [5],

– defaultUL-BH-RLC-Channel, which is configured by RRC on the IAB-node in TS 38.331[3].

Each entry of the Uplink Traffic to BH RLC Channel Mapping Configuration contains:

– a traffic type specifier, which is indicated by UL UP TNL Information IE for F1-U packets or Non-UP Traffic Type IE for non-F1-U packets in TS 38.473 [5],

– an egress link ID, which is indicated by Next-Hop BAP address IE in BH Information IE in TS 38.473 [5], belonging to topology indicated by Egress Non-F1-terminating IAB-donor Topology Indicator IE in F1AP if configured, and

– an egress BH RLC channel ID, which is indicated by BH RLC CH ID IE in BH Information IE in TS 38.473 [5].

For a BAP SDU received from upper layers at the IAB-node and to be transmitted in upstream direction, whose egress link has been selected as specified in clause 5.2.1.3, the BAP entity shall:

– if the Uplink Traffic to BH RLC Channel Mapping Configuration has not been (re)configured by F1AP after the last (re)configuration of defaultUL-BH-RLC-Channel by RRC:

– select the egress BH RLC channel corresponding to defaultUL-BH-RLC-Channel configured in TS 38.331 [3] for non-F1-U packets;

– else:

– for the BAP SDU encapsulating an F1-U packet:

– if there is an entry in the Uplink Traffic to BH RLC Channel Mapping Configuration with its traffic type specifier corresponds to the destination IP address and TEID of this BAP SDU and its egress link ID corresponding to the selected egress link;

– select the egress BH RLC channel corresponding to the egress BH RLC channel ID of this entry;

– else:

– select any egress BH RLC channel on the selected egress link;

– for the BAP SDU encapsulating a non-F1-U packet:

– if there is an entry from the Uplink Traffic to BH RLC Channel Mapping Configuration with its traffic type specifier corresponds to the traffic type of this BAP SDU and its egress link ID corresponding to the selected egress link;

– select the egress BH RLC channel corresponding to the egress BH RLC channel ID of this entry;

– else:

– select any egress BH RLC channel on the selected egress link;

NOTE: Uplink Traffic to BH RLC Channel Mapping Configuration may contain multiple entries for F1-C traffic. It is up to IAB node’s implementation to decide which entry is selected, but the selected entry has to match the BAP routing ID selected in 5.2.1.2.1, i.e. BAP routing ID and BH RLC channel must be derived from the same BH Information IE.

5.2.1.4.3 Mapping to BH RLC Channel at IAB-donor-DU

For a BAP SDU received from upper layers at the IAB-donor-DU, the BAP entity performs mapping to an egress BH RLC channel based on:

– Downlink Traffic to BH RLC Channel Mapping Configuration, which is derived from IP-to-layer-2 traffic mapping Information List IE, and optionally together with the Configured BAP address IE and the BH RLC Channel to be Setup/Modified List IE, as configured on the IAB-donor-DU in TS 38.473 [5].

Each entry of the Downlink Traffic to BH RLC Channel Mapping Configuration contains:

– a destination IP address, which is indicated by Destination IAB TNL Address IE in IP header information IE including an IPv4 address or IPv6 address or an IPv6 address prefix,

– an IPv6 flow label, if configured, which is indicated by IPv6 Flow Label IE in IP header information IE,

– a DSCP, if configured, which is indicated by DSCP IE in DS Information List IE in IP header information IE,

– an egress link ID, which is indicated by Next-Hop BAP Address IE in BH Information IE, or by the Configured BAP address IE in UE-associated F1AP message, and

– an egress BH RLC channel ID, which is indicated by Egress BH RLC CH ID IE in BH Information IE, or by the BH RLC CH ID IE in UE-associated F1AP message.

At the IAB-donor-DU, for a BAP SDU received from upper layers and to be transmitted in downstream direction, whose egress link has been selected as specified in clause 5.2.1.3, the BAP entity shall:

– for the BAP SDU encapsulating an IPv6 packet:

– if there is an entry in the Downlink Traffic to BH RLC Channel Mapping Configuration with its egress link ID corresponding to the selected egress link, and the entry fulfils the following conditions:

– the Destination IP address of this BAP SDU matches the destination IP address in this entry; and

– the IPv6 Flow Label of this BAP SDU matches IPv6 flow label in this entry if configured; and

– the DSCP of this BAP SDU matches DSCP in this entry if configured:

– select the egress BH RLC channel corresponding to egress BH RLC channel ID of this entry;

– else:

– select any egress BH RLC channel on the selected egress link;

– for the BAP SDU encapsulating an IPv4 packet:

– the Destination IP address of this BAP SDU matches the destination IP address in this entry; and

– the DSCP of this BAP SDU matches DSCP in this entry if configured:

– select the egress BH RLC channel corresponding to egress BH RLC channel ID of this entry;

– else:

– select any egress BH RLC channel on the selected egress link;

5.2.1.5 BAP header rewriting operation

The BAP entity performs BAP header rewriting based on:

– the Header Rewriting Configuration derived from an F1AP message as specified in TS 38.473 [5].

Each entry of the Header Rewriting Configuration contains:

– an Ingress Routing ID consisting of a BAP address and a BAP path identity of the BAP Data PDU, which is indicated by Ingress BAP Routing ID IE, and

– an Egress Routing ID consisting of a BAP address and a BAP path identity of the BAP Data PDU, which is indicated by Egress BAP Routing ID IE, and

– a Topology indicator, indicating whether the Egress Routing ID belongs to the non-F1-terminating donor topology, which is optionally indicated by Non-F1-terminating IAB-donor Topology Indicator IE.

For each BAP Data PDU to be considered for BAP header rewriting, the BAP entity shall:

– for the IAB-MT of boundary IAB-node, if there is an entry in the Header Rewriting Configuration configured with Non-F1-terminating IAB-donor Topology Indicator IE, whose BAP address of Ingress Routing ID matches the DESTINATION field, and whose BAP path identity of Ingress Routing ID matches the PATH field:

– rewrite the BAP header of this BAP Data PDU, where the DESTINATION field is set to the leftmost 10 bits of Egress Routing ID of the entry (i.e. BAP address), and the PATH field is set to the rightmost 10 bits of Egress Routing ID of the entry (i.e. BAP path identity);

– consider this BAP Data PDU as data to be routed to non-F1-terminating donor topology;

– for the IAB-DU of boundary IAB-node, if the ingress link of this BAP Data PDU belongs to non-F1-terminating donor’s topology of the boundary IAB-node; and

– if there is an entry in the Header Rewriting Configuration not configured with Non-F1-terminating IAB-donor Topology Indicator IE, whose BAP address of Ingress Routing ID matches the DESTINATION field, and whose BAP path identity of Ingress Routing ID matches the PATH field:

NOTE: In this specification, a BH link belongs to the topology of the IAB-donor that provides the configuration of that BH link, as specified in TS 38.331 [3].

5.2.2 Receiving operation

Upon receiving a BAP Data PDU from lower layer (i.e. ingress BH RLC channel), the receiving part of the BAP entity shall:

– if DESTINATION field of this BAP Data PDU matches the BAP address, which is configured for this IAB-node by the IAB-donor providing this ingress BH RLC channel configuration:

– remove the BAP header of this BAP Data PDU and deliver the BAP SDU to upper layers;

– else:

– deliver the BAP Data Packet to the transmitting part of the collocated BAP entity.

5.3 Flow control

5.3.1 Flow control feedback

5.3.1.1 Transmitting operation

For a link, the BAP entity at the IAB-MT shall:

– when a flow control feedback is triggered due to the buffer load exceeding a certain level, or

– when a BAP Control PDU for flow control polling is received at the receiving part, the transmitting part of this BAP entity shall:

– construct a BAP Control PDU for flow control feedback per BH RLC channel, if configured by RRC, in accordance with clause 6.2.3;

– construct a BAP Control PDU for flow control feedback per BAP routing ID, if configured by RRC, in accordance with clause 6.2.3;

– if the egress BH RLC channel for the BAP Control PDU is configured as specified in TS 38.473 [5]:

– submit the BAP Control PDU(s) to the configured egress BH RLC channel of the egress link, indicated by Egress BH RLC CH ID IE in BH Information IE associated with Non-UP Traffic Type IE set to BAP control PDU in TS 38.473 [5];

– else:

– submit the BAP Control PDU(s) to any egress BH RLC channel of the egress link.

NOTE: The BH RLC channel(s) and BAP routing ID(s) to be included in the flow control feedback is up to IAB node implementation, once triggered.

5.3.1.2 Receiving operation

For a link, the BAP entity at the IAB-DU or IAB-donor-DU may:

– if the available buffer size as indicated by the received BAP Control PDU for flow control feedback per BAP routing ID is less than the Buffer Size Threshold IE, configured by F1AP in TS 38.473 [5]:

– consider the BH link as congested for this BAP routing ID (for routing defined in accordance with clause 5.2.1.3).

5.3.2 Flow control polling

When a flow control polling is to be transmitted over an egress link, the transmitting part of the BAP entity at the IAB-DU or IAB-donor-DU:

– construct a BAP Control PDU for flow control polling in accordance with clause 6.2.3:

– if the egress BH RLC channel for the BAP Control PDU is configured as specified in TS 38.473 [5]:

– submit this BAP Control PDU to the configured egress BH RLC channel of the egress link, indicated by BH RLC CH ID IE which is associated with BAP Control PDU Channel IE that is set to true in TS 38.473[5];

– else:

– submit this BAP Control PDU to any egress BH RLC channel of the egress link.

5.4 BH RLF related indications

5.4.1 Transmitting operation

When a BH RLF recovery failure is detected at the IAB-MT, for each egress link associated with the IAB-DU, the transmitting part of the collocated BAP entity at the IAB-DU may:

– construct a BAP Control PDU for BH RLF indication in accordance with clause 6.2.3.3;

When BH RLF(s) occur at the IAB-MT on all the link(s) providing F1 interface over BAP, for each egress link associated with the IAB-DU, the transmitting part of the collocated BAP entity at the IAB-DU may:

– construct a BAP Control PDU for BH RLF detection indication in accordance with clause 6.2.3.4;

When BH RLF recovery is successful at the IAB-MT, for each egress link associated with the IAB-DU on which a BAP Control PDU for BH RLF detection indication was transmitted, the transmitting part of the collocated BAP entity at the IAB-DU may:

– construct a BAP Control PDU for BH RLF recovery indication in accordance with clause 6.2.3.5;

For any constructed BAP Control PDU above, the BAP entity shall:

– if the egress BH RLC channel for the BAP control PDU is configured as specified in TS 38.473 [5]:

– submit this BAP Control PDU to the configured egress BH RLC channel of the egress link, indicated by BH RLC CH ID IE which is associated with BAP Control PDU Channel that is set to true in TS 38.473 [5];

– else:

– submit this BAP Control PDU to any egress BH RLC channel of the egress link.

5.4.2 Receiving operation

Upon receiving a BAP Control PDU for BH RLF indication from lower layer (i.e. ingress BH RLC channel), the receiving part of the BAP entity shall:

– indicate to upper layers that the BH RLF indication has been received for the ingress link where this BAP Control PDU is received.

Upon receiving a BAP Control PDU for BH RLF detection indication from lower layer (i.e. ingress BH RLC channel), the receiving part of the BAP entity shall:

– consider the BH link, from which this BAP Control PDU is received, as not available (for routing defined in accordance with clause 5.2.1.3).

Upon receiving a BAP Control PDU for BH RLF recovery indication from lower layer (i.e. ingress BH RLC channel), the receiving part of the BAP entity shall:

– consider the BH link, from which this BAP Control PDU is received, to be available again (for routing defined in accordance with clause 5.2.1.3).

5.5 Handling of unknown, unforeseen, and erroneous protocol data

When a BAP Data PDU contains a BAP address which is not included in the applied entries of BH Routing Configuration during Routing operation in accordance with clause 5.2.1.3 and the BAP Data PDU contains a BAP address which is not the BAP address of this node in accordance with clause 5.2.2 upon receiving; or

when a BAP Control PDU that contains reserved or invalid values is received the BAP entity shall:

– discard the received BAP PDU.