The transport layer for data streams over F1 is an IP based Transport. The following figure shows the transport protocol stacks over F1.
The GTP-U (TS 29.281
) protocol over UDP over IP shall be supported as the transport for data streams on the F1 interface. The data link layer is as specified in clause 4
The transport bearer is identified by the GTP-U TEID (TS 29.281
) and the IP address (source TEID, destination TEID, source IP address, destination IP address).
The GTP-U (TS 29.281
) protocol shall be used over the F1 interface between gNB-DU and gNB-CU.
The path protocol used shall be UDP (RFC 768
The UDP port number for GTP-U shall be as defined in TS 29.281
The gNB-DU and the gNB-CU shall support fragmentation and assembly of GTP packets at the IP layer.
The gNB-DU and the gNB-CU shall support Ipv6 (RFC 8200
) and/or Ipv4 (RFC 791
There may be one or several IP addresses in the gNB-DU and in the gNB-CU. The packet processing function in the gNB-CU shall send downstream packets of a given bearer to the gNB-DU IP address (received in F1AP) associated to that particular bearer. The packet processing function in the gNB-DU shall send upstream packets of a given bearer to the gNB-CU IP address (received in F1AP) associated to that particular bearer.
The Transport Layer Address signalled in F1AP messages is a bit string of:
32 bits in case of Ipv4 address according to RFC 791; or
128 bits in case of Ipv6 address according to RFC 8200; or
160 bits if both Ipv4 and Ipv6 addresses are signalled, in which case the Ipv4 address is contained in the first 32 bits.
IP Differentiated Services code point marking (RFC 2474
) shall be supported. Except the downlink F1-U to the IAB node, the mapping between traffic categories and Diffserv code points shall be configurable by O&M based on 5G QoS Identifier (5QI) , the Priority Level (if explicitly signalled), and other NG-RAN traffic parameters (e.g. ARP). Traffic categories are implementation-specific and may be determined from the application parameters. In the IAB system, the Diffserv code points used for the downlink F1-U shall be configurable by the CU-CP.