The present document specifies the standards for Signalling Transport to be used across the F1 interface. The F1 interface provides means for interconnecting a gNB-CU and a gNB-DU of a gNB within an NG-RAN, or for interconnecting a gNB-CU and a gNB-DU of an en-gNB within an E-UTRAN. The present document describes how the F1AP signalling messages are transported over F1.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

The support of any suitable Data Link Layer protocol, e.g. PPP, Ethernet, etc., shall not be prevented.

The gNB-CU and gNB-DU shall support IPv6 (RFC 8200) and/or IPv4 (RFC 791). The IP layer of F1-C only supports point-to-point transmission for delivering F1AP message. The gNB-CU and gNB-DU shall support the Diffserv Code Point marking as described in RFC 2474.

SCTP (RFC 4960) shall be supported as the transport layer of F1-C signalling bearer. The Payload Protocol Identifier (ppid) to be used by SCTP for the application layer protocol F1AP and for DTLS over SCTP (RFC 6083) is assigned by IANA in [11]. The byte order of the ppid shall be big-endian. SCTP refers to the Stream Control Transmission Protocol developed by the Sigtran working group of the IETF for the purpose of transporting various signalling protocols over IP network. The gNB-DU and gNB-CU shall support a configuration with a single SCTP association per gNB-DU/gNB-CU pair. Configurations with multiple SCTP endpoints per gNB-DU/gNB-CU pair should be supported. When configurations with multiple SCTP associations are supported, the gNB-CU/gNB-DU may request to dynamically add/remove SCTP associations between the gNB-DU/gNB-CU pair. The gNB-DU shall establish the SCTP association. The SCTP Destination Port number value to be used by F1AP is assigned by IANA in [12]. When the gNB-CU requests to dynamically add additional SCTP associations between the gNB-DU/gNB-CU pair, the SCTP Destination Port number value may be the one assigned by IANA in [12], or any dynamic port value (RFC 6335). Within the set of SCTP associations established between one gNB-CU and gNB-DU pair, a single SCTP association shall be employed for F1AP elementary procedures that utilize non-UE-associated signalling with the possibility of fail-over to a new association to enable robustness. Selection of the SCTP association by the gNB-DU and the gNB-CU is specified in TS 38.401. When the configuration with multiple SCTP endpoints per gNB-DU is supported and gNB-DU wants to add additional SCTP endpoints, the gNB-DU Configuration Update procedure shall be the first F1AP procedure triggered on an additional TNLA of an already setup F1-C interface instance after the TNL association has become operational, and the gNB-CU shall associate the TNLA to the F1-C interface instance using the included gNB-DU ID. Between one gNB-CU and gNB-DU pair:

• A single pair of stream identifiers shall be reserved over an SCTP association for the sole use of F1AP elementary procedures that utilize non UE-associated signalling.
• At least one pair of stream identifiers over one or several SCTP associations shall be reserved for the sole use of F1AP elementary procedures that utilize UE-associated signalling. However, a few pairs (i.e. more than one) should be reserved.
• For a single UE-associated signalling, the gNB-DU shall use one SCTP association and one SCTP stream, and the SCTP association/stream should not be changed during the communication of the UE-associated signalling until after current SCTP association is failed or removed, or TNL binding update is performed.

Transport network redundancy may be achieved by SCTP multi-homing between two end-points, of which one or both is assigned with multiple IP addresses. SCTP end-points shall support a multi-homed remote SCTP end-point. For SCTP endpoint redundancy an INIT may be sent from a gNB-CU or gNB-DU, at any time for an already established SCTP association, which shall be handled as defined in Section 5.2 of RFC 4960. The SCTP congestion control may, using an implementation specific mechanism, initiate higher layer protocols to reduce the signalling traffic at the source and prioritise certain messages. For MBS-associated signalling, principles specified above for UE-associated signalling shall apply.