The present document specifies the standards for signalling transport to be used across Xn interface. The Xn interface provides means for interconnecting two NG-RAN nodes. The Xn interface is a logical interface between two nodes of the NG-RAN. The present document describes how the XnAP signalling messages are transported over Xn.

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 NG-RAN node shall support IPv6 (IETF RFC 8200) and/or IPv4 (IETF RFC 791). The IP layer of Xn-C only supports point-to-point transmission for delivering XnAP message. The NG-RAN node shall support the Diffserv Code Point marking as described in IETF RFC 2474.

SCTP (RFC 4960) shall be supported as the transport layer of Xn-C signalling bearer. The Payload Protocol Identifier (ppid) to be used by SCTP for the application layer protocol XnAP and for DTLS over SCTP (RFC 6083) is assigned by IANA in [9]. 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. An NG RAN node shall support a configuration with a single SCTP association per NG RAN node pair. Configurations with multiple SCTP endpoints per the NG RAN node pair should be supported. When configurations with multiple SCTP associations are supported, the NG RAN node may request to dynamically add/remove SCTP associations between the NG RAN node pair. Within the set of SCTP associations established between one NG RAN node pair, a single SCTP association shall be employed for XnAP elementary procedures that utilize non-UE-associated signalling with the possibility of fail-over to a new association to enable robustness. The SCTP Destination Port number value to be used by XnAP is assigned by IANA in [10]. When a NG-RAN node requests to dynamically add additional SCTP associations between the NG RAN node pair, the SCTP Destination Port number value may be the one assigned by IANA in [10], or any dynamic port value (RFC 6335). The NG RAN node configuration update procedure shall be the first XnAP procedure triggered on an additional TNLA of an already setup Xn-C interface instance after the TNL association has become operational, and the peer NG RAN node shall associate the TNLA to the Xn-C interface instance using the included Global NG-RAN Node ID. Between one pair of two NG-RAN nodes:

• a single pair of stream identifiers shall be reserved over an SCTP association for the sole use of XnAP 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 XnAP elementary procedures that utilize UE-associated signallings. However, a few pairs (i.e. more than one) should be reserved.
• For a single UE-associated signalling, the NG RAN nodes 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 unless 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 one NG-RAN node, 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.