The present document specifies the standards for Signalling Transport to be used across X2 interface. X2 interface is a logical interface between eNBs, or an eNB and an en-gNB. The present document describes how the X2-AP signalling messages are transported over X2.

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 eNB and the en-gNB shall support IPv6 (RFC 2460) and/or IPv4 (RFC 791). The IP layer of X2 only supports point-to-point transmission for delivering X2-AP message. The eNB and the en-gNB shall support the Diffserv Code Point marking as described in RFC 2474.

SCTP (RFC 4960) shall be supported as the transport layer of X2 signalling bearer. The Payload Protocol Identifier (ppid) to be used by SCTP for the application layer protocol X2AP is assigned by IANA in [8]. 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. There shall be only one SCTP association established between one eNB pair. An eNB and an en-gNB shall support a configuration with a single SCTP association per one eNB and en-gNB pair. Configurations with multiple SCTP endpoints per one eNB and en-gNB pair should be supported. When configurations with multiple SCTP associations are supported, en-gNB may request to dynamically add/remove SCTP associations between one eNB and en-gNB pair. Within the set of SCTP associations established between one eNB and en-gNB pair, a single SCTP association shall be employed for X2AP elementary procedures that utilize non-UE-associated signalling with the possibility of fail-over to a new association to enable robustness. An eNB and an en-gNB shall use the Destination Port Number value assigned by IANA [9] to be used for X2AP and this value shall also be used in Source Port Number by all eNBs and en-gNBs within a network. When the en-gNB request to dynamically add additional SCTP association between one eNB and en-gNB pair, the SCTP Destination Port number value may be the one assigned by IANA in [9], or any dynamic port value (RFC 6335). An arbitrary eNB or en-gNB shall be able to initiate the INIT procedure towards another en-gNB or eNB for establishing the SCTP association. Within the SCTP association established between one eNB pair;

• A single pair of stream identifiers shall be reserved for the sole use of X2AP elementary procedures that utilize non UE-associated signalling.
• At least one pair of stream identifiers shall be reserved for the sole use of X2AP elementary procedures that utilize UE-associated signalling. However a few pairs (i.e. more than one) should be reserved.
• A single UE-associated signalling shall use one SCTP stream and the stream should not be changed during the communication of the UE-associated signalling.

Between one eNB and en-gNB pair;

• A single pair of stream identifiers shall be reserved for the sole use of X2AP elementary procedures that utilize non UE-associated signalling.
• At least one pair of stream identifiers shall be reserved for the sole use of X2AP 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 eNB and the en-gNB 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 by the eNB or the en-gNB. The eNB (or en-gNB) can update the UE TNLA binding by sending the first available X2AP message for the UE to the peer en-gNB (or eNB) via a different TNLA, and the peer en-gNB (or eNB) shall update the X2AP UE TNLA binding with the new TNLA.

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 either of the eNBs or an en-gNB, 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.