38.4723GPPF1 signalling transportNG-RANRelease 16TS
SCTP (IETF RFC 4960 ) shall be supported as the transport layer of F1-C signalling bearer. The Payload Protocol Identifier (ppid) assigned by IANA to be used by SCTP for the application layer protocol F1AP is 62, and 68 for DTLS over SCTP (IETF RFC 6083 ). 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 assigned by IANA to be used for F1AP is 38472. 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 38472, or any dynamic port value (IETF 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 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 IETF RFC 4960  in sub clause 5.2.
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.
Annex A (informative):
Incorporated agreed TPs from R3 NR#2 Adhoc
Incorporated Rapporteur’s cleanup
Incorporated agreed TPs from R3#97
Incorporated agreed TPs from R3#97b
Incorporated agreed TPs from R3#98
Submitted for approval to RAN
TR approved by RAN plenary
Clarifications on multiple TNL associations
NR Corrections (38.472 Baseline CR covering RAN3-101 agreements)
Multiple TNLA over F1 transport
Removal of Multiple TNLA(s)
Addition of PPID for DTLS over SCTP for 38.472
Use of SCTP ports for multiple TNLA
Ambiguity with multiple SCTP associations in 38.472
Rapporteur’s Update for 38.472
SCTP association change when current SCTP association is failed