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TS 38.472
NG-RAN — F1 interface –
F1 Signalling Transport

Use "3GPP‑Page" to get the Word version, and "ETSI‑search" to get the PDF version
V16.1.0 (PDF)2020/09  … p.
V15.7.02020/09  … p.
Rapporteur:
Mr. Miller, JamesInterdigital Asia LLC

Content for  TS 38.472  Word version:  16.1.0

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1  ScopeWord‑p. 5
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.

2  References

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.
[1]  TR 21.905   "Vocabulary for 3GPP Specifications".
[2]  RFC 8200  (2017-07): "Internet Protocol, Version 6 (IPv6) Specification".
[3]  RFC 791  (1981-09): "Internet Protocol".
[4]  RFC 2474  (1998-12): "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers".
[5]  RFC 4960  (2007-09): "Stream Control Transmission Protocol".
[6]  TS 38.300   "NR; Overall description; Stage-2".
[7]  TS 38.401   "NG-RAN; Architecture description".
[8]  TS 37.340   "NR; Multi-connectivity; Overall description; Stage-2".
[9]  RFC 6083  (2011-01): "Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)".
[10]  RFC 6335  (2011-08): " Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry".
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3  Definitions and abbreviations

3.1  Definitions

For the purposes of the present document, the terms and definitions given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905.
en-gNB:
as defined in TS 37.340
F1:
interface between a gNB-CU and a gNB-DU, providing an interconnection point between the gNB-CU and the gNB-DU.
F1-C:
Reference point for the control plane protocol between gNB-CU and gNB-DU.
gNB-CU:
as defined in TS 38.401
gNB-DU:
as defined in TS 38.401
gNB:
as defined in TS 38.300
SCTP endpoint:
as defined in IETF RFC 4960 (2007-09) [5]
SCTP association:
as defined in IETF RFC 4960 (2007-09) [5]
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3.3  AbbreviationsWord‑p. 6
For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.
DiffServ
Differentiated Service
IANA
Internet Assigned Number Authority
IP
Internet Protocol
PPP
Point to Point Protocol
SCTP
Stream Control Transmission Protocol

4  F1-C signalling bearer

4.1  Function and protocol stack

The F1-C signalling bearer provides the following functions:
  • Provision of reliable transfer of F1AP messages over the F1-C interface.
  • Provision of networking and routeing function.
  • Provision of redundancy in the signalling network.
  • Support for flow control and congestion control.
The protocol stack for F1-C Signalling Bearer is shown in figure 4.1-1 and details on each protocol are described in the following clauses.
Reproduction of 3GPP TS 38.472, Figure 4.1-1: F1-C signalling bearer protocol stack
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The Transport Network Layer is based on IP transport, comprising SCTP on top of IP.

5  Data link layerWord‑p. 7
The support of any suitable Data Link Layer protocol, e.g. PPP, Ethernet, etc., shall not be prevented.

6  IP layer

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.

7  Transport layer

SCTP (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 (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 (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 RFC 4960 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.
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$  Change HistoryWord‑p. 9

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