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TS 38.474
NG-RAN — F1 interface –
F1 Data Transport

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V16.0.0 (PDF)  2020/06  10 p.
V15.3.0  2019/09  10 p.
Rapporteur:
Mr. Sirotkin, SashaIntel Corporation (UK) Ltd

Content for  TS 38.474  Word version:  16.0.0

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1  ScopeWord‑p. 5
The present document specifies the standards for user data transport protocols and related signalling protocols to establish user plane transport bearers over 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 interconnection a gNB-CU and a gNB-DU of an en-gNB within an E-UTRAN.

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]  TS 29.281   "General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)".
[3]  RFC 768  (1980-08): "User Datagram Protocol".
[4]  RFC 2474  (1998-12): "Definition of the Differentiated Services Field (DS Field) in the Ipv4 and Ipv6 Headers".
[5]  RFC 8200  (2017-07): "Internet Protocol, Version 6 (Ipv6) Specification".
[6]  RFC 791  (1981-09): "Internet Protocol".
[7]  TS 38.300   "NR; Overall description; Stage-2".
[8]  TS 38.401   "NG-RAN; Architecture Description".
[9]  TS 37.340   "NR; Multi-connectivity; Overall description; Stage-2".
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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-DU and a gNB-CU, providing an interconnection point between the gNB-DU and the gNB-CU.
gNB-CU:
as defined in TS 38.401.
gNB-DU:
as defined in TS 38.401.
gNB:
as defined in TS 38.300.
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3.2  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.
GTP
GPRS Tunnelling Protocol
IAB
Integrated Access and Backhaul
IP
Internet Protocol
TEID
Tunnel Endpoint Identifier
UDP
User Datagram Protocol

4  Data Link Layer

Any data link protocol that fulfils the requirements toward the upper layer may be used.

5  F1 Interface user plane protocol

5.1  General

The transport layer for data streams over F1 is an IP based Transport. The following figure shows the transport protocol stacks over F1.
Reproduction of 3GPP TS 38.474, Figure 5.1: Transport network layer for data streams over F1
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The GTP-U (TS 29.281) protocol over UDP over IP shall be supported as the transport for data streams on the F1 interface. The data link layer is as specified in clause 4.
The transport bearer is identified by the GTP-U TEID (TS 29.281) and the IP address (source TEID, destination TEID, source IP address, destination IP address).
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5.2  GTP-U

The GTP-U (TS 29.281) protocol shall be used over the F1 interface between gNB-DU and gNB-CU.

5.3  UDP/IP

The path protocol used shall be UDP (RFC 768).
The UDP port number for GTP-U shall be as defined in TS 29.281.
The gNB-DU and the gNB-CU shall support fragmentation and assembly of GTP packets at the IP layer.
The gNB-DU and the gNB-CU shall support Ipv6 (RFC 8200) and/or Ipv4 (RFC 791).
There may be one or several IP addresses in the gNB-DU and in the gNB-CU. The packet processing function in the gNB-CU shall send downstream packets of a given bearer to the gNB-DU IP address (received in F1AP) associated to that particular bearer. The packet processing function in the gNB-DU shall send upstream packets of a given bearer to the gNB-CU IP address (received in F1AP) associated to that particular bearer.
The Transport Layer Address signalled in F1AP messages is a bit string of:
  1. 32 bits in case of Ipv4 address according to RFC 791; or
  2. 128 bits in case of Ipv6 address according to RFC 8200; or
  3. 160 bits if both Ipv4 and Ipv6 addresses are signalled, in which case the Ipv4 address is contained in the first 32 bits.
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5.4  Diffserv code point markingWord‑p. 7
IP Differentiated Services code point marking (RFC 2474) shall be supported. Except the downlink F1-U to the IAB node, the mapping between traffic categories and Diffserv code points shall be configurable by O&M based on 5G QoS Identifier (5QI) , the Priority Level (if explicitly signalled), and other NG-RAN traffic parameters (e.g. ARP). Traffic categories are implementation-specific and may be determined from the application parameters. In the IAB system, the Diffserv code points used for the downlink F1-U shall be configurable by the CU-CP.
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A  Change historyWord‑p. 8

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