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TS 36.424
E-UTRAN – X2 Interface
Data Transport

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Rapporteur:
Dr. Reininger, Philippe

X2 signalling transport in X2 User Plane protocol stack

Content for  TS 36.424  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 X2 interface.

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 2460  (1998-12): "Internet Protocol, Version 6 (IPv6) Specification".
[6]
RFC 791  (1981-09): "Internet Protocol".
[7]
TS 36.401: "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Architecture Description".
[8]
TS 36.300: "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2".
[9]
TS 36.425: "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 interface user plane protocol".
[10]
TS 37.340: "NR; Multi-connectivity; Overall description; Stage-2".
[11]
TS 38.425: "NG-RAN; NR user plane protocol".
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3  Definitions, symbols and abbreviations

3.1  Definitions

For the purposes of the present document, the following terms and definitions below apply. Terms and definitions not defined below can be found in TR 21.905.
Corresponding E-UTRAN node:
Used in this specification according to the definition of the corresponding node in TS 38.425.
Dual Connectivity:
Defined in TS 36.300.
EN-DC:
Defined in TS 37.340.
E-RAB:
Defined in TS 36.401.
X2:
logical interface between two eNBs. Whilst logically representing a point to point link between eNBs, the physical realisation need not be a point to point link.

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.
DC
Dual Connectivity
eNB
E-UTRAN Node B
EN-DC
E-UTRA-NR Dual Connectivity
E-RAB
E-UTRAN Radio Access Bearer
E-UTRAN
Evolved UTRAN
GTP
GPRS Tunnelling Protocol
IP
Internet Protocol
MeNB
Master eNB
PDCP
Packet Data Convergence Protocol
PDU
Protocol Data Unit
SCG
Secondary Cell Group
SeNB
Secondary eNB
TEID
Tunnel Endpoint Identifier
UDP
User Datagram Protocol
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4  Data link layer

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

5  X2 interface user plane protocol

5.1  General

The transport layer for data streams over X2 is an IP based Transport. The following figure shows the transport protocol stacks over X2.
(not reproduced yet)
Figure 5.1: Transport network layer for data streams over X2
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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 X2 interface. The data link layer is as specified in clause 4.
There may be zero or one UL data stream and zero or one DL data stream per E-RAB at the X2 interface.
  • The DL data stream is used for DL data forwarding from the source eNB to the target eNB.
  • The UL data stream is used for UL data forwarding from the source eNB to the target eNB.
Each data stream is carried on a dedicated transport bearer.
The identity of a transport bearer signalled in the RNL control plane consists of the IP address and the TEID of the corresponding GTP tunnel, allocated by the target eNB (see TS 29.281).
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5.2  GTP-UWord‑p. 7
The GTP-U (TS 29.281) protocol shall be used over the X2 interface between two eNBs.

5.3  UDP/IP

The path protocol used shall be UDP (IETF RFC 768 [3]).
The UDP port number for GTP-U shall be as defined in TS 29.281.
The eNBs over the X2 interface shall support fragmentation and assembly of GTP packets at the IP layer.
The eNB shall support IPv6 (IETF RFC 2460 [5]) and/or IPv4 (IETF RFC 791 [6]).
There may be one or several IP addresses in the both eNBs. The packet processing function in the source eNB shall send downstream packets of a given E-RAB to the target eNB IP address (received in X2AP) associated to the DL transport bearer of that particular E-RAB. The packet processing function in the source eNB shall send upstream packets of a given E-RAB to the target eNB IP address (received in X2AP) associated to the UL transport bearer of that particular E-RAB.
The Transport Layer Address signalled in X2AP messages is a bit string of
  1. 32 bits in case of IPv4 address according to IETF RFC 791 [6]; or
  2. 128 bits in case of IPv6 address according to IETF RFC 2460 [5] ; 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 marking

IP Differentiated Services code point marking (IETF RFC 2474 [4]) shall be supported. The mapping between traffic categories and Diffserv code points shall be configurable by O&M based on QoS Class Identifier (QCI)/ Label Characteristics and other E-UTRAN traffic parameters (e.g. ARP). Traffic categories are implementation-specific and may be determined from the application parameters.

5.5  Dual Connectivity

For the SCG bearer option, user data forwarding may be performed. The behaviour of the eNB from which user data is forwarded is the same as specified for the "source eNB", the behaviour of the eNB to which user data is forwarded is the same as specified for the "target eNB".
For the split bearer option:
  • the GTP-U (TS 29.281) protocol over UDP over IP shall be supported as the transport for the data stream of PDCP PDUs on the X2 interface. The GTP-U PDU may include a RAN Container with flow control information as specified in TS 36.425 which is carried in the GTP-U extension header. The transport bearer is identified by the GTP-U TEID (TS 29.281) and the IP address of the MeNB and SeNB respectively. There may be zero or one UL data stream and there is one DL data stream per E-RAB at the X2 interface;
    • The DL data stream is used for DL data transmission from the MeNB to the SeNB;
    • The UL data stream is used for UL data transmission from the SeNB to the MeNB;
  • the packet processing function in the MeNB shall send downstream packets of a given E-RAB to the SeNB IP address (received in X2AP) associated to the DL transport bearer of that particular E-RAB. The packet processing function in the SeNB shall send upstream packets of a given E-RAB to the MeNB IP address (received in X2AP) associated to the UL transport bearer of that particular E-RAB;
  • data forwarding may be performed by MeNB providing GTP-U TEID to receive the DL data forwarded by the SeNB.
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5.6  E-UTRA-NR Dual ConnectivityWord‑p. 8
User data forwarding may be performed for each E-RAB configured for EN-DC, towards or from the node hosting the PDCP entity. The behaviour of the E-UTRAN node from which user data is forwarded is the same as specified for the "source eNB", the behaviour of the E-UTRAN node to which user data is forwarded is the same as specified for the "target eNB".
If X2-U data bearer resources are allocated for EN-DC:
  • the GTP-U (TS 29.281) protocol over UDP over IP shall be supported as the transport for the data stream of PDCP PDUs on the X2 interface. The GTP-U PDU may include an NR RAN Container with flow control information as specified in TS 38.425 which is carried in the GTP-U extension header. The transport bearer is identified by the GTP-U TEID (TS 29.281) and the IP address of the E-UTRAN nodes involved in EN-DC;
  • the packet processing function in the E-UTRAN node hosting the PDCP entity shall send downstream packets of a given E-RAB to the IP address indicated by the corresponding E-UTRAN node in X2AP associated to the DL transport bearer of that particular E-RAB. The packet processing function in the corresponding E-UTRAN node shall send upstream packets of a given E-RAB to the IP address indicated by the E-UTRAN node hosting the PDCP entity in X2AP associated to the UL transport bearer of that particular E-RAB;
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$  Change HistoryWord‑p. 9

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