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: Dual Connectivity: EN-DC: E-RAB: X2:

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 eNB EN-DC E-RAB E-UTRAN GTP IP MeNB PDCP PDU SCG SeNB TEID UDP

The transport layer for data streams over X2 is an IP based Transport. The following Figure shows the transport protocol stacks over X2.

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).

The GTP-U (TS 29.281) protocol shall be used over the X2 interface between two eNBs.

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 eNBs over the X2 interface shall support fragmentation and assembly of GTP packets at the IP layer. The eNB shall support IPv6 (RFC 2460) and/or IPv4 (RFC 791). 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 RFC 791; or
2. 128 bits in case of IPv6 address according to RFC 2460 ; 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.

IP Differentiated Services code point marking (RFC 2474) 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.

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.

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;