The transport layer for data streams over Xn is an IP based Transport. The following Figure shows the transport protocol stacks over Xn.
The GTP-U (TS 29.281
) protocol over UDP over IP shall be supported as the transport for data streams on the Xn 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).
The GTP-U (TS 29.281
) protocol shall be used over the Xn interface between two NG-RAN nodes.
The path protocol used shall be UDP (IETF RFC 768
The UDP port number for GTP-U shall be as defined in TS 29.281
The NG-RAN nodes over the Xn interface shall support fragmentation and assembly of GTP packets at the IP layer.
The NG-RAN nodes shall support IPv6 (IETF RFC 8200
) and/or IPv4 (IETF RFC 791
There may be one or several IP addresses in both NG-RAN nodes. The packet processing function in the source NG-RAN node shall send downstream packets of a given flow to the target NG-RAN node IP address (received in XnAP) associated to the DL transport bearer of that particular flow. The packet processing function in the source NG-RAN node shall send upstream packets of a given flow to the target NG-RAN node IP address (received in XnAP) associated to the UL transport bearer of that particular flow.
The Transport Layer Address signalled in XnAP messages is a bit string of:
32 bits in case of IPv4 address according to IETF RFC 791; or
128 bits in case of IPv6 address according to IETF RFC 8200; or
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 (IETF RFC 2474
) shall be supported. The mapping between traffic categories and DiffServ code points shall be configurable by O&M based on 5G QoS Class 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.