The X2 user plane (X2 UP) protocol is located in the User Plane of the Radio Network layer over the X2 interface.
The X2 UP protocol is used to convey control information related to the user data flow management of E-RABs.
Each X2 UP protocol instance is associated to one E-RAB only.
This version of the present document defines the X2 UP protocol in the context of dual connectivity only, more
specifically, only for X2 user data bearers setup for E-RABs configured with the split bearer option.
If configured, X2 UP protocol instances exist at the eNBs between which the X2 user data bearers are setup, specifically
for dual connectivity between the MeNB and the SeNB.
NOTE: X2 user data bearers may be setup for data forwarding purposes during X2 HO or during DC related
mobility, however, these X2 user data bearers do not require the execution of any additional E-RAB
related UP protocol functions related to an X2 UP protocol instance.
In this version of the present document, X2 UP protocol data is conveyed by GTP-U protocol means, more specifically,
by means of the "RAN Container" GTP-U extension header as defined in TS 29.281