The first node a: this node is a child node of the migrated relay node, such as the relay node b or c in 
The first node b: the parent node directly connected or indirectly connected to the migrated relay node (indirect connection refers to the parent node connected through one or more nodes), for example, the relay node 1 or 2 referring to 
The third node: this node is the migrated relay node, or the child node of the migrated node, or the parent node of the migrated node. The fourth node is the target receiving node (which may also be called as a destination receiving node) of the user data packet corresponding to the third node. Certainly, it may also be the distributed unit function part of the node or may be the mobile terminal function part of the node. As an example, the third node may be a migrated node.