The AKARI Project, one of clean slate approaches to design New Generation Network (NwGN), is proposing a six-layer network architecture called ZNA (Z Network Architecture). One of the features of ZNA is that the session layer (L5) is inserted between the application layer (L6) and the transport layer (L4) to provide the application layer with more sophisticated communication paths. The API and protocols of L5 have been proposed, however the behavior has not been confirmed by implementation. This paper designs in detail the protocols and APIs of L5, and verifies the feasibility by implementing a prototype of L5 in the user space before implementing it in the kernel. In this prototype, two new types of paths can be established: the bundled path and the spatiallyspliced path. In the former, bandwidth aggregation or fault tolerance is supported by using multiple L4 paths. In the latter, end-to-end communication via middle boxes is supported by splicing two L4 paths at the middle boxes after negotiation between the end node and the middle boxes.