The document provides an overview of the transport layer (L4) of the MIPI UniPro protocol. It discusses the functionality of the transport layer, including segmentation and reassembly, addressing, connections, error handling, and flow control. It also describes the structure of transport protocol data units and some of the protocol's features, such as address translation, segmentation, reassembly, and explicit flow control. The implementation of the transport layer in Arrow's layered building block framework is also briefly mentioned.