Which layer of the TCP/IP model is directly responsible for end-to-end communication?

The Transport layer is directly responsible for end-to-end communication in the TCP/IP model. This layer's primary function is to ensure that data is transmitted between two endpoints in a reliable or unreliably fashion, depending on the protocol used. It manages segmentation of data into packets, flow control, and error correction. Protocols such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) operate at this layer; TCP provides reliable communication with error checking and correction, while UDP offers a faster but less reliable service.

In contrast, the other layers serve different roles within the network architecture. The Application layer interacts directly with user applications, providing services for email, file transfer, and web browsing among others. The Internet layer is responsible for the logical addressing and routing of packets across the network, ensuring the data can find its way from source to destination across multiple hops. The Network interface layer deals with the hardware specifics of networking, such as framing and physical transmission of data. Thus, while all these layers are essential to network communication, the Transport layer specifically focuses on ensuring reliable delivery and management of data between hosts, making it the correct choice for this question.