Transport Layer — Layer 4

The Transport Layer is the highest layer in the OSI model which is directly involved with data communications. The network service provides routeing and relaying across real subnetworks, possibly involving the use of intermediate open systems; the transport service, in contrast, is concerned only with communication between end open systems and has no interest in the route actually taken by data travelling between those end systems. Its responsibility is to provide session entities with a reliable, cost-effective means of transferring data, protecting them from needing to know anything about the underlying real data communications system. The Transport Layer thus links the layers below, which are responsible for transmitting data across real subnetworks, with the layers above, which are responsible for maintaining an orderly dialogue between application processes in end open systems.