Transport layer

Layer 4: Transport layer

This layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail.

Although it was not developed under the OSI Reference Model and does not strictly conform to the OSI definition of the Transport Service, the best known example of a layer 4 protocol is the TCP( Transmission Control Protocol). The transport layer is the layer that converts messages into TCP segments or UDP ( User Datagram Protocol) , SCTP (Stream Control Transmission Protocol), etc. packets.

Of the actual OSI protocols, not merely protocols developed under the model, there are five classes of transport protocols, ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (which is also known as TP4 and is designed for less reliable networks, similar to the Internet). Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer.

Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols over an IP network, or end-to-end encryption . While GRE (Generic Routing Encapsulation) might seem to be a network layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint.