TCP/IP Guide

Transport Layer (Layer 4)

The fourth layer of the OSI Reference Model protocol stack is the transport layer, also called the middle layer. The transport layer is in some ways part of both the lower and upper groups of layers in the OSI model. It is more often associated with the lower layers, because it concerns itself with the transport of data, but its functions are also somewhat high level, resulting in its having a fair bit in common with layers 5 through 7 as well.

You'll recall that layers 1 through 3 are concerned with the actual packaging, addressing, routing, and delivery of data. The physical layer handles the bits, the data link layer deals with local networks, and the network layer handles routing between networks. The transport layer, ...