How Peer OSI Layers Communicate

Communication between OSI layers is both vertical within the OSI layers, and also horizontal between peer layers in another computer (see Figure). This is important to understand, because it affects how data is passed within a computer, as well as between two computers. When information is passed within the OSI model on a computer, each protocol layer adds its own information to the message being sent. This information takes the form of a header added to the beginning of the original message. The sending of a message always goes down the OSI stack, and hence headers are added from the top to the bottom (see Figure). When the message is received by the destination computer, eachlayer removes the header from its peer layer. Thus at each layer headers are removed (stripped ) by the receiving computer after the information in the header has been utilized. Stripped headers are removed in the reverse order in which they were added. That is, the last header added by the sending computer, is the ?rst one stripped off and read by the receiving computer. In summary, the information between the layers is passed along vertically. The information between computers is essentially horizontal, though, because each layer in one computer talks to its respective layer in the other computer.