OSPF Network Design Solutions, Second Edition

OSI Reference Model Layers and Information Exchange

The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. Control information typically takes one of two forms:

• Headers— Appended to the front of data passed down from upper layers

• Trailers— Appended to the back of data passed down from upper layers

OSI layers are not necessarily required to attach a header or trailer to upper-layer data, but they typically do.

Headers, Trailers, and Data

Headers (and trailers) and data are relative concepts, depending on the layer that is analyzing the information unit at the ...