As repeatedly emphasized in computer networks, communication between two remote hosts may involve sending network messages, which are enclosed in packets exchange. In general, the packet transmitted over the network regardless of protocol used and the type of network (Token Ring, Ethernet, X.25, etc.) consists of a packet header and data fields. In the packet header is usually entered the service information, depending on the software protocol and required to address the package, its identification, transformation, etc.
In the data field are placed either directly to the data, or other package of higher-level OSI. For example, the packet transport layer can be embedded into the packet network level, which, in turn, is embedded in the packet data link layer. Sproe-to classify this statement to the network operating system that use the TCP / IP, it can be argued that the package of TCP (transport layer) is embedded in the package IP (network layer), which, in turn, is embedded in the package Ethernet (link layer).