The headers applied by the various protocols implement the specific functions carried out by those protocols. In addition to communicating horizontally with the same protocol in the other system, the header information will enable each layer to communicate with the layers above and below. For example, the network layer will communicate with the data link layer and transport layer.
The inter layer communication is called as the vertical communication. When a system receives a packet and passes it up through various layers, the data link layer protocol header includes a field which specifies the name of network layer protocol to be used to process the packets. Similarly, the network layer protocol header will specify the name of transport layer to be used to process the packet.
Due to vertical communication, it becomes possible for a computer to supply multiple protocols at each layer simultaneously.