While it may seem that only one protocol is necessary on a network, multiple protocols are needed in order to provide a variety of clearly defined and managed services. In fact, multiple protocols are usually found within a single packet.
Each protocol has its own purpose and defines its own portion of the control information for the packet. The control information for the highest level protocol is first placed around the data, then the control information for each subsequent protocol in the protocol stack is added to the beginning or end of the packet.