Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 29
Data Encapsulation by Layer

IP will add its control information (in the form of headers), specific to the IP layer only, to the data received by the upper layer (transport layer). Once this is accomplished, it will inform the datalink (layer 2) that it has a message to send to the network. At the network layer, encapsulated data is known as a datagram (rumor has it that this term was coined referring to a similar message delivery system known as the telegram). This datagram may be transferred over high–speed networks (Ethernet, Token Ring, FDDI). When the datalink layer adds its headers and trailers it is called a packet (a term referring to a small package). When transmitted onto the cable, the physical layer frames (basically with signaling information such as the preamble for Ethernet or the flag field for Frame Relay and X.25) the information it has received from the datalink layer; therefore, it is called a frame. For most of us, the terms frame and packet are interchangeable. If you want to get into an argument about those terms you need to go find the people who are still arguing about baud and bits per second (bps). For simplicity, considering that the primary focus of the book is network protocols over high–speed networks, packets and frames will be synonymous. Frames will not be mentioned unless the original specification mandated that term. It is important to remember that IP presents datagrams to its lower layer (the datalink layer). When I talk about a datagram, I am specifically talking about the IP layer. When I talk about a packet, I am specifically talking about the access layer (data link and physical).

The IP protocol does not care what kind of data is in the datagram. All it knows is that it must apply some control information, called an IP header, to the data received from the upper–layer protocol (presumably TCP or UDP) and try to deliver it to some station on the network or internet.

Data Encapsulation by Layer

The IP protocol is not completely without merit. It does provide mechanisms on how hosts and routers should process transmitted or received datagrams, or when an error should be generated, and when an IP datagram may be discarded. To understand the IP functionality, a brief look at the control information it adds (the IP header) to the packet will be shown.