The Illustrated Network- P8:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 1 Protocols and Layers 39 From Application Layer To Application Layer To Network Layer From Network Layer FIGURE The transport layer showing how data are broken up if necessary and reassembled at the destination. In TCP IP it is often said that the network layer IP itself offers an unreliable or best effort service while the transport layer adds reliability in the form of flow and error control. Later in this book we ll see why these terms are unfortunate and what they really mean. The network layer gets a single packet to the right system and the transport layer gets the entire message to the right process. Figure shows the transport layer breaking up a message at the sender into three pieces each labeled TL data for transport-layer data and TH for transport-layer header . The figure then shows the transport layer reassembling the message at the receiver from the various segments that make up a message. In TCP IP there are also data units known as datagrams which are always handled as self-contained units. There are profound differences between how the transport layer treats segments and datagrams but this figure is just a general illustration of segment handling. The functions that the transport layer which in some protocols is called the end-to-end layer might have to include follow Process addressing and multiplexing Also known as service-point addressing the transport layer has to decide which process originated the message and to which process the message must be delivered. These are also known as port addresses in TCP IP. Port addresses are an important portion of the application socket in TCP IP. Segment handling In cases where each message is divided into segments each segment has a sequence number used to put the message back together at the destination. When datagrams are used each data unit is handled independently and sequencing is not necessary. 40 PART I Networking Basics I b h Process on System A Process on System B Internetwork .
