There are many interconnections in the field of networking, as in most technical fields, and it is difficult to find an order of presentation that does not involve endless “forward references” to future chapters; this is true even if – as is done here – a largely bottom-up ordering is followed. I have therefore taken here a different approach: this first chapter is a summary of the essentials – LANs, IP and TCP – across the board, and later chapters expand on the material here.
Local Area Networks, or LANs, are the “physical” networks that provide the connection between machines within, say, a home, school or corporation. LANs are, as the name says, “local”; it is the IP, or Internet Protocol, layer that provides an abstraction for connecting multiple LANs into, well, the Internet. Finally, TCP deals with transport and connections and actually sending user data.
This chapter also contains some important other material. The section on datagram forwarding, central to packet-based switching and routing, is essential. This chapter also discusses packets generally, congestion, and sliding windows, but those topics are revisited in later chapters. Firewalls and network address translation are also covered here and not elsewhere.
- 1.3: Packets
- Packets are modest-sized buffers of data, transmitted as a unit through some shared set of links. Of necessity, packets need to be prefixed with a header containing delivery information. In the common case known as datagram forwarding, the header contains a destination address; headers in networks using so-called virtual-circuit forwarding contain instead an identifier for the connection. Almost all networking today (and for the past 50 years) is packet-based.
- 1.5: Topology
- In the network diagrammed in the previous section, there are no loops; graph theorists might describe this by saying the network graph is acyclic, or is a tree. In a loop-free network there is a unique path between any pair of nodes. The forwarding-table algorithm has only to make sure that every destination appears in the forwarding tables; the issue of choosing between alternative paths does not arise.