Computer Network Fundamentals

1Introduction

This lecture explains the basic structure of a computer networkComputer network, in which multiple computers exchange data over communication links. It distinguishes addresses that identify communicating entities, protocols that define communication procedures, and routes along which packets are forwarded. These concepts provide a foundation for the subsequent lectures on TCP, IP, DNS, and HTTP.

2Components and Layers

A network consists of end systemsEnd system that run user programs, linksLink that connect adjacent devices, and routersRouter that relay packets between links. Clients, servers, and smartphones are examples of end systems.

Communication functions are divided into layersLayer according to their roles. Each layer uses functions provided by the layer below and provides functions to the layer above. Layering separates, for example, the communication rules of an application from the rules for forwarding packets through a network.

IPInternet Protocol is a network-layer protocol that forwards packets, while HTTPHypertext Transfer Protocol is an application-layer protocol that defines request and response semantics. They serve here as examples of layering; subsequent lectures explain them in detail.

3Basic Terms

A protocolProtocol is a set of rules that specifies the syntax and semantics of messages exchanged by communicating entities, as well as the order in which those messages are exchanged.

An addressAddress is information that identifies a communicating entity or attachment point at a particular layer. The object identified and the scope of an address depend on the layer. An IP address, for example, identifies a network-layer interface, and its destination prefix is used in a router's forwarding decision.

A packetPacket is a finite unit of data transferred by a network. A packet contains a header carrying control information and data received from an upper layer.

A routeRoute is a sequence of links and routers through which a packet passes from its source to its destination.

4Packet Forwarding

An end system sends data onto a link as packets. A router consults the destination information in a received packet and a forwarding tableForwarding table to select an outgoing interface or next hopNext hop. This operation is called forwardingForwarding. Repeated forwarding at successive routers moves a packet in the direction of its destination.

RoutingRouting is the process that constructs the route information used by routers. Routing constructs route information, whereas forwarding applies that information to select a next hop for each packet.

In a packet-switched network, multiple communications share links. Packets can therefore experience delay, reordering, or loss. The guarantees provided by the network layer or an upper layer depend on the protocols in use.

5Layering and Encapsulation

At the sender, each layer adds control information to data received from the layer above and passes the result to the layer below. This process is called encapsulationEncapsulation. At the receiver, the corresponding layers interpret the control information and pass the data upward.

For example, an HTTP message is passed to the transport layer and is then forwarded through the network in IP packets. Layering allows HTTP to define the semantics of requests and responses while IP handles packet forwarding toward a destination.

6Distinguishing the Roles

• An address identifies a communicating entity or attachment point at a particular layer.
• A protocol defines message formats, semantics, and exchange procedures.
• Routing constructs route information, while forwarding uses that information to select the next hop for each packet.
• Layering separates application semantics from data-transfer mechanisms.

7Key Points

• A computer network consists of end systems, links, routers, and layered protocols.
• A packet is forwarded toward its destination through successive local next-hop decisions by routers.
• Distinguishing addresses, protocols, routing, and forwarding prepares for the division of responsibility between TCP and IP.

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