Chapter 1

What is the Internet?

A worldwide "network of networks" consisting of billions of connected computing devices, communication links, packet switches, and individual networks.

Host / End System

A computing device connected to the Internet that runs network applications. Hosts are also called end systems and operate at the Internet's edge.

Network edge

The part of the Internet containing end systems such as clients and servers. Servers are often located in data centers.

Network core

The interconnected routers that form a "network of networks" and move packets between end systems.

Client vs. Server

A client typically requests information or services; a server provides information or services to clients.

Packet

A chunk of data transmitted through a network. Application messages are divided into packets for transmission.

Packet switch

A networking device that forwards packets toward their destination. Routers and switches are packet switches.

Router

A packet switch that receives packets on input links and forwards them onto appropriate output links.

Communication link

A connection used to transmit data between network devices. Examples include fiber, copper, radio, and satellite.

Bandwidth / Transmission rate

The rate at which bits can be transmitted over a communication link, usually measured in bits per second (bps).

Network

A collection of devices, routers, and links managed by an organization.

Protocol

A set of rules governing communication between network entities. It defines the format and order of messages sent and received and the actions taken when messages are transmitted, received, or other events occur.

Why are network protocols necessary?

Devices need agreed-upon communication rules so they know what messages mean, when messages should be sent, and what actions should occur after receiving them.

Access network

The network that connects an end system to its first router, called the edge router.

Main types of access networks

Residential access networks, institutional networks such as schools/companies, and mobile access networks such as WiFi and 4G/5G.

Important characteristics of an access network

Its transmission rate and whether access is shared among users or dedicated to an individual user.

Cable Internet / HFC

Cable Internet commonly uses Hybrid Fiber Coax (HFC), combining fiber and coaxial cable. Homes share the access network to the cable headend.

Is cable Internet symmetric?

No. Cable access is generally asymmetric, meaning downstream speed is higher than upstream speed.

DSL

Digital Subscriber Line. Residential Internet technology that uses an existing telephone line to connect a home to the telephone company's central office.

How does DSL carry voice and Internet simultaneously?

Voice and Internet data use different frequencies on the same telephone line. Data is directed toward the Internet while voice is directed toward the telephone network.

DSLAM

Digital Subscriber Line Access Multiplexer. Equipment at the central office that connects DSL users and directs their Internet data toward the Internet.

Cable vs. DSL access

Cable access is shared among homes connected to a cable headend, while a DSL line between a home and the central office is dedicated.

Home network

A typical home network contains a cable/DSL modem, router, firewall/NAT functionality, Ethernet, WiFi access point, and wired or wireless end devices. Several functions may be combined into one device.

Ethernet

A wired networking technology used in home, enterprise, and data-center networks.

WiFi

A wireless local-area networking technology based on IEEE 802.11 standards.

Wireless access network

A shared wireless network that connects an end system to a router through a base station or access point.

WLAN

Wireless Local Area Network. A wireless network, such as WiFi, generally covering a relatively small geographic area.

Wide-area cellular network

A wireless access network provided by a cellular operator, such as 4G or 5G, covering a much larger geographic area than a WLAN.

Enterprise network

A network used by organizations such as companies or universities, generally combining Ethernet, WiFi, switches, routers, servers, and an Internet connection.

Data center network

A high-bandwidth network connecting hundreds or thousands of servers to each other and to the Internet.

Physical media

The actual medium through which network signals travel. Examples include twisted-pair copper, coaxial cable, fiber-optic cable, and wireless radio.

Twisted-pair cable

A physical medium consisting of two insulated copper wires twisted together. It is commonly used for Ethernet.

Coaxial cable

A physical medium containing two concentric copper conductors. It can carry multiple frequency channels over the same cable.

Fiber-optic cable

A physical medium consisting of glass fiber that carries light pulses representing bits and supports extremely high transmission rates.

Wireless radio

A physical medium in which signals travel through the electromagnetic spectrum without physical wires. Signals may be affected by reflection, obstacles, interference, and noise.

Satellite communication

Wireless communication using satellites. Satellite links can cover large geographic areas but may experience significant propagation delay.

What happens to an application message before transmission?

The host breaks the application message into packets and transmits the packets into the network.

L

Packet length measured in bits.

R

Link transmission rate measured in bits per second (bps).

Forwarding

The local action performed by a router to move an arriving packet from an input link to the appropriate output link.

Routing

The global process of determining the source-to-destination paths packets take through a network.

Forwarding vs. Routing

Forwarding is a local router action that selects the appropriate output link; routing determines the overall path packets take through the network.

Forwarding table

A table used by a router to determine which output link should be used for a packet based on information such as its destination address.

Routing algorithm

An algorithm used to determine source-to-destination paths through a network.

Packet switching

A networking method in which data is divided into packets that dynamically share network resources while traveling through routers.

Circuit switching

A networking method in which end-to-end resources are reserved for communication between a source and destination.

Packet switching vs. Circuit switching

Packet switching dynamically shares resources and works well for bursty data but can experience congestion, delay, and loss. Circuit switching reserves dedicated resources and can provide predictable performance but wastes capacity when reserved resources are idle.

Why is packet switching good for bursty data?

Users do not continuously transmit data, so dynamically sharing resources allows unused capacity to be used by other users.

Advantages of packet switching

Better resource sharing, good scalability for bursty traffic, and no call setup is required.

Disadvantages of packet switching

Heavy traffic can cause congestion, queueing delay, and packet loss due to buffer overflow.

Store-and-forward

A packet-switching method in which the entire packet must arrive at a router before the router can begin transmitting it onto the next link.

Transmission delay

The time required to push all bits of a packet onto a communication link.

Transmission delay formula

d_trans = L/R, where L is packet length in bits and R is link transmission rate in bits per second.

Four sources of nodal delay

Processing delay, queueing delay, transmission delay, and propagation delay.

Nodal delay formula

d_nodal = d_proc + d_queue + d_trans + d_prop.

Processing delay (d_proc)

Time needed by a node to process a packet, such as checking for bit errors and determining the appropriate output link.

Queueing delay (d_queue)

Time a packet spends waiting in a router's output queue before it can be transmitted. It depends heavily on network congestion.

Transmission delay (d_trans)

Time needed to push an entire packet onto a link. d_trans = L/R.

Propagation delay (d_prop)

Time required for a signal to physically travel across a communication link.

Propagation delay formula

d_prop = d/s, where d is the physical length of the link and s is the signal's propagation speed.

Transmission delay vs. Propagation delay

Transmission delay depends on packet length and transmission rate (L/R), while propagation delay depends on physical distance and propagation speed (d/s).

Does transmission delay depend on distance?

No. Transmission delay depends on packet length L and transmission rate R.

Does propagation delay depend on packet size?

No. Propagation delay depends on distance d and propagation speed s.

Queueing

Waiting that occurs when packets arrive faster than an output link can transmit them.

Router buffer

Memory used by a router to temporarily store packets waiting to be transmitted.

Packet loss

A packet is dropped when a router's buffer is full and there is no available space for the arriving packet.

Why does a router's queue grow?

The queue grows when packets arrive faster than the output link can transmit them.

Traffic intensity

A measure of how heavily a link is being used, calculated as La/R, where a is average packet arrival rate, L is packet length, and R is transmission rate.

Traffic intensity formula

La/R.

What happens as traffic intensity (La/R) changes?

When La/R is near 0, average queueing delay is small. As La/R approaches 1, queueing delay becomes very large. If La/R exceeds 1, work arrives faster than it can be serviced and the model gives an infinite average delay.

Traceroute

A program that measures delays from a source to routers along an end-to-end Internet path toward a destination.

How does traceroute work?

It sends probe packets with increasing time-to-live (TTL) values so they reach successive routers. Routers return responses, allowing the sender to measure delays along the path.

TTL

Time-to-live. A packet field used by traceroute to control how far a probe travels before a router returns a response.

RTT

Round-Trip Time. The time required for information to travel from a sender to a network point/destination and for a response to return.

Throughput

The rate at which bits are successfully sent from a sender to a receiver, measured in bits per unit time.

Instantaneous vs. Average throughput

Instantaneous throughput is the rate at a particular moment; average throughput is the rate measured over a longer period.

Bottleneck link

The link on an end-to-end path that limits the overall end-to-end throughput.

Basic end-to-end throughput formula

For links with rates Rs and Rc, end-to-end throughput is min(Rs, Rc). The slower link becomes the bottleneck.

Throughput with a shared backbone

If 10 connections fairly share a backbone of rate R, per-connection throughput is min(Rc, Rs, R/10).

Layer

A level in a networking architecture that provides a particular service while relying on services provided by the layer below it.

Why is networking organized into layers?

Layering gives a complex system an explicit structure and provides modularity, making it easier to understand, maintain, and update.

Modularity in networking

A layer's internal implementation can change without affecting the rest of the system as long as the service it provides remains consistent.

Five layers of the Internet protocol stack

Application → Transport → Network → Link → Physical.

Application layer

Supports network applications. Protocol examples include HTTP, IMAP, SMTP, and DNS.

Transport layer

Provides process-to-process data transfer. Protocol examples include TCP and UDP.

Network layer

Routes datagrams from a source host to a destination host. Includes IP and routing protocols.

Link layer

Provides data transfer between neighboring network elements. Examples include Ethernet, 802.11 WiFi, and PPP.

Physical layer

Responsible for transmitting individual bits over the physical medium—bits "on the wire."

Encapsulation

The process in which each networking layer adds its own header to data received from the layer above before passing it downward.

Data units of the protocol stack

Application = message; Transport = segment; Network = datagram; Link = frame.

Transport-layer encapsulation

The transport layer adds header Ht to application message M, producing the segment [Ht | M].

Network-layer encapsulation

The network layer adds header Hn to the transport segment, producing the datagram [Hn | Ht | M].

Link-layer encapsulation

The link layer adds header Hl to the network datagram, producing the frame [Hl | Hn | Ht | M].

Complete encapsulation order

Message M → Segment [Ht | M] → Datagram [Hn | Ht | M] → Frame [Hl | Hn | Ht | M] → bits transmitted by the physical layer.

How do layers depend on one another?

The application layer uses transport services; transport uses network services; network uses link services; and the link layer ultimately relies on the physical layer for transmission.

1961–1972: Early packet-switching era

Kleinrock demonstrated the effectiveness of packet switching through queueing theory; Baran worked on packet switching for military networks; ARPAnet was conceived in 1967; its first node became operational in 1969; and ARPAnet was publicly demonstrated in 1972.

1972–1980: Internetworking era

ALOHAnet appeared, Cerf and Kahn developed an architecture for interconnecting networks, Ethernet was developed at Xerox PARC, and multiple proprietary network architectures emerged.

Cerf and Kahn's internetworking principles

Minimalism/autonomy, best-effort service, stateless routing, and decentralized control. These principles helped define today's Internet architecture.

1980–1990: New Internet protocols

TCP/IP was deployed in 1983; SMTP, DNS, and FTP were defined during this period; TCP congestion control appeared in 1988; and the Internet expanded rapidly.

1990s–2000s: Web and commercialization

ARPAnet was decommissioned, restrictions on commercial use of NSFnet were lifted, the Web emerged with HTML and HTTP, browsers such as Mosaic appeared, and the Web became commercialized.