Computer Networks Introduction

What are the three main networking topic areas?

Communications (signals/bits), Networking (packets/Internet), Distributed Systems (apps like gaming/streaming)

What is a Host (end-system)?

A device that supports applications; examples: smartphones, laptops, desktops

What is a Router?

A network device that relays messages between links; examples: access points, cable/DSL modems

What is a Link (channel)?

A connection between nodes; can be wired or wireless

What is Full-Duplex?

A link that is bi-directional — both sides can send simultaneously

What is Half-Duplex?

A link that is bi-directional but not simultaneously — e.g., wireless

What is Simplex?

A link that is unidirectional — data flows only one way at all times

How do wireless links transmit messages?

Messages are broadcast and received by all nodes within the transmission range

What is a PAN?

Personal Area Network; close-proximity scale; example: Bluetooth

What is a LAN?

Local Area Network; building scale; examples: WiFi, Ethernet

What is a MAN?

Metropolitan Area Network; city/campus scale; examples: Cable, DSL

What is a WAN?

Wide Area Network; country scale; example: Large ISP

What is the Internet?

A planet-scale network of all networks

Why do networks need modularity (protocols and layers)?

To manage complexity and support reuse across diverse network functions

How do protocols communicate?

Horizontally — a protocol at one node talks to the same protocol at a peer node

How do layers communicate?

Vertically — a higher layer protocol uses the services of the lower layer on the same node

What is encapsulation?

The mechanism where each lower layer wraps the higher layer content by adding its own header to create a new message for delivery

What is the order of headers added during encapsulation (bottom to top on wire)?

802.11 → IP → TCP → HTTP (first bit to last bit on the wire)

What is demultiplexing?

The process of directing an incoming message to the correct protocol using demultiplexing keys in headers

What are the three demultiplexing keys used in a typical incoming message?

Ethertype value (Ethernet layer), IP protocol field (IP layer), TCP port number (TCP layer)

What is an advantage of layering?

Information hiding and reuse — the same upper layers (HTTP, TCP, IP) can work over different lower layers (Ethernet or 802.11)

What are disadvantages of layering?

Adds overhead; hides information (e.g., app can't easily tell if it's on wired or wireless)

What are the 7 layers of the OSI model (top to bottom)?

Application, Presentation, Session, Transport, Network, Data Link, Physical

What are the 4 layers of the Internet Reference Model (top to bottom)?

Application, Transport, Internet (IP), Link

What protocols are at the Application layer of the Internet model?

HTTP, RTP, SMTP, DNS

What protocols are at the Transport layer of the Internet model?

TCP, UDP

What protocol is at the Internet (Network) layer?

IP

What protocols are at the Link layer of the Internet model?

3G/4G/5G cellular, Ethernet, DSL, Cable, 802.11

What is the "narrow waist" of the Internet and why?

IP — it supports many different link technologies below and many applications above

What is the unit of data at the Application layer?

Message

What is the unit of data at the Transport layer?

Segment

What is the unit of data at the Network layer?

Packet

What is the unit of data at the Link layer?

Frame

What is the unit of data at the Physical layer?

Bit

What layers does a Repeater/Hub operate at?

Physical layer only

What layers does a Switch/Bridge operate at?

Physical and Link layers

What layers does a Router operate at?

Physical, Link, and Network layers

What layers does a Proxy/Middlebox/Gateway operate at?

All layers (Application through Physical)

What body standardizes Internet protocols (e.g., HTTP, DNS)?

IETF (Internet Engineering Task Force)

What body standardizes communications (e.g., Ethernet, WiFi)?

IEEE

What body standardizes web technologies (e.g., HTML5, CSS)?

W3C

What body standardizes telecom (e.g., ADSL, H.264)?

ITU

What happened in 1969 regarding the Internet?

The first ARPAnet node became operational

What were Cerf and Kahn's key internetworking principles (1974)?

Minimalism/autonomy, best-effort service model, stateless routers, decentralized control

When was TCP/IP deployed and DNS defined?

TCP/IP deployed in 1983; DNS also defined in 1983

Who created HTML and HTTP, and when was the Web commercialized?

Tim Berners-Lee; commercialized in the late 1990s

What major developments occurred from 2005–2015 in Internet history?

~750M hosts, smartphone/tablet growth, broadband expansion, social networks (Facebook), cloud computing (Amazon EC2), provider private networks (Google, Microsoft)