Computer Networking: A Top-Down Approach - Chapter 1 Notes

Introduction

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Chapter 1: Introduction

  • Goal: To gain a foundational understanding of networking terminology and concepts.
  • Overview:
    • What is the Internet and its protocols?
    • Network Edge: Hosts, access networks, and physical media.
    • Network Core: Packet/circuit switching, Internet structure.
    • Performance metrics: Loss, delay, and throughput.
    • Protocol layers and service models.
    • Security considerations.
    • Historical context.

Internet: A Nuts and Bolts View

  • The Internet consists of billions of connected computing devices (hosts or end systems) running network applications at the edge.
  • Networks are collections of devices, routers, and links managed by organizations.
  • Packet switches forward data in chunks (packets) using routers and switches.
  • Communication links use fiber, copper, radio, or satellite, with transmission rate measured by bandwidth.

Internet-Connected Devices

  • Examples include web-enabled toasters, Internet phones, security cameras, IP picture frames, refrigerators, and various sensors and monitors.
  • Also includes gaming devices, cars, scooters, bikes, AR devices, and health monitors.

Internet: Network of Networks

  • The Internet is an interconnected network of ISPs.
  • Protocols govern message sending and receiving (e.g., HTTP, streaming video, Skype, TCP, IP, WiFi, Ethernet, 4/5G).
  • Internet standards are documented in RFCs (Request for Comments) and managed by the IETF (Internet Engineering Task Force).

Internet: A Services View

  • The Internet provides services to applications like web browsing, streaming video, teleconferencing, email, games, e-commerce, and social media.
  • It offers a programming interface for distributed applications with “hooks” for sending/receiving apps to use Internet transport services.
  • Service options are provided, similar to postal service options.

What’s a Protocol?

  • Protocols define the format and order of messages exchanged among network entities, as well as actions taken upon message transmission or receipt.
  • They establish rules for specific messages sent and actions triggered by received messages or other events.

Protocol Examples

  • Human protocol: "What's the time?" followed by a response.
  • Computer network protocol: TCP connection requests and responses, HTTP GET requests, etc.

Roadmap

  • Topics to be covered:
    • What is the Internet?
    • What is a protocol?
    • Network edge: hosts, access network, physical media
    • Network core: packet/circuit switching, internet structure
    • Performance: loss, delay, throughput
    • Security
    • Protocol layers, service models
    • History

Internet Structure: Network Edge

  • Network edge includes hosts (clients and servers), with servers often located in data centers.

Internet Structure: Access Networks & Physical Media

  • Access networks and physical media involve wired and wireless communication links.

Internet Structure: Network Core

  • The network core comprises interconnected routers forming a network of networks.

Access Networks and Physical Media

  • Methods for connecting end systems to the edge router include residential access networks, institutional access networks (schools, companies), and mobile access networks (WiFi, 4G/5G).

Access Networks: Cable-Based

  • Cable modem connects homes to the cable headend using a shared cable distribution network.
  • HFC (hybrid fiber coax) provides asymmetric transmission rates (up to 40 Mbps – 1.2 Gbps downstream, 30-100 Mbps upstream).
  • Frequency Division Multiplexing (FDM) transmits different channels in different frequency bands.

Access Networks: DSL

  • DSL uses existing telephone lines to connect homes to the central office DSLAM.
  • Voice and data are transmitted at different frequencies.
  • Dedicated transmission rates of 24-52 Mbps downstream and 3.5-16 Mbps upstream.

Access Networks: Home Networks

  • Home networks use cable or DSL modems, routers, firewalls, and NAT.
  • Wired Ethernet (1 Gbps) and WiFi (54, 450 Mbps) are common.
  • Wireless and wired devices are often combined in a single box.

Wireless Access Networks

  • Shared wireless access networks connect end systems to a router via a base station (access point).
  • Wireless LANs (WLANs) use 802.11b/g/n (WiFi) with transmission rates of 11, 54, 450 Mbps.
  • Wide-area cellular access networks are provided by mobile operators (4G/5G) offering speeds of 10's Mbps.

Access Networks: Enterprise Networks

  • Companies and universities use a mix of wired and wireless technologies, including Ethernet (100Mbps, 1Gbps, 10Gbps) and WiFi (11, 54, 450 Mbps).

Access Networks: Data Center Networks

  • Data center networks use high-bandwidth links (10s to 100s Gbps) to connect thousands of servers together and to the Internet.

Host: Sending Packets of Data

  • The sending host takes an application message, breaks it into packets of L bits, and transmits them into the access network at a transmission rate R.
  • Packet transmission delay is calculated as \frac{L}{R}.

Links: Physical Media

  • Bits propagate between transmitter/receiver pairs over a physical link.
  • Guided media include copper, fiber, and coax cables.
  • Unguided media involve wireless signals.
  • Twisted pair (TP) cables (Category 5, 6) support 100 Mbps to 10Gbps Ethernet.

Links: Coaxial Cable

  • Coaxial cable consists of two concentric copper conductors and supports bidirectional communication with multiple frequency channels (100’s Mbps per channel).

Links: Fiber Optic Cable

  • Fiber optic cables transmit light pulses through glass fiber, enabling high-speed point-to-point transmission (10’s-100’s Gbps).
  • They have low error rates and are immune to electromagnetic noise.

Links: Wireless Radio

  • Wireless radio transmits signals in various bands of the electromagnetic spectrum without physical wires.
  • It uses broadcast and half-duplex communication, subject to reflection, obstruction, and interference.
  • Types include:
    • Wireless LAN (WiFi): 10-100’s Mbps over 10’s of meters.
    • Wide-area (4G/5G cellular): 10’s Mbps over ~10 Km.
    • Bluetooth: Short distances, limited rates.
    • Terrestrial microwave: Point-to-point; 45 Mbps channels.
    • Satellite: Up to < 100 Mbps downlink with 270 msec delay.

Network Core: Packet Switching

  • The network core uses packet switching, where hosts break application-layer messages into packets.
  • Packets are forwarded from one router to the next across links from source to destination.

Network-Core Functions

  • Forwarding: Moving packets from a router’s input link to the appropriate output link based on the destination address.
  • Routing: Determining the source-destination paths taken by packets using routing algorithms.

Packet-Switching: Store-and-Forward

  • Packet transmission delay involves pushing an L-bit packet into a link at R bps, taking \frac{L}{R} seconds.
  • Store-and-forward requires the entire packet to arrive at a router before it can be transmitted on the next link.
  • Numerical example: For L = 10 Kbits and R = 100 Mbps, one-hop transmission delay is 0.1 msec.

Packet-Switching: Queueing

  • Queueing occurs when the arrival rate exceeds the service rate, leading to packets waiting for transmission over the output link.

Packet-Switching: Queueing and Loss

  • If the arrival rate (in bps) to a link exceeds the transmission rate (bps) for a period:
    • Packets queue, waiting to be transmitted on the output link.
    • Packets can be dropped (lost) if the router's memory (buffer) fills up.

Circuit Switching

  • Circuit switching allocates end-end resources for a call between source and destination.
  • Dedicated resources ensure guaranteed performance, but circuit segments remain idle if not used, resulting in no sharing.
  • Commonly used in traditional telephone networks.

Circuit Switching: FDM and TDM

  • Frequency Division Multiplexing (FDM) divides optical or electromagnetic frequencies into narrow bands.
  • Time Division Multiplexing (TDM) divides time into slots, allocating each call to its own slot(s) for transmission at the maximum rate.

Packet Switching vs. Circuit Switching

  • Example: 1 Gb/s link with users needing 100 Mb/s when active (10% of the time).
    • Circuit-switching: Supports 10 users.
    • Packet switching: Can support more users with a small probability of congestion.
  • Packet switching is great for "bursty" data, offering resource sharing and simplicity but potential congestion.

Internet Structure: A Network of Networks

  • Hosts connect to the Internet via access ISPs, which interconnect to allow any two hosts to send packets to each other.
  • The resulting network is complex, driven by economics and national policies.

Internet Structure: Connecting Access ISPs

  • Connecting each access ISP directly to each other doesn't scale due to O(N^2) connections.

Internet Structure: Global Transit ISPs

  • Connect each access ISP to one global transit ISP, forming customer and provider agreements.

Internet Structure: Competitors and IXPs

  • Multiple global ISPs compete and connect to each other through Internet Exchange Points (IXPs).

Internet Structure: Regional Networks and Content Providers

  • Regional networks connect access nets to ISPs.
  • Content provider networks (e.g., Google, Microsoft, Akamai) run their own networks to bring services and content closer to end-users.

Internet Structure: Tier-1 ISPs and Content Networks

  • The center consists of well-connected large networks, including tier-1 commercial ISPs and content provider networks.

Packet Delay and Loss

  • Packets queue in router buffers, and queue length grows when the arrival rate temporarily exceeds the output link capacity.
  • Packet loss occurs when the memory fills up.

Packet Delay: Four Sources

  • d_{\text{proc}}: Nodal processing delay (check bit errors, determine output link).
  • d_{\text{queue}}: Queueing delay (time waiting at the output link).
  • d_{\text{trans}}: Transmission delay (\frac{L}{R}, where L is the packet length and R is the link transmission rate).
  • d_{\text{prop}}: Propagation delay (\frac{d}{s}, where d is the link length and s is the propagation speed).

Caravan Analogy

  • Illustrates transmission vs. propagation delay using a caravan of cars passing through toll booths.

Packet Queueing Delay (Revisited)

  • Traffic intensity is defined as \frac{La}{R}, where a is the average packet arrival rate.
  • If \frac{La}{R} \approx 0, average queueing delay is small.
  • If \frac{La}{R} \rightarrow 1, average queueing delay becomes large.
  • If \frac{La}{R} > 1, the average delay is infinite.

Real Internet Delays and Routes

  • The traceroute program provides delay measurements from source to router along the end-end path.

Packet Loss

  • Packets arriving to a full queue are dropped (lost).
  • Lost packets may be retransmitted or not, depending on the protocol.

Throughput

  • Throughput is the rate at which bits are sent from sender to receiver.
  • Bottleneck link: The link on the end-end path that constrains the end-end throughput.

Throughput: Network Scenario

  • Each connection end-to-end throughput: \min(Rc, Rs, \frac{R}{10})

Network Security

  • The Internet was not originally designed with strong security.
  • Security considerations are now crucial at all layers.

Network Security Threats

  • Packet sniffing: Interception of packets.
  • IP spoofing: Injection of packets with a false source address.
  • Denial of Service (DoS): Overwhelming resources with bogus traffic.

Lines of Defense

  • Authentication: Proving identity.
  • Confidentiality: Using encryption.
  • Integrity checks: Using digital signatures.
  • Access restrictions: Password-protected VPNs.
  • Firewalls: Filtering packets to restrict senders, receivers, and applications.

Protocol Layers and Reference Models

  • Networks are complex, and protocol layers help organize network structure.

Example: Organization of Air Travel

  • Layers: Ticketing service, baggage service, gate service, runway service, routing service. Each layer offers specific services.

Why Layering

  • Explicit Structure
  • Modularity

Layered Internet Protocol Stack

  • Application: (HTTP, IMAP, SMTP, DNS).
  • Transport: (TCP, UDP).
  • Network: (IP, routing protocols).
  • Link: (Ethernet, 802.11, PPP).
  • Physical: Bits on the wire.

Services, Layering and Encapsulation

  • Each layer encapsulates data from the layer above by adding a header.

Internet History: Early Packet-Switching

  • 1961: Kleinrock's queueing theory.
  • 1964: Baran's packet-switching.
  • 1969: First ARPAnet node operational.
  • 1972: ARPAnet public demo and first e-mail program.

Internet History: Internetworking

  • 1974: Cerf and Kahn's architecture for interconnecting networks.
  • 1976: Ethernet at Xerox PARC.
  • Cerf and Kahn’s Internetworking Principles:
    • Minimalism, autonomy.
    • Best-effort service model.
    • Stateless routing.
    • Decentralized control.

Internet History: Proliferation of Networks

  • 1983: Deployment of TCP/IP.
  • 1983: DNS defined for name-to-IP address translation.
  • 1988: TCP congestion control.

Internet History: Commercialization

  • Early 1990s: NSF lifts restrictions on commercial use of NSFnet.
  • Early 1990s: Web (HTML, HTTP, Berners-Lee).
  • Late 1990s: Commercialization of the Web.

Internet History: Scale, SDN, Mobility, Cloud

  • Aggressive deployment of broadband access.
  • 2008: Software-defined networking (SDN).
  • Increasing ubiquity of high-speed wireless access.
  • Service providers create their own networks, and enterprises run services in the cloud.
  • Rise of smartphones.

Chapter Summary

  • Overview of the Internet, protocols, network edge, core, packet vs circuit switching, performance, layering, security, and history.

ISO/OSI Reference Model

  • Presentation: (Encryption, Compression, Machine-Specific Conventions).
  • Session: (Synchronization, Checkpointing, Recovery of Data Exchange).

Wireshark

  • Transport, Network, Link, and Physical layers used to capture network packets.