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

Introduction

  • 1-1 Chapter 1 Introduction
    • Slides are freely available for educational use.
    • Attribution is requested if used in classes or online.
    • Copyright 1996-2023 J.F Kurose and K.W. Ross.
    • Computer Networking: A Top-Down Approach, 8th edition, Jim Kurose, Keith Ross, Pearson, 2020.

Overview

  • Chapter 1 Goals
    • To gain a feel and big-picture understanding of networking terminology.
    • Deeper dives come later.
  • Roadmap:
    • 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.
    • Protocol layers, service models.
    • Security.
    • History.

Internet: Nuts and Bolts View

  • Internet: A network of networks.
    • Includes mobile, home, enterprise, and data center networks, interconnected via ISPs.
  • Packet Switches: Forward packets (data chunks).
    • Includes routers and switches.
  • Communication Links: Various media with different transmission rates (bandwidth).
    • Fiber, copper, radio, satellite.
  • Connected Devices: Billions of devices.
    • Hosts or end systems run network applications at the Internet's edge.
  • Networks: Collections of devices, routers, and links managed by organizations.

Internet-Connected Devices

  • Examples of connected devices:
    • Web-enabled toaster, weather forecaster, Internet phones.
    • Slingbox, security cameras, IP picture frames, Internet refrigerator.
    • Tweet-a-watt (energy monitor), sensorized bed mattress, Amazon Echo.
    • Gaming devices, cars, scooters, bikes.
    • Pacemaker, monitors, AR devices, Fitbit, diapers.

Internet: Network of Networks

  • Internet is a network of interconnected ISPs.
  • Protocols are Essential
    • Control sending and receiving of messages.
    • Examples: HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4/5G, Ethernet.
  • Internet Standards
    • RFC: Request for Comments.
    • IETF: Internet Engineering Task Force.

Internet: Services View

  • Infrastructure for applications
    • Supports web, streaming video, teleconferencing, email, games, e-commerce, social media, interconnected appliances.
  • Programming Interface
    • Provides "hooks" for apps to connect to and use Internet transport services.
    • Offers service options similar to postal service.

What's a Protocol?

  • Human Protocols
    • Examples: Asking the time, asking a question, introductions.
  • Network Protocols
    • Govern all communication activity in the Internet.
    • Define format, order of messages, and actions taken on transmission/receipt.
  • Rules
    • Specify messages sent and actions taken upon receipt or other events.

Protocol Examples

  • Human Protocol
    • A: Hi
    • B: Hi
    • A: Got the time?
    • B: 2:00
  • Computer Network Protocol
    • TCP connection request.
    • TCP connection response.
    • GET http://gaia.cs.umass.edu/kurose_ross

Network Edge

  • The network edge comprises hosts (clients and servers).
  • Servers are often located in data centers.

Access Networks and Physical Media

  • Network Edge
    • Hosts: clients and servers.
    • Servers: often in data centers.
  • Access Networks, Physical Media
    • Wired, wireless communication links.
  • Network Core
    • Interconnected routers.
    • Network of networks.

Connecting to the Edge Router

  • How do end systems connect to the edge router?
    • Residential access networks.
    • Institutional access networks (school, company).
    • Mobile access networks (WiFi, 4G/5G).

Cable-Based Access Networks

  • Cable Modem
    • Connects homes to the cable headend.
  • Frequency Division Multiplexing (FDM)
    • Different channels transmitted in different frequency bands.

Cable-Based Access Details

  • Data and TV transmitted at different frequencies over a shared cable distribution network.
  • HFC: Hybrid Fiber Coax
    • Asymmetric: Up to 40 Mbps – 1.2 Gbps downstream, 30-100 Mbps upstream.
  • Network of cable and fiber connects homes to ISP router.
  • Homes share access network to the cable headend.
  • CMTS: Cable Modem Termination System

Digital Subscriber Line (DSL) Access

  • Central Office
    • Connects homes to the DSLAM.
  • Voice and data transmitted at different frequencies over a dedicated line to the central office.
  • Use existing telephone lines to central office DSLAM.
  • Data over DSL goes to the Internet; voice goes to the telephone net.
  • 24-52 Mbps dedicated downstream.
  • 3.5-16 Mbps dedicated upstream.
  • DSL modem and DSL access multiplexer.

Home Networks

  • Connections:
    • To/from headend or central office.
    • Cable or DSL modem.
    • Router, firewall, NAT.
    • Wired Ethernet (1 Gbps).
    • WiFi wireless access point (54, 450 Mbps).
  • Wireless and wired devices are often combined in a single box.

Wireless Access Networks

  • Shared wireless access network connects end system to router via a base station (access point).
  • Wireless Local Area Networks (WLANs)
    • Typically within or around a building (~100 ft).
    • 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate.
  • Wide-Area Cellular Access Networks
    • Provided by mobile, cellular network operator (10’s km).
    • 10’s Mbps.
    • 4G/5G cellular networks.

Enterprise Networks

  • Used by companies, universities, etc.
  • Mix of wired, wireless link technologies.
  • Connects a mix of switches and routers.
  • Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps.
  • WiFi: wireless access points at 11, 54, 450 Mbps.
  • Institutional mail, web servers and router.
  • Enterprise link to ISP (Internet).

Data Center Networks

  • High-bandwidth links (10s to 100s Gbps).
  • Connect hundreds to thousands of servers together and to the Internet.

Host Sending Data

  • Host: Sends packets of data.
  • Sending Function
    • Takes application message.
    • Breaks into smaller chunks, known as packets, of length L bits.
    • Transmits packet into the access network at transmission rate R. Where
      R is the link transmission rate, also known as link capacity or link bandwidth.
  • Packet transmission delay (time needed to transmit L-bit packet into link) is calculated as:
    • \frac{L \text{ (bits)}}{R \text{ (bits/sec)}}
  • Bit: Propagates between transmitter/receiver pairs.
  • Physical Link: What lies between transmitter & receiver.
  • Guided Media: Signals propagate in solid media (copper, fiber, coax).
  • Unguided Media: Signals propagate freely (radio).
  • Twisted Pair (TP)
    • Two insulated copper wires.
    • Category 5: 100 Mbps, 1 Gbps Ethernet.
    • Category 6: 10Gbps Ethernet.

Coaxial and Fiber Optic Cables

  • Coaxial Cable:
    • Two concentric copper conductors.
    • Bidirectional.
    • Broadband: multiple frequency channels.
    • 100’s Mbps per channel.
  • Fiber Optic Cable:
    • Glass fiber carrying light pulses.
    • High-speed: 10’s-100’s Gbps.
    • Low error rate.
    • Immune to electromagnetic noise.
  • Signal carried in electromagnetic spectrum bands.
  • No physical "wire."
  • Broadcast, half-duplex (sender to receiver).
  • Propagation environment effects: reflection, obstruction, interference.
  • Radio Link Types:
    • Wireless LAN (WiFi): 10-100’s Mbps; 10’s of meters.
    • Wide-area (e.g., 4G/5G cellular): 10’s Mbps (4G) over ~10 Km.
    • Bluetooth: short distances, limited rates.
    • Terrestrial microwave: point-to-point; 45 Mbps channels.
    • Satellite: up to < 100 Mbps (Starlink) downlink, 270 msec delay.

Network Core Overview

  • The network core is a mesh of interconnected routers.
  • Packet-switching: Hosts break application-layer messages into packets.
    • The network forwards packets from one router to the next across links on the path from source to destination.

Key Network Core Functions

  • Forwarding (Switching)
    • Local action: move arriving packets from router’s input link to appropriate router output link.
    • Uses a local forwarding table.
  • Routing
    • Global action: Determine source-destination paths taken by packets.
    • Uses routing algorithms to update the forwarding table.

Packet-Switching: Store-and-Forward

  • Packet transmission delay: Time to transmit (push out) an L-bit packet into a link at rate R bps.
  • Store and forward: Entire packet must arrive at the router before it can be transmitted on the next link.
  • One-hop transmission delay: If L = 10 Kbits and R = 100 Mbps, the one-hop transmission delay = 0.1 msec.

Packet-Switching: Queueing

  • Queueing occurs when work arrives faster than it can be serviced.

Packet Queueing and Loss

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

Circuit Switching

  • End-to-end resources are allocated to and reserved for a "call" between source and destination.
  • Dedicated resources: no sharing.
  • Circuit segment is idle if not used by the call (no sharing).
  • Commonly used in traditional telephone networks.

Circuit Switching: FDM and TDM

  • Frequency Division Multiplexing (FDM)
    • Optical, electromagnetic frequencies divided into narrow frequency bands.
    • Each call gets its own band and can transmit at the max rate of that band.
  • Time Division Multiplexing (TDM)
    • Time is divided into slots.
    • Each call is allocated periodic slots and can transmit at the maximum rate during its time slot.

Packet Switching vs. Circuit Switching

  • Example:
    • 1 Gb/s link.
    • Each user:
      • 100 Mb/s when active.
      • Active 10% of the time.
  • Question: How many users can use this network under circuit-switching and packet switching?
    • Circuit-switching: 10 users.
    • Packet switching: with 35 users, the probability of > 10 active users at the same time is less than 0.0004.

Packet Switching Advantages

  • Great for "bursty" data with intermittent transmission needs.
    • Resource sharing.
    • Simpler, no call setup required.
  • Excessive congestion is possible.
    • Packet delay and loss due to buffer overflow.
    • Protocols needed for reliable data transfer and congestion control.
  • Question: How to provide circuit-like behavior with packet-switching?

Internet Structure

  • Hosts connect to the Internet via access Internet Service Providers (ISPs).
  • Access ISPs must be interconnected.
    • So that any two hosts can send packets to each other.
  • The resulting network of networks is very complex.
    • Evolution driven by economics and national policies.

Connecting Access ISPs

  • Challenge: How to connect millions of access ISPs?
  • Connecting each access ISP to each other directly doesn’t scale: O(N^2) connections.

Transit ISPs

  • Option: Connect each access ISP to one global transit ISP?
  • Customer and provider ISPs have economic agreements.

Internet Exchange Points (IXP)

  • If one global ISP is viable, there will be competitors.
    • Competitors will want to be connected.
  • Internet exchange point peering link (IXP)

Regional Networks and Content Providers

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

Tier-1 ISPs and Content Provider Networks

  • At the center: small # of well-connected large networks.
    • Tier-1 commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT): national & international coverage.
    • Content provider networks (e.g., Google, Facebook): private network that connects its data centers to the Internet, often bypassing tier-1, regional ISPs.

Packet Delay and Loss

  • Packets queue in router buffers, waiting for their turn for transmission.
  • Queue length grows when the arrival rate to the link (temporarily) exceeds the output link capacity.
  • Packet loss occurs when memory to hold queued packets fills up.

Packet Delay: Four Sources

  • d{\text{nodal}} = d{\text{proc}} + d{\text{queue}} + d{\text{trans}} + d_{\text{prop}}
  • d_{\text{proc}}: Nodal Processing
    • Check bit errors.
    • Determine output link.
    • Typically < microsecs.
  • d_{\text{queue}}: Queueing Delay
    • Time waiting at output link for transmission.
    • Depends on congestion level of router.

Transmission vs. Propagation Delay

  • d_{\text{trans}}: Transmission Delay
    • L: Packet length (bits).
    • R: Link transmission rate (bps).
    • d_{\text{trans}} = L/R
  • d_{\text{prop}}: Propagation Delay
    • d: Length of physical link.
    • s: Propagation speed (~2x108 m/sec).
    • d_{\text{prop}} = d/s
  • Transmission and propagation delays are very different.

Caravan Analogy

  • Car ~ bit; caravan ~ packet; toll service ~ link transmission.
  • The toll booth takes 12 seconds to service a car (bit transmission time).
  • "Propagate" at 100 km/hr.
  • Question: How long until the caravan is lined up before the 2nd toll booth?
    • Time to “push” the entire caravan through the toll booth onto the highway = 12 * 10 = 120 seconds.
    • Time for the last car to propagate from the 1st to the 2nd toll booth: 100km / (100km/hr) = 1 hour.
    • Answer: 62 minutes.

Caravan Analogy Revisited

  • Cars "propagate" at 1000 km/hr.
  • Toll booth now takes one minute to service a car.
  • Question: Will cars arrive at the 2nd booth before all cars are serviced at the first booth?
    • Answer: Yes! After 7 minutes, the first car arrives at the second booth; three cars are still at the first booth.

Packet Queueing Delay

  • Variables:
    • a: Average packet arrival rate.
    • L: Packet length (bits).
    • R: Link bandwidth (bit transmission rate).
  • Traffic intensity, \frac{La}{R} : average queueing delay large, greater than 1 then average delay is infinite

Real Internet Delays and Routes

  • Traceroute program: Measures delay from source to router along the Internet path to the destination.
    • Sends three packets to router i on the path towards the destination.
    • Router i returns packets to the sender.
    • The sender measures the time interval between transmission and reply.

Traceroute Example

  • Example output shows delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu.
  • Traceroute output indicates delays decrease.
  • Also, the presence of trans-oceanic links introduces more delay.
  • * = probe lost or router not replying

Packet Loss

  • A buffer, preceding link, has finite capacity.
  • Packet arriving to full queue dropped (lost).
  • Lost packet may be retransmitted: may not be retransmitted at all.

Throughput

  • Throughput: Rate (bits/time unit) at which bits are sent from sender to receiver.
    • Instantaneous: Rate at a given point in time.
    • Average: Rate over a longer period of time.
  • Scenario: A server with a file of F bits to send to a client.
  • Server sends bits (fluid) into pipe.
  • End-to-end throughput is constrained by the bottleneck link.

Network Scenario & Throughput

  • Scenario: 10 connections share a backbone bottleneck link of R bits/sec. Rs and Rc are link capacities.
  • Per-connection end-to-end throughput: min(Rc, Rs, R/10).
  • In practice: Rc or Rs is often the bottleneck.

Network Security Concerns

  • Internet originally designed with minimal security.
    • Original vision: "a group of mutually trusting users attached to a transparent network."
  • Internet protocol designers are playing "catch-up" regarding security.
  • Security considerations are now present in all layers.
  • Now think about:
    • How bad guys can attack computer networks.
    • How to defend networks against attacks.
    • How to design architectures immune to attacks.

Packet Sniffing

  • Broadcast media (shared Ethernet, wireless).
  • Promiscuous network interface reads/records all packets (e.g., including passwords!) passing by.
  • Wireshark software is a free packet-sniffer.

IP Spoofing

  • Injection of packet with false source address.

Denial of Service (DoS)

  • Attackers make resources (server, bandwidth) unavailable to legitimate traffic.
  • Overwhelm resource with bogus traffic.
    1. Select target
    2. Break into hosts around the network
    3. Send packets to target from compromised hosts

Lines of Defense

  • Authentication: Proving you are who you say you are.
    • Cellular networks use SIM cards for hardware identity; traditional Internet lacks this.
  • Confidentiality: Via encryption.
  • Integrity checks: Digital signatures prevent/detect tampering.
  • Access restrictions: Password-protected VPNs.
  • Firewalls: specialized “middleboxes”.
    • Off-by-default: filter incoming packets to restrict senders, receivers, applications.
    • Detecting/reacting to DOS attacks.
  • Lots more on security.

Protocol Layers & Reference Models

  • Networks are complex, with many pieces.
  • Question: Is there hope of organizing structure of network?
  • Is there hope for our discussion of networks?

Organization of Air Travel

  • A series of steps involving many services:
    • Ticketing, baggage check, gate loading, runway takeoff, airplane routing.
    • Ticketing, baggage claim, gate unloading, runway landing, airplane routing.
  • Layers: each layer implements a service via its own actions while relying on the services of the layer below.

Benefits of Layering

  • Explicit structure facilitates identification and relationship of system pieces.
  • Modularization eases maintenance and updating.
    • A change in a layer's service implementation is transparent to the rest of the system.

Layered Internet Protocol Stack

  • Application: Supporting network applications (HTTP, IMAP, SMTP, DNS).
  • Transport: Process-process data transfer (TCP, UDP).
  • Network: Routing of datagrams from source to destination (IP, routing protocols).
  • Link: Data transfer between neighboring network elements (Ethernet, 802.11 (WiFi), PPP).
  • Physical: Bits “on the wire”.

Services, Layering, and Encapsulation

  • Transport-layer protocol encapsulates application-layer message M with transport layer header H_t to create a transport-layer segment.
  • Network-layer protocol encapsulates transport-layer segment [Ht | M] with network layer header Hn to create a network-layer datagram.
  • Link-layer protocol encapsulates network datagram [Hn| [Ht |M]] with link-layer header H_l to create a link-layer frame.

Encapsulation

  • message
  • segment
  • datagram
  • frame

Internet History: Early Packet Switching Principles (1961-1972)

  • 1961: Kleinrock - queueing theory shows effectiveness of packet-switching.
  • 1964: Baran - packet-switching in military nets.
  • 1967: ARPAnet conceived by Advanced Research Projects Agency.
  • 1969: First ARPAnet node operational.
  • 1972:
    • ARPAnet public demo.
    • NCP (Network Control Protocol) first host-host protocol.
    • First e-mail program.
    • ARPAnet has 15 nodes.

Internet History: Internetworking, New and Proprietary Networks (1972-1980)

  • 1970: ALOHAnet satellite network in Hawaii.
  • 1974: Cerf and Kahn - architecture for interconnecting networks.
  • 1976: Ethernet at Xerox PARC.
  • Late 70’s: Proprietary architectures: DECnet, SNA, XNA.
  • 1979: ARPAnet has 200 nodes.
  • Cerf and Kahn’s internetworking principles:
    • Minimalism, autonomy - no internal changes required to interconnect networks.
    • Best-effort service model.
    • Stateless routing.
    • Decentralized control.

Internet History: New Protocols, a Proliferation of Networks (1980-1990)

  • 1983: Deployment of TCP/IP.
  • 1982: smtp e-mail protocol defined.
  • 1983: DNS defined for name-to -IP-address translation.
  • 1985: ftp protocol defined.
  • 1988: TCP congestion control.
  • New national networks: CSnet, BITnet, NSFnet, Minitel.
  • 100,000 hosts connected to confederation of networks.

Internet History: Commercialization, the Web, New Applications (1990, 2000s)

  • Early 1990s: ARPAnet decommissioned.
  • 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995).
  • Early 1990s: Web.
    • Hypertext [Bush 1945, Nelson 1960’s].
    • HTML, HTTP: Berners-Lee.
    • 1994: Mosaic, later Netscape.
  • Late 1990s: Commercialization of the Web.
  • Late 1990s – 2000s:
    • More killer apps: instant messaging, P2P file sharing.
    • Network security to the forefront.
    • Est. 50 million host, 100 million+ users.
    • Backbone links running at Gbps.

Internet History: Scale, SDN, Mobility, Cloud (2005-Present)

  • Aggressive deployment of broadband home access (10-100’s Mbps).
  • 2008: Software-defined networking (SDN).
  • Increasing ubiquity of high-speed wireless access: 4G/5G, WiFi.
  • Service providers (Google, FB, Microsoft) create their own networks.
    • Bypass commercial Internet to connect “close” to end-user, providing “instantaneous” access to social media, search, video content.
  • Enterprises run their services in “cloud” (e.g., Amazon Web Services, Microsoft Azure).
  • Rise of smartphones: more mobile than fixed devices on the Internet (2017).
  • ~15B devices attached to Internet (2023, statista.com).

Additional Slides

  • Additional slides for Chapter 1.

ISO/OSI Reference Model

  • Two layers not found in Internet protocol stack!
    • Presentation: Allow applications to interpret the meaning of data.
    • Session: Synchronization, checkpointing, recovery of data exchange.
  • Internet stack “missing” these layers!
    • These services, if needed, must be implemented in the application.

More Than Seven OSI Layers!

  • Political, Financial, Here

Wireshark

  • How to display transport, network, link and physical and application layers.