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

Introduction

• These slides are available for free use, modification, and distribution.
• Attribution is requested when used in classes or posted online.
• All material is copyrighted by J.F Kurose and K.W. Ross.

Chapter 1 Overview

• Goal: To gain a foundational understanding and terminology related to computer networking.
• 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

The Internet: A Nuts and Bolts View

• Billions of connected computing devices (hosts or end systems) running network applications at the Internet’s edge.
• Networks are collections of devices, routers, and links managed by an organization.
• Packet switches (routers, switches) forward packets (chunks of data).
• Communication links use fiber, copper, radio, satellite with varying transmission rates (bandwidth).
• The Internet is a “network of networks,” interconnected ISPs.
• Protocols govern sending and receiving messages, including HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4/5G, Ethernet.
• Internet standards are defined in RFCs (Request for Comments) and managed by the IETF (Internet Engineering Task Force).

The Internet: A Services View

• The Internet provides services to applications like web browsing, streaming video, multimedia teleconferencing, email, games, e-commerce, social media, and interconnected appliances.
• It offers a programming interface to distributed applications, allowing them to connect to and use Internet transport services.
• Service options are analogous to postal service options.

What’s a Protocol?

• Protocols define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt.
  • It is a set of rules governing communication.
• Human protocols example:
  • “what’s the time?”
  • “I have a question”
  • introductions
• Network protocols example:
  • Computers (devices) rather than humans
  • Specific messages sent
  • Specific actions taken when a message is received, or other events
• Examples:
  • Human: "Hi," "Got the time?"
  • Computer: TCP connection request, TCP connection response, HTTP GET request, file transfer.

Network Edge

• Hosts (clients and servers) are located at the network edge.
• Servers are often located in data centers.

Access Networks and Physical Media

• Access networks connect end systems to the edge router.
• Physical media includes wired and wireless communication links.
• Types of access networks:
  • Residential access networks
  • Institutional access networks (school, company)
  • Mobile access networks (WiFi, 4G/5G)

Access Networks: Cable-Based

• Cable modem uses frequency division multiplexing (FDM) to transmit different channels in different frequency bands.
• HFC (hybrid fiber coax) systems offer asymmetric transmission rates (up to 40 Mbps – 1.2 Gbps downstream, 30-100 Mbps upstream).
• Homes share access network to cable headend.
• Cable modem termination system (CMTS) connects cable network to ISP.

Access Networks: Digital Subscriber Line (DSL)

• DSL uses existing telephone lines to connect to a central office DSLAM.
• Voice and data are transmitted at different frequencies over a dedicated line.
• Offers dedicated downstream (24-52 Mbps) and upstream (3.5-16 Mbps) transmission rates.
• DSL modem and splitter are used in homes.

Access Networks: Home Networks

• Combine wired (Ethernet) and wireless (WiFi) devices with a router, firewall, and NAT.

Wireless Access Networks

• Shared wireless access networks connect end systems to a router via a base station or access point.
• Wireless LANs (WLANs) typically operate within a building (~100 ft) using 802.11b/g/n (WiFi) standards.
• Wide-area cellular access networks are provided by mobile operators (10’s km) using 4G/5G cellular networks.

Access Networks: Enterprise Networks

• Used by companies, universities, etc.
• Mix of wired (Ethernet at 100Mbps, 1Gbps, 10Gbps) and wireless (WiFi at 11, 54, 450 Mbps) technologies.

Access Networks: Data Center Networks

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

Host: Sending Packets of Data

• Host sending function:
  • Takes application message
  • Breaks into smaller chunks, known as packets, of length L bits
  • Transmits packet into access network at transmission rate R.
  • Link transmission rate, aka link capacity, aka link bandwidth
• Packet transmission delay = \frac{L}{R} where L is the packet length in bits and R is the transmission rate in bits/sec.

Links: Physical Media

• Bit propagates between transmitter/receiver pairs.
• Physical link lies between transmitter & receiver.
• Guided media: signals propagate in solid media (copper, fiber, coax).
• Unguided media: signals propagate freely (radio).

Links: Physical Media - Twisted Pair (TP)

• Two insulated copper wires.
• Category 5: 100 Mbps, 1 Gbps Ethernet.
• Category 6: 10Gbps Ethernet.

Links: Physical Media - Coaxial Cable

• Two concentric copper conductors.
• Bidirectional.
• Broadband: multiple frequency channels (100’s Mbps per channel).

Links: Physical Media - Fiber Optic Cable

• Glass fiber carrying light pulses.
• High-speed operation (10’s-100’s Gbps).
• Low error rate, immune to electromagnetic noise.

Links: Physical Media - Wireless Radio

• Signal carried in various “bands” in the electromagnetic spectrum.
• No physical “wire”.
• Broadcast, “half-duplex” (sender to receiver).
• Propagation environment effects: reflection, obstruction, interference/noise.
• Radio link types:
  • Wireless LAN (WiFi): 10-100’s Mbps; 10’s of meters.
  • Wide-area (4G/5G cellular): 10’s Mbps (4G) over ~10 Km.
  • Bluetooth: cable replacement; short distances, limited rates.
  • Terrestrial microwave: point-to-point; 45 Mbps channels.
  • Satellite: up to < 100 Mbps (Starlink) downlink; 270 msec end-end delay (geostationary).

The Network Core

• Mesh of interconnected routers.
• Packet-switching: hosts break application-layer messages into packets, and the network forwards packets from one router to the next.

Two Key Network-Core Functions

• Forwarding (Switching):
  • Local action to move arriving packets from router’s input link to the appropriate output link.
• Routing:
  • Global action to determine source-destination paths taken by packets.
  • Routing algorithms.

Packet-Switching: Store-and-Forward

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

Packet-Switching: Queueing

• Queueing occurs when the arrival rate is faster than it can be serviced.

Packet-Switching: Queueing and Loss

• If the arrival rate (in bps) to a link exceeds the transmission rate (bps) of the link for some period of 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.

Alternative to Packet Switching: Circuit Switching

• End-end resources are allocated to, reserved for a “call” between source and destination.
• Dedicated resources with no sharing resulting in circuit-like (guaranteed) performance.
• In circuit switching, a circuit segment is idle if not used by a 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 allocated its own band and can transmit at the max rate of that narrow band.
• Time Division Multiplexing (TDM):
  • Time is divided into slots.
  • Each call has periodic slot(s) allocated, and can transmit at the maximum rate of the assigned frequency band only during its time slot(s).

Packet Switching Versus Circuit Switching

• Example: 1 Gb/s link, each user requires 100 Mb/s when “active,” and is active 10% of the time.
  • Circuit-switching: 10 users can be supported.
  • Packet switching: with 35 users, the probability of >10 being active at the same time is less than 0.0004.
• Packet switching is great for “bursty” data with resource sharing and simple implementation, but can experience packet delay and loss due to congestion.
• Circuit switching guarantees dedicated resources but can be inefficient for bursty data.

Internet Structure: A “Network of Networks”

• Hosts connect to the Internet via access Internet Service Providers (ISPs).
• Access ISPs must be interconnected so hosts can send packets to each other.
• The resulting network of networks is complex due to economics and national policies.

Internet Structure: Connecting Access ISPs

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

Internet Structure: Global Transit ISP

• Connect each access ISP to one global transit ISP, with customer and provider ISPs having an economic agreement.

Internet Structure: Competitors and IXPs

• If one global ISP is viable, there will be competitors.
• ISPs will want to be connected via peering links and Internet exchange points (IXP).

Internet Structure: Regional Networks

• Regional networks may arise to connect access nets to ISPs.

Internet Structure: Content Provider Networks

• Content provider networks (e.g., Google, Microsoft, Akamai) may run their own network to bring services and content close to end users.

Internet Structure: Tier-1 ISPs and Content Providers

• At the “center” are a small number of well-connected large networks, including “tier-1” commercial ISPs and content provider networks.
• Tier 1 ISPs have national & international coverage.
• Content providers connect their 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 a link exceeds the output link capacity.
• Packet loss occurs when the memory to hold queued packets fills up.

Packet Delay: Four Sources

• d{nodal} = d{proc} + d{queue} + d{trans} + d_{prop}

• d_{proc}: nodal processing (check bit errors, determine output link) typically < microsecs.

• d_{queue}: queueing delay (time waiting at output link for transmission) depends on congestion level.

• d_{trans}: transmission delay: L (packet length in bits) / R (link transmission rate in bps).

• d_{prop}: propagation delay: d (length of physical link) / s (propagation speed ~2x10^8 m/sec).

Packet Queueing Delay

• a: average packet arrival rate
• L: packet length (bits)
• R: link bandwidth (bit transmission rate)
• Traffic intensity = \frac{La}{R}:
  • \frac{La}{R} \approx 0: avg. queueing delay small
  • \frac{La}{R} \rightarrow 1: avg. queueing delay large
  • \frac{La}{R} > 1: average delay infinite!

“Real” Internet Delays and Routes

• Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination.
• Sends three packets that will reach router i on path towards destination (with time-to-live field value of i).
• Router i will return packets to sender.
• Sender measures time interval between transmission and reply.

Packet Loss

• Packet arriving to full queue dropped (aka lost).
• Lost packet may be retransmitted by previous node, by source end system, or not at all.

Throughput

• Throughput: rate (bits/time unit) at which bits are being sent from sender to receiver.
  • Instantaneous: rate at a given point in time.
  • Average: rate over a longer period of time.

Throughput: Bottleneck Link

• If Rs < Rc, average end-end throughput is R_s.
• If Rs > Rc, average end-end throughput is R_c.

Network Security

• The Internet was not originally designed with security in mind.
• Security considerations are now present in all layers.

Bad Guys: Packet Interception

• Packet “sniffing” in broadcast media (shared Ethernet, wireless).
• Promiscuous network interface reads/records all packets (e.g., including passwords!).

Bad Guys: Fake Identity

• IP spoofing: injection of packet with false source address.

Bad Guys: Denial of Service

• Denial of Service (DoS): attackers make resources unavailable to legitimate traffic by overwhelming the resource with bogus traffic.
• Involves selecting a target, breaking into hosts around the network (botnet), and sending packets to the target.

Lines of Defense

• Authentication: proving you are who you say you are.
• Confidentiality: via encryption.
• Integrity checks: digital signatures to prevent/detect tampering.
• Access restrictions: password-protected VPNs.
• Firewalls: specialized “middleboxes” in access and core networks to filter incoming packets.

Protocol “Layers” and Reference Models

• Networks are complex with many “pieces” and it is beneficial to organize their structure.

Example: Organization of Air Travel

• Air travel can be organized into layers, each implementing a service via its own internal actions and relying on services provided by the layer below.

Why Layering?

• Explicit structure allows identification and relationship of system’s pieces.
• Modularization eases maintenance and updating of 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
  Transport-layer protocol transfers M from one process to another, using services of network layer.

• 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

Internet History

• 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: 1972-1980

• 1970: ALOHAnet satellite network in Hawaii
• 1974: Cerf and Kahn - architecture for interconnecting networks
• 1976: Ethernet at Xerox PARC
• late70’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
  • define today’s Internet architecture

Internet History: 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: 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 forefront
  • est. 50 million host, 100 million+ users
  • backbone links running at Gbps

Internet History: 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 Internet (2017)
• ~15B devices attached to Internet (2023, statista.com)

Chapter 1: Summary

• Internet overview
• what’s a protocol?
• network edge, access network, core
  • packet-switching versus circuit-switching
  • Internet structure
• performance: loss, delay, throughput
• layering, service models
• security
• history

ISO/OSI Reference Model

• Two layers not found in Internet protocol stack!
  • presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions
  • session: synchronization, checkpointing, recovery of data exchange
• Internet stack “missing” these layers!
  • these services, if needed, must be implemented in application