Computer Networking Chapter 1 Introduction

1.1 What is the Internet?

1.1.1 A Nuts-and Bolts Description

Figure 1.1 Some pieces of the Internet - hosts = end systems - connected computing devices like smartphone, laptops… - communication links

  • connect end systems
  • made up of different physical materials like fiber, copper, radio, satellite…
  • transmission rate: bandwidth (bits/second)
  • packet switches

    • packet: sending system segments the data and adds the header bytes to each segment
    • store and forward packet
    • two prominent types: routers and link-layer switches
  • path or route

    • from the sending end system to the receiving system
  • Internet Service Providers (ISPs)

    • end systems access the Internet by ISPs, also connect Internet to content providers

    • such as residential ISPs, university ISPs…
    • a network of packet switches and communication links
    • different types of access like residential broadband access (DSL or cable modem), wireless access…
  • protocols

    • control the sending and receiving of information within the Internet
    • TCP/IP
      • Transmission Control Protocol (TCP) and Internet Protocol (IP)
      • IP specifies the format of the packets that are sent and received
  • Internet standards

    • requests for comments (RFCs)

1.1.2 A Services Description

describe the Internet from a different angle—–an infrastructure that provides services to applications

  • applications—–distributed applications
  • Internet apps run on end systems instead of in the packet switches in the network core
  • services = programming interface
  • Application Programming Interface (API)
    • describe how to ask Internet to deliver data to a specific destination program
    • Internet API
      • a set of rules that the sending program must follow

1.1.3 What is a Protocol?

Figure 1.2 A human protocol and a computer network protocol
Figure 1.2 A human protocol and a computer network protocol

A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

1.2 Network Edge

at the edge of the Internet, also hosts = end systems

  • two categories: clients and servers

1.2.1 Access Networks

  1. Home Access: DSL, Cable, FTTH, Dial-Up, and Satellite
    • digital subscriber line (DSL)
      • a customer's telco is its ISP
      • up to 56Kbps direct access to router
      • use existing telephone line to central office DSLAM
        • data over DSL phone line goes to Internet
        • voice over DSL phone line goes to telephone net
        • cannot surf and phone at same time: can't be "always on"
      • Figure 1.3 DSL Internet access
    • ADSL: asymmetric digital subscriber line
      • up to 1Mbps upstream (today typically < 25kbps)
      • up to 8Mbps downstream (today typically < 1Mbps)
      • FDM: 50 kHz - 1 MHz for downstream
        • 4kHz - 50kHz for upstream
        • 0kHz - 4kHz for ordinary telephone
    • cable Internet access
      • use the cable Internet access from access television company's existing cable television infrastructure
      • use fiber and coaxial cable—–hybrid fiber coax (HFC)
        • asymmetric: up to 30Mbps downstream transmission rate, 2Mbps upstream transmission rate
      • Figure 1.4 A hybrid fiber-coaxial access network
      • a shared broadcast medium
      • issues: congestion, dimensioning
    • fiber to the home (FTTH)
      • provide an optical fiber path from the central office (CO) directly to the homes
  2. Access in the Enterprise (and the Home): Ethernet and WiFi
    • company / university local area network (LAN)
    • Ethernet:
      • shared or dedicated link connects end system and router
      • 10Mbs, 100Mbs, Gigabit Ethernet
  3. Wire-Area Wireless Access: 3G and LTE
    • shared wireless access network

1.2.2 Physical Media

  • Bits: propagates between transmitter / receiver pairs
  • guided media and unguided media
    • guided media: the waves are guided along a solid media, such as a fiber-optic cable, a twisted-pair
    • unguided media: the waves propagate in the atmosphere like wireless LAN
  • twisted-pair (TP) copper wire
    • two insulated copper wires
    • category 3: traditional phone wires, 10Mbps Ethernet
    • category 5 TP: 100Mbps Ethernet
  • coaxial cable
    • two concentric copper conductors
    • bidirectional
    • baseband:
      • single channel on cable
      • legacy Ethernet
    • broadband:
      • multiple channels on cable
      • HFC
  • fiber optics
    • glass fiber carrying light pulses, each pulse a bit
    • high-speed operation: high-speed point-to-point transmission
    • lower error rate: repeaters spaced far apart; immune to electromagnetic noise
  • terrestrial radio channels
  • satellite radio channels

1.3 Network Core

1.3.1 Packet Switching

  • hosts break application-layer messages into packets
  • two main types: routers and link-layer switches
  • each packet transmitted at full link capacity
  1. Store-and-Forward Transmission
    • entire packet must arrive at router before transmitted
    • takes L/R seconds to transmit (push out) L-bit packet into link at R bps (assuming 0 propagation delay)
    • Figure 1.5 Store-and-forward packet switching
    • \(d_{end-to-end}=N\frac{L}{R}\)
  2. Queuing Delays and Packet Loss
    • Figure 1.6 Queuing and Loss
    • each packet switch has multiple links, for each link, the packet switch has an output queue (also buffer), which stores packets that is about to be sent
    • packet must wait in the queue if the link is busy, this is queuing delays
    • packet loss will occur if buffer fills up
  3. Forwarding Tables and Routing Protocols
    • routing: determine source-destination route taken by packets
    • forwarding: move packets from router’s input to appropriate router output

1.3.2 Circuit Switching

  • another way to move data through a network of links and switches

  • Figure 1.7 A simple circuit-switched network consisting of four switches and four links
    Figure 1.7 A simple circuit-switched network consisting of four switches and four links
    • in diagram, each link has four circuit, call gets 2nd circuit in top link and 1st circuit in right link
    • each circuit is a dedicated end-to-end connection (no sharing)
    • path are reserved for the communication
  • Multiplexing in Circuit-Switched Networks

    Figure 1.8 FDM versus TDM
    Figure 1.8 FDM versus TDM
    • frequency-division multiplexing (FDM)
      • the link delicates a frequency band to each connection
    • time-division multiplexing (TDM)
      • time is divided into frame of fixed duration, each frames divided into a fixed number of time slots

1.3.3 A Network of Networks

  • end systems connect to Internet via access ISPs
  • access ISPs in turn must be interconnected.

1.4 delay, loss, throughput in networks

1.4.1 Overview of Delay in Packet-Switched Networks

  • packets queue in router buffers
    • packet arrival rate link (temporarily) exceeds output link capacity
    • packets queue, wait for turn
Figure 1.9 Loss and Delay
Figure 1.9 Loss and Delay
  • four sources of packet delay

    Figure 1.10 four sources of packet delay
    Figure 1.10 four sources of packet delay
    • dproc: nodal processing
      • check bit errors
      • determine output link
      • typically < msec
    • dqueue: queuing delay
      • time waiting at output link for transmission
      • depends on congestion level of router
    • dtrans: transmission delay
      • L: packet length (bits)
      • R: link bandwidth (bps)
      • dtrans = L/R
    • dprop: propadation delay
      • d: length of physical link
      • s: propagation speed in medium (2x108 m/sec)
      • dprop = d/s

1.4.2 Queuing Delay and Packet Loss

  • queueing delay (revisited)
    • R = link bandwidth (bps)
    • L = packet length (bits)
    • a = average packet arrival rate
    • L * a / R = traffic intensity
    • L * a / R = 0: average queueing delay small
    • L * a / R -> 1: delays become large
    • L * a / R > 1: more "work" arriving than can be served, average delay infinite
  • packet loss
    • queue (aka buffer) preceding link in buffer has finite capacity
    • packet arriving to full queue dropped (aka lost)
    • lost packet may be retransmitted by previous node, by source end system, or not at all

1.4.3 End-to-End Delay

  • dend-end = N (dproc + dqueue + dtrans + dprop)
  • traceroute: a service to feel end-to-end delay, in wins cmd: tracert

1.4.4 Throughput in Computer Networks

  • throughput: the amount of data per second that can be transferred, also rate (bits / time unit) at which bits transferred between sender / receiver
  • Figure 1.11 Throughput for a file
    • throughput = \(min\{R_c, R_s\}, min\{R_1,R_2...,R_N\}\)

1.5 protocol layers, service models

1.5.1 Layered Architecture

Figure 1.12 The Internet protocol stack and OSI reference model
Figure 1.12 The Internet protocol stack and OSI reference model
  • Internet protocol stack
    • application: supporting network applications
      • FTP, SMTP, HTTP
    • 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, 802.111 (WiFi), PPP
    • physical: bits "on the wire"
  • ISO/OSI reference model
    • presentation: allow applications to interpret meaning to interpret meaning of data
      • encryption, compression, machine-specific, conventions
    • session: synchronization, checkpointing, recovery of data exchange

1.5.2 Encapsulation

Figure 1.13 Encapsulation
Figure 1.13 Encapsulation

1.6 Networks Under Attack

Internet not originally designed with (much) security in mind

original vision "a group of mutually trusting users attached to a transparent network"

  • Bad guys: put malware into hosts via Internet
    • malware can get in host form:
      • virus: self-replicating infection by receiving/executing object
      • worm: self-replicating infection by passively receiving object that gets itself executed
    • spyware malware can record keystrokes, web sites visited, upload info to collection site
    • infected host can be enrolled in botnet, used for spam
  • Bad guys: attack server, network infrastructure
    • Denial of Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic
    • steps:
      1. select target
      2. break into hosts around the network
      3. send packets to target from compromised hosts
  • Bad guys: sniff packets
  • Bad guys: use fake address

1.7 History

Read by yourself


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