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The Internet

(Usage hints for this presentation)

VM Neuland im Internet 2021
Dr. Jens Lechtenbörger (License Information)

1 Introduction

1.1 Today’s Core Questions

  • What is the Internet?
  • How to provide global connectivity in view of heterogeneous network technologies, diverse devices, and novel (and forthcoming) applications?
  • How to cope with complexity?

1.2 Learning Objectives

  • Explain and contrast Internet and OSI architectures
  • Explain layers in Internet architecture
    • Roles and interplay for communication
    • Basic properties of IP, UDP, TCP
  • Explain forwarding of Internet messages based on (IP and MAC) addresses and demux keys
    • Use Wireshark to inspect network traffic

1.3 General Importance of Internet

  • The Internet is everywhere
  • IT permeates our life
  • How does that really work?
    • Complexity? Functionality?
    • Security? Privacy?

2 Basics

2.1 (Computer) Networks

[PD11]: A network can be defined recursively as

  • two or more nodes/devices/hosts connected by a link
    • (e.g., copper, fibre, nothing)

  • or two or more networks connected by one or more nodes (with necessary links)
    • (e.g., gateway, router)

2.1.1 On Routers

  • Previous slide mentions routers as nodes that connect networks
    • One example: router at home that connects home network to ISP’s network
    • Other example: router that connects “large” networks at backbone of Internet
      • Independently managed networks are called autonomous systems
      • Routers exchange information about reachable networks with protocols such as BGP

2.2 Internet vs Web

  • The Internet is a network of networks
    • Connectivity for heterogeneous devices
    • Various protocols, some details on later slide
      • IPv4 and IPv6 to send messages between devices on the Internet
      • TCP and UDP to send messages between processes on Internet devices
        • (E.g., process of Web browser talks with remote process of Web server)
        • TCP: Reliable full-duplex byte streams
        • UDP: Unreliable message transfer
  • The Web is an application using the Internet
    • Clients and servers talking HTTP over TCP/IP
      • E.g., GET requests asking for HTML pages (separate presentation)
      • Web servers provide resources to Web clients (browsers, apps)
  • Internet and Web are and contain DSs

2.3 Heterogeneity

  • Internet is network of networks
  • Potentially each network with
    • independent administrative control
    • different applications and protocols
    • different performance and security requirements
    • different technologies (fiber, copper, wired, wireless)
    • different hardware and operating systems
  • How to overcome heterogeneity?

3 Layering and Protocols

3.1 Layering

General technique in Software Engineering and Information Systems

  • Use abstractions to hide complexity
    • Abstractions naturally lead to layering
    • Alternative abstractions at each layer
      • Abstractions specified by standards/protocols/APIs
  • Thus, problem at hand is decomposed into manageable components
    • Design becomes (more) modular

3.2 Network Models/Architectures

  • Models frequently have different layers of abstraction
    • Goal of layering: Reduce complexity
      • Each layer offers services to higher layers
        • Semantics: What does the layer do?
      • Layer interface defines how to access its services from higher layers
        • Parameters and results
        • Implementation details are hidden
        • (Think of class with interface describing method signatures while code is hidden)
  • Peer entities, located at same layer on different machines, communicate with each other
    • Protocols describe rules and conventions of communication
      • E.g., message formats, sequencing of events
  • Network architecture = set of layers and protocols

(Based on: [Tan02])

3.3 Protocol Layers

  • Each protocol instance talks virtually to its peer

    Layered Communication in OSI Model

    Layered Communication in OSI Model” by Runtux under Public domain; from Wikimedia Commons

    • E.g., HTTP GET request from Web browser to Web server
  • Each layer communicates only by using the one below
    • E.g., Web browser asks lower layer to transmit GET request to Web server
    • Lower layer service accessed by an interface
  • At bottom, messages are carried by the medium

(Based on: [Tan02])

3.4 Famous Models/Architectures

  • ISO OSI Reference Model
    • Mostly a model, describes what each layer should do
      • But no specification of services and protocols (thus, no real architecture)
    • Predates real systems/networks
  • TCP/IP Reference Model
    • Originally, no clear distinction between services, interfaces, and protocols
      • Instead, focus on protocols
    • Model a la OSI as afterthought

(Based on: [Tan02])

4 Internet and OSI Models

4.1 OSI Reference Model

  • International standard
    • Seven layer model to connect different systems
      • Media Layers
        1. Sends bits as signals
        2. Sends frames of information
        3. Sends packets from source host over multiple links to destination host
      • Host layers
        1. Provides end-to-end delivery
        2. Manages task dialogs
        3. Converts different representations
        4. Provides functions needed by users/applications

OSI Model

OSI Model” by Offnfopt under CC0 1.0; from Wikimedia Commons

4.1.1 Drawing for OSI Model

Networking layers

Networking layers

Figure © 2016 Julia Evans, all rights reserved; from julia's drawings. Displayed here with personal permission.

4.1.2 Where are Top and Bottom?

  • In layered architectures, lower layers represent more technical details while higher layers abstract away details
    • E.g., in the OSI model the top layer (7) is the application layer, which does not care about technical communication details
  • The previous drawing does not follow that convention when showing layers, but implicitly assumes it anyways (layer 3 “ignores layers 4 and above”)

4.2 OSI Model on Internet

OSI Model

OSI Model” by Offnfopt under CC0 1.0; from Wikimedia Commons

  • Internet architecture involves following subset of OSI layers
    • Application layer
      • E.g., Web (HTTP), e-mail (SMTP), naming (DNS)
      • (Presentation and session omitted; part of application protocols)
    • Transport layer
      • E.g., TCP, UDP
    • Network layer
      • Unifying standard: Internet Protocol (IP; v4, v6)
      • Everything over IP, IP over everything
    • Data link layer
      • E.g., Ethernet, WiFi, cellular phone network, satellite link

4.3 Internet Standards

4.3.1 Internet Architecture

  • “Hourglass design”

    Internet Architecture with narrow waist

  • IP is focal point
    • “Narrow waist”
    • Application independent!
      • Everything over IP
    • Network independent!
      • IP over everything

4.3.2 IP, UDP, and TCP

  • IP (Internet protocol)
    • Offers best-effort host-to-host connectivity
      • Best effort: Try once, no effort to recover from transmission errors
      • Connection-less delivery of datagrams
  • Transport layer alternatives
    • UDP (User Datagram Protocol)
      • Extends IP towards best-effort application-to-application connectivity
        • Ports identify applications/processes (e.g., 53 for DNS)
        • Connection-less
    • TCP (Transmission Control Protocol)
      • Offers reliable application-to-application connectivity
        • Ports identify applications/processes (e.g., 80/443 for Web servers)
        • Full-duplex byte stream
        • Three-way handshake to establish connection
        • Acknowledgements and timeouts for retransmissions

4.3.3 Drawing on TCP

TCP basics!

TCP basics!

Figure © 2016 Julia Evans, all rights reserved; from julia's drawings. Displayed here with personal permission.

5 Internet Communication

5.1 IP Stack Connections

IP stack connections

IP stack connections” by Jens Lechtenbörger under CC BY-SA 4.0; based on work under CC BY-SA 3.0 by en:User:Kbrose and en:User:Cburnett by changing arrow labels; from GitLab

5.1.1 Drawing on MAC Addresses

What's a MAC address?

What's a MAC address?

Figure © 2016 Julia Evans, all rights reserved; from julia's drawings. Displayed here with personal permission.

5.1.2 Drawing of Packet

Anatomy of a packet

Anatomy of a packet

Figure © 2016 Julia Evans, all rights reserved; from julia's drawings. Displayed here with personal permission.

5.1.3 Typical Communication Steps (0/2)

  • Prerequisites

    • Internet communication requires numeric IP addresses
      • Lookup of IP addresses for human readable names via DNS
        • DNS is request-reply protocol
        • DNS client (e.g., the browser) asks DNS server for IP address of name, e.g., query for may result in
        • (And more)
    • LAN communication requires MAC addresses
      • MAC (media access control) address: Hardware address of network card, e.g., for Ethernet, WiFi
        • Typical format with hexadecimal digits: 02:42:fa:5c:4a:4a
      • Lookup of MAC addresses for IP addresses via ARP (Address Resolution P.)
        • Send ARP request into local network: “If you have IP addresses x, what is your MAC address?
        • ARP request is a broadcast: Sent to every device in LAN
        • Device that has IP address x replies with its MAC address

5.1.4 Typical Communication Steps (1/2)

  • Ex.: Send HTTP message M to host
    1. Perform DNS lookup for
      • Returns IP address
    2. Encapsulate M by adding TCP header
      • Source and destination TCP ports: Numbers that identify processes
        • Typically, destination port 80 for Web servers with HTTP (443 for HTTPS)
        • Random source ports for Web browsers
    3. Encapsulate TCP segment by adding IP header
      • Source and destination IP addresses
      • Demux key to indicate that TCP segment is contained

5.1.5 Typical Communication Steps (2/2)

  • Ex.: Send HTTP message M to host

    1. Perform DNS lookup for
    2. Encapsulate with TCP header
    3. Encapsulate with IP header
    1. Routing decision to determine IP address of next hop router
      • Returns IP address IPR within sender’s network
      • E.g., at my work, at home
    1. ARP lookup to determine MAC address for IPR
      • E.g., 0:0:c:7:ac:0
    1. Encapsulate IP datagram with LAN-specific header with MAC address, send via LAN to router
    • Routers repeat steps (4) - (6) to forward M to final destination

5.2 Encapsulation

Sample encapsulation of GET request

5.3 Encapsulation and Demux Keys

  • Encapsulation
    • Protocol specific header added for each layer
      • Starting from “pure” application message
      • Headers prepended when moving down the protocol stack
    • Headers “unwrapped” when moving up again
  • Demux key
    • Identifies recipient protocol at next higher layer
    • Different protocols use different forms of demux keys (see previous slide)
      • Ethernet header contains type field (IPv4 = 0x0800, ARP = 0x0806)
      • IP header contains protocol field (TCP = 6, UDP = 17)
      • TCP header contains port (application id) as demux key

5.4 Review Questions

6 Conclusions

6.1 Summary

  • Computer networks are general purpose networks
    • The Internet forms the backbone for modern communication and collaboration
  • Complexity reduced via layered architecture
    • Modular design
    • Internet vs OSI architecture
    • Encapsulation and demux keys


License Information

This document is part of a larger course. Source code and source files are available on GitLab under free licenses.

Except where otherwise noted, the work “The Internet”, © 2018-2021 Jens Lechtenbörger, is published under the Creative Commons license CC BY-SA 4.0.