1. Introduction to the Internet

1. Computer Networks
2. The Internet
3. Circuit Switching
4. Packet Switching
5. Packet Switching Example
6. Brief History of the Internet
7. Communication Protocols
8. Protocols and Layering
9. TCP/IP 5-layer Reference Model
10. TCP/IP layers with some protocols
11. Data Passing Through Layers
12. Headers and Layers
13. Internet Communication Paradigms
14. Connection-Oriented Communication
15. Designing Applications
16. Client-Server Model
17. Client Software
18. Server Software
19. Server Identification
20. IPv6 Addresses
21. Service Identification
22. TCP Demultiplexing
23. A specific example
24. Links to more information

1.1. Computer Networks

• computer networks are everywhere
• they form an essential part of our infrastructure
• used
  • at home
  • at work
  • by governments
  • on the move
  • ...
• there are many different types of networks and standards
• we will concentrate on the Internet

1.2. The Internet

1.3. Circuit Switching

• early communication networks evolved from telephone systems
• used physical pair of wires between two parties to form a dedicated circuit
• circuit switching was the task of deciding which circuit to use when two parties wanted to communicate
• the circuit is reserved for the two parties during communication
• so it is not available to other parties

1.4. Packet Switching

• the Internet uses packet switching which is considered more efficient
• packet switching
  • divides data into small blocks, called packets
  • allows multiple users to share a network
  • includes identification of the intended recipient in each packet
  • routers each have information about where to send each packet next

1.5. Packet Switching Example

1.6. Brief History of the Internet

• (1957) Advanced Research Projects Agency (ARPA) established by US Department of Defense
• (1968-9) first packet-switching networks
• (1972) Telnet
• (1973) File Transfer Protocol (FTP); ARPANET goes international:
  • University College, London (UK)
  • Royal Radar Establishment (Norway)
• (1974) design of TCP (Transmission Control Protocol)
• (1977) email
• (1982) TCP and IP (Internet Protocol) used for ARPANET
• (1984) DNS (Domain Name Service) introduced
• (1991) WWW released

1.7. Communication Protocols

• communication always involves at least two entities
  • one that sends information and another that receives it
• all entities in a network must agree on how information will be represented and communicated
  • the way that electrical signals are used to represent data
  • procedures used to initiate and conduct communication
  • the format of messages
• all communicating parties follow the same set of rules, a set of specifications
• a specification for network communication is called a communication protocol

1.8. Protocols and Layering

• computer networks are complex systems including both hardware and software
• rather than a single, huge specification for all possible forms of communication, designers divide the communication problem into subparts, called layers
• the interfaces between the layers are defined by protocols
• layers provide for modularity, making implementation and changes easier
• the combination of layers is sometimes called a protocol stack

1.9. TCP/IP 5-layer Reference Model

1.10. TCP/IP layers with some protocols

1.11. Data Passing Through Layers

1.12. Headers and Layers

1.13. Internet Communication Paradigms

• Internet supports two basic communication paradigms:
  • stream paradigm
  • message paradigm

• we will focus mostly on the stream paradigm

1.14. Connection-Oriented Communication

• the Internet stream service is connection-oriented
• two applications must request that a connection be created
• by contrast, connectionless communication allows messages to be sent at any time
• once it has been established, the connection allows the applications to send data in either direction
• each pair of applications has its own (logical) connection
• finally, when they finish communicating, the applications request that the connection be terminated

1.15. Designing Applications

• networked applications follow a small number of design patterns
• two most common are client-server and peer-to-peer
• in client-server, there is usually a single server and many clients
• in peer-to-peer (e.g. Skype, BitTorrent) there is no single server
• we will focus on client-server

1.16. Client-Server Model

1.17. Client Software

• is an arbitrary application program that becomes a client temporarily when remote access is needed, but also performs other computation
• is invoked directly by a user, and executes only for one session
• runs locally on a user's personal computer
• actively initiates contact with a server
• can access multiple services as needed, but usually contacts one remote server at a time

1.18. Server Software

• is a special-purpose, privileged program
• is dedicated to providing one service that can handle multiple remote clients at the same time
• is invoked automatically when a system boots, and continues to execute through many sessions
• runs on a large, powerful computer
• waits passively for contact from arbitrary remote clients
• accepts contact from arbitrary clients, but offers a single service

1.19. Server Identification

• Internet protocols divide identification into two pieces:
  • an identifier for the computer on which a server runs
  • an identifier for a service on the computer
• identifying a computer
  • each computer on the Internet is assigned a unique identifier known as an Internet Protocol address (IP address)
    • for IPv4, this is a 32-bit quantity
    • for IPv6, this is a 128-bit quantity
  • 4 bytes of an IPv4 address are written as n1.n2.n3.n4 where each ni is a decimal number, e.g., 18.23.0.22
  • a client process must specify the IP address of the machine on which the server process is running
  • to make server identification easy for humans, each computer is also assigned a domain name
  • the Domain Name System (DNS) is used to translate a name into an IP address (see later)
  • so a user specifies a name such as www.dcs.bbk.ac.uk rather than an IP address

1.20. IPv6 Addresses

• IPv6 addresses are 128-bits
• conventional notation is a series of (up to 8) blocks or fields of 4 hexadecimal numbers each
• example is: 5f05:2000:80ad:5800:0058:0800:2023:1d71
• easy to convert hexadecimal numbers to binary
• there are a number of standardised simplifications:
  • leading zeroes of a block can be omitted, so 0058 can be written as 58
  • blocks of all zeroes can be replaced by ::, so 0:0:0:0:0:0:0:1 can be written as ::1
  • an IPv4-compatible IPv6 address can be written as :: followed by an IPv4 address in dotted decimal notation

1.21. Service Identification

• each service available on the Internet is assigned a unique 16-bit identifier
• this identifier is known as a protocol port number (or port number), e.g.,
  • (sending) email uses port number 25 by default
  • the web uses port number 80 by default
• when a server process begins execution
  • it registers with its local OS by specifying the port number for its service
• when a client process contacts a computer to request a service
  • the request contains the port number for the service
  • as well as the IP address for the computer
• when a request arrives at a server computer
  • TCP/IP software uses the port number in the request to determine which application (process) should handle the request

1.22. TCP Demultiplexing

1.23. A specific example

• suppose I want to retrieve a web page from www.w3.org
• my browser will use DNS to find the IP address 128.30.52.37
• my browser will compose a message based on HTTP asking to get the page
• HTTP will ask TCP to connect to port 80 on 128.30.52.37
• TCP will ask IP to send the message to 128.30.52.37
• IP will send the message to a router on the local network
• this router will send the message to another router
• ...
• the router on the local network for 128.30.52.37 will receive the message
• it will send the message to 128.30.52.37
• IP will receive it and pass it up to TCP
• TCP will see that it is for port 80 and will pass it to the web server process
• the web server will interpret the HTTP and send the page to my browser

1.24. Links to more information

• Brief History of the Internet
• The web resources for Tanenbaum's Computer Networks book, Chapter 1
• Wikipedia articles on the Internet, packet switching and IP addresses.