• Basic principles of Engineering:
    • Reproducibility (do the same thing more than once),
    • Measurability (define what is same),
    • Portability (as a result of the above),
    • Standardization (formal or industrial),
    • Documentation (what else is needed for reproducibility),
    • Organization (no randomness).
  • Apply these principles in Software Engineering.

Computer networks

  • Systems for transmitting data between computers
  • Layered architecture (different approaches):
    • Physical level – hardware, wires, wireless
    • Logical level:
      • Network organization and topology
      • Addressing and communication protocols
    • Application level:
      • Services and applications offered to end-users


  • Client-Server is a relation between two subjects.
  • Server is passive (serves), fulfills client requests.
  • The relation can be observed on many levels:
    • Relation between entire Systems,
    • Relation between application parts, computer processes.
  • Examples:
    • Web Browser is a client to a Web Server,
    • Web Server is a client to a Database Server,
    • Web Browser is a server to an end user,
    • Computer is a server to an end user.
  • Opposite: P2P architecture.

Computer networks

  • Need a communication protocol (set of rules).
  • In applications we are not interested in the physical layer.
  • A computer network from an application viewpoint:

A random network schema

Internet protocols

  • Most networks use protocols from the TCP/IP protocol suite.
  • Internet is every network using the TCP/IP protocol suite.
    • The Internet vs. internet vs. intranet
  • IP (Internet Protocol) – A basic transmission protocol (packets).
  • TCP (Transmission Control Protocol) – A reliable protocol.
  • UDP (User Datagram Protocol) – An unreliable protocol.
  • Dozens of protocols for network services (DNS, DHCP, HTTP, SMTP, SSH, …).
  • Internet standards are marked as RFC (industry standards).

Internet Addresses

  • Hardware address (MAC address) – unusable in the application layer:
    • Assigned by the device manufacturer,
    • Reasonably worldwide unique.
  • Logical address (IP Address) – most important for transmission:
    • Assigned by the network administrator,
    • Respects topology of the network and can be used to find the physical device location.
  • Name address (Domain Name) – used by end-users only:
    • Assigned through DNS (service/server/system),
    • Must be translated (M:N) to an IP Address for transmission.

Addressing hosts

  • For communication between two hosts, the logical addresses of both devices must be known.
  • Domain names must be translated to logical addresses.
  • One host may run multiple applications, a socket must be used to distinguish between them.
  • For communication between applications, both the source and destination socket must be known.
  • Socket = IP Address + Port.
  • Port is a numeric identifier of the application (1-65535).

Internet Network Services

  • Network Services are various applications offered to the end-users of the internet network
  • Each service has a network protocol (and therefore the port) assigned to it:
    • Destination ports are well-known,
    • Source ports are ephemeral.

Connection example

  • HTTP protocol uses port 80:
    • apache.exe TCP server:80 server:0 LISTENING
    • opera.exe TCP pc19:4307 server:80 ESTABLISHED
    • apache.exe TCP server:80 pc19:4307 ESTABLISHED

Internet Address Format

  • URL (Uniform Resource Locator) is used by many services.
  • protocol://user:password@address:port/path?query
    • protocol – a registered protocol name (required),
    • user and password – optional credentials (discouraged),
    • address – either an IP address or a name address of the destination host (required),
    • port – a socket port (required),
    • path – a directory to the actual requested file,
    • queryname-value pairs optionally supplied to the requested file.

WWW Service

  • WWW (World Wide Web) Service is a system for providing linked text documents.
  • The service offers access to published documents – web pages.
  • Web pages can be:
    • static – must be manually changed on the server,
    • dynamic – generated by an application on the web server.
  • WWW service uses the HTTP (Hyper-text transfer) protocol for transmitting the web pages.
  • Web pages are usually created using the HTML language.

HTTP Protocol

  • HTTP is a stateless protocol:
    • Communication is split into transactions,
    • Each transaction creates a new HTTP connection,
    • Together with terminating the transaction, all state information is lost:
      • Theoretically this prevents someone from logging in, but there is a way around this.
      • Requests from a single client are isolated on the server and they are difficult to be connected together.
  • HTTP is a text protocol.

HTTP Transaction

  • An HTTP Transaction is composed of the steps:
    • Establish a connection (client),
    • Send a request (client),
    • Send a response (server),
    • Terminate the connection (server).
  • Both the request and the response consist of headers and body.
    • An HTTP Transaction fulfills a single HTTP request.
  • Many HTTP transactions are required to load a single page.

HTTP Transactions

HTTP Requests


  • What is the difference between the web and the internet?
  • How many different TCP/IP protocols do you use in a day?
  • How many HTTP request have you sent during reading these slides?
  • What use is an unreliable protocol?
  • Is it more important to know the protocol or the port?
  • Is it possible to download a page without creating a socket?
  • Can you visit a page by knowing just the MAC address of the server?
  • Can a server be a client?