Computer Networks and Systems

  • A computing device is a physical artifact that can run a program.
  • A computer network is a type of computing system
  • A path between two computing devices on a computer network (a sender and a reciever) is a sequence of directly connected computing devices that begins at the sender and ends at the receiver.
  • Routing is the process of finding a path from sender to receiver.
  • The bandwidth of a computer network is the maximum amount of data that can be sent in a fixed amount of time.
  • Bandwidth is usually measured in bits per second.

How Computers Are Connected

  • A computing system is a group of computing devices and programs working together for a common purpose.
  • A computer network is a group of interconnected computing devices capable of sending or receiving data.

Packets

  • A packet is a small amount of data sent over a network. Each packet also includes the source and the destination information.
  • The message (file) is broken up into packets and sent in any order. The packets are reassembled by the recipient’s device. (Packet Switching)

The Internet

  • The Internet is a computer network consisting of interconnected networks that use standardized, open (nonpropritary) communication protocols.
  • Access to the Internet depends on the ability to connact a computing dvice to an internet connected device.
  • Routing on the Internet is usually dynamic; it is not specified in advance
  • Information is passed through the Internet as a data stream. Data streams contain chunks of data, which are encapsulated in packets
  • Packets contain a chunk of data and metadata used for routing the packet between the origin and the destination on the Internet, as well as for data reassembly
  • Packets may arrive at the destination in order, our of order, or not at all
  • The World Wide Web is a system of linked pages, programs, and files
  • The World Wide Web uses the Internet

Protocols

  • A protocol is an agreed-upon set of rules that specify the behavior of a system.
  • The protocols used in the Internet are open, which allows users to easily connect additional computing devices to the Internet.
  • IP, TCP, and UDP are common protocols used on the Internet
  • TCP does error checking and error recovery so it is slower
  • UDP performs error checking, but it discards erroneous packets
  • HTTP is a protocol used by the World Wide Web

Internet and Transport Layers

  • Transport Layer - How the packets are sent, and for what
  • Reliability - reliable - sender gets a receipt back, resend as needed TCP
  • Unreliability - send and forget. UDP

Internet Layer

  • IPv4: Example 192.168.24.1 32 bits, separated into 4 “octets” 8 bits long (0-255)
  • Separated into 2 parts:
  • Network (1-31)
  • Device (32-network bits)
  • Three targets that can be addressed using IP
  • Unicast - A specific device. Internet wide access. TCP is used.
  • Multicast - A group of devices. It is specific range of IP addresses. Internet-wide access. UDP is used.
  • Broadcast - All devices. LAN-wide. Data stops at the router. UDP is used.

Application Layer

  • Key protocols used:

  • http. How to ask and receive data from web servers. Normally uses TCP, Port 80 at the transport layer
  • https. Like http but with security. Usually uses TCP Port 443

Web Servers:

  • Programs running on machines connected to the Internet
  • Provides clients web pages - data linked to other pages using URLs (Uniform Resource Locations)
  • The World Wide Web (WWW) is this network of linked data and programs, running over the Internet

DNS - Domain Name Service

  • Applications that translate a human readable URL (e.g. www.mycompany.com) to an IP address
  • DNS holds a database of mapping of names to IPs. Servers running DNS are spread across the internet
  • There are large computers serving and managing these name/IP Mappings for each Domain (.com, .net, .gov, etc.)
  • You use a DNS server every time you ask for a web page on a browser

Scalibility of the Internet

  • The scalability of a system is the capacity for the system to change in size and scale to meet new demands.
  • Able to change in size and scale to meet new demands

Local Area Network (LAN)

  • Physical Connections, limited by hardware
  • 1 to 100s of systems

Intranet:

  • LANs connected by Routers within an organization

Autonomous Systems (AS):

  • Large intranets linked together under the control and policies of major organizations. Large routers link networks with large telecommunications connections

The Internet consists of:

  • Autonomous Systems linked together
  • Large Routers linking Ass via special telecom means (Fiber, T3, Satellites)
  • Major infrastructures - DNS (.com, .net, etc.), Cyber Operations
  • Authentication measures such as:
  • Strong passwords
  • Multifactor authentication.

    Encryption

  • Encryption: the process of encoding data to prevent unauthorized access
  • Decryption: the process of decoding the data.
  • Two common encryption approaches are:
  • Symmetric key encryption involves one key for both encryption and decryption
  • Public key encryption pairs a public key for encryption and a private key for decryption.
  • The sender does not need the receiver’s private key to encrypt a message, but the receiver’s private key is required to decrypt the message
  • Certificate authorities issue digital certificates that validates the ownership of encryption keys used in secure communications and are based on a trust model.