Computer Networks and the Internet

References:

1. James Kurose and Keith Ross, "Comptuer Networking - A Top-Down Approach", Sixth Edition, Chapter 1

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1.1 - What is the Internet?

• Largest engineered structure on ( around ) the planet.
• May be viewed as edges ( applications, user interfaces, end-point connections ) connecte by a core ( underlying structure providing ( long distance ) communication services. )
• TCP/IP are the two most important protocols on the Internet.
• The Internet Engineering Task Force, IETF maintains the database of RFCs, Requests for Comments that define all network protocols.
  • Sample: Hypertext Transfer Prococol -- HTTP/1.1

1.2 - The Network Edge

1.3 - The Network Core

1.4 - Delay, Loss, and Throughput in Packet-Switched Newtorks

• Four types of delay:
  • Processing delay, as the router checks headers, determines outgoing route, etc.
  • Queueing delay, as packets wait their turn on the transmission medium.
  • Transmission delay, limited by the speed at which the router can put bits on the "wire".
  • Propagation delay, limited by the speed at which bits travel down the "wire".
• Any one of these types may be the limiting factor in different circumstances.
• Total delay at a node is the sum of all four delay types. Overall end-to-end delay is the sum of all delays over all nodes on the path from source to destination.
• If the incoming rate of data exceeds the capacity of a node to process, transmit, and queue it, then packets may be dropped ( ignored ) by the node.
  • Dropped packets may be re-transmitted, either by the immediately preceding node or by the original sender of the message. This unfortunately can add to the overall congestion on the network, and actually make things worse in some cases.
• Other delays may be introduced, by the application or by any of the intermediate layers. ( e.g. ALOHA protocol. )

1.5 - Protocol Layers and Their Service Models

1.6 Networks Under Attack

1.7 History of Computer Networking and the Internet

Precursors to the World Wide Web

• File Transfer Protocol, FTP - Basic utility still used for transferring files from one computer to another, Originally required a login account and password on both computers in order to access files.
• Anonymous FTP - Allows access to selected repositories of files without requiring an account or password.
  • First source of "publicly" available information on the Internet. ( Though Internet access itself was still restricted at that time. )
  • User would enter "anonymous" or "ftp" as the user name at the login prompt. Courtesy called for using your e-mail address as the password, but anything would work.
  • For security purposes, anonymous ftp servers used "chroot" to re-define the "root" of the directory tree, thereby eliminating any possibility of anonymous access to files other than those intended to be publicly available.
  • Still used today ( e.g. in EVL for sharing files too large to be e-mailed. )
• Gopher - Text & command-line version of today's modern WWW.
  • Developed at the U of MN. ( Gopher mascot )
  • Users would connect to gopher servers using a text-based interface, with a command-line prompt for requesting files or directories.
  • Gopher sites were linked, and users could follow the links with the appropriate menu selections. Users would bounce around the Internet and around the world, often without knowing what computers they were connected to, much like today's GUI-based WWW.
  • The Internet was still restricted to non-commercial use and limited access, so gopher sites were typically information repositories, e.g. universities and government public information.