Chapter 58 Intro

• He provides his definition of internet
  • A internet connects different computer networks, allowing hosts on all of the networks to communicate with one another.
  • A subnet is one such network.
  • This means that any host on any of the connected subnetworks has the potential to contact any other host on the connected networks with a single address.
• Request for Comment
  • This is a set of documents that describe how the the internet is/can/should/might be built.
  • Some RFCs are adopted by the Internet Engineering Task Force to become internet standards.
  • Others are only for amusement A Standard for the Transmission of IP Datagrams on Avian Carriers".
  • These contain the "official" standards for the internet.
  • And are worth getting to know.
• Framework
  • There have been multiple networks designed.
    • Most of us have suffered the OSI model.
  • The current standard is TCP/IP
• TCP/IP
  • Was a ARPA/DARPA project in the Early 1970s.
  • Is a series of protocols at different layers.
  • Different pieces of hardware/software handle the transmission of data.
• Generally a host is located on a single network.
  • A host that is connected to two networks is multihomed
    • It has a connection to each network.
    • It is able to communicate in the lower level protocols of each network.
    • It has at least two addresses.
  • A special multihomed task is that of a router
    • This device is responsible forwarding packets between the networks to which it is connected.
    • But it must filter, block, ....
      • Yes, modern routers do this, but the primary function is to transfer packets.
• TCP/IP
  • This is a multi layer model
  • 
    1. Each physical network has a protocol.
       • What is a 0/1 in terms of voltage, duration, ....
       • How to tell if a collision has occurred
       • This is probably a more physical type thing.
       • And might be over wire, fiber optic cable, radio waves, even pigeons.
    2. Then there is the data link layer
       • This is the network card and the driver(s) that goes with it.
       • It uses the underlying hardware, but adds some logical organization to it (like frames)
       • Generally data is transferred in terms of frames.
       • It can perform error detection, retrransmission, flow control.
       • This is not a physical connection, but a logical level.
       • But it is very close to the hardware.
       • We probably don't have to worry about this as programmers.
    3. The IP layer is a networking layer.
       • This is a get packets to the right place layer.
       • Hosts have an IP address.
       • Networks via routers transfer packets from one address to another.
       • Packet delivery is unreliable and connectionless
       • Packets have a maximum size, so data from a higher level may be fragmented.
       • Packets are encapsulated or padded with possible headers and footers.
    4. The Transport layer (TCP, UDP, ...)
       • This is a host to host communication layer.
       • Ideas such as sessions/connections and reliability are implemented here.
       • TCP supports ports.
    5. Application Layer (http, ftp, ssh, ...)
       • This is per application.
       • You must design how messages will be sent, what are the borers, what should I receive.
  • Notice in the picture, it looks like logically each layer is "talking" to its corresponding level on the other machine.
    • This is not true.
    • We pass things down through the layers and back up for real
    • 
  • And we employ encapsulation
    • 
  • I like these pictures from wikipedia as well
    • 
    •