Notes On Chapter Twenty -- Internetworking: Concepts, Architecture, and Protocols

• 20.1 Introduction
  
  
  • Motivation
  • Hardware Used
  • Architecture
  • Significance
  
  
• 20.2 The Motivation for Internetworking
  
  
  • There are different kinds of networks for different needs.
  
  
• 20.3 The Concept of Universal Service
  
  
  • It can be difficult or impossible to figure out ways for two computers to communicate when they each belong to a different and incompatible type of network.
  • However, people naturally want universal service - they want service that allow any pair of computers to communicate.
  • Part of the reason why universal service is desirable is because it enables people to be more productive.
  
  
• 20.4 Universal Service in a Heterogeneous World
  
  
  • One cannot join incompatible networks simply by connecting media together. Even if the media used are the same, there will be other incompatibilities such as differing frequencies, voltages, and coding techniques.
  • Nor is it enough to employ bridge technology to connect incompatible networks. Bridges work by forwarding frames, and frame formats vary dramatically from network to network.
  
  
• 20.5 Internetworking
  
  
  • By using a combination of hardware and software, one may form a network of networks - an internetwork or internet.
  
  
• 20.6 Physical Network Connection with Routers
  
  
  • The basic unit of hardware used to form an internet is a router.
  • A router has some characteristics in common with a bridge:
    • A router contains a CPU and memory - so like a bridge it is a type of special-purpose computer.
    • A router has connections to two or more networks.
    • The interfaces used in a router to connect to networks are ordinary network interfaces - they are just like the interfaces that all other computers use to connect to that type of network.
    
    
  • Typically a bridge connects two network segments that are exactly the the same type of network.
  • Unlike a bridge, a router is typically connected to two or more networks that are NOT the same type of network. For example, a router might connect:
    • an Ethernet to a WAN, or
    • an Ethernet to a WiFi network, or
    • a WiFi network to an IBM Token Ring network.
  
  
• 20.7 Internet Architecture
  
  
  • The job of a router is to forward packets. To forward a packet, a router receives a packet on one of its network interfaces and transmits it out over another of its network interfaces.
  • A router has to process information about a packet in its CPU to decide on which interface to use to transmit the packet.
  • The more interfaces a router has, the more incoming packets it may be required to process per unit of time.
  • To keep their workload manageable, routers are usually built with a fairly small number of network interfaces.
  • It's good to build an internet with redundant links and routers - so that the network is not likely to be partitioned if just a handful of routers and/or links fail.
  • Internet architects must consider:
    • need for reliability
    • need for channel capacity
    • cost constraints
    • expected traffic
    • expected performance characteristics of available router hardware
  
  
• 20.8 Achieving Universal Service
  
  
  • The job of an internet is to allow any pair of computers to communicate.
  • Therefore the system of routers has to assure that packets are forwarded all the way from the source to the destination.
  • How can this be done, when the frame formats and addressing techniques on the various networks are hopelessly incompatible?
  • It can be done through the use of protocol software.
  
  
• 20.9 A Virtual Network
  
  
  • Through the use of routers and Internet protocol software the Internet creates a virtual network that overlays the physical networks it joins together.
  • Users and applications programs are able to ignore the physical networks, and just work with functionality provided by the virtual network.
  
  
• 20.10 Protocols for Internetworking
  
  
  • The protocol family utilized to construct the Internet is the TCP/IP protocol suite.
  
  
• 20.11 Review of TCP/IP Layering
  
  
  • The IP layer is layer 3 in the TCP/IP networking model.
  • IP specifies Internet packet format and mechanisms for forwarding packets.
  • TCP is layer 4. It deals with the messages and procedures that ensure reliable transfer.
  
  
• 20.12 Host Computers, Routers, and Protocol Layers
  
  
  • A host computer (also known simply as a host) is a computer that connects to the Internet and runs applications.
  • The design of the Internet requires that host computers and routers execute TCP/IP protocol software.