Notes On Chapter Seventeen -- Internetworking: Concepts, Architecture, And Protocols

• 17.1 Introduction
  
  
  • This chapter is about how networks can be joined together into an internet.
    
    
• 17.2 The Motivation For Internetworking
  
  
  • It is natural that there will be very different networks for different needs.
    
    
  • It is also natural that people want to inter-connect these different networks -- no one wants to be an "island" all of the time.
    
    
• 17.3 The Concept Of Universal Service
  
  
  • Workers are more productive if they can achieve all "connectivity" using a single computer.
    
    
  • The concept of universal service is to allow any two computers to communicate.
    
    
• 17.4 Universal Service In A Heterogeneous World
  
  
  • Problem: different WAN's have incompatible packet formats, addressing schemes, and protocols.
    
    
  • It takes more than splicing wires together or the use of bridges to connect incompatible networks.
    
    
• 17.5 Internetworking
  
  
  • On the other hand, people can and do create hardware and software tools that allow them to connect diverse networks together.
    
    
  • An internetwork or internet is a heterogeneous set of inter-connected networks -- different kinds of networks all connected together.
    
    
• 17.6 Physical Network Connection With Routers
  
  
  • Two different kinds of networks can be joined by connecting each of them to a special kind of packet switch called a router.
    
    
  • A router is like a bridge in that each network connection is a standard NIC -- the kind that would be used by any other computer on the same network.
    
    
  • A router is unlike a bridge in that it often connects two or more completely different kinds of networks -- like a fast Ethernet and a Token Ring.
    
    
• 17.7 Internet Architecture
  
  
  • An internet is a bunch of networks connected by routers.
    
    
  • The layout of the connections is chosen to achieve an adequate tradeoff among performance, reliability, and cost.
    
    
  • Redundant connections are desirable to improve performance and reliability.
    
    
    • The job of a router is CPU-intensive so it should not be burdened with having a large number of network connections.
      
      
    • There should be enough extra connections so that the network will not be partitioned if just a few routers fail.
      
      
• 17.8 Achieving Universal Service
  
  
  • Besides routers the other major ingredient required for building an internet is internet protocol software.
    
    
  • Internet protocol software is discussed in detail later.
    
    
• 17.9 A Virtual Network
  
  
  • Through the use of software a collection of dissimilar networks can be made to appear to be one large network offering universal service with a common interface to all users -- a virtual network.
    
    
• 17.10 Protocols For Internetworking
  
  
  • TCP/IP is the standout protocol family for internetworking.
    
    
  • However other protocols have been used.
    
    
  • TCP/IP was first and was developed by the same group that created the internetworking concept.
    
    
  • The original work on TCP/IP was funded by the (Defense) Advanced Research Projects Agency (DARPA) and then by the National Science Foundation (NSF) and other U.S. government agencies.
    
    
• 17.11 Significance Of Internetworking And TCP/IP
  
  
  • The great success of the Internet has influenced network software companies to make products that are compatible with internetting.
    
    
  • For example it is very common now for computers to "speak" more than one family of networking protocols.
    
    
• 17.12 Layering And TCP/IP Protocols
  
  
  • The TCP/IP layering model is:
    
    
    • Layer 5: Application: Each layer 5 protocol specifies how one application uses an internet.
      
      
    • Layer 4: Transport: Specify how to ensure reliable transfer.
      
      
    • Layer 3: Internet: Specifies format of internet packets and rules for forwarding internet packets from router to router.
      
      
    • Layer 2: Network Interface: specify format of LAN frames and transmission details.
      
      
    • Layer 1: Physical Layer: basic level of bit-by-bit transmission.
      
      
  • (Parts of) layers in the TCP/IP stack are similar to (parts of) layers in the OSI stack:

    TCP/IP                       OSI	  
    Application               Application, Presentation
    Transport                 Transport
    Internet                  Network
    Network Interface         Data Link
    Physical                  Physical

  • The ISO stack does not have an Internet Layer, but the ISO Network Layer is similar to parts of the TCP/IP Internet Layer.
    
    
  • The TCP/IP stack does not have a Session Layer, but parts of the TCP/IP Transport and Application Layers contain some of the functionality of a Session Layer.
    
    
• 17.13 Host Computers, Routers, And Protocol Layers
  
  
  • A host is anything that is connected to the internet and runs applications software.
    
    
  • Most routers are dedicated routers. A dedicated router is not a host. It does not run applications software. It only performs routing tasks.
    
    
  • To make communication across the internet possible, all routers and hosts must run internet protocol software.
    
    
  • Dedicated routers do not need layer 5 software.
    
    
  • Hosts usually must run software at all five levels.
    
    
• 17.14 Summary