structure.txt.svn-base

网络模拟器

  How to build a network for simulation in JNS
  Christian Nentwich ([e-mail])
  06/03/1999

  This file describes how to build the static structure of a network
  (nodes, links, etc.) from the classes available in JNS.
  ______________________________________________________________________

  Table of Contents


  1. Prerequisites

  2. Introduction

  3. Nodes

  4. Interfaces

     4.1 SimplexInterface
     4.2 DuplexInterface

  5. Links

     5.1 SimplexLink
     5.2 DuplexLink

  6. An Example

  7. The whole example code

  8. Additional Comments



  ______________________________________________________________________

  1.  Prerequisites


  Before reading this document, you should ask yourself the following
  questions:

  o  Does the term "subnet" mean anything to you? Are you scared of the
     word "netmask" ?

  o  Can you tell a "router" from a "host"? Is a "host" someone who
     provides hospitality?

  o  Does the expression public static void main(String args[]) look
     meaningful?

  o  Does MTU mean "Military Telecommunications Unit"?


  If the answer to any of those is "yes", you will probably have to do
  some reading before you can use JNS. You will have to read up on the
  IP protocol, especially subnetting, the Java programming language
  and/or basic networking terms.



  2.  Introduction

  This document will describe how you can build the network you want to
  simulate in JNS. It is concerned with the static structure only, not
  with dynamic things such as tracing or sending packets. Consequently,
  when functions of several classes are described, those concerned with
  data transfer are left out.


  The functions presented here are not recommended for "daily use". JNS
  was designed with many options in mind, thus you have quite a bit of
  setup work to do in order to build your network. If you want to just
  quickly build a network without much customisation you are strongly
  advised to write some code that will take away most details (For
  examples, a node together with an interface could be built
  automatically and returned).


  Again, this file describes how to build a network from the basic
  building blocks:

  o  Nodes - think of this as computers. The box on your desk or under
     your desk.

  o  Interfaces - the equivalent of network cards, modems, you name it.

  o  Links - the cable that runs out of your network card into the next
     one, your phone-line, etc.. Note, however, that links are between
     two interfaces, not like an Ethernet cable that can connect more
     than two computers.


  Each of these will be described in detail in the next sections. Nearly
  all of them can be attached to each other. As a reminder from here on,
  JNS has a uniform naming scheme for attaching elements to others, the
  function to do this is always called 'attach', regardless of what is
  attached to what.



  3.  Nodes


  Nodes represent a computer. The class that implements a node is
  called..  Node. In JNS, a node contains a couple of essential
  components:

  o  Zero or more Interfaces - Any computer needs an interface to a
     network to participate. You can have zero interfaces if you find a
     use for it.

  o  One IP Handler object - you will not see this object if you do not
     look for it actively. It takes care of sending and receiving IP
     packets and dispatching them to higher level protocols. There is
     only one of those in each node and it is attached to every
     interface. It also contains the routing table.

  o  A name - be imaginative ;)


  That's about all the elements a node contains. The functionality
  provided by nodes includes:

  o  Attaching Interfaces - the most obvious functionality needed.

  o  Adding a routing table entry - you can add a route to a subnet.

  o  Adding a default route - the route to use if no other routes seem
     to match.

  It is worth pointing out that nodes are not particularly useful in any
  simulation, there is nothing going in on inside that class. The only
  purpose it has is to hold together its components, so the actual class
  looks pretty boring.


  The first thing you want to do when creating your network is create a
  couple of Node objects to add interfaces to later.



  4.  Interfaces


  An interface lets a node connect to a link, i.e. it is a bit like a
  network card. The class Interface is actually an abstract class, so
  you cannot use it when building your network. Instead, you have to
  choose between SimplexInterface and DuplexInterface


  Those two classes share the important functionality you need, so it is
  described here. An interface (regardless of whether it is simplex or
  duplex) consists of:

  o  An IP address - a globally unique IP address by which this
     interface can be identified.

  o  A bandwidth - the bandwidth is given in bps. Most of the time, you
     will prefer to assign a bandwidth to a link and let all interfaces
     that connect to the link inherit the bandwidth.

  o  A reference to an IP handler - this is invisible to you. The
     interface knows which handler gets the packets that arrive.

  o  A queue - incoming and outgoing packets will be held in a queue
     until they can receive further treatment.

  o  A maximum transfer unit (MTU) - the maximum packet length in bytes
     of a packet this interface can send. Defaults to 1500. Override if
     you want to see some IP fragmentation.


  There are several functions provided by all interfaces which are used
  almost all of the time.

  o  Attaching a link - You will need this after creating a link when
     you want to connect it to an interface. You can choose here whether
     the interface will take its bandwidth from the link.

  o  Attaching a queue - An interface contains a queue for packets. You
     can override the default queue here. The behaviour differs between
     SimplexInterface and DuplexInterface, check in the relevant
     sections.


  4.1.  SimplexInterface

  A SimplexInterface is a unidirectional interface, which means it can
  either be a sender or a receiver, but not both. This class is used
  heavily internally but it is generally not very useful for simulation.
  Note that you can only attach a SimplexLink to a SimplexInterface.


  Simplex Interfaces contain a queue. By default, when a simplex
  interface is created, a drop-tail queue is attached to it without any
  further interaction from you. If you decide to attach your own queue
  (e.g. a RED queue you implemented) this default queue will be
  overwritten.


  4.2.  DuplexInterface

  The DuplexInterface is much more like your network card. It really
  consists of two simplex interfaces internally, but most of the time
  you can ignore this. It is made of by a sender and a receiver
  interface.


  Note that attaching a queue to this interface works a bit differently.
  If you choose to do this, the clone() method of the queue you are
  attaching will be called and the two queues will be attach to the two
  simplex interfaces.



  5.  Links

  Setting up links is the last step in building a network. Similar to
  the Interface class, the Link class is abstract so you cannot
  instantiate it. You have to make a choice between SimplexLink and
  DuplexLink.


  Links in JNS have the following common characteristics:

  o  A Bandwidth - the bandwidth of a link is specified in bytes per
     second (bps). You should know the bandwidth you want to use.

  o  A propagation delay - this value specifies how long it takes for
     one bit to travel from one length of the link to the other. You
     will normally either know the propagation delay or calculate it
     from 2/3 the speed of light (around the speed of an electron going
     down a wire) and the length of the link.

  o  An error rate - This is a value between 0 and 1. 0 means error free
     and 1 means "ruins everything". JNS will generate a random variable
     and if it is smaller than this value, it will corrupt the CRC of
     the current packet the link is sending.


  5.1.  SimplexLink


  Most of the time, you will not use simplex links, so they are not very
  interesting (although all processing is done internally using simplex
  links anyway). If you do decide to use one, keep in mind that it can
  only attach to a SimplexInterface.


  5.2.  DuplexLink


  A duplex link encapsulates two simplex links and can be attached to a
  DuplexInterface. There is not much more to say about it but this is
  definitely the kind of link you should use if you want to be sane
  after laying out your network.






  6.  An Example

  We will now provide some annotated example code that shows how to
  create a static network structure in JNS. We will create a simple
  network of three nodes: Two hosts joined by a router. The way to
  proceed is to show bits of code and explain them. What all steps will
  have in common is that every element that is generated has to be
  attached to the simulator.


  Step 1: Obtain a reference to the Simulator

  ______________________________________________________________________
      Simulator sim=Simulator.getInstance();
  ______________________________________________________________________



  Note that only one instance of the simulator can ever exist. That's
  why the constructor is private and you have to use this method to
  obtain the singleton instance.


  Step 2: Create the three nodes (the code should be self-explanatory)
  and attach them to the simulator:

  ______________________________________________________________________
      Node src=new Node("Source node");
      Node router=new Node("Router");
      Node dest=new Node("Destination node");

      sim.attach(src);
      sim.attach(router);
      sim.attach(dest);