@c ======================================================================== @c Other Network Topologies @c ======================================================================== @node Other-network-topologies @chapter Other Network Topologies @cindex topology @cindex Channel @cindex NetDevice @cindex topology!bus @cindex topology!point-to-point @cindex PointToPointChannel @cindex PointToPointNetDevice @emph{Network topology} is the study of the arrangement of of the elements (in @command{ns-3} represented by the classes @code{Channel} and @code{Node}) of a network. Two fundamental types of physical topologies are the @emph{point-to-point} and @emph{bus} topologies. We have already been exposed to the @command{ns-3} channel specialization named @code{CsmaChannel}. This is a simulation of a bus network. We also provide a simulation of a point-to-point channel with associated net devices. As described previously, the associated C++ classes specialize the @command{ns-3} base classes @code{NetDevice} and @code{Channel} and are called @code{PointToPointNetDevice} and @code{PointToPointChannel} respectively. We will use combinations of these bus and point-to-point topology elements to show how to create several commonly seen network topologies. @section A Point-to-Point Network We're going to take what might be seen as a step backward and look at a simple point-to-point network. We will be building the simplest network you can imagine. A serial link (point to point) between two computers. When you see this point-to-point network, you can think of an RS-422 (or RS-232 for you old-timers) cable. This topology is shown below. @sp 1 @center @image{pp,,,,png} We have provided a file for you in the @code{tutorial} directory called @code{point-to-point.cc}. You should now be familiar enough with the system to pick out fairly easily what has been changed. Let's focus on the following lines: @verbatim Ptr n0 = Create (); Ptr n1 = Create (); Ptr link = PointToPointTopology::AddPointToPointLink ( n0, n1, DataRate (38400), MilliSeconds (20)); PointToPointTopology::AddIpv4Addresses (link, n0, "10.1.1.1", n1, "10.1.1.2"); @end verbatim You can see that we created two @code{InternetNode} objects in the usual way. Then, instead of creating a @code{CsmaChannel} we create a @code{PointToPointChannel}. This point-to-point channel, which we call @code{link}, connects node zero (@code{n0}) and node one (@code{n1}) over a simulated link that runs at 38400 bits per second and has a 20 millisecond simulated speed-of-light delay. This call also creates appropriate net devices and attaches them to nodes zero and one. We then add IP addresses to the net devices we just created using the topology helper @code{AddIpv4Addresses}. Node zero gets the IP address 10.1.1.1 and node one gets the IP address 10.1.1.2 assigned. The alert tutorial user may wonder what the network number or prefix is of those IP addresses. The point-to-point topology assumes that you want a @code{/30} subnet and assigns an appropriate net mask for you. It then then @emph{asserts} that the network numbers of the two net devices match. So there is an implicit network mask created down in the topology code that looks like, @verbatim Ipv4Mask netmask("255.255.255.252"); @end verbatim The rest of the code you should recognize and understand. We are just going to echo one packet across the point-to-point link. You should be now be able to build and run this example and to locate and interpret the ASCII trace file. This is left as an exercise for you. The file @code{point-to-point.cc} is reproduced here for your convenience: @verbatim /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */ /* * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation; * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include "ns3/log.h" #include "ns3/ptr.h" #include "ns3/internet-node.h" #include "ns3/point-to-point-channel.h" #include "ns3/mac48-address.h" #include "ns3/point-to-point-net-device.h" #include "ns3/point-to-point-topology.h" #include "ns3/udp-echo-client.h" #include "ns3/udp-echo-server.h" #include "ns3/simulator.h" #include "ns3/nstime.h" #include "ns3/ascii-trace.h" #include "ns3/pcap-trace.h" #include "ns3/global-route-manager.h" NS_LOG_COMPONENT_DEFINE ("PointToPointSimulation"); using namespace ns3; // Network topology // // point to point // +--------------+ // | | // n0 n1 // int main (int argc, char *argv[]) { LogComponentEnable ("PointToPointSimulation", LOG_LEVEL_INFO); NS_LOG_INFO ("Point to Point Topology Simulation"); Ptr n0 = Create (); Ptr n1 = Create (); Ptr link = PointToPointTopology::AddPointToPointLink ( n0, n1, DataRate (38400), MilliSeconds (20)); PointToPointTopology::AddIpv4Addresses (link, n0, "10.1.1.1", n1, "10.1.1.2"); uint16_t port = 7; Ptr client = Create (n0, "10.1.1.2", port, 1, Seconds(1.), 1024); Ptr server = Create (n1, port); server->Start(Seconds(1.)); client->Start(Seconds(2.)); server->Stop (Seconds(10.)); client->Stop (Seconds(10.)); AsciiTrace asciitrace ("tutorial.tr"); asciitrace.TraceAllQueues (); asciitrace.TraceAllNetDeviceRx (); Simulator::Run (); Simulator::Destroy (); } @end verbatim @section A Star Network A point-to-point network is considered a special case of a star network. As you might expect, the process of constructing a star network is an extension of the very simple process used for a point-to-point link. We have provided a file for you in the @code{tutorial} directory called @code{star.cc} that implements a simple star network as seen below. @sp 1 @center @image{star,,,,png} In order to create a star network, we need to be able to instantiate some number (greater than one) of net devices on a node. In the name of simplicity of use, the @code{PointToPointTopology} topology helper does not allow one to do this. We provided a separate topology helper class, the @code{PointToPointIpv4Topology} helper class that provides the slightly finer granularity we need to accomplish a star network. In order to use this new helper we have to load the definitions by including the appropriate file. @verbatim #include "ns3/point-to-point-ipv4-topology.h" @end verbatim The star that we're going to create has a node in the center (@code{n0}) with six nodes surrounding (@code{n1} - @code{n6}). You should be able to easily find and understand the code that creates these nodes. @verbatim Ptr n0 = Create (); Ptr n1 = Create (); Ptr n2 = Create (); Ptr n3 = Create (); Ptr n4 = Create (); Ptr n5 = Create (); Ptr n6 = Create (); @end verbatim Next, we get into the differences between the @code{PointToPointTopology} helper and the @code{PointToPointIpv4Topology} helper. The @code{PointToPointIpv4Topology} helper looks and feels a little like the @code{CsmaIpv4Topology} helper. Just like you created a CSMA channel previously, you need to create a point-to-point channel. The following code creates a @code{PointToPointChannel} and calls it @code{link01}. You can interpret this name as being the channel (or @emph{link}) from node zero to node one. @verbatim Ptr link01 = PointToPointIpv4Topology::CreateChannel (DataRate (38400), MilliSeconds (20)); @end verbatim You need to provide a data rate for the channel which we set at 38400 bits per second. You must also provide a speed-of-light delay which we set at 20 milliseconds. Just as you added a net device to the nodes in the CSMA tutorial section, you do the same here but with a point-to-point net device. The following code illustrates how we do that: @verbatim uint32_t nd01 = PointToPointIpv4Topology::AddNetDevice (n0, link01); @end verbatim We call the @code{PointToPointIpv4Topology} helper and ask it to add a net device to node zero (@code{n0}) and connect it to the appropriate point-to-point link (@code{link01}) which you will recall is the serial link from node zero to node one. If you look at the following code, you will see the same calls are repeated to create the remaining five point-to-point channels and connect them to net devices on node zero. The next new code is found after the ``spokes'' of the star have been created. It looks like the following: @verbatim uint32_t nd1 = PointToPointIpv4Topology::AddNetDevice (n1, link01); uint32_t nd2 = PointToPointIpv4Topology::AddNetDevice (n2, link02); uint32_t nd3 = PointToPointIpv4Topology::AddNetDevice (n3, link03); uint32_t nd4 = PointToPointIpv4Topology::AddNetDevice (n4, link04); uint32_t nd5 = PointToPointIpv4Topology::AddNetDevice (n5, link05); uint32_t nd6 = PointToPointIpv4Topology::AddNetDevice (n6, link06); @end verbatim Here we are creating the net devices on the nodes surrounding the center node. In the first call, we are adding a net device on node one (@code{n1}) and connecting that net device to the channel named @code{link01}. Remember that we created the channel @code{link01} as the channel connecting node zero and node one. We previously created a net device on node zero and attached that device to @code{link01}. Here we are connecting the other side of that link to node one. The return value from this call is the net device index of the created net device. The next section of code adds addresses to the net devices we just created. The first call adds the IP address 10.1.1.1 to the net device going from node zero to node one. Recall that we first created a node named @code{n0} and a channel called @code{link01}. We added a net device to @code{n0} and remembered the net device index as the @code{uint32_t nd01}. This meant the net device @emph{nd} on node @emph{0} that we connected to node @emph{1}. We call @code{AddAddress} to add an IP address (10.1.1.1) to the net device on node zero identified by the net device index @code{nd01}. We provide a net mask suitable for a point to point network. This is typically a /30 address but we don't force that in this API. After setting up the address on node zero, we do the same for the node on the other end of the ``spoke'' --- in this case node one, with its single net device. Note that the network number is the same on both sides of this network. @verbatim PointToPointIpv4Topology::AddAddress (n0, nd01, "10.1.1.1", ``255.255.255.252''); PointToPointIpv4Topology::AddAddress (n1, nd1, "10.1.1.2", ``255.255.255.252''); @end verbatim The following code repeats this pattern assining similar IP addresses to the remaining net devices. Note that there are no @code{Mac48Address} address assignments --- they are not required. The rest of the code you should recognize and understand. We are just going to echo one packet across the point-to-point link. You should be now be able to build and run this example and to locate and interpret the ASCII trace file. This is left as an exercise for you. The file @code{star.cc} is reproduced here for your convenience: @verbatim /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */ /* * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation; * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include "ns3/log.h" #include "ns3/ptr.h" #include "ns3/internet-node.h" #include "ns3/point-to-point-channel.h" #include "ns3/mac48-address.h" #include "ns3/point-to-point-net-device.h" #include "ns3/point-to-point-ipv4-topology.h" #include "ns3/udp-echo-client.h" #include "ns3/udp-echo-server.h" #include "ns3/simulator.h" #include "ns3/nstime.h" #include "ns3/ascii-trace.h" #include "ns3/pcap-trace.h" #include "ns3/global-route-manager.h" NS_LOG_COMPONENT_DEFINE ("StarSimulation"); using namespace ns3; // Network topology // // n3 n2 // | / // | / // n4 --- n0 --- n1 // / | // / | // n5 n6 int main (int argc, char *argv[]) { LogComponentEnable ("StarSimulation", LOG_LEVEL_INFO); NS_LOG_INFO ("Star Topology Simulation"); Ptr n0 = Create (); Ptr n1 = Create (); Ptr n2 = Create (); Ptr n3 = Create (); Ptr n4 = Create (); Ptr n5 = Create (); Ptr n6 = Create (); Ptr link01 = PointToPointIpv4Topology::CreateChannel (DataRate (38400), MilliSeconds (20)); uint32_t nd01 = PointToPointIpv4Topology::AddNetDevice (n0, link01); Ptr link02 = PointToPointIpv4Topology::CreateChannel (DataRate (38400), MilliSeconds (20)); uint32_t nd02 = PointToPointIpv4Topology::AddNetDevice (n0, link02); Ptr link03 = PointToPointIpv4Topology::CreateChannel (DataRate (38400), MilliSeconds (20)); uint32_t nd03 = PointToPointIpv4Topology::AddNetDevice (n0, link03); Ptr link04 = PointToPointIpv4Topology::CreateChannel (DataRate (38400), MilliSeconds (20)); uint32_t nd04 = PointToPointIpv4Topology::AddNetDevice (n0, link04); Ptr link05 = PointToPointIpv4Topology::CreateChannel (DataRate (38400), MilliSeconds (20)); uint32_t nd05 = PointToPointIpv4Topology::AddNetDevice (n0, link05); Ptr link06 = PointToPointIpv4Topology::CreateChannel (DataRate (38400), MilliSeconds (20)); uint32_t nd06 = PointToPointIpv4Topology::AddNetDevice (n0, link06); uint32_t nd1 = PointToPointIpv4Topology::AddNetDevice (n1, link01); uint32_t nd2 = PointToPointIpv4Topology::AddNetDevice (n2, link02); uint32_t nd3 = PointToPointIpv4Topology::AddNetDevice (n3, link03); uint32_t nd4 = PointToPointIpv4Topology::AddNetDevice (n4, link04); uint32_t nd5 = PointToPointIpv4Topology::AddNetDevice (n5, link05); uint32_t nd6 = PointToPointIpv4Topology::AddNetDevice (n6, link06); PointToPointIpv4Topology::AddAddress (n0, nd01, "10.1.1.1", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n1, nd1, "10.1.1.2", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n0, nd02, "10.1.2.1", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n2, nd2, "10.1.2.2", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n0, nd03, "10.1.3.1", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n3, nd3, "10.1.2.2", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n0, nd04, "10.1.4.1", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n4, nd4, "10.1.4.2", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n0, nd05, "10.1.5.1", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n5, nd5, "10.1.5.2", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n0, nd06, "10.1.6.1", "255.255.255.252"); PointToPointIpv4Topology::AddAddress (n6, nd6, "10.1.6.2", "255.255.255.252"); uint16_t port = 7; Ptr client = Create (n0, "10.1.1.2", port, 1, Seconds(1.), 1024); Ptr server = Create (n1, port); server->Start(Seconds(1.)); client->Start(Seconds(2.)); server->Stop (Seconds(10.)); client->Stop (Seconds(10.)); AsciiTrace asciitrace ("tutorial.tr"); asciitrace.TraceAllQueues (); asciitrace.TraceAllNetDeviceRx (); Simulator::Run (); Simulator::Destroy (); } @end verbatim @subsection Routing If you are really excited about this simulator you may have already tried to modify the scripts outside the tutorial. I know that one of the first things that would have occurred to me when I saw the star network would have been to start trying to add applications to echo packets from nodes other than zero. If you tried, for example, to start the echo client on node one instead of node zero, you would have found an empty trace file. The reason for this is that you have now created an internetwork. This means you will need to enable internetwork routing. We have provided a file for you in the @code{tutorial} directory called @code{star-routing.cc} to show you how this is done. This extremely tricky and difficult change is shown below: @verbatim GlobalRouteManager::PopulateRoutingTables (); @end verbatim This one-line addition, located just before the simulation runs, tells the @command{ns-3} @emph{global route manager} to walk the topology you created and build internetwork routing tables for all of the nodes in the simulation. We changed the client application so that it runs on node four: @verbatim Ptr client = Create (n4, "10.1.1.2", port, 1, Seconds(1.), 1024); @end verbatim Now if you build and run @code{star-routing.cc} you can examine the @code{tutorial.tr} file and see that your UDP echo packets are now correctly routed through the topology. @section A Dumbbell Network One of the most interesting simple topologies (from a phenomenological point of view) is commonly called a dumbbell network. The name derives from a superficial similarity in form to a piece of exercise equipment. The dumbbell model is typically composed of two bus or star network elements connected via a point-to-point link. The point-to-point link is usually configured with a lower bandwidth than the bus elements to provide a @emph{choke point}. The following is a representation of the topology. @sp 1 @center @image{dumbbell,,,,png} We have provided a file that constructs this dumbbell network and creates enough data flowing across the choke point that some packets will be dropped. The file is called @code{linear-dumbbell.cc} and is located in the @code{tutorial} directory. We have already covered all of the code used to create this network, so we will just quickly go over the main sections of the script. The first section creates a CSMA lan that will become the left side of the dumbbell network. This code should be very familiar since we used the same process to create our first example. @verbatim // // Create the lan on the left side of the dumbbell. // Ptr n0 = Create (); Ptr n1 = Create (); Ptr n2 = Create (); Ptr n3 = Create (); Ptr lan1 = CsmaTopology::CreateCsmaChannel (DataRate (10000000), MilliSeconds (2)); uint32_t nd0 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n0, lan1, "08:00:2e:00:00:00"); uint32_t nd1 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n1, lan1, "08:00:2e:00:00:01"); uint32_t nd2 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n2, lan1, "08:00:2e:00:00:02"); uint32_t nd3 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n3, lan1, "08:00:2e:00:00:03"); CsmaIpv4Topology::AddIpv4Address (n0, nd0, "10.1.1.1", "255.255.255.0"); CsmaIpv4Topology::AddIpv4Address (n1, nd1, "10.1.1.2", "255.255.255.0"); CsmaIpv4Topology::AddIpv4Address (n2, nd2, "10.1.1.3", "255.255.255.0"); CsmaIpv4Topology::AddIpv4Address (n3, nd3, "10.1.1.4", "255.255.255.0"); @end verbatim The code to generate the CSMA lan on the right side is similar; only the names have been changed. @verbatim // // Create the lan on the right side of the dumbbell. // Ptr n4 = Create (); Ptr n5 = Create (); Ptr n6 = Create (); Ptr n7 = Create (); Ptr lan2 = CsmaTopology::CreateCsmaChannel (DataRate (10000000), MilliSeconds (2)); uint32_t nd4 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n4, lan2, "08:00:2e:00:00:04"); uint32_t nd5 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n5, lan2, "08:00:2e:00:00:05"); uint32_t nd6 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n6, lan2, "08:00:2e:00:00:06"); uint32_t nd7 = CsmaIpv4Topology::AddIpv4CsmaNetDevice (n7, lan2, "08:00:2e:00:00:07"); CsmaIpv4Topology::AddIpv4Address (n4, nd4, "10.1.2.1", "255.255.255.0"); CsmaIpv4Topology::AddIpv4Address (n5, nd5, "10.1.2.2", "255.255.255.0"); CsmaIpv4Topology::AddIpv4Address (n6, nd6, "10.1.2.3", "255.255.255.0"); CsmaIpv4Topology::AddIpv4Address (n7, nd7, "10.1.2.4", "255.255.255.0"); @end verbatim Next, we create a point to point link to connect the two lans. We connect the point-to-point channel between nodes three (on the left lan) and four (on the right lan). You should recoginze this as substantially similar to the link setup from the @code{point-to-point} example. @verbatim // // Create the point-to-point link to connect the two lans. // Ptr link = PointToPointTopology::AddPointToPointLink ( n3, n4, DataRate (38400), MilliSeconds (20)); PointToPointTopology::AddIpv4Addresses (link, n3, "10.1.3.1", n4, "10.1.3.2"); @end verbatim Then we configure data flows. We create four echo clients that send UDP packets from the left side lan to servers created on the right side lan. Notice that we send 100 packets with an inter-packet gap of ten milliseconds instead of the single packet we have previously used. This data rate is sufficient to saturate the point-to-point link and will cause packets to be dropped when the queue on the link net devices overflows (the default maximum queue depth is 100 packets). Note that we stagger the start of the echo clients to slowly bring up the data rates. @verbatim // // Create data flows across the link: // n0 ==> n4 ==> n0 // n1 ==> n5 ==> n1 // n2 ==> n6 ==> n2 // n3 ==> n7 ==> n3 // uint16_t port = 7; Ptr client0 = Create (n0, "10.1.2.1", port, 100, Seconds(.01), 1024); Ptr client1 = Create (n1, "10.1.2.2", port, 100, Seconds(.01), 1024); Ptr client2 = Create (n2, "10.1.2.3", port, 100, Seconds(.01), 1024); Ptr client3 = Create (n3, "10.1.2.4", port, 100, Seconds(.01), 1024); Ptr server4 = Create (n4, port); Ptr server5 = Create (n5, port); Ptr server6 = Create (n6, port); Ptr server7 = Create (n7, port); server4->Start(Seconds(1.)); server5->Start(Seconds(1.)); server6->Start(Seconds(1.)); server7->Start(Seconds(1.)); client0->Start(Seconds(2.)); client1->Start(Seconds(2.1)); client2->Start(Seconds(2.2)); client3->Start(Seconds(2.3)); server4->Stop (Seconds(10.)); server5->Stop (Seconds(10.)); server6->Stop (Seconds(10.)); server7->Stop (Seconds(10.)); client0->Stop (Seconds(10.)); client1->Stop (Seconds(10.)); client2->Stop (Seconds(10.)); client3->Stop (Seconds(10.)); @end verbatim The remainder of the file should be quite familiar to you. Go ahead and run @code{linear-dumbbell}. Now take a look at the trace (@code{tutorial.tr}) file. You will now see trace lines that begin with @code{d}. Alternatively you can search for the string ``queue-drop'' which is the expansion of the drop code. Interpretation of a dropped packet is straightforward. We have expanded the first @code{queue-drop} trace for you below. See the section on ASCII tracing for details. @verbatim 00 d 01 2.40938 02 nodeid=3 03 device=1 04 queue-drop 05 pkt-uid=124 06 LLCSNAP(type 0x800) 07 IPV4( 08 tos 0x0 09 ttl 63 10 id 20 11 offset 0 12 flags [none] 13 length: 1052) 10.1.1.3 > 10.1.2.3 14 UDP(length: 1032) 15 49153 > 7 16 DATA (length 1024) @end verbatim We leave it as an exercise to examine the trace files in more detail. @c ======================================================================== @c Nonlinear Thinking @c ======================================================================== @node Nonlinear-Thinking @chapter Nonlinear Thinking One thing that all of our examples so far have in common is that they are composed of a linear collection of calls into the @command{ns-3} system. The programmers among the readers may have wondered why there is not as much as a for-loop in all of the examples. The answer is that we wanted to introduce you to @command{ns-3} scripting with a minimum of conceptual overhead. We're going to remedy that situation shortly. We have written a number of @command{ns-3} scripts in C++. Although we have been perfectly linear in our script implementations, just like any other C++ program, an @command{ns-3} script can use any features of the language you desire. If you will look back at the @code{linear-dumbbell.cc} example, you may notice that the code to create the left and right sides of the dumbbell is operationally identical --- only the names change. An obvious improvement of this program would be to use subroutines to create the sides. Since we are working with C++, we should probably do this in an object-oriented way. Since object-oriented design is somewhat of a black art to some people, we'll take some time here and outline a simple methodology you can follow. @section Object Design 101 --- Class Ipv4BusNetwork If you are a master of object oriented design, feel free to skip or skim this section, in which we derive a simplistic but fully operational bus network class. So you want to create a BusNetwork class. Often the biggest hurdle in a design is figuring out how to get started. One of the simplest and most straightforward ways to do an object decomposition of a problem is to simply write down a description of the problem and take a look at the words you used. Let's take some time and do that, first at a very high level. @example A bus network is an implementation of a particular network topology that contains some number of nodes. Each of these nodes is attached to a single multi-drop channel. The network itself has some attributes independent of the topology such as a network mask, network number (prefix) and base IP address. @end example The first thing to do is to focus on the nouns and adjectives. These will give you a starting point for required classes and member variables. Immediately we can notice that at the highest level we are talking about the noun @emph{network}. This probably won't surprise you. We also have an adjective that modifies the noun --- @emph{bus}. This should lead us to our first class defintion. Usually class names are constructed in the same way as an English language sentence would be spoken. For example, one would speak of a @emph{bus network} in conversation, so we would normally create a @code{class BusNetwork} to represent it. One thing to note is that we have used two words in our description quite naturally: @emph{is} and @emph{has}. When you see these words should should immediately think of the object-oriented concepts of @emph{ISA} (inheritance) and @emph{HASA} (containment) respectively. We wrote that a bus network @emph{is} an implementation of a particular network topology. Perhaps you will agree that there is a natural base class called @code{Network} that @emph{has} the attributes discussed above. The fact that a @code{BusNetwork} @emph{ISA} kind of @code{Network} suggests inheritance. Let's capture that thought right away remembering that we're focused on IP version four here: @verbatim class Ipv4Network { public: Ipv4Address m_network; Ipv4Mask m_mask; Ipv4Address m_baseAddress; }; class Ipv4BusNetwork : public Ipv4Network { }; @end verbatim Let's take a look at the @emph{HASA} relationships of the bus network. Clearly it will @emph{have} a reference to the underlying channel that implements the actual communications medium. We use smart pointers for those references, so one member variable is obvious: @verbatim Ptr m_channel; @end verbatim A bus network will also need to contain references to all of the nodes we eventually want to create. If you are working in C++ and see the words contain or container, you should immediately think of the Standard Template Library or STL. A quick search of the available containers there will probably lead you to consider the vector class. A vector is a container that looks like an array. This is just what we need here. Again, we want to use smart pointers to reference our nodes, so the declaration of the vector would look like, @verbatim std::vector > m_nodes; @end verbatim It will save you headaches in the future if you notice that the space between the two right brackets is required to differentiate this situation from a right-shift operator. So we have a pretty good start already after just a little work. Now we need to turn our attention to actions. Let's write another little description of the things you consider doing to a Bus network. @example We need to be able to create a bus network. We need to be able to delete a bus network. We need to be able to get a handle to a node in order to add applications. We need to be able to set the network, mask and base address somehow, specify how many nodes to create and provide the underlying channel its required bandwidth and delay parameters. @end example We now look at the @emph{verbs} in that sentence. These will give a good starting point for the methods of the classes. For example, the verbs @emph{create} and @emph{delete} should suggest @emph{constructor} and @emph{destructor}. The verb @emph{get} leads us to providing a method called @code{GetNode}. We have to provide a number of parameters so we can either provide @emph{setters} or we can simply pass them in as parameters to our constructors. Since this is a simple example, we won't bother to implement getters and setters (methods to get and set member variables to enhance data hiding). Let's use this guidance to finish up our class declarations: @verbatim class Ipv4Network { public: Ipv4Network (Ipv4Address network, Ipv4Mask mask, Ipv4Address address); virtual ~Ipv4Network (); Ipv4Address m_network; Ipv4Mask m_mask; Ipv4Address m_baseAddress; }; class Ipv4BusNetwork : public Ipv4Network { public: Ipv4BusNetwork ( Ipv4Address network, Ipv4Mask mask, Ipv4Address startAddress, DataRate bps, Time delay, uint32_t n); virtual ~Ipv4BusNetwork (); Ptr GetNode (uint32_t n); private: std::vector > m_nodes; Ptr m_channel; }; @end verbatim That's it. We have actually already walked through almost all of the code required to construct a bus network in our @code{csma-echo.cc} example, so let's just jump forward and take a look at an implementation of this thing. We provide an implementation for you in the files @code{ipv4-bus-network.h} and @code{ipv4-bus-network.cc} located in the @code{tutorial} directory. We also provide an example that uses the new class in the file @code{bus-network.cc}. The interesting method from our current perspective is the Ipv4BusNetwork constructor, shown below: @verbatim Ipv4BusNetwork::Ipv4BusNetwork ( Ipv4Address network, Ipv4Mask mask, Ipv4Address baseAddress, DataRate bps, Time delay, uint32_t n) : Ipv4Network (network, mask, baseAddress) { Ipv4AddressGenerator::SeedNetwork (mask, network); Ipv4AddressGenerator::SeedAddress (mask, baseAddress); m_channel = CsmaTopology::CreateCsmaChannel (bps, delay); for (uint32_t i = 0; i < n; ++i) { Ptr node = Create (); uint32_t nd = CsmaIpv4Topology::AddIpv4CsmaNetDevice (node, m_channel, Mac48Address::Allocate ()); Ipv4Address address = Ipv4AddressGenerator::AllocateAddress (mask, network); CsmaIpv4Topology::AddIpv4Address (node, nd, address, mask); m_nodes.push_back (node); } } @end verbatim Notice that we do the simple and straightforward thing and pass all of our parameters to the constructor. For those unfamiliar with C++, the line after the colon and before the opening brace (shown below), @verbatim : Ipv4Network (network, mask, baseAddress) { @end verbatim Passes the appropriate parameters to the constructor of the base class @code{Ipv4Network}. There are two new calls that we haven't seen immediately after this initialization. They are: @verbatim Ipv4AddressGenerator::SeedNetwork (mask, network); Ipv4AddressGenerator::SeedAddress (mask, baseAddress); @end verbatim We provide an IP address generator class to allow us to programatically allocate IP addresses. The first call to @code{SeedNetwork} gives the address generator a starting network number to use when generating addresses. The second call to @code{SeedAddress} gives the address generator a starting IP address to use. There is a starting network and starting address for each of the 32 possible network masks. Later in the for loop, you will see a call to @code{AllocateAddress} in which the IP address for each node created in the loop is actually generated. The only unfamiliar call in the reset of the constructor will be: @verbatim m_nodes.push_back (node); @end verbatim This is the STL code to add the newly created node to the vector of nodes attached to the bus. For your convenience, we reproduce the entire bus network implementation below: @verbatim /* -*- Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil; -*- */ /* * Copyright (c) 2007 University of Washington * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation; * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include "ns3/mac48-address.h" #include "ns3/csma-net-device.h" #include "ns3/csma-topology.h" #include "ns3/csma-ipv4-topology.h" #include "ipv4-bus-network.h" #include "ipv4-address-generator.h" namespace ns3 { Ipv4Network::Ipv4Network ( Ipv4Address network, Ipv4Mask mask, Ipv4Address address) : m_network (network), m_mask (mask), m_baseAddress (address) { } Ipv4Network::~Ipv4Network () { } Ipv4BusNetwork::Ipv4BusNetwork ( Ipv4Address network, Ipv4Mask mask, Ipv4Address baseAddress, DataRate bps, Time delay, uint32_t n) : Ipv4Network (network, mask, baseAddress) { Ipv4AddressGenerator::SeedNetwork (mask, network); Ipv4AddressGenerator::SeedAddress (mask, baseAddress); m_channel = CsmaTopology::CreateCsmaChannel (bps, delay); for (uint32_t i = 0; i < n; ++i) { Ptr node = Create (); uint32_t nd = CsmaIpv4Topology::AddIpv4CsmaNetDevice (node, m_channel, Mac48Address::Allocate ()); Ipv4Address address = Ipv4AddressGenerator::AllocateAddress (mask, network); CsmaIpv4Topology::AddIpv4Address (node, nd, address, mask); m_nodes.push_back (node); } } Ipv4BusNetwork::~Ipv4BusNetwork () { } Ptr Ipv4BusNetwork::GetNode (uint32_t n) { return m_nodes[n]; } }; // namespace ns3 @end verbatim @section Using Ipv4BusNetwork If all you ever want to do with a bus network can be captured in a topology with four nodes on the bus, the preceeding section may seem like a colossal waste of time. This is probably not the case, though. Now that we have a relatively abstract bus class, we can create bus networks with 4, 40 or 4000 nodes with no additional effort. A use of the bus network class is shown in the file @code{bus-netowrk.cc} located in the @code{tutorial} directory. The interesting code is, @verbatim Ipv4BusNetwork bus ("10.1.0.0", "255.255.0.0", "0.0.0.3", DataRate(10000000), MilliSeconds(20), 10); @end verbatim Here we create a bus network with the network number ``10.1.0.0'' and the network mask ``255.255.0.0'' that completes the IP network definition. You can consider these together as ``10.1.0.0/16'' if you prefer. The next parameter tells the bus to start numbering IP addresses of contained nodes at ``10.1.0.3'' (remember the network number will be combined). We provided a data rate of 10 megabits per second and a latency of 20 milliseconds. Finally, we ask the @code{Ipv4BusNetwork} object to create ten nodes in the network. If you are feeling brave, go ahead and change the number of nodes to be 100, 1000, 10,000 or more to generate larger and larger networks. Before you go too far, remember that a trace file will be generated when you run your resulting program and ee asked the trace facility to trace all net device receive events. This will include the reception of the broadcast ARP request by all of the nodes in the simulation, so this can add up quickly. @c ======================================================================== @c Summary @c ======================================================================== @node Summary @chapter Summary This concludes the first part of the tutorial. We have focused on using the @command{ns-3} system to construct various network topologies and to simulate sendng data across the networks; and we've shown you how to use the trace facility to get access to simulation results. We now encourage you to play with the system a little. Experiment with what we have provided. Build a hierarchical network simulation. Perhaps exercise your object design skills and create a new @code{Ipv4DumbbellNetwork} class to create dumbbell networks using the Ipv4BusNetwork class we just created. Hint: An Ipv4DumbbellNetwork @emph{has} two @code{Ipv4BusNetwork} objects; a left side and a right side. In the next part of the tutorial we are going to drop down a level and begin examining the lower levels of the system in more detail. We are going to explain how to change the behavior of the system and eventually how to write new models and applications. This is a good time to make sure that you thorougly understand what we've gone over so far. @c ======================================================================== @c Object Model @c ======================================================================== @node Object-Model @chapter Object Model There are two distinctly different meanings associated with the term Object Model. The first speaks to the implementation of an object system --- a system view; and the second speaks to the application programming interface (classes or objects) one uses to access some service or system --- an application view. As an example of the system view sense of the term, the C++ language has an associated object model that describes how objects are laid out in memory, how virtual functions work, how inheritance is implemented, constructor and destructor execution ordering, template instantiation, etc. In the case of the application view, the Document Object Model is a good example. In the words of W3C, the Document Object Model (DOM) is an application programming interface (API) for HTML and XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated. The Component Object Model (COM) from Microsoft actually spans both meanings of the term and extends further into policy statements. From a system perspective, COM specifies an interface definition language, the layout of objects virtual function tables, the formats of Globally Unique Identifiers and also specifies lifetime management mechanisms for objects via reference counting. From the point of view of the API, COM specifies a number of Interfaces as well as functions such as CoCreateInstance and various threading models. The COM specification extends to policy by disallowing implementation inheritance. The @command{ns-3} object model takes the C++ language (system level) object model as its basis, and extends that model by providing an API for software componentry. You will find terms like Component, Interface and QueryInterface in the following discussion. It is important to understand from the outset that this is the @command{ns-3} object model, and not any other object model. Richard Feynman (an American physicist) once described the behavior of matter and light on a very small scale in the following way, @quotation ``They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.'' @end quotation Just as students of quantum mechanics must rid themselves of preconceptions regarding the behavior of matter at small scales, you should rid yourself of any preconceptions you may have about components, interfaces and APIs for software componentry before continuing. To paraphrase Feynman, @command{ns-3} components do not behave like COM Components, or Java Beans, or CORBA objects, or clouds or weights on springs, or like anything that you have ever seen they are @command{ns-3} components. @section The C++ Object Model is the Root of all Things @command{Ns-3} is primarily a C++ system. The system is written in C++ and one can use standard C++ mechanisms for creating and using ns-3 objects. We do not change this at all, nor do we make any pronouncements about the superiority of one mechanism or another. What we will do is provide convenience functions that we think will make creating and managing simulation objects easier. Previously, you have seen objects created using the template function @code{Create} as in the following example: @verbatim Ptr n0 = Create (); @end verbatim This line of code, while it may be unfamiliar to some, is pure C++. If you were to look in the header file ptr.h, you would find the following definition of the @code{Create} template. @verbatim template Ptr Create (void) { T *obj = new T (); Ptr p = obj; obj->Unref (); return p; } @end verbatim As you can see, this template creates objects of type @code{T} using the operator @code{new}. Its a little harder to find the corresponding delete --- it's in the file @code{object.cc} inside the method @code{Object::MaybeDelete}, but when that @code{Ptr} which you see above goes out of scope it will call @code{Unref} and ultimately the C++ @code{delete} operator will be called. The ns-3 system uses the C++ @code{new} and @code{delete} operators, so there is really no reason that you as a user of the ns-3 system are forbidden from using these or any other C++ mechanism. If you so desire, you can take on the responsibility for managing object lifetime (i.e., do not use the @code{Ptr} smart pointer), work directly with the @code{new} and @code{delete} operators and call methods like any C++ object as in the following example: @verbatim MyClass *obj = new MyClass (); obj->Method(); delete obj; @end verbatim You, as a competent model author, are encouraged to use whatever methods you think are appropriate in your private code. Remember, however, that the public ns-3 APIs do use smart pointers to pass objects around in an effort to reduce the burden of object lifetime management. If you do intend to export an API publicly, you should use the same object lifetime management approaches as those found in the ns-3 public API if only for consistency. These APIs are there for convenience and consistency, but do not change the fact that in ns-3 all of the objects are really just C++ objects, ultimately created using the C++ new operator with C++ constructor semantics and are ultimately deleted using the C++ delete operator, following C++ destructor semantics. Although it may sometimes appear so, there is really no system- level magic going on in ns-3. Ns-3 components and interfaces are C++ objects just like any other object and our object model is simply a collection of APIs built on the normal C++ object model. @section Interfaces There are many different ideas floating around of what exactly the term @emph{Interface} means. Originally an interface just meant a communication boundary between two entities. As the concepts of object oriented programming (OOP) were surfacing in the 1980s, the term interface was applied to the collection of access methods for the modular entities that were being defined. Two distinct approaches developed regarding specifying access mechanisms for objects. The OOP purists were very concerned about object reuse and were led to Abstract Data Types (ADT). These were eventually implemented in the case of C++, as pure virtual methods in Abstract Base Classes (ABC). Another group of folks was more interested in simply specifying object access methods in one place and using inheritance as the primary reuse mechanism. Bjarne Stroustroup, the creator of C++, embraced both approaches. He makes the following interesting observation: @quotation ``Many classes [@dots{}] are useful both as themselves and also as bases for derived classes. [@dots{}] Some classes, such as class @strong{Shape}, represent abstract concepts for which objects cannot exist.'' @end quotation @command{Ns-3} does not pick and enforce a particular approach. In @command{ns-3} an interface is determined completely by a class declaration just as any C++ object interface is declared. If you think of an object as an abstract concept that should be implemented by derived classes, by all means, use the Abstract Base Class approach to interface declaration. If you think that an object should be completely concrete and you foresee no need to ever modify its behavior, feel free to avoid declaring any methods virtual. If you think that an object could be useful as a base class, feel free to declare its methods virtual. If you like to use the PIMPL idiom, again, feel free. If you want to use any combination of these techniques, feel free. We make no restrictions. When we speak of an ns-3 interface, we do not worry about interface definition languages, or pure virtual classes, or registries we just think about C++ object declarations and their associated methods. When we instantiate an @command{ns-3} Interface, it is the C++ object model that dictates how that object is brought into existence. When a method is called on an @command{ns-3} Interface, it is the C++ object model that dictates how that method is dispatched. The only difference between a vanilla C++ object and an ns-3 Interface, is that an object acting as an ns-3 Interface must inherit from the base class Object. This inheritance gives the Interface object a very useful capability. @section The Ns-3 Capital I Interface and QueryInterface One thing that Microsoft got right in the Component Object Model was the idea of Interface aggregation and discovery via QueryInterface. We have embraced these ideas in @command{ns-3}. This was done primarily to address a common problem in large software systems. A good example of this problem happens in the @command{ns-3} Node class. If one were to take the standard OOP view of specializing a @code{Node} into an internet host, for example, one would typically inherit from the @code{Node} base class and include functionality to implement such things as internet routing and a TCP / IP protocol stack. Other types of @code{Node}s might inherit from the node class and specialize in different ways, or further specialize the internet host class, treating it as a base class. This can result in a complicated inheritance tree in which some specializations are simply not available to other branches of the tree which can make reuse difficult or impossible. This is known as the @emph{weak base class} problem and creates pressure to drive functionality up the inheritance tree into the base classes. This, in turn, results in @emph{base class bloat} and the resulting @emph{swiss army knife} base classes which end up trying to do everything in one place. Even if one successfully avoided these swiss army knife base classes, one would also want to be able to treat new specializations of @code{Node} generically in the system. This means one would pass references to the base class (@code{Node}) across public APIs. This introduces @emph{upcasts} prior to passing across public APIs and corresponding @emph{downcasts} on the other side in order to gain access to required specialized functions. As the inheritance tree becomes more complicated, this approach can cause another related problem known as the @emph{fragile base class} problem. This happens when changes to the base class cause unexpected problems in the various and sundry subclasses. These effects seem always to result in a positive feedback loop driving everything into the base class and destroying much of the encapsulation which is a hallmark of the object oriented approach. @subsection Interface Composition There is a completely different way to address the Node specialization problem. Instead of approaching the situation using inheritance, one can look at the problem as one of composition. We can look at the @code{Node} class as a container of sorts that holds other objects. In this case, the objects would be instances of the classes implementing the internetwork routing code, or the TCP / IP protocol stack described above. This approach preserves the encapsulation and solves the weak base class, base class bloat and fragile base class problems; but the question of method dispatch immediately comes to mind. In many systems, @emph{delegation} is used. The base class, @code{Node}, in this approach would provide methods that simply forward to the objects implementing the desired functionality. This situation clearly does not address the base class bloat problem since dispatch methods must be added to the base class. The situation is mitigated somewhat by pushing the implementation of the dispatch methods to contained objects, but the fundamental problems are still present. What is really needed is a way to compose objects but at the same time keep the interfaces to those objects separated. Composition, usually called @emph{aggregation}, along with runtime Interface discovery is the solution that Microsoft originally championed and that @command{ns-3} has adopted. In our example a @code{Node} would contain separate Interface objects implementing internetwork routing and TCP/IP. These contained objects have interfaces in the C++ sense of collections of method signatures. When objects are capable of participating in this aggregation process, they are called @command{ns-3} Interfaces and they receive the functionality required for this participation by inheriting from the base class @code{Object}. @subsection Object, interfaces and Interfaces As mentioned above, the class that implements the aggregation mechanism for @command{ns-3} objects is called @code{Object}. The class named @code{Object} is simply a base class that you will inherit from if you want your objects to support aggregation and QueryInterface. Many systems have a base class that implements common functionality and these base classes are typically called Object. The @command{ns-3} version of this object base class relates primarily to Interface aggregation, although it does provide methods to help with intrusive reference counting and tracing as well. When a C++ object inherits from the ns-3 Object base class, it is conceptually promoted to an ns-3 Interface (note the capital I in Interface) irrespective of how the object was declared (e.g., as an abstract base class, concrete class, with virtual methods, etc.). In ns-3, you should associate inheritance from the class named @code{Object} with promotion of an object to the status of Interface rather than the form of the Interface declaration. When you inherit from @code{Object}, you will get new methods and an Interface Identifier. The Interface Identifer, or @emph{iid}, is the @command{ns-3} version of the @emph{Universally Unique ID} (UUID) or @emph{Globally Unique ID} (GUID) found in other systems. Unlike the GUID, it is really a dynamically created process-local ID. For now, consider it as simply a number which the system will generate for you that uniquely identifies an Interface class within the ns-3 system and allows you to specify an interface type to @code{QueryInterface}. To summarize, when you instantiate an object that inherits from the @code{Object} class, you will have a C++ object that has four important properties: @itemize @bullet @item The object has a C++ interface defined by the collection of method signatures in its inheritance tree; @item The object has an Interface ID that uniquely identifies the C++ interface of its class; @item The object is a container that has the ability to aggregate other interfaces; @item The object exports a method that allows for discovery of aggregated interfaces (@code{QueryInterface}) according to Interface ID. @end itemize It is crucially important to understand what we have described here. A given C++ class has an object access interface that is essentially the collection of method signatures specified in its inheritance tree. This is a C++ object model thing. Ns-3 provides a base class from which the class in question can inherit and be promoted to the status of Interface. Once a class becomes an Interface it has inherited the ability to set its own interface identifier (@code{iid}), and exports methods to aggregate and search other Interfaces that are added to its aggregation. That last detail is important. In @command{ns-3} Interfaces are both containers and specifications for object method access. We have previously mentioned the @code{Node} class acts as a container. In fact, the @code{Node} class inherits from @code{Object} and is itself also an @command{ns-3} Interface. When the @code{Node} object is created it is really an aggregation of one Interface, the @code{Node} Interface. This is generally true --- Interfaces are both containers and Interfaces. @subsection Aggregations The figure below shows how an Interface could be illustrated in detail. The line with the circle at the top of the diagram represents the appearance of the Interface to the external world. This circle and line are called a lollipop because of its superficial similarity to a kind of childs candy. @sp 1 @center @image{oneif,,,,png} You could declare this interface quite simply using a non-virtual class as follows, @verbatim class A : public Object { public: static const InterfaceId iid; void MethodA (void); }; @end verbatim The methods that are then available via the Interface labeled @code{A} in the figure above are the methods inherited from the @code{Object} base class ( @code{QueryInterface}, @code{Ref}, and @code{Unref}) and those from class @code{A} (@code{MethodA}). Note that you must declare an @code{InterfaceId} for your Interface class, and it must be declared static to make it class-wide in scope. This @code{iid} can be thought of as a kind of type information that uniquely identifies objects as being instantiated from this class. You can think of the arc and arrow device coming off each side of the Interface as part of a connector. These connectors allow @code{QueryInterface} to search aggregations for a particular @code{iid}. The figure below shows an aggregation of three Interfaces: A, B and C. The class declarations for classes @code{B} and @code{C} are substantially similar to that of class @code{A}. @sp 1 @center @image{threeif,,,,png} You can visualize these Interfaces as being snapped together like Lego building blocks if you like. When the Interfaces are aggregated, a @code{QueryInterface} search path is formed through the connectors. In order to create this aggregation we first need to create the Interface objects. These are just normal, everyday C++ objects that we can create using the @code{Create} template function and manage using smart pointers. The following code should be obvious to you by now: @verbatim Ptr a = Create (); Ptr b = Create (); Ptr c = Create (); @end verbatim When you create an aggregation, you pick one of the Interfaces to act as the container. In this case well pick Interface A. In order to aggregate an Interface, you simply call the method @code{AddInterface} that your class inherited from @code{Object}. The following code will aggregate Interface @code{B} and Interface @code{C} onto the Interface (and container) @code{A}. @verbatim a->AddInterface (b); a->AddInterface (c); @end verbatim Thats all there is to it. Now that you have those connectors snapped together, you can ask each of the Interfaces in the aggregation for any of the Interfaces in the aggregation. Lets look at a simple example: @verbatim Ptr newB = a->QueryInterface (B:iid); @end verbatim The left hand side of this assignment declares a smart pointer to the class @code{B} to help with memory management of the returned Interface pointer. Object lifetime management is very important when dealing with Interfaces and our smart pointer will simply take care of it all for you. The right hand side illustrates the basic idea of @code{QueryInterface}. We take a take a (smart) pointer to Interface @code{A} and ask it to search the aggregation for an interface associated with an interface identifier with the value of @code{B:iid} which is passed as a parameter. Recall that @code{B::iid} is the @code{static InterfaceId} of the Interface class @code{B}. Observe that @code{QueryInterface} is a template function and the type specified in the angle brackets, here @code{}, tells it what kind of smart pointer to return. In this case @code{QueryInterface} will find an Interface object of type @code{B::iid} in its list of Interfaces and return a smart pointer to @code{B} as instructed. Now that you have those connectors snapped together, you can ask each of the Interfaces in the aggregation for any of the Interfaces in the aggregation. For example we could walk the Interfaces asking each for the next in the aggregation. First we would ask the Interface pointed to by the smart pointer a to look for the InterfaceId representing @code{B}: @verbatim Ptr newB = a->QueryInterface (B:iid); @end verbatim Next, we can ask the Interface pointed to by the smart pointer @code{newB} to look for the @code{InterfaceId} representing @code{C}: @verbatim Ptr newC = newB->QueryInterface (C:iid); @end verbatim Then, we can ask the Interface pointed to by the smart pointer @code{newC} to look for the InterfaceId representing A and complete our circuit of the aggregation: @verbatim Ptr newA = newC->QueryInterface (A:iid); @end verbatim @code{QueryInterface} (often abbreviated QI) has some important properties that we need to go over. Technically, QI is a @emph{symmetric}, @emph{reflexive} and @emph{transitive} operation with respect to the set of aggregated Interfaces. @subsubsection Symmetry The symmetric nature of QI guarantees that if one performs a QI on a given Interface for the Interface Id of that same interface, that @code{QueryInterface} must succeed. The existence of interface A in the aggregation implies the reachability of Interface A in the aggregation. This is usually written (by Microsoft) as, @center must succeed (A >> A) We can illustrate this property with the code snippet, @verbatim Ptr symmetricA = a->QueryInterface (A:iid); NS_ASSERT (symmetricA); @end verbatim Here we take as given an interface (smart) pointer named a on which we perform a QI looking for the InterfaceId of that same Interface. This call must always succeed and a smart pointer to the Interface a is returned by QI. @subsubsection Reflexivity Calls to QI must also be reflexive. This means that if you successfully QI for interface B from interface A, then you must always be able to QI for A from B. This is usually written as, @center must succeed (A >> B, then B >> A) This property can be illustrated with the code snippet, @verbatim Ptr b = a->QueryInterface (B:iid); Ptr reflexiveA = b->QueryInterface (A:iid); NS_ASSERT (reflexiveA); @end verbatim If the first @code{QueryInterface} on Interface A looking for Interface B succeeds, then a @code{QueryInterface} on Interface B looking for Interface A must succeed. @subsubsection Transitivity @code{QueryInteface} must also be transitive. This means that if one can find Interface B from Interface A, and Interface C from Interface B, then one must also be able to find interface C from Interface A. This is usually written as, @center must succeed (A >> B, and B >> C, then A >> C) This property can be illustrated with the code snippet, @verbatim Ptr b = a->QueryInterface (B:iid); Ptr c = b->QueryInterface (C:iid); Ptr transitiveC = a->QueryInterface (C:iid); NS_ASSERT (transitiveC); @end verbatim If you can get to Interface B from Interface A, and you can get to Interface C from Interface B, then a QueryInterface on Interface A looking for Interface C must also succeed. @subsection Creating the InterfaceId The final piece of this puzzle is to locate where the interface Ids actually come from. The answer is from a static initializer that must be located in the @code{.cc} file corresponding to the Interface. For example, to initialize the Interface Id for the class A above, you would simply add the following code to the source file that implements class A, @verbatim const InterfaceId A::iid = MakeInterfaceId (``A'', Object::iid); @end verbatim This code is guaranteed by the C++ language definition to be executed before your main procedure is entered. The call to MakeInterfaceId will assign a process-local unique identifier to your class and associate your interface with the name (string) ``A.'' This allows you to look up an InterfaceId by a human readable string. An advanced ns-3 specific feature of QueryInterface is exposed here. @code{MakeInterfaceId} takes an @code{InterfaceId} as a parameter. This is the @code{iid} of the base class from which you inherited. In most cases this will be @code{Object::iid}, which is the @code{InterfaceId} of the @code{Object} base class. In @command{ns-3}, the @code{Object} base class has its own @code{iid} and you can QI for that @code{iid}. The @code{Object} base class has a rough equivalence to the @emph{IUnknown} Interface in Microsofts COM, so you can QI for @code{Object::iid} in @command{ns-3}f just as you might QI for IID_IUnknown in COM. The InterfaceId you pass to @code{MakeInterfaceId} is used to create an inheritance tree in the ns-3 interface manager. This inheritance tree is also walked in @code{QueryInterface} Interface searches. Consider a simple case of a base class and a derived class as shown below, @verbatim class Base : public Object { public: static const InterfaceId iid; ... }; class Derived : public Base { public: static const InterfaceId iid; ... }; @end verbatim To assign the InterfaceId for each of these classes, we could add two calls to @code{MakeInterfaceId} reflecting the class hierarchy we just created. @verbatim const InterfaceId Base::iid = MakeInterfaceId (``Base'', Object::iid); const InterfaceId Derived::iid = MakeInterfaceId (``Derived'', Base::iid); @end verbatim The first Interface is shown to inherit from class @code{Object} and the second inherits from class @code{Base}. We could create these interfaces as we usually do, @verbatim Ptr base = Create (); Ptr derived = Create (); @end verbatim The derived and base @code{InterfaceIds} are either present or not present based on the inheritance tree. For example, a QI for the @code{Base InterfaceId} must succeed when done against a @code{Ptr}; but a QI for the @code{Derived InterfaceId} must fail when done against a @code{Ptr}. However, a QI for the @code{Base InterfaceId} must succeed when done against a @code{Ptr}; and a QI for the @code{Derived InterfaceId} must succeed when done against a @code{Ptr}. This feature allows you to use implementation inheritance to easily create new Interfaces. You are prevented from doing so in Microsoft COM, but this was almost universally identified as a problem. @subsection A Real Example At this point you may be asking yourself what the point of all of this is, since you already had those pointers laying around when you created the objects. The typical case is that you would forget about the pointers to the contained objects and only export a single Interface. Other Interfaces could be discovered using QI. Generally one tends to think of one of the Interfaces in the aggregation as being the container and other Interfaces being aggregated to that container. In the case of a Node, for example, it is quite natural to think of the Node as being the container which contains Interfaces for the protocol stacks, internet routing, etc. So, lets start developing an example by calling the container Interface Node instead of A. The creation of this Interface is found all over our example programs. For example, you will find code like the following in @code{samples/simple-point-to-point.cc}: @verbatim Ptr n = Create (); @end verbatim This code is described in detail in previous sections, but the important thing to realize here is that the resulting @code{Node} is an @command{ns-3} Interface. This is not at all obvious -- you must look at the source code to see that this is true. Take a look at @code{src/node/node.h} and find the class declaration for class @code{Node}. There you will find, @verbatim class Node : public Object { public: static const InterfaceId iid; ... }; @end verbatim Class @code{Node} inherits from class @code{Object} and provides an @code{InterfaceId}, therefore it is an @command{ns-3} interface. You now know you can use @code{AddInterface} for aggregation and @code{QueryInterface} for Interface discovery against any @code{Node} in the system. We spoke of a protocol stack that is aggregated to a @code{Node} in our discussions above, what we see in the real @command{ns-3} code is that this is represented by the @code{Ipv4} Interface. If you look in @code{src/node/ipv4.h} you will find, @verbatim class Ipv4 : public Object { public: static const InterfaceId iid; ... }; @end verbatim Since class @code{Ipv4} inherits from class @code{Object} and has a @code{static InterfaceId}, it is an @command{ns-3} Interface. If you look in @code{src/node/ipv4.cc} you will find, @verbatim const InterfaceId Ipv4::iid = MakeInterfaceId (``Ipv4'', Object::iid); @end verbatim After all of this reading you now know that this code snippet is asking the system to create a unique @code{InterfaceId} for the @code{Ipv4} class and declares that @code{Ipv4} inherits from class @code{Object}. It turns out that the Ipv4 class is an abstract base class (ABC). There are a number of pure virtual methods declared in that class. This means that an @code{Ipv4} object may not be instantiated. What is instantiated is an implementation class, called @code{Ipv4Impl}. This class inherits from @code{Ipv4} and provides the required virtual methods. This is where understanding what is an Interface and what is not gets tricky. The Interface is the @code{Ipv4} class since that is where the @code{InterfaceId} is found. The fact that you see @code{ipv4::iid} tells you that the @code{Ipv4} class is the Interface and has the associated @code{InterfaceId}. The class @code{Ipv4Impl} provides an implementation for the pure virtual methods in @code{Ipv4}. Since class @code{Ipv4} cannot be instantiated, one instantiates the @code{Ipv4Impl} class to create an @code{Ipv4} Interface. Once the @code{Ipv4Impl} class is instantiated, the pointer to it is immediately cast to an @code{Ipv4} pointer. Clients will then use the @code{Ipv4} object access methods (see @code{ipv4.h}) to talk to the @code{Ipv4Impl} object over the @code{Ipv4} Interface. I urge you to not go any further until you thoroughly understand what youve just read. If you now look in the file, @code{src/internet-node/internet-node.cc} you will see the following code in @code{InternetNode::Construct} that creates the @code{Ipv4} Interface and aggregates it to the @code{Node} interface (recall that class @code{Node} is an Interface and class @code{InternetNode} inherits from class @code{Node}): @verbatim Ptr ipv4Impl = Create (ipv4); ... Object::AddInterface (ipv4Impl); @end verbatim Note that the parameter @code{ipv4} passed to the @code{Create} template function is actually a pointer to an @code{Ipv4L3Protocol} which you can ignore at this point --- it doesn't really have anything to do with the @code{Ipv4} Interface. This last example does illustrate that the fact that whether an @command{ns-3} object is or is not an Interface can be quite well hidden. The designers of the system had long and involved discussions on this issue and in the end decided that mnemonic aids such as Hungarian notation were a stylistic thing and you should just refer to the system documentation to determine what objects are ns-3 Interfaces and what those Interfaces actually are (RTFM --- Read the Fine Manual). In this case, you know that the class @code{Ipv4Impl} inherits from some Interface since there is a call to @code{AddInterface} that refers to it. You can go to the header file @code{src/internet-node/ipv4-impl.h} and find that @code{Ipv4Impl} inherits from class @code{Ipv4}. You then go to file @code{src/node/ipv4.h} and see that it inherits from @code{Object} and contains an @code{InterfaceId}. Thus the Interface added is really the @code{Ipv4} Interface with the interface Id @code{Ipv4::iid}. Returning to some @command{ns-3} example code, lets take a look at @code{src/examples/simple-point-to-point.cc} again. You will find the following code: @verbatim Ptr n0 = Create (); ... Ptr ipv4; ipv4 = n0->QueryInterface (Ipv4::iid); ipv4->SetDefaultRoute (Ipv4Address (``10.1.1.2''), 1); @end verbatim The first line creates an @code{InternetNode} object and casts the resulting smart pointer to a @code{Node}. The next line declares a smart pointer to an @code{Ipv4} object. Because youve been through the code with us, you know that both the @code{Node} and the @code{Ipv4} objects are Interfaces. They should be able to participate in a @code{QueryInterface}. The next line confirms it. We do a @code{QueryInterface} on the @code{Node}, looking for the @code{Ipv4} Interface (@code{Ipv4::iid}). @code{QueryInterface} then returns a smart pointer to its aggregated @code{Ipv4} Interface. [Recall that this Interface was aggregated in @code{InternetNode::Construct}. We knew to start looking for the aggregation in @code{InternetNode} since we originally created an @code{InternetNode} in the @code{Create} template function and then implicitly cast it to a @code{Node}.] Once you have the @code{Ipv4} smart pointer, you simply use it as if it were any other C++ object. The last line shows this by setting the default route for the node. @section Caveats There are a few things that you should remember but which may not be immediately obvious. @subsection Interface Ids are Associated with Classes not Objects Interfaces are identified by an @code{InterfaceId} that is associated with the Interface class, not the Interface object. That is indicated by the @code{static} keyword in the declaration of the @code{iid} in the class. The interface Id for a given Interface class exists independently of any objects of that class that you may instantiate; and all objects of a given Interface type share the same @code{InterfaceId}. You cannot add more than one Interface of a given type (@code{iid}) to an aggregation. If you need to contain a number of Interfaces of the same type in the same aggregation, you will need to provide a separate container over which you can iterate. For example, the @code{Node} class provides methods, @verbatim uint32_t GetNDevices (void) const; Ptr GetDevice (uint32_t index) const; @end verbatim that are used iterate over the multiple @code{NetDevice} Interfaces associated with it. @emph{Interface Ids do not identify objects.} @subsection Dont use QI to Check Your Own Type. It is tempting to use @code{QueryInterface} as a form of runtime type information. Dont do it. You have no control over what other object may be added to your aggregation and this may cause problems. Someone else may have appropriated (reimplemented) your type and aggregated themselves onto your aggregation. Consider a socket factory implementation. Sockets can be either UDP sockets or TCP sockets. A socket factory will have a generic @code{SocketFactory} Interface and either a UDP specific interface for setting UDP parameters or a similar TCP-specific interface. Consider what might happen if you declared your socket factory as a partially abstract base class, and then provided separate implementations for UDP and TCP specific methods of this factory in separate concrete classes. Now consider what might happen if you used QueryInterface in your base class to determine if you were a UDP or a TCP factory. If a factory, say the UDP version, were not aggregated to any other Interface, the base class could QueryInterface on itself for the UDP-specific interface. It could then infer that it was a UDP implementation and would then do any UDP-specific tasks it could. [Experienced C++ folks are cringing about how horrible this design is, but bear with me --- its a simple illustration of a specific and perhaps not-too-obvious problem.] If another factory, say the TCP version, were not aggregated to any other Interface, the base class could QueryInterface on itself for the UDP-specific interface. If this failed, it could then infer that it had a TCP implementation and would then do any TCP-specific tasks it could. Now, what happens when these two working objects are aggregated together. Since the Interfaces are conceptually snapped together the TCP implementation would suddenly begin finding the UDP Interface from the other class factory and fail. @emph{Interface Ids should not be used as run-time type information.} @section Connecting the Dots This may all sound very complicated to you if this is your first exposure to these concepts. It may be annoying if I tell you that its really not as hard as it sounds. Rest assured that if you take some time, look at and understand the examples and write a little test code it will all come together for you. Grep around the system for AddInterface and QueryInterface and take a look at how we have used them. This will also give you a good idea of what our core Interfaces are. If you grep for @code{::iid} you will find most, if not all of the interface declarations in the system. The more you see this idiom in use, the more comfortable you will be with the idea and the more you will see how this addresses the weak base class, swiss army knife base class, and fragile base class problems I explained at the beginning. As I alluded to earlier, the developers had long discussions regarding how to make navigating the QueryInterface environment easier. The primary issue was how we could make it easier to convey to you, the model writer, that an object was an Interface. One suggestion was to adopt the convention that classes that implement Interfaces begin with the letter I. Microsoft does this, as exemplified by the class IUnknown. We also toyed with the idea of beginning our header files with i- as in i-ipv4.h. We considered forcing some structure on Interfaces with a pure virtual class specification, the names of which begin with an I; and corresponding implementations, the names of which begin with a C. In the end we decided that we were really discussing issues of programming style, and we really could not come up with a strong reason to impose any particular solution. In the end, we decided that we would not impose any structure on the source code, nor impose any naming convention. We will rely on our documentation system (Doxygen) to break out all objects with InterfaceIds in their class hierarchy into a separate section. For now, until this is implemented, grep is your friend. @c ======================================================================== @c Doxygen @c ======================================================================== @node The-Doxygen-Documentation-System @chapter The Doxygen Documentation System @node How-To-Change-Things @chapter How to Change Things @node How-To-Set-Default-Values @chapter How to Set Default Values @node How-To-Write-A-New-Application @chapter How to Write a New Application