--- a/doc/tutorial/building-topologies.texi Sat Jun 28 20:02:25 2008 -0700
+++ b/doc/tutorial/building-topologies.texi Sat Jun 28 22:39:06 2008 -0700
@@ -16,285 +16,437 @@
@chapter Building Topologies
@menu
-* Building a Star Topology
+* Building a Bus Network Topology
@end menu
@c ========================================================================
-@c Building a Star Topology
+@c Building a Bus Network Topology
@c ========================================================================
-@node Building a Star Topology
-@section Building a Star Topology
+@node Building a Bus Network Topology
+@section Building a Bus Network Topology
@cindex topology
@cindex topology|star
-In this section we are going to use the point-to-point topology primitive we
-have explored in quite some detail to construct a star network. In the star,
-there is a central node that is connected to other nodes in a ``hub and spoke''
-fashion, with each ``spoke'' being a point-to-point link.
+In this section we are going to expand our mastery of ns-3 network devices and
+channels to cover an example of a bus network. Ns-3 provides a net device and
+channel we call CSMA (Carrier Sense Multiple Access).
+
+The ns-3 CSMA device models a simple network in the spirit of Ethernet. A real
+Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
+scheme with exponentially increasing backoff to contend for the shared
+transmission medium. The ns-3 CSMA device and channel models only a
+subset of this.
+
+Just as we have seen point-to-point topology helper objects when constructing
+point-to-point topologies, we will see equivalent CSMA topology helpers in
+this section. The appearance and operation of these helpers should look
+quite familiar to you.
-We provide an example of a star topology simulation script on our
-@code{examples} directory. Go ahead and open @code{examples/star.cc} in your
-favorite editor. You will have already seen enough ns-3 code to understand
-this example, but we will go over the script briefly and examine some of the
-output.
+We provide an example script in our @code{examples} directory. This script
+builds on the @code{first.cc} script and adds a CSMA network to the
+point-to-point simulation we've already considered. Go ahead and open
+@code{examples/second.cc} in your favorite editor. You will have already seen
+enough ns-3 code to understand most of what is going on in this example, but
+we will go over the entire script and examine some of the output.
+
+Just as in the @code{first.cc} example (and in all ns-3 examples) the file
+begins with an emacs mode line and some GPL boilerplate.
-There is an amazingly beautiful bit of ASCII art at the beginning of the file
-that shows the topology used in the script. Most ns-3 example programs
-provide this, which can be surprisingly useful in understanding the code.
+One thing that can be surprisingly useful is a small bit of ASCII art that
+shows a cartoon of the network topology constructed in the example. You will
+find a similar ``drawing'' in most of our examples.
+
+In this case, you can see that we are going to extend our point-to-point
+example (the link between the nodes n0 and n1 below) by hanging a bus network
+off of the right side. Notice that this is the default network topology
+since you can actually vary the number of nodes created on the LAN. If you
+set nCsma to one, there will be a total of two nodes on the LAN (CSMA
+channel) --- one required node and one ``extra'' node. By default there are
+thee ``extra'' nodes as seen below:
@verbatim
- // Network topology
- //
- // n3 n2
- // | /
- // | /
- // n4 --- n0 --- n1
- // / |
- // / |
- // n5 n6
+// Default Network Topology
+//
+// 10.1.1.0
+// n0 -------------- n1 n2 n3 n4
+// point-to-point | | | |
+// ================
+// LAN 10.1.2.0
@end verbatim
-There are seven nodes in this simulation, nodes zero through six. Node zero is
-the hub of this star network. We are going to discover that there will be a
-flow of echo packets between node four and node one, between node three and
-node six, and node five and node two.
-
-The script starts by creating seven nodes in a @code{NodeContainer} and
-installs internet protocol stacks on all seven.
+The actual code begins by loading module include files just as was done in the
+@code{first.cc} example. Then the ns-3 namespace is @code{used} and a logging
+component is defined. This is all just as it was in @code{first.cc}, so there
+is nothing new yet.
@verbatim
- int
+ #include "ns3/core-module.h"
+ #include "ns3/simulator-module.h"
+ #include "ns3/node-module.h"
+ #include "ns3/helper-module.h"
+ #include "ns3/global-routing-module.h"
+
+ using namespace ns3;
+
+ NS_LOG_COMPONENT_DEFINE ("SecondScriptExample");
+@end verbatim
+
+The main program begins by enabling the @code{UdpEchoClientApplication} and
+@code{UdpEchoServerApplication} logging components at @code{INFO} level so
+we can see some output when we run the example. This should be entirely
+familiar to you so far.
+
+@verbatim
+ int
main (int argc, char *argv[])
{
- NodeContainer nodes;
- nodes.Create (7);
-
- InternetStackHelper stack;
- stack.Install (nodes);
-
- ...
+ LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);
+ LogComponentEnable("UdpEchoServerApplication", LOG_LEVEL_INFO);
@end verbatim
-It then creates six more @code{NodeContainer}s representing the nodes at each
-end of the six spokes. In all cases, the first node in the spoke is node zero.
+Next, you will see some familiar code that will allow you to change the number
+of devices on the CSMA network via command line argument. We did something
+similar when we allowed the number of packets sent to be changed in the section
+on command line arguments.
+
+@verbatim
+ uint32_t nCsma = 3;
+ CommandLine cmd;
+ cmd.AddValue ("nCsma", "Number of \"extra\" CSMA nodes/devices", nCsma);
+ cmd.Parse (argc,argv);
+@end verbatim
+
+The next step is to create two nodes that we will connect via the
+point-to-point link. The @code{NodeContainer} is used to do this just as was
+done in @code{first.cc}.
@verbatim
- NodeContainer nodes0and1 = NodeContainer (nodes.Get (0), nodes.Get (1));
- NodeContainer nodes0and2 = NodeContainer (nodes.Get (0), nodes.Get (2));
- NodeContainer nodes0and3 = NodeContainer (nodes.Get (0), nodes.Get (3));
- NodeContainer nodes0and4 = NodeContainer (nodes.Get (0), nodes.Get (4));
- NodeContainer nodes0and5 = NodeContainer (nodes.Get (0), nodes.Get (5));
- NodeContainer nodes0and6 = NodeContainer (nodes.Get (0), nodes.Get (6));
+ NodeContainer p2pNodes;
+ p2pNodes.Create (2);
@end verbatim
-These node containers will be used during the creation of the point-to-point
-links by the @code{PointToPointHelper}. The next bit of code instantiates
-a helper and sets the default values for the device @code{DataRate} and the
-channel @code{Delay} attributes as we have previously seen.
+Next, we delare another @code{NodeContainer} to hold the nodes that will be
+part of the bus (CSMA) network. First we just instantiate the container
+object itself.
+
+@verbatim
+ NodeContainer csmaNodes;
+ csmaNodes.Add (p2pNodes.Get (1));
+ csmaNodes.Create (nCsma);
+@end verbatim
+
+The next line of code @code{Get}s the first node (as in having an index of one)
+from the point-to-point node container and adds it to the container of nodes
+that will get CSMA devices. The node in question is going to end up with a
+point-to-point device and a CSMA device. We then create a number of ``extra''
+nodes that compose the remainder of the CSMA network.
+
+The next bit of code should be quite familiar by now. We instantiate a
+@code{PointToPointHelper} and set the associated default attributes so that
+we create a five megabit per second transmitter on devices created using the
+helper and a two millisecond delay on channels created by the helper.
@verbatim
PointToPointHelper pointToPoint;
pointToPoint.SetDeviceParameter ("DataRate", StringValue ("5Mbps"));
pointToPoint.SetChannelParameter ("Delay", StringValue ("2ms"));
+
+ NetDeviceContainer p2pDevices;
+ p2pDevices = pointToPoint.Install (p2pNodes);
@end verbatim
-The next step is to install the net devices on the nodes referenced by the
-six node containers and connect them via channels. We used one instance
-of the @code{pointToPoint.Install} method to construct our single
-point-to-point link in the @code{first.cc} script. This code just does that
-six times between different combinations of nodes.
+We then instantiate a @code{NetDeviceContainer} to keep track of the
+point-to-point net devices and we install devices on the ``point-to-point
+nodes.
+
+We mentioned above that you were going to see a helper for CSMA devices and
+channels, and the next lines introduce them. The @code{CsmaHelper} works just
+like a @code{PointToPointHelper}, but it creates and connects CSMA devices and
+channels.
@verbatim
- NetDeviceContainer devicesOnNodes0and1 = pointToPoint.Install (nodes0and1);
- NetDeviceContainer devicesOnNodes0and2 = pointToPoint.Install (nodes0and2);
- NetDeviceContainer devicesOnNodes0and3 = pointToPoint.Install (nodes0and3);
- NetDeviceContainer devicesOnNodes0and4 = pointToPoint.Install (nodes0and4);
- NetDeviceContainer devicesOnNodes0and5 = pointToPoint.Install (nodes0and5);
- NetDeviceContainer devicesOnNodes0and6 = pointToPoint.Install (nodes0and6);
+ CsmaHelper csma;
+
+ NetDeviceContainer csmaDevices;
+ csmaDevices = csma.Install (csmaNodes);
@end verbatim
-Note that this does mean that node zero will have six net devices after this
-code executes. Recall that we need to keep containers holding the net devices
-we created in order to do IP address assignment. This is the next step in the
-script.
+Just as we created a @code{NetDeviceContainer} to hold the devices created by
+the @code{PointToPointHelper} we create a @code{NetDeviceContainer} to hold
+the devices created by our @code{CsmaHelper}. We call the @code{Install}
+method of the @code{CsmaHelper} to install the devices into the nodes of the
+@code{csmaNodes NodeContainer}.
+
+We now have our nodes, devices and channels created, but we have no protocol
+stacks present. Just as in the @code{first.cc} script, we will use the
+@code{InternetStackHelper} to install these stacks.
-Just as in @code{first.cc} we use the Ipv4AddressHelper to make it easy to
-deal with addressing. We need to create different networks for each stub, so
-we have to call the @code{SetBase} method and provide a different network
-number before assigning addresses to the devices on each network. We make it
-easy to remember what is what by setting the least significant bits of the
-network number to the unique node on the spoke. For example, the spoke between
-nodes zero and one is network 10.1.1; and the spoke between nodes zero and
-two is 10.1.2, etc. Note that the network mask is set to 255.255.255.252 in
-all cases.
+@verbatim
+ InternetStackHelper stack;
+ stack.Install (p2pNodes.Get (0));
+ stack.Install (csmaNodes);
+@end verbatim
+
+Recall that we took one of the nodes from the @code{p2pNodes} container and
+added it to the @code{csmaNodes} container. Thus we only need to install
+the stacks on the remaining @code{p2pNodes} node, and all of the nodes in the
+@code{csmaNodes} container.
+
+Just as in the @code{first.cc} example script, we are going to use the
+@code{Ipv4AddressHelper} to assign IP addresses to our device interfaces.
+First we use the network 10.1.1.0 to create the two addresses needed for our
+two point-to-point devices.
@verbatim
- Ipv4AddressHelper ipv4;
-
- ipv4.SetBase ("10.1.1.0", "255.255.255.252");
- Ipv4InterfaceContainer interfacesOnNodes0and1 =
- ipv4.Assign (devicesOnNodes0and1);
-
- ipv4.SetBase ("10.1.2.0", "255.255.255.252");
- Ipv4InterfaceContainer interfacesOnNodes0and2 =
- ipv4.Assign (devicesOnNodes0and2);
-
- ipv4.SetBase ("10.1.3.0", "255.255.255.252");
- Ipv4InterfaceContainer interfacesOnNodes0and3 =
- ipv4.Assign (devicesOnNodes0and3);
-
- ipv4.SetBase ("10.1.4.0", "255.255.255.252");
- Ipv4InterfaceContainer interfacesOnNodes0and4 =
- ipv4.Assign (devicesOnNodes0and4);
-
- ipv4.SetBase ("10.1.5.0", "255.255.255.252");
- Ipv4InterfaceContainer interfacesOnNodes0and5 =
- ipv4.Assign (devicesOnNodes0and5);
-
- ipv4.SetBase ("10.1.6.0", "255.255.255.252");
- Ipv4InterfaceContainer interfacesOnNodes0and6 =
- ipv4.Assign (devicesOnNodes0and6);
+ Ipv4AddressHelper address;
+ address.SetBase ("10.1.1.0", "255.255.255.0");
+ Ipv4InterfaceContainer p2pInterfaces;
+ p2pInterfaces = address.Assign (p2pDevices);
@end verbatim
-When the above code has executed, we will have the topology complete and then
-need to move on to applications. The code to do this should be familiar to
-you, but we have rearranged it slightly.
+Recall that we save the created interfaces in a container to make it easy to
+pull out addressing information later.
+
+We then need to assign IP addresses to our CSMA device interfaces. The
+operation works just as it did for the point-to-point case, except we now
+are performing the operation on a container that has a variable number of
+CSMA devices --- remember we made that number changeable by command line
+argument. So the CSMA devices will be associated with IP addresses from
+network number 10.1.2.0 in this case.
-We use an ApplicationContainer to hold the results of the helper operations,
-and use the UdpEchoServerHelper and UdpEchoClient server to configure the
-apps just as we did in the @code{first.cc} script.
+@verbatim
+ address.SetBase ("10.1.2.0", "255.255.255.0");
+ Ipv4InterfaceContainer csmaInterfaces;
+ csmaInterfaces = address.Assign (csmaDevices);
+@end verbatim
+
+Now, we have a topology built, but we need applications. This section is
+going to be fundamentally similar to the applications section of
+@code{first.cc} but we are going to instantiate the server on one of the
+nodes that has a CSMA node and the client on the node having only a
+point-to-point device.
+
+You should completely understand the code for setting up the server since we
+have seen this before.
@verbatim
- uint16_t port = 9;
-
- ApplicationContainer apps;
-
- UdpEchoServerHelper server;
- server.SetPort (port);
-
- UdpEchoClientHelper client;
- client.SetAppAttribute ("MaxPackets", UintegerValue (10));
- client.SetAppAttribute ("Interval", StringValue ("10ms"));
- client.SetAppAttribute ("PacketSize", UintegerValue (137));
+ ApplicationContainer serverApps = echoServer.Install (csmaNodes.Get (nCsma));
+ serverApps.Start (Seconds (1.0));
+ serverApps.Stop (Seconds (10.0));
@end verbatim
-The next section of code is repeated three times, once for each data flow.
-We'll skim over the first instance quickly. The other two differ only in
-the nodes selected and the starting times of the flows.
+Recall that the @code{csmaNodes NodeContainer} contains one of the
+nodes created for the point-to-point network and @code{nCsma} extra nodes.
+What we want to get is the last of the ``extra'' nodes. The zeroth entry of
+the @code{csmaNodes} container will the the point-to-point node. The easy
+way to think of this, then, is if we create one ``extra'' CSMA node, then it
+will be be at index one of the @code{csmaNodes} container and, by induction,
+if we create @code{nCsma} ``extra'' nodes the last one will be at index
+@code{nCsma}. You see this exhibited in the first line of code.
-Just as we did in the @code{first.cc} script, we use the
-@code{UdpEchoServerHelper} to install the echo server on a node (in this case,
-node number one -- the rightmost spoke in the topology). We tell the server
-application to start at a simulation time of one second. We use the
-@code{UdpEchoClientHelper} to install the echo client on a node (in this case,
-node number four -- the leftmost spoke in the topology). We tell the client
-to start sending data at a simulation time of two seconds.
+The client application is set up exactly as we did in the @code{first.cc}
+example script. We point the client to the server we set up on the last of
+the ``extra'' CSMA nodes and install the client onto the point-to-point node
+that is not associated with any CSMA device.
@verbatim
- //
- // Echo from node 4 to node 1 starting at 2 sec
- //
- apps = server.Install (nodes.Get (1));
- apps.Start (Seconds (1.0));
+ UdpEchoClientHelper echoClient;
+ echoClient.SetRemote (csmaInterfaces.GetAddress (nCsma), 9);
+ echoClient.SetAppAttribute ("MaxPackets", UintegerValue (1));
+ echoClient.SetAppAttribute ("Interval", TimeValue (Seconds (1.)));
+ echoClient.SetAppAttribute ("PacketSize", UintegerValue (1024));
- client.SetRemote (interfacesOnNodes0and1.GetAddress (1), port);
- apps = client.Install (nodes.Get (4));
- apps.Start (Seconds (2.0));
+ ApplicationContainer clientApps = echoClient.Install (p2pNodes.Get (0));
+ clientApps.Start (Seconds (2.0));
+ clientApps.Stop (Seconds (10.0));
@end verbatim
-The next sections do the same thing, but for nodes three and six with a flow
-starting at 2.1 seconds, and for nodes five and two with flow starting at 2.2
-seconds.
-
-The only piece of code you may not have seen before follows. Since we have
-built an internetwork, we need some form of internetwork routing. Ns-3
-provides what we call a global route manager to set up the routing tables on
-nodes. This route manager has a global function that runs though the nodes
-of the simulation and does the hard work of setting up routing for you.
+Since we have actually built an internetwork here, we need some form of
+internetwork routing. Ns-3 provides what we call a global route manager to
+set up the routing tables on nodes. This route manager has a global function
+ that runs though the nodes created for the simulation and does the hard work
+of setting up routing for you.
Basically, what happens is that each node behaves as if it were an OSPF router
that communicates instantly and magically with all other routers. Each node
-generates link advertisements to the global route manager, which uses this
-information to construct the routing tables for each node.
+generates link advertisements and communicates them directly to a global route
+ manager, which uses this global information to construct the routing tables
+for each node. Setting up this form of routing is a one-liner:
@verbatim
GlobalRouteManager::PopulateRoutingTables ();
@end verbatim
The remainder of the script should be very familiar to you. We just enable
-ASCII and pcap tracing and run the simulation:
+pcap tracing, run the simulation and exit the script. Notice that enabling
+pcap tracing using the CSMA helper is done in the same way as for the pcap
+tracing with the point-to-point helper.
@verbatim
- std::ofstream ascii;
- ascii.open (``star.tr'');
- PointToPointHelper::EnableAsciiAll (ascii);
- PointToPointHelper::EnablePcapAll (``star'');
+ PointToPointHelper::EnablePcapAll ("second");
+ CsmaHelper::EnablePcapAll ("second");
Simulator::Run ();
Simulator::Destroy ();
+ return 0;
}
@end verbatim
-In order to run this example, you use waf. All of our example programs are
-built by default, so all you have to do is run it. Now, you did not build
-this script by copying it into the scratch directory, so you don't have to
-add the scratch directory to the run command. You just do the following,
+In order to run this example, you have to copy the @code{second.cc} example
+script into the scratch directory and use waf to build just as you did with
+the @code{first.cc} example. If you are in the top-level directory of the
+repository you would type,
@verbatim
- ./waf --run star
+ cp examples/second.cc scratch/
+ ./waf
+ ./waf --run scratch/second
@end verbatim
-There is no output from this script by default. It will just go off and
-silently do its thing. All you will see are waf messages:
+Since we have set up the UDP echo applications just as we did in the
+@code{first.cc} script, you will see similar output.
@verbatim
- ~/repos/ns-3-tutorial > ./waf --run star
- Entering directory `/home/craigdo/repos/ns-3-tutorial/build'
+ ~/repos/ns-3-dev > ./waf --run scratch/second
+ Entering directory `/home/craigdo/repos/ns-3-dev/build'
Compilation finished successfully
- ~/repos/ns-3-tutorial >
+ Sent 1024 bytes to 10.1.2.4
+ Received 1024 bytes from 10.1.1.1
+ Received 1024 bytes from 10.1.2.4
+ ~/repos/ns-3-dev >
@end verbatim
+Recall that the first message, @code{Sent 1024 bytes to 10.1.2.4} is the
+UDP echo client sending a packet to the server. In this case, the server
+is on a different network (10.1.2.0). The second message, @code{Received 1024
+bytes from 10.1.1.1}, is from the UDP echo server, generated when it receives
+the echo packet. The final message, @code{Received 1024 bytes from 10.1.2.4}
+is from the echo client, indicating that it has received its echo back from
+the server.
+
If you now go and look in the top level directory, you will find a number of
trace files:
@verbatim
- ~/repos/ns-3-tutorial > ls *.pcap *.tr
- star-0-0.pcap star-0-3.pcap star-1-0.pcap star-4-0.pcap star.tr
- star-0-1.pcap star-0-4.pcap star-2-0.pcap star-5-0.pcap
- star-0-2.pcap star-0-5.pcap star-3-0.pcap star-6-0.pcap
- ~/repos/ns-3-tutorial >
+ ~/repos/ns-3-dev > ls *.pcap
+ second-0-0.pcap second-1-1.pcap second-3-0.pcap
+ second-1-0.pcap second-2-0.pcap second-4-0.pcap
+ ~/repos/ns-3-dev >
@end verbatim
-If you have gone through the tracing section of this tutorial, you know how to
-interpret all of these files. For your reading pleasure, we reproduce a
-tcpdump from the net device on node four:
+Let's take a moment to look at the naming of these files. They all have the
+same form --- @code{<name>-<node>-<device>.pcap}. For example, the first file
+in the listing is @code{second-0-0.pcap} which is the pcap trace from node
+zero, device zero. There are no other devices on node zero so this is the
+only trace from that node.
+
+Now look at @code{second-1-0.pcap} and @code{second-1-1.pcap}. The former is
+the pcap trace for device zero on node one and the latter is the trace file
+for device one on node one. If you refer back to the topology cartoon at
+the start of the section, you will see that node one is the node that has
+both a point-to-point device and a CSMA device, so we should expect two pcap
+traces for that node.
+
+Now, let's follow the echo packet through the internetwork. First, do a
+tcpdump of the trace file for the leftmost point-to-point node --- node zero.
+
+@verbatim
+ ~/repos/ns-3-dev > tcpdump -r second-0-0.pcap -nn -tt
+ reading from file second-0-0.pcap, link-type PPP (PPP)
+ 2.000000 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
+ 2.007382 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
+ ~/repos/ns-3-dev >
+@end verbatim
+
+The first line of the dump indicates that the link type is PPP (point-to-point)
+which we should expect. You then see the echo packet leaving node zero on
+via the device associated with IP address 10.1.1.1 headed for IP address
+10.1.2.4 (the rightmost CSMA node).
+
+This packet will move over the point-to-point link and be received by the
+point-to-point net device on node one. Let's take a look:
+
+@verbatim
+ ~/repos/ns-3-dev > tcpdump -r second-1-0.pcap -nn -tt
+ reading from file second-1-0.pcap, link-type PPP (PPP)
+ 2.003686 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
+ 2.003695 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
+ ~/repos/ns-3-dev >
+@end verbatim
+
+Here we see that the link type is also PPP as we would expect. You see the
+packet from IP address 10.1.1.1 headed toward 10.1.2.4 appear on this
+interface. Now, internally to this node, the packet will be forwarded to the
+CSMA interface and we should see it pop out the other device headed for its
+ultimate destination. Let's then look at second-1-1.pcap and see if its there.
@verbatim
- [ns-regression] ~/repos/ns-3-tutorial > tcpdump -r star-4-0.pcap -nn -tt
- reading from file star-4-0.pcap, link-type PPP (PPP)
- 2.000000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.009068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.010000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.019068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.020000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.029068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.030000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.039068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.040000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.049068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.050000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.059068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.060000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.069068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.070000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.079068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.080000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.089068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- 2.090000 IP 10.1.4.2.49153 > 10.1.1.2.9: UDP, length 137
- 2.099068 IP 10.1.1.2.9 > 10.1.4.2.49153: UDP, length 137
- [ns-regression] ~/repos/ns-3-tutorial >
+ ~/repos/ns-3-dev > tcpdump -r second-1-1.pcap -nn -tt
+ reading from file second-1-1.pcap, link-type EN10MB (Ethernet)
+ 2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
+ 2.003687 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
+ 2.003687 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
+ 2.003691 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
+ 2.003691 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
+ 2.003695 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
+ ~/repos/ns-3-dev >
+@end verbatim
+
+As you can see, the link type is now ``Ethernet.'' Something new has appeared,
+though. The bus network needs @code{ARP}, the Address Resolution Protocol.
+The node knows it needs to send the packet to IP address 10.1.2.4, but it
+doesn't know the MAC address of the corresponding node. It broadcasts on the
+CSMA network (ff:ff:ff:ff:ff:ff) asking for the device that has IP address
+10.1.2.4. In this case, the rightmost node replies saying it is at MAC address
+00:00:00:00:00:06. This exchange is seen in the following lines,
+
+@verbatim
+ 2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
+ 2.003687 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
@end verbatim
-You are encouraged to think through the pcap traces and map them back to the
-script. Also, take a look at the @code{star.tr} ASCII trace file (which is
-quite large) to make sure you understand what it is telling you.
+Then node one, device one goes ahead and sends the echo packet to the UDP echo
+server at IP address 10.1.2.4. We can now look at the pcap trace for the
+echo server,
+
+@verbatim
+ ~/repos/ns-3-dev > tcpdump -r second-4-0.pcap -nn -tt
+ reading from file second-4-0.pcap, link-type EN10MB (Ethernet)
+ 2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
+ 2.003686 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
+ 2.003690 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
+ 2.003690 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
+ 2.003692 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
+ 2.003692 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
+ ~/repos/ns-3-dev >
+@end verbatim
+
+Again, you see that the link type is ``Ethernet.'' The first two entries are
+the ARP exchange we just explained. The third packet is the echo packet
+being delivered to its final destination.
+
+The echo server turns the packet around and needs to send it back to the echo
+cleint on 10.1.1.1 but it knows that this address is on another network that
+it reaches via IP address 10.1.2.1. This is because we initialized the global
+routing and it has figured all of this out for us. But, the echo server node
+doesn't know the MAC address of the first CSMA node, so it has to ARP for it
+just like the first CSMA node had to do. We leave it as an exercise for you
+to find the entries corresponding to the packet returning back on its way to
+the client (we have already dumped the traces and you can find them in those
+tcpdumps above.
+
+Finally, let's take a look at one of the CSMA nodes that wasn't involved in
+the packet exchange:
+
+@verbatim
+ ~/repos/ns-3-dev > tcpdump -r second-2-0.pcap -nn -tt
+ reading from file second-2-0.pcap, link-type EN10MB (Ethernet)
+ 2.003686 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
+ 2.003691 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
+ ~/repos/ns-3-dev >
+@end verbatim
+
+You can see that the CSMA channel is a broadcast medium and so all of the
+devices see the ARP requests involved in the packet exchange. The remaining
+pcap trace will be identical to this one.
+