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+# Network Simulation: Labwork 3
+## Bus Network Topology
+Build a bus network topology of three nodes (0, 1, 2)
+on a LAN using CSMA channel:
+
+```c++
+CsmaHelper csma;
+// Commonly found Ethernet data rate
+csma.SetChannelAttribute ("DataRate", StringValue ("100Mbps"));
+// Speed of light, over maximum distance of 2000 meters
+csma.SetChannelAttribute ("Delay", TimeValue (NanoSeconds (6560)));
+
+NodeContainer nodes;
+nodes.Create (3);
+NetDeviceContainer csmaDevices = csma.Install (nodes);
+
+InternetStackHelper stack;
+stack.Install (nodes);
+
+Ipv4AddressHelper address;
+address.SetBase ("10.1.1.0", "255.255.255.0");
+Ipv4InterfaceContainer csmaInterfaces = address.Assign (csmaDevices);
+Ipv4GlobalRoutingHelper::PopulateRoutingTables ();
+
+Simulator::Run ();
+Simulator::Destroy ();
+```
+
+## Packet Sniffing
+Implement scenario of node 0 is a client and node 2 is an echo server,
+which exchange 100 packets within 10 seconds:
+
+```c++
+// Set up ping server at port 9
+UdpEchoServerHelper echoServer (9);
+ApplicationContainer serverApps = echoServer.Install (nodes.Get (2));
+serverApps.Start (Seconds (1.0));
+serverApps.Stop (Seconds (12.0));
+
+UdpEchoClientHelper echoClient (csmaInterfaces.GetAddress (2), 9);
+echoClient.SetAttribute ("MaxPackets", UintegerValue (100));
+// For the ease of comparison with the later section
+echoClient.SetAttribute ("Interval", TimeValue (MilliSeconds (100)));
+echoClient.SetAttribute ("PacketSize", UintegerValue (1024));
+
+ApplicationContainer clientApps = echoClient.Install (nodes.Get (0));
+clientApps.Start (Seconds (2.0));
+clientApps.Stop (Seconds (12.0));
+
+Ipv4GlobalRoutingHelper::PopulateRoutingTables ();
+```
+
+Capture the PCAP trace from node 1:
+
+```c++
+csma.EnablePcap ("second", csmaDevices.Get (1), true);
+```
+
+## Grow the Topology
+Implement the scenario of five nodes
+
+```c++
+nodes.Create (5);
+```
+
+on which there are 2 pairs of client-server (0-2) and (1-3) exchanging packets
+at the same time, by adding a server at node 3 and a client at node 1:
+
+```c++
+serverApps = echoServer.Install (nodes.Get (3));
+serverApps.Start (Seconds (1.0));
+serverApps.Stop (Seconds (12.0));
+
+echoClient = UdpEchoClientHelper(csmaInterfaces.GetAddress (3), 9);
+echoClient.SetAttribute ("MaxPackets", UintegerValue (100));
+echoClient.SetAttribute ("Interval", TimeValue (MilliSeconds (100)));
+echoClient.SetAttribute ("PacketSize", UintegerValue (1024));
+
+clientApps = echoClient.Install (nodes.Get (1));
+clientApps.Start (Seconds (2.0));
+clientApps.Stop (Seconds (12.0));
+```
+
+The exchange rate is kept at 10 packets/s for 10s.
+Node 4 is used instead for sniffing:
+
+```c++
+csma.EnablePcap ("second", csmaDevices.Get (4), true);
+```
+
+At the interval of 100 ms (compared to the expected delivery delay
+of under 93 μm), it is unsurprising that for both cases of one and two
+echo pairs the packet delivery rate is perfectly 100 %.
+
+However, in the case with two pairs, both clients send at the same time
+and one server receives a few hundred microseconds later than the other
+due to queuing.  To estimated by the average delay, we only need the time
+the clients receive the echo packet from the server:
+
+```console
+$ tcpdump -nn -tt -r ../../second-4-0.pcap |
+> grep 'IP 10.1.1.[34]' |
+> awk 'match($1, /[0-9]+\.[0-9]/) {sum += $1 - substr($1, RSTART, RLENGTH)}
+> END {print sum/NR/2*1e6, "μs"}'
+235.213 μs
+```
+
+In the original case, the average delay is almost a half:
+
+```console
+$ tcpdump -nn -tt -r ../../second-1-0.pcap |
+> grep 'IP 10.1.1.3' |
+> awk 'match($1, /[0-9]+\.[0-9]/) {sum += $1 - substr($1, RSTART, RLENGTH)}
+> END {print sum/NR/2*1e6, "μs"}'
+137.75 μs
+```