Mininet Assignment

For this project you are to install a Mininet virtual machine (see mininet_basics.html), and complete two of the experiments below. Neither involves "serious" programming, but you will have to use Python as a configuration language, and maybe be able to do some basic shell scripting.

1. TCP Simultaneous Open

For this project, you are to arrange so that you can view and record TCP simultaneous open. Here is a suitable network configuration:

h1 ---- R ---- h2

The main purpose of the router R is to have a place where you can monitor the traffic, using tcpdump or wireshark.

Start xterms (x-terminals) on each of h1, h2, and R. I will assume that h1's IPv4 address is and h2's is; these are the assignments if you run python -N 1. Then run the following:

But this doesn't get you a simultaneous open. To do that, you'll have to run netcat -p 5432 ip-address 5432 on both h1 and h2 simultaneously. The -p 5432 specifies the source port; you can also do netcat -p 2000 h2 3000 on h1, and netcat -p 3000 h1 2000 on h2, for example.

To run the two commands simultaneously is impossible, of course. However, you can add some delay to the h1--R and R--h2 links, at least 1000 ms worth. You will probably also want to use ssh from, say, R to h1 and h2, to run the commands in quick succession.

To set the delay, see the file, which unfortunately I did not get pre-installed in the virtual machine (use ssh or scp to transfer, or copy/paste if you are able to get that working, or just open a browser within the virtual machine and download it from this page). I've set DELAY='250ms' in the file, and, if mininet is running and I type h1 ping h3, I get

PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=63 time=250 ms
64 bytes from icmp_seq=2 ttl=63 time=250 ms
64 bytes from icmp_seq=3 ttl=63 time=250 ms

which is exactly what it is supposed to be.

I did this experiment using a modified version of, which I named (Specifically, I increased the DELAY variable and made sure r's interfaces were being assigned IP addresses (in my version I also eliminated h2, but that was unnecessary).) The file creates a topology h1 ---- r ---- h3, with a one-way delay on the r---h3 link. The delay applies only in the h3-to-r direction, though, so to arrange a packet crossing you will have to take that into account. But this is good enough, provided you start at h3. Here's the basic idea:

One way to do this quickly is to type both commands, but don't press Enter. Then give the h3 window the focus (that is, click on it), and move the mouse over the h1 window, but don't click yet. Press ENTER, and the h3 command will start, because that window still has the focus. Now click -- on the h1 window -- and press ENTER again to start the previously typed command on h1. In other words, the sequence is ENTER-CLICK-ENTER. That's easy to do quickly; there's no mouse motion in that sequence.

It took me a fair number of tries, but I finally got it to work (almost all my failed attempts were because I typed a port number or IP address wrong). One bright spot: if you get the simultaneous open to work, then typing a string on either side will cause it to appear on the other side. I had thought that netcat would simply create the connection (visible in tcpdump) and then lock up.

How much delay do you need? 3000 ms is easy; can you do it in 1000?

Bidirectional delay is straightforward to set up, but we don't need it here. All we need is for packets to cross in the wire. So start on h3, and while that SYN packet is making its way to h1, start on h1 before it gets there. Note that if you start on h1, the SYN packet arrives at h3 essentially immediately, and there is no hope of arranging a simultaneous open.

There's a problem with the commands to set up node r are commented out, and they use the wrong interfaces. Here are the commented-out lines:

    # r.cmd('ifconfig r-eth0')
    # r.cmd('ifconfig r-eth1')
    # r.cmd('ifconfig r-eth2')

But at some point an update to Mininet changed the interface numbering, so instead of eth0,eth1,eth2 it's now eth1,eth2,eth3, and I did not notice this. Change the interface names as follows:

    r.cmd('ifconfig r-eth1')
    r.cmd('ifconfig r-eth2')
    r.cmd('ifconfig r-eth3')

Bonus points: close the connection on h3 first and then as soon as possible on h1 in order to generate a simultaneous close.

Alternative method: set up ssh so, from r, you can put ssh commands in a shell script to start netcat on h1 and then netcat on h3. But you may need to do a moderate bit of ssh configuration.

To submit, save the .pcap file from tcpdump (it may have a .pcapng extension; ng for "next generation")

2. TCP Reno vs Cubic

The goal is to compare the total throughput for the two TCP congestion mechanisms. For this example, the relevant Python file is TCP Reno and TCP Cubic are both available.

Choose a time interval, eg 10 seconds (maybe less for trial runs), and compare the throughput of two connections sharing a link. Do a few runs to get an idea of how consistent your results are. Actually, your time interval will usually be until one connection transfers all of its blocks, so what you're really doing is choosing a value for blockcount large enough that the test runs for 10 seconds (or 30 seconds or whatever).

I recommend comparing Reno to Cubic and also, as a control, Reno to Reno.

It is important to try different RTTs, eg 50 ms, 100 ms and 200 ms (for the basic tests, you can implement delay on the common link; be careful whether it is bidirectional or not). Both connections will usually use the same RTT for any given run, though.

You might also try different delays on the two paths. TCP Cubic is supposed to handle this reasonably well for cubic-to-cubic competition. But this is optional.

This one is probably the most straightforward: use, with running on h3 and two instances of on each of h1 and h2 (or both running on h1, which is what I usually do). Note that takes command-line arguments to control its behavior: the following

python3 10000 5430 reno

sends 10000 blocks to h3 at port 5430 (one of the two used by default in using TCP Reno. You can replace 'reno' by 'cubic' to get TCP Cubic. takes an optional block-count parameter, which should match that of the senders, in which case it calculates the bandwidth ratio for you:

    python3 100000

Phase effects

Strictly speaking, the setup here will be influenced by phase effects. You are not required to adjust for these in this assignment. But the way to address these would be to introduce some random traffic to break up the "phase locks". This can be done using You should verify the following variable values: BottleneckBW=40 (40 mbit/sec) and density=0.02 (2%) (since two connections are using the same h1--r link). The version of linked to here has these values preset.

This uses UDP because the small TCP packets were running together. With the values above, the mean gap between packets is something like 2.5ms, and TCP tends to combine packets sent within 5-10ms of one another.

The random-telnet solution is still imperfect; there is still a great deal of variability. But it's better than without.

We would run on h1; the existing parameters should be good though you will have to specify the destination address:


To receive this traffic, run the following on the destination host, eg h3 (5433 is the port number, and we're redirecting output to /dev/null because we're ignoring it)

    netcat -l -u 5433 > /dev/null

I usually find it easiest to create a separate pair of xterm windows on h1 and h3 for the purposes of running this.

To submit, send me (a) a description of the setup you actually used, (b) some of your output, and (c) a brief discussion of how you determined the relative throughputs of reno and cubic from that output. Note that you'll need to do this for each RTT you used.

3. Bandwidth limits

Set up a topology like this, based on the file:


Run htb on R, and set separate classes for traffic from h1/h2/h3 to h4. Run on each of h1/h2/h3, and a receiver on h4 (I'm still working on that; only receives from two and that should be good enough to get started while I work out a more general solution).

Set appropriate bandwidth limits for each of h1/h2/h3, and show that they are obeyed.

Experiment with a large burst for h1, but not h2/h3. What happens?

Can you change the bandwidth while traffic is being sent, and see a change in the received bandwidth?

To do this you must set up the Linux bandwidth-throttling system, known as htb for hierarchical token bucket. The command you use is tc, for traffic control.

I will for the time being stick with the configuration with one less host:

         r ---- h3

You run htb on r. If we're trying to control traffic through r, we must apply htb to the downstream interface, which for h1->h3 and h2->h3 traffic is named r-eth3. Here are the commands necessary to limit h1->h3 traffic to 1 mbit/sec, and h2->h3 traffic to 10 mbit/sec. When specifying rates, mbit is megabit/sec and mbps is megabyte/sec. Also, these units are written adjacent to the numeric value, with no intervening space!

First, set up the htb "root", and a default class (which we eventually do not use, but the point is that traffic goes through the default class until everything else is set up, so traffic is never blocked).

tc qdisc del dev r-eth3 root        # delete any previous htb stuff on r-eth3

tc qdisc add dev r-eth3 root handle 1: htb default 10    # class 10 is the default class

Now we add two classes, one for h1 (class 1) and one for h2 (class 2). At this point you can't tell yet that class 1 is for h1; that's in the following paragraph.

tc class add dev r-eth3 parent 1: classid 1:1 htb rate 1mbit    # '1mbit' has no space! The classes are 1:1 and (next line) 1:2

tc class add dev r-eth3 parent 1: classid 1:2 htb rate 10mbit

Now we have to create filters that assign traffic to one class or another. The flowid of the filter matches the classid above. We'll assign traffic from h1 ( to classid 1:1, and traffic from h2 to classid 1:2. The "parent 1:" identifies the root above with handle 1:. The "u32" refers to the so-called u32 classifier; the name comes from unsigned 32-bit.

tc filter add dev r-eth3 protocol ip  parent 1: u32 match ip src flowid 1:1

tc filter add dev r-eth3 protocol ip  parent 1: u32 match ip src flowid 1:2

Now if we start the senders, we should see h1 getting 1/10 the bandwidth! Also, each sender should get 1mbit and 10mbit respectively. We can also just drop the 1:2 class (and its filter), and have that traffic go to the default. In this case, h1 is limited to 1mbit and h2 is not limited at all.

We can also apply filters to traffic from selected ports, so different traffic from the same host is treated differently.

We can also replace "add" by "change" to update the rules. This makes the most sense for the "tc class" statements that assign rates. (We can also use "del" to delete classes.)

To submit, send me your own list of tc/htb commands, some output (from, eg, showing the bandwidth throttling, and a brief discussion.

4. Routing Merry-go-round

Set up four routers in a loop:

       [r1-eth1]                             [r2-eth1]                    
          |                                     |
       10.0.0                                10.0.2
          |                                     |                    
       [r4-eth1]                             [r3-eth0]

(You don't actually need any non-router hosts here.) For the r1--r2 link, r1 would have address and r2 would have address, and so on for each of the other four links (so on the r1--r4 link, r1 would have address and r4 would have address

[There is a catch here: if you create an interface, then subsequent interfaces have to have a higher number. Thus, starting at r1 and working clockwise, we must always create the eth0 interface before the eth1 interface. But when we come back to r1, under the "natural" ordering we would want to create the eth1 interface first, which we cannot do. So we reverse the interface numbering at r1.]

Then set up forwarding so that each router knows how to reach its two immediate neighbors, and the default route is to the clockwise neighbor.

Almost all this will be done in your python config file. You might start by copying to, and then making the appropriate edits. You will probably need to call rp_disable() on each router (this is found in

At this point we have an example of asymmetric routing: r2 reaches r4 via r3, but r4 reaches r2 via r1. Verify that each router can ping every other router.

The command ip route get addr is useful for determining the next hop to the destination. For example, on r1, ip route get returns via dev r1-eth1 src, with as the next hop.

Now send a packet to an unknown IP network (if you've used addresses for everything, you can send to What happens? [I was expecting a routing loop, but I have not yet been able to get this to happen, probably because r1, etc, won't forward a packet if the source address is r1.] Document the packet behavior using wireshark on one of the routers.