Today I have been mostly teaching or preparing upcoming courses. I also
had a nice lunch discussion with
colleagues on DNS and the
role of transaction IDs, but that story will have to wait until tomorrow!
I gave another networking course for first year's students today. This
was the first practical session where they actually had to plug some
cables around: you can imagine the excitement but also the mess! To make
things even easier, the course was in a new networking lab I had never
been before, so I had to improvise with the hardware lying around.
The students learnt how to configure network interfaces (
netstat on FreeBSD), and they had to use their prior knowledge
of packet capture and
ping to troubleshoot when things didn't work as
expected. They had to form a simple "chain" topology (shown below) with
two subnets, and the computer in the middle needed to be configured as a
router. They needed to figure out that static routes were required on
both edge computers, so that they knew how to reach the remote subnet
through the router. Finally, they looked in details at the behaviour of
ARP and the scope of MAC addresses.
Network security course
I then prepared an upcoming practical session on network security with a
colleague working for Quarkslab. I already
have a good part of the course ready from last year on firewalling and
advanced uses of
iptables (including compiling custom BPF programs!).
My colleague wants to add a part where students will practice ARP
spoofing, so we looked at how to integrate that with the existing content.
Interestingly, he showed me how to automate virtual machine generation
using Packer. This should be really helpful for
future teachers in this course: they will be able to easily customize and
rebuild the virtual machine images used by the students! Last year, I
installed and configured the virtual machine manually, which makes it hard
to update it or apply the same modifications to a new VM image.
Today I mostly worked on my current research project, a simulator of
multipath multistream scheduling algorithms.
Multipath scheduling is needed when you want to transmit data over several
concurrent paths: which piece of data should be sent on which path? This
problem has been made visible by Multipath
TCP, since the Linux implementation includes
that can be changed at runtime. Several new schedulers for MPTCP are
being proposed in the academic literature every year: the original
and its evaluation,
All these algorithms mostly differ in the objective function they try to
optimize or in assumptions that can be made for specific data flows
(e.g. video streaming traffic). However, they all adopt the semantic of
TCP, which transports a single flow of data. I am interested in
extending the problem for several streams (have you heard of QUIC?)
that need to be scheduled on multiple paths. Instead of a single
optimisation problem, you now end up with several concurrent streams,
where each stream wants to complete as soon as possible!
Writing a simulator
The goal of my simulator is to quickly obtain an intuition on the
behaviour of scheduling algorithms: it provides a graphical and animated
visualisation of what's going on over time. The simulator also allows for
more in-depth exploration, for instance comparing the completion times of
streams for different scheduling algorithms.
Below is a screenshot of the current simulator: it is not very pretty
because that's not its goal! The streams are represented by vertical bars
whose size equals the amount of data remaining to be transmitted, and the
darker parts represent in-flight data. Below are two paths, each modelled
as a packet queue with a constant-rate service (link capacity) and a fixed
I am writing this simulator in Python thanks to
salabim: this is a really well designed,
easy-to-use and well-documented simulation framework. I had an initial
simulation prototype working in less than one day, and it took only an
additional day to add graphical visualisation. One of the reasons it's so
easy to use is thanks to Python: I didn't want to spend days implementing
complex algorithms in NS-3, even though it would be much more realistic.
At the same time, salabim is reasonably fast once you disable logging and
After working with salabim some more, I did find some limitations: the
programming style around salabim is fine for small simulations, but
quickly becomes a mess for larger projects. All examples use lots of
global variables, which
encourages you to write all your code in one file (after all, this is how
salabim itself is
with its 15k lines in a single file...)