Tierless Programming for Software-Defined Networks

Posted on 30 September 2014.

Network devices like switches and routers update their behavior in real-time. For instance, a router may change how it forwards traffic to address an outage or congestion. In a traditional network, devices use distributed protocols to decide on mutually consistent behavior, but Software-Defined Networks (SDN) operate differently. Switches are no longer fully autonomous agents, but instead receive instructions from logically centralized controller applications running on separate hardware. Since these applications can be arbitrary programs, SDN operators gain tremendous flexibility in customizing their network.

The most popular SDN standard in current use is OpenFlow. With OpenFlow, Controller applications install persistent forwarding rules on the switches that match on packet header fields and list actions to take on a match. These actions can include header modifications, forwarding, and even sending packets to the controller for further evaluation. When a packet arrives without a matching rule installed, the switch defaults to sending the packet to the controller for instructions.

Let's write a small controller application. It should (1) record the addresses of machines sending packets on the network and (2) cause each switch to forward traffic by flooding (i.e., sending out on all ports except the arrival port). This is simple enough to write in POX, a controller platform for Python. The core of this program is a function that reacts to packets as they arrive at the controller (we have removed some boilerplate and initialization):

def _handle_PacketIn (self, event):
    packet = event.parsed

    def install_nomore ():
      msg = of.ofp_flow_mod()
      msg.match = of.ofp_match(dl_src = packet.src)
      msg.buffer_id = event.ofp.buffer_id
      msg.actions.append(of.ofp_action_output(port = of.OFPP_FLOOD))

    def do_flood ():
      msg = of.ofp_packet_out()
      msg.actions.append(of.ofp_action_output(port = of.OFPP_FLOOD))
      msg.data = event.ofp
      msg.buffer_id = None
      msg.in_port = event.port


First, the controller records the packet's source in its internal table. Next, the install_nomore function adds a rule to the switch saying that packets with this source should be flooded. Once the rule is installed, the switch will not send packets with the same source to the controller again. Finally, the do_flood function sends a reply telling the switch to flood the packet.

This style of programming may remind you of the standard three-tier web-programming architecture. Much like a web program generates JavaScript or SQL strings, controller programs produce new switch rules in real-time. One major difference is that switch rules are much less expressive than JavaScript, which means that less computation can be delegated to the switches. A bug in a controller program can throw the entire network's behavior off. But it's easy to introduce bugs when every program produces switch rules in real-time, effectively requiring its own mini-compiler!

SDN Programming Without Tiers

We've been working on a tierless language for SDN controllers: Flowlog. In Flowlog, you write programs as if the controller sees every packet, and never have to worry about the underlying switch rules. This means that some common bugs in controller/switch interaction can never occur, but it also means that the programming experience is simpler. In Flowlog, our single-switch address-monitoring program is just:

TABLE seen(macaddr);
ON ip_packet(p):
  INSERT (p.dlSrc) INTO seen;
  DO forward(new) WHERE new.locPt != p.locPt;

The first line declares a one-column database table, "seen". Line 2 says that the following two lines are triggered by IP packets. Line 3 adds those packets' source addresses to the table, and line 4 sends the packets out all other ports.

As soon as this program runs, the Flowlog runtime proactively installs switch rules to match the current controller state and automatically ensures consistency. As the controller sees more addresses, the switch sends fewer packets back to the controller—but this is entirely transparent to the programmer, whose job is simplified by the abstraction of an all-seeing controller.

Examples and Further Reading

Flowlog is good for more than just toy examples. We've used Flowlog for many different network applications: ARP-caching, network address translation, and even mediating discovery and content-streaming for devices like Apple TVs. You can read more about Flowlog and Flowlog applications in our paper here.

Next time, we'll talk more about what you can use Flowlog to write, and also see how Flowlog allows programs to call out to external libraries in other languages.

CS Student Work/Sleep Habits Revealed As Possibly Dangerously Normal

Posted on 14 June 2014. Written by Jesse Polhemus, and originally posted at the Brown CS blog

Imagine a first-year computer science concentrator (let’s call him Luis) e-mailing friends and family back home after a few weeks with Brown Computer Science (BrownCS). Everything he expected to be challenging is even tougher than anticipated: generative recursion, writing specifications instead of implementations, learning how to test his code instead of just writing it. Worst of all is the workload. On any given night, he’s averaging –this seems too cruel to be possible– no more than eight or nine hours of sleep.

Wait, what? Everyone knows that CS students don't get any sleep, so eight or nine hours is out of the question. Or is it? Recent findings from PhD student Joseph Gibbs Politz, adjunct professor Kathi Fisler, and professor Shriram Krishnamurthi analyze when students completed tasks in two different BrownCS classes, shedding interesting light on an age-old question: when do our students work, and when (if ever) do they sleep? The question calls to mind a popular conception of the computer scientist that Luis has likely seen in countless movies and books:

  • Hours are late. (A recent poster to boardgames@lists.cs.brown.edu requests a 2 PM start time in order to avoid being “ridiculously early” for prospective players.)
  • Sleep is minimal. BrownCS alumnus Andy Hertzfeld, writing about the early days of Apple Computer in Revolution in the Valley, describes the “gigantic bag of chocolate-covered espresso beans” and “medicinal quantities of caffeinated beverages” that allowed days of uninterrupted coding.

Part 1: Deadline Experiments

The story begins a few years before Luis’s arrival, when Shriram would routinely schedule his assignments to be due at the 11:00 AM start of class. “Students looked exhausted,” he remembers. “They were clearly staying up all night in order to complete the assignment just prior to class.”

Initially, he moved the deadline to 2:00 AM, figuring that night owl students would finish work in the early hours of the morning and then get some sleep. This was effective, but someone pointed out that it was unfair to other professors who taught earlier classes and were forced to deal with tired students who had finished Shriram’s assignment but not slept sufficiently.

“My final step,” he explains, “was to change deadlines to midnight. I also began penalizing late assignments on a 24-hour basis instead of an hourly one. This encourages students to get a full night’s sleep even if they miss a deadline.”

This was the situation when Luis arrives. The next task was to start measuring the results.

Part 2: Tracking Events

Shriram, Kathi, and Joe analyzed two of Shriram’s classes, CS 019 and CS 1730. For each class, Luis must submit test suites at any time he chooses, then read reviews of his work from fellow students. He then continues working on the solution, eventually producing a final implementation that must be submitted prior to the midnight deadline.

Part 3: Reality And Mythology

Given these parameters, what work and sleep patterns would you expect? We asked professor Tom Doeppner to reflect on Luis and share his experience of working closely with students as Director of Undergraduate Studies and Director of the Master’s Program. “Do students work late? I know I get e-mail from students at all hours of the night,” he says, “and I found out quickly that morning classes are unpopular, which is why I teach in the afternoon. Maybe it’s associated with age? I liked to work late when I was young, but I got out of the habit in my thirties.”

Asked about the possible mythologizing of late nights and sleeplessness, Tom tells a story from his own teaching: “Before we broke up CS 169 into two classes, the students had t-shirts made: ‘CS 169: Because There Are Only 168 Hours In A Week’. I think there’s definitely a widespread belief that you’re not really working hard unless you’re pulling multiple all-nighters.”

This doesn’t exactly sound like Luis’s sleep habits! Take a look at the graphs below to see how mythology and reality compare.

Part 4: Results And Conclusions

The graphs below depict test suite submissions, with time displayed in six-hour segments. For example, between 6 PM and the midnight deadline (“6-M”), 50 CS 173 students are submitting tests.

This graph is hypothetical, showing Joe, Kathi, and Shriram’s expectations for submission activity. They expected activity to be slow and increase steadily, culminating in frantic late-night activity just before the deadline. Generally taller “M-6” (midnight to 6 AM) bars indicate late-night work and a corresponding flurry of submissions, followed by generally shorter “6-N” (6 AM to noon) bars when students tried to get a few winks in. Cumulatively, these two trends depict the popular conception of the computer science student who favors late hours and perpetually lacks sleep.

These graphs show actual submissions. As expected, activity generally increases over time and the last day contains the majority of submissions. However, unexpectedly, the “N-6” (noon to 6 PM) and “6-M” (6 PM to midnight) segments are universally the most active. In the case of the CS 173 graph, this morning segment contains far more submissions than any other of the day’s three segments. In both of these graphs, the “M-6” (midnight to 6 AM) segments are universally the least active, even the day the assignment is due. For example, the final segment of this type, which represents the last available span of early morning hours, is among the lowest of all segments, with only ten submissions occurring. In contrast, the corresponding “6-N” (6 AM to noon) shows more than four times as many submissions, suggesting that most students do their work before or after the pre-dawn hours but not during them.

“I wouldn’t have expected that,” Joe comments. “I think of the stories folks tell of when they work not lining up with that, in terms of staying up late and getting up just in time for class. Our students have something important to do at midnight other than work: they cut off their work before midnight and do something else. For the majority it’s probably sleep, but it could just be social time or other coursework. Either way, it’s an interesting across-the-board behavior.”

If word of these results gets out, what can Luis and his fellow students expect? “People will realize,” Shriram says, “that despite what everyone likes to claim, students even in challenging courses really are getting sleep, so it’s okay for them to, too.” Joe agrees: “There isn’t so much work in CS that you have to sacrifice normal sleeping hours for it.”

Luis, his family, and his well-rested classmates will undoubtedly be glad to hear it. The only question is: will their own descriptions of their work/sleep habits change to match reality, or are tales of hyper-caffeinated heroics too tempting to resist?


The graphs above are simplified for readability, and aggregated into 6-hour increments. Below we include graphs of the raw data in 3-hour increments. This shows that there is some work going on from 12am-3am the night before assignments are due, but essentially nothing from 3am-6am.

In both of these classes, we were also performing experiments on code review, so the raw data includes when students read the code reviews they received, in addition to when they submitted their work. Since the review necessarily happens after submission, and the reading of the review after that, we see many more “late” events for reading reviews.

CS019 in 3-hour increments:

CS173 in 3-hour increments:

Parley: User Studies for Syntax Design

Posted on 01 April 2014.

Programming languages' syntax choices have always been cause for spirited, yet unresolvable, debate. Opinions and so-called best practices dominate discussion, with little empirical evidence to justify them. As part of the Pyret project, we've been performing one of the most comprehensive empirical studies on programming language syntax.

To recap some of the issues: Many languages repurpose plain English words for keywords, and run afoul of the impedance mismatch between, for instance, the dictionary meaning of switch and its semantics within the language (see Language-Independent Conceptual "Bugs" in Novice Programming for a much more detailed discussion). Another alternative, using non-ASCII symbols which cannot have their meaning conflated, is promising but doesn't work well with traditional editors (APL, we're looking at you).

We are, instead, evalauting the use of unconventional syntax motivated by a non-technical, easily-interpreted lexicon. We refer to these cohesive lexicons as lingos, and have begun experimenting with one lingo-based syntax for Pyret, which we call Parley. It is best to see the Parley lingo in action, compared to the more traditional syntax in early versions of Pyret:

var sum = 0
var arr = [1,2,3,4,5,6,7,8]
for each(piece from arr):
  sum := sum + piece
yar sum be 0
yar arr be [1,2,3,4,5,6,7,8]
fer each(piece of arr):
  sum now be sum + piece

While it should seem obvious to the casual reader that Parley lingo is a strict improvement, this is not a substitute for empirical evaluation. To this end, we have been running a rummy series of user studies to validate the new syntax. Our latest experiments are testing program comprehension using an aye-tracker. The results, which will be submitted to the Principles of Pirate Lingo conference, are pending clearance from our Aye Arr Bee.