In Portus, a congestion control algorithm is represented as a python class and a new instance of this class is created for each flow (and deleted when this flow ends). This class must be a subclass of `portus.AlgBase` and must implement the following 3 methods:

1. init_programs(): return all DATAPATH programs that your algorithm will use.
   • Returns a list of 2-tuples, where each tuple consists of (1) a unique (string-literal) name for the program, and (2) the program itself (also a string-literal).
   • Note: this is a static/class method, it does not take self, and thus will produce the same result for all flows
2. on_create(self): initialize any per-flow state (and store it in self), and choose an initial datapath program to use.
   • Doesn't return anything.
3. on_report(self, r): all algorithm logic goes here, called each time your datapath progarm executes (report).
   • The report parameter r is a report containing all of the fields you defined in your datapath program Report structure, plus two permanent fields, Cwnd and Rate, which are the current congestion window and rate in the datapath at the time of this report.
   • Doesn't return anything.

Since python (2) doesn't have type annotations, we use our own runtime type checker to ensure:

• Your class is a subclass of portus.AlgBase.
• All 3 methods are implemented.
• Each method takes the correct parameters (names must match exactly).
• Each method returns the correct type (mainly relevant for init_programs).

Thus, the minimal working example that will pass our type checker is as follows:

import portus

class AIMD(portus.AlgBase):          # <-- subclass of AlgBase

    def init_programs(self):
        return [("name", "program")] # <-- note the example return type

    def on_create(self):
        # TODO initial state
        # TODO self.datapath.set_program(...)
        pass

    def on_report(self, r):
        # TODO do stuff with r
        # TODO maybe change cwnd or rate: self.datapath.update_field(...)
        pass

Starting with this base, we will go through the implementation of AIMD one function at a time...

Some rust stuff here...