asf-probing-module.cpp

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/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
#include "asf-probing-module.hpp"
#include "core/random.hpp"

namespace nfd {
namespace fw {
namespace asf {

constexpr time::seconds ProbingModule::DEFAULT_PROBING_INTERVAL;

static_assert(ProbingModule::DEFAULT_PROBING_INTERVAL < AsfMeasurements::MEASUREMENTS_LIFETIME,
              "ProbingModule::DEFAULT_PROBING_INTERVAL must be less than AsfMeasurements::MEASUREMENTS_LIFETIME");

ProbingModule::ProbingModule(AsfMeasurements& measurements)
  : m_probingInterval(DEFAULT_PROBING_INTERVAL)
  , m_measurements(measurements)
{
}

void
ProbingModule::scheduleProbe(const fib::Entry& fibEntry, const time::milliseconds& interval)
{
  Name prefix = fibEntry.getPrefix();

  // Set the probing flag for the namespace to true after passed interval of time
  scheduler::schedule(interval, [this, prefix] {
    NamespaceInfo* info = m_measurements.getNamespaceInfo(prefix);

    if (info == nullptr) {
      // fib::Entry with the passed prefix has been removed or the fib::Entry has
      // a name that is not controlled by the AsfStrategy
      return;
    }
    else {
      info->setIsProbingDue(true);
    }
  });
}

Face*
ProbingModule::getFaceToProbe(const Face& inFace,
                              const Interest& interest,
                              const fib::Entry& fibEntry,
                              const Face& faceUsed)
{
  FaceInfoFacePairSet rankedFaces(
    [] (FaceInfoFacePair pairLhs, FaceInfoFacePair pairRhs) -> bool {
      // Sort by RTT
      // If a face has timed-out, rank it behind non-timed-out faces
      FaceInfo& lhs = *pairLhs.first;
      FaceInfo& rhs = *pairRhs.first;

      return (!lhs.isTimeout() && rhs.isTimeout()) ||
             (lhs.isTimeout() == rhs.isTimeout() && lhs.getSrtt() < rhs.getSrtt());
  });

  // Put eligible faces into rankedFaces. If a face does not have an RTT measurement,
  // immediately pick the face for probing
  for (const fib::NextHop& hop : fibEntry.getNextHops()) {
    Face& hopFace = hop.getFace();

    // Don't send probe Interest back to the incoming face or use the same face
    // as the forwarded Interest
    if (hopFace.getId() == inFace.getId() || hopFace.getId() == faceUsed.getId()) {
      continue;
    }

    FaceInfo* info = m_measurements.getFaceInfo(fibEntry, interest, hopFace);

    // If no RTT has been recorded, probe this face
    if (info == nullptr || !info->hasSrttMeasurement()) {
      return &hopFace;
    }

    // Add FaceInfo to container sorted by RTT
    rankedFaces.insert(std::make_pair(info, &hopFace));
  }

  if (rankedFaces.empty()) {
    // No Face to probe
    return nullptr;
  }

  return getFaceBasedOnProbability(rankedFaces);
}

bool
ProbingModule::isProbingNeeded(const fib::Entry& fibEntry, const Interest& interest)
{
  // Return the probing status flag for a namespace
  NamespaceInfo& info = m_measurements.getOrCreateNamespaceInfo(fibEntry, interest);

  // If a first probe has not been scheduled for a namespace
  if (!info.isFirstProbeScheduled()) {
    // Schedule first probe between 0 and 5 seconds
    uint64_t interval = getRandomNumber(0, 5000);
    scheduleProbe(fibEntry, time::milliseconds(interval));

    info.setHasFirstProbeBeenScheduled(true);
  }

  return info.isProbingDue();
}

void
ProbingModule::afterForwardingProbe(const fib::Entry& fibEntry, const Interest& interest)
{
  // After probing is done, need to set probing flag to false and
  // schedule another future probe
  NamespaceInfo& info = m_measurements.getOrCreateNamespaceInfo(fibEntry, interest);
  info.setIsProbingDue(false);

  scheduleProbe(fibEntry, m_probingInterval);
}

Face*
ProbingModule::getFaceBasedOnProbability(const FaceInfoFacePairSet& rankedFaces)
{
  double randomNumber = getRandomNumber(0, 1);
  uint64_t rankSum = ((rankedFaces.size() + 1) * rankedFaces.size()) / 2;

  uint64_t rank = 1;
  double offset = 0.0;

  for (const FaceInfoFacePair pair : rankedFaces) {
    double probability = getProbingProbability(rank++, rankSum, rankedFaces.size());

    // Is the random number within the bounds of this face's probability + the previous faces'
    // probability?
    //
    // e.g. (FaceId: 1, p=0.5), (FaceId: 2, p=0.33), (FaceId: 3, p=0.17)
    //      randomNumber = 0.92
    //
    //      The face with FaceId: 3 should be picked
    //      (0.68 < 0.5 + 0.33 + 0.17) == true
    //
    if (randomNumber <= offset + probability) {
      // Found face to probe
      return pair.second;
    }

    offset += probability;
  }

  // Given a set of Faces, this method should always select a Face to probe
  BOOST_ASSERT(false);
  return nullptr;
}

double
ProbingModule::getProbingProbability(uint64_t rank, uint64_t rankSum, uint64_t nFaces)
{
  // p = n + 1 - j ; n: # faces
  //     ---------
  //     sum(ranks)
  return static_cast<double>(nFaces + 1 - rank) / rankSum;
}

double
ProbingModule::getRandomNumber(double start, double end)
{
  std::uniform_real_distribution<double> dist(start, end);
  return dist(getGlobalRng());
}

} // namespace asf
} // namespace fw
} // namespace nfd