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fw: ASF: Fix implementation not matching technical report
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This commit resolves several issues. First, a bug was discovered
regarding the usage of std::set objects when ranking faces for
forwarding and probing purposes. These objects enforce uniqueness;
however, this is defined based on the comparison function provided.
As no fields that were inherently unique were used to rank faces, this
would mean that a face with matching ranking information as one
inserted previously would be blocked from being inserted into face
ranking for forwarding and probing. We add faceId as a tiebreaker to
ensure deterministic behavior in these cases. We also fix an issue
where the count of consecutive silent timeouts was not properly cleared
upon the receipt of data or when a timeout was marked.

Additionally, this commit replaces previous implementations of the
ranking code for forwarding and probing. This is partially in order
to eliminate bugs introduced by incorrect implementations of the ranking
for probing and forwarding, as well as trying to make the code easier
to understand and maintain. This included removing a specific
optimization in probing by including unmeasured faces in the
ranking rather than have probes to unmeasured faces be ordered
based on undefined metrics (in practice, tied faces were inserted
in route creation order). However, we did make the choice to add
cost as a tiebreaker for working measured faces, as was the case
with the initial implementation for forwarding.

Refs #5310

Change-Id: Iabfdafea764fe24fe6c478a6073734e51aaf9fa1
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@awlane
awlane committed Feb 5, 2024
1 parent 9acba9c commit 5fdcbec
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Showing 6 changed files with 495 additions and 130 deletions.
17 changes: 7 additions & 10 deletions daemon/fw/asf-measurements.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -35,8 +35,9 @@

namespace nfd::fw::asf {

/** \brief Strategy information for each face in a namespace
*/
/**
* \brief Strategy information for each face in a namespace.
*/
class FaceInfo
{
public:
Expand Down Expand Up @@ -72,12 +73,6 @@ class FaceInfo
cancelTimeout(interestName);
}

bool
hasTimeout() const
{
return getLastRtt() == RTT_TIMEOUT;
}

time::nanoseconds
getLastRtt() const
{
Expand Down Expand Up @@ -123,7 +118,8 @@ class FaceInfo
////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////

/** \brief Stores strategy information about each face in this namespace
/**
* \brief Stores strategy information about each face in this namespace.
*/
class NamespaceInfo final : public StrategyInfo
{
Expand Down Expand Up @@ -183,7 +179,8 @@ class NamespaceInfo final : public StrategyInfo
////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////

/** \brief Helper class to retrieve and create strategy measurements
/**
* \brief Helper class to retrieve and create strategy measurements.
*/
class AsfMeasurements : noncopyable
{
Expand Down
146 changes: 95 additions & 51 deletions daemon/fw/asf-probing-module.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -57,14 +57,94 @@ ProbingModule::scheduleProbe(const fib::Entry& fibEntry, time::milliseconds inte
});
}

static auto
getFaceRankForProbing(const FaceStats& fs) noexcept
{
// The RTT is used to store the status of the face:
// - A positive value indicates data was received and is assumed to indicate a working face (group 1),
// - RTT_NO_MEASUREMENT indicates a face is unmeasured (group 2),
// - RTT_TIMEOUT indicates a face is timed out (group 3).
// These groups are defined in the technical report.
//
// Unlike during forwarding, we adjust the ranking such that unmeasured faces (group 2)
// are prioritized before working faces (group 1), and working faces are prioritized
// before timed out faces (group 3). We assign each group a priority value from 1-3
// to ensure lowest-to-highest ordering consistent with this logic.
// Additionally, unmeasured faces will always be chosen to probe if they exist.

// Working faces are ranked second in priority; if RTT is not
// a special value, we assume the face to be in this group.
int priority = 2;
if (fs.rtt == FaceInfo::RTT_NO_MEASUREMENT) {
priority = 1;
}
else if (fs.rtt == FaceInfo::RTT_TIMEOUT) {
priority = 3;
}

// We set SRTT by default to the max value; if a face is working, we instead set it to the actual value.
// Unmeasured and timed out faces are not sorted by SRTT.
auto srtt = priority == 2 ? fs.srtt : time::nanoseconds::max();

// For ranking, group takes the priority over SRTT (if present) or cost, SRTT (if present)
// takes priority over cost, and cost takes priority over FaceId.
// FaceId is included to ensure all unique entries are included in the ranking (see #5310).
return std::tuple(priority, srtt, fs.cost, fs.face->getId());
}

bool
ProbingModule::FaceStatsProbingCompare::operator()(const FaceStats& lhs, const FaceStats& rhs) const noexcept
{
return getFaceRankForProbing(lhs) < getFaceRankForProbing(rhs);
}

static Face*
chooseFace(const ProbingModule::FaceStatsProbingSet& rankedFaces)
{
static std::uniform_real_distribution<> randDist;
static auto& rng = ndn::random::getRandomNumberEngine();
const double randomNumber = randDist(rng);

const auto nFaces = rankedFaces.size();
const double rankSum = (nFaces + 1) * nFaces / 2;
size_t rank = 1;
double offset = 0.0;

for (const auto& faceStat : rankedFaces) {
// n + 1 - j
// p = ---------
// sum(ranks)
double probability = static_cast<double>(nFaces + 1 - rank) / rankSum;
rank++;

// 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
//
offset += probability;
if (randomNumber <= offset) {
// Found face to probe
return faceStat.face;
}
}

// Given a set of Faces, this method should always select a Face to probe
NDN_CXX_UNREACHABLE;
}

Face*
ProbingModule::getFaceToProbe(const Face& inFace, const Interest& interest,
const fib::Entry& fibEntry, const Face& faceUsed)
{
FaceInfoFacePairSet rankedFaces;
FaceStatsProbingSet rankedFaces;

// Put eligible faces into rankedFaces. If a face does not have an RTT measurement,
// immediately pick the face for probing
// Put eligible faces into rankedFaces. If one or more faces do not have an RTT measurement,
// the lowest ranked one will always be returned.
for (const auto& hop : fibEntry.getNextHops()) {
Face& hopFace = hop.getFace();

Expand All @@ -76,20 +156,25 @@ ProbingModule::getFaceToProbe(const Face& inFace, const Interest& interest,
}

FaceInfo* info = m_measurements.getFaceInfo(fibEntry, interest.getName(), hopFace.getId());
// If no RTT has been recorded, probe this face
if (info == nullptr || info->getLastRtt() == FaceInfo::RTT_NO_MEASUREMENT) {
return &hopFace;
rankedFaces.insert({&hopFace, FaceInfo::RTT_NO_MEASUREMENT,
FaceInfo::RTT_NO_MEASUREMENT, hop.getCost()});
}
else {
rankedFaces.insert({&hopFace, info->getLastRtt(), info->getSrtt(), hop.getCost()});
}

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

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

// If the top face is unmeasured, immediately return it.
if (rankedFaces.begin()->rtt == FaceInfo::RTT_NO_MEASUREMENT) {
return rankedFaces.begin()->face;
}

return chooseFace(rankedFaces);
}

Expand All @@ -103,7 +188,8 @@ ProbingModule::isProbingNeeded(const fib::Entry& fibEntry, const Name& interestN
if (!info.isFirstProbeScheduled()) {
// Schedule first probe between 0 and 5 seconds
static std::uniform_int_distribution<> randDist(0, 5000);
auto interval = randDist(ndn::random::getRandomNumberEngine());
static auto& rng = ndn::random::getRandomNumberEngine();
auto interval = randDist(rng);
scheduleProbe(fibEntry, time::milliseconds(interval));
info.setIsFirstProbeScheduled(true);
}
Expand All @@ -122,48 +208,6 @@ ProbingModule::afterForwardingProbe(const fib::Entry& fibEntry, const Name& inte
scheduleProbe(fibEntry, m_probingInterval);
}

Face*
ProbingModule::chooseFace(const FaceInfoFacePairSet& rankedFaces)
{
static std::uniform_real_distribution<> randDist;
double randomNumber = randDist(ndn::random::getRandomNumberEngine());
uint64_t rankSum = (rankedFaces.size() + 1) * rankedFaces.size() / 2;

uint64_t rank = 1;
double offset = 0.0;

for (const auto& 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
NDN_CXX_UNREACHABLE;
}

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;
}

void
ProbingModule::setProbingInterval(time::milliseconds probingInterval)
{
Expand Down
47 changes: 20 additions & 27 deletions daemon/fw/asf-probing-module.hpp
View file
Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
/*
* Copyright (c) 2014-2022, Regents of the University of California,
* Copyright (c) 2014-2024, Regents of the University of California,
* Arizona Board of Regents,
* Colorado State University,
* University Pierre & Marie Curie, Sorbonne University,
Expand Down Expand Up @@ -30,7 +30,19 @@

namespace nfd::fw::asf {

/** \brief ASF Probing Module
/**
* \brief Container for ranking-related values.
*/
struct FaceStats
{
Face* face = nullptr;
time::nanoseconds rtt = 0_ns;
time::nanoseconds srtt = 0_ns;
uint64_t cost = 0;
};

/**
* \brief ASF Probing Module.
*/
class ProbingModule
{
Expand Down Expand Up @@ -60,35 +72,16 @@ class ProbingModule
return m_probingInterval;
}

private:
// Used to associate FaceInfo with the face in a NextHop
using FaceInfoFacePair = std::pair<FaceInfo*, Face*>;
public:
static constexpr time::milliseconds DEFAULT_PROBING_INTERVAL = 1_min;
static constexpr time::milliseconds MIN_PROBING_INTERVAL = 1_s;

struct FaceInfoCompare
struct FaceStatsProbingCompare
{
bool
operator()(const FaceInfoFacePair& leftPair, const FaceInfoFacePair& rightPair) const
{
const FaceInfo& lhs = *leftPair.first;
const FaceInfo& rhs = *rightPair.first;

// Sort by RTT: if a face has timed-out, rank it behind non-timed-out faces
return (!lhs.hasTimeout() && rhs.hasTimeout()) ||
(lhs.hasTimeout() == rhs.hasTimeout() && lhs.getSrtt() < rhs.getSrtt());
}
operator()(const FaceStats& lhs, const FaceStats& rhs) const noexcept;
};

using FaceInfoFacePairSet = std::set<FaceInfoFacePair, FaceInfoCompare>;

static Face*
chooseFace(const FaceInfoFacePairSet& rankedFaces);

static double
getProbingProbability(uint64_t rank, uint64_t rankSum, uint64_t nFaces);

public:
static constexpr time::milliseconds DEFAULT_PROBING_INTERVAL = 1_min;
static constexpr time::milliseconds MIN_PROBING_INTERVAL = 1_s;
using FaceStatsProbingSet = std::set<FaceStats, FaceStatsProbingCompare>;

private:
time::milliseconds m_probingInterval;
Expand Down
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