daemon/face/lp-reliability.cpp - NFD - Gitiles

 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
  * Copyright (c) 2014-2024,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
  *                           Washington University in St. Louis,
  *                           Beijing Institute of Technology,
  *                           The University of Memphis.
  *
  * This file is part of NFD (Named Data Networking Forwarding Daemon).
  * See AUTHORS.md for complete list of NFD authors and contributors.
  *
  * NFD is free software: you can redistribute it and/or modify it under the terms
  * of the GNU General Public License as published by the Free Software Foundation,
  * either version 3 of the License, or (at your option) any later version.
  *
  * NFD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
  * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
  * PURPOSE.  See the GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License along with
  * NFD, e.g., in COPYING.md file.  If not, see <http://www.gnu.org/licenses/>.
  */

 #include "lp-reliability.hpp"
 #include "common/global.hpp"
 #include "generic-link-service.hpp"
 #include "transport.hpp"

 #include <ndn-cxx/lp/fields.hpp>

 #include <set>

 namespace nfd::face {

 NFD_LOG_INIT(LpReliability);

 LpReliability::LpReliability(const LpReliability::Options& options, GenericLinkService* linkService)
   : m_options(options)
   , m_linkService(linkService)
   , m_firstUnackedFrag(m_unackedFrags.begin())
   , m_lastTxSeqNo(-1) // set to "-1" to start TxSequence numbers at 0
 {
   BOOST_ASSERT(m_linkService != nullptr);
   BOOST_ASSERT(m_options.idleAckTimerPeriod > 0_ns);
 }

 void
 LpReliability::setOptions(const Options& options)
 {
   BOOST_ASSERT(options.idleAckTimerPeriod > 0_ns);

   if (m_options.isEnabled && !options.isEnabled) {
     m_idleAckTimer.cancel();
   }

   m_options = options;
 }

 const GenericLinkService*
 LpReliability::getLinkService() const
 {
   return m_linkService;
 }

 void
 LpReliability::handleOutgoing(std::vector<lp::Packet>& frags, lp::Packet&& pkt, bool isInterest)
 {
   BOOST_ASSERT(m_options.isEnabled);

   auto unackedFragsIt = m_unackedFrags.begin();
   auto sendTime = time::steady_clock::now();

   auto netPkt = make_shared<NetPkt>(std::move(pkt), isInterest);
   netPkt->unackedFrags.reserve(frags.size());

   for (lp::Packet& frag : frags) {
     // Non-IDLE packets are required to have assigned Sequence numbers with LpReliability enabled
     BOOST_ASSERT(frag.has<lp::SequenceField>());

     // Assign TxSequence number
     lp::Sequence txSeq = assignTxSequence(frag);

     // Store LpPacket for future retransmissions
     unackedFragsIt = m_unackedFrags.try_emplace(unackedFragsIt, txSeq, frag);
     unackedFragsIt->second.sendTime = sendTime;
     auto rto = m_rttEst.getEstimatedRto();
     lp::Sequence seq = frag.get<lp::SequenceField>();
     NFD_LOG_FACE_TRACE("transmitting seq=" << seq << ", txseq=" << txSeq << ", rto=" <<
                        time::duration_cast<time::milliseconds>(rto).count() << "ms");
     unackedFragsIt->second.rtoTimer = getScheduler().schedule(rto, [=] {
       onLpPacketLost(txSeq, true);
     });
     unackedFragsIt->second.netPkt = netPkt;

     if (m_unackedFrags.size() == 1) {
       m_firstUnackedFrag = m_unackedFrags.begin();
     }

     // Add to associated NetPkt
     netPkt->unackedFrags.push_back(unackedFragsIt);
   }
 }

 bool
 LpReliability::processIncomingPacket(const lp::Packet& pkt)
 {
   BOOST_ASSERT(m_options.isEnabled);

   bool isDuplicate = false;
   auto now = time::steady_clock::now();

   // Extract and parse Acks
   for (lp::Sequence ackTxSeq : pkt.list<lp::AckField>()) {
     auto fragIt = m_unackedFrags.find(ackTxSeq);
     if (fragIt == m_unackedFrags.end()) {
       // Ignore an Ack for an unknown TxSequence number
       NFD_LOG_FACE_DEBUG("received ack for unknown txseq=" << ackTxSeq);
       continue;
     }
     auto& frag = fragIt->second;

     // Cancel the RTO timer for the acknowledged fragment
     frag.rtoTimer.cancel();

     if (frag.retxCount == 0) {
       NFD_LOG_FACE_TRACE("received ack for seq=" << frag.pkt.get<lp::SequenceField>() << ", txseq=" <<
                          ackTxSeq << ", retx=0, rtt=" <<
                          time::duration_cast<time::milliseconds>(now - frag.sendTime).count() << "ms");
       // This sequence had no retransmissions, so use it to estimate the RTO
       m_rttEst.addMeasurement(now - frag.sendTime);
     }
     else {
       NFD_LOG_FACE_TRACE("received ack for seq=" << frag.pkt.get<lp::SequenceField>() << ", txseq=" <<
                          ackTxSeq << ", retx=" << frag.retxCount);
     }

     // Look for frags with TxSequence numbers < ackTxSeq (allowing for wraparound) and consider
     // them lost if a configurable number of Acks containing greater TxSequence numbers have been
     // received.
     auto lostLpPackets = findLostLpPackets(fragIt);

     // Remove the fragment from the map of unacknowledged fragments and from its associated network
     // packet. Potentially increment the start of the window.
     onLpPacketAcknowledged(fragIt);

     // This set contains TxSequences that have been removed by onLpPacketLost below because they
     // were part of a network packet that was removed due to a fragment exceeding retx, as well as
     // any other TxSequences removed by onLpPacketLost. This prevents onLpPacketLost from being
     // called later for an invalid iterator.
     std::set<lp::Sequence> removedLpPackets;

     // Resend or fail fragments considered lost. Potentially increment the start of the window.
     for (lp::Sequence txSeq : lostLpPackets) {
       if (removedLpPackets.find(txSeq) == removedLpPackets.end()) {
         auto removedTxSeqs = onLpPacketLost(txSeq, false);
         for (auto removedTxSeq : removedTxSeqs) {
           removedLpPackets.insert(removedTxSeq);
         }
       }
     }
   }

   // If packet has Fragment and TxSequence fields, extract TxSequence and add to AckQueue
   if (pkt.has<lp::FragmentField>() && pkt.has<lp::TxSequenceField>()) {
     NFD_LOG_FACE_TRACE("queueing ack for remote txseq=" << pkt.get<lp::TxSequenceField>());
     m_ackQueue.push(pkt.get<lp::TxSequenceField>());

     // Check for received frames with duplicate Sequences
     if (pkt.has<lp::SequenceField>()) {
       lp::Sequence pktSequence = pkt.get<lp::SequenceField>();
       isDuplicate = m_recentRecvSeqs.count(pktSequence) > 0;
       // Check for recent received Sequences to remove
       auto now = time::steady_clock::now();
       auto rto = m_rttEst.getEstimatedRto();
       while (!m_recentRecvSeqsQueue.empty() &&
              now > m_recentRecvSeqs[m_recentRecvSeqsQueue.front()] + rto) {
         m_recentRecvSeqs.erase(m_recentRecvSeqsQueue.front());
         m_recentRecvSeqsQueue.pop();
       }
       m_recentRecvSeqs.try_emplace(pktSequence, now);
       m_recentRecvSeqsQueue.push(pktSequence);
     }

     startIdleAckTimer();
   }

   return !isDuplicate;
 }

 void
 LpReliability::piggyback(lp::Packet& pkt, ssize_t mtu)
 {
   BOOST_ASSERT(m_options.isEnabled);
   BOOST_ASSERT(pkt.wireEncode().type() == lp::tlv::LpPacket);

   // up to 2 extra octets reserved for potential TLV-LENGTH size increases
   ssize_t pktSize = pkt.wireEncode().size();
   ssize_t reservedSpace = tlv::sizeOfVarNumber(ndn::MAX_NDN_PACKET_SIZE) -
                           tlv::sizeOfVarNumber(pktSize);
   ssize_t remainingSpace = (mtu == MTU_UNLIMITED ? ndn::MAX_NDN_PACKET_SIZE : mtu) - reservedSpace;
   remainingSpace -= pktSize;

   while (!m_ackQueue.empty()) {
     lp::Sequence ackTxSeq = m_ackQueue.front();
     // Ack size = Ack TLV-TYPE (3 octets) + TLV-LENGTH (1 octet) + lp::Sequence (8 octets)
     const ssize_t ackSize = tlv::sizeOfVarNumber(lp::tlv::Ack) +
                             tlv::sizeOfVarNumber(sizeof(lp::Sequence)) +
                             sizeof(lp::Sequence);

     if (ackSize > remainingSpace) {
       break;
     }

     NFD_LOG_FACE_TRACE("piggybacking ack for remote txseq=" << ackTxSeq);

     pkt.add<lp::AckField>(ackTxSeq);
     m_ackQueue.pop();
     remainingSpace -= ackSize;
   }
 }

 lp::Sequence
 LpReliability::assignTxSequence(lp::Packet& frag)
 {
   lp::Sequence txSeq = ++m_lastTxSeqNo;
   frag.set<lp::TxSequenceField>(txSeq);
   if (!m_unackedFrags.empty() && m_lastTxSeqNo == m_firstUnackedFrag->first) {
     NDN_THROW(std::length_error("TxSequence range exceeded"));
   }
   return m_lastTxSeqNo;
 }

 void
 LpReliability::startIdleAckTimer()
 {
   if (m_idleAckTimer) {
     // timer is already running, do nothing
     return;
   }

   m_idleAckTimer = getScheduler().schedule(m_options.idleAckTimerPeriod, [this] {
     while (!m_ackQueue.empty()) {
       m_linkService->requestIdlePacket();
     }
   });
 }

 std::vector<lp::Sequence>
 LpReliability::findLostLpPackets(LpReliability::UnackedFrags::iterator ackIt)
 {
   std::vector<lp::Sequence> lostLpPackets;

   for (auto it = m_firstUnackedFrag; ; ++it) {
     if (it == m_unackedFrags.end()) {
       it = m_unackedFrags.begin();
     }

     if (it->first == ackIt->first) {
       break;
     }

     auto& unackedFrag = it->second;
     unackedFrag.nGreaterSeqAcks++;
     NFD_LOG_FACE_TRACE("received ack=" << ackIt->first << " before=" << it->first <<
                        ", before count=" << unackedFrag.nGreaterSeqAcks);

     if (unackedFrag.nGreaterSeqAcks >= m_options.seqNumLossThreshold) {
       lostLpPackets.push_back(it->first);
     }
   }

   return lostLpPackets;
 }

 std::vector<lp::Sequence>
 LpReliability::onLpPacketLost(lp::Sequence txSeq, bool isTimeout)
 {
   BOOST_ASSERT(m_unackedFrags.count(txSeq) > 0);
   auto txSeqIt = m_unackedFrags.find(txSeq);

   auto& txFrag = txSeqIt->second;
   txFrag.rtoTimer.cancel();
   auto netPkt = txFrag.netPkt;
   std::vector<lp::Sequence> removedThisTxSeq;
   lp::Sequence seq = txFrag.pkt.get<lp::SequenceField>();

   if (isTimeout) {
     NFD_LOG_FACE_TRACE("rto timer expired for seq=" << seq << ", txseq=" << txSeq);
   }
   else { // lost due to out-of-order TxSeqs
     NFD_LOG_FACE_TRACE("seq=" << seq << ", txseq=" << txSeq <<
                        " considered lost from acks for more recent txseqs");
   }

   // Check if maximum number of retransmissions exceeded
   if (txFrag.retxCount >= m_options.maxRetx) {
     NFD_LOG_FACE_DEBUG("seq=" << seq << " exceeded allowed retransmissions: DROP");
     // Delete all LpPackets of NetPkt from m_unackedFrags (except this one)
     for (size_t i = 0; i < netPkt->unackedFrags.size(); i++) {
       if (netPkt->unackedFrags[i] != txSeqIt) {
         removedThisTxSeq.push_back(netPkt->unackedFrags[i]->first);
         deleteUnackedFrag(netPkt->unackedFrags[i]);
       }
     }

     ++m_linkService->nRetxExhausted;

     // Notify strategy of dropped Interest (if any)
     if (netPkt->isInterest) {
       BOOST_ASSERT(netPkt->pkt.has<lp::FragmentField>());
       auto frag = netPkt->pkt.get<lp::FragmentField>();
       onDroppedInterest(Interest(Block({frag.first, frag.second})));
     }

     // Delete this LpPacket from m_unackedFrags
     removedThisTxSeq.push_back(txSeqIt->first);
     deleteUnackedFrag(txSeqIt);
   }
   else {
     // Assign new TxSequence
     lp::Sequence newTxSeq = assignTxSequence(txFrag.pkt);
     netPkt->didRetx = true;

     // Move fragment to new TxSequence mapping
     auto hint = m_firstUnackedFrag != m_unackedFrags.end() && m_firstUnackedFrag->first > newTxSeq
                 ? m_firstUnackedFrag
                 : m_unackedFrags.end();
     auto newTxFragIt = m_unackedFrags.try_emplace(hint, newTxSeq, txFrag.pkt);
     auto& newTxFrag = newTxFragIt->second;
     newTxFrag.retxCount = txFrag.retxCount + 1;
     newTxFrag.netPkt = netPkt;

     // Update associated NetPkt
     auto fragInNetPkt = std::find(netPkt->unackedFrags.begin(), netPkt->unackedFrags.end(), txSeqIt);
     BOOST_ASSERT(fragInNetPkt != netPkt->unackedFrags.end());
     *fragInNetPkt = newTxFragIt;

     removedThisTxSeq.push_back(txSeqIt->first);
     deleteUnackedFrag(txSeqIt);

     // Retransmit fragment
     m_linkService->sendLpPacket(lp::Packet(newTxFrag.pkt));

     auto rto = m_rttEst.getEstimatedRto();
     NFD_LOG_FACE_TRACE("retransmitting seq=" << seq << ", txseq=" << newTxSeq << ", retx=" <<
                        txFrag.retxCount << ", rto=" <<
                        time::duration_cast<time::milliseconds>(rto).count() << "ms");

     // Start RTO timer for this sequence
     newTxFrag.rtoTimer = getScheduler().schedule(rto, [=] {
       onLpPacketLost(newTxSeq, true);
     });
   }

   return removedThisTxSeq;
 }

 void
 LpReliability::onLpPacketAcknowledged(UnackedFrags::iterator fragIt)
 {
   auto netPkt = fragIt->second.netPkt;

   // Remove from NetPkt unacked fragment list
   auto fragInNetPkt = std::find(netPkt->unackedFrags.begin(), netPkt->unackedFrags.end(), fragIt);
   BOOST_ASSERT(fragInNetPkt != netPkt->unackedFrags.end());
   *fragInNetPkt = netPkt->unackedFrags.back();
   netPkt->unackedFrags.pop_back();

   // Check if network-layer packet completely received. If so, increment counters
   if (netPkt->unackedFrags.empty()) {
     if (netPkt->didRetx) {
       ++m_linkService->nRetransmitted;
     }
     else {
       ++m_linkService->nAcknowledged;
     }
   }

   deleteUnackedFrag(fragIt);
 }

 void
 LpReliability::deleteUnackedFrag(UnackedFrags::iterator fragIt)
 {
   lp::Sequence firstUnackedTxSeq = m_firstUnackedFrag->first;
   lp::Sequence currentTxSeq = fragIt->first;
   auto nextFragIt = m_unackedFrags.erase(fragIt);

   if (!m_unackedFrags.empty() && firstUnackedTxSeq == currentTxSeq) {
     // If "first" fragment in send window (allowing for wraparound), increment window begin
     if (nextFragIt == m_unackedFrags.end()) {
       m_firstUnackedFrag = m_unackedFrags.begin();
     }
     else {
       m_firstUnackedFrag = nextFragIt;
     }
   }
   else if (m_unackedFrags.empty()) {
     m_firstUnackedFrag = m_unackedFrags.end();
   }
 }

 std::ostream&
 operator<<(std::ostream& os, const FaceLogHelper<LpReliability>& flh)
 {
   if (flh.obj.getLinkService() == nullptr) {
     os << "[id=0,local=unknown,remote=unknown] ";
   }
   else {
     os << FaceLogHelper<LinkService>(*flh.obj.getLinkService());
   }
   return os;
 }

 } // namespace nfd::face
	/* -- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -- */
	/*
	* Copyright (c) 2014-2024, Regents of the University of California,
	* Arizona Board of Regents,
	* Colorado State University,
	* University Pierre & Marie Curie, Sorbonne University,
	* Washington University in St. Louis,
	* Beijing Institute of Technology,
	* The University of Memphis.
	*
	* This file is part of NFD (Named Data Networking Forwarding Daemon).
	* See AUTHORS.md for complete list of NFD authors and contributors.
	*
	* NFD is free software: you can redistribute it and/or modify it under the terms
	* of the GNU General Public License as published by the Free Software Foundation,
	* either version 3 of the License, or (at your option) any later version.
	*
	* NFD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
	* without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
	* PURPOSE. See the GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License along with
	* NFD, e.g., in COPYING.md file. If not, see <http://www.gnu.org/licenses/>.
	*/

	#include "lp-reliability.hpp"
	#include "common/global.hpp"
	#include "generic-link-service.hpp"
	#include "transport.hpp"

	#include <ndn-cxx/lp/fields.hpp>

	#include <set>

	namespace nfd::face {

	NFD_LOG_INIT(LpReliability);

	LpReliability::LpReliability(const LpReliability::Options& options, GenericLinkService* linkService)
	: m_options(options)
	, m_linkService(linkService)
	, m_firstUnackedFrag(m_unackedFrags.begin())
	, m_lastTxSeqNo(-1) // set to "-1" to start TxSequence numbers at 0
	{
	BOOST_ASSERT(m_linkService != nullptr);
	BOOST_ASSERT(m_options.idleAckTimerPeriod > 0_ns);
	}

	void
	LpReliability::setOptions(const Options& options)
	{
	BOOST_ASSERT(options.idleAckTimerPeriod > 0_ns);

	if (m_options.isEnabled && !options.isEnabled) {
	m_idleAckTimer.cancel();
	}

	m_options = options;
	}

	const GenericLinkService*
	LpReliability::getLinkService() const
	{
	return m_linkService;
	}

	void
	LpReliability::handleOutgoing(std::vector<lp::Packet>& frags, lp::Packet&& pkt, bool isInterest)
	{
	BOOST_ASSERT(m_options.isEnabled);

	auto unackedFragsIt = m_unackedFrags.begin();
	auto sendTime = time::steady_clock::now();

	auto netPkt = make_shared<NetPkt>(std::move(pkt), isInterest);
	netPkt->unackedFrags.reserve(frags.size());

	for (lp::Packet& frag : frags) {
	// Non-IDLE packets are required to have assigned Sequence numbers with LpReliability enabled
	BOOST_ASSERT(frag.has<lp::SequenceField>());

	// Assign TxSequence number
	lp::Sequence txSeq = assignTxSequence(frag);

	// Store LpPacket for future retransmissions
	unackedFragsIt = m_unackedFrags.try_emplace(unackedFragsIt, txSeq, frag);
	unackedFragsIt->second.sendTime = sendTime;
	auto rto = m_rttEst.getEstimatedRto();
	lp::Sequence seq = frag.get<lp::SequenceField>();
	NFD_LOG_FACE_TRACE("transmitting seq=" << seq << ", txseq=" << txSeq << ", rto=" <<
	time::duration_cast<time::milliseconds>(rto).count() << "ms");
	unackedFragsIt->second.rtoTimer = getScheduler().schedule(rto, [=] {
	onLpPacketLost(txSeq, true);
	});
	unackedFragsIt->second.netPkt = netPkt;

	if (m_unackedFrags.size() == 1) {
	m_firstUnackedFrag = m_unackedFrags.begin();
	}

	// Add to associated NetPkt
	netPkt->unackedFrags.push_back(unackedFragsIt);
	}
	}

	bool
	LpReliability::processIncomingPacket(const lp::Packet& pkt)
	{
	BOOST_ASSERT(m_options.isEnabled);

	bool isDuplicate = false;
	auto now = time::steady_clock::now();

	// Extract and parse Acks
	for (lp::Sequence ackTxSeq : pkt.list<lp::AckField>()) {
	auto fragIt = m_unackedFrags.find(ackTxSeq);
	if (fragIt == m_unackedFrags.end()) {
	// Ignore an Ack for an unknown TxSequence number
	NFD_LOG_FACE_DEBUG("received ack for unknown txseq=" << ackTxSeq);
	continue;
	}
	auto& frag = fragIt->second;

	// Cancel the RTO timer for the acknowledged fragment
	frag.rtoTimer.cancel();

	if (frag.retxCount == 0) {
	NFD_LOG_FACE_TRACE("received ack for seq=" << frag.pkt.get<lp::SequenceField>() << ", txseq=" <<
	ackTxSeq << ", retx=0, rtt=" <<
	time::duration_cast<time::milliseconds>(now - frag.sendTime).count() << "ms");
	// This sequence had no retransmissions, so use it to estimate the RTO
	m_rttEst.addMeasurement(now - frag.sendTime);
	}
	else {
	NFD_LOG_FACE_TRACE("received ack for seq=" << frag.pkt.get<lp::SequenceField>() << ", txseq=" <<
	ackTxSeq << ", retx=" << frag.retxCount);
	}

	// Look for frags with TxSequence numbers < ackTxSeq (allowing for wraparound) and consider
	// them lost if a configurable number of Acks containing greater TxSequence numbers have been
	// received.
	auto lostLpPackets = findLostLpPackets(fragIt);

	// Remove the fragment from the map of unacknowledged fragments and from its associated network
	// packet. Potentially increment the start of the window.
	onLpPacketAcknowledged(fragIt);

	// This set contains TxSequences that have been removed by onLpPacketLost below because they
	// were part of a network packet that was removed due to a fragment exceeding retx, as well as
	// any other TxSequences removed by onLpPacketLost. This prevents onLpPacketLost from being
	// called later for an invalid iterator.
	std::set<lp::Sequence> removedLpPackets;

	// Resend or fail fragments considered lost. Potentially increment the start of the window.
	for (lp::Sequence txSeq : lostLpPackets) {
	if (removedLpPackets.find(txSeq) == removedLpPackets.end()) {
	auto removedTxSeqs = onLpPacketLost(txSeq, false);
	for (auto removedTxSeq : removedTxSeqs) {
	removedLpPackets.insert(removedTxSeq);
	}
	}
	}
	}

	// If packet has Fragment and TxSequence fields, extract TxSequence and add to AckQueue
	if (pkt.has<lp::FragmentField>() && pkt.has<lp::TxSequenceField>()) {
	NFD_LOG_FACE_TRACE("queueing ack for remote txseq=" << pkt.get<lp::TxSequenceField>());
	m_ackQueue.push(pkt.get<lp::TxSequenceField>());

	// Check for received frames with duplicate Sequences
	if (pkt.has<lp::SequenceField>()) {
	lp::Sequence pktSequence = pkt.get<lp::SequenceField>();
	isDuplicate = m_recentRecvSeqs.count(pktSequence) > 0;
	// Check for recent received Sequences to remove
	auto now = time::steady_clock::now();
	auto rto = m_rttEst.getEstimatedRto();
	while (!m_recentRecvSeqsQueue.empty() &&
	now > m_recentRecvSeqs[m_recentRecvSeqsQueue.front()] + rto) {
	m_recentRecvSeqs.erase(m_recentRecvSeqsQueue.front());
	m_recentRecvSeqsQueue.pop();
	}
	m_recentRecvSeqs.try_emplace(pktSequence, now);
	m_recentRecvSeqsQueue.push(pktSequence);
	}

	startIdleAckTimer();
	}

	return !isDuplicate;
	}

	void
	LpReliability::piggyback(lp::Packet& pkt, ssize_t mtu)
	{
	BOOST_ASSERT(m_options.isEnabled);
	BOOST_ASSERT(pkt.wireEncode().type() == lp::tlv::LpPacket);

	// up to 2 extra octets reserved for potential TLV-LENGTH size increases
	ssize_t pktSize = pkt.wireEncode().size();
	ssize_t reservedSpace = tlv::sizeOfVarNumber(ndn::MAX_NDN_PACKET_SIZE) -
	tlv::sizeOfVarNumber(pktSize);
	ssize_t remainingSpace = (mtu == MTU_UNLIMITED ? ndn::MAX_NDN_PACKET_SIZE : mtu) - reservedSpace;
	remainingSpace -= pktSize;

	while (!m_ackQueue.empty()) {
	lp::Sequence ackTxSeq = m_ackQueue.front();
	// Ack size = Ack TLV-TYPE (3 octets) + TLV-LENGTH (1 octet) + lp::Sequence (8 octets)
	const ssize_t ackSize = tlv::sizeOfVarNumber(lp::tlv::Ack) +
	tlv::sizeOfVarNumber(sizeof(lp::Sequence)) +
	sizeof(lp::Sequence);

	if (ackSize > remainingSpace) {
	break;
	}

	NFD_LOG_FACE_TRACE("piggybacking ack for remote txseq=" << ackTxSeq);

	pkt.add<lp::AckField>(ackTxSeq);
	m_ackQueue.pop();
	remainingSpace -= ackSize;
	}
	}

	lp::Sequence
	LpReliability::assignTxSequence(lp::Packet& frag)
	{
	lp::Sequence txSeq = ++m_lastTxSeqNo;
	frag.set<lp::TxSequenceField>(txSeq);
	if (!m_unackedFrags.empty() && m_lastTxSeqNo == m_firstUnackedFrag->first) {
	NDN_THROW(std::length_error("TxSequence range exceeded"));
	}
	return m_lastTxSeqNo;
	}

	void
	LpReliability::startIdleAckTimer()
	{
	if (m_idleAckTimer) {
	// timer is already running, do nothing
	return;
	}

	m_idleAckTimer = getScheduler().schedule(m_options.idleAckTimerPeriod, [this] {
	while (!m_ackQueue.empty()) {
	m_linkService->requestIdlePacket();
	}
	});
	}

	std::vector<lp::Sequence>
	LpReliability::findLostLpPackets(LpReliability::UnackedFrags::iterator ackIt)
	{
	std::vector<lp::Sequence> lostLpPackets;

	for (auto it = m_firstUnackedFrag; ; ++it) {
	if (it == m_unackedFrags.end()) {
	it = m_unackedFrags.begin();
	}

	if (it->first == ackIt->first) {
	break;
	}

	auto& unackedFrag = it->second;
	unackedFrag.nGreaterSeqAcks++;
	NFD_LOG_FACE_TRACE("received ack=" << ackIt->first << " before=" << it->first <<
	", before count=" << unackedFrag.nGreaterSeqAcks);

	if (unackedFrag.nGreaterSeqAcks >= m_options.seqNumLossThreshold) {
	lostLpPackets.push_back(it->first);
	}
	}

	return lostLpPackets;
	}

	std::vector<lp::Sequence>
	LpReliability::onLpPacketLost(lp::Sequence txSeq, bool isTimeout)
	{
	BOOST_ASSERT(m_unackedFrags.count(txSeq) > 0);
	auto txSeqIt = m_unackedFrags.find(txSeq);

	auto& txFrag = txSeqIt->second;
	txFrag.rtoTimer.cancel();
	auto netPkt = txFrag.netPkt;
	std::vector<lp::Sequence> removedThisTxSeq;
	lp::Sequence seq = txFrag.pkt.get<lp::SequenceField>();

	if (isTimeout) {
	NFD_LOG_FACE_TRACE("rto timer expired for seq=" << seq << ", txseq=" << txSeq);
	}
	else { // lost due to out-of-order TxSeqs
	NFD_LOG_FACE_TRACE("seq=" << seq << ", txseq=" << txSeq <<
	" considered lost from acks for more recent txseqs");
	}

	// Check if maximum number of retransmissions exceeded
	if (txFrag.retxCount >= m_options.maxRetx) {
	NFD_LOG_FACE_DEBUG("seq=" << seq << " exceeded allowed retransmissions: DROP");
	// Delete all LpPackets of NetPkt from m_unackedFrags (except this one)
	for (size_t i = 0; i < netPkt->unackedFrags.size(); i++) {
	if (netPkt->unackedFrags[i] != txSeqIt) {
	removedThisTxSeq.push_back(netPkt->unackedFrags[i]->first);
	deleteUnackedFrag(netPkt->unackedFrags[i]);
	}
	}

	++m_linkService->nRetxExhausted;

	// Notify strategy of dropped Interest (if any)
	if (netPkt->isInterest) {
	BOOST_ASSERT(netPkt->pkt.has<lp::FragmentField>());
	auto frag = netPkt->pkt.get<lp::FragmentField>();
	onDroppedInterest(Interest(Block({frag.first, frag.second})));
	}

	// Delete this LpPacket from m_unackedFrags
	removedThisTxSeq.push_back(txSeqIt->first);
	deleteUnackedFrag(txSeqIt);
	}
	else {
	// Assign new TxSequence
	lp::Sequence newTxSeq = assignTxSequence(txFrag.pkt);
	netPkt->didRetx = true;

	// Move fragment to new TxSequence mapping
	auto hint = m_firstUnackedFrag != m_unackedFrags.end() && m_firstUnackedFrag->first > newTxSeq
	? m_firstUnackedFrag
	: m_unackedFrags.end();
	auto newTxFragIt = m_unackedFrags.try_emplace(hint, newTxSeq, txFrag.pkt);
	auto& newTxFrag = newTxFragIt->second;
	newTxFrag.retxCount = txFrag.retxCount + 1;
	newTxFrag.netPkt = netPkt;

	// Update associated NetPkt
	auto fragInNetPkt = std::find(netPkt->unackedFrags.begin(), netPkt->unackedFrags.end(), txSeqIt);
	BOOST_ASSERT(fragInNetPkt != netPkt->unackedFrags.end());
	*fragInNetPkt = newTxFragIt;

	removedThisTxSeq.push_back(txSeqIt->first);
	deleteUnackedFrag(txSeqIt);

	// Retransmit fragment
	m_linkService->sendLpPacket(lp::Packet(newTxFrag.pkt));

	auto rto = m_rttEst.getEstimatedRto();
	NFD_LOG_FACE_TRACE("retransmitting seq=" << seq << ", txseq=" << newTxSeq << ", retx=" <<
	txFrag.retxCount << ", rto=" <<
	time::duration_cast<time::milliseconds>(rto).count() << "ms");

	// Start RTO timer for this sequence
	newTxFrag.rtoTimer = getScheduler().schedule(rto, [=] {
	onLpPacketLost(newTxSeq, true);
	});
	}

	return removedThisTxSeq;
	}

	void
	LpReliability::onLpPacketAcknowledged(UnackedFrags::iterator fragIt)
	{
	auto netPkt = fragIt->second.netPkt;

	// Remove from NetPkt unacked fragment list
	auto fragInNetPkt = std::find(netPkt->unackedFrags.begin(), netPkt->unackedFrags.end(), fragIt);
	BOOST_ASSERT(fragInNetPkt != netPkt->unackedFrags.end());
	*fragInNetPkt = netPkt->unackedFrags.back();
	netPkt->unackedFrags.pop_back();

	// Check if network-layer packet completely received. If so, increment counters
	if (netPkt->unackedFrags.empty()) {
	if (netPkt->didRetx) {
	++m_linkService->nRetransmitted;
	}
	else {
	++m_linkService->nAcknowledged;
	}
	}

	deleteUnackedFrag(fragIt);
	}

	void
	LpReliability::deleteUnackedFrag(UnackedFrags::iterator fragIt)
	{
	lp::Sequence firstUnackedTxSeq = m_firstUnackedFrag->first;
	lp::Sequence currentTxSeq = fragIt->first;
	auto nextFragIt = m_unackedFrags.erase(fragIt);

	if (!m_unackedFrags.empty() && firstUnackedTxSeq == currentTxSeq) {
	// If "first" fragment in send window (allowing for wraparound), increment window begin
	if (nextFragIt == m_unackedFrags.end()) {
	m_firstUnackedFrag = m_unackedFrags.begin();
	}
	else {
	m_firstUnackedFrag = nextFragIt;
	}
	}
	else if (m_unackedFrags.empty()) {
	m_firstUnackedFrag = m_unackedFrags.end();
	}
	}

	std::ostream&
	operator<<(std::ostream& os, const FaceLogHelper<LpReliability>& flh)
	{
	if (flh.obj.getLinkService() == nullptr) {
	os << "[id=0,local=unknown,remote=unknown] ";
	}
	else {
	os << FaceLogHelper<LinkService>(*flh.obj.getLinkService());
	}
	return os;
	}

	} // namespace nfd::face