net/dcsctp/tx/outstanding_data.cc - src - Git at Google

 /*
  *  Copyright (c) 2021 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 #include "net/dcsctp/tx/outstanding_data.h"

 #include <algorithm>
 #include <set>
 #include <utility>
 #include <vector>

 #include "net/dcsctp/common/math.h"
 #include "net/dcsctp/common/sequence_numbers.h"
 #include "net/dcsctp/public/types.h"
 #include "rtc_base/logging.h"

 namespace dcsctp {

 // The number of times a packet must be NACKed before it's retransmitted.
 // See https://tools.ietf.org/html/rfc4960#section-7.2.4
 constexpr uint8_t kNumberOfNacksForRetransmission = 3;

 // Returns how large a chunk will be, serialized, carrying the data
 size_t OutstandingData::GetSerializedChunkSize(const Data& data) const {
   return RoundUpTo4(data_chunk_header_size_ + data.size());
 }

 void OutstandingData::Item::Ack() {
   ack_state_ = AckState::kAcked;
   should_be_retransmitted_ = false;
 }

 OutstandingData::Item::NackAction OutstandingData::Item::Nack(
     bool retransmit_now) {
   ack_state_ = AckState::kNacked;
   ++nack_count_;
   if ((retransmit_now || nack_count_ >= kNumberOfNacksForRetransmission) &&
       !is_abandoned_) {
     // Nacked enough times - it's considered lost.
     if (num_retransmissions_ < *max_retransmissions_) {
       should_be_retransmitted_ = true;
       return NackAction::kRetransmit;
     }
     Abandon();
     return NackAction::kAbandon;
   }
   return NackAction::kNothing;
 }

 void OutstandingData::Item::Retransmit() {
   ack_state_ = AckState::kUnacked;
   should_be_retransmitted_ = false;

   nack_count_ = 0;
   ++num_retransmissions_;
 }

 void OutstandingData::Item::Abandon() {
   is_abandoned_ = true;
   should_be_retransmitted_ = false;
 }

 bool OutstandingData::Item::has_expired(TimeMs now) const {
   return expires_at_ <= now;
 }

 bool OutstandingData::IsConsistent() const {
   size_t actual_outstanding_bytes = 0;
   size_t actual_outstanding_items = 0;

   std::set<UnwrappedTSN> actual_to_be_retransmitted;
   for (const auto& elem : outstanding_data_) {
     if (elem.second.is_outstanding()) {
       actual_outstanding_bytes += GetSerializedChunkSize(elem.second.data());
       ++actual_outstanding_items;
     }

     if (elem.second.should_be_retransmitted()) {
       actual_to_be_retransmitted.insert(elem.first);
     }
   }

   if (outstanding_data_.empty() &&
       next_tsn_ != last_cumulative_tsn_ack_.next_value()) {
     return false;
   }

   return actual_outstanding_bytes == outstanding_bytes_ &&
          actual_outstanding_items == outstanding_items_ &&
          actual_to_be_retransmitted == to_be_retransmitted_;
 }

 void OutstandingData::AckChunk(AckInfo& ack_info,
                                std::map<UnwrappedTSN, Item>::iterator iter) {
   if (!iter->second.is_acked()) {
     size_t serialized_size = GetSerializedChunkSize(iter->second.data());
     ack_info.bytes_acked += serialized_size;
     if (iter->second.is_outstanding()) {
       outstanding_bytes_ -= serialized_size;
       --outstanding_items_;
     }
     if (iter->second.should_be_retransmitted()) {
       to_be_retransmitted_.erase(iter->first);
     }
     iter->second.Ack();
     ack_info.highest_tsn_acked =
         std::max(ack_info.highest_tsn_acked, iter->first);
   }
 }

 OutstandingData::AckInfo OutstandingData::HandleSack(
     UnwrappedTSN cumulative_tsn_ack,
     rtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
     bool is_in_fast_retransmit) {
   OutstandingData::AckInfo ack_info(cumulative_tsn_ack);
   // Erase all items up to cumulative_tsn_ack.
   RemoveAcked(cumulative_tsn_ack, ack_info);

   // ACK packets reported in the gap ack blocks
   AckGapBlocks(cumulative_tsn_ack, gap_ack_blocks, ack_info);

   // NACK and possibly mark for retransmit chunks that weren't acked.
   NackBetweenAckBlocks(cumulative_tsn_ack, gap_ack_blocks,
                        is_in_fast_retransmit, ack_info);

   RTC_DCHECK(IsConsistent());
   return ack_info;
 }

 void OutstandingData::RemoveAcked(UnwrappedTSN cumulative_tsn_ack,
                                   AckInfo& ack_info) {
   auto first_unacked = outstanding_data_.upper_bound(cumulative_tsn_ack);

   for (auto iter = outstanding_data_.begin(); iter != first_unacked; ++iter) {
     AckChunk(ack_info, iter);
   }

   outstanding_data_.erase(outstanding_data_.begin(), first_unacked);
   last_cumulative_tsn_ack_ = cumulative_tsn_ack;
 }

 void OutstandingData::AckGapBlocks(
     UnwrappedTSN cumulative_tsn_ack,
     rtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
     AckInfo& ack_info) {
   // Mark all non-gaps as ACKED (but they can't be removed) as (from RFC)
   // "SCTP considers the information carried in the Gap Ack Blocks in the
   // SACK chunk as advisory.". Note that when NR-SACK is supported, this can be
   // handled differently.

   for (auto& block : gap_ack_blocks) {
     auto start = outstanding_data_.lower_bound(
         UnwrappedTSN::AddTo(cumulative_tsn_ack, block.start));
     auto end = outstanding_data_.upper_bound(
         UnwrappedTSN::AddTo(cumulative_tsn_ack, block.end));
     for (auto iter = start; iter != end; ++iter) {
       AckChunk(ack_info, iter);
     }
   }
 }

 void OutstandingData::NackBetweenAckBlocks(
     UnwrappedTSN cumulative_tsn_ack,
     rtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
     bool is_in_fast_recovery,
     OutstandingData::AckInfo& ack_info) {
   // Mark everything between the blocks as NACKED/TO_BE_RETRANSMITTED.
   // https://tools.ietf.org/html/rfc4960#section-7.2.4
   // "Mark the DATA chunk(s) with three miss indications for retransmission."
   // "For each incoming SACK, miss indications are incremented only for
   // missing TSNs prior to the highest TSN newly acknowledged in the SACK."
   //
   // What this means is that only when there is a increasing stream of data
   // received and there are new packets seen (since last time), packets that are
   // in-flight and between gaps should be nacked. This means that SCTP relies on
   // the T3-RTX-timer to re-send packets otherwise.
   UnwrappedTSN max_tsn_to_nack = ack_info.highest_tsn_acked;
   if (is_in_fast_recovery && cumulative_tsn_ack > last_cumulative_tsn_ack_) {
     // https://tools.ietf.org/html/rfc4960#section-7.2.4
     // "If an endpoint is in Fast Recovery and a SACK arrives that advances
     // the Cumulative TSN Ack Point, the miss indications are incremented for
     // all TSNs reported missing in the SACK."
     max_tsn_to_nack = UnwrappedTSN::AddTo(
         cumulative_tsn_ack,
         gap_ack_blocks.empty() ? 0 : gap_ack_blocks.rbegin()->end);
   }

   UnwrappedTSN prev_block_last_acked = cumulative_tsn_ack;
   for (auto& block : gap_ack_blocks) {
     UnwrappedTSN cur_block_first_acked =
         UnwrappedTSN::AddTo(cumulative_tsn_ack, block.start);
     for (auto iter = outstanding_data_.upper_bound(prev_block_last_acked);
          iter != outstanding_data_.lower_bound(cur_block_first_acked); ++iter) {
       if (iter->first <= max_tsn_to_nack) {
         ack_info.has_packet_loss =
             NackItem(iter->first, iter->second, /*retransmit_now=*/false);
       }
     }
     prev_block_last_acked = UnwrappedTSN::AddTo(cumulative_tsn_ack, block.end);
   }

   // Note that packets are not NACKED which are above the highest gap-ack-block
   // (or above the cumulative ack TSN if no gap-ack-blocks) as only packets
   // up until the highest_tsn_acked (see above) should be considered when
   // NACKing.
 }

 bool OutstandingData::NackItem(UnwrappedTSN tsn,
                                Item& item,
                                bool retransmit_now) {
   if (item.is_outstanding()) {
     outstanding_bytes_ -= GetSerializedChunkSize(item.data());
     --outstanding_items_;
   }

   switch (item.Nack(retransmit_now)) {
     case Item::NackAction::kNothing:
       return false;
     case Item::NackAction::kRetransmit:
       to_be_retransmitted_.insert(tsn);
       RTC_DLOG(LS_VERBOSE) << *tsn.Wrap() << " marked for retransmission";
       break;
     case Item::NackAction::kAbandon:
       AbandonAllFor(item);
       break;
   }
   return true;
 }

 void OutstandingData::AbandonAllFor(const Item& item) {
   // Erase all remaining chunks from the producer, if any.
   if (discard_from_send_queue_(item.data().is_unordered, item.data().stream_id,
                                item.data().message_id)) {
     // There were remaining chunks to be produced for this message. Since the
     // receiver may have already received all chunks (up till now) for this
     // message, we can't just FORWARD-TSN to the last fragment in this
     // (abandoned) message and start sending a new message, as the receiver will
     // then see a new message before the end of the previous one was seen (or
     // skipped over). So create a new fragment, representing the end, that the
     // received will never see as it is abandoned immediately and used as cum
     // TSN in the sent FORWARD-TSN.
     UnwrappedTSN tsn = next_tsn_;
     next_tsn_.Increment();
     Data message_end(item.data().stream_id, item.data().ssn,
                      item.data().message_id, item.data().fsn, item.data().ppid,
                      std::vector<uint8_t>(), Data::IsBeginning(false),
                      Data::IsEnd(true), item.data().is_unordered);
     Item& added_item =
         outstanding_data_
             .emplace(tsn,
                      Item(std::move(message_end), MaxRetransmits::NoLimit(),
                           TimeMs(0), TimeMs::InfiniteFuture()))
             .first->second;
     // The added chunk shouldn't be included in `outstanding_bytes`, so set it
     // as acked.
     added_item.Ack();
     RTC_DLOG(LS_VERBOSE) << "Adding unsent end placeholder for message at tsn="
                          << *tsn.Wrap();
   }

   for (auto& elem : outstanding_data_) {
     UnwrappedTSN tsn = elem.first;
     Item& other = elem.second;

     if (!other.is_abandoned() &&
         other.data().stream_id == item.data().stream_id &&
         other.data().is_unordered == item.data().is_unordered &&
         other.data().message_id == item.data().message_id) {
       RTC_DLOG(LS_VERBOSE) << "Marking chunk " << *tsn.Wrap()
                            << " as abandoned";
       if (other.should_be_retransmitted()) {
         to_be_retransmitted_.erase(tsn);
       }
       other.Abandon();
     }
   }
 }

 std::vector<std::pair<TSN, Data>> OutstandingData::GetChunksToBeRetransmitted(
     size_t max_size) {
   std::vector<std::pair<TSN, Data>> result;

   for (auto it = to_be_retransmitted_.begin();
        it != to_be_retransmitted_.end();) {
     UnwrappedTSN tsn = *it;
     auto elem = outstanding_data_.find(tsn);
     RTC_DCHECK(elem != outstanding_data_.end());
     Item& item = elem->second;
     RTC_DCHECK(item.should_be_retransmitted());
     RTC_DCHECK(!item.is_outstanding());
     RTC_DCHECK(!item.is_abandoned());
     RTC_DCHECK(!item.is_acked());

     size_t serialized_size = GetSerializedChunkSize(item.data());
     if (serialized_size <= max_size) {
       item.Retransmit();
       result.emplace_back(tsn.Wrap(), item.data().Clone());
       max_size -= serialized_size;
       outstanding_bytes_ += serialized_size;
       ++outstanding_items_;
       it = to_be_retransmitted_.erase(it);
     } else {
       ++it;
     }
     // No point in continuing if the packet is full.
     if (max_size <= data_chunk_header_size_) {
       break;
     }
   }

   RTC_DCHECK(IsConsistent());
   return result;
 }

 void OutstandingData::ExpireOutstandingChunks(TimeMs now) {
   for (const auto& elem : outstanding_data_) {
     UnwrappedTSN tsn = elem.first;
     const Item& item = elem.second;

     // Chunks that are nacked can be expired. Care should be taken not to expire
     // unacked (in-flight) chunks as they might have been received, but the SACK
     // is either delayed or in-flight and may be received later.
     if (item.is_abandoned()) {
       // Already abandoned.
     } else if (item.is_nacked() && item.has_expired(now)) {
       RTC_DLOG(LS_VERBOSE) << "Marking nacked chunk " << *tsn.Wrap()
                            << " and message " << *item.data().message_id
                            << " as expired";
       AbandonAllFor(item);
     } else {
       // A non-expired chunk. No need to iterate any further.
       break;
     }
   }
   RTC_DCHECK(IsConsistent());
 }

 UnwrappedTSN OutstandingData::highest_outstanding_tsn() const {
   return outstanding_data_.empty() ? last_cumulative_tsn_ack_
                                    : outstanding_data_.rbegin()->first;
 }

 absl::optional<UnwrappedTSN> OutstandingData::Insert(
     const Data& data,
     MaxRetransmits max_retransmissions,
     TimeMs time_sent,
     TimeMs expires_at) {
   UnwrappedTSN tsn = next_tsn_;
   next_tsn_.Increment();

   // All chunks are always padded to be even divisible by 4.
   size_t chunk_size = GetSerializedChunkSize(data);
   outstanding_bytes_ += chunk_size;
   ++outstanding_items_;
   auto it = outstanding_data_
                 .emplace(tsn, Item(data.Clone(), max_retransmissions, time_sent,
                                    expires_at))
                 .first;

   if (it->second.has_expired(time_sent)) {
     // No need to send it - it was expired when it was in the send
     // queue.
     RTC_DLOG(LS_VERBOSE) << "Marking freshly produced chunk "
                          << *it->first.Wrap() << " and message "
                          << *it->second.data().message_id << " as expired";
     AbandonAllFor(it->second);
     RTC_DCHECK(IsConsistent());
     return absl::nullopt;
   }

   RTC_DCHECK(IsConsistent());
   return tsn;
 }

 void OutstandingData::NackAll() {
   for (auto& elem : outstanding_data_) {
     UnwrappedTSN tsn = elem.first;
     Item& item = elem.second;
     if (!item.is_acked()) {
       NackItem(tsn, item, /*retransmit_now=*/true);
     }
   }
   RTC_DCHECK(IsConsistent());
 }

 absl::optional<DurationMs> OutstandingData::MeasureRTT(TimeMs now,
                                                        UnwrappedTSN tsn) const {
   auto it = outstanding_data_.find(tsn);
   if (it != outstanding_data_.end() && !it->second.has_been_retransmitted()) {
     // https://tools.ietf.org/html/rfc4960#section-6.3.1
     // "Karn's algorithm: RTT measurements MUST NOT be made using
     // packets that were retransmitted (and thus for which it is ambiguous
     // whether the reply was for the first instance of the chunk or for a
     // later instance)"
     return now - it->second.time_sent();
   }
   return absl::nullopt;
 }

 std::vector<std::pair<TSN, OutstandingData::State>>
 OutstandingData::GetChunkStatesForTesting() const {
   std::vector<std::pair<TSN, State>> states;
   states.emplace_back(last_cumulative_tsn_ack_.Wrap(), State::kAcked);
   for (const auto& elem : outstanding_data_) {
     State state;
     if (elem.second.is_abandoned()) {
       state = State::kAbandoned;
     } else if (elem.second.should_be_retransmitted()) {
       state = State::kToBeRetransmitted;
     } else if (elem.second.is_acked()) {
       state = State::kAcked;
     } else if (elem.second.is_outstanding()) {
       state = State::kInFlight;
     } else {
       state = State::kNacked;
     }

     states.emplace_back(elem.first.Wrap(), state);
   }
   return states;
 }

 bool OutstandingData::ShouldSendForwardTsn() const {
   if (!outstanding_data_.empty()) {
     auto it = outstanding_data_.begin();
     return it->first == last_cumulative_tsn_ack_.next_value() &&
            it->second.is_abandoned();
   }
   return false;
 }

 ForwardTsnChunk OutstandingData::CreateForwardTsn() const {
   std::map<StreamID, SSN> skipped_per_ordered_stream;
   UnwrappedTSN new_cumulative_ack = last_cumulative_tsn_ack_;

   for (const auto& elem : outstanding_data_) {
     UnwrappedTSN tsn = elem.first;
     const Item& item = elem.second;

     if ((tsn != new_cumulative_ack.next_value()) || !item.is_abandoned()) {
       break;
     }
     new_cumulative_ack = tsn;
     if (!item.data().is_unordered &&
         item.data().ssn > skipped_per_ordered_stream[item.data().stream_id]) {
       skipped_per_ordered_stream[item.data().stream_id] = item.data().ssn;
     }
   }

   std::vector<ForwardTsnChunk::SkippedStream> skipped_streams;
   skipped_streams.reserve(skipped_per_ordered_stream.size());
   for (const auto& elem : skipped_per_ordered_stream) {
     skipped_streams.emplace_back(elem.first, elem.second);
   }
   return ForwardTsnChunk(new_cumulative_ack.Wrap(), std::move(skipped_streams));
 }

 IForwardTsnChunk OutstandingData::CreateIForwardTsn() const {
   std::map<std::pair<IsUnordered, StreamID>, MID> skipped_per_stream;
   UnwrappedTSN new_cumulative_ack = last_cumulative_tsn_ack_;

   for (const auto& elem : outstanding_data_) {
     UnwrappedTSN tsn = elem.first;
     const Item& item = elem.second;

     if ((tsn != new_cumulative_ack.next_value()) || !item.is_abandoned()) {
       break;
     }
     new_cumulative_ack = tsn;
     std::pair<IsUnordered, StreamID> stream_id =
         std::make_pair(item.data().is_unordered, item.data().stream_id);

     if (item.data().message_id > skipped_per_stream[stream_id]) {
       skipped_per_stream[stream_id] = item.data().message_id;
     }
   }

   std::vector<IForwardTsnChunk::SkippedStream> skipped_streams;
   skipped_streams.reserve(skipped_per_stream.size());
   for (const auto& elem : skipped_per_stream) {
     const std::pair<IsUnordered, StreamID>& stream = elem.first;
     MID message_id = elem.second;
     skipped_streams.emplace_back(stream.first, stream.second, message_id);
   }

   return IForwardTsnChunk(new_cumulative_ack.Wrap(),
                           std::move(skipped_streams));
 }

 }  // namespace dcsctp
	/*
	* Copyright (c) 2021 The WebRTC project authors. All Rights Reserved.
	*
	* Use of this source code is governed by a BSD-style license
	* that can be found in the LICENSE file in the root of the source
	* tree. An additional intellectual property rights grant can be found
	* in the file PATENTS. All contributing project authors may
	* be found in the AUTHORS file in the root of the source tree.
	*/
	#include "net/dcsctp/tx/outstanding_data.h"

	#include <algorithm>
	#include <set>
	#include <utility>
	#include <vector>

	#include "net/dcsctp/common/math.h"
	#include "net/dcsctp/common/sequence_numbers.h"
	#include "net/dcsctp/public/types.h"
	#include "rtc_base/logging.h"

	namespace dcsctp {

	// The number of times a packet must be NACKed before it's retransmitted.
	// See https://tools.ietf.org/html/rfc4960#section-7.2.4
	constexpr uint8_t kNumberOfNacksForRetransmission = 3;

	// Returns how large a chunk will be, serialized, carrying the data
	size_t OutstandingData::GetSerializedChunkSize(const Data& data) const {
	return RoundUpTo4(data_chunk_header_size_ + data.size());
	}

	void OutstandingData::Item::Ack() {
	ack_state_ = AckState::kAcked;
	should_be_retransmitted_ = false;
	}

	OutstandingData::Item::NackAction OutstandingData::Item::Nack(
	bool retransmit_now) {
	ack_state_ = AckState::kNacked;
	++nack_count_;
	if ((retransmit_now \|\| nack_count_ >= kNumberOfNacksForRetransmission) &&
	!is_abandoned_) {
	// Nacked enough times - it's considered lost.
	if (num_retransmissions_ < *max_retransmissions_) {
	should_be_retransmitted_ = true;
	return NackAction::kRetransmit;
	}
	Abandon();
	return NackAction::kAbandon;
	}
	return NackAction::kNothing;
	}

	void OutstandingData::Item::Retransmit() {
	ack_state_ = AckState::kUnacked;
	should_be_retransmitted_ = false;

	nack_count_ = 0;
	++num_retransmissions_;
	}

	void OutstandingData::Item::Abandon() {
	is_abandoned_ = true;
	should_be_retransmitted_ = false;
	}

	bool OutstandingData::Item::has_expired(TimeMs now) const {
	return expires_at_ <= now;
	}

	bool OutstandingData::IsConsistent() const {
	size_t actual_outstanding_bytes = 0;
	size_t actual_outstanding_items = 0;

	std::set<UnwrappedTSN> actual_to_be_retransmitted;
	for (const auto& elem : outstanding_data_) {
	if (elem.second.is_outstanding()) {
	actual_outstanding_bytes += GetSerializedChunkSize(elem.second.data());
	++actual_outstanding_items;
	}

	if (elem.second.should_be_retransmitted()) {
	actual_to_be_retransmitted.insert(elem.first);
	}
	}

	if (outstanding_data_.empty() &&
	next_tsn_ != last_cumulative_tsn_ack_.next_value()) {
	return false;
	}

	return actual_outstanding_bytes == outstanding_bytes_ &&
	actual_outstanding_items == outstanding_items_ &&
	actual_to_be_retransmitted == to_be_retransmitted_;
	}

	void OutstandingData::AckChunk(AckInfo& ack_info,
	std::map<UnwrappedTSN, Item>::iterator iter) {
	if (!iter->second.is_acked()) {
	size_t serialized_size = GetSerializedChunkSize(iter->second.data());
	ack_info.bytes_acked += serialized_size;
	if (iter->second.is_outstanding()) {
	outstanding_bytes_ -= serialized_size;
	--outstanding_items_;
	}
	if (iter->second.should_be_retransmitted()) {
	to_be_retransmitted_.erase(iter->first);
	}
	iter->second.Ack();
	ack_info.highest_tsn_acked =
	std::max(ack_info.highest_tsn_acked, iter->first);
	}
	}

	OutstandingData::AckInfo OutstandingData::HandleSack(
	UnwrappedTSN cumulative_tsn_ack,
	rtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
	bool is_in_fast_retransmit) {
	OutstandingData::AckInfo ack_info(cumulative_tsn_ack);
	// Erase all items up to cumulative_tsn_ack.
	RemoveAcked(cumulative_tsn_ack, ack_info);

	// ACK packets reported in the gap ack blocks
	AckGapBlocks(cumulative_tsn_ack, gap_ack_blocks, ack_info);

	// NACK and possibly mark for retransmit chunks that weren't acked.
	NackBetweenAckBlocks(cumulative_tsn_ack, gap_ack_blocks,
	is_in_fast_retransmit, ack_info);

	RTC_DCHECK(IsConsistent());
	return ack_info;
	}

	void OutstandingData::RemoveAcked(UnwrappedTSN cumulative_tsn_ack,
	AckInfo& ack_info) {
	auto first_unacked = outstanding_data_.upper_bound(cumulative_tsn_ack);

	for (auto iter = outstanding_data_.begin(); iter != first_unacked; ++iter) {
	AckChunk(ack_info, iter);
	}

	outstanding_data_.erase(outstanding_data_.begin(), first_unacked);
	last_cumulative_tsn_ack_ = cumulative_tsn_ack;
	}

	void OutstandingData::AckGapBlocks(
	UnwrappedTSN cumulative_tsn_ack,
	rtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
	AckInfo& ack_info) {
	// Mark all non-gaps as ACKED (but they can't be removed) as (from RFC)
	// "SCTP considers the information carried in the Gap Ack Blocks in the
	// SACK chunk as advisory.". Note that when NR-SACK is supported, this can be
	// handled differently.

	for (auto& block : gap_ack_blocks) {
	auto start = outstanding_data_.lower_bound(
	UnwrappedTSN::AddTo(cumulative_tsn_ack, block.start));
	auto end = outstanding_data_.upper_bound(
	UnwrappedTSN::AddTo(cumulative_tsn_ack, block.end));
	for (auto iter = start; iter != end; ++iter) {
	AckChunk(ack_info, iter);
	}
	}
	}

	void OutstandingData::NackBetweenAckBlocks(
	UnwrappedTSN cumulative_tsn_ack,
	rtc::ArrayView<const SackChunk::GapAckBlock> gap_ack_blocks,
	bool is_in_fast_recovery,
	OutstandingData::AckInfo& ack_info) {
	// Mark everything between the blocks as NACKED/TO_BE_RETRANSMITTED.
	// https://tools.ietf.org/html/rfc4960#section-7.2.4
	// "Mark the DATA chunk(s) with three miss indications for retransmission."
	// "For each incoming SACK, miss indications are incremented only for
	// missing TSNs prior to the highest TSN newly acknowledged in the SACK."
	//
	// What this means is that only when there is a increasing stream of data
	// received and there are new packets seen (since last time), packets that are
	// in-flight and between gaps should be nacked. This means that SCTP relies on
	// the T3-RTX-timer to re-send packets otherwise.
	UnwrappedTSN max_tsn_to_nack = ack_info.highest_tsn_acked;
	if (is_in_fast_recovery && cumulative_tsn_ack > last_cumulative_tsn_ack_) {
	// https://tools.ietf.org/html/rfc4960#section-7.2.4
	// "If an endpoint is in Fast Recovery and a SACK arrives that advances
	// the Cumulative TSN Ack Point, the miss indications are incremented for
	// all TSNs reported missing in the SACK."
	max_tsn_to_nack = UnwrappedTSN::AddTo(
	cumulative_tsn_ack,
	gap_ack_blocks.empty() ? 0 : gap_ack_blocks.rbegin()->end);
	}

	UnwrappedTSN prev_block_last_acked = cumulative_tsn_ack;
	for (auto& block : gap_ack_blocks) {
	UnwrappedTSN cur_block_first_acked =
	UnwrappedTSN::AddTo(cumulative_tsn_ack, block.start);
	for (auto iter = outstanding_data_.upper_bound(prev_block_last_acked);
	iter != outstanding_data_.lower_bound(cur_block_first_acked); ++iter) {
	if (iter->first <= max_tsn_to_nack) {
	ack_info.has_packet_loss =
	NackItem(iter->first, iter->second, /retransmit_now=/false);
	}
	}
	prev_block_last_acked = UnwrappedTSN::AddTo(cumulative_tsn_ack, block.end);
	}

	// Note that packets are not NACKED which are above the highest gap-ack-block
	// (or above the cumulative ack TSN if no gap-ack-blocks) as only packets
	// up until the highest_tsn_acked (see above) should be considered when
	// NACKing.
	}

	bool OutstandingData::NackItem(UnwrappedTSN tsn,
	Item& item,
	bool retransmit_now) {
	if (item.is_outstanding()) {
	outstanding_bytes_ -= GetSerializedChunkSize(item.data());
	--outstanding_items_;
	}

	switch (item.Nack(retransmit_now)) {
	case Item::NackAction::kNothing:
	return false;
	case Item::NackAction::kRetransmit:
	to_be_retransmitted_.insert(tsn);
	RTC_DLOG(LS_VERBOSE) << *tsn.Wrap() << " marked for retransmission";
	break;
	case Item::NackAction::kAbandon:
	AbandonAllFor(item);
	break;
	}
	return true;
	}

	void OutstandingData::AbandonAllFor(const Item& item) {
	// Erase all remaining chunks from the producer, if any.
	if (discard_from_send_queue_(item.data().is_unordered, item.data().stream_id,
	item.data().message_id)) {
	// There were remaining chunks to be produced for this message. Since the
	// receiver may have already received all chunks (up till now) for this
	// message, we can't just FORWARD-TSN to the last fragment in this
	// (abandoned) message and start sending a new message, as the receiver will
	// then see a new message before the end of the previous one was seen (or
	// skipped over). So create a new fragment, representing the end, that the
	// received will never see as it is abandoned immediately and used as cum
	// TSN in the sent FORWARD-TSN.
	UnwrappedTSN tsn = next_tsn_;
	next_tsn_.Increment();
	Data message_end(item.data().stream_id, item.data().ssn,
	item.data().message_id, item.data().fsn, item.data().ppid,
	std::vector<uint8_t>(), Data::IsBeginning(false),
	Data::IsEnd(true), item.data().is_unordered);
	Item& added_item =
	outstanding_data_
	.emplace(tsn,
	Item(std::move(message_end), MaxRetransmits::NoLimit(),
	TimeMs(0), TimeMs::InfiniteFuture()))
	.first->second;
	// The added chunk shouldn't be included in `outstanding_bytes`, so set it
	// as acked.
	added_item.Ack();
	RTC_DLOG(LS_VERBOSE) << "Adding unsent end placeholder for message at tsn="
	<< *tsn.Wrap();
	}

	for (auto& elem : outstanding_data_) {
	UnwrappedTSN tsn = elem.first;
	Item& other = elem.second;

	if (!other.is_abandoned() &&
	other.data().stream_id == item.data().stream_id &&
	other.data().is_unordered == item.data().is_unordered &&
	other.data().message_id == item.data().message_id) {
	RTC_DLOG(LS_VERBOSE) << "Marking chunk " << *tsn.Wrap()
	<< " as abandoned";
	if (other.should_be_retransmitted()) {
	to_be_retransmitted_.erase(tsn);
	}
	other.Abandon();
	}
	}
	}

	std::vector<std::pair<TSN, Data>> OutstandingData::GetChunksToBeRetransmitted(
	size_t max_size) {
	std::vector<std::pair<TSN, Data>> result;

	for (auto it = to_be_retransmitted_.begin();
	it != to_be_retransmitted_.end();) {
	UnwrappedTSN tsn = *it;
	auto elem = outstanding_data_.find(tsn);
	RTC_DCHECK(elem != outstanding_data_.end());
	Item& item = elem->second;
	RTC_DCHECK(item.should_be_retransmitted());
	RTC_DCHECK(!item.is_outstanding());
	RTC_DCHECK(!item.is_abandoned());
	RTC_DCHECK(!item.is_acked());

	size_t serialized_size = GetSerializedChunkSize(item.data());
	if (serialized_size <= max_size) {
	item.Retransmit();
	result.emplace_back(tsn.Wrap(), item.data().Clone());
	max_size -= serialized_size;
	outstanding_bytes_ += serialized_size;
	++outstanding_items_;
	it = to_be_retransmitted_.erase(it);
	} else {
	++it;
	}
	// No point in continuing if the packet is full.
	if (max_size <= data_chunk_header_size_) {
	break;
	}
	}

	RTC_DCHECK(IsConsistent());
	return result;
	}

	void OutstandingData::ExpireOutstandingChunks(TimeMs now) {
	for (const auto& elem : outstanding_data_) {
	UnwrappedTSN tsn = elem.first;
	const Item& item = elem.second;

	// Chunks that are nacked can be expired. Care should be taken not to expire
	// unacked (in-flight) chunks as they might have been received, but the SACK
	// is either delayed or in-flight and may be received later.
	if (item.is_abandoned()) {
	// Already abandoned.
	} else if (item.is_nacked() && item.has_expired(now)) {
	RTC_DLOG(LS_VERBOSE) << "Marking nacked chunk " << *tsn.Wrap()
	<< " and message " << *item.data().message_id
	<< " as expired";
	AbandonAllFor(item);
	} else {
	// A non-expired chunk. No need to iterate any further.
	break;
	}
	}
	RTC_DCHECK(IsConsistent());
	}

	UnwrappedTSN OutstandingData::highest_outstanding_tsn() const {
	return outstanding_data_.empty() ? last_cumulative_tsn_ack_
	: outstanding_data_.rbegin()->first;
	}

	absl::optional<UnwrappedTSN> OutstandingData::Insert(
	const Data& data,
	MaxRetransmits max_retransmissions,
	TimeMs time_sent,
	TimeMs expires_at) {
	UnwrappedTSN tsn = next_tsn_;
	next_tsn_.Increment();

	// All chunks are always padded to be even divisible by 4.
	size_t chunk_size = GetSerializedChunkSize(data);
	outstanding_bytes_ += chunk_size;
	++outstanding_items_;
	auto it = outstanding_data_
	.emplace(tsn, Item(data.Clone(), max_retransmissions, time_sent,
	expires_at))
	.first;

	if (it->second.has_expired(time_sent)) {
	// No need to send it - it was expired when it was in the send
	// queue.
	RTC_DLOG(LS_VERBOSE) << "Marking freshly produced chunk "
	<< *it->first.Wrap() << " and message "
	<< *it->second.data().message_id << " as expired";
	AbandonAllFor(it->second);
	RTC_DCHECK(IsConsistent());
	return absl::nullopt;
	}

	RTC_DCHECK(IsConsistent());
	return tsn;
	}

	void OutstandingData::NackAll() {
	for (auto& elem : outstanding_data_) {
	UnwrappedTSN tsn = elem.first;
	Item& item = elem.second;
	if (!item.is_acked()) {
	NackItem(tsn, item, /retransmit_now=/true);
	}
	}
	RTC_DCHECK(IsConsistent());
	}

	absl::optional<DurationMs> OutstandingData::MeasureRTT(TimeMs now,
	UnwrappedTSN tsn) const {
	auto it = outstanding_data_.find(tsn);
	if (it != outstanding_data_.end() && !it->second.has_been_retransmitted()) {
	// https://tools.ietf.org/html/rfc4960#section-6.3.1
	// "Karn's algorithm: RTT measurements MUST NOT be made using
	// packets that were retransmitted (and thus for which it is ambiguous
	// whether the reply was for the first instance of the chunk or for a
	// later instance)"
	return now - it->second.time_sent();
	}
	return absl::nullopt;
	}

	std::vector<std::pair<TSN, OutstandingData::State>>
	OutstandingData::GetChunkStatesForTesting() const {
	std::vector<std::pair<TSN, State>> states;
	states.emplace_back(last_cumulative_tsn_ack_.Wrap(), State::kAcked);
	for (const auto& elem : outstanding_data_) {
	State state;
	if (elem.second.is_abandoned()) {
	state = State::kAbandoned;
	} else if (elem.second.should_be_retransmitted()) {
	state = State::kToBeRetransmitted;
	} else if (elem.second.is_acked()) {
	state = State::kAcked;
	} else if (elem.second.is_outstanding()) {
	state = State::kInFlight;
	} else {
	state = State::kNacked;
	}

	states.emplace_back(elem.first.Wrap(), state);
	}
	return states;
	}

	bool OutstandingData::ShouldSendForwardTsn() const {
	if (!outstanding_data_.empty()) {
	auto it = outstanding_data_.begin();
	return it->first == last_cumulative_tsn_ack_.next_value() &&
	it->second.is_abandoned();
	}
	return false;
	}

	ForwardTsnChunk OutstandingData::CreateForwardTsn() const {
	std::map<StreamID, SSN> skipped_per_ordered_stream;
	UnwrappedTSN new_cumulative_ack = last_cumulative_tsn_ack_;

	for (const auto& elem : outstanding_data_) {
	UnwrappedTSN tsn = elem.first;
	const Item& item = elem.second;

	if ((tsn != new_cumulative_ack.next_value()) \|\| !item.is_abandoned()) {
	break;
	}
	new_cumulative_ack = tsn;
	if (!item.data().is_unordered &&
	item.data().ssn > skipped_per_ordered_stream[item.data().stream_id]) {
	skipped_per_ordered_stream[item.data().stream_id] = item.data().ssn;
	}
	}

	std::vector<ForwardTsnChunk::SkippedStream> skipped_streams;
	skipped_streams.reserve(skipped_per_ordered_stream.size());
	for (const auto& elem : skipped_per_ordered_stream) {
	skipped_streams.emplace_back(elem.first, elem.second);
	}
	return ForwardTsnChunk(new_cumulative_ack.Wrap(), std::move(skipped_streams));
	}

	IForwardTsnChunk OutstandingData::CreateIForwardTsn() const {
	std::map<std::pair<IsUnordered, StreamID>, MID> skipped_per_stream;
	UnwrappedTSN new_cumulative_ack = last_cumulative_tsn_ack_;

	for (const auto& elem : outstanding_data_) {
	UnwrappedTSN tsn = elem.first;
	const Item& item = elem.second;

	if ((tsn != new_cumulative_ack.next_value()) \|\| !item.is_abandoned()) {
	break;
	}
	new_cumulative_ack = tsn;
	std::pair<IsUnordered, StreamID> stream_id =
	std::make_pair(item.data().is_unordered, item.data().stream_id);

	if (item.data().message_id > skipped_per_stream[stream_id]) {
	skipped_per_stream[stream_id] = item.data().message_id;
	}
	}

	std::vector<IForwardTsnChunk::SkippedStream> skipped_streams;
	skipped_streams.reserve(skipped_per_stream.size());
	for (const auto& elem : skipped_per_stream) {
	const std::pair<IsUnordered, StreamID>& stream = elem.first;
	MID message_id = elem.second;
	skipped_streams.emplace_back(stream.first, stream.second, message_id);
	}

	return IForwardTsnChunk(new_cumulative_ack.Wrap(),
	std::move(skipped_streams));
	}

	} // namespace dcsctp