net/dcsctp/tx/rr_send_queue.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/rr_send_queue.h"

 #include <cstdint>
 #include <deque>
 #include <limits>
 #include <map>
 #include <utility>
 #include <vector>

 #include "absl/algorithm/container.h"
 #include "absl/types/optional.h"
 #include "api/array_view.h"
 #include "net/dcsctp/packet/data.h"
 #include "net/dcsctp/public/dcsctp_message.h"
 #include "net/dcsctp/public/dcsctp_socket.h"
 #include "net/dcsctp/public/types.h"
 #include "net/dcsctp/tx/send_queue.h"
 #include "rtc_base/logging.h"

 namespace dcsctp {

 RRSendQueue::RRSendQueue(absl::string_view log_prefix,
                          size_t buffer_size,
                          StreamPriority default_priority,
                          std::function<void(StreamID)> on_buffered_amount_low,
                          size_t total_buffered_amount_low_threshold,
                          std::function<void()> on_total_buffered_amount_low)
     : log_prefix_(std::string(log_prefix) + "fcfs: "),
       buffer_size_(buffer_size),
       default_priority_(default_priority),
       on_buffered_amount_low_(std::move(on_buffered_amount_low)),
       total_buffered_amount_(std::move(on_total_buffered_amount_low)) {
   total_buffered_amount_.SetLowThreshold(total_buffered_amount_low_threshold);
 }

 bool RRSendQueue::OutgoingStream::HasDataToSend() const {
   if (pause_state_ == PauseState::kPaused ||
       pause_state_ == PauseState::kResetting) {
     // The stream has paused (and there is no partially sent message).
     return false;
   }

   if (items_.empty()) {
     return false;
   }

   return true;
 }

 void RRSendQueue::OutgoingStream::AddHandoverState(
     DcSctpSocketHandoverState::OutgoingStream& state) const {
   state.next_ssn = next_ssn_.value();
   state.next_ordered_mid = next_ordered_mid_.value();
   state.next_unordered_mid = next_unordered_mid_.value();
   state.priority = *priority_;
 }

 bool RRSendQueue::IsConsistent() const {
   size_t total_buffered_amount = 0;
   for (const auto& [unused, stream] : streams_) {
     total_buffered_amount += stream.buffered_amount().value();
   }

   if (previous_message_has_ended_) {
     auto it = streams_.find(current_stream_id_);
     if (it != streams_.end() && it->second.has_partially_sent_message()) {
       RTC_DLOG(LS_ERROR)
           << "Previous message has ended, but still partial message in stream";
       return false;
     }
   } else {
     auto it = streams_.find(current_stream_id_);
     if (it == streams_.end() || !it->second.has_partially_sent_message()) {
       RTC_DLOG(LS_ERROR)
           << "Previous message has NOT ended, but there is no partial message";
       return false;
     }
   }

   return total_buffered_amount == total_buffered_amount_.value();
 }

 bool RRSendQueue::OutgoingStream::IsConsistent() const {
   size_t bytes = 0;
   for (const auto& item : items_) {
     bytes += item.remaining_size;
   }
   return bytes == buffered_amount_.value();
 }

 void RRSendQueue::ThresholdWatcher::Decrease(size_t bytes) {
   RTC_DCHECK(bytes <= value_);
   size_t old_value = value_;
   value_ -= bytes;

   if (old_value > low_threshold_ && value_ <= low_threshold_) {
     on_threshold_reached_();
   }
 }

 void RRSendQueue::ThresholdWatcher::SetLowThreshold(size_t low_threshold) {
   // Betting on https://github.com/w3c/webrtc-pc/issues/2654 being accepted.
   if (low_threshold_ < value_ && low_threshold >= value_) {
     on_threshold_reached_();
   }
   low_threshold_ = low_threshold;
 }

 void RRSendQueue::OutgoingStream::Add(DcSctpMessage message,
                                       TimeMs expires_at,
                                       const SendOptions& send_options) {
   buffered_amount_.Increase(message.payload().size());
   total_buffered_amount_.Increase(message.payload().size());
   items_.emplace_back(std::move(message), expires_at, send_options);

   RTC_DCHECK(IsConsistent());
 }

 absl::optional<SendQueue::DataToSend> RRSendQueue::OutgoingStream::Produce(
     TimeMs now,
     size_t max_size) {
   RTC_DCHECK(pause_state_ != PauseState::kPaused &&
              pause_state_ != PauseState::kResetting);

   while (!items_.empty()) {
     Item& item = items_.front();
     DcSctpMessage& message = item.message;

     // Allocate Message ID and SSN when the first fragment is sent.
     if (!item.message_id.has_value()) {
       // Oops, this entire message has already expired. Try the next one.
       if (item.expires_at <= now) {
         buffered_amount_.Decrease(item.remaining_size);
         total_buffered_amount_.Decrease(item.remaining_size);
         items_.pop_front();
         continue;
       }

       MID& mid =
           item.send_options.unordered ? next_unordered_mid_ : next_ordered_mid_;
       item.message_id = mid;
       mid = MID(*mid + 1);
     }
     if (!item.send_options.unordered && !item.ssn.has_value()) {
       item.ssn = next_ssn_;
       next_ssn_ = SSN(*next_ssn_ + 1);
     }

     // Grab the next `max_size` fragment from this message and calculate flags.
     rtc::ArrayView<const uint8_t> chunk_payload =
         item.message.payload().subview(item.remaining_offset, max_size);
     rtc::ArrayView<const uint8_t> message_payload = message.payload();
     Data::IsBeginning is_beginning(chunk_payload.data() ==
                                    message_payload.data());
     Data::IsEnd is_end((chunk_payload.data() + chunk_payload.size()) ==
                        (message_payload.data() + message_payload.size()));

     StreamID stream_id = message.stream_id();
     PPID ppid = message.ppid();

     // Zero-copy the payload if the message fits in a single chunk.
     std::vector<uint8_t> payload =
         is_beginning && is_end
             ? std::move(message).ReleasePayload()
             : std::vector<uint8_t>(chunk_payload.begin(), chunk_payload.end());

     FSN fsn(item.current_fsn);
     item.current_fsn = FSN(*item.current_fsn + 1);
     buffered_amount_.Decrease(payload.size());
     total_buffered_amount_.Decrease(payload.size());

     SendQueue::DataToSend chunk(Data(stream_id, item.ssn.value_or(SSN(0)),
                                      item.message_id.value(), fsn, ppid,
                                      std::move(payload), is_beginning, is_end,
                                      item.send_options.unordered));
     if (item.send_options.max_retransmissions.has_value() &&
         *item.send_options.max_retransmissions >=
             std::numeric_limits<MaxRetransmits::UnderlyingType>::min() &&
         *item.send_options.max_retransmissions <=
             std::numeric_limits<MaxRetransmits::UnderlyingType>::max()) {
       chunk.max_retransmissions =
           MaxRetransmits(*item.send_options.max_retransmissions);
     }
     chunk.expires_at = item.expires_at;

     if (is_end) {
       // The entire message has been sent, and its last data copied to `chunk`,
       // so it can safely be discarded.
       items_.pop_front();

       if (pause_state_ == PauseState::kPending) {
         RTC_DLOG(LS_VERBOSE) << "Pause state on " << *stream_id
                              << " is moving from pending to paused";
         pause_state_ = PauseState::kPaused;
       }
     } else {
       item.remaining_offset += chunk_payload.size();
       item.remaining_size -= chunk_payload.size();
       RTC_DCHECK(item.remaining_offset + item.remaining_size ==
                  item.message.payload().size());
       RTC_DCHECK(item.remaining_size > 0);
     }
     RTC_DCHECK(IsConsistent());
     return chunk;
   }
   RTC_DCHECK(IsConsistent());
   return absl::nullopt;
 }

 bool RRSendQueue::OutgoingStream::Discard(IsUnordered unordered,
                                           MID message_id) {
   bool result = false;
   if (!items_.empty()) {
     Item& item = items_.front();
     if (item.send_options.unordered == unordered &&
         item.message_id.has_value() && *item.message_id == message_id) {
       buffered_amount_.Decrease(item.remaining_size);
       total_buffered_amount_.Decrease(item.remaining_size);
       items_.pop_front();

       if (pause_state_ == PauseState::kPending) {
         pause_state_ = PauseState::kPaused;
       }

       // As the item still existed, it had unsent data.
       result = true;
     }
   }
   RTC_DCHECK(IsConsistent());
   return result;
 }

 void RRSendQueue::OutgoingStream::Pause() {
   if (pause_state_ != PauseState::kNotPaused) {
     // Already in progress.
     return;
   }

   bool had_pending_items = !items_.empty();

   // https://datatracker.ietf.org/doc/html/rfc8831#section-6.7
   // "Closing of a data channel MUST be signaled by resetting the corresponding
   // outgoing streams [RFC6525].  This means that if one side decides to close
   // the data channel, it resets the corresponding outgoing stream."
   // ... "[RFC6525] also guarantees that all the messages are delivered (or
   // abandoned) before the stream is reset."

   // A stream is paused when it's about to be reset. In this implementation,
   // it will throw away all non-partially send messages - they will be abandoned
   // as noted above. This is subject to change. It will however not discard any
   // partially sent messages - only whole messages. Partially delivered messages
   // (at the time of receiving a Stream Reset command) will always deliver all
   // the fragments before actually resetting the stream.
   for (auto it = items_.begin(); it != items_.end();) {
     if (it->remaining_offset == 0) {
       buffered_amount_.Decrease(it->remaining_size);
       total_buffered_amount_.Decrease(it->remaining_size);
       it = items_.erase(it);
     } else {
       ++it;
     }
   }

   pause_state_ = (items_.empty() || items_.front().remaining_offset == 0)
                      ? PauseState::kPaused
                      : PauseState::kPending;

   if (had_pending_items && pause_state_ == PauseState::kPaused) {
     RTC_DLOG(LS_VERBOSE) << "Stream " << *stream_id()
                          << " was previously active, but is now paused.";
   }

   RTC_DCHECK(IsConsistent());
 }

 void RRSendQueue::OutgoingStream::Resume() {
   RTC_DCHECK(pause_state_ == PauseState::kResetting);
   if (!items_.empty()) {
     RTC_DLOG(LS_VERBOSE) << "Stream " << *stream_id()
                          << " was previously paused, but is now active.";
   }
   pause_state_ = PauseState::kNotPaused;
   RTC_DCHECK(IsConsistent());
 }

 void RRSendQueue::OutgoingStream::Reset() {
   // This can be called both when an outgoing stream reset has been responded
   // to, or when the entire SendQueue is reset due to detecting the peer having
   // restarted. The stream may be in any state at this time.
   if (!items_.empty()) {
     // If this message has been partially sent, reset it so that it will be
     // re-sent.
     auto& item = items_.front();
     buffered_amount_.Increase(item.message.payload().size() -
                               item.remaining_size);
     total_buffered_amount_.Increase(item.message.payload().size() -
                                     item.remaining_size);
     item.remaining_offset = 0;
     item.remaining_size = item.message.payload().size();
     item.message_id = absl::nullopt;
     item.ssn = absl::nullopt;
     item.current_fsn = FSN(0);
   }
   pause_state_ = PauseState::kNotPaused;
   next_ordered_mid_ = MID(0);
   next_unordered_mid_ = MID(0);
   next_ssn_ = SSN(0);
   RTC_DCHECK(IsConsistent());
 }

 bool RRSendQueue::OutgoingStream::has_partially_sent_message() const {
   if (items_.empty()) {
     return false;
   }
   return items_.front().message_id.has_value();
 }

 void RRSendQueue::Add(TimeMs now,
                       DcSctpMessage message,
                       const SendOptions& send_options) {
   RTC_DCHECK(!message.payload().empty());
   // Any limited lifetime should start counting from now - when the message
   // has been added to the queue.
   TimeMs expires_at = TimeMs::InfiniteFuture();
   if (send_options.lifetime.has_value()) {
     // `expires_at` is the time when it expires. Which is slightly larger than
     // the message's lifetime, as the message is alive during its entire
     // lifetime (which may be zero).
     expires_at = now + *send_options.lifetime + DurationMs(1);
   }
   GetOrCreateStreamInfo(message.stream_id())
       .Add(std::move(message), expires_at, send_options);
   RTC_DCHECK(IsConsistent());
 }

 bool RRSendQueue::IsFull() const {
   return total_buffered_amount() >= buffer_size_;
 }

 bool RRSendQueue::IsEmpty() const {
   return total_buffered_amount() == 0;
 }

 std::map<StreamID, RRSendQueue::OutgoingStream>::iterator
 RRSendQueue::GetNextStream() {
   auto start_it = streams_.lower_bound(StreamID(*current_stream_id_ + 1));

   for (auto it = start_it; it != streams_.end(); ++it) {
     if (it->second.HasDataToSend()) {
       current_stream_id_ = it->first;
       return it;
     }
   }

   for (auto it = streams_.begin(); it != start_it; ++it) {
     if (it->second.HasDataToSend()) {
       current_stream_id_ = it->first;
       return it;
     }
   }
   return streams_.end();
 }

 absl::optional<SendQueue::DataToSend> RRSendQueue::Produce(TimeMs now,
                                                            size_t max_size) {
   std::map<StreamID, RRSendQueue::OutgoingStream>::iterator stream_it;

   for (;;) {
     if (previous_message_has_ended_) {
       // Previous message has ended. Round-robin to a different stream, if there
       // even is one with data to send.
       stream_it = GetNextStream();
       if (stream_it == streams_.end()) {
         RTC_DLOG(LS_VERBOSE)
             << log_prefix_
             << "There is no stream with data; Can't produce any data.";
         return absl::nullopt;
       }
     } else {
       // The previous message has not ended; Continue from the current stream.
       stream_it = streams_.find(current_stream_id_);
       RTC_DCHECK(stream_it != streams_.end());
     }

     absl::optional<DataToSend> data = stream_it->second.Produce(now, max_size);
     if (!data.has_value()) {
       continue;
     }
     RTC_DLOG(LS_VERBOSE) << log_prefix_ << "Producing DATA, type="
                          << (data->data.is_unordered ? "unordered" : "ordered")
                          << "::"
                          << (*data->data.is_beginning && *data->data.is_end
                                  ? "complete"
                              : *data->data.is_beginning ? "first"
                              : *data->data.is_end       ? "last"
                                                         : "middle")
                          << ", stream_id=" << *stream_it->first
                          << ", ppid=" << *data->data.ppid
                          << ", length=" << data->data.payload.size();

     previous_message_has_ended_ = *data->data.is_end;
     RTC_DCHECK(IsConsistent());
     return data;
   }
 }

 bool RRSendQueue::Discard(IsUnordered unordered,
                           StreamID stream_id,
                           MID message_id) {
   bool has_discarded =
       GetOrCreateStreamInfo(stream_id).Discard(unordered, message_id);
   if (has_discarded) {
     // Only partially sent messages are discarded, so if a message was
     // discarded, then it was the currently sent message.
     previous_message_has_ended_ = true;
   }

   return has_discarded;
 }

 void RRSendQueue::PrepareResetStream(StreamID stream_id) {
   GetOrCreateStreamInfo(stream_id).Pause();
   RTC_DCHECK(IsConsistent());
 }

 bool RRSendQueue::HasStreamsReadyToBeReset() const {
   for (auto& [unused, stream] : streams_) {
     if (stream.IsReadyToBeReset()) {
       return true;
     }
   }
   return false;
 }
 std::vector<StreamID> RRSendQueue::GetStreamsReadyToBeReset() {
   RTC_DCHECK(absl::c_count_if(streams_, [](const auto& p) {
                return p.second.IsResetting();
              }) == 0);
   std::vector<StreamID> ready;
   for (auto& [stream_id, stream] : streams_) {
     if (stream.IsReadyToBeReset()) {
       stream.SetAsResetting();
       ready.push_back(stream_id);
     }
   }
   return ready;
 }

 void RRSendQueue::CommitResetStreams() {
   RTC_DCHECK(absl::c_count_if(streams_, [](const auto& p) {
                return p.second.IsResetting();
              }) > 0);
   for (auto& [unused, stream] : streams_) {
     if (stream.IsResetting()) {
       stream.Reset();
     }
   }
   RTC_DCHECK(IsConsistent());
 }

 void RRSendQueue::RollbackResetStreams() {
   RTC_DCHECK(absl::c_count_if(streams_, [](const auto& p) {
                return p.second.IsResetting();
              }) > 0);
   for (auto& [unused, stream] : streams_) {
     if (stream.IsResetting()) {
       stream.Resume();
     }
   }
   RTC_DCHECK(IsConsistent());
 }

 void RRSendQueue::Reset() {
   // Recalculate buffered amount, as partially sent messages may have been put
   // fully back in the queue.
   for (auto& [unused, stream] : streams_) {
     stream.Reset();
   }
   previous_message_has_ended_ = true;
 }

 size_t RRSendQueue::buffered_amount(StreamID stream_id) const {
   auto it = streams_.find(stream_id);
   if (it == streams_.end()) {
     return 0;
   }
   return it->second.buffered_amount().value();
 }

 size_t RRSendQueue::buffered_amount_low_threshold(StreamID stream_id) const {
   auto it = streams_.find(stream_id);
   if (it == streams_.end()) {
     return 0;
   }
   return it->second.buffered_amount().low_threshold();
 }

 void RRSendQueue::SetBufferedAmountLowThreshold(StreamID stream_id,
                                                 size_t bytes) {
   GetOrCreateStreamInfo(stream_id).buffered_amount().SetLowThreshold(bytes);
 }

 RRSendQueue::OutgoingStream& RRSendQueue::GetOrCreateStreamInfo(
     StreamID stream_id) {
   auto it = streams_.find(stream_id);
   if (it != streams_.end()) {
     return it->second;
   }

   return streams_
       .emplace(stream_id,
                OutgoingStream(
                    stream_id, default_priority_,
                    [this, stream_id]() { on_buffered_amount_low_(stream_id); },
                    total_buffered_amount_))
       .first->second;
 }

 void RRSendQueue::SetStreamPriority(StreamID stream_id,
                                     StreamPriority priority) {
   OutgoingStream& stream = GetOrCreateStreamInfo(stream_id);

   stream.set_priority(priority);
   RTC_DCHECK(IsConsistent());
 }

 StreamPriority RRSendQueue::GetStreamPriority(StreamID stream_id) const {
   auto stream_it = streams_.find(stream_id);
   if (stream_it == streams_.end()) {
     return default_priority_;
   }
   return stream_it->second.priority();
 }

 HandoverReadinessStatus RRSendQueue::GetHandoverReadiness() const {
   HandoverReadinessStatus status;
   if (!IsEmpty()) {
     status.Add(HandoverUnreadinessReason::kSendQueueNotEmpty);
   }
   return status;
 }

 void RRSendQueue::AddHandoverState(DcSctpSocketHandoverState& state) {
   for (const auto& [stream_id, stream] : streams_) {
     DcSctpSocketHandoverState::OutgoingStream state_stream;
     state_stream.id = stream_id.value();
     stream.AddHandoverState(state_stream);
     state.tx.streams.push_back(std::move(state_stream));
   }
 }

 void RRSendQueue::RestoreFromState(const DcSctpSocketHandoverState& state) {
   for (const DcSctpSocketHandoverState::OutgoingStream& state_stream :
        state.tx.streams) {
     StreamID stream_id(state_stream.id);
     streams_.emplace(
         stream_id,
         OutgoingStream(
             stream_id, StreamPriority(state_stream.priority),
             [this, stream_id]() { on_buffered_amount_low_(stream_id); },
             total_buffered_amount_, &state_stream));
   }
 }
 }  // 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/rr_send_queue.h"

	#include <cstdint>
	#include <deque>
	#include <limits>
	#include <map>
	#include <utility>
	#include <vector>

	#include "absl/algorithm/container.h"
	#include "absl/types/optional.h"
	#include "api/array_view.h"
	#include "net/dcsctp/packet/data.h"
	#include "net/dcsctp/public/dcsctp_message.h"
	#include "net/dcsctp/public/dcsctp_socket.h"
	#include "net/dcsctp/public/types.h"
	#include "net/dcsctp/tx/send_queue.h"
	#include "rtc_base/logging.h"

	namespace dcsctp {

	RRSendQueue::RRSendQueue(absl::string_view log_prefix,
	size_t buffer_size,
	StreamPriority default_priority,
	std::function<void(StreamID)> on_buffered_amount_low,
	size_t total_buffered_amount_low_threshold,
	std::function<void()> on_total_buffered_amount_low)
	: log_prefix_(std::string(log_prefix) + "fcfs: "),
	buffer_size_(buffer_size),
	default_priority_(default_priority),
	on_buffered_amount_low_(std::move(on_buffered_amount_low)),
	total_buffered_amount_(std::move(on_total_buffered_amount_low)) {
	total_buffered_amount_.SetLowThreshold(total_buffered_amount_low_threshold);
	}

	bool RRSendQueue::OutgoingStream::HasDataToSend() const {
	if (pause_state_ == PauseState::kPaused \|\|
	pause_state_ == PauseState::kResetting) {
	// The stream has paused (and there is no partially sent message).
	return false;
	}

	if (items_.empty()) {
	return false;
	}

	return true;
	}

	void RRSendQueue::OutgoingStream::AddHandoverState(
	DcSctpSocketHandoverState::OutgoingStream& state) const {
	state.next_ssn = next_ssn_.value();
	state.next_ordered_mid = next_ordered_mid_.value();
	state.next_unordered_mid = next_unordered_mid_.value();
	state.priority = *priority_;
	}

	bool RRSendQueue::IsConsistent() const {
	size_t total_buffered_amount = 0;
	for (const auto& [unused, stream] : streams_) {
	total_buffered_amount += stream.buffered_amount().value();
	}

	if (previous_message_has_ended_) {
	auto it = streams_.find(current_stream_id_);
	if (it != streams_.end() && it->second.has_partially_sent_message()) {
	RTC_DLOG(LS_ERROR)
	<< "Previous message has ended, but still partial message in stream";
	return false;
	}
	} else {
	auto it = streams_.find(current_stream_id_);
	if (it == streams_.end() \|\| !it->second.has_partially_sent_message()) {
	RTC_DLOG(LS_ERROR)
	<< "Previous message has NOT ended, but there is no partial message";
	return false;
	}
	}

	return total_buffered_amount == total_buffered_amount_.value();
	}

	bool RRSendQueue::OutgoingStream::IsConsistent() const {
	size_t bytes = 0;
	for (const auto& item : items_) {
	bytes += item.remaining_size;
	}
	return bytes == buffered_amount_.value();
	}

	void RRSendQueue::ThresholdWatcher::Decrease(size_t bytes) {
	RTC_DCHECK(bytes <= value_);
	size_t old_value = value_;
	value_ -= bytes;

	if (old_value > low_threshold_ && value_ <= low_threshold_) {
	on_threshold_reached_();
	}
	}

	void RRSendQueue::ThresholdWatcher::SetLowThreshold(size_t low_threshold) {
	// Betting on https://github.com/w3c/webrtc-pc/issues/2654 being accepted.
	if (low_threshold_ < value_ && low_threshold >= value_) {
	on_threshold_reached_();
	}
	low_threshold_ = low_threshold;
	}

	void RRSendQueue::OutgoingStream::Add(DcSctpMessage message,
	TimeMs expires_at,
	const SendOptions& send_options) {
	buffered_amount_.Increase(message.payload().size());
	total_buffered_amount_.Increase(message.payload().size());
	items_.emplace_back(std::move(message), expires_at, send_options);

	RTC_DCHECK(IsConsistent());
	}

	absl::optional<SendQueue::DataToSend> RRSendQueue::OutgoingStream::Produce(
	TimeMs now,
	size_t max_size) {
	RTC_DCHECK(pause_state_ != PauseState::kPaused &&
	pause_state_ != PauseState::kResetting);

	while (!items_.empty()) {
	Item& item = items_.front();
	DcSctpMessage& message = item.message;

	// Allocate Message ID and SSN when the first fragment is sent.
	if (!item.message_id.has_value()) {
	// Oops, this entire message has already expired. Try the next one.
	if (item.expires_at <= now) {
	buffered_amount_.Decrease(item.remaining_size);
	total_buffered_amount_.Decrease(item.remaining_size);
	items_.pop_front();
	continue;
	}

	MID& mid =
	item.send_options.unordered ? next_unordered_mid_ : next_ordered_mid_;
	item.message_id = mid;
	mid = MID(*mid + 1);
	}
	if (!item.send_options.unordered && !item.ssn.has_value()) {
	item.ssn = next_ssn_;
	next_ssn_ = SSN(*next_ssn_ + 1);
	}

	// Grab the next `max_size` fragment from this message and calculate flags.
	rtc::ArrayView<const uint8_t> chunk_payload =
	item.message.payload().subview(item.remaining_offset, max_size);
	rtc::ArrayView<const uint8_t> message_payload = message.payload();
	Data::IsBeginning is_beginning(chunk_payload.data() ==
	message_payload.data());
	Data::IsEnd is_end((chunk_payload.data() + chunk_payload.size()) ==
	(message_payload.data() + message_payload.size()));

	StreamID stream_id = message.stream_id();
	PPID ppid = message.ppid();

	// Zero-copy the payload if the message fits in a single chunk.
	std::vector<uint8_t> payload =
	is_beginning && is_end
	? std::move(message).ReleasePayload()
	: std::vector<uint8_t>(chunk_payload.begin(), chunk_payload.end());

	FSN fsn(item.current_fsn);
	item.current_fsn = FSN(*item.current_fsn + 1);
	buffered_amount_.Decrease(payload.size());
	total_buffered_amount_.Decrease(payload.size());

	SendQueue::DataToSend chunk(Data(stream_id, item.ssn.value_or(SSN(0)),
	item.message_id.value(), fsn, ppid,
	std::move(payload), is_beginning, is_end,
	item.send_options.unordered));
	if (item.send_options.max_retransmissions.has_value() &&
	*item.send_options.max_retransmissions >=
	std::numeric_limits<MaxRetransmits::UnderlyingType>::min() &&
	*item.send_options.max_retransmissions <=
	std::numeric_limits<MaxRetransmits::UnderlyingType>::max()) {
	chunk.max_retransmissions =
	MaxRetransmits(*item.send_options.max_retransmissions);
	}
	chunk.expires_at = item.expires_at;

	if (is_end) {
	// The entire message has been sent, and its last data copied to `chunk`,
	// so it can safely be discarded.
	items_.pop_front();

	if (pause_state_ == PauseState::kPending) {
	RTC_DLOG(LS_VERBOSE) << "Pause state on " << *stream_id
	<< " is moving from pending to paused";
	pause_state_ = PauseState::kPaused;
	}
	} else {
	item.remaining_offset += chunk_payload.size();
	item.remaining_size -= chunk_payload.size();
	RTC_DCHECK(item.remaining_offset + item.remaining_size ==
	item.message.payload().size());
	RTC_DCHECK(item.remaining_size > 0);
	}
	RTC_DCHECK(IsConsistent());
	return chunk;
	}
	RTC_DCHECK(IsConsistent());
	return absl::nullopt;
	}

	bool RRSendQueue::OutgoingStream::Discard(IsUnordered unordered,
	MID message_id) {
	bool result = false;
	if (!items_.empty()) {
	Item& item = items_.front();
	if (item.send_options.unordered == unordered &&
	item.message_id.has_value() && *item.message_id == message_id) {
	buffered_amount_.Decrease(item.remaining_size);
	total_buffered_amount_.Decrease(item.remaining_size);
	items_.pop_front();

	if (pause_state_ == PauseState::kPending) {
	pause_state_ = PauseState::kPaused;
	}

	// As the item still existed, it had unsent data.
	result = true;
	}
	}
	RTC_DCHECK(IsConsistent());
	return result;
	}

	void RRSendQueue::OutgoingStream::Pause() {
	if (pause_state_ != PauseState::kNotPaused) {
	// Already in progress.
	return;
	}

	bool had_pending_items = !items_.empty();

	// https://datatracker.ietf.org/doc/html/rfc8831#section-6.7
	// "Closing of a data channel MUST be signaled by resetting the corresponding
	// outgoing streams [RFC6525]. This means that if one side decides to close
	// the data channel, it resets the corresponding outgoing stream."
	// ... "[RFC6525] also guarantees that all the messages are delivered (or
	// abandoned) before the stream is reset."

	// A stream is paused when it's about to be reset. In this implementation,
	// it will throw away all non-partially send messages - they will be abandoned
	// as noted above. This is subject to change. It will however not discard any
	// partially sent messages - only whole messages. Partially delivered messages
	// (at the time of receiving a Stream Reset command) will always deliver all
	// the fragments before actually resetting the stream.
	for (auto it = items_.begin(); it != items_.end();) {
	if (it->remaining_offset == 0) {
	buffered_amount_.Decrease(it->remaining_size);
	total_buffered_amount_.Decrease(it->remaining_size);
	it = items_.erase(it);
	} else {
	++it;
	}
	}

	pause_state_ = (items_.empty() \|\| items_.front().remaining_offset == 0)
	? PauseState::kPaused
	: PauseState::kPending;

	if (had_pending_items && pause_state_ == PauseState::kPaused) {
	RTC_DLOG(LS_VERBOSE) << "Stream " << *stream_id()
	<< " was previously active, but is now paused.";
	}

	RTC_DCHECK(IsConsistent());
	}

	void RRSendQueue::OutgoingStream::Resume() {
	RTC_DCHECK(pause_state_ == PauseState::kResetting);
	if (!items_.empty()) {
	RTC_DLOG(LS_VERBOSE) << "Stream " << *stream_id()
	<< " was previously paused, but is now active.";
	}
	pause_state_ = PauseState::kNotPaused;
	RTC_DCHECK(IsConsistent());
	}

	void RRSendQueue::OutgoingStream::Reset() {
	// This can be called both when an outgoing stream reset has been responded
	// to, or when the entire SendQueue is reset due to detecting the peer having
	// restarted. The stream may be in any state at this time.
	if (!items_.empty()) {
	// If this message has been partially sent, reset it so that it will be
	// re-sent.
	auto& item = items_.front();
	buffered_amount_.Increase(item.message.payload().size() -
	item.remaining_size);
	total_buffered_amount_.Increase(item.message.payload().size() -
	item.remaining_size);
	item.remaining_offset = 0;
	item.remaining_size = item.message.payload().size();
	item.message_id = absl::nullopt;
	item.ssn = absl::nullopt;
	item.current_fsn = FSN(0);
	}
	pause_state_ = PauseState::kNotPaused;
	next_ordered_mid_ = MID(0);
	next_unordered_mid_ = MID(0);
	next_ssn_ = SSN(0);
	RTC_DCHECK(IsConsistent());
	}

	bool RRSendQueue::OutgoingStream::has_partially_sent_message() const {
	if (items_.empty()) {
	return false;
	}
	return items_.front().message_id.has_value();
	}

	void RRSendQueue::Add(TimeMs now,
	DcSctpMessage message,
	const SendOptions& send_options) {
	RTC_DCHECK(!message.payload().empty());
	// Any limited lifetime should start counting from now - when the message
	// has been added to the queue.
	TimeMs expires_at = TimeMs::InfiniteFuture();
	if (send_options.lifetime.has_value()) {
	// `expires_at` is the time when it expires. Which is slightly larger than
	// the message's lifetime, as the message is alive during its entire
	// lifetime (which may be zero).
	expires_at = now + *send_options.lifetime + DurationMs(1);
	}
	GetOrCreateStreamInfo(message.stream_id())
	.Add(std::move(message), expires_at, send_options);
	RTC_DCHECK(IsConsistent());
	}

	bool RRSendQueue::IsFull() const {
	return total_buffered_amount() >= buffer_size_;
	}

	bool RRSendQueue::IsEmpty() const {
	return total_buffered_amount() == 0;
	}

	std::map<StreamID, RRSendQueue::OutgoingStream>::iterator
	RRSendQueue::GetNextStream() {
	auto start_it = streams_.lower_bound(StreamID(*current_stream_id_ + 1));

	for (auto it = start_it; it != streams_.end(); ++it) {
	if (it->second.HasDataToSend()) {
	current_stream_id_ = it->first;
	return it;
	}
	}

	for (auto it = streams_.begin(); it != start_it; ++it) {
	if (it->second.HasDataToSend()) {
	current_stream_id_ = it->first;
	return it;
	}
	}
	return streams_.end();
	}

	absl::optional<SendQueue::DataToSend> RRSendQueue::Produce(TimeMs now,
	size_t max_size) {
	std::map<StreamID, RRSendQueue::OutgoingStream>::iterator stream_it;

	for (;;) {
	if (previous_message_has_ended_) {
	// Previous message has ended. Round-robin to a different stream, if there
	// even is one with data to send.
	stream_it = GetNextStream();
	if (stream_it == streams_.end()) {
	RTC_DLOG(LS_VERBOSE)
	<< log_prefix_
	<< "There is no stream with data; Can't produce any data.";
	return absl::nullopt;
	}
	} else {
	// The previous message has not ended; Continue from the current stream.
	stream_it = streams_.find(current_stream_id_);
	RTC_DCHECK(stream_it != streams_.end());
	}

	absl::optional<DataToSend> data = stream_it->second.Produce(now, max_size);
	if (!data.has_value()) {
	continue;
	}
	RTC_DLOG(LS_VERBOSE) << log_prefix_ << "Producing DATA, type="
	<< (data->data.is_unordered ? "unordered" : "ordered")
	<< "::"
	<< (data->data.is_beginning && data->data.is_end
	? "complete"
	: *data->data.is_beginning ? "first"
	: *data->data.is_end ? "last"
	: "middle")
	<< ", stream_id=" << *stream_it->first
	<< ", ppid=" << *data->data.ppid
	<< ", length=" << data->data.payload.size();

	previous_message_has_ended_ = *data->data.is_end;
	RTC_DCHECK(IsConsistent());
	return data;
	}
	}

	bool RRSendQueue::Discard(IsUnordered unordered,
	StreamID stream_id,
	MID message_id) {
	bool has_discarded =
	GetOrCreateStreamInfo(stream_id).Discard(unordered, message_id);
	if (has_discarded) {
	// Only partially sent messages are discarded, so if a message was
	// discarded, then it was the currently sent message.
	previous_message_has_ended_ = true;
	}

	return has_discarded;
	}

	void RRSendQueue::PrepareResetStream(StreamID stream_id) {
	GetOrCreateStreamInfo(stream_id).Pause();
	RTC_DCHECK(IsConsistent());
	}

	bool RRSendQueue::HasStreamsReadyToBeReset() const {
	for (auto& [unused, stream] : streams_) {
	if (stream.IsReadyToBeReset()) {
	return true;
	}
	}
	return false;
	}
	std::vector<StreamID> RRSendQueue::GetStreamsReadyToBeReset() {
	RTC_DCHECK(absl::c_count_if(streams_, [](const auto& p) {
	return p.second.IsResetting();
	}) == 0);
	std::vector<StreamID> ready;
	for (auto& [stream_id, stream] : streams_) {
	if (stream.IsReadyToBeReset()) {
	stream.SetAsResetting();
	ready.push_back(stream_id);
	}
	}
	return ready;
	}

	void RRSendQueue::CommitResetStreams() {
	RTC_DCHECK(absl::c_count_if(streams_, [](const auto& p) {
	return p.second.IsResetting();
	}) > 0);
	for (auto& [unused, stream] : streams_) {
	if (stream.IsResetting()) {
	stream.Reset();
	}
	}
	RTC_DCHECK(IsConsistent());
	}

	void RRSendQueue::RollbackResetStreams() {
	RTC_DCHECK(absl::c_count_if(streams_, [](const auto& p) {
	return p.second.IsResetting();
	}) > 0);
	for (auto& [unused, stream] : streams_) {
	if (stream.IsResetting()) {
	stream.Resume();
	}
	}
	RTC_DCHECK(IsConsistent());
	}

	void RRSendQueue::Reset() {
	// Recalculate buffered amount, as partially sent messages may have been put
	// fully back in the queue.
	for (auto& [unused, stream] : streams_) {
	stream.Reset();
	}
	previous_message_has_ended_ = true;
	}

	size_t RRSendQueue::buffered_amount(StreamID stream_id) const {
	auto it = streams_.find(stream_id);
	if (it == streams_.end()) {
	return 0;
	}
	return it->second.buffered_amount().value();
	}

	size_t RRSendQueue::buffered_amount_low_threshold(StreamID stream_id) const {
	auto it = streams_.find(stream_id);
	if (it == streams_.end()) {
	return 0;
	}
	return it->second.buffered_amount().low_threshold();
	}

	void RRSendQueue::SetBufferedAmountLowThreshold(StreamID stream_id,
	size_t bytes) {
	GetOrCreateStreamInfo(stream_id).buffered_amount().SetLowThreshold(bytes);
	}

	RRSendQueue::OutgoingStream& RRSendQueue::GetOrCreateStreamInfo(
	StreamID stream_id) {
	auto it = streams_.find(stream_id);
	if (it != streams_.end()) {
	return it->second;
	}

	return streams_
	.emplace(stream_id,
	OutgoingStream(
	stream_id, default_priority_,
	[this, stream_id]() { on_buffered_amount_low_(stream_id); },
	total_buffered_amount_))
	.first->second;
	}

	void RRSendQueue::SetStreamPriority(StreamID stream_id,
	StreamPriority priority) {
	OutgoingStream& stream = GetOrCreateStreamInfo(stream_id);

	stream.set_priority(priority);
	RTC_DCHECK(IsConsistent());
	}

	StreamPriority RRSendQueue::GetStreamPriority(StreamID stream_id) const {
	auto stream_it = streams_.find(stream_id);
	if (stream_it == streams_.end()) {
	return default_priority_;
	}
	return stream_it->second.priority();
	}

	HandoverReadinessStatus RRSendQueue::GetHandoverReadiness() const {
	HandoverReadinessStatus status;
	if (!IsEmpty()) {
	status.Add(HandoverUnreadinessReason::kSendQueueNotEmpty);
	}
	return status;
	}

	void RRSendQueue::AddHandoverState(DcSctpSocketHandoverState& state) {
	for (const auto& [stream_id, stream] : streams_) {
	DcSctpSocketHandoverState::OutgoingStream state_stream;
	state_stream.id = stream_id.value();
	stream.AddHandoverState(state_stream);
	state.tx.streams.push_back(std::move(state_stream));
	}
	}

	void RRSendQueue::RestoreFromState(const DcSctpSocketHandoverState& state) {
	for (const DcSctpSocketHandoverState::OutgoingStream& state_stream :
	state.tx.streams) {
	StreamID stream_id(state_stream.id);
	streams_.emplace(
	stream_id,
	OutgoingStream(
	stream_id, StreamPriority(state_stream.priority),
	[this, stream_id]() { on_buffered_amount_low_(stream_id); },
	total_buffered_amount_, &state_stream));
	}
	}
	} // namespace dcsctp