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

 #include "absl/algorithm/container.h"
 #include "absl/types/optional.h"
 #include "api/array_view.h"
 #include "net/dcsctp/common/internal_types.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"
 #include "rtc_base/strings/str_join.h"

 namespace dcsctp {
 using ::webrtc::TimeDelta;
 using ::webrtc::Timestamp;

 RRSendQueue::RRSendQueue(absl::string_view log_prefix,
                          DcSctpSocketCallbacks* callbacks,
                          size_t mtu,
                          StreamPriority default_priority,
                          size_t total_buffered_amount_low_threshold)
     : log_prefix_(log_prefix),
       callbacks_(*callbacks),
       default_priority_(default_priority),
       scheduler_(log_prefix_, mtu),
       total_buffered_amount_(
           [this]() { callbacks_.OnTotalBufferedAmountLow(); }) {
   total_buffered_amount_.SetLowThreshold(total_buffered_amount_low_threshold);
 }

 size_t RRSendQueue::OutgoingStream::bytes_to_send_in_next_message() const {
   if (pause_state_ == PauseState::kPaused ||
       pause_state_ == PauseState::kResetting) {
     // The stream has paused (and there is no partially sent message).
     return 0;
   }

   if (items_.empty()) {
     return 0;
   }

   return items_.front().remaining_size;
 }

 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 = *scheduler_stream_->priority();
 }

 bool RRSendQueue::IsConsistent() const {
   std::set<StreamID> expected_active_streams;
   std::set<StreamID> actual_active_streams =
       scheduler_.ActiveStreamsForTesting();

   size_t total_buffered_amount = 0;
   for (const auto& [stream_id, stream] : streams_) {
     total_buffered_amount += stream.buffered_amount().value();
     if (stream.bytes_to_send_in_next_message() > 0) {
       expected_active_streams.emplace(stream_id);
     }
   }
   if (expected_active_streams != actual_active_streams) {
     auto fn = [&](rtc::StringBuilder& sb, const auto& p) { sb << *p; };
     RTC_DLOG(LS_ERROR) << "Active streams mismatch, is=["
                        << StrJoin(actual_active_streams, ",", fn)
                        << "], expected=["
                        << StrJoin(expected_active_streams, ",", fn) << "]";
     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,
                                       MessageAttributes attributes) {
   bool was_active = bytes_to_send_in_next_message() > 0;
   buffered_amount_.Increase(message.payload().size());
   parent_.total_buffered_amount_.Increase(message.payload().size());
   OutgoingMessageId message_id = parent_.current_message_id;
   parent_.current_message_id =
       OutgoingMessageId(*parent_.current_message_id + 1);
   items_.emplace_back(message_id, std::move(message), std::move(attributes));

   if (!was_active) {
     scheduler_stream_->MaybeMakeActive();
   }

   RTC_DCHECK(IsConsistent());
 }

 absl::optional<SendQueue::DataToSend> RRSendQueue::OutgoingStream::Produce(
     Timestamp 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.mid.has_value()) {
       // Oops, this entire message has already expired. Try the next one.
       if (item.attributes.expires_at <= now) {
         HandleMessageExpired(item);
         items_.pop_front();
         continue;
       }

       MID& mid =
           item.attributes.unordered ? next_unordered_mid_ : next_ordered_mid_;
       item.mid = mid;
       mid = MID(*mid + 1);
     }
     if (!item.attributes.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());
     parent_.total_buffered_amount_.Decrease(payload.size());

     SendQueue::DataToSend chunk(
         item.message_id, Data(stream_id, item.ssn.value_or(SSN(0)), *item.mid,
                               fsn, ppid, std::move(payload), is_beginning,
                               is_end, item.attributes.unordered));
     chunk.max_retransmissions = item.attributes.max_retransmissions;
     chunk.expires_at = item.attributes.expires_at;
     chunk.lifecycle_id =
         is_end ? item.attributes.lifecycle_id : LifecycleId::NotSet();

     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;
 }

 void RRSendQueue::OutgoingStream::HandleMessageExpired(
     OutgoingStream::Item& item) {
   buffered_amount_.Decrease(item.remaining_size);
   parent_.total_buffered_amount_.Decrease(item.remaining_size);
   if (item.attributes.lifecycle_id.IsSet()) {
     RTC_DLOG(LS_VERBOSE) << "Triggering OnLifecycleMessageExpired("
                          << item.attributes.lifecycle_id.value() << ", false)";

     parent_.callbacks_.OnLifecycleMessageExpired(item.attributes.lifecycle_id,
                                                  /*maybe_delivered=*/false);
     parent_.callbacks_.OnLifecycleEnd(item.attributes.lifecycle_id);
   }
 }

 bool RRSendQueue::OutgoingStream::Discard(OutgoingMessageId message_id) {
   bool result = false;
   if (!items_.empty()) {
     Item& item = items_.front();
     if (item.message_id == message_id) {
       HandleMessageExpired(item);
       items_.pop_front();

       // Only partially sent messages are discarded, so if a message was
       // discarded, then it was the currently sent message.
       scheduler_stream_->ForceReschedule();

       if (pause_state_ == PauseState::kPending) {
         pause_state_ = PauseState::kPaused;
         scheduler_stream_->MakeInactive();
       } else if (bytes_to_send_in_next_message() == 0) {
         scheduler_stream_->MakeInactive();
       }

       // 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) {
       HandleMessageExpired(*it);
       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.";
     scheduler_stream_->MakeInactive();
   }

   RTC_DCHECK(IsConsistent());
 }

 void RRSendQueue::OutgoingStream::Resume() {
   RTC_DCHECK(pause_state_ == PauseState::kResetting);
   pause_state_ = PauseState::kNotPaused;
   scheduler_stream_->MaybeMakeActive();
   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.
   PauseState old_pause_state = pause_state_;
   pause_state_ = PauseState::kNotPaused;
   next_ordered_mid_ = MID(0);
   next_unordered_mid_ = MID(0);
   next_ssn_ = SSN(0);
   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);
     parent_.total_buffered_amount_.Increase(item.message.payload().size() -
                                             item.remaining_size);
     item.remaining_offset = 0;
     item.remaining_size = item.message.payload().size();
     item.mid = absl::nullopt;
     item.ssn = absl::nullopt;
     item.current_fsn = FSN(0);
     if (old_pause_state == PauseState::kPaused ||
         old_pause_state == PauseState::kResetting) {
       scheduler_stream_->MaybeMakeActive();
     }
   }
   RTC_DCHECK(IsConsistent());
 }

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

 void RRSendQueue::Add(Timestamp 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.

   // `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).
   MessageAttributes attributes = {
       .unordered = send_options.unordered,
       .max_retransmissions =
           send_options.max_retransmissions.has_value()
               ? MaxRetransmits(send_options.max_retransmissions.value())
               : MaxRetransmits::NoLimit(),
       .expires_at = send_options.lifetime.has_value()
                         ? now + send_options.lifetime->ToTimeDelta() +
                               TimeDelta::Millis(1)
                         : Timestamp::PlusInfinity(),
       .lifecycle_id = send_options.lifecycle_id,
   };
   StreamID stream_id = message.stream_id();
   GetOrCreateStreamInfo(stream_id).Add(std::move(message),
                                        std::move(attributes));
   RTC_DCHECK(IsConsistent());
 }

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

 absl::optional<SendQueue::DataToSend> RRSendQueue::Produce(Timestamp now,
                                                            size_t max_size) {
   return scheduler_.Produce(now, max_size);
 }

 bool RRSendQueue::Discard(StreamID stream_id, OutgoingMessageId message_id) {
   bool has_discarded = GetOrCreateStreamInfo(stream_id).Discard(message_id);

   RTC_DCHECK(IsConsistent());
   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();
   }
   scheduler_.ForceReschedule();
 }

 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(
           std::piecewise_construct, std::forward_as_tuple(stream_id),
           std::forward_as_tuple(this, &scheduler_, stream_id, default_priority_,
                                 [this, stream_id]() {
                                   callbacks_.OnBufferedAmountLow(stream_id);
                                 }))
       .first->second;
 }

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

   stream.SetPriority(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(
         std::piecewise_construct, std::forward_as_tuple(stream_id),
         std::forward_as_tuple(
             this, &scheduler_, stream_id, StreamPriority(state_stream.priority),
             [this, stream_id]() { callbacks_.OnBufferedAmountLow(stream_id); },
             &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 <set>
	#include <utility>
	#include <vector>

	#include "absl/algorithm/container.h"
	#include "absl/types/optional.h"
	#include "api/array_view.h"
	#include "net/dcsctp/common/internal_types.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"
	#include "rtc_base/strings/str_join.h"

	namespace dcsctp {
	using ::webrtc::TimeDelta;
	using ::webrtc::Timestamp;

	RRSendQueue::RRSendQueue(absl::string_view log_prefix,
	DcSctpSocketCallbacks* callbacks,
	size_t mtu,
	StreamPriority default_priority,
	size_t total_buffered_amount_low_threshold)
	: log_prefix_(log_prefix),
	callbacks_(*callbacks),
	default_priority_(default_priority),
	scheduler_(log_prefix_, mtu),
	total_buffered_amount_(
	[this]() { callbacks_.OnTotalBufferedAmountLow(); }) {
	total_buffered_amount_.SetLowThreshold(total_buffered_amount_low_threshold);
	}

	size_t RRSendQueue::OutgoingStream::bytes_to_send_in_next_message() const {
	if (pause_state_ == PauseState::kPaused \|\|
	pause_state_ == PauseState::kResetting) {
	// The stream has paused (and there is no partially sent message).
	return 0;
	}

	if (items_.empty()) {
	return 0;
	}

	return items_.front().remaining_size;
	}

	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 = *scheduler_stream_->priority();
	}

	bool RRSendQueue::IsConsistent() const {
	std::set<StreamID> expected_active_streams;
	std::set<StreamID> actual_active_streams =
	scheduler_.ActiveStreamsForTesting();

	size_t total_buffered_amount = 0;
	for (const auto& [stream_id, stream] : streams_) {
	total_buffered_amount += stream.buffered_amount().value();
	if (stream.bytes_to_send_in_next_message() > 0) {
	expected_active_streams.emplace(stream_id);
	}
	}
	if (expected_active_streams != actual_active_streams) {
	auto fn = [&](rtc::StringBuilder& sb, const auto& p) { sb << *p; };
	RTC_DLOG(LS_ERROR) << "Active streams mismatch, is=["
	<< StrJoin(actual_active_streams, ",", fn)
	<< "], expected=["
	<< StrJoin(expected_active_streams, ",", fn) << "]";
	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,
	MessageAttributes attributes) {
	bool was_active = bytes_to_send_in_next_message() > 0;
	buffered_amount_.Increase(message.payload().size());
	parent_.total_buffered_amount_.Increase(message.payload().size());
	OutgoingMessageId message_id = parent_.current_message_id;
	parent_.current_message_id =
	OutgoingMessageId(*parent_.current_message_id + 1);
	items_.emplace_back(message_id, std::move(message), std::move(attributes));

	if (!was_active) {
	scheduler_stream_->MaybeMakeActive();
	}

	RTC_DCHECK(IsConsistent());
	}

	absl::optional<SendQueue::DataToSend> RRSendQueue::OutgoingStream::Produce(
	Timestamp 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.mid.has_value()) {
	// Oops, this entire message has already expired. Try the next one.
	if (item.attributes.expires_at <= now) {
	HandleMessageExpired(item);
	items_.pop_front();
	continue;
	}

	MID& mid =
	item.attributes.unordered ? next_unordered_mid_ : next_ordered_mid_;
	item.mid = mid;
	mid = MID(*mid + 1);
	}
	if (!item.attributes.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());
	parent_.total_buffered_amount_.Decrease(payload.size());

	SendQueue::DataToSend chunk(
	item.message_id, Data(stream_id, item.ssn.value_or(SSN(0)), *item.mid,
	fsn, ppid, std::move(payload), is_beginning,
	is_end, item.attributes.unordered));
	chunk.max_retransmissions = item.attributes.max_retransmissions;
	chunk.expires_at = item.attributes.expires_at;
	chunk.lifecycle_id =
	is_end ? item.attributes.lifecycle_id : LifecycleId::NotSet();

	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;
	}

	void RRSendQueue::OutgoingStream::HandleMessageExpired(
	OutgoingStream::Item& item) {
	buffered_amount_.Decrease(item.remaining_size);
	parent_.total_buffered_amount_.Decrease(item.remaining_size);
	if (item.attributes.lifecycle_id.IsSet()) {
	RTC_DLOG(LS_VERBOSE) << "Triggering OnLifecycleMessageExpired("
	<< item.attributes.lifecycle_id.value() << ", false)";

	parent_.callbacks_.OnLifecycleMessageExpired(item.attributes.lifecycle_id,
	/maybe_delivered=/false);
	parent_.callbacks_.OnLifecycleEnd(item.attributes.lifecycle_id);
	}
	}

	bool RRSendQueue::OutgoingStream::Discard(OutgoingMessageId message_id) {
	bool result = false;
	if (!items_.empty()) {
	Item& item = items_.front();
	if (item.message_id == message_id) {
	HandleMessageExpired(item);
	items_.pop_front();

	// Only partially sent messages are discarded, so if a message was
	// discarded, then it was the currently sent message.
	scheduler_stream_->ForceReschedule();

	if (pause_state_ == PauseState::kPending) {
	pause_state_ = PauseState::kPaused;
	scheduler_stream_->MakeInactive();
	} else if (bytes_to_send_in_next_message() == 0) {
	scheduler_stream_->MakeInactive();
	}

	// 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) {
	HandleMessageExpired(*it);
	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.";
	scheduler_stream_->MakeInactive();
	}

	RTC_DCHECK(IsConsistent());
	}

	void RRSendQueue::OutgoingStream::Resume() {
	RTC_DCHECK(pause_state_ == PauseState::kResetting);
	pause_state_ = PauseState::kNotPaused;
	scheduler_stream_->MaybeMakeActive();
	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.
	PauseState old_pause_state = pause_state_;
	pause_state_ = PauseState::kNotPaused;
	next_ordered_mid_ = MID(0);
	next_unordered_mid_ = MID(0);
	next_ssn_ = SSN(0);
	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);
	parent_.total_buffered_amount_.Increase(item.message.payload().size() -
	item.remaining_size);
	item.remaining_offset = 0;
	item.remaining_size = item.message.payload().size();
	item.mid = absl::nullopt;
	item.ssn = absl::nullopt;
	item.current_fsn = FSN(0);
	if (old_pause_state == PauseState::kPaused \|\|
	old_pause_state == PauseState::kResetting) {
	scheduler_stream_->MaybeMakeActive();
	}
	}
	RTC_DCHECK(IsConsistent());
	}

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

	void RRSendQueue::Add(Timestamp 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.

	// `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).
	MessageAttributes attributes = {
	.unordered = send_options.unordered,
	.max_retransmissions =
	send_options.max_retransmissions.has_value()
	? MaxRetransmits(send_options.max_retransmissions.value())
	: MaxRetransmits::NoLimit(),
	.expires_at = send_options.lifetime.has_value()
	? now + send_options.lifetime->ToTimeDelta() +
	TimeDelta::Millis(1)
	: Timestamp::PlusInfinity(),
	.lifecycle_id = send_options.lifecycle_id,
	};
	StreamID stream_id = message.stream_id();
	GetOrCreateStreamInfo(stream_id).Add(std::move(message),
	std::move(attributes));
	RTC_DCHECK(IsConsistent());
	}

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

	absl::optional<SendQueue::DataToSend> RRSendQueue::Produce(Timestamp now,
	size_t max_size) {
	return scheduler_.Produce(now, max_size);
	}

	bool RRSendQueue::Discard(StreamID stream_id, OutgoingMessageId message_id) {
	bool has_discarded = GetOrCreateStreamInfo(stream_id).Discard(message_id);

	RTC_DCHECK(IsConsistent());
	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();
	}
	scheduler_.ForceReschedule();
	}

	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(
	std::piecewise_construct, std::forward_as_tuple(stream_id),
	std::forward_as_tuple(this, &scheduler_, stream_id, default_priority_,
	[this, stream_id]() {
	callbacks_.OnBufferedAmountLow(stream_id);
	}))
	.first->second;
	}

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

	stream.SetPriority(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(
	std::piecewise_construct, std::forward_as_tuple(stream_id),
	std::forward_as_tuple(
	this, &scheduler_, stream_id, StreamPriority(state_stream.priority),
	[this, stream_id]() { callbacks_.OnBufferedAmountLow(stream_id); },
	&state_stream));
	}
	}
	} // namespace dcsctp