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/*
* 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/retransmission_queue.h"
#include <algorithm>
#include <cstdint>
#include <functional>
#include <iterator>
#include <map>
#include <optional>
#include <set>
#include <string>
#include <utility>
#include <vector>
#include "absl/algorithm/container.h"
#include "absl/strings/string_view.h"
#include "api/array_view.h"
#include "net/dcsctp/common/math.h"
#include "net/dcsctp/common/sequence_numbers.h"
#include "net/dcsctp/packet/chunk/data_chunk.h"
#include "net/dcsctp/packet/chunk/forward_tsn_chunk.h"
#include "net/dcsctp/packet/chunk/forward_tsn_common.h"
#include "net/dcsctp/packet/chunk/idata_chunk.h"
#include "net/dcsctp/packet/chunk/iforward_tsn_chunk.h"
#include "net/dcsctp/packet/chunk/sack_chunk.h"
#include "net/dcsctp/packet/data.h"
#include "net/dcsctp/public/dcsctp_options.h"
#include "net/dcsctp/public/types.h"
#include "net/dcsctp/timer/timer.h"
#include "net/dcsctp/tx/outstanding_data.h"
#include "net/dcsctp/tx/send_queue.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
#include "rtc_base/strings/str_join.h"
#include "rtc_base/strings/string_builder.h"
namespace dcsctp {
namespace {
using ::webrtc::TimeDelta;
using ::webrtc::Timestamp;
} // namespace
RetransmissionQueue::RetransmissionQueue(
absl::string_view log_prefix,
DcSctpSocketCallbacks* callbacks,
TSN my_initial_tsn,
size_t a_rwnd,
SendQueue& send_queue,
std::function<void(TimeDelta rtt)> on_new_rtt,
std::function<void()> on_clear_retransmission_counter,
Timer& t3_rtx,
const DcSctpOptions& options,
bool supports_partial_reliability,
bool use_message_interleaving)
: callbacks_(*callbacks),
options_(options),
partial_reliability_(supports_partial_reliability),
log_prefix_(log_prefix),
data_chunk_header_size_(use_message_interleaving
? IDataChunk::kHeaderSize
: DataChunk::kHeaderSize),
on_new_rtt_(std::move(on_new_rtt)),
on_clear_retransmission_counter_(
std::move(on_clear_retransmission_counter)),
t3_rtx_(t3_rtx),
cwnd_(options_.cwnd_mtus_initial * options_.mtu),
rwnd_(a_rwnd),
// "The initial value of ssthresh MAY be arbitrarily high (for
// example, implementations MAY use the size of the receiver advertised
// window).""
ssthresh_(rwnd_),
partial_bytes_acked_(0),
send_queue_(send_queue),
outstanding_data_(
data_chunk_header_size_,
tsn_unwrapper_.Unwrap(TSN(*my_initial_tsn - 1)),
[this](StreamID stream_id, OutgoingMessageId message_id) {
return send_queue_.Discard(stream_id, message_id);
}) {}
bool RetransmissionQueue::IsConsistent() const {
return true;
}
// Returns how large a chunk will be, serialized, carrying the data
size_t RetransmissionQueue::GetSerializedChunkSize(const Data& data) const {
return RoundUpTo4(data_chunk_header_size_ + data.size());
}
void RetransmissionQueue::MaybeExitFastRecovery(
UnwrappedTSN cumulative_tsn_ack) {
// "When a SACK acknowledges all TSNs up to and including this [fast
// recovery] exit point, Fast Recovery is exited."
if (fast_recovery_exit_tsn_.has_value() &&
cumulative_tsn_ack >= *fast_recovery_exit_tsn_) {
RTC_DLOG(LS_VERBOSE) << log_prefix_
<< "exit_point=" << *fast_recovery_exit_tsn_->Wrap()
<< " reached - exiting fast recovery";
fast_recovery_exit_tsn_ = std::nullopt;
}
}
void RetransmissionQueue::HandleIncreasedCumulativeTsnAck(
size_t unacked_bytes,
size_t total_bytes_acked) {
// Allow some margin for classifying as fully utilized, due to e.g. that too
// small packets (less than kMinimumFragmentedPayload) are not sent +
// overhead.
bool is_fully_utilized = unacked_bytes + options_.mtu >= cwnd_;
size_t old_cwnd = cwnd_;
if (phase() == CongestionAlgorithmPhase::kSlowStart) {
if (is_fully_utilized && !is_in_fast_recovery()) {
// "Only when these three conditions are met can the cwnd be
// increased; otherwise, the cwnd MUST not be increased. If these
// conditions are met, then cwnd MUST be increased by, at most, the
// lesser of 1) the total size of the previously outstanding DATA
// chunk(s) acknowledged, and 2) the destination's path MTU."
cwnd_ += std::min(total_bytes_acked, options_.mtu);
RTC_DLOG(LS_VERBOSE) << log_prefix_ << "SS increase cwnd=" << cwnd_
<< " (" << old_cwnd << ")";
}
} else if (phase() == CongestionAlgorithmPhase::kCongestionAvoidance) {
// "Whenever cwnd is greater than ssthresh, upon each SACK arrival
// that advances the Cumulative TSN Ack Point, increase
// partial_bytes_acked by the total number of bytes of all new chunks
// acknowledged in that SACK including chunks acknowledged by the new
// Cumulative TSN Ack and by Gap Ack Blocks."
size_t old_pba = partial_bytes_acked_;
partial_bytes_acked_ += total_bytes_acked;
if (partial_bytes_acked_ >= cwnd_ && is_fully_utilized) {
// "When partial_bytes_acked is equal to or greater than cwnd and
// before the arrival of the SACK the sender had cwnd or more bytes of
// data outstanding (i.e., before arrival of the SACK, flightsize was
// greater than or equal to cwnd), increase cwnd by MTU, and reset
// partial_bytes_acked to (partial_bytes_acked - cwnd)."
partial_bytes_acked_ -= cwnd_;
cwnd_ += options_.mtu;
RTC_DLOG(LS_VERBOSE) << log_prefix_ << "CA increase cwnd=" << cwnd_
<< " (" << old_cwnd << ") ssthresh=" << ssthresh_
<< ", pba=" << partial_bytes_acked_ << " ("
<< old_pba << ")";
} else {
RTC_DLOG(LS_VERBOSE) << log_prefix_ << "CA unchanged cwnd=" << cwnd_
<< " (" << old_cwnd << ") ssthresh=" << ssthresh_
<< ", pba=" << partial_bytes_acked_ << " ("
<< old_pba << ")";
}
}
}
void RetransmissionQueue::HandlePacketLoss(UnwrappedTSN highest_tsn_acked) {
if (!is_in_fast_recovery()) {
// "If not in Fast Recovery, adjust the ssthresh and cwnd of the
// destination address(es) to which the missing DATA chunks were last
// sent, according to the formula described in Section 7.2.3."
size_t old_cwnd = cwnd_;
size_t old_pba = partial_bytes_acked_;
ssthresh_ = std::max(cwnd_ / 2, options_.cwnd_mtus_min * options_.mtu);
cwnd_ = ssthresh_;
partial_bytes_acked_ = 0;
RTC_DLOG(LS_VERBOSE) << log_prefix_
<< "packet loss detected (not fast recovery). cwnd="
<< cwnd_ << " (" << old_cwnd
<< "), ssthresh=" << ssthresh_
<< ", pba=" << partial_bytes_acked_ << " (" << old_pba
<< ")";
// "If not in Fast Recovery, enter Fast Recovery and mark the highest
// outstanding TSN as the Fast Recovery exit point."
fast_recovery_exit_tsn_ = outstanding_data_.highest_outstanding_tsn();
RTC_DLOG(LS_VERBOSE) << log_prefix_
<< "fast recovery initiated with exit_point="
<< *fast_recovery_exit_tsn_->Wrap();
} else {
// "While in Fast Recovery, the ssthresh and cwnd SHOULD NOT change for
// any destinations due to a subsequent Fast Recovery event (i.e., one
// SHOULD NOT reduce the cwnd further due to a subsequent Fast Retransmit)."
RTC_DLOG(LS_VERBOSE) << log_prefix_
<< "packet loss detected (fast recovery). No changes.";
}
}
void RetransmissionQueue::UpdateReceiverWindow(uint32_t a_rwnd) {
rwnd_ = outstanding_data_.unacked_bytes() >= a_rwnd
? 0
: a_rwnd - outstanding_data_.unacked_bytes();
}
void RetransmissionQueue::StartT3RtxTimerIfOutstandingData() {
// Note: Can't use `unacked_bytes()` as that one doesn't count chunks to
// be retransmitted.
if (outstanding_data_.empty()) {
// "Whenever all outstanding data sent to an address have been
// acknowledged, turn off the T3-rtx timer of that address.
// Note: Already stopped in `StopT3RtxTimerOnIncreasedCumulativeTsnAck`."
} else {
// "Whenever a SACK is received that acknowledges the DATA chunk
// with the earliest outstanding TSN for that address, restart the T3-rtx
// timer for that address with its current RTO (if there is still
// outstanding data on that address)."
// "Whenever a SACK is received missing a TSN that was previously
// acknowledged via a Gap Ack Block, start the T3-rtx for the destination
// address to which the DATA chunk was originally transmitted if it is not
// already running."
if (!t3_rtx_.is_running()) {
t3_rtx_.Start();
}
}
}
bool RetransmissionQueue::IsSackValid(const SackChunk& sack) const {
// "If Cumulative TSN Ack is less than the Cumulative TSN Ack Point,
// then drop the SACK. Since Cumulative TSN Ack is monotonically increasing,
// a SACK whose Cumulative TSN Ack is less than the Cumulative TSN Ack Point
// indicates an out-of- order SACK."
//
// Note: Important not to drop SACKs with identical TSN to that previously
// received, as the gap ack blocks or dup tsn fields may have changed.
UnwrappedTSN cumulative_tsn_ack =
tsn_unwrapper_.PeekUnwrap(sack.cumulative_tsn_ack());
if (cumulative_tsn_ack < outstanding_data_.last_cumulative_tsn_ack()) {
// "If Cumulative TSN Ack is less than the Cumulative TSN Ack Point,
// then drop the SACK. Since Cumulative TSN Ack is monotonically
// increasing, a SACK whose Cumulative TSN Ack is less than the Cumulative
// TSN Ack Point indicates an out-of- order SACK."
return false;
} else if (cumulative_tsn_ack > outstanding_data_.highest_outstanding_tsn()) {
return false;
}
return true;
}
bool RetransmissionQueue::HandleSack(Timestamp now, const SackChunk& sack) {
if (!IsSackValid(sack)) {
return false;
}
UnwrappedTSN old_last_cumulative_tsn_ack =
outstanding_data_.last_cumulative_tsn_ack();
size_t old_unacked_bytes = outstanding_data_.unacked_bytes();
size_t old_rwnd = rwnd_;
UnwrappedTSN cumulative_tsn_ack =
tsn_unwrapper_.Unwrap(sack.cumulative_tsn_ack());
if (sack.gap_ack_blocks().empty()) {
UpdateRTT(now, cumulative_tsn_ack);
}
// Exit fast recovery before continuing processing, in case it needs to go
// into fast recovery again due to new reported packet loss.
MaybeExitFastRecovery(cumulative_tsn_ack);
OutstandingData::AckInfo ack_info = outstanding_data_.HandleSack(
cumulative_tsn_ack, sack.gap_ack_blocks(), is_in_fast_recovery());
// Add lifecycle events for delivered messages.
for (LifecycleId lifecycle_id : ack_info.acked_lifecycle_ids) {
RTC_DLOG(LS_VERBOSE) << "Triggering OnLifecycleMessageDelivered("
<< lifecycle_id.value() << ")";
callbacks_.OnLifecycleMessageDelivered(lifecycle_id);
callbacks_.OnLifecycleEnd(lifecycle_id);
}
for (LifecycleId lifecycle_id : ack_info.abandoned_lifecycle_ids) {
RTC_DLOG(LS_VERBOSE) << "Triggering OnLifecycleMessageExpired("
<< lifecycle_id.value() << ", true)";
callbacks_.OnLifecycleMessageExpired(lifecycle_id,
/*maybe_delivered=*/true);
callbacks_.OnLifecycleEnd(lifecycle_id);
}
// Update of outstanding_data_ is now done. Congestion control remains.
UpdateReceiverWindow(sack.a_rwnd());
RTC_DLOG(LS_VERBOSE) << log_prefix_ << "Received SACK, cum_tsn_ack="
<< *cumulative_tsn_ack.Wrap() << " ("
<< *old_last_cumulative_tsn_ack.Wrap()
<< "), unacked_bytes="
<< outstanding_data_.unacked_bytes() << " ("
<< old_unacked_bytes << "), rwnd=" << rwnd_ << " ("
<< old_rwnd << ")";
if (cumulative_tsn_ack > old_last_cumulative_tsn_ack) {
// "Whenever a SACK is received that acknowledges the DATA chunk
// with the earliest outstanding TSN for that address, restart the T3-rtx
// timer for that address with its current RTO (if there is still
// outstanding data on that address)."
// Note: It may be started again in a bit further down.
t3_rtx_.Stop();
HandleIncreasedCumulativeTsnAck(old_unacked_bytes, ack_info.bytes_acked);
}
if (ack_info.has_packet_loss) {
HandlePacketLoss(ack_info.highest_tsn_acked);
}
// "When an outstanding TSN is acknowledged [...] the endpoint shall clear
// the error counter ..."
if (ack_info.bytes_acked > 0) {
on_clear_retransmission_counter_();
}
StartT3RtxTimerIfOutstandingData();
RTC_DCHECK(IsConsistent());
return true;
}
void RetransmissionQueue::UpdateRTT(Timestamp now,
UnwrappedTSN cumulative_tsn_ack) {
// RTT updating is flawed in SCTP, as explained in e.g. Pedersen J, Griwodz C,
// Halvorsen P (2006) Considerations of SCTP retransmission delays for thin
// streams.
// Due to delayed acknowledgement, the SACK may be sent much later which
// increases the calculated RTT.
// TODO(boivie): Consider occasionally sending DATA chunks with I-bit set and
// use only those packets for measurement.
TimeDelta rtt = outstanding_data_.MeasureRTT(now, cumulative_tsn_ack);
if (rtt.IsFinite()) {
on_new_rtt_(rtt);
}
}
void RetransmissionQueue::HandleT3RtxTimerExpiry() {
size_t old_cwnd = cwnd_;
size_t old_unacked_bytes = unacked_bytes();
// "For the destination address for which the timer expires, adjust
// its ssthresh with rules defined in Section 7.2.3 and set the cwnd <- MTU."
ssthresh_ = std::max(cwnd_ / 2, 4 * options_.mtu);
cwnd_ = 1 * options_.mtu;
partial_bytes_acked_ = 0;
// "For the destination address for which the timer expires, set RTO
// <- RTO * 2 ("back off the timer"). The maximum value discussed in rule C7
// above (RTO.max) may be used to provide an upper bound to this doubling
// operation."
// Already done by the Timer implementation.
// "Determine how many of the earliest (i.e., lowest TSN) outstanding
// DATA chunks for the address for which the T3-rtx has expired will fit into
// a single packet"
// "Note: Any DATA chunks that were sent to the address for which the
// T3-rtx timer expired but did not fit in one MTU (rule E3 above) should be
// marked for retransmission and sent as soon as cwnd allows (normally, when a
// SACK arrives)."
outstanding_data_.NackAll();
// "Start the retransmission timer T3-rtx on the destination address
// to which the retransmission is sent, if rule R1 above indicates to do so."
// Already done by the Timer implementation.
RTC_DLOG(LS_INFO) << log_prefix_ << "t3-rtx expired. new cwnd=" << cwnd_
<< " (" << old_cwnd << "), ssthresh=" << ssthresh_
<< ", unacked_bytes " << unacked_bytes() << " ("
<< old_unacked_bytes << ")";
RTC_DCHECK(IsConsistent());
}
std::vector<std::pair<TSN, Data>>
RetransmissionQueue::GetChunksForFastRetransmit(size_t bytes_in_packet) {
RTC_DCHECK(outstanding_data_.has_data_to_be_fast_retransmitted());
RTC_DCHECK(IsDivisibleBy4(bytes_in_packet));
std::vector<std::pair<TSN, Data>> to_be_sent;
size_t old_unacked_bytes = unacked_bytes();
to_be_sent =
outstanding_data_.GetChunksToBeFastRetransmitted(bytes_in_packet);
RTC_DCHECK(!to_be_sent.empty());
// "4) Restart the T3-rtx timer only if ... the endpoint is retransmitting
// the first outstanding DATA chunk sent to that address."
if (to_be_sent[0].first ==
outstanding_data_.last_cumulative_tsn_ack().next_value().Wrap()) {
RTC_DLOG(LS_VERBOSE)
<< log_prefix_
<< "First outstanding DATA to be retransmitted - restarting T3-RTX";
t3_rtx_.Stop();
}
// "Every time a DATA chunk is sent to any address (including a
// retransmission), if the T3-rtx timer of that address is not running,
// start it running so that it will expire after the RTO of that address."
if (!t3_rtx_.is_running()) {
t3_rtx_.Start();
}
size_t bytes_retransmitted = absl::c_accumulate(
to_be_sent, 0, [&](size_t r, const std::pair<TSN, Data>& d) {
return r + GetSerializedChunkSize(d.second);
});
++rtx_packets_count_;
rtx_bytes_count_ += bytes_retransmitted;
RTC_DLOG(LS_VERBOSE) << log_prefix_ << "Fast-retransmitting TSN "
<< webrtc::StrJoin(to_be_sent, ",",
[&](rtc::StringBuilder& sb,
const std::pair<TSN, Data>& c) {
sb << *c.first;
})
<< " - " << bytes_retransmitted
<< " bytes. unacked_bytes=" << unacked_bytes() << " ("
<< old_unacked_bytes << ")";
RTC_DCHECK(IsConsistent());
return to_be_sent;
}
std::vector<std::pair<TSN, Data>> RetransmissionQueue::GetChunksToSend(
Timestamp now,
size_t bytes_remaining_in_packet) {
// Chunks are always padded to even divisible by four.
RTC_DCHECK(IsDivisibleBy4(bytes_remaining_in_packet));
std::vector<std::pair<TSN, Data>> to_be_sent;
size_t old_unacked_bytes = unacked_bytes();
size_t old_rwnd = rwnd_;
// Calculate the bandwidth budget (how many bytes that is
// allowed to be sent), and fill that up first with chunks that are
// scheduled to be retransmitted. If there is still budget, send new chunks
// (which will have their TSN assigned here.)
size_t max_bytes =
RoundDownTo4(std::min(max_bytes_to_send(), bytes_remaining_in_packet));
to_be_sent = outstanding_data_.GetChunksToBeRetransmitted(max_bytes);
size_t bytes_retransmitted = absl::c_accumulate(
to_be_sent, 0, [&](size_t r, const std::pair<TSN, Data>& d) {
return r + GetSerializedChunkSize(d.second);
});
max_bytes -= bytes_retransmitted;
if (!to_be_sent.empty()) {
++rtx_packets_count_;
rtx_bytes_count_ += bytes_retransmitted;
}
while (max_bytes > data_chunk_header_size_) {
RTC_DCHECK(IsDivisibleBy4(max_bytes));
std::optional<SendQueue::DataToSend> chunk_opt =
send_queue_.Produce(now, max_bytes - data_chunk_header_size_);
if (!chunk_opt.has_value()) {
break;
}
size_t chunk_size = GetSerializedChunkSize(chunk_opt->data);
max_bytes -= chunk_size;
rwnd_ -= chunk_size;
std::optional<UnwrappedTSN> tsn = outstanding_data_.Insert(
chunk_opt->message_id, chunk_opt->data, now,
partial_reliability_ ? chunk_opt->max_retransmissions
: MaxRetransmits::NoLimit(),
partial_reliability_ ? chunk_opt->expires_at
: Timestamp::PlusInfinity(),
chunk_opt->lifecycle_id);
if (tsn.has_value()) {
if (chunk_opt->lifecycle_id.IsSet()) {
RTC_DCHECK(chunk_opt->data.is_end);
callbacks_.OnLifecycleMessageFullySent(chunk_opt->lifecycle_id);
}
to_be_sent.emplace_back(tsn->Wrap(), std::move(chunk_opt->data));
}
}
if (!to_be_sent.empty()) {
// "Every time a DATA chunk is sent to any address (including a
// retransmission), if the T3-rtx timer of that address is not running,
// start it running so that it will expire after the RTO of that address."
if (!t3_rtx_.is_running()) {
t3_rtx_.Start();
}
RTC_DLOG(LS_VERBOSE) << log_prefix_ << "Sending TSN "
<< webrtc::StrJoin(to_be_sent, ",",
[&](rtc::StringBuilder& sb,
const std::pair<TSN, Data>& c) {
sb << *c.first;
})
<< " - "
<< absl::c_accumulate(
to_be_sent, 0,
[&](size_t r, const std::pair<TSN, Data>& d) {
return r + GetSerializedChunkSize(d.second);
})
<< " bytes. unacked_bytes=" << unacked_bytes() << " ("
<< old_unacked_bytes << "), cwnd=" << cwnd_
<< ", rwnd=" << rwnd_ << " (" << old_rwnd << ")";
}
RTC_DCHECK(IsConsistent());
return to_be_sent;
}
bool RetransmissionQueue::ShouldSendForwardTsn(Timestamp now) {
if (!partial_reliability_) {
return false;
}
outstanding_data_.ExpireOutstandingChunks(now);
bool ret = outstanding_data_.ShouldSendForwardTsn();
RTC_DCHECK(IsConsistent());
return ret;
}
size_t RetransmissionQueue::max_bytes_to_send() const {
size_t left = unacked_bytes() >= cwnd_ ? 0 : cwnd_ - unacked_bytes();
if (unacked_bytes() == 0) {
// ... However, regardless of the value of rwnd (including if it is 0), the
// data sender can always have one DATA chunk in flight to the receiver if
// allowed by cwnd (see rule B, below).
return left;
}
return std::min(rwnd(), left);
}
void RetransmissionQueue::PrepareResetStream(StreamID stream_id) {
// TODO(boivie): These calls are now only affecting the send queue. The
// packet buffer can also change behavior - for example draining the chunk
// producer and eagerly assign TSNs so that an "Outgoing SSN Reset Request"
// can be sent quickly, with a known `sender_last_assigned_tsn`.
send_queue_.PrepareResetStream(stream_id);
}
bool RetransmissionQueue::HasStreamsReadyToBeReset() const {
return send_queue_.HasStreamsReadyToBeReset();
}
std::vector<StreamID> RetransmissionQueue::BeginResetStreams() {
outstanding_data_.BeginResetStreams();
return send_queue_.GetStreamsReadyToBeReset();
}
void RetransmissionQueue::CommitResetStreams() {
send_queue_.CommitResetStreams();
}
void RetransmissionQueue::RollbackResetStreams() {
send_queue_.RollbackResetStreams();
}
HandoverReadinessStatus RetransmissionQueue::GetHandoverReadiness() const {
HandoverReadinessStatus status;
if (!outstanding_data_.empty()) {
status.Add(HandoverUnreadinessReason::kRetransmissionQueueOutstandingData);
}
if (fast_recovery_exit_tsn_.has_value()) {
status.Add(HandoverUnreadinessReason::kRetransmissionQueueFastRecovery);
}
if (outstanding_data_.has_data_to_be_retransmitted()) {
status.Add(HandoverUnreadinessReason::kRetransmissionQueueNotEmpty);
}
return status;
}
void RetransmissionQueue::AddHandoverState(DcSctpSocketHandoverState& state) {
state.tx.next_tsn = next_tsn().value();
state.tx.rwnd = rwnd_;
state.tx.cwnd = cwnd_;
state.tx.ssthresh = ssthresh_;
state.tx.partial_bytes_acked = partial_bytes_acked_;
}
void RetransmissionQueue::RestoreFromState(
const DcSctpSocketHandoverState& state) {
// Validate that the component is in pristine state.
RTC_DCHECK(outstanding_data_.empty());
RTC_DCHECK(!t3_rtx_.is_running());
RTC_DCHECK(partial_bytes_acked_ == 0);
cwnd_ = state.tx.cwnd;
rwnd_ = state.tx.rwnd;
ssthresh_ = state.tx.ssthresh;
partial_bytes_acked_ = state.tx.partial_bytes_acked;
outstanding_data_.ResetSequenceNumbers(
tsn_unwrapper_.Unwrap(TSN(state.tx.next_tsn - 1)));
}
} // namespace dcsctp