net/dcsctp/rx/data_tracker.cc

/*
 *  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/rx/data_tracker.h"

#include <cstdint>
#include <iterator>
#include <set>
#include <string>
#include <utility>
#include <vector>

#include "absl/strings/string_view.h"
#include "absl/types/optional.h"
#include "net/dcsctp/common/sequence_numbers.h"
#include "net/dcsctp/packet/chunk/sack_chunk.h"
#include "net/dcsctp/timer/timer.h"
#include "rtc_base/logging.h"
#include "rtc_base/strings/string_builder.h"

namespace dcsctp {

bool DataTracker::IsTSNValid(TSN tsn) const {
  UnwrappedTSN unwrapped_tsn = tsn_unwrapper_.PeekUnwrap(tsn);

  // Note that this method doesn't return `false` for old DATA chunks, as those
  // are actually valid, and receiving those may affect the generated SACK
  // response (by setting "duplicate TSNs").

  uint32_t difference =
      UnwrappedTSN::Difference(unwrapped_tsn, last_cumulative_acked_tsn_);
  if (difference > kMaxAcceptedOutstandingFragments) {
    return false;
  }
  return true;
}

void DataTracker::Observe(TSN tsn,
                          AnyDataChunk::ImmediateAckFlag immediate_ack) {
  UnwrappedTSN unwrapped_tsn = tsn_unwrapper_.Unwrap(tsn);

  // IsTSNValid must be called prior to calling this method.
  RTC_DCHECK(
      UnwrappedTSN::Difference(unwrapped_tsn, last_cumulative_acked_tsn_) <=
      kMaxAcceptedOutstandingFragments);

  // Old chunk already seen before?
  if (unwrapped_tsn <= last_cumulative_acked_tsn_) {
    // TODO(boivie) Set duplicate TSN, even if it's not used in SCTP yet.
    return;
  }

  if (unwrapped_tsn == last_cumulative_acked_tsn_.next_value()) {
    last_cumulative_acked_tsn_ = unwrapped_tsn;
    // The cumulative acked tsn may be moved even further, if a gap was filled.
    while (!additional_tsns_.empty() &&
           *additional_tsns_.begin() ==
               last_cumulative_acked_tsn_.next_value()) {
      last_cumulative_acked_tsn_.Increment();
      additional_tsns_.erase(additional_tsns_.begin());
    }
  } else {
    additional_tsns_.insert(unwrapped_tsn);
  }

  // https://tools.ietf.org/html/rfc4960#section-6.7
  // "Upon the reception of a new DATA chunk, an endpoint shall examine the
  // continuity of the TSNs received.  If the endpoint detects a gap in
  // the received DATA chunk sequence, it SHOULD send a SACK with Gap Ack
  // Blocks immediately.  The data receiver continues sending a SACK after
  // receipt of each SCTP packet that doesn't fill the gap."
  if (!additional_tsns_.empty()) {
    UpdateAckState(AckState::kImmediate, "packet loss");
  }

  // https://tools.ietf.org/html/rfc7053#section-5.2
  // "Upon receipt of an SCTP packet containing a DATA chunk with the I
  // bit set, the receiver SHOULD NOT delay the sending of the corresponding
  // SACK chunk, i.e., the receiver SHOULD immediately respond with the
  // corresponding SACK chunk."
  if (*immediate_ack) {
    UpdateAckState(AckState::kImmediate, "immediate-ack bit set");
  }

  if (!seen_packet_) {
    // https://tools.ietf.org/html/rfc4960#section-5.1
    // "After the reception of the first DATA chunk in an association the
    // endpoint MUST immediately respond with a SACK to acknowledge the DATA
    // chunk."
    seen_packet_ = true;
    UpdateAckState(AckState::kImmediate, "first DATA chunk");
  }

  // https://tools.ietf.org/html/rfc4960#section-6.2
  // "Specifically, an acknowledgement SHOULD be generated for at least
  // every second packet (not every second DATA chunk) received, and SHOULD be
  // generated within 200 ms of the arrival of any unacknowledged DATA chunk."
  if (ack_state_ == AckState::kIdle) {
    UpdateAckState(AckState::kBecomingDelayed, "received DATA when idle");
  } else if (ack_state_ == AckState::kDelayed) {
    UpdateAckState(AckState::kImmediate, "received DATA when already delayed");
  }
}

void DataTracker::HandleForwardTsn(TSN new_cumulative_ack) {
  // ForwardTSN is sent to make the receiver (this socket) "forget" about partly
  // received (or not received at all) data, up until `new_cumulative_ack`.

  UnwrappedTSN unwrapped_tsn = tsn_unwrapper_.Unwrap(new_cumulative_ack);
  UnwrappedTSN prev_last_cum_ack_tsn = last_cumulative_acked_tsn_;

  // Old chunk already seen before?
  if (unwrapped_tsn <= last_cumulative_acked_tsn_) {
    // https://tools.ietf.org/html/rfc3758#section-3.6
    // "Note, if the "New Cumulative TSN" value carried in the arrived
    // FORWARD TSN chunk is found to be behind or at the current cumulative TSN
    // point, the data receiver MUST treat this FORWARD TSN as out-of-date and
    // MUST NOT update its Cumulative TSN.  The receiver SHOULD send a SACK to
    // its peer (the sender of the FORWARD TSN) since such a duplicate may
    // indicate the previous SACK was lost in the network."
    UpdateAckState(AckState::kImmediate,
                   "FORWARD_TSN new_cumulative_tsn was behind");
    return;
  }

  // https://tools.ietf.org/html/rfc3758#section-3.6
  // "When a FORWARD TSN chunk arrives, the data receiver MUST first update
  // its cumulative TSN point to the value carried in the FORWARD TSN chunk, and
  // then MUST further advance its cumulative TSN point locally if possible, as
  // shown by the following example..."

  // The `new_cumulative_ack` will become the current
  // `last_cumulative_acked_tsn_`, and if there have been prior "gaps" that are
  // now overlapping with the new value, remove them.
  last_cumulative_acked_tsn_ = unwrapped_tsn;
  int erased_additional_tsns = std::distance(
      additional_tsns_.begin(), additional_tsns_.upper_bound(unwrapped_tsn));
  additional_tsns_.erase(additional_tsns_.begin(),
                         additional_tsns_.upper_bound(unwrapped_tsn));

  // See if the `last_cumulative_acked_tsn_` can be moved even further:
  while (!additional_tsns_.empty() &&
         *additional_tsns_.begin() == last_cumulative_acked_tsn_.next_value()) {
    last_cumulative_acked_tsn_.Increment();
    additional_tsns_.erase(additional_tsns_.begin());
    ++erased_additional_tsns;
  }

  RTC_DLOG(LS_VERBOSE) << log_prefix_ << "FORWARD_TSN, cum_ack_tsn="
                       << *prev_last_cum_ack_tsn.Wrap() << "->"
                       << *new_cumulative_ack << "->"
                       << *last_cumulative_acked_tsn_.Wrap() << ", removed "
                       << erased_additional_tsns << " additional TSNs";

  // https://tools.ietf.org/html/rfc3758#section-3.6
  // "Any time a FORWARD TSN chunk arrives, for the purposes of sending a
  // SACK, the receiver MUST follow the same rules as if a DATA chunk had been
  // received (i.e., follow the delayed sack rules specified in ..."
  if (ack_state_ == AckState::kIdle) {
    UpdateAckState(AckState::kBecomingDelayed,
                   "received FORWARD_TSN when idle");
  } else if (ack_state_ == AckState::kDelayed) {
    UpdateAckState(AckState::kImmediate,
                   "received FORWARD_TSN when already delayed");
  }
}

SackChunk DataTracker::CreateSelectiveAck(size_t a_rwnd) {
  // Note that in SCTP, the receiver side is allowed to discard received data
  // and signal that to the sender, but only chunks that have previously been
  // reported in the gap-ack-blocks. However, this implementation will never do
  // that. So this SACK produced is more like a NR-SACK as explained in
  // https://ieeexplore.ieee.org/document/4697037 and which there is an RFC
  // draft at https://tools.ietf.org/html/draft-tuexen-tsvwg-sctp-multipath-17.
  std::vector<TSN> duplicate_tsns;
  duplicate_tsns.reserve(duplicates_.size());
  for (UnwrappedTSN tsn : duplicates_) {
    duplicate_tsns.push_back(tsn.Wrap());
  }
  duplicates_.clear();

  return SackChunk(last_cumulative_acked_tsn_.Wrap(), a_rwnd,
                   CreateGapAckBlocks(), duplicate_tsns);
}

std::vector<SackChunk::GapAckBlock> DataTracker::CreateGapAckBlocks() const {
  // This method will calculate the gaps between blocks of contiguous values in
  // `additional_tsns_`, in the same format as the SACK chunk expects it;
  // offsets from the "cumulative ack TSN value".
  std::vector<SackChunk::GapAckBlock> gap_ack_blocks;

  absl::optional<UnwrappedTSN> first_tsn_in_block = absl::nullopt;
  absl::optional<UnwrappedTSN> last_tsn_in_block = absl::nullopt;

  auto flush = [&]() {
    if (first_tsn_in_block.has_value()) {
      auto start_diff = UnwrappedTSN::Difference(*first_tsn_in_block,
                                                 last_cumulative_acked_tsn_);
      auto end_diff = UnwrappedTSN::Difference(*last_tsn_in_block,
                                               last_cumulative_acked_tsn_);
      gap_ack_blocks.emplace_back(static_cast<uint16_t>(start_diff),
                                  static_cast<uint16_t>(end_diff));
      first_tsn_in_block = absl::nullopt;
      last_tsn_in_block = absl::nullopt;
    }
  };
  for (UnwrappedTSN tsn : additional_tsns_) {
    if (last_tsn_in_block.has_value() &&
        last_tsn_in_block->next_value() == tsn) {
      // Continuing the same block.
      last_tsn_in_block = tsn;
    } else {
      // New block, or a gap from the old block's last value.
      flush();
      first_tsn_in_block = tsn;
      last_tsn_in_block = tsn;
    }
  }
  flush();
  return gap_ack_blocks;
}

bool DataTracker::ShouldSendAck(bool also_if_delayed) {
  if (ack_state_ == AckState::kImmediate ||
      (also_if_delayed && (ack_state_ == AckState::kBecomingDelayed ||
                           ack_state_ == AckState::kDelayed))) {
    UpdateAckState(AckState::kIdle, "sending SACK");
    return true;
  }

  return false;
}

bool DataTracker::will_increase_cum_ack_tsn(TSN tsn) const {
  UnwrappedTSN unwrapped = tsn_unwrapper_.PeekUnwrap(tsn);
  return unwrapped == last_cumulative_acked_tsn_.next_value();
}

void DataTracker::ForceImmediateSack() {
  ack_state_ = AckState::kImmediate;
}

void DataTracker::HandleDelayedAckTimerExpiry() {
  UpdateAckState(AckState::kImmediate, "delayed ack timer expired");
}

void DataTracker::ObservePacketEnd() {
  if (ack_state_ == AckState::kBecomingDelayed) {
    UpdateAckState(AckState::kDelayed, "packet end");
  }
}

void DataTracker::UpdateAckState(AckState new_state, absl::string_view reason) {
  if (new_state != ack_state_) {
    RTC_DLOG(LS_VERBOSE) << log_prefix_ << "State changed from "
                         << ToString(ack_state_) << " to "
                         << ToString(new_state) << " due to " << reason;
    if (ack_state_ == AckState::kDelayed) {
      delayed_ack_timer_.Stop();
    } else if (new_state == AckState::kDelayed) {
      delayed_ack_timer_.Start();
    }
    ack_state_ = new_state;
  }
}

absl::string_view DataTracker::ToString(AckState ack_state) {
  switch (ack_state) {
    case AckState::kIdle:
      return "IDLE";
    case AckState::kBecomingDelayed:
      return "BECOMING_DELAYED";
    case AckState::kDelayed:
      return "DELAYED";
    case AckState::kImmediate:
      return "IMMEDIATE";
  }
}

}  // namespace dcsctp