ICE 连接排序规则

// 组标识为 udpcandidate，使用 RTP 协议传输数据，基于 UDP 协议，
// 优先级为 1076558079，地址为 172.217.27.142:31269，类型为 host；
// host 表示客户端与该 candidate 位于同一个 LAN 或者 NAT，因此可以直连
a=candidate:udpcandidate 1 udp 1076558079 172.217.27.142 31269 typ host

// 组标识为 tcpcandidate，使用 RTP 协议传输数据，基于 TCP 协议，
// 优先级为 1076302079，地址为 172.217.27.142:31368，类型为 host；
// passive（被动）这个概念仅属于 TCP candidates，
// 表示该 candidate 不会主动发起连接，只能被动等待连接
a=candidate:tcpcandidate 1 tcp 1076302079 172.217.27.142 31368 typ host tcptype passive

IceControllerInterface::SwitchResult
BasicIceController::SortAndSwitchConnection(IceControllerEvent reason) {
  // Find the best alternative connection by sorting.  It is important to note
  // that amongst equal preference, writable connections, this will choose the
  // one whose estimated latency is lowest.  So it is the only one that we
  // need to consider switching to.
  // TODO(honghaiz): Don't sort;  Just use std::max_element in the right places.
  absl::c_stable_sort(
      connections_, [this](const Connection* a, const Connection* b) {
        int cmp = CompareConnections(a, b, absl::nullopt, nullptr);
        if (cmp != 0) {
          return cmp > 0;
        }
        // Otherwise, sort based on latency estimate.
        return a->rtt() < b->rtt();
      });

  // method body...

  const Connection* top_connection =
      (!connections_.empty()) ? connections_[0] : nullptr;

  return ShouldSwitchConnection(reason, top_connection);
}

int BasicIceController::CompareConnections(
    const Connection* a,
    const Connection* b,
    absl::optional<int64_t> receiving_unchanged_threshold,
    bool* missed_receiving_unchanged_threshold) const {
  RTC_CHECK(a != nullptr);
  RTC_CHECK(b != nullptr);

  // We prefer to switch to a writable and receiving connection over a
  // non-writable or non-receiving connection, even if the latter has
  // been nominated by the controlling side.
  int state_cmp = CompareConnectionStates(a, b, receiving_unchanged_threshold,
                                          missed_receiving_unchanged_threshold);
  if (state_cmp != 0) {
    return state_cmp;
  }

  if (ice_role_func_() == ICEROLE_CONTROLLED) {
    // Compare the connections based on the nomination states and the last data
    // received time if this is on the controlled side.
    if (a->remote_nomination() > b->remote_nomination()) {
      return a_is_better;
    }
    if (a->remote_nomination() < b->remote_nomination()) {
      return b_is_better;
    }

    if (a->last_data_received() > b->last_data_received()) {
      return a_is_better;
    }
    if (a->last_data_received() < b->last_data_received()) {
      return b_is_better;
    }
  }

  // Compare the network cost and priority.
  return CompareConnectionCandidates(a, b);
}

bool BasicIceController::PresumedWritable(const Connection* conn) const {
  return (conn->write_state() == Connection::STATE_WRITE_INIT &&
          config_.presume_writable_when_fully_relayed &&
          conn->local_candidate().type() == RELAY_PORT_TYPE &&
          (conn->remote_candidate().type() == RELAY_PORT_TYPE ||
           conn->remote_candidate().type() == PRFLX_PORT_TYPE));
}

// Compare two connections based on their writing, receiving, and connected
// states.
int BasicIceController::CompareConnectionStates(
    const Connection* a,
    const Connection* b,
    absl::optional<int64_t> receiving_unchanged_threshold,
    bool* missed_receiving_unchanged_threshold) const {
  // First, prefer a connection that's writable or presumed writable over
  // one that's not writable.
  bool a_writable = a->writable() || PresumedWritable(a);
  bool b_writable = b->writable() || PresumedWritable(b);
  if (a_writable && !b_writable) {
    return a_is_better;
  }
  if (!a_writable && b_writable) {
    return b_is_better;
  }

  // Sort based on write-state. Better states have lower values.
  if (a->write_state() < b->write_state()) {
    return a_is_better;
  }
  if (b->write_state() < a->write_state()) {
    return b_is_better;
  }

  // We prefer a receiving connection to a non-receiving, higher-priority
  // connection when sorting connections and choosing which connection to
  // switch to.
  if (a->receiving() && !b->receiving()) {
    return a_is_better;
  }
  if (!a->receiving() && b->receiving()) {
    if (!receiving_unchanged_threshold ||
        (a->receiving_unchanged_since() <= *receiving_unchanged_threshold &&
         b->receiving_unchanged_since() <= *receiving_unchanged_threshold)) {
      return b_is_better;
    }
    *missed_receiving_unchanged_threshold = true;
  }

  // some comments...

  // In the case where we reconnect TCP connections, the original best
  // connection is disconnected without changing to WRITE_TIMEOUT. In this case,
  // the new connection, when it becomes writable, should have higher priority.
  if (a->write_state() == Connection::STATE_WRITABLE &&
      b->write_state() == Connection::STATE_WRITABLE) {
    if (a->connected() && !b->connected()) {
      return a_is_better;
    }
    if (!a->connected() && b->connected()) {
      return b_is_better;
    }
  }

  return 0;
}

// Compares two connections based only on the candidate and network information.
// Returns positive if |a| is better than |b|.
int BasicIceController::CompareConnectionCandidates(const Connection* a,
                                                    const Connection* b) const {
  int compare_a_b_by_networks =
      CompareCandidatePairNetworks(a, b, config_.network_preference);
  if (compare_a_b_by_networks != a_and_b_equal) {
    return compare_a_b_by_networks;
  }

  // Compare connection priority. Lower values get sorted last.
  if (a->priority() > b->priority()) {
    return a_is_better;
  }
  if (a->priority() < b->priority()) {
    return b_is_better;
  }

  // If we're still tied at this point, prefer a younger generation.
  // (Younger generation means a larger generation number).
  int cmp = (a->remote_candidate().generation() + a->generation()) -
            (b->remote_candidate().generation() + b->generation());
  if (cmp != 0) {
    return cmp;
  }

  // A periodic regather (triggered by the regather_all_networks_interval_range)
  // will produce candidates that appear the same but would use a new port. We
  // want to use the new candidates and purge the old candidates as they come
  // in, so use the fact that the old ports get pruned immediately to rank the
  // candidates with an active port/remote candidate higher.
  bool a_pruned = is_connection_pruned_func_(a);
  bool b_pruned = is_connection_pruned_func_(b);
  if (!a_pruned && b_pruned) {
    return a_is_better;
  }
  if (a_pruned && !b_pruned) {
    return b_is_better;
  }

  // Otherwise, must be equal
  return 0;
}

IceControllerInterface::SwitchResult BasicIceController::ShouldSwitchConnection(
    IceControllerEvent reason,
    const Connection* new_connection) {
  if (!ReadyToSend(new_connection) || selected_connection_ == new_connection) {
    return {absl::nullopt, absl::nullopt};
  }

  if (selected_connection_ == nullptr) {
    return HandleInitialSelectDampening(reason, new_connection);
  }

  // Do not switch to a connection that is not receiving if it is not on a
  // preferred network or it has higher cost because it may be just spuriously
  // better.
  int compare_a_b_by_networks = CompareCandidatePairNetworks(
      new_connection, selected_connection_, config_.network_preference);
  if (compare_a_b_by_networks == b_is_better && !new_connection->receiving()) {
    return {absl::nullopt, absl::nullopt};
  }

  bool missed_receiving_unchanged_threshold = false;
  absl::optional<int64_t> receiving_unchanged_threshold(
      rtc::TimeMillis() - config_.receiving_switching_delay_or_default());
  int cmp = CompareConnections(selected_connection_, new_connection,
                               receiving_unchanged_threshold,
                               &missed_receiving_unchanged_threshold);

  // method body...

  if (cmp < 0) {
    return {new_connection, absl::nullopt};
  } else if (cmp > 0) {
    return {absl::nullopt, recheck_event};
  }

  // If everything else is the same, switch only if rtt has improved by
  // a margin.
  if (new_connection->rtt() <= selected_connection_->rtt() - kMinImprovement) {
    return {new_connection, absl::nullopt};
  }

  return {absl::nullopt, recheck_event};
}