rhubarb-lip-sync/rhubarb/lib/webrtc-8d2248ff/webrtc/p2p/base/p2ptransportchannel.cc

/*
 *  Copyright 2004 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 "webrtc/p2p/base/p2ptransportchannel.h"

#include <algorithm>
#include <set>

#include "webrtc/base/common.h"
#include "webrtc/base/crc32.h"
#include "webrtc/base/logging.h"
#include "webrtc/base/stringencode.h"
#include "webrtc/p2p/base/candidate.h"
#include "webrtc/p2p/base/candidatepairinterface.h"
#include "webrtc/p2p/base/common.h"
#include "webrtc/p2p/base/relayport.h"  // For RELAY_PORT_TYPE.
#include "webrtc/p2p/base/stunport.h"   // For STUN_PORT_TYPE.
#include "webrtc/system_wrappers/include/field_trial.h"

namespace {

// messages for queuing up work for ourselves
enum { MSG_SORT = 1, MSG_CHECK_AND_PING };

// The minimum improvement in RTT that justifies a switch.
static const double kMinImprovement = 10;

bool IsRelayRelay(cricket::Connection* conn) {
  return conn->local_candidate().type() == cricket::RELAY_PORT_TYPE &&
         conn->remote_candidate().type() == cricket::RELAY_PORT_TYPE;
}

bool IsUdp(cricket::Connection* conn) {
  return conn->local_candidate().relay_protocol() == cricket::UDP_PROTOCOL_NAME;
}

cricket::PortInterface::CandidateOrigin GetOrigin(cricket::PortInterface* port,
                                         cricket::PortInterface* origin_port) {
  if (!origin_port)
    return cricket::PortInterface::ORIGIN_MESSAGE;
  else if (port == origin_port)
    return cricket::PortInterface::ORIGIN_THIS_PORT;
  else
    return cricket::PortInterface::ORIGIN_OTHER_PORT;
}

// Compares two connections based only on the candidate and network information.
// Returns positive if |a| is better than |b|.
int CompareConnectionCandidates(cricket::Connection* a,
                                cricket::Connection* b) {
  uint32_t a_cost = a->ComputeNetworkCost();
  uint32_t b_cost = b->ComputeNetworkCost();
  // Smaller cost is better.
  if (a_cost < b_cost) {
    return 1;
  }
  if (a_cost > b_cost) {
    return -1;
  }

  // Compare connection priority. Lower values get sorted last.
  if (a->priority() > b->priority())
    return 1;
  if (a->priority() < b->priority())
    return -1;

  // If we're still tied at this point, prefer a younger generation.
  return (a->remote_candidate().generation() + a->port()->generation()) -
         (b->remote_candidate().generation() + b->port()->generation());
}

// Compare two connections based on their writing, receiving, and connected
// states.
int CompareConnectionStates(cricket::Connection* a, cricket::Connection* b) {
  // Sort based on write-state.  Better states have lower values.
  if (a->write_state() < b->write_state())
    return 1;
  if (a->write_state() > b->write_state())
    return -1;

  // 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 1;
  if (!a->receiving() && b->receiving())
    return -1;

  // WARNING: Some complexity here about TCP reconnecting.
  // When a TCP connection fails because of a TCP socket disconnecting, the
  // active side of the connection will attempt to reconnect for 5 seconds while
  // pretending to be writable (the connection is not set to the unwritable
  // state).  On the passive side, the connection also remains writable even
  // though it is disconnected, and a new connection is created when the active
  // side connects.  At that point, there are two TCP connections on the passive
  // side: 1. the old, disconnected one that is pretending to be writable, and
  // 2.  the new, connected one that is maybe not yet writable.  For purposes of
  // pruning, pinging, and selecting the best connection, we want to treat the
  // new connection as "better" than the old one.  We could add a method called
  // something like Connection::ImReallyBadEvenThoughImWritable, but that is
  // equivalent to the existing Connection::connected(), which we already have.
  // So, in code throughout this file, we'll check whether the connection is
  // connected() or not, and if it is not, treat it as "worse" than a connected
  // one, even though it's writable.  In the code below, we're doing so to make
  // sure we treat a new writable connection as better than an old disconnected
  // connection.

  // 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() == cricket::Connection::STATE_WRITABLE &&
      b->write_state() == cricket::Connection::STATE_WRITABLE) {
    if (a->connected() && !b->connected()) {
      return 1;
    }
    if (!a->connected() && b->connected()) {
      return -1;
    }
  }
  return 0;
}

int CompareConnections(cricket::Connection* a, cricket::Connection* b) {
  int state_cmp = CompareConnectionStates(a, b);
  if (state_cmp != 0) {
    return state_cmp;
  }
  // Compare the candidate information.
  return CompareConnectionCandidates(a, b);
}

// Wraps the comparison connection into a less than operator that puts higher
// priority writable connections first.
class ConnectionCompare {
 public:
  bool operator()(const cricket::Connection *ca,
                  const cricket::Connection *cb) {
    cricket::Connection* a = const_cast<cricket::Connection*>(ca);
    cricket::Connection* b = const_cast<cricket::Connection*>(cb);

    // Compare first on writability and static preferences.
    int cmp = CompareConnections(a, b);
    if (cmp > 0)
      return true;
    if (cmp < 0)
      return false;

    // Otherwise, sort based on latency estimate.
    return a->rtt() < b->rtt();

    // Should we bother checking for the last connection that last received
    // data? It would help rendezvous on the connection that is also receiving
    // packets.
    //
    // TODO: Yes we should definitely do this.  The TCP protocol gains
    // efficiency by being used bidirectionally, as opposed to two separate
    // unidirectional streams.  This test should probably occur before
    // comparison of local prefs (assuming combined prefs are the same).  We
    // need to be careful though, not to bounce back and forth with both sides
    // trying to rendevous with the other.
  }
};

// Determines whether we should switch between two connections, based first on
// connection states, static preferences, and then (if those are equal) on
// latency estimates.
bool ShouldSwitch(cricket::Connection* a_conn,
                  cricket::Connection* b_conn,
                  cricket::IceRole ice_role) {
  if (a_conn == b_conn)
    return false;

  if (!a_conn || !b_conn)  // don't think the latter should happen
    return true;

  // 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_conn, b_conn);
  if (state_cmp != 0) {
    return state_cmp < 0;
  }
  if (ice_role == cricket::ICEROLE_CONTROLLED && a_conn->nominated()) {
    LOG(LS_VERBOSE) << "Controlled side did not switch due to nominated status";
    return false;
  }

  int prefs_cmp = CompareConnectionCandidates(a_conn, b_conn);
  if (prefs_cmp != 0) {
    return prefs_cmp < 0;
  }

  return b_conn->rtt() <= a_conn->rtt() + kMinImprovement;
}

}  // unnamed namespace

namespace cricket {

// When the socket is unwritable, we will use 10 Kbps (ignoring IP+UDP headers)
// for pinging.  When the socket is writable, we will use only 1 Kbps because
// we don't want to degrade the quality on a modem.  These numbers should work
// well on a 28.8K modem, which is the slowest connection on which the voice
// quality is reasonable at all.
static const int PING_PACKET_SIZE = 60 * 8;
// STRONG_PING_INTERVAL (480ms) is applied when the best connection is both
// writable and receiving.
static const int STRONG_PING_INTERVAL = 1000 * PING_PACKET_SIZE / 1000;
// WEAK_PING_INTERVAL (48ms) is applied when the best connection is either not
// writable or not receiving.
const int WEAK_PING_INTERVAL = 1000 * PING_PACKET_SIZE / 10000;

// If the current best connection is both writable and receiving, then we will
// also try hard to make sure it is pinged at this rate (a little less than
// 2 * STRONG_PING_INTERVAL).
static const int MAX_CURRENT_STRONG_INTERVAL = 900;  // ms

static const int MIN_CHECK_RECEIVING_INTERVAL = 50;  // ms

P2PTransportChannel::P2PTransportChannel(const std::string& transport_name,
                                         int component,
                                         P2PTransport* transport,
                                         PortAllocator* allocator)
    : P2PTransportChannel(transport_name, component, allocator) {}

P2PTransportChannel::P2PTransportChannel(const std::string& transport_name,
                                         int component,
                                         PortAllocator* allocator)
    : TransportChannelImpl(transport_name, component),
      allocator_(allocator),
      worker_thread_(rtc::Thread::Current()),
      incoming_only_(false),
      error_(0),
      best_connection_(NULL),
      pending_best_connection_(NULL),
      sort_dirty_(false),
      remote_ice_mode_(ICEMODE_FULL),
      ice_role_(ICEROLE_UNKNOWN),
      tiebreaker_(0),
      gathering_state_(kIceGatheringNew),
      check_receiving_interval_(MIN_CHECK_RECEIVING_INTERVAL * 5),
      config_(MIN_CHECK_RECEIVING_INTERVAL * 50 /* receiving_timeout */,
              0 /* backup_connection_ping_interval */,
              false /* gather_continually */,
              false /* prioritize_most_likely_candidate_pairs */,
              MAX_CURRENT_STRONG_INTERVAL /* max_strong_interval */) {
  uint32_t weak_ping_interval = ::strtoul(
      webrtc::field_trial::FindFullName("WebRTC-StunInterPacketDelay").c_str(),
      nullptr, 10);
  if (weak_ping_interval) {
    weak_ping_interval_ = static_cast<int>(weak_ping_interval);
  }
}

P2PTransportChannel::~P2PTransportChannel() {
  ASSERT(worker_thread_ == rtc::Thread::Current());
}

// Add the allocator session to our list so that we know which sessions
// are still active.
void P2PTransportChannel::AddAllocatorSession(
    std::unique_ptr<PortAllocatorSession> session) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  session->set_generation(static_cast<uint32_t>(allocator_sessions_.size()));
  session->SignalPortReady.connect(this, &P2PTransportChannel::OnPortReady);
  session->SignalCandidatesReady.connect(
      this, &P2PTransportChannel::OnCandidatesReady);
  session->SignalCandidatesAllocationDone.connect(
      this, &P2PTransportChannel::OnCandidatesAllocationDone);

  // We now only want to apply new candidates that we receive to the ports
  // created by this new session because these are replacing those of the
  // previous sessions.
  ports_.clear();

  allocator_sessions_.push_back(std::move(session));
}

void P2PTransportChannel::AddConnection(Connection* connection) {
  connections_.push_back(connection);
  unpinged_connections_.insert(connection);
  connection->set_remote_ice_mode(remote_ice_mode_);
  connection->set_receiving_timeout(config_.receiving_timeout);
  connection->SignalReadPacket.connect(
      this, &P2PTransportChannel::OnReadPacket);
  connection->SignalReadyToSend.connect(
      this, &P2PTransportChannel::OnReadyToSend);
  connection->SignalStateChange.connect(
      this, &P2PTransportChannel::OnConnectionStateChange);
  connection->SignalDestroyed.connect(
      this, &P2PTransportChannel::OnConnectionDestroyed);
  connection->SignalNominated.connect(this, &P2PTransportChannel::OnNominated);
  had_connection_ = true;
}

void P2PTransportChannel::SetIceRole(IceRole ice_role) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  if (ice_role_ != ice_role) {
    ice_role_ = ice_role;
    for (std::vector<PortInterface *>::iterator it = ports_.begin();
         it != ports_.end(); ++it) {
      (*it)->SetIceRole(ice_role);
    }
  }
}

void P2PTransportChannel::SetIceTiebreaker(uint64_t tiebreaker) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  if (!ports_.empty()) {
    LOG(LS_ERROR)
        << "Attempt to change tiebreaker after Port has been allocated.";
    return;
  }

  tiebreaker_ = tiebreaker;
}

TransportChannelState P2PTransportChannel::GetState() const {
  return state_;
}

// A channel is considered ICE completed once there is at most one active
// connection per network and at least one active connection.
TransportChannelState P2PTransportChannel::ComputeState() const {
  if (!had_connection_) {
    return TransportChannelState::STATE_INIT;
  }

  std::vector<Connection*> active_connections;
  for (Connection* connection : connections_) {
    if (connection->active()) {
      active_connections.push_back(connection);
    }
  }
  if (active_connections.empty()) {
    return TransportChannelState::STATE_FAILED;
  }

  std::set<rtc::Network*> networks;
  for (Connection* connection : active_connections) {
    rtc::Network* network = connection->port()->Network();
    if (networks.find(network) == networks.end()) {
      networks.insert(network);
    } else {
      LOG_J(LS_VERBOSE, this) << "Ice not completed yet for this channel as "
                              << network->ToString()
                              << " has more than 1 connection.";
      return TransportChannelState::STATE_CONNECTING;
    }
  }

  return TransportChannelState::STATE_COMPLETED;
}

void P2PTransportChannel::SetIceCredentials(const std::string& ice_ufrag,
                                            const std::string& ice_pwd) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  ice_ufrag_ = ice_ufrag;
  ice_pwd_ = ice_pwd;
  // Note: Candidate gathering will restart when MaybeStartGathering is next
  // called.
}

void P2PTransportChannel::SetRemoteIceCredentials(const std::string& ice_ufrag,
                                                  const std::string& ice_pwd) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  IceParameters* current_ice = remote_ice();
  IceParameters new_ice(ice_ufrag, ice_pwd);
  if (!current_ice || *current_ice != new_ice) {
    // Keep the ICE credentials so that newer connections
    // are prioritized over the older ones.
    remote_ice_parameters_.push_back(new_ice);
  }

  // Update the pwd of remote candidate if needed.
  for (RemoteCandidate& candidate : remote_candidates_) {
    if (candidate.username() == ice_ufrag && candidate.password().empty()) {
      candidate.set_password(ice_pwd);
    }
  }
  // We need to update the credentials and generation for any peer reflexive
  // candidates.
  for (Connection* conn : connections_) {
    conn->MaybeSetRemoteIceCredentialsAndGeneration(
        ice_ufrag, ice_pwd,
        static_cast<int>(remote_ice_parameters_.size() - 1));
  }
  // Updating the remote ICE candidate generation could change the sort order.
  RequestSort();
}

void P2PTransportChannel::SetRemoteIceMode(IceMode mode) {
  remote_ice_mode_ = mode;
}

void P2PTransportChannel::SetIceConfig(const IceConfig& config) {
  config_.gather_continually = config.gather_continually;
  LOG(LS_INFO) << "Set gather_continually to " << config_.gather_continually;

  if (config.backup_connection_ping_interval >= 0 &&
      config_.backup_connection_ping_interval !=
          config.backup_connection_ping_interval) {
    config_.backup_connection_ping_interval =
        config.backup_connection_ping_interval;
    LOG(LS_INFO) << "Set backup connection ping interval to "
                 << config_.backup_connection_ping_interval << " milliseconds.";
  }

  if (config.receiving_timeout >= 0 &&
      config_.receiving_timeout != config.receiving_timeout) {
    config_.receiving_timeout = config.receiving_timeout;
    check_receiving_interval_ =
        std::max(MIN_CHECK_RECEIVING_INTERVAL, config_.receiving_timeout / 10);

    for (Connection* connection : connections_) {
      connection->set_receiving_timeout(config_.receiving_timeout);
    }
    LOG(LS_INFO) << "Set ICE receiving timeout to " << config_.receiving_timeout
                 << " milliseconds";
  }

  config_.prioritize_most_likely_candidate_pairs =
      config.prioritize_most_likely_candidate_pairs;
  LOG(LS_INFO) << "Set ping most likely connection to "
               << config_.prioritize_most_likely_candidate_pairs;

  if (config.max_strong_interval >= 0 &&
      config_.max_strong_interval != config.max_strong_interval) {
    config_.max_strong_interval = config.max_strong_interval;
    LOG(LS_INFO) << "Set max strong interval to "
                 << config_.max_strong_interval;
  }
}

const IceConfig& P2PTransportChannel::config() const {
  return config_;
}

// Go into the state of processing candidates, and running in general
void P2PTransportChannel::Connect() {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  if (ice_ufrag_.empty() || ice_pwd_.empty()) {
    ASSERT(false);
    LOG(LS_ERROR) << "P2PTransportChannel::Connect: The ice_ufrag_ and the "
                  << "ice_pwd_ are not set.";
    return;
  }

  // Start checking and pinging as the ports come in.
  thread()->Post(RTC_FROM_HERE, this, MSG_CHECK_AND_PING);
}

void P2PTransportChannel::MaybeStartGathering() {
  // Start gathering if we never started before, or if an ICE restart occurred.
  if (allocator_sessions_.empty() ||
      IceCredentialsChanged(allocator_sessions_.back()->ice_ufrag(),
                            allocator_sessions_.back()->ice_pwd(), ice_ufrag_,
                            ice_pwd_)) {
    if (gathering_state_ != kIceGatheringGathering) {
      gathering_state_ = kIceGatheringGathering;
      SignalGatheringState(this);
    }
    // Time for a new allocator.
    std::unique_ptr<PortAllocatorSession> pooled_session =
        allocator_->TakePooledSession(transport_name(), component(), ice_ufrag_,
                                      ice_pwd_);
    if (pooled_session) {
      AddAllocatorSession(std::move(pooled_session));
      PortAllocatorSession* raw_pooled_session =
          allocator_sessions_.back().get();
      // Process the pooled session's existing candidates/ports, if they exist.
      OnCandidatesReady(raw_pooled_session,
                        raw_pooled_session->ReadyCandidates());
      for (PortInterface* port : allocator_sessions_.back()->ReadyPorts()) {
        OnPortReady(raw_pooled_session, port);
      }
      if (allocator_sessions_.back()->CandidatesAllocationDone()) {
        OnCandidatesAllocationDone(raw_pooled_session);
      }
    } else {
      AddAllocatorSession(allocator_->CreateSession(
          SessionId(), transport_name(), component(), ice_ufrag_, ice_pwd_));
      allocator_sessions_.back()->StartGettingPorts();
    }
  }
}

// A new port is available, attempt to make connections for it
void P2PTransportChannel::OnPortReady(PortAllocatorSession *session,
                                      PortInterface* port) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // Set in-effect options on the new port
  for (OptionMap::const_iterator it = options_.begin();
       it != options_.end();
       ++it) {
    int val = port->SetOption(it->first, it->second);
    if (val < 0) {
      LOG_J(LS_WARNING, port) << "SetOption(" << it->first
                              << ", " << it->second
                              << ") failed: " << port->GetError();
    }
  }

  // Remember the ports and candidates, and signal that candidates are ready.
  // The session will handle this, and send an initiate/accept/modify message
  // if one is pending.

  port->SetIceRole(ice_role_);
  port->SetIceTiebreaker(tiebreaker_);
  ports_.push_back(port);
  port->SignalUnknownAddress.connect(
      this, &P2PTransportChannel::OnUnknownAddress);
  port->SignalDestroyed.connect(this, &P2PTransportChannel::OnPortDestroyed);
  port->SignalNetworkInactive.connect(
      this, &P2PTransportChannel::OnPortNetworkInactive);
  port->SignalRoleConflict.connect(
      this, &P2PTransportChannel::OnRoleConflict);
  port->SignalSentPacket.connect(this, &P2PTransportChannel::OnSentPacket);

  // Attempt to create a connection from this new port to all of the remote
  // candidates that we were given so far.

  std::vector<RemoteCandidate>::iterator iter;
  for (iter = remote_candidates_.begin(); iter != remote_candidates_.end();
       ++iter) {
    CreateConnection(port, *iter, iter->origin_port());
  }

  SortConnections();
}

// A new candidate is available, let listeners know
void P2PTransportChannel::OnCandidatesReady(
    PortAllocatorSession* session,
    const std::vector<Candidate>& candidates) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  for (size_t i = 0; i < candidates.size(); ++i) {
    SignalCandidateGathered(this, candidates[i]);
  }
}

void P2PTransportChannel::OnCandidatesAllocationDone(
    PortAllocatorSession* session) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  gathering_state_ = kIceGatheringComplete;
  LOG(LS_INFO) << "P2PTransportChannel: " << transport_name() << ", component "
               << component() << " gathering complete";
  SignalGatheringState(this);
}

// Handle stun packets
void P2PTransportChannel::OnUnknownAddress(
    PortInterface* port,
    const rtc::SocketAddress& address, ProtocolType proto,
    IceMessage* stun_msg, const std::string &remote_username,
    bool port_muxed) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // Port has received a valid stun packet from an address that no Connection
  // is currently available for. See if we already have a candidate with the
  // address. If it isn't we need to create new candidate for it.

  // Determine if the remote candidates use shared ufrag.
  bool ufrag_per_port = false;
  std::vector<RemoteCandidate>::iterator it;
  if (remote_candidates_.size() > 0) {
    it = remote_candidates_.begin();
    std::string username = it->username();
    for (; it != remote_candidates_.end(); ++it) {
      if (it->username() != username) {
        ufrag_per_port = true;
        break;
      }
    }
  }

  const Candidate* candidate = NULL;
  std::string remote_password;
  for (it = remote_candidates_.begin(); it != remote_candidates_.end(); ++it) {
    if (it->username() == remote_username) {
      remote_password = it->password();
      if (ufrag_per_port ||
          (it->address() == address &&
           it->protocol() == ProtoToString(proto))) {
        candidate = &(*it);
        break;
      }
      // We don't want to break here because we may find a match of the address
      // later.
    }
  }

  uint32_t remote_generation = 0;
  // The STUN binding request may arrive after setRemoteDescription and before
  // adding remote candidate, so we need to set the password to the shared
  // password if the user name matches.
  if (remote_password.empty()) {
    const IceParameters* ice_param =
        FindRemoteIceFromUfrag(remote_username, &remote_generation);
    // Note: if not found, the remote_generation will still be 0.
    if (ice_param != nullptr) {
      remote_password = ice_param->pwd;
    }
  }

  Candidate remote_candidate;
  bool remote_candidate_is_new = (candidate == nullptr);
  if (!remote_candidate_is_new) {
    remote_candidate = *candidate;
    if (ufrag_per_port) {
      remote_candidate.set_address(address);
    }
  } else {
    // Create a new candidate with this address.
    // The priority of the candidate is set to the PRIORITY attribute
    // from the request.
    const StunUInt32Attribute* priority_attr =
        stun_msg->GetUInt32(STUN_ATTR_PRIORITY);
    if (!priority_attr) {
      LOG(LS_WARNING) << "P2PTransportChannel::OnUnknownAddress - "
                      << "No STUN_ATTR_PRIORITY found in the "
                      << "stun request message";
      port->SendBindingErrorResponse(stun_msg, address, STUN_ERROR_BAD_REQUEST,
                                     STUN_ERROR_REASON_BAD_REQUEST);
      return;
    }
    int remote_candidate_priority = priority_attr->value();

    uint16_t network_id = 0;
    uint16_t network_cost = 0;
    const StunUInt32Attribute* network_attr =
        stun_msg->GetUInt32(STUN_ATTR_NETWORK_INFO);
    if (network_attr) {
      uint32_t network_info = network_attr->value();
      network_id = static_cast<uint16_t>(network_info >> 16);
      network_cost = static_cast<uint16_t>(network_info);
    }

    // RFC 5245
    // If the source transport address of the request does not match any
    // existing remote candidates, it represents a new peer reflexive remote
    // candidate.
    remote_candidate = Candidate(
        component(), ProtoToString(proto), address, remote_candidate_priority,
        remote_username, remote_password, PRFLX_PORT_TYPE, remote_generation,
        "", network_id, network_cost);

    // From RFC 5245, section-7.2.1.3:
    // The foundation of the candidate is set to an arbitrary value, different
    // from the foundation for all other remote candidates.
    remote_candidate.set_foundation(
        rtc::ToString<uint32_t>(rtc::ComputeCrc32(remote_candidate.id())));
  }

  // RFC5245, the agent constructs a pair whose local candidate is equal to
  // the transport address on which the STUN request was received, and a
  // remote candidate equal to the source transport address where the
  // request came from.

  // There shouldn't be an existing connection with this remote address.
  // When ports are muxed, this channel might get multiple unknown address
  // signals. In that case if the connection is already exists, we should
  // simply ignore the signal otherwise send server error.
  if (port->GetConnection(remote_candidate.address())) {
    if (port_muxed) {
      LOG(LS_INFO) << "Connection already exists for peer reflexive "
                   << "candidate: " << remote_candidate.ToString();
      return;
    } else {
      ASSERT(false);
      port->SendBindingErrorResponse(stun_msg, address,
                                     STUN_ERROR_SERVER_ERROR,
                                     STUN_ERROR_REASON_SERVER_ERROR);
      return;
    }
  }

  Connection* connection =
      port->CreateConnection(remote_candidate, PortInterface::ORIGIN_THIS_PORT);
  if (!connection) {
    ASSERT(false);
    port->SendBindingErrorResponse(stun_msg, address, STUN_ERROR_SERVER_ERROR,
                                   STUN_ERROR_REASON_SERVER_ERROR);
    return;
  }

  LOG(LS_INFO) << "Adding connection from "
               << (remote_candidate_is_new ? "peer reflexive" : "resurrected")
               << " candidate: " << remote_candidate.ToString();
  AddConnection(connection);
  connection->HandleBindingRequest(stun_msg);

  // Update the list of connections since we just added another.  We do this
  // after sending the response since it could (in principle) delete the
  // connection in question.
  SortConnections();
}

void P2PTransportChannel::OnRoleConflict(PortInterface* port) {
  SignalRoleConflict(this);  // STUN ping will be sent when SetRole is called
                             // from Transport.
}

const IceParameters* P2PTransportChannel::FindRemoteIceFromUfrag(
    const std::string& ufrag,
    uint32_t* generation) {
  const auto& params = remote_ice_parameters_;
  auto it = std::find_if(
      params.rbegin(), params.rend(),
      [ufrag](const IceParameters& param) { return param.ufrag == ufrag; });
  if (it == params.rend()) {
    // Not found.
    return nullptr;
  }
  *generation = params.rend() - it - 1;
  return &(*it);
}

void P2PTransportChannel::OnNominated(Connection* conn) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  ASSERT(ice_role_ == ICEROLE_CONTROLLED);

  if (conn->write_state() == Connection::STATE_WRITABLE) {
    if (best_connection_ != conn) {
      pending_best_connection_ = NULL;
      LOG(LS_INFO) << "Switching best connection on controlled side: "
                   << conn->ToString();
      SwitchBestConnectionTo(conn);
      // Now we have selected the best connection, time to prune other existing
      // connections and update the read/write state of the channel.
      RequestSort();
    }
  } else {
    LOG(LS_INFO) << "Not switching the best connection on controlled side yet,"
                 << " because it's not writable: " << conn->ToString();
    pending_best_connection_ = conn;
  }
}

void P2PTransportChannel::AddRemoteCandidate(const Candidate& candidate) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  uint32_t generation = GetRemoteCandidateGeneration(candidate);
  // If a remote candidate with a previous generation arrives, drop it.
  if (generation < remote_ice_generation()) {
    LOG(LS_WARNING) << "Dropping a remote candidate because its ufrag "
                    << candidate.username()
                    << " indicates it was for a previous generation.";
    return;
  }

  Candidate new_remote_candidate(candidate);
  new_remote_candidate.set_generation(generation);
  // ICE candidates don't need to have username and password set, but
  // the code below this (specifically, ConnectionRequest::Prepare in
  // port.cc) uses the remote candidates's username.  So, we set it
  // here.
  if (remote_ice()) {
    if (candidate.username().empty()) {
      new_remote_candidate.set_username(remote_ice()->ufrag);
    }
    if (new_remote_candidate.username() == remote_ice()->ufrag) {
      if (candidate.password().empty()) {
        new_remote_candidate.set_password(remote_ice()->pwd);
      }
    } else {
      // The candidate belongs to the next generation. Its pwd will be set
      // when the new remote ICE credentials arrive.
      LOG(LS_WARNING) << "A remote candidate arrives with an unknown ufrag: "
                      << candidate.username();
    }
  }

  // Create connections to this remote candidate.
  CreateConnections(new_remote_candidate, NULL);

  // Resort the connections list, which may have new elements.
  SortConnections();
}

void P2PTransportChannel::RemoveRemoteCandidate(
    const Candidate& cand_to_remove) {
  auto iter =
      std::remove_if(remote_candidates_.begin(), remote_candidates_.end(),
                     [cand_to_remove](const Candidate& candidate) {
                       return cand_to_remove.MatchesForRemoval(candidate);
                     });
  if (iter != remote_candidates_.end()) {
    LOG(LS_VERBOSE) << "Removed remote candidate " << cand_to_remove.ToString();
    remote_candidates_.erase(iter, remote_candidates_.end());
  }
}

// Creates connections from all of the ports that we care about to the given
// remote candidate.  The return value is true if we created a connection from
// the origin port.
bool P2PTransportChannel::CreateConnections(const Candidate& remote_candidate,
                                            PortInterface* origin_port) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // If we've already seen the new remote candidate (in the current candidate
  // generation), then we shouldn't try creating connections for it.
  // We either already have a connection for it, or we previously created one
  // and then later pruned it. If we don't return, the channel will again
  // re-create any connections that were previously pruned, which will then
  // immediately be re-pruned, churning the network for no purpose.
  // This only applies to candidates received over signaling (i.e. origin_port
  // is NULL).
  if (!origin_port && IsDuplicateRemoteCandidate(remote_candidate)) {
    // return true to indicate success, without creating any new connections.
    return true;
  }

  // Add a new connection for this candidate to every port that allows such a
  // connection (i.e., if they have compatible protocols) and that does not
  // already have a connection to an equivalent candidate.  We must be careful
  // to make sure that the origin port is included, even if it was pruned,
  // since that may be the only port that can create this connection.
  bool created = false;
  std::vector<PortInterface *>::reverse_iterator it;
  for (it = ports_.rbegin(); it != ports_.rend(); ++it) {
    if (CreateConnection(*it, remote_candidate, origin_port)) {
      if (*it == origin_port)
        created = true;
    }
  }

  if ((origin_port != NULL) &&
      std::find(ports_.begin(), ports_.end(), origin_port) == ports_.end()) {
    if (CreateConnection(origin_port, remote_candidate, origin_port))
      created = true;
  }

  // Remember this remote candidate so that we can add it to future ports.
  RememberRemoteCandidate(remote_candidate, origin_port);

  return created;
}

// Setup a connection object for the local and remote candidate combination.
// And then listen to connection object for changes.
bool P2PTransportChannel::CreateConnection(PortInterface* port,
                                           const Candidate& remote_candidate,
                                           PortInterface* origin_port) {
  if (!port->SupportsProtocol(remote_candidate.protocol())) {
    return false;
  }
  // Look for an existing connection with this remote address.  If one is not
  // found or it is found but the existing remote candidate has an older
  // generation, then we can create a new connection for this address.
  Connection* connection = port->GetConnection(remote_candidate.address());
  if (connection == nullptr ||
      connection->remote_candidate().generation() <
          remote_candidate.generation()) {
    // Don't create a connection if this is a candidate we received in a
    // message and we are not allowed to make outgoing connections.
    PortInterface::CandidateOrigin origin = GetOrigin(port, origin_port);
    if (origin == PortInterface::ORIGIN_MESSAGE && incoming_only_) {
      return false;
    }
    Connection* connection = port->CreateConnection(remote_candidate, origin);
    if (!connection) {
      return false;
    }
    AddConnection(connection);
    LOG_J(LS_INFO, this) << "Created connection with origin=" << origin << ", ("
                         << connections_.size() << " total)";
    return true;
  }

  // No new connection was created.
  // Check if this is a peer reflexive candidate.
  connection->MaybeUpdatePeerReflexiveCandidate(remote_candidate);

  // It is not legal to try to change any of the parameters of an existing
  // connection; however, the other side can send a duplicate candidate.
  if (!remote_candidate.IsEquivalent(connection->remote_candidate())) {
    LOG(INFO) << "Attempt to change a remote candidate."
              << " Existing remote candidate: "
              << connection->remote_candidate().ToString()
              << "New remote candidate: " << remote_candidate.ToString();
  }
  return false;
}

bool P2PTransportChannel::FindConnection(Connection* connection) const {
  std::vector<Connection*>::const_iterator citer =
      std::find(connections_.begin(), connections_.end(), connection);
  return citer != connections_.end();
}

uint32_t P2PTransportChannel::GetRemoteCandidateGeneration(
    const Candidate& candidate) {
  // If the candidate has a ufrag, use it to find the generation.
  if (!candidate.username().empty()) {
    uint32_t generation = 0;
    if (!FindRemoteIceFromUfrag(candidate.username(), &generation)) {
      // If the ufrag is not found, assume the next/future generation.
      generation = static_cast<uint32_t>(remote_ice_parameters_.size());
    }
    return generation;
  }
  // If candidate generation is set, use that.
  if (candidate.generation() > 0) {
    return candidate.generation();
  }
  // Otherwise, assume the generation from remote ice parameters.
  return remote_ice_generation();
}

// Check if remote candidate is already cached.
bool P2PTransportChannel::IsDuplicateRemoteCandidate(
    const Candidate& candidate) {
  for (size_t i = 0; i < remote_candidates_.size(); ++i) {
    if (remote_candidates_[i].IsEquivalent(candidate)) {
      return true;
    }
  }
  return false;
}

// Maintain our remote candidate list, adding this new remote one.
void P2PTransportChannel::RememberRemoteCandidate(
    const Candidate& remote_candidate, PortInterface* origin_port) {
  // Remove any candidates whose generation is older than this one.  The
  // presence of a new generation indicates that the old ones are not useful.
  size_t i = 0;
  while (i < remote_candidates_.size()) {
    if (remote_candidates_[i].generation() < remote_candidate.generation()) {
      LOG(INFO) << "Pruning candidate from old generation: "
                << remote_candidates_[i].address().ToSensitiveString();
      remote_candidates_.erase(remote_candidates_.begin() + i);
    } else {
      i += 1;
    }
  }

  // Make sure this candidate is not a duplicate.
  if (IsDuplicateRemoteCandidate(remote_candidate)) {
    LOG(INFO) << "Duplicate candidate: " << remote_candidate.ToString();
    return;
  }

  // Try this candidate for all future ports.
  remote_candidates_.push_back(RemoteCandidate(remote_candidate, origin_port));
}

// Set options on ourselves is simply setting options on all of our available
// port objects.
int P2PTransportChannel::SetOption(rtc::Socket::Option opt, int value) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  OptionMap::iterator it = options_.find(opt);
  if (it == options_.end()) {
    options_.insert(std::make_pair(opt, value));
  } else if (it->second == value) {
    return 0;
  } else {
    it->second = value;
  }

  for (size_t i = 0; i < ports_.size(); ++i) {
    int val = ports_[i]->SetOption(opt, value);
    if (val < 0) {
      // Because this also occurs deferred, probably no point in reporting an
      // error
      LOG(WARNING) << "SetOption(" << opt << ", " << value << ") failed: "
                   << ports_[i]->GetError();
    }
  }
  return 0;
}

bool P2PTransportChannel::GetOption(rtc::Socket::Option opt, int* value) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  const auto& found = options_.find(opt);
  if (found == options_.end()) {
    return false;
  }
  *value = found->second;
  return true;
}

// Send data to the other side, using our best connection.
int P2PTransportChannel::SendPacket(const char *data, size_t len,
                                    const rtc::PacketOptions& options,
                                    int flags) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  if (flags != 0) {
    error_ = EINVAL;
    return -1;
  }
  if (best_connection_ == NULL) {
    error_ = EWOULDBLOCK;
    return -1;
  }

  last_sent_packet_id_ = options.packet_id;
  int sent = best_connection_->Send(data, len, options);
  if (sent <= 0) {
    ASSERT(sent < 0);
    error_ = best_connection_->GetError();
  }
  return sent;
}

bool P2PTransportChannel::GetStats(ConnectionInfos *infos) {
  ASSERT(worker_thread_ == rtc::Thread::Current());
  // Gather connection infos.
  infos->clear();

  for (Connection* connection : connections_) {
    ConnectionInfo info = connection->stats();
    info.best_connection = (best_connection_ == connection);
    info.receiving = connection->receiving();
    info.writable = (connection->write_state() == Connection::STATE_WRITABLE);
    info.timeout =
        (connection->write_state() == Connection::STATE_WRITE_TIMEOUT);
    info.new_connection = !connection->reported();
    connection->set_reported(true);
    info.rtt = connection->rtt();
    info.local_candidate = connection->local_candidate();
    info.remote_candidate = connection->remote_candidate();
    info.key = connection;
    infos->push_back(info);
  }

  return true;
}

rtc::DiffServCodePoint P2PTransportChannel::DefaultDscpValue() const {
  OptionMap::const_iterator it = options_.find(rtc::Socket::OPT_DSCP);
  if (it == options_.end()) {
    return rtc::DSCP_NO_CHANGE;
  }
  return static_cast<rtc::DiffServCodePoint> (it->second);
}

// Monitor connection states.
void P2PTransportChannel::UpdateConnectionStates() {
  int64_t now = rtc::TimeMillis();

  // We need to copy the list of connections since some may delete themselves
  // when we call UpdateState.
  for (size_t i = 0; i < connections_.size(); ++i)
    connections_[i]->UpdateState(now);
}

// Prepare for best candidate sorting.
void P2PTransportChannel::RequestSort() {
  if (!sort_dirty_) {
    worker_thread_->Post(RTC_FROM_HERE, this, MSG_SORT);
    sort_dirty_ = true;
  }
}

// Sort the available connections to find the best one.  We also monitor
// the number of available connections and the current state.
void P2PTransportChannel::SortConnections() {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // Make sure the connection states are up-to-date since this affects how they
  // will be sorted.
  UpdateConnectionStates();

  // Any changes after this point will require a re-sort.
  sort_dirty_ = false;

  // 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.
  ConnectionCompare cmp;
  std::stable_sort(connections_.begin(), connections_.end(), cmp);
  LOG(LS_VERBOSE) << "Sorting " << connections_.size()
                  << " available connections:";
  for (size_t i = 0; i < connections_.size(); ++i) {
    LOG(LS_VERBOSE) << connections_[i]->ToString();
  }

  Connection* top_connection =
      (connections_.size() > 0) ? connections_[0] : nullptr;

  // If necessary, switch to the new choice.
  // Note that |top_connection| doesn't have to be writable to become the best
  // connection although it will have higher priority if it is writable.
  if (ShouldSwitch(best_connection_, top_connection, ice_role_)) {
    LOG(LS_INFO) << "Switching best connection: " << top_connection->ToString();
    SwitchBestConnectionTo(top_connection);
  }

  // Controlled side can prune only if the best connection has been nominated.
  // because otherwise it may delete the connection that will be selected by
  // the controlling side.
  if (ice_role_ == ICEROLE_CONTROLLING || best_nominated_connection()) {
    PruneConnections();
  }

  // Check if all connections are timedout.
  bool all_connections_timedout = true;
  for (size_t i = 0; i < connections_.size(); ++i) {
    if (connections_[i]->write_state() != Connection::STATE_WRITE_TIMEOUT) {
      all_connections_timedout = false;
      break;
    }
  }

  // Now update the writable state of the channel with the information we have
  // so far.
  if (all_connections_timedout) {
    HandleAllTimedOut();
  }

  // Update the state of this channel.  This method is called whenever the
  // state of any connection changes, so this is a good place to do this.
  UpdateState();
}

Connection* P2PTransportChannel::best_nominated_connection() const {
  return (best_connection_ && best_connection_->nominated()) ? best_connection_
                                                             : nullptr;
}

void P2PTransportChannel::PruneConnections() {
  // We can prune any connection for which there is a connected, writable
  // connection on the same network with better or equal priority.  We leave
  // those with better priority just in case they become writable later (at
  // which point, we would prune out the current best connection).  We leave
  // connections on other networks because they may not be using the same
  // resources and they may represent very distinct paths over which we can
  // switch. If the |premier| connection is not connected, we may be
  // reconnecting a TCP connection and temporarily do not prune connections in
  // this network. See the big comment in CompareConnections.

  // Get a list of the networks that we are using.
  std::set<rtc::Network*> networks;
  for (const Connection* conn : connections_) {
    networks.insert(conn->port()->Network());
  }
  for (rtc::Network* network : networks) {
    Connection* premier = GetBestConnectionOnNetwork(network);
    // Do not prune connections if the current best connection is weak on this
    // network. Otherwise, it may delete connections prematurely.
    if (!premier || premier->weak()) {
      continue;
    }

    for (Connection* conn : connections_) {
      if ((conn != premier) && (conn->port()->Network() == network) &&
          (CompareConnectionCandidates(premier, conn) >= 0)) {
        conn->Prune();
      }
    }
  }
}

// Track the best connection, and let listeners know
void P2PTransportChannel::SwitchBestConnectionTo(Connection* conn) {
  // Note: if conn is NULL, the previous best_connection_ has been destroyed,
  // so don't use it.
  Connection* old_best_connection = best_connection_;
  best_connection_ = conn;
  if (best_connection_) {
    if (old_best_connection) {
      LOG_J(LS_INFO, this) << "Previous best connection: "
                           << old_best_connection->ToString();
    }
    LOG_J(LS_INFO, this) << "New best connection: "
                         << best_connection_->ToString();
    SignalRouteChange(this, best_connection_->remote_candidate());
    // This is a temporary, but safe fix to webrtc issue 5705.
    // TODO(honghaiz): Make all EWOULDBLOCK error routed through the transport
    // channel so that it knows whether the media channel is allowed to
    // send; then it will only signal ready-to-send if the media channel
    // has been disallowed to send.
    if (best_connection_->writable()) {
      SignalReadyToSend(this);
    }
  } else {
    LOG_J(LS_INFO, this) << "No best connection";
  }
  // TODO(honghaiz): rename best_connection_ with selected_connection_ or
  // selected_candidate pair_.
  SignalSelectedCandidatePairChanged(this, best_connection_,
                                     last_sent_packet_id_);
}

// Warning: UpdateState should eventually be called whenever a connection
// is added, deleted, or the write state of any connection changes so that the
// transport controller will get the up-to-date channel state. However it
// should not be called too often; in the case that multiple connection states
// change, it should be called after all the connection states have changed. For
// example, we call this at the end of SortConnections.
void P2PTransportChannel::UpdateState() {
  TransportChannelState state = ComputeState();
  if (state_ != state) {
    LOG_J(LS_INFO, this) << "Transport channel state changed from " << state_
                         << " to " << state;
    // Check that the requested transition is allowed. Note that
    // P2PTransportChannel does not (yet) implement a direct mapping of the ICE
    // states from the standard; the difference is covered by
    // TransportController and PeerConnection.
    switch (state_) {
      case STATE_INIT:
        // TODO(deadbeef): Once we implement end-of-candidates signaling,
        // we shouldn't go from INIT to COMPLETED.
        RTC_DCHECK(state == STATE_CONNECTING || state == STATE_COMPLETED);
        break;
      case STATE_CONNECTING:
        RTC_DCHECK(state == STATE_COMPLETED || state == STATE_FAILED);
        break;
      case STATE_COMPLETED:
        // TODO(deadbeef): Once we implement end-of-candidates signaling,
        // we shouldn't go from COMPLETED to CONNECTING.
        // Though we *can* go from COMPlETED to FAILED, if consent expires.
        RTC_DCHECK(state == STATE_CONNECTING || state == STATE_FAILED);
        break;
      case STATE_FAILED:
        // TODO(deadbeef): Once we implement end-of-candidates signaling,
        // we shouldn't go from FAILED to CONNECTING or COMPLETED.
        RTC_DCHECK(state == STATE_CONNECTING || state == STATE_COMPLETED);
        break;
      default:
        RTC_DCHECK(false);
        break;
    }
    state_ = state;
    SignalStateChanged(this);
  }

  bool writable = best_connection_ && best_connection_->writable();
  set_writable(writable);

  bool receiving = false;
  for (const Connection* connection : connections_) {
    if (connection->receiving()) {
      receiving = true;
      break;
    }
  }
  set_receiving(receiving);
}

void P2PTransportChannel::MaybeStopPortAllocatorSessions() {
  if (!IsGettingPorts()) {
    return;
  }

  for (const auto& session : allocator_sessions_) {
    if (!session->IsGettingPorts()) {
      continue;
    }
    // If gathering continually, keep the last session running so that it
    // will gather candidates if the networks change.
    if (config_.gather_continually && session == allocator_sessions_.back()) {
      session->ClearGettingPorts();
      break;
    }
    session->StopGettingPorts();
  }
}

// If all connections timed out, delete them all.
void P2PTransportChannel::HandleAllTimedOut() {
  for (Connection* connection : connections_) {
    connection->Destroy();
  }
}

bool P2PTransportChannel::weak() const {
  return !best_connection_ || best_connection_->weak();
}

// If we have a best connection, return it, otherwise return top one in the
// list (later we will mark it best).
Connection* P2PTransportChannel::GetBestConnectionOnNetwork(
    rtc::Network* network) const {
  // If the best connection is on this network, then it wins.
  if (best_connection_ && (best_connection_->port()->Network() == network))
    return best_connection_;

  // Otherwise, we return the top-most in sorted order.
  for (size_t i = 0; i < connections_.size(); ++i) {
    if (connections_[i]->port()->Network() == network)
      return connections_[i];
  }

  return NULL;
}

// Handle any queued up requests
void P2PTransportChannel::OnMessage(rtc::Message *pmsg) {
  switch (pmsg->message_id) {
    case MSG_SORT:
      OnSort();
      break;
    case MSG_CHECK_AND_PING:
      OnCheckAndPing();
      break;
    default:
      ASSERT(false);
      break;
  }
}

// Handle queued up sort request
void P2PTransportChannel::OnSort() {
  // Resort the connections based on the new statistics.
  SortConnections();
}

// Handle queued up check-and-ping request
void P2PTransportChannel::OnCheckAndPing() {
  // Make sure the states of the connections are up-to-date (since this affects
  // which ones are pingable).
  UpdateConnectionStates();
  // When the best connection is not receiving or not writable, or any active
  // connection has not been pinged enough times, use the weak ping interval.
  bool need_more_pings_at_weak_interval = std::any_of(
      connections_.begin(), connections_.end(), [](Connection* conn) {
        return conn->active() &&
               conn->num_pings_sent() < MIN_PINGS_AT_WEAK_PING_INTERVAL;
      });
  int ping_interval = (weak() || need_more_pings_at_weak_interval)
                          ? weak_ping_interval_
                          : STRONG_PING_INTERVAL;
  if (rtc::TimeMillis() >= last_ping_sent_ms_ + ping_interval) {
    Connection* conn = FindNextPingableConnection();
    if (conn) {
      PingConnection(conn);
      MarkConnectionPinged(conn);
    }
  }
  int delay = std::min(ping_interval, check_receiving_interval_);
  thread()->PostDelayed(RTC_FROM_HERE, delay, this, MSG_CHECK_AND_PING);
}

// A connection is considered a backup connection if the channel state
// is completed, the connection is not the best connection and it is active.
bool P2PTransportChannel::IsBackupConnection(Connection* conn) const {
  return state_ == STATE_COMPLETED && conn != best_connection_ &&
         conn->active();
}

// Is the connection in a state for us to even consider pinging the other side?
// We consider a connection pingable even if it's not connected because that's
// how a TCP connection is kicked into reconnecting on the active side.
bool P2PTransportChannel::IsPingable(Connection* conn, int64_t now) {
  const Candidate& remote = conn->remote_candidate();
  // We should never get this far with an empty remote ufrag.
  ASSERT(!remote.username().empty());
  if (remote.username().empty() || remote.password().empty()) {
    // If we don't have an ICE ufrag and pwd, there's no way we can ping.
    return false;
  }

  // An never connected connection cannot be written to at all, so pinging is
  // out of the question. However, if it has become WRITABLE, it is in the
  // reconnecting state so ping is needed.
  if (!conn->connected() && !conn->writable()) {
    return false;
  }

  // If the channel is weakly connected, ping all connections.
  if (weak()) {
    return true;
  }

  // Always ping active connections regardless whether the channel is completed
  // or not, but backup connections are pinged at a slower rate.
  if (IsBackupConnection(conn)) {
    return (now >= conn->last_ping_response_received() +
                       config_.backup_connection_ping_interval);
  }
  return conn->active();
}

// Returns the next pingable connection to ping.  This will be the oldest
// pingable connection unless we have a connected, writable connection that is
// past the maximum acceptable ping interval. When reconnecting a TCP
// connection, the best connection is disconnected, although still WRITABLE
// while reconnecting. The newly created connection should be selected as the
// ping target to become writable instead. See the big comment in
// CompareConnections.
Connection* P2PTransportChannel::FindNextPingableConnection() {
  int64_t now = rtc::TimeMillis();
  Connection* conn_to_ping = nullptr;
  if (best_connection_ && best_connection_->connected() &&
      best_connection_->writable() &&
      (best_connection_->last_ping_sent() + config_.max_strong_interval <=
       now)) {
    conn_to_ping = best_connection_;
  } else {
    conn_to_ping = FindConnectionToPing(now);
  }
  return conn_to_ping;
}

void P2PTransportChannel::MarkConnectionPinged(Connection* conn) {
  if (conn && pinged_connections_.insert(conn).second) {
    unpinged_connections_.erase(conn);
  }
}

// Apart from sending ping from |conn| this method also updates
// |use_candidate_attr| flag. The criteria to update this flag is
// explained below.
// Set USE-CANDIDATE if doing ICE AND this channel is in CONTROLLING AND
//    a) Channel is in FULL ICE AND
//      a.1) |conn| is the best connection OR
//      a.2) there is no best connection OR
//      a.3) the best connection is unwritable OR
//      a.4) |conn| has higher priority than best_connection.
//    b) we're doing LITE ICE AND
//      b.1) |conn| is the best_connection AND
//      b.2) |conn| is writable.
void P2PTransportChannel::PingConnection(Connection* conn) {
  bool use_candidate = false;
  if (remote_ice_mode_ == ICEMODE_FULL && ice_role_ == ICEROLE_CONTROLLING) {
    use_candidate = (conn == best_connection_) || (best_connection_ == NULL) ||
                    (!best_connection_->writable()) ||
                    (CompareConnectionCandidates(best_connection_, conn) < 0);
  } else if (remote_ice_mode_ == ICEMODE_LITE && conn == best_connection_) {
    use_candidate = best_connection_->writable();
  }
  conn->set_use_candidate_attr(use_candidate);
  last_ping_sent_ms_ = rtc::TimeMillis();
  conn->Ping(last_ping_sent_ms_);
}

// When a connection's state changes, we need to figure out who to use as
// the best connection again.  It could have become usable, or become unusable.
void P2PTransportChannel::OnConnectionStateChange(Connection* connection) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // Update the best connection if the state change is from pending best
  // connection and role is controlled.
  if (ice_role_ == ICEROLE_CONTROLLED) {
    if (connection == pending_best_connection_ && connection->writable()) {
      pending_best_connection_ = NULL;
      LOG(LS_INFO) << "Switching best connection on controlled side"
                   << " because it's now writable: " << connection->ToString();
      SwitchBestConnectionTo(connection);
    }
  }

  // May stop the allocator session when at least one connection becomes
  // strongly connected after starting to get ports and the local candidate of
  // the connection is at the latest generation. It is not enough to check
  // that the connection becomes weakly connected because the connection may be
  // changing from (writable, receiving) to (writable, not receiving).
  bool strongly_connected = !connection->weak();
  bool latest_generation = connection->local_candidate().generation() >=
                           allocator_session()->generation();
  if (strongly_connected && latest_generation) {
    MaybeStopPortAllocatorSessions();
  }

  // We have to unroll the stack before doing this because we may be changing
  // the state of connections while sorting.
  RequestSort();
}

// When a connection is removed, edit it out, and then update our best
// connection.
void P2PTransportChannel::OnConnectionDestroyed(Connection* connection) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // Note: the previous best_connection_ may be destroyed by now, so don't
  // use it.

  // Remove this connection from the list.
  std::vector<Connection*>::iterator iter =
      std::find(connections_.begin(), connections_.end(), connection);
  ASSERT(iter != connections_.end());
  pinged_connections_.erase(*iter);
  unpinged_connections_.erase(*iter);
  connections_.erase(iter);

  LOG_J(LS_INFO, this) << "Removed connection ("
    << static_cast<int>(connections_.size()) << " remaining)";

  if (pending_best_connection_ == connection) {
    pending_best_connection_ = NULL;
  }

  // If this is currently the best connection, then we need to pick a new one.
  // The call to SortConnections will pick a new one.  It looks at the current
  // best connection in order to avoid switching between fairly similar ones.
  // Since this connection is no longer an option, we can just set best to NULL
  // and re-choose a best assuming that there was no best connection.
  if (best_connection_ == connection) {
    LOG(LS_INFO) << "Best connection destroyed.  Will choose a new one.";
    SwitchBestConnectionTo(NULL);
    RequestSort();
  }

  UpdateState();
}

// When a port is destroyed remove it from our list of ports to use for
// connection attempts.
void P2PTransportChannel::OnPortDestroyed(PortInterface* port) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // Remove this port from the list (if we didn't drop it already).
  std::vector<PortInterface*>::iterator iter =
      std::find(ports_.begin(), ports_.end(), port);
  if (iter != ports_.end())
    ports_.erase(iter);

  LOG(INFO) << "Removed port from p2p socket: "
            << static_cast<int>(ports_.size()) << " remaining";
}

void P2PTransportChannel::OnPortNetworkInactive(PortInterface* port) {
  // If it does not gather continually, the port will be removed from the list
  // when ICE restarts.
  if (!config_.gather_continually) {
    return;
  }
  auto it = std::find(ports_.begin(), ports_.end(), port);
  // Don't need to do anything if the port has been deleted from the port list.
  if (it == ports_.end()) {
    return;
  }
  ports_.erase(it);
  LOG(INFO) << "Removed port due to inactive networks: " << ports_.size()
            << " remaining";
  std::vector<Candidate> candidates = port->Candidates();
  for (Candidate& candidate : candidates) {
    candidate.set_transport_name(transport_name());
  }
  SignalCandidatesRemoved(this, candidates);
}

// We data is available, let listeners know
void P2PTransportChannel::OnReadPacket(Connection* connection,
                                       const char* data,
                                       size_t len,
                                       const rtc::PacketTime& packet_time) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  // Do not deliver, if packet doesn't belong to the correct transport channel.
  if (!FindConnection(connection))
    return;

  // Let the client know of an incoming packet
  SignalReadPacket(this, data, len, packet_time, 0);

  // May need to switch the sending connection based on the receiving media path
  // if this is the controlled side.
  if (ice_role_ == ICEROLE_CONTROLLED && !best_nominated_connection() &&
      connection->writable() && best_connection_ != connection) {
    SwitchBestConnectionTo(connection);
  }
}

void P2PTransportChannel::OnSentPacket(const rtc::SentPacket& sent_packet) {
  ASSERT(worker_thread_ == rtc::Thread::Current());

  SignalSentPacket(this, sent_packet);
}

void P2PTransportChannel::OnReadyToSend(Connection* connection) {
  if (connection == best_connection_ && writable()) {
    SignalReadyToSend(this);
  }
}

// Find "triggered checks".  We ping first those connections that have
// received a ping but have not sent a ping since receiving it
// (last_received_ping > last_sent_ping).  But we shouldn't do
// triggered checks if the connection is already writable.
Connection* P2PTransportChannel::FindOldestConnectionNeedingTriggeredCheck(
    int64_t now) {
  Connection* oldest_needing_triggered_check = nullptr;
  for (auto conn : connections_) {
    if (!IsPingable(conn, now)) {
      continue;
    }
    bool needs_triggered_check =
        (!conn->writable() &&
         conn->last_ping_received() > conn->last_ping_sent());
    if (needs_triggered_check &&
        (!oldest_needing_triggered_check ||
         (conn->last_ping_received() <
          oldest_needing_triggered_check->last_ping_received()))) {
      oldest_needing_triggered_check = conn;
    }
  }

  if (oldest_needing_triggered_check) {
    LOG(LS_INFO) << "Selecting connection for triggered check: "
                 << oldest_needing_triggered_check->ToString();
  }
  return oldest_needing_triggered_check;
}

Connection* P2PTransportChannel::FindConnectionToPing(int64_t now) {
  RTC_CHECK(connections_.size() ==
            pinged_connections_.size() + unpinged_connections_.size());

  // If there is nothing pingable in the |unpinged_connections_|, copy
  // over from |pinged_connections_|. We do this here such that the
  // new connection will take precedence.
  if (std::find_if(unpinged_connections_.begin(), unpinged_connections_.end(),
                   [this, now](Connection* conn) {
                     return this->IsPingable(conn, now);
                   }) == unpinged_connections_.end()) {
    unpinged_connections_.insert(pinged_connections_.begin(),
                                 pinged_connections_.end());
    pinged_connections_.clear();
  }

  Connection* conn_to_ping = FindOldestConnectionNeedingTriggeredCheck(now);
  if (conn_to_ping) {
    return conn_to_ping;
  }

  for (Connection* conn : unpinged_connections_) {
    if (!IsPingable(conn, now)) {
      continue;
    }
    if (!conn_to_ping ||
        SelectMostPingableConnection(conn_to_ping, conn) == conn) {
      conn_to_ping = conn;
    }
  }
  return conn_to_ping;
}

Connection* P2PTransportChannel::MostLikelyToWork(Connection* conn1,
                                                  Connection* conn2) {
  bool rr1 = IsRelayRelay(conn1);
  bool rr2 = IsRelayRelay(conn2);
  if (rr1 && !rr2) {
    return conn1;
  } else if (rr2 && !rr1) {
    return conn2;
  } else if (rr1 && rr2) {
    bool udp1 = IsUdp(conn1);
    bool udp2 = IsUdp(conn2);
    if (udp1 && !udp2) {
      return conn1;
    } else if (udp2 && udp1) {
      return conn2;
    }
  }
  return nullptr;
}

Connection* P2PTransportChannel::LeastRecentlyPinged(Connection* conn1,
                                                     Connection* conn2) {
  if (conn1->last_ping_sent() < conn2->last_ping_sent()) {
    return conn1;
  }
  if (conn1->last_ping_sent() > conn2->last_ping_sent()) {
    return conn2;
  }
  return nullptr;
}

Connection* P2PTransportChannel::SelectMostPingableConnection(
    Connection* conn1,
    Connection* conn2) {
  RTC_DCHECK(conn1 != conn2);
  if (config_.prioritize_most_likely_candidate_pairs) {
    Connection* most_likely_to_work_conn = MostLikelyToWork(conn1, conn2);
    if (most_likely_to_work_conn) {
      return most_likely_to_work_conn;
    }
  }

  Connection* least_recently_pinged_conn = LeastRecentlyPinged(conn1, conn2);
  if (least_recently_pinged_conn) {
    return least_recently_pinged_conn;
  }

  // During the initial state when nothing has been pinged yet, return the first
  // one in the ordered |connections_|.
  return *(std::find_if(connections_.begin(), connections_.end(),
                        [conn1, conn2](Connection* conn) {
                          return conn == conn1 || conn == conn2;
                        }));
}

}  // namespace cricket