/*

Copyright (c) 2015, Mikhail Titov

Copyright (c) 2004-2020, Arvid Norberg

Copyright (c) 2016-2018, 2020, Alden Torres

Copyright (c) 2016, Pavel Pimenov

Copyright (c) 2016-2017, Steven Siloti

All rights reserved.

Redistribution and use in source and binary forms, with or without

modification, are permitted provided that the following conditions

are met:

    * Redistributions of source code must retain the above copyright

      notice, this list of conditions and the following disclaimer.

    * Redistributions in binary form must reproduce the above copyright

      notice, this list of conditions and the following disclaimer in

      the documentation and/or other materials provided with the distribution.

    * Neither the name of the author nor the names of its

      contributors may be used to endorse or promote products derived

      from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE

LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE

POSSIBILITY OF SUCH DAMAGE.

*/

#include <cctype>

#include <functional>

#include <tuple>

#include "libtorrent/parse_url.hpp"

#include "libtorrent/udp_tracker_connection.hpp"

#include "libtorrent/hex.hpp"

#include "libtorrent/random.hpp"

#include "libtorrent/aux_/session_settings.hpp"

#include "libtorrent/aux_/resolver_interface.hpp"

#include "libtorrent/ip_filter.hpp"

#include "libtorrent/aux_/time.hpp"

#include "libtorrent/aux_/io.hpp"

#include "libtorrent/aux_/ip_helpers.hpp" // for is_v6

#include "libtorrent/peer.hpp"

#include "libtorrent/error_code.hpp"

#ifndef TORRENT_DISABLE_LOGGING

#include "libtorrent/socket_io.hpp"

#endif

namespace libtorrent {

  std::map<address, udp_tracker_connection::connection_cache_entry>

    udp_tracker_connection::m_connection_cache;

  std::mutex udp_tracker_connection::m_cache_mutex;

  udp_tracker_connection::udp_tracker_connection(

    io_context& ios

    , tracker_manager& man

    , tracker_request const& req

    , std::weak_ptr<request_callback> c)

    : tracker_connection(man, req, ios, std::move(c))

    , m_transaction_id(0)

    , m_attempts(0)

    , m_state(action_t::error)

    , m_abort(false)

  {

    update_transaction_id();

  }

  void udp_tracker_connection::start()

  {

    // TODO: 2 support authentication here. tracker_req().auth

    std::string hostname;

    std::string protocol;

    int port;

    error_code ec;

    std::tie(protocol, std::ignore, hostname, port, std::ignore)

      = parse_url_components(tracker_req().url, ec);

    if (port == -1) port = protocol == "http" ? 80 : 443;

    if (ec)

    {

      tracker_connection::fail(ec, operation_t::parse_address);

      return;

    }

    aux::session_settings const& settings = m_man.settings();

    int const proxy_type = settings.get_int(settings_pack::proxy_type);

    if (settings.get_bool(settings_pack::proxy_hostnames)

      && (proxy_type == settings_pack::socks5

        || proxy_type == settings_pack::socks5_pw))

    {

      m_hostname = hostname;

      m_target.port(std::uint16_t(port));

      start_announce();

    }

    else

    {

      using namespace std::placeholders;

      ADD_OUTSTANDING_ASYNC("udp_tracker_connection::name_lookup");

      // when stopping, pass in the cache-only flag, because we

      // don't want to get stuck on DNS lookups when shutting down

      m_man.host_resolver().async_resolve(hostname

        , (tracker_req().event == event_t::stopped

          ? aux::resolver_interface::cache_only : aux::resolver_flags{})

          | aux::resolver_interface::abort_on_shutdown

        , std::bind(&udp_tracker_connection::name_lookup

          , shared_from_this(), _1, _2, port));

#ifndef TORRENT_DISABLE_LOGGING

      std::shared_ptr<request_callback> cb = requester();

      if (cb) cb->debug_log("*** UDP_TRACKER [ initiating name lookup: \"%s\" ]"

        , hostname.c_str());

#endif

    }

    set_timeout(tracker_req().event == event_t::stopped

      ? settings.get_int(settings_pack::stop_tracker_timeout)

      : settings.get_int(settings_pack::tracker_completion_timeout)

      , settings.get_int(settings_pack::tracker_receive_timeout));

  }

  void udp_tracker_connection::fail(error_code const& ec, operation_t const op

    , char const* msg, seconds32 const interval, seconds32 const min_interval)

  {

    // m_target failed. remove it from the endpoint list

    auto const i = std::find(m_endpoints.begin()

      , m_endpoints.end(), make_tcp(m_target));

    if (i != m_endpoints.end()) m_endpoints.erase(i);

    // if that was the last one, or the listen socket was closed

    // fail the whole announce

    if (m_endpoints.empty() || !tracker_req().outgoing_socket)

    {

      tracker_connection::fail(ec, op, msg, interval, min_interval);

      return;

    }

#ifndef TORRENT_DISABLE_LOGGING

    std::shared_ptr<request_callback> cb = requester();

    if (cb && cb->should_log())

    {

      cb->debug_log(R"(*** UDP_TRACKER [ host: "%s" ip: "%s" | ERROR: "%s" ])"

        , m_hostname.c_str(), print_endpoint(m_target).c_str(), ec.message().c_str());

    }

#endif

    // pick another target endpoint and try again

    m_target = make_udp(m_endpoints.front());

#ifndef TORRENT_DISABLE_LOGGING

    if (cb && cb->should_log())

    {

      cb->debug_log(R"(*** UDP_TRACKER trying next IP [ host: "%s" ip: "%s" ])"

        , m_hostname.c_str(), print_endpoint(m_target).c_str());

    }

#endif

    post(get_executor(), std::bind(

      &udp_tracker_connection::start_announce, shared_from_this()));

    aux::session_settings const& settings = m_man.settings();

    set_timeout(tracker_req().event == event_t::stopped

      ? settings.get_int(settings_pack::stop_tracker_timeout)

      : settings.get_int(settings_pack::tracker_completion_timeout)

      , settings.get_int(settings_pack::tracker_receive_timeout));

  }

  void udp_tracker_connection::name_lookup(error_code const& error

    , std::vector<address> const& addresses, int port)

  {

    COMPLETE_ASYNC("udp_tracker_connection::name_lookup");

    if (m_abort) return;

    if (error == boost::asio::error::operation_aborted) return;

    if (error || addresses.empty())

    {

      fail(error, operation_t::hostname_lookup);

      return;

    }

    std::shared_ptr<request_callback> cb = requester();

#ifndef TORRENT_DISABLE_LOGGING

    if (cb) cb->debug_log("*** UDP_TRACKER [ name lookup successful ]");

#endif

    if (cancelled())

    {

      fail(error_code(errors::torrent_aborted), operation_t::hostname_lookup);

      return;

    }

    restart_read_timeout();

    if (!tracker_req().outgoing_socket)

    {

      fail(error_code(errors::invalid_listen_socket), operation_t::hostname_lookup);

      return;

    }

    auto const listen_socket = bind_socket();

    // filter all endpoints we cannot reach from this listen socket, which may

    // be all of them, in which case we should not announce this listen socket

    // to this tracker

    for (auto const& addr : addresses)

    {

      if (!listen_socket.can_route(addr)) continue;

      m_endpoints.emplace_back(addr, std::uint16_t(port));

    }

    if (m_endpoints.empty())

    {

      fail(lt::errors::announce_skipped, operation_t::hostname_lookup);

      return;

    }

    if (tracker_req().filter)

    {

      // remove endpoints that are filtered by the IP filter

      for (auto k = m_endpoints.begin(); k != m_endpoints.end();)

      {

        if (tracker_req().filter->access(k->address()) == ip_filter::blocked)

        {

#ifndef TORRENT_DISABLE_LOGGING

          if (cb && cb->should_log())

          {

            cb->debug_log("*** UDP_TRACKER [ IP blocked by filter: %s ]"

              , print_address(k->address()).c_str());

          }

#endif

          k = m_endpoints.erase(k);

        }

        else

          ++k;

      }

    }

    // if all endpoints were filtered by the IP filter, we can't connect

    if (m_endpoints.empty())

    {

      fail(error_code(errors::banned_by_ip_filter), operation_t::hostname_lookup);

      return;

    }

    m_target = make_udp(m_endpoints.front());

    start_announce();

  }

  void udp_tracker_connection::start_announce()

  {

    std::unique_lock<std::mutex> l(m_cache_mutex);

    auto const cc = m_connection_cache.find(m_target.address());

    if (cc != m_connection_cache.end())

    {

      // we found a cached entry! Now, we can only

      // use if if it hasn't expired

      if (aux::time_now() < cc->second.expires)

      {

        if (tracker_req().kind & tracker_request::scrape_request)

          send_udp_scrape();

        else

          send_udp_announce();

        return;

      }

      // if it expired, remove it from the cache

      m_connection_cache.erase(cc);

    }

    l.unlock();

    send_udp_connect();

  }

  void udp_tracker_connection::on_timeout(error_code const& ec)

  {

    if (ec)

    {

      fail(ec, operation_t::timer);

      return;

    }

#ifndef TORRENT_DISABLE_LOGGING

    std::shared_ptr<request_callback> cb = requester();

    if (cb) cb->debug_log("*** UDP_TRACKER [ timed out url: %s ]", tracker_req().url.c_str());

#endif

    fail(error_code(errors::timed_out), operation_t::timer);

  }

  void udp_tracker_connection::close()

  {

    cancel();

    m_man.remove_request(this);

  }

  bool udp_tracker_connection::on_receive_hostname(string_view const hostname

    , span<char const> buf)

  {

    TORRENT_UNUSED(hostname);

    // just ignore the hostname this came from, pretend that

    // it's from the same endpoint we sent it to (i.e. the same

    // port). We have so many other ways of confirming this packet

    // comes from the tracker anyway, so it's not a big deal

    return on_receive(m_target, buf);

  }

  bool udp_tracker_connection::on_receive(udp::endpoint const& ep

    , span<char const> const buf)

  {

#ifndef TORRENT_DISABLE_LOGGING

    std::shared_ptr<request_callback> cb = requester();

#endif

    // ignore responses before we've sent any requests

    if (m_state == action_t::error)

    {

#ifndef TORRENT_DISABLE_LOGGING

      if (cb) cb->debug_log("<== UDP_TRACKER [ m_action == error ]");

#endif

      return false;

    }

    if (m_abort)

    {

#ifndef TORRENT_DISABLE_LOGGING

      if (cb) cb->debug_log("<== UDP_TRACKER [ aborted]");

#endif

      return false;

    }

    // ignore packet not sent from the tracker

    // if m_target is inaddr_any, it suggests that we

    // sent the packet through a proxy only knowing

    // the hostname, in which case this packet might be for us

    if (!m_target.address().is_unspecified() && m_target != ep)

    {

#ifndef TORRENT_DISABLE_LOGGING

      if (cb && cb->should_log())

      {

        cb->debug_log("<== UDP_TRACKER [ unexpected source IP: %s "

          "expected: %s ]"

          , print_endpoint(ep).c_str()

          , print_endpoint(m_target).c_str());

      }

#endif

      return false;

    }

#ifndef TORRENT_DISABLE_LOGGING

    if (cb) cb->debug_log("<== UDP_TRACKER_PACKET [ size: %d ]"

      , int(buf.size()));

#endif

    // ignore packets smaller than 8 bytes

    if (buf.size() < 8) return false;

    span<const char> ptr = buf;

    auto const action = static_cast<action_t>(aux::read_int32(ptr));

    std::uint32_t const transaction = aux::read_uint32(ptr);

#ifndef TORRENT_DISABLE_LOGGING

    if (cb) cb->debug_log("*** UDP_TRACKER_PACKET [ action: %d ]"

      , static_cast<int>(action));

#endif

    // ignore packets with incorrect transaction id

    if (m_transaction_id != transaction)

    {

#ifndef TORRENT_DISABLE_LOGGING

    if (cb) cb->debug_log("*** UDP_TRACKER_PACKET [ tid: %x ]"

        , int(transaction));

#endif

      return false;

    }

    if (action == action_t::error)

    {

      fail(error_code(errors::tracker_failure), operation_t::bittorrent

        , std::string(buf.data(), static_cast<std::size_t>(buf.size())).c_str());

      return true;

    }

    // ignore packets that's not a response to our message

    if (action != m_state)

    {

#ifndef TORRENT_DISABLE_LOGGING

      if (cb) cb->debug_log("*** UDP_TRACKER_PACKET [ unexpected action: %d "

        " expected: %d ]", static_cast<int>(action), static_cast<int>(m_state));

#endif

      return false;

    }

    restart_read_timeout();

#ifndef TORRENT_DISABLE_LOGGING

    if (cb)

      cb->debug_log("*** UDP_TRACKER_RESPONSE [ tid: %x ]"

        , int(transaction));

#endif

    switch (m_state)

    {

      case action_t::connect:

        return on_connect_response(buf);

      case action_t::announce:

        return on_announce_response(buf);

      case action_t::scrape:

        return on_scrape_response(buf);

      case action_t::error:

        return false;

    }

    return false;

  }

  void udp_tracker_connection::update_transaction_id()

  {

    // don't use 0, because that has special meaning (uninitialized)

    std::uint32_t const new_tid = random(0xfffffffe) + 1;

    if (m_transaction_id != 0)

      m_man.update_transaction_id(shared_from_this(), new_tid);

    m_transaction_id = new_tid;

  }

  bool udp_tracker_connection::on_connect_response(span<char const> buf)

  {

    // ignore packets smaller than 16 bytes

    if (buf.size() < 16) return false;

    restart_read_timeout();

    // skip header

    buf = buf.subspan(8);

    // reset transaction

    update_transaction_id();

    std::int64_t const connection_id = aux::read_int64(buf);

    std::lock_guard<std::mutex> l(m_cache_mutex);

    connection_cache_entry& cce = m_connection_cache[m_target.address()];

    cce.connection_id = connection_id;

    cce.expires = aux::time_now() + seconds(m_man.settings().get_int(settings_pack::udp_tracker_token_expiry));

    if (!(tracker_req().kind & tracker_request::scrape_request))

      send_udp_announce();

    else if (tracker_req().kind & tracker_request::scrape_request)

      send_udp_scrape();

    return true;

  }

  void udp_tracker_connection::send_udp_connect()

  {

#ifndef TORRENT_DISABLE_LOGGING

    std::shared_ptr<request_callback> cb = requester();

#endif

    if (m_abort)

    {

#ifndef TORRENT_DISABLE_LOGGING

      if (cb) cb->debug_log("==> UDP_TRACKER_CONNECT [ skipped, m_abort ]");

#endif

      return;

    }

    std::size_t const connect_packet_size = 16;

    std::array<char, connect_packet_size> buf;

    span<char> view = buf;

    TORRENT_ASSERT(m_transaction_id != 0);

    aux::write_uint32(0x417, view);

    aux::write_uint32(0x27101980, view); // connection_id

    aux::write_int32(action_t::connect, view); // action (connect)

    aux::write_int32(m_transaction_id, view); // transaction_id

    TORRENT_ASSERT(view.empty());

    error_code ec;

    if (!m_hostname.empty())

    {

      m_man.send_hostname(bind_socket(), m_hostname.c_str()

        , m_target.port(), buf, ec

        , udp_socket::tracker_connection);

    }

    else

    {

      m_man.send(bind_socket(), m_target, buf, ec

        , udp_socket::tracker_connection);

    }

    ++m_attempts;

    if (ec)

    {

#ifndef TORRENT_DISABLE_LOGGING

      if (cb && cb->should_log())

      {

        cb->debug_log("==> UDP_TRACKER_CONNECT [ failed: %s ]"

          , ec.message().c_str());

      }

#endif

      fail(ec, operation_t::sock_write);

      return;

    }

#ifndef TORRENT_DISABLE_LOGGING

    if (cb && cb->should_log())

    {

      cb->debug_log("==> UDP_TRACKER_CONNECT [ to: %s ih: %s ]"

        , m_hostname.empty()

          ? print_endpoint(m_target).c_str()

          : (m_hostname + ":" + to_string(m_target.port()).data()).c_str()

        , aux::to_hex(tracker_req().info_hash).c_str());

    }

#endif

    m_state = action_t::connect;

    sent_bytes(16 + 28); // assuming UDP/IP header

  }

  void udp_tracker_connection::send_udp_scrape()

  {

    if (m_abort) return;

    auto const i = m_connection_cache.find(m_target.address());

    // this isn't really supposed to happen

    TORRENT_ASSERT(i != m_connection_cache.end());

    if (i == m_connection_cache.end()) return;

    char buf[8 + 4 + 4 + 20];

    span<char> view = buf;

    aux::write_int64(i->second.connection_id, view); // connection_id

    aux::write_int32(action_t::scrape, view); // action (scrape)

    aux::write_int32(m_transaction_id, view); // transaction_id

    // info_hash

    std::copy(tracker_req().info_hash.begin(), tracker_req().info_hash.end()

      , view.data());

#if TORRENT_USE_ASSERTS

    TORRENT_ASSERT(view.size() == 20);

#endif

    error_code ec;

    if (!m_hostname.empty())

    {

      m_man.send_hostname(bind_socket(), m_hostname.c_str(), m_target.port()

        , buf, ec, udp_socket::tracker_connection);

    }

    else

    {

      m_man.send(bind_socket(), m_target, buf, ec

        , udp_socket::tracker_connection);

    }

    m_state = action_t::scrape;

    sent_bytes(sizeof(buf) + 28); // assuming UDP/IP header

    ++m_attempts;

    if (ec)

    {

      fail(ec, operation_t::sock_write);

      return;

    }

  }

  bool udp_tracker_connection::on_announce_response(span<char const> buf)

  {

    if (buf.size() < 20) return false;

    buf = buf.subspan(8);

    restart_read_timeout();

    tracker_response resp;

    resp.interval = seconds32(aux::read_int32(buf));

    resp.min_interval = seconds32(60);

    resp.incomplete = aux::read_int32(buf);

    resp.complete = aux::read_int32(buf);

    int const ip_stride = aux::is_v6(m_target) ? 18 : 6;

    auto const num_peers = buf.size() / ip_stride;

    if (buf.size() % ip_stride != 0)

    {

      fail(error_code(errors::invalid_tracker_response_length), operation_t::bittorrent);

      return false;

    }

    std::shared_ptr<request_callback> cb = requester();

#ifndef TORRENT_DISABLE_LOGGING

    if (cb)

    {

      cb->debug_log("<== UDP_TRACKER_RESPONSE [ url: %s ]", tracker_req().url.c_str());

    }

#endif

    if (!cb)

    {

      close();

      return true;

    }

    if (aux::is_v6(m_target))

    {

      resp.peers6.reserve(static_cast<std::size_t>(num_peers));

      for (int i = 0; i < num_peers; ++i)

      {

        ipv6_peer_entry e{};

        std::memcpy(e.ip.data(), buf.data(), 16);

        buf = buf.subspan(16);

        e.port = aux::read_uint16(buf);

        resp.peers6.push_back(e);

      }

    }

    else

    {

      resp.peers4.reserve(static_cast<std::size_t>(num_peers));

      for (int i = 0; i < num_peers; ++i)

      {

        ipv4_peer_entry e{};

        std::memcpy(e.ip.data(), buf.data(), 4);

        buf = buf.subspan(4);

        e.port = aux::read_uint16(buf);

        resp.peers4.push_back(e);

      }

    }

    // TODO: why is this a linked list?

    std::list<address> ip_list;

    std::transform(m_endpoints.begin(), m_endpoints.end(), std::back_inserter(ip_list)

      , [](tcp::endpoint const& ep) { return ep.address(); } );

    cb->tracker_response(tracker_req(), m_target.address(), ip_list, resp);

    close();

    return true;

  }

  bool udp_tracker_connection::on_scrape_response(span<char const> buf)

  {

    restart_read_timeout();

    auto const action = static_cast<action_t>(aux::read_int32(buf));

    std::uint32_t const transaction = aux::read_uint32(buf);

    if (transaction != m_transaction_id)

    {

      fail(error_code(errors::invalid_tracker_transaction_id), operation_t::bittorrent);

      return false;

    }

    if (action == action_t::error)

    {

      fail(error_code(errors::tracker_failure), operation_t::bittorrent

        , std::string(buf.data(), static_cast<std::size_t>(buf.size())).c_str());

      return true;

    }

    if (action != action_t::scrape)

    {

      fail(error_code(errors::invalid_tracker_action), operation_t::bittorrent);

      return true;

    }

    if (buf.size() < 12)

    {

      fail(error_code(errors::invalid_tracker_response_length), operation_t::bittorrent);

      return true;

    }

    int const complete = aux::read_int32(buf);

    int const downloaded = aux::read_int32(buf);

    int const incomplete = aux::read_int32(buf);

    std::shared_ptr<request_callback> cb = requester();

    if (!cb)

    {

      close();

      return true;

    }

    cb->tracker_scrape_response(tracker_req()

      , complete, incomplete, downloaded, -1);

    close();

    return true;

  }

  void udp_tracker_connection::send_udp_announce()

  {

    if (m_abort) return;

    char buf[800];

    span<char> out = buf;

    tracker_request const& req = tracker_req();

    aux::session_settings const& settings = m_man.settings();

    auto const i = m_connection_cache.find(m_target.address());

    // this isn't really supposed to happen

    TORRENT_ASSERT(i != m_connection_cache.end());

    if (i == m_connection_cache.end()) return;

    aux::write_int64(i->second.connection_id, out); // connection_id

    aux::write_int32(action_t::announce, out); // action (announce)

    aux::write_int32(m_transaction_id, out); // transaction_id

    std::copy(req.info_hash.begin(), req.info_hash.end(), out.data()); // info_hash

    out = out.subspan(20);

    std::copy(req.pid.begin(), req.pid.end(), out.data()); // peer_id

    out = out.subspan(20);

    aux::write_int64(req.downloaded, out); // downloaded

    aux::write_int64(req.left, out); // left

    aux::write_int64(req.uploaded, out); // uploaded

    aux::write_int32(req.event, out); // event

    // ip address

    address_v4 announce_ip;

    if (!settings.get_bool(settings_pack::anonymous_mode)

      && !settings.get_str(settings_pack::announce_ip).empty())

    {

      error_code ec;

      address ip = make_address(settings.get_str(settings_pack::announce_ip).c_str(), ec);

      if (!ec && ip.is_v4()) announce_ip = ip.to_v4();

    }

    aux::write_uint32(announce_ip.to_uint(), out);

    aux::write_int32(req.key, out); // key

    aux::write_int32(req.num_want, out); // num_want

    aux::write_uint16(req.listen_port, out); // port

    std::string request_string;

    error_code ec;

    using std::ignore;

    std::tie(ignore, ignore, ignore, ignore, request_string)

      = parse_url_components(req.url, ec);

    if (ec) request_string.clear();

    if (!request_string.empty())

    {

      std::size_t str_len = std::min(request_string.size(), std::size_t(255));

      request_string.resize(str_len);

      aux::write_uint8(2, out);

      aux::write_uint8(str_len, out);

      aux::write_string(request_string, out);

    }

#ifndef TORRENT_DISABLE_LOGGING

    std::shared_ptr<request_callback> cb = requester();

    if (cb && cb->should_log())

    {

      cb->debug_log("==> UDP_TRACKER_ANNOUNCE [%s]", aux::to_hex(req.info_hash).c_str());

    }

#endif

    if (!m_hostname.empty())

    {

      m_man.send_hostname(bind_socket(), m_hostname.c_str()

        , m_target.port(), {buf, int(sizeof(buf)) - out.size()}, ec

        , udp_socket::tracker_connection);

    }

    else

    {

      m_man.send(bind_socket(), m_target, {buf, int(sizeof(buf)) - out.size()}, ec

        , udp_socket::tracker_connection);

    }

    m_state = action_t::announce;

    sent_bytes(int(sizeof(buf)) - int(out.size()) + 28); // assuming UDP/IP header

    ++m_attempts;

    if (ec)

    {

      fail(ec, operation_t::sock_write);

      return;

    }

  }

}