src/net/dns/dns_session.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "net/dns/dns_session.h"
   6
   7 #include "base/basictypes.h"
   8 #include "base/bind.h"
   9 #include "base/lazy_instance.h"
  10 #include "base/metrics/histogram.h"
  11 #include "base/metrics/sample_vector.h"
  12 #include "base/rand_util.h"
  13 #include "base/stl_util.h"
  14 #include "base/time/time.h"
  15 #include "net/base/ip_endpoint.h"
  16 #include "net/base/net_errors.h"
  17 #include "net/dns/dns_config_service.h"
  18 #include "net/dns/dns_socket_pool.h"
  19 #include "net/socket/stream_socket.h"
  20 #include "net/udp/datagram_client_socket.h"
  21
  22 namespace net {
  23
  24 namespace {
  25 // Never exceed max timeout.
  26 const unsigned kMaxTimeoutMs = 5000;
  27 // Set min timeout, in case we are talking to a local DNS proxy.
  28 const unsigned kMinTimeoutMs = 10;
  29
  30 // Number of buckets in the histogram of observed RTTs.
  31 const size_t kRTTBucketCount = 100;
  32 // Target percentile in the RTT histogram used for retransmission timeout.
  33 const unsigned kRTOPercentile = 99;
  34 }  // namespace
  35
  36 // Runtime statistics of DNS server.
  37 struct DnsSession::ServerStats {
  38   ServerStats(base::TimeDelta rtt_estimate_param, RttBuckets* buckets)
  39     : last_failure_count(0), rtt_estimate(rtt_estimate_param) {
  40     rtt_histogram.reset(new base::SampleVector(buckets));
  41     // Seed histogram with 2 samples at |rtt_estimate| timeout.
  42     rtt_histogram->Accumulate(rtt_estimate.InMilliseconds(), 2);
  43   }
  44
  45   // Count of consecutive failures after last success.
  46   int last_failure_count;
  47
  48   // Last time when server returned failure or timeout.
  49   base::Time last_failure;
  50   // Last time when server returned success.
  51   base::Time last_success;
  52
  53   // Estimated RTT using moving average.
  54   base::TimeDelta rtt_estimate;
  55   // Estimated error in the above.
  56   base::TimeDelta rtt_deviation;
  57
  58   // A histogram of observed RTT .
  59   scoped_ptr<base::SampleVector> rtt_histogram;
  60
  61   DISALLOW_COPY_AND_ASSIGN(ServerStats);
  62 };
  63
  64 // static
  65 base::LazyInstance<DnsSession::RttBuckets>::Leaky DnsSession::rtt_buckets_ =
  66     LAZY_INSTANCE_INITIALIZER;
  67
  68 DnsSession::RttBuckets::RttBuckets() : base::BucketRanges(kRTTBucketCount + 1) {
  69   base::Histogram::InitializeBucketRanges(1, 5000, this);
  70 }
  71
  72 DnsSession::SocketLease::SocketLease(scoped_refptr<DnsSession> session,
  73                                      unsigned server_index,
  74                                      scoped_ptr<DatagramClientSocket> socket)
  75     : session_(session), server_index_(server_index), socket_(socket.Pass()) {}
  76
  77 DnsSession::SocketLease::~SocketLease() {
  78   session_->FreeSocket(server_index_, socket_.Pass());
  79 }
  80
  81 DnsSession::DnsSession(const DnsConfig& config,
  82                        scoped_ptr<DnsSocketPool> socket_pool,
  83                        const RandIntCallback& rand_int_callback,
  84                        NetLog* net_log)
  85     : config_(config),
  86       socket_pool_(socket_pool.Pass()),
  87       rand_callback_(base::Bind(rand_int_callback, 0, kuint16max)),
  88       net_log_(net_log),
  89       server_index_(0) {
  90   socket_pool_->Initialize(&config_.nameservers, net_log);
  91   UMA_HISTOGRAM_CUSTOM_COUNTS(
  92       "AsyncDNS.ServerCount", config_.nameservers.size(), 0, 10, 11);
  93   for (size_t i = 0; i < config_.nameservers.size(); ++i) {
  94     server_stats_.push_back(new ServerStats(config_.timeout,
  95                                             rtt_buckets_.Pointer()));
  96   }
  97 }
  98
  99 DnsSession::~DnsSession() {
 100   RecordServerStats();
 101 }
 102
 103 int DnsSession::NextQueryId() const { return rand_callback_.Run(); }
 104
 105 unsigned DnsSession::NextFirstServerIndex() {
 106   unsigned index = NextGoodServerIndex(server_index_);
 107   if (config_.rotate)
 108     server_index_ = (server_index_ + 1) % config_.nameservers.size();
 109   return index;
 110 }
 111
 112 unsigned DnsSession::NextGoodServerIndex(unsigned server_index) {
 113   unsigned index = server_index;
 114   base::Time oldest_server_failure(base::Time::Now());
 115   unsigned oldest_server_failure_index = 0;
 116
 117   UMA_HISTOGRAM_BOOLEAN("AsyncDNS.ServerIsGood",
 118                         server_stats_[server_index]->last_failure.is_null());
 119
 120   do {
 121     base::Time cur_server_failure = server_stats_[index]->last_failure;
 122     // If number of failures on this server doesn't exceed number of allowed
 123     // attempts, return its index.
 124     if (server_stats_[server_index]->last_failure_count < config_.attempts) {
 125       return index;
 126     }
 127     // Track oldest failed server.
 128     if (cur_server_failure < oldest_server_failure) {
 129       oldest_server_failure = cur_server_failure;
 130       oldest_server_failure_index = index;
 131     }
 132     index = (index + 1) % config_.nameservers.size();
 133   } while (index != server_index);
 134
 135   // If we are here it means that there are no successful servers, so we have
 136   // to use one that has failed oldest.
 137   return oldest_server_failure_index;
 138 }
 139
 140 void DnsSession::RecordServerFailure(unsigned server_index) {
 141   UMA_HISTOGRAM_CUSTOM_COUNTS(
 142       "AsyncDNS.ServerFailureIndex", server_index, 0, 10, 11);
 143   ++(server_stats_[server_index]->last_failure_count);
 144   server_stats_[server_index]->last_failure = base::Time::Now();
 145 }
 146
 147 void DnsSession::RecordServerSuccess(unsigned server_index) {
 148   if (server_stats_[server_index]->last_success.is_null()) {
 149     UMA_HISTOGRAM_COUNTS_100("AsyncDNS.ServerFailuresAfterNetworkChange",
 150                            server_stats_[server_index]->last_failure_count);
 151   } else {
 152     UMA_HISTOGRAM_COUNTS_100("AsyncDNS.ServerFailuresBeforeSuccess",
 153                            server_stats_[server_index]->last_failure_count);
 154   }
 155   server_stats_[server_index]->last_failure_count = 0;
 156   server_stats_[server_index]->last_failure = base::Time();
 157   server_stats_[server_index]->last_success = base::Time::Now();
 158 }
 159
 160 void DnsSession::RecordRTT(unsigned server_index, base::TimeDelta rtt) {
 161   DCHECK_LT(server_index, server_stats_.size());
 162
 163   // For measurement, assume it is the first attempt (no backoff).
 164   base::TimeDelta timeout_jacobson = NextTimeoutFromJacobson(server_index, 0);
 165   base::TimeDelta timeout_histogram = NextTimeoutFromHistogram(server_index, 0);
 166   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorJacobson", rtt - timeout_jacobson);
 167   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorHistogram",
 168                       rtt - timeout_histogram);
 169   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorJacobsonUnder",
 170                       timeout_jacobson - rtt);
 171   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorHistogramUnder",
 172                       timeout_histogram - rtt);
 173
 174   // Jacobson/Karels algorithm for TCP.
 175   // Using parameters: alpha = 1/8, delta = 1/4, beta = 4
 176   base::TimeDelta& estimate = server_stats_[server_index]->rtt_estimate;
 177   base::TimeDelta& deviation = server_stats_[server_index]->rtt_deviation;
 178   base::TimeDelta current_error = rtt - estimate;
 179   estimate += current_error / 8;  // * alpha
 180   base::TimeDelta abs_error = base::TimeDelta::FromInternalValue(
 181       std::abs(current_error.ToInternalValue()));
 182   deviation += (abs_error - deviation) / 4;  // * delta
 183
 184   // Histogram-based method.
 185   server_stats_[server_index]->rtt_histogram
 186       ->Accumulate(rtt.InMilliseconds(), 1);
 187 }
 188
 189 void DnsSession::RecordLostPacket(unsigned server_index, int attempt) {
 190   base::TimeDelta timeout_jacobson =
 191       NextTimeoutFromJacobson(server_index, attempt);
 192   base::TimeDelta timeout_histogram =
 193       NextTimeoutFromHistogram(server_index, attempt);
 194   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutSpentJacobson", timeout_jacobson);
 195   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutSpentHistogram", timeout_histogram);
 196 }
 197
 198 void DnsSession::RecordServerStats() {
 199   for (size_t index = 0; index < server_stats_.size(); ++index) {
 200     if (server_stats_[index]->last_failure_count) {
 201       if (server_stats_[index]->last_success.is_null()) {
 202         UMA_HISTOGRAM_COUNTS("AsyncDNS.ServerFailuresWithoutSuccess",
 203                              server_stats_[index]->last_failure_count);
 204       } else {
 205         UMA_HISTOGRAM_COUNTS("AsyncDNS.ServerFailuresAfterSuccess",
 206                              server_stats_[index]->last_failure_count);
 207       }
 208     }
 209   }
 210 }
 211
 212
 213 base::TimeDelta DnsSession::NextTimeout(unsigned server_index, int attempt) {
 214   // Respect config timeout if it exceeds |kMaxTimeoutMs|.
 215   if (config_.timeout.InMilliseconds() >= kMaxTimeoutMs)
 216     return config_.timeout;
 217   return NextTimeoutFromHistogram(server_index, attempt);
 218 }
 219
 220 // Allocate a socket, already connected to the server address.
 221 scoped_ptr<DnsSession::SocketLease> DnsSession::AllocateSocket(
 222     unsigned server_index, const NetLog::Source& source) {
 223   scoped_ptr<DatagramClientSocket> socket;
 224
 225   socket = socket_pool_->AllocateSocket(server_index);
 226   if (!socket.get())
 227     return scoped_ptr<SocketLease>();
 228
 229   socket->NetLog().BeginEvent(NetLog::TYPE_SOCKET_IN_USE,
 230                               source.ToEventParametersCallback());
 231
 232   SocketLease* lease = new SocketLease(this, server_index, socket.Pass());
 233   return scoped_ptr<SocketLease>(lease);
 234 }
 235
 236 scoped_ptr<StreamSocket> DnsSession::CreateTCPSocket(
 237     unsigned server_index, const NetLog::Source& source) {
 238   return socket_pool_->CreateTCPSocket(server_index, source);
 239 }
 240
 241 // Release a socket.
 242 void DnsSession::FreeSocket(unsigned server_index,
 243                             scoped_ptr<DatagramClientSocket> socket) {
 244   DCHECK(socket.get());
 245
 246   socket->NetLog().EndEvent(NetLog::TYPE_SOCKET_IN_USE);
 247
 248   socket_pool_->FreeSocket(server_index, socket.Pass());
 249 }
 250
 251 base::TimeDelta DnsSession::NextTimeoutFromJacobson(unsigned server_index,
 252                                                     int attempt) {
 253   DCHECK_LT(server_index, server_stats_.size());
 254
 255   base::TimeDelta timeout = server_stats_[server_index]->rtt_estimate +
 256                             4 * server_stats_[server_index]->rtt_deviation;
 257
 258   timeout = std::max(timeout, base::TimeDelta::FromMilliseconds(kMinTimeoutMs));
 259
 260   // The timeout doubles every full round.
 261   unsigned num_backoffs = attempt / config_.nameservers.size();
 262
 263   return std::min(timeout * (1 << num_backoffs),
 264                   base::TimeDelta::FromMilliseconds(kMaxTimeoutMs));
 265 }
 266
 267 base::TimeDelta DnsSession::NextTimeoutFromHistogram(unsigned server_index,
 268                                                      int attempt) {
 269   DCHECK_LT(server_index, server_stats_.size());
 270
 271   COMPILE_ASSERT(std::numeric_limits<base::HistogramBase::Count>::is_signed,
 272                  histogram_base_count_assumed_to_be_signed);
 273
 274   // Use fixed percentile of observed samples.
 275   const base::SampleVector& samples =
 276       *server_stats_[server_index]->rtt_histogram;
 277
 278   base::HistogramBase::Count total = samples.TotalCount();
 279   base::HistogramBase::Count remaining_count = kRTOPercentile * total / 100;
 280   size_t index = 0;
 281   while (remaining_count > 0 && index < rtt_buckets_.Get().size()) {
 282     remaining_count -= samples.GetCountAtIndex(index);
 283     ++index;
 284   }
 285
 286   base::TimeDelta timeout =
 287       base::TimeDelta::FromMilliseconds(rtt_buckets_.Get().range(index));
 288
 289   timeout = std::max(timeout, base::TimeDelta::FromMilliseconds(kMinTimeoutMs));
 290
 291   // The timeout still doubles every full round.
 292   unsigned num_backoffs = attempt / config_.nameservers.size();
 293
 294   return std::min(timeout * (1 << num_backoffs),
 295                   base::TimeDelta::FromMilliseconds(kMaxTimeoutMs));
 296 }
 297
 298 }  // namespace net