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(
  43         static_cast<base::HistogramBase::Sample>(rtt_estimate.InMilliseconds()),
  44         2);
  45   }
  46
  47   // Count of consecutive failures after last success.
  48   int last_failure_count;
  49
  50   // Last time when server returned failure or timeout.
  51   base::Time last_failure;
  52   // Last time when server returned success.
  53   base::Time last_success;
  54
  55   // Estimated RTT using moving average.
  56   base::TimeDelta rtt_estimate;
  57   // Estimated error in the above.
  58   base::TimeDelta rtt_deviation;
  59
  60   // A histogram of observed RTT .
  61   scoped_ptr<base::SampleVector> rtt_histogram;
  62
  63   DISALLOW_COPY_AND_ASSIGN(ServerStats);
  64 };
  65
  66 // static
  67 base::LazyInstance<DnsSession::RttBuckets>::Leaky DnsSession::rtt_buckets_ =
  68     LAZY_INSTANCE_INITIALIZER;
  69
  70 DnsSession::RttBuckets::RttBuckets() : base::BucketRanges(kRTTBucketCount + 1) {
  71   base::Histogram::InitializeBucketRanges(1, 5000, this);
  72 }
  73
  74 DnsSession::SocketLease::SocketLease(scoped_refptr<DnsSession> session,
  75                                      unsigned server_index,
  76                                      scoped_ptr<DatagramClientSocket> socket)
  77     : session_(session), server_index_(server_index), socket_(socket.Pass()) {}
  78
  79 DnsSession::SocketLease::~SocketLease() {
  80   session_->FreeSocket(server_index_, socket_.Pass());
  81 }
  82
  83 DnsSession::DnsSession(const DnsConfig& config,
  84                        scoped_ptr<DnsSocketPool> socket_pool,
  85                        const RandIntCallback& rand_int_callback,
  86                        NetLog* net_log)
  87     : config_(config),
  88       socket_pool_(socket_pool.Pass()),
  89       rand_callback_(base::Bind(rand_int_callback, 0, kuint16max)),
  90       net_log_(net_log),
  91       server_index_(0) {
  92   socket_pool_->Initialize(&config_.nameservers, net_log);
  93   UMA_HISTOGRAM_CUSTOM_COUNTS(
  94       "AsyncDNS.ServerCount", config_.nameservers.size(), 0, 10, 11);
  95   for (size_t i = 0; i < config_.nameservers.size(); ++i) {
  96     server_stats_.push_back(new ServerStats(config_.timeout,
  97                                             rtt_buckets_.Pointer()));
  98   }
  99 }
 100
 101 DnsSession::~DnsSession() {
 102   RecordServerStats();
 103 }
 104
 105 uint16 DnsSession::NextQueryId() const {
 106   return static_cast<uint16>(rand_callback_.Run());
 107 }
 108
 109 unsigned DnsSession::NextFirstServerIndex() {
 110   unsigned index = NextGoodServerIndex(server_index_);
 111   if (config_.rotate)
 112     server_index_ = (server_index_ + 1) % config_.nameservers.size();
 113   return index;
 114 }
 115
 116 unsigned DnsSession::NextGoodServerIndex(unsigned server_index) {
 117   unsigned index = server_index;
 118   base::Time oldest_server_failure(base::Time::Now());
 119   unsigned oldest_server_failure_index = 0;
 120
 121   UMA_HISTOGRAM_BOOLEAN("AsyncDNS.ServerIsGood",
 122                         server_stats_[server_index]->last_failure.is_null());
 123
 124   do {
 125     base::Time cur_server_failure = server_stats_[index]->last_failure;
 126     // If number of failures on this server doesn't exceed number of allowed
 127     // attempts, return its index.
 128     if (server_stats_[server_index]->last_failure_count < config_.attempts) {
 129       return index;
 130     }
 131     // Track oldest failed server.
 132     if (cur_server_failure < oldest_server_failure) {
 133       oldest_server_failure = cur_server_failure;
 134       oldest_server_failure_index = index;
 135     }
 136     index = (index + 1) % config_.nameservers.size();
 137   } while (index != server_index);
 138
 139   // If we are here it means that there are no successful servers, so we have
 140   // to use one that has failed oldest.
 141   return oldest_server_failure_index;
 142 }
 143
 144 void DnsSession::RecordServerFailure(unsigned server_index) {
 145   UMA_HISTOGRAM_CUSTOM_COUNTS(
 146       "AsyncDNS.ServerFailureIndex", server_index, 0, 10, 11);
 147   ++(server_stats_[server_index]->last_failure_count);
 148   server_stats_[server_index]->last_failure = base::Time::Now();
 149 }
 150
 151 void DnsSession::RecordServerSuccess(unsigned server_index) {
 152   if (server_stats_[server_index]->last_success.is_null()) {
 153     UMA_HISTOGRAM_COUNTS_100("AsyncDNS.ServerFailuresAfterNetworkChange",
 154                            server_stats_[server_index]->last_failure_count);
 155   } else {
 156     UMA_HISTOGRAM_COUNTS_100("AsyncDNS.ServerFailuresBeforeSuccess",
 157                            server_stats_[server_index]->last_failure_count);
 158   }
 159   server_stats_[server_index]->last_failure_count = 0;
 160   server_stats_[server_index]->last_failure = base::Time();
 161   server_stats_[server_index]->last_success = base::Time::Now();
 162 }
 163
 164 void DnsSession::RecordRTT(unsigned server_index, base::TimeDelta rtt) {
 165   DCHECK_LT(server_index, server_stats_.size());
 166
 167   // For measurement, assume it is the first attempt (no backoff).
 168   base::TimeDelta timeout_jacobson = NextTimeoutFromJacobson(server_index, 0);
 169   base::TimeDelta timeout_histogram = NextTimeoutFromHistogram(server_index, 0);
 170   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorJacobson", rtt - timeout_jacobson);
 171   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorHistogram",
 172                       rtt - timeout_histogram);
 173   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorJacobsonUnder",
 174                       timeout_jacobson - rtt);
 175   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutErrorHistogramUnder",
 176                       timeout_histogram - rtt);
 177
 178   // Jacobson/Karels algorithm for TCP.
 179   // Using parameters: alpha = 1/8, delta = 1/4, beta = 4
 180   base::TimeDelta& estimate = server_stats_[server_index]->rtt_estimate;
 181   base::TimeDelta& deviation = server_stats_[server_index]->rtt_deviation;
 182   base::TimeDelta current_error = rtt - estimate;
 183   estimate += current_error / 8;  // * alpha
 184   base::TimeDelta abs_error = base::TimeDelta::FromInternalValue(
 185       std::abs(current_error.ToInternalValue()));
 186   deviation += (abs_error - deviation) / 4;  // * delta
 187
 188   // Histogram-based method.
 189   server_stats_[server_index]->rtt_histogram->Accumulate(
 190       static_cast<base::HistogramBase::Sample>(rtt.InMilliseconds()), 1);
 191 }
 192
 193 void DnsSession::RecordLostPacket(unsigned server_index, int attempt) {
 194   base::TimeDelta timeout_jacobson =
 195       NextTimeoutFromJacobson(server_index, attempt);
 196   base::TimeDelta timeout_histogram =
 197       NextTimeoutFromHistogram(server_index, attempt);
 198   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutSpentJacobson", timeout_jacobson);
 199   UMA_HISTOGRAM_TIMES("AsyncDNS.TimeoutSpentHistogram", timeout_histogram);
 200 }
 201
 202 void DnsSession::RecordServerStats() {
 203   for (size_t index = 0; index < server_stats_.size(); ++index) {
 204     if (server_stats_[index]->last_failure_count) {
 205       if (server_stats_[index]->last_success.is_null()) {
 206         UMA_HISTOGRAM_COUNTS("AsyncDNS.ServerFailuresWithoutSuccess",
 207                              server_stats_[index]->last_failure_count);
 208       } else {
 209         UMA_HISTOGRAM_COUNTS("AsyncDNS.ServerFailuresAfterSuccess",
 210                              server_stats_[index]->last_failure_count);
 211       }
 212     }
 213   }
 214 }
 215
 216
 217 base::TimeDelta DnsSession::NextTimeout(unsigned server_index, int attempt) {
 218   // Respect config timeout if it exceeds |kMaxTimeoutMs|.
 219   if (config_.timeout.InMilliseconds() >= kMaxTimeoutMs)
 220     return config_.timeout;
 221   return NextTimeoutFromHistogram(server_index, attempt);
 222 }
 223
 224 // Allocate a socket, already connected to the server address.
 225 scoped_ptr<DnsSession::SocketLease> DnsSession::AllocateSocket(
 226     unsigned server_index, const NetLog::Source& source) {
 227   scoped_ptr<DatagramClientSocket> socket;
 228
 229   socket = socket_pool_->AllocateSocket(server_index);
 230   if (!socket.get())
 231     return scoped_ptr<SocketLease>();
 232
 233   socket->NetLog().BeginEvent(NetLog::TYPE_SOCKET_IN_USE,
 234                               source.ToEventParametersCallback());
 235
 236   SocketLease* lease = new SocketLease(this, server_index, socket.Pass());
 237   return scoped_ptr<SocketLease>(lease);
 238 }
 239
 240 scoped_ptr<StreamSocket> DnsSession::CreateTCPSocket(
 241     unsigned server_index, const NetLog::Source& source) {
 242   return socket_pool_->CreateTCPSocket(server_index, source);
 243 }
 244
 245 // Release a socket.
 246 void DnsSession::FreeSocket(unsigned server_index,
 247                             scoped_ptr<DatagramClientSocket> socket) {
 248   DCHECK(socket.get());
 249
 250   socket->NetLog().EndEvent(NetLog::TYPE_SOCKET_IN_USE);
 251
 252   socket_pool_->FreeSocket(server_index, socket.Pass());
 253 }
 254
 255 base::TimeDelta DnsSession::NextTimeoutFromJacobson(unsigned server_index,
 256                                                     int attempt) {
 257   DCHECK_LT(server_index, server_stats_.size());
 258
 259   base::TimeDelta timeout = server_stats_[server_index]->rtt_estimate +
 260                             4 * server_stats_[server_index]->rtt_deviation;
 261
 262   timeout = std::max(timeout, base::TimeDelta::FromMilliseconds(kMinTimeoutMs));
 263
 264   // The timeout doubles every full round.
 265   unsigned num_backoffs = attempt / config_.nameservers.size();
 266
 267   return std::min(timeout * (1 << num_backoffs),
 268                   base::TimeDelta::FromMilliseconds(kMaxTimeoutMs));
 269 }
 270
 271 base::TimeDelta DnsSession::NextTimeoutFromHistogram(unsigned server_index,
 272                                                      int attempt) {
 273   DCHECK_LT(server_index, server_stats_.size());
 274
 275   static_assert(std::numeric_limits<base::HistogramBase::Count>::is_signed,
 276                 "histogram base count assumed to be signed");
 277
 278   // Use fixed percentile of observed samples.
 279   const base::SampleVector& samples =
 280       *server_stats_[server_index]->rtt_histogram;
 281
 282   base::HistogramBase::Count total = samples.TotalCount();
 283   base::HistogramBase::Count remaining_count = kRTOPercentile * total / 100;
 284   size_t index = 0;
 285   while (remaining_count > 0 && index < rtt_buckets_.Get().size()) {
 286     remaining_count -= samples.GetCountAtIndex(index);
 287     ++index;
 288   }
 289
 290   base::TimeDelta timeout =
 291       base::TimeDelta::FromMilliseconds(rtt_buckets_.Get().range(index));
 292
 293   timeout = std::max(timeout, base::TimeDelta::FromMilliseconds(kMinTimeoutMs));
 294
 295   // The timeout still doubles every full round.
 296   unsigned num_backoffs = attempt / config_.nameservers.size();
 297
 298   return std::min(timeout * (1 << num_backoffs),
 299                   base::TimeDelta::FromMilliseconds(kMaxTimeoutMs));
 300 }
 301
 302 }  // namespace net