Anti-DDoS enhancements: Log to net log, UMA stats, improved policy.

net/url_request/url_request_throttler_entry.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/url_request/url_request_throttler_entry.h"
 
 #include <cmath>
 
 #include "base/logging.h"
+#include "base/metrics/field_trial.h"
+#include "base/metrics/histogram.h"
 #include "base/rand_util.h"
 #include "base/string_number_conversions.h"
+#include "base/time.h"
+#include "base/values.h"
+#include "net/base/net_log.h"
 #include "net/url_request/url_request_throttler_header_interface.h"
 #include "net/url_request/url_request_throttler_manager.h"
 
 namespace net {
 
 const int URLRequestThrottlerEntry::kDefaultSlidingWindowPeriodMs = 2000;
 const int URLRequestThrottlerEntry::kDefaultMaxSendThreshold = 20;
 
 // This set of back-off parameters will (at maximum values, i.e. without
 // the reduction caused by jitter) add 0-41% (distributed uniformly
 // in that range) to the "perceived downtime" of the remote server, once
 // exponential back-off kicks in and is throttling requests for more than
 // about a second at a time.  Once the maximum back-off is reached, the added
 // perceived downtime decreases rapidly, percentage-wise.
 //
 // Another way to put it is that the maximum additional perceived downtime
 // with these numbers is a couple of seconds shy of 15 minutes, and such
 // a delay would not occur until the remote server has been actually
 // unavailable at the end of each back-off period for a total of about
 // 48 minutes.
 //
-// Ignoring the first 4 errors helps avoid back-off from kicking in on
-// flaky connections.
-const int URLRequestThrottlerEntry::kDefaultNumErrorsToIgnore = 4;
+// Ignoring the first couple of errors is just a conservative measure to
+// avoid false positives.  It should help avoid back-off from kicking in e.g.
+// on flaky connections.
+const int URLRequestThrottlerEntry::kDefaultNumErrorsToIgnore = 2;
 const int URLRequestThrottlerEntry::kDefaultInitialBackoffMs = 700;
 const double URLRequestThrottlerEntry::kDefaultMultiplyFactor = 1.4;
 const double URLRequestThrottlerEntry::kDefaultJitterFactor = 0.4;
 const int URLRequestThrottlerEntry::kDefaultMaximumBackoffMs = 15 * 60 * 1000;
 const int URLRequestThrottlerEntry::kDefaultEntryLifetimeMs = 2 * 60 * 1000;
 const char URLRequestThrottlerEntry::kRetryHeaderName[] = "X-Retry-After";
 const char URLRequestThrottlerEntry::kExponentialThrottlingHeader[] =
     "X-Chrome-Exponential-Throttling";
 const char URLRequestThrottlerEntry::kExponentialThrottlingDisableValue[] =
     "disable";
 
+// NetLog parameters when a request is rejected by throttling.
+class RejectedRequestParameters : public NetLog::EventParameters {
+ public:
+  RejectedRequestParameters(const std::string& url_id,
+                            int num_failures,
+                            int release_after_ms)
+      : url_id_(url_id),
+        num_failures_(num_failures),
+        release_after_ms_(release_after_ms) {
+  }
+
+  virtual Value* ToValue() const {
+    DictionaryValue* dict = new DictionaryValue();
+    dict->SetString("url", url_id_);
+    dict->SetInteger("num_failures", num_failures_);
+    dict->SetInteger("release_after_ms", release_after_ms_);
+    return dict;
+  }
+
+ private:
+  std::string url_id_;
+  int num_failures_;
+  int release_after_ms_;
+};
+
+// NetLog parameters when a response contains an X-Retry-After header.
+class RetryAfterParameters : public NetLog::EventParameters {
+ public:
+  RetryAfterParameters(const std::string& url_id,
+                            int retry_after_ms)

[yzshen1, 2011/05/25 03:49:06]

Wrong indent.

[Jói, 2011/05/25 14:30:35]

Done.

+      : url_id_(url_id),
+        retry_after_ms_(retry_after_ms) {
+  }
+
+  virtual Value* ToValue() const {
+    DictionaryValue* dict = new DictionaryValue();
+    dict->SetString("url", url_id_);
+    dict->SetInteger("retry_after_ms", retry_after_ms_);
+    return dict;
+  }
+
+ private:
+  std::string url_id_;
+  int retry_after_ms_;
+};
+
 URLRequestThrottlerEntry::URLRequestThrottlerEntry(
-    URLRequestThrottlerManager* manager)
+    URLRequestThrottlerManager* manager,
+    const std::string& url_id)
     : sliding_window_period_(
           base::TimeDelta::FromMilliseconds(kDefaultSlidingWindowPeriodMs)),
       max_send_threshold_(kDefaultMaxSendThreshold),
       is_backoff_disabled_(false),
       backoff_entry_(&backoff_policy_),
-      manager_(manager) {
+      manager_(manager),
+      url_id_(url_id),
+      net_log_(BoundNetLog::Make(
+          manager->net_log(), NetLog::SOURCE_EXPONENTIAL_BACKOFF_THROTTLING)) {
   DCHECK(manager_);
   Initialize();
 }
 
 URLRequestThrottlerEntry::URLRequestThrottlerEntry(
     URLRequestThrottlerManager* manager,
+    const std::string& url_id,
     int sliding_window_period_ms,
     int max_send_threshold,
     int initial_backoff_ms,
     double multiply_factor,
     double jitter_factor,
     int maximum_backoff_ms)
     : sliding_window_period_(
           base::TimeDelta::FromMilliseconds(sliding_window_period_ms)),
       max_send_threshold_(max_send_threshold),
       is_backoff_disabled_(false),
       backoff_entry_(&backoff_policy_),
-      manager_(manager) {
+      manager_(manager),
+      url_id_(url_id) {
   DCHECK_GT(sliding_window_period_ms, 0);
   DCHECK_GT(max_send_threshold_, 0);
   DCHECK_GE(initial_backoff_ms, 0);
   DCHECK_GT(multiply_factor, 0);
   DCHECK_GE(jitter_factor, 0.0);
   DCHECK_LT(jitter_factor, 1.0);
   DCHECK_GE(maximum_backoff_ms, 0);
   DCHECK(manager_);
 
   Initialize();
@@ skipping 16 matching lines @@
   // it should not be considered outdated.
   //
   // TODO(joi): Once the manager is not a Singleton, revisit whether
   // refcounting is needed at all.
   if (!HasOneRef())
     return false;
 
   // If there are send events in the sliding window period, we still need this
   // entry.
   if (!send_log_.empty() &&
-      send_log_.back() + sliding_window_period_ > GetTimeNow()) {
+      send_log_.back() + sliding_window_period_ > ImplGetTimeNow()) {
     return false;
   }
 
   return GetBackoffEntry()->CanDiscard();
 }
 
 void URLRequestThrottlerEntry::DisableBackoffThrottling() {
   is_backoff_disabled_ = true;
 }
 
 void URLRequestThrottlerEntry::DetachManager() {
   manager_ = NULL;
 }
 
 bool URLRequestThrottlerEntry::IsDuringExponentialBackoff() const {
-  if (is_backoff_disabled_)
-    return false;
+  bool reject_request = false;
+  if (!is_backoff_disabled_ && GetBackoffEntry()->ShouldRejectRequest()) {
+    int num_failures = GetBackoffEntry()->failure_count();
+    int release_after_ms =
+        (GetBackoffEntry()->GetReleaseTime() - base::TimeTicks::Now())
+            .InMilliseconds();
 
-  return GetBackoffEntry()->ShouldRejectRequest();
+    net_log_.AddEvent(
+      NetLog::TYPE_THROTTLING_REJECTED_REQUEST,

[yzshen1, 2011/05/25 03:49:06]

Wrong indent.

[Jói, 2011/05/25 14:30:35]

Done.

+      make_scoped_refptr(
+          new RejectedRequestParameters(url_id_,
+                                        num_failures,
+                                        release_after_ms)));
+
+    reject_request = true;
+  }
+
+  int reject_count = reject_request ? 1 : 0;
+  UMA_HISTOGRAM_ENUMERATION(
+      "Throttling.RequestThrottled", reject_count, 2);
+  if (base::FieldTrialList::TrialExists("ThrottlingEnabled")) {
+    UMA_HISTOGRAM_ENUMERATION(base::FieldTrial::MakeName(
+        "Throttling.RequestThrottled", "ThrottlingEnabled"), reject_count, 2);
+  }
+
+  return reject_request;
 }
 
 int64 URLRequestThrottlerEntry::ReserveSendingTimeForNextRequest(
     const base::TimeTicks& earliest_time) {
-  base::TimeTicks now = GetTimeNow();
+  base::TimeTicks now = ImplGetTimeNow();
 
   // If a lot of requests were successfully made recently,
   // sliding_window_release_time_ may be greater than
   // exponential_backoff_release_time_.
   base::TimeTicks recommended_sending_time =
       std::max(std::max(now, earliest_time),
                std::max(GetBackoffEntry()->GetReleaseTime(),
                         sliding_window_release_time_));
 
   DCHECK(send_log_.empty() ||
@@ skipping 20 matching lines @@
 }
 
 base::TimeTicks
     URLRequestThrottlerEntry::GetExponentialBackoffReleaseTime() const {
   // If a site opts out, it's likely because they have problems that trigger
   // the back-off mechanism when it shouldn't be triggered, in which case
   // returning the calculated back-off release time would probably be the
   // wrong thing to do (i.e. it would likely be too long).  Therefore, we
   // return "now" so that retries are not delayed.
   if (is_backoff_disabled_)
-    return GetTimeNow();
+    return ImplGetTimeNow();
 
   return GetBackoffEntry()->GetReleaseTime();
 }
 
 void URLRequestThrottlerEntry::UpdateWithResponse(
     const std::string& host,
     const URLRequestThrottlerHeaderInterface* response) {
-  if (response->GetResponseCode() >= 500) {
+  int status_code = response->GetResponseCode();
+  HandleMetricsTracking(status_code);
+
+  if (IsConsideredError(status_code)) {
     GetBackoffEntry()->InformOfRequest(false);
   } else {
     GetBackoffEntry()->InformOfRequest(true);
 
     std::string retry_header = response->GetNormalizedValue(kRetryHeaderName);
     if (!retry_header.empty())
       HandleCustomRetryAfter(retry_header);
 
     std::string throttling_header = response->GetNormalizedValue(
         kExponentialThrottlingHeader);
     if (!throttling_header.empty())
       HandleThrottlingHeader(throttling_header, host);
   }
 }
 
-void URLRequestThrottlerEntry::ReceivedContentWasMalformed() {
+void URLRequestThrottlerEntry::ReceivedContentWasMalformed(int status_code) {
   // A malformed body can only occur when the request to fetch a resource
   // was successful.  Therefore, in such a situation, we will receive one
-  // call to ReceivedContentWasMalformed() and one call to UpdateWithResponse()
-  // with a response categorized as "good".  To end up counting one failure,
-  // we need to count two failures here against the one success in
-  // UpdateWithResponse().
-  GetBackoffEntry()->InformOfRequest(false);
-  GetBackoffEntry()->InformOfRequest(false);
+  // call to ReceivedContentWasMalformed() and one call to
+  // UpdateWithResponse() with a response categorized as "good".  To end
+  // up counting one failure, we need to count two failures here against
+  // the one success in UpdateWithResponse().
+  //
+  // We do nothing for a response that is already being considered an error
+  // based on its status code (otherwise we would count 3 errors instead of 1).
+  if (!IsConsideredError(status_code)) {
+    GetBackoffEntry()->InformOfRequest(false);
+    GetBackoffEntry()->InformOfRequest(false);
+  }
 }
 
 URLRequestThrottlerEntry::~URLRequestThrottlerEntry() {
 }
 
 void URLRequestThrottlerEntry::Initialize() {
   sliding_window_release_time_ = base::TimeTicks::Now();
   backoff_policy_.num_errors_to_ignore = kDefaultNumErrorsToIgnore;
   backoff_policy_.initial_backoff_ms = kDefaultInitialBackoffMs;
   backoff_policy_.multiply_factor = kDefaultMultiplyFactor;
   backoff_policy_.jitter_factor = kDefaultJitterFactor;
   backoff_policy_.maximum_backoff_ms = kDefaultMaximumBackoffMs;
   backoff_policy_.entry_lifetime_ms = kDefaultEntryLifetimeMs;
+
+  // We pretend we just had a successful response so that we have a
+  // starting point to our tracking.
+  last_successful_response_time_ = base::TimeTicks::Now();

[yzshen1, 2011/05/25 03:49:06]

It would be good to comment about why ImplGetTimeNow is not used here.

[Jói, 2011/05/25 14:30:35]

Done.

+  last_response_was_success_ = true;
 }
 
-base::TimeTicks URLRequestThrottlerEntry::GetTimeNow() const {
+bool URLRequestThrottlerEntry::IsConsideredError(int status_code) {
+  // We throttle only for the status codes most likely to indicate the server
+  // is failing because it is too busy or otherwise are likely to be
+  // because of DDoS.
+  //
+  // 500 is the generic error when no better message is suitable, and
+  //     as such does not necessarily indicate a temporary state, but
+  //     other status codes cover most of the permanent error states.
+  // 503 is explicitly documented as a temporary state where the server
+  //     is either overloaded or down for maintenance.
+  // 509 is the (non-standard but widely implemented) Bandwidth Limit Exceeded
+  //     status code, which might indicate DDoS.
+  //
+  // We do not back off on 502 or 504, which are reported by gateways
+  // (proxies) on timeouts or failures, because in many cases these requests
+  // have not made it to the destination server and so we do not actually
+  // know that it is down or busy.  One degenerate case could be a proxy on
+  // localhost, where you are not actually connected to the network.
+  return (status_code == 500 ||
+          status_code == 503 ||
+          status_code == 509);
+}
+
+base::TimeTicks URLRequestThrottlerEntry::ImplGetTimeNow() const {
   return base::TimeTicks::Now();
 }
 
 void URLRequestThrottlerEntry::HandleCustomRetryAfter(
     const std::string& header_value) {
   // Input parameter is the number of seconds to wait in a floating point value.
   double time_in_sec = 0;
   bool conversion_is_ok = base::StringToDouble(header_value, &time_in_sec);
 
   // Conversion of custom retry-after header value failed.
   if (!conversion_is_ok)
     return;
 
   // We must use an int value later so we transform this in milliseconds.
   int64 value_ms = static_cast<int64>(0.5 + time_in_sec * 1000);
 
   // We do not check for an upper bound; the server can set any Retry-After it
   // desires. Recovery from error would involve restarting the browser.
   if (value_ms < 0)
     return;
 
-  GetBackoffEntry()->SetCustomReleaseTime(
-      GetTimeNow() + base::TimeDelta::FromMilliseconds(value_ms));
+  net_log_.AddEvent(
+      NetLog::TYPE_THROTTLING_GOT_CUSTOM_RETRY_AFTER,
+      make_scoped_refptr(new RetryAfterParameters(url_id_, value_ms)));
+
+  base::TimeDelta value = base::TimeDelta::FromMilliseconds(value_ms);
+  GetBackoffEntry()->SetCustomReleaseTime(ImplGetTimeNow() + value);
+
+  UMA_HISTOGRAM_CUSTOM_TIMES(
+      "Throttling.CustomRetryAfterMs", value,
+      base::TimeDelta::FromSeconds(1), base::TimeDelta::FromHours(12), 50);
+  if (base::FieldTrialList::TrialExists("ThrottlingEnabled")) {
+    UMA_HISTOGRAM_CUSTOM_TIMES(
+        base::FieldTrial::MakeName("Throttling.CustomRetryAfterMs",
+                                   "ThrottlingEnabled"), value,
+        base::TimeDelta::FromSeconds(1), base::TimeDelta::FromHours(12), 50);
+  }
 }
 
 void URLRequestThrottlerEntry::HandleThrottlingHeader(
     const std::string& header_value,
     const std::string& host) {
   if (header_value == kExponentialThrottlingDisableValue) {
     DisableBackoffThrottling();
     if (manager_)
       manager_->AddToOptOutList(host);
   } else {
     // TODO(joi): Log this.
   }
 }
 
+void URLRequestThrottlerEntry::HandleMetricsTracking(int status_code) {
+  // This is essentially the same as the "Net.HttpResponseCode" UMA stat
+  // but we are tracking it separately here for the throttling experiment
+  // to make sure we count only the responses seen by throttling.
+  // TODO(joi): Remove after experiment.
+  UMA_HISTOGRAM_ENUMERATION("Throttling.HttpResponseCode", status_code, 600);
+  if (base::FieldTrialList::TrialExists("ThrottlingEnabled")) {
+    UMA_HISTOGRAM_ENUMERATION(base::FieldTrial::MakeName(
+        "Throttling.HttpResponseCode", "ThrottlingEnabled"), status_code, 600);
+  }
+
+  // Note that we are not interested in whether the code is considered
+  // an error for the backoff logic, but whether it is a 5xx error in
+  // general.  This is because here, we are tracking the apparent total
+  // downtime of a server.
+  if (status_code >= 500) {
+    last_response_was_success_ = false;
+  } else {
+    base::TimeTicks now = base::TimeTicks::Now();

[yzshen1, 2011/05/25 03:49:06]

Is it intended to not use ImplGetTimeNow here?

[Jói, 2011/05/25 14:30:35]

No, thanks for catching that.  I am writing tests in my next change that will
assert on perceived downtime in a simulated scenario - you may have saved me
from scratching my head over why they were failing :)

+    if (!last_response_was_success_) {
+      // We are transitioning from failure to success, so generate our stats.
+      base::TimeDelta down_time = now - last_successful_response_time_;
+      int failure_count = GetBackoffEntry()->failure_count();
+
+      UMA_HISTOGRAM_COUNTS("Throttling.FailureCountAtSuccess", failure_count);
+      UMA_HISTOGRAM_CUSTOM_TIMES(
+          "Throttling.PerceivedDowntime", down_time,
+          base::TimeDelta::FromMilliseconds(10),
+          base::TimeDelta::FromHours(6), 50);
+
+      if (base::FieldTrialList::TrialExists("ThrottlingEnabled")) {
+        UMA_HISTOGRAM_COUNTS(base::FieldTrial::MakeName(
+            "Throttling.FailureCountAtSuccess", "ThrottlingEnabled"),
+            failure_count);
+        UMA_HISTOGRAM_CUSTOM_TIMES(base::FieldTrial::MakeName(
+            "Throttling.PerceivedDowntime", "ThrottlingEnabled"), down_time,
+            base::TimeDelta::FromMilliseconds(10),
+            base::TimeDelta::FromHours(6), 50);
+      }
+    }
+
+    last_successful_response_time_ = now;
+    last_response_was_success_ = true;
+  }
+}
+
 const BackoffEntry* URLRequestThrottlerEntry::GetBackoffEntry() const {
   return &backoff_entry_;
 }
 
 BackoffEntry* URLRequestThrottlerEntry::GetBackoffEntry() {
   return &backoff_entry_;
 }
 
 }  // namespace net