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// Copyright (c) 2012 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.
// A Predictor object is instantiated once in the browser process, and manages
// both preresolution of hostnames, as well as TCP/IP preconnection to expected
// subresources.
// Most hostname lists are provided by the renderer processes, and include URLs
// that *might* be used in the near future by the browsing user. One goal of
// this class is to cause the underlying DNS structure to lookup a hostname
// before it is really needed, and hence reduce latency in the standard lookup
// paths.
// Subresource relationships are usually acquired from the referrer field in a
// navigation. A subresource URL may be associated with a referrer URL. Later
// navigations may, if the likelihood of needing the subresource is high enough,
// cause this module to speculatively create a TCP/IP connection. If there is
// only a low likelihood, then a DNS pre-resolution operation may be performed.
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>
#include "base/gtest_prod_util.h"
#include "base/memory/scoped_ptr.h"
#include "base/memory/weak_ptr.h"
#include "chrome/browser/net/referrer.h"
#include "chrome/browser/net/spdyproxy/proxy_advisor.h"
#include "chrome/browser/net/timed_cache.h"
#include "chrome/browser/net/url_info.h"
#include "chrome/common/net/predictor_common.h"
#include "net/base/host_port_pair.h"
class IOThread;
class PrefService;
class Profile;
namespace base {
class ListValue;
class WaitableEvent;
namespace net {
class HostResolver;
class SSLConfigService;
class TransportSecurityState;
class URLRequestContextGetter;
namespace user_prefs {
class PrefRegistrySyncable;
namespace chrome_browser_net {
typedef chrome_common_net::UrlList UrlList;
typedef chrome_common_net::NameList NameList;
typedef std::map<GURL, UrlInfo> Results;
// An observer for testing.
class PredictorObserver {
virtual ~PredictorObserver() {}
virtual void OnPreconnectUrl(const GURL& original_url,
const GURL& first_party_for_cookies,
UrlInfo::ResolutionMotivation motivation,
int count) = 0;
// Predictor is constructed during Profile construction (on the UI thread),
// but it is destroyed on the IO thread when ProfileIOData goes away. All of
// its core state and functionality happens on the IO thread. The only UI
// methods are initialization / shutdown related (including preconnect
// initialization), or convenience methods that internally forward calls to
// the IO thread.
class Predictor {
// Enum describing when to allow network predictions based on connection type.
// The same enum must be used by the platform-dependent components.
// TODO(bnc): implement as per
enum NetworkPredictionOptions {
// A version number for prefs that are saved. This should be incremented when
// we change the format so that we discard old data.
static const int kPredictorReferrerVersion;
// Given that the underlying Chromium resolver defaults to a total maximum of
// 8 paralell resolutions, we will avoid any chance of starving navigational
// resolutions by limiting the number of paralell speculative resolutions.
// This is used in the field trials and testing.
// TODO(jar): Move this limitation into the resolver.
static const size_t kMaxSpeculativeParallelResolves;
// To control the congestion avoidance system, we need an estimate of how
// many speculative requests may arrive at once. Since we currently only
// keep 8 subresource names for each frame, we'll use that as our basis.
// Note that when scanning search results lists, we might actually get 10 at
// a time, and wikipedia can often supply (during a page scan) upwards of 50.
// In those odd cases, we may discard some of the later speculative requests
// mistakenly assuming that the resolutions took too long.
static const int kTypicalSpeculativeGroupSize;
// The next constant specifies an amount of queueing delay that is
// "too large," and indicative of problems with resolutions (perhaps due to
// an overloaded router, or such). When we exceed this delay, congestion
// avoidance will kick in and all speculations in the queue will be discarded.
static const int kMaxSpeculativeResolveQueueDelayMs;
// We don't bother learning to preconnect via a GET if the original URL
// navigation was so long ago, that a preconnection would have been dropped
// anyway. We believe most servers will drop the connection in 10 seconds, so
// we currently estimate this time-till-drop at 10 seconds.
// TODO(jar): We should do a persistent field trial to validate/optimize this.
static const int kMaxUnusedSocketLifetimeSecondsWithoutAGet;
// |max_concurrent| specifies how many concurrent (parallel) prefetches will
// be performed. Host lookups will be issued through |host_resolver|.
explicit Predictor(bool preconnect_enabled);
virtual ~Predictor();
// This function is used to create a predictor. For testing, we can create
// a version which does a simpler shutdown.
static Predictor* CreatePredictor(bool preconnect_enabled,
bool simple_shutdown);
static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
// ------------- Start UI thread methods.
virtual void InitNetworkPredictor(PrefService* user_prefs,
PrefService* local_state,
IOThread* io_thread,
net::URLRequestContextGetter* getter);
// The Omnibox has proposed a given url to the user, and if it is a search
// URL, then it also indicates that this is preconnectable (i.e., we could
// preconnect to the search server).
void AnticipateOmniboxUrl(const GURL& url, bool preconnectable);
// Preconnect a URL and all of its subresource domains.
void PreconnectUrlAndSubresources(const GURL& url,
const GURL& first_party_for_cookies);
static UrlList GetPredictedUrlListAtStartup(PrefService* user_prefs,
PrefService* local_state);
static void set_max_queueing_delay(int max_queueing_delay_ms);
static void set_max_parallel_resolves(size_t max_parallel_resolves);
virtual void ShutdownOnUIThread();
// ------------- End UI thread methods.
// ------------- Start IO thread methods.
// Cancel pending requests and prevent new ones from being made.
void Shutdown();
// In some circumstances, for privacy reasons, all results should be
// discarded. This method gracefully handles that activity.
// Destroy all our internal state, which shows what names we've looked up, and
// how long each has taken, etc. etc. We also destroy records of suggesses
// (cache hits etc.).
void DiscardAllResults();
// Add hostname(s) to the queue for processing.
void ResolveList(const UrlList& urls,
UrlInfo::ResolutionMotivation motivation);
void Resolve(const GURL& url, UrlInfo::ResolutionMotivation motivation);
// Record details of a navigation so that we can preresolve the host name
// ahead of time the next time the users navigates to the indicated host.
// Should only be called when urls are distinct, and they should already be
// canonicalized to not have a path.
void LearnFromNavigation(const GURL& referring_url, const GURL& target_url);
// When displaying info in about:dns, the following API is called.
static void PredictorGetHtmlInfo(Predictor* predictor, std::string* output);
// Dump HTML table containing list of referrers for about:dns.
void GetHtmlReferrerLists(std::string* output);
// Dump the list of currently known referrer domains and related prefetchable
// domains for about:dns.
void GetHtmlInfo(std::string* output);
// Discards any referrer for which all the suggested host names are currently
// annotated with negligible expected-use. Scales down (diminishes) the
// expected-use of those that remain, so that their use will go down by a
// factor each time we trim (moving the referrer closer to being discarded in
// a future call).
// The task is performed synchronously and completes before returing.
void TrimReferrersNow();
// Construct a ListValue object that contains all the data in the referrers_
// so that it can be persisted in a pref.
void SerializeReferrers(base::ListValue* referral_list);
// Process a ListValue that contains all the data from a previous reference
// list, as constructed by SerializeReferrers(), and add all the identified
// values into the current referrer list.
void DeserializeReferrers(const base::ListValue& referral_list);
void DeserializeReferrersThenDelete(base::ListValue* referral_list);
void DiscardInitialNavigationHistory();
void FinalizeInitializationOnIOThread(
const std::vector<GURL>& urls_to_prefetch,
base::ListValue* referral_list,
IOThread* io_thread,
bool predictor_enabled);
// During startup, we learn what the first N urls visited are, and then
// resolve the associated hosts ASAP during our next startup.
void LearnAboutInitialNavigation(const GURL& url);
// Renderer bundles up list and sends to this browser API via IPC.
// TODO(jar): Use UrlList instead to include port and scheme.
void DnsPrefetchList(const NameList& hostnames);
// May be called from either the IO or UI thread and will PostTask
// to the IO thread if necessary.
void DnsPrefetchMotivatedList(const UrlList& urls,
UrlInfo::ResolutionMotivation motivation);
// May be called from either the IO or UI thread and will PostTask
// to the IO thread if necessary.
void SaveStateForNextStartupAndTrim(PrefService* prefs);
void SaveDnsPrefetchStateForNextStartupAndTrim(
base::ListValue* startup_list,
base::ListValue* referral_list,
base::WaitableEvent* completion);
// May be called from either the IO or UI thread and will PostTask
// to the IO thread if necessary.
void EnablePredictor(bool enable);
void EnablePredictorOnIOThread(bool enable);
// May be called from either the IO or UI thread and will PostTask
// to the IO thread if necessary.
void PreconnectUrl(const GURL& url, const GURL& first_party_for_cookies,
UrlInfo::ResolutionMotivation motivation, int count);
void PreconnectUrlOnIOThread(const GURL& url,
const GURL& first_party_for_cookies,
UrlInfo::ResolutionMotivation motivation,
int count);
void RecordPreconnectTrigger(const GURL& url);
void RecordPreconnectNavigationStat(const std::vector<GURL>& url_chain,
bool is_subresource);
void RecordLinkNavigation(const GURL& url);
// ------------- End IO thread methods.
// The following methods may be called on either the IO or UI threads.
// Instigate pre-connection to any URLs, or pre-resolution of related host,
// that we predict will be needed after this navigation (typically
// more-embedded resources on a page). This method will actually post a task
// to do the actual work, so as not to jump ahead of the frame navigation that
// instigated this activity.
void PredictFrameSubresources(const GURL& url,
const GURL& first_party_for_cookies);
// Put URL in canonical form, including a scheme, host, and port.
// Returns GURL::EmptyGURL() if the scheme is not http/https or if the url
// cannot be otherwise canonicalized.
static GURL CanonicalizeUrl(const GURL& url);
// Used for testing.
void SetHostResolver(net::HostResolver* host_resolver) {
host_resolver_ = host_resolver;
// Used for testing.
void SetTransportSecurityState(
net::TransportSecurityState* transport_security_state) {
transport_security_state_ = transport_security_state;
// Used for testing.
void SetProxyAdvisor(ProxyAdvisor* proxy_advisor) {
// Used for testing.
size_t max_concurrent_dns_lookups() const {
return max_concurrent_dns_lookups_;
// Used for testing.
void SetShutdown(bool shutdown) {
shutdown_ = shutdown;
// Used for testing.
void SetObserver(PredictorObserver* observer) {
observer_ = observer;
// Flag setting to use preconnection instead of just DNS pre-fetching.
bool preconnect_enabled() const {
return preconnect_enabled_;
// Flag setting for whether we are prefetching dns lookups.
bool predictor_enabled() const {
return predictor_enabled_;
FRIEND_TEST_ALL_PREFIXES(PredictorTest, BenefitLookupTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, ShutdownWhenResolutionIsPendingTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, SingleLookupTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, ConcurrentLookupTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, MassiveConcurrentLookupTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, PriorityQueuePushPopTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, PriorityQueueReorderTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, ReferrerSerializationTrimTest);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, SingleLookupTestWithDisabledAdvisor);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, SingleLookupTestWithEnabledAdvisor);
FRIEND_TEST_ALL_PREFIXES(PredictorTest, TestSimplePreconnectAdvisor);
friend class WaitForResolutionHelper; // For testing.
class LookupRequest;
// A simple priority queue for handling host names.
// Some names that are queued up have |motivation| that requires very rapid
// handling. For example, a sub-resource name lookup MUST be done before the
// actual sub-resource is fetched. In contrast, a name that was speculatively
// noted in a page has to be resolved before the user "gets around to"
// clicking on a link. By tagging (with a motivation) each push we make into
// this FIFO queue, the queue can re-order the more important names to service
// them sooner (relative to some low priority background resolutions).
class HostNameQueue {
void Push(const GURL& url,
UrlInfo::ResolutionMotivation motivation);
bool IsEmpty() const;
GURL Pop();
// The names in the queue that should be serviced (popped) ASAP.
std::queue<GURL> rush_queue_;
// The names in the queue that should only be serviced when rush_queue is
// empty.
std::queue<GURL> background_queue_;
// The InitialObserver monitors navigations made by the network stack. This
// is only used to identify startup time resolutions (for re-resolution
// during our next process startup).
// TODO(jar): Consider preconnecting at startup, which may be faster than
// waiting for render process to start and request a connection.
class InitialObserver {
// Recording of when we observed each navigation.
typedef std::map<GURL, base::TimeTicks> FirstNavigations;
// Potentially add a new URL to our startup list.
void Append(const GURL& url, Predictor* predictor);
// Get an HTML version of our current planned first_navigations_.
void GetFirstResolutionsHtml(std::string* output);
// Persist the current first_navigations_ for storage in a list.
void GetInitialDnsResolutionList(base::ListValue* startup_list);
// Discards all initial loading history.
void DiscardInitialNavigationHistory() { first_navigations_.clear(); }
// List of the first N URL resolutions observed in this run.
FirstNavigations first_navigations_;
// The number of URLs we'll save for pre-resolving at next startup.
static const size_t kStartupResolutionCount = 10;
// A map that is keyed with the host/port that we've learned were the cause
// of loading additional URLs. The list of additional targets is held
// in a Referrer instance, which is a value in this map.
typedef std::map<GURL, Referrer> Referrers;
// Depending on the expected_subresource_use_, we may either make a TCP/IP
// preconnection, or merely pre-resolve the hostname via DNS (or even do
// nothing). The following are the threasholds for taking those actions.
static const double kPreconnectWorthyExpectedValue;
static const double kDNSPreresolutionWorthyExpectedValue;
// Referred hosts with a subresource_use_rate_ that are less than the
// following threshold will be discarded when we Trim() the list.
static const double kDiscardableExpectedValue;
// During trimming operation to discard hosts for which we don't have likely
// subresources, we multiply the expected_subresource_use_ value by the
// following ratio until that value is less than kDiscardableExpectedValue.
// This number should always be less than 1, an more than 0.
static const double kReferrerTrimRatio;
// Interval between periodic trimming of our whole referrer list.
// We only do a major trimming about once an hour, and then only when the user
// is actively browsing.
static const int64 kDurationBetweenTrimmingsHours;
// Interval between incremental trimmings (to avoid inducing Jank).
static const int64 kDurationBetweenTrimmingIncrementsSeconds;
// Number of referring URLs processed in an incremental trimming.
static const size_t kUrlsTrimmedPerIncrement;
// Only for testing. Returns true if hostname has been successfully resolved
// (name found).
bool WasFound(const GURL& url) const {
Results::const_iterator it(results_.find(url));
return (it != results_.end()) &&
// Only for testing. Return how long was the resolution
// or UrlInfo::NullDuration() if it hasn't been resolved yet.
base::TimeDelta GetResolutionDuration(const GURL& url) {
if (results_.find(url) == results_.end())
return UrlInfo::NullDuration();
return results_[url].resolve_duration();
// Only for testing;
size_t peak_pending_lookups() const { return peak_pending_lookups_; }
// If a proxy advisor is defined, let it know that |url| will be prefetched or
// preconnected to. Can be called on either UI or IO threads and will post to
// the IO thread if necessary, invoking AdviseProxyOnIOThread().
void AdviseProxy(const GURL& url,
UrlInfo::ResolutionMotivation motivation,
bool is_preconnect);
// ------------- Start IO thread methods.
// Perform actual resolution or preconnection to subresources now. This is
// an internal worker method that is reached via a post task from
// PredictFrameSubresources().
void PrepareFrameSubresources(const GURL& url,
const GURL& first_party_for_cookies);
// Access method for use by async lookup request to pass resolution result.
void OnLookupFinished(LookupRequest* request, const GURL& url, bool found);
// Underlying method for both async and synchronous lookup to update state.
void LookupFinished(LookupRequest* request,
const GURL& url, bool found);
// Queue hostname for resolution. If queueing was done, return the pointer
// to the queued instance, otherwise return NULL. If the proxy advisor is
// enabled, and |url| is likely to be proxied, the hostname will not be
// queued as the browser is not expected to fetch it directly.
UrlInfo* AppendToResolutionQueue(const GURL& url,
UrlInfo::ResolutionMotivation motivation);
// Check to see if too much queuing delay has been noted for the given info,
// which indicates that there is "congestion" or growing delay in handling the
// resolution of names. Rather than letting this congestion potentially grow
// without bounds, we abandon our queued efforts at pre-resolutions in such a
// case.
// To do this, we will recycle |info|, as well as all queued items, back to
// the state they had before they were queued up. We can't do anything about
// the resolutions we've already sent off for processing on another thread, so
// we just let them complete. On a slow system, subject to congestion, this
// will greatly reduce the number of resolutions done, but it will assure that
// any resolutions that are done, are in a timely and hence potentially
// helpful manner.
bool CongestionControlPerformed(UrlInfo* info);
// Take lookup requests from work_queue_ and tell HostResolver to look them up
// asynchronously, provided we don't exceed concurrent resolution limit.
void StartSomeQueuedResolutions();
// Performs trimming similar to TrimReferrersNow(), except it does it as a
// series of short tasks by posting continuations again an again until done.
void TrimReferrers();
// Loads urls_being_trimmed_ from keys of current referrers_.
void LoadUrlsForTrimming();
// Posts a task to do additional incremental trimming of referrers_.
void PostIncrementalTrimTask();
// Calls Trim() on some or all of urls_being_trimmed_.
// If it does not process all the URLs in that vector, it posts a task to
// continue with them shortly (i.e., it yeilds and continues).
void IncrementalTrimReferrers(bool trim_all_now);
// If a proxy advisor is defined, let it know that |url| will be prefetched or
// preconnected to.
void AdviseProxyOnIOThread(const GURL& url,
UrlInfo::ResolutionMotivation motivation,
bool is_preconnect);
// Applies the HSTS redirect for |url|, if any.
GURL GetHSTSRedirectOnIOThread(const GURL& url);
// ------------- End IO thread methods.
scoped_ptr<InitialObserver> initial_observer_;
// Reference to URLRequestContextGetter from the Profile which owns the
// predictor. Used by Preconnect.
scoped_refptr<net::URLRequestContextGetter> url_request_context_getter_;
// Status of speculative DNS resolution and speculative TCP/IP connection
// feature.
bool predictor_enabled_;
// work_queue_ holds a list of names we need to look up.
HostNameQueue work_queue_;
// results_ contains information for existing/prior prefetches.
Results results_;
std::set<LookupRequest*> pending_lookups_;
// For testing, to verify that we don't exceed the limit.
size_t peak_pending_lookups_;
// When true, we don't make new lookup requests.
bool shutdown_;
// The number of concurrent speculative lookups currently allowed to be sent
// to the resolver. Any additional lookups will be queued to avoid exceeding
// this value. The queue is a priority queue that will accelerate
// sub-resource speculation, and retard resolutions suggested by page scans.
const size_t max_concurrent_dns_lookups_;
// The maximum queueing delay that is acceptable before we enter congestion
// reduction mode, and discard all queued (but not yet assigned) resolutions.
const base::TimeDelta max_dns_queue_delay_;
// The host resolver we warm DNS entries for.
net::HostResolver* host_resolver_;
// The TransportSecurityState instance we query HSTS redirects from.
net::TransportSecurityState* transport_security_state_;
// The SSLConfigService we query SNI support from (used in querying HSTS
// redirects).
net::SSLConfigService* ssl_config_service_;
// Are we currently using preconnection, rather than just DNS resolution, for
// subresources and omni-box search URLs.
bool preconnect_enabled_;
// Most recent suggestion from Omnibox provided via AnticipateOmniboxUrl().
std::string last_omnibox_host_;
// The time when the last preresolve was done for last_omnibox_host_.
base::TimeTicks last_omnibox_preresolve_;
// The number of consecutive requests to AnticipateOmniboxUrl() that suggested
// preconnecting (because it was to a search service).
int consecutive_omnibox_preconnect_count_;
// The time when the last preconnection was requested to a search service.
base::TimeTicks last_omnibox_preconnect_;
class PreconnectUsage;
scoped_ptr<PreconnectUsage> preconnect_usage_;
// For each URL that we might navigate to (that we've "learned about")
// we have a Referrer list. Each Referrer list has all hostnames we might
// need to pre-resolve or pre-connect to when there is a navigation to the
// orginial hostname.
Referrers referrers_;
// List of URLs in referrers_ currently being trimmed (scaled down to
// eventually be aged out of use).
std::vector<GURL> urls_being_trimmed_;
// A time after which we need to do more trimming of referrers.
base::TimeTicks next_trim_time_;
scoped_ptr<base::WeakPtrFactory<Predictor> > weak_factory_;
scoped_ptr<ProxyAdvisor> proxy_advisor_;
// An observer for testing.
PredictorObserver* observer_;
// This version of the predictor is used for testing.
class SimplePredictor : public Predictor {
explicit SimplePredictor(bool preconnect_enabled)
: Predictor(preconnect_enabled) {}
virtual ~SimplePredictor() {}
virtual void InitNetworkPredictor(
PrefService* user_prefs,
PrefService* local_state,
IOThread* io_thread,
net::URLRequestContextGetter* getter) OVERRIDE;
virtual void ShutdownOnUIThread() OVERRIDE;
} // namespace chrome_browser_net