HTTP Cache Age calculation mechanism changed with RFC 7234

net/http/http_response_headers.cc

 // 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.
 
 // The rules for header parsing were borrowed from Firefox:
 // http://lxr.mozilla.org/seamonkey/source/netwerk/protocol/http/src/nsHttpResponseHead.cpp
 // The rules for parsing content-types were also borrowed from Firefox:
 // http://lxr.mozilla.org/mozilla/source/netwerk/base/src/nsURLHelper.cpp#834
 
 #include "net/http/http_response_headers.h"
@@ skipping 1085 matching lines @@
     return lifetimes;
   }
 
   // Our heuristic freshness estimate for this resource is 0 seconds, in
   // accordance with common browser behaviour. However, stale-while-revalidate
   // may still apply.
   DCHECK_EQ(TimeDelta(), lifetimes.freshness);
   return lifetimes;
 }
 
-// From RFC 2616 section 13.2.3:
+// From RFC 7234 section 4.2.3:
 //
-// Summary of age calculation algorithm, when a cache receives a response:
+// The following data is used for the age calculation:
 //
-//   /*
-//    * age_value
-//    *      is the value of Age: header received by the cache with
-//    *              this response.
-//    * date_value
-//    *      is the value of the origin server's Date: header
-//    * request_time
-//    *      is the (local) time when the cache made the request
-//    *              that resulted in this cached response
-//    * response_time
-//    *      is the (local) time when the cache received the
-//    *              response
-//    * now
-//    *      is the current (local) time
-//    */
-//   apparent_age = max(0, response_time - date_value);
-//   corrected_received_age = max(apparent_age, age_value);
-//   response_delay = response_time - request_time;
-//   corrected_initial_age = corrected_received_age + response_delay;
-//   resident_time = now - response_time;
-//   current_age   = corrected_initial_age + resident_time;
+//    age_value
+//
+//       The term "age_value" denotes the value of the Age header field
+//       (Section 5.1), in a form appropriate for arithmetic operation; or
+//       0, if not available.
+//
+//    date_value
+//
+//       The term "date_value" denotes the value of the Date header field,
+//       in a form appropriate for arithmetic operations.  See Section
+//       7.1.1.2 of [RFC7231] for the definition of the Date header field,
+//       and for requirements regarding responses without it.
+//
+//    now
+//
+//       The term "now" means "the current value of the clock at the host
+//       performing the calculation".  A host ought to use NTP ([RFC5905])
+//       or some similar protocol to synchronize its clocks to Coordinated
+//       Universal Time.
+//
+//    request_time
+//
+//       The current value of the clock at the host at the time the request
+//       resulting in the stored response was made.
+//
+//    response_time
+//
+//       The current value of the clock at the host at the time the
+//       response was received.
+//
+//    The age is then calculated as
+//
+//     apparent_age = max(0, response_time - date_value);
+//     response_delay = response_time - request_time;
+//     corrected_age_value = age_value + response_delay;
+//     corrected_initial_age = max(apparent_age, corrected_age_value);
+//     resident_time = now - response_time;
+//     current_age = corrected_initial_age + resident_time;
 //
 TimeDelta HttpResponseHeaders::GetCurrentAge(const Time& request_time,
                                              const Time& response_time,
                                              const Time& current_time) const {
   // If there is no Date header, then assume that the server response was
   // generated at the time when we received the response.
   Time date_value;
   if (!GetDateValue(&date_value))
     date_value = response_time;
 
   // If there is no Age header, then assume age is zero.  GetAgeValue does not
   // modify its out param if the value does not exist.
   TimeDelta age_value;
   GetAgeValue(&age_value);
 
   TimeDelta apparent_age = std::max(TimeDelta(), response_time - date_value);
-  TimeDelta corrected_received_age = std::max(apparent_age, age_value);
   TimeDelta response_delay = response_time - request_time;
-  TimeDelta corrected_initial_age = corrected_received_age + response_delay;
+  TimeDelta corrected_age_value = age_value + response_delay;
+  TimeDelta corrected_initial_age = std::max(apparent_age, corrected_age_value);
   TimeDelta resident_time = current_time - response_time;
   TimeDelta current_age = corrected_initial_age + resident_time;
 
   return current_age;
 }
 
 bool HttpResponseHeaders::GetMaxAgeValue(TimeDelta* result) const {
   return GetCacheControlDirective("max-age", result);
 }
 
@@ skipping 308 matching lines @@
   return true;
 }
 
 bool HttpResponseHeaders::IsChunkEncoded() const {
   // Ignore spurious chunked responses from HTTP/1.0 servers and proxies.
   return GetHttpVersion() >= HttpVersion(1, 1) &&
       HasHeaderValue("Transfer-Encoding", "chunked");
 }
 
 }  // namespace net