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diff --git a/netwerk/base/nsIClassOfService.idl b/netwerk/base/nsIClassOfService.idl
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+/**
+ * nsIClassOfService.idl
+ *
+ * Used to express class dependencies and characteristics - complimentary to
+ * nsISupportsPriority which is used to express weight
+ *
+ * Channels that implement this interface may make use of this
+ * information in different ways.
+ */
+
+[scriptable, uuid(1ccb58ec-5e07-4cf9-a30d-ac5490d23b41)]
+interface nsIClassOfService : nsISupports
+{
+  attribute unsigned long classFlags;
+
+  void clearClassFlags(in unsigned long flags);
+  void addClassFlags(in unsigned long flags);
+
+  // All these flags have a (de)prioritization effect.
+
+  // In the HTTP/1 world the priority is considered for all requests inside a so
+  // called 'Request Context' which is a context common to all sub-resources
+  // belonging to a single top level window (RequestContextService). Requests
+  // marked with the Leader flag are blocking (preventing from being sent to the
+  // server) all other resource loads except those marked with the Unblocked
+  // flag.  Other classes run in parallel - neither being blocked nor blocking.
+  // The Leader flag is used only for <head> blocking resources (sync and
+  // defer javascript resources and stylesheets.) Purpose is to deliver these
+  // first-paint and domcontentloaded blocking resources as soon as possbile.
+
+  // In the HTTP/2 world it's different. Priorities are done only per HTTP/2
+  // session, normally we have one session per one origin (including origin
+  // attributes!) Requests are dispatched (sent) immediately on an HTTP/2
+  // session. Each session has artificial streams (groups) relating to the class
+  // of service flags (Leader, Other, Background, Speculative, Follower,
+  // UrgentStart), each such a stream is given a different weight (only way to
+  // give a stream a priority in HTTP/2) reflecting the desired request group
+  // priority.  Actual request streams are then dependent on these artificial
+  // streams (groups). nsISupportsPriority of each request is passed as a weight
+  // on the HTTP/2 stream to prioritize streams in the same group. A stream can
+  // also be dependent on other stream. We have dependency of Followers on
+  // Leaders, hence everything set the Follower flag should be processed by the
+  // server after Leaders. Same for Speculative being dependent on Background. The
+  // tree is created and used here:
+  // https://searchfox.org/mozilla-central/rev/cc280c4be94ff8cf64a27cc9b3d6831ffa49fa45/netwerk/protocol/http/Http2Session.cpp#1053-1070
+  // https://searchfox.org/mozilla-central/rev/cc280c4be94ff8cf64a27cc9b3d6831ffa49fa45/netwerk/protocol/http/Http2Stream.cpp#1338
+  // For detailed description of how HTTP/2 server should handle the priorities
+  // and dependencies see:
+  // https://developers.google.com/web/fundamentals/performance/http2/#stream_prioritization
+  // Please note that the dependecies and weights we are sending to the server
+  // are only suggestions, the server may completely ignore it.
+
+  // Leaders (should) block all other resources except Unblocked.  This flag
+  // also priortizes HTTP cache reading queue by blocking all other cache
+  // requests.
+  const unsigned long Leader = 1 << 0;
+  // The Follower flag is currently unused!
+  const unsigned long Follower = 1 << 1;
+  // The Speculative flag is currently unused!
+  const unsigned long Speculative = 1 << 2;
+  // The Background flag is currently only used for Beacon.
+  const unsigned long Background = 1 << 3;
+  // Requests marked with this flag are not blocked by Leaders.  This is mainly
+  // used for probing-like XMLHttpRequests that may block delivery of head
+  // blocking resources, e.g. CSS files tailored for the UA.
+  const unsigned long Unblocked = 1 << 4;
+  // Throttleable flag allows response throttling of the resource load. Note
+  // that this functionality is currently disabled.
+  const unsigned long Throttleable = 1 << 5;
+  // UrgentStart makes the request temporarily extend HTTP/1 connection
+  // parallelism limits.  Used mainly for navigational requests (top level html)
+  // and any request considered coming from a user interaction to make reaction
+  // of the browser as fast as possible and not blocked.
+  const unsigned long UrgentStart = 1 << 6;
+  // Specifically disables throttling under any circumstances, used for media
+  // responses mainly.
+  const unsigned long DontThrottle = 1 << 7;
+  // Enforce tailing on this load; any of Leader, Unblocked, UrgentStart,
+  // TailForbidden overrule this flag (disable tailing.)
+  const unsigned long Tail = 1 << 8;
+  // Tailing may be engaged regardless if the load is marked Unblocked when some
+  // other conditions are met later, like when the load is found to be a
+  // tracker.
+  const unsigned long TailAllowed = 1 << 9;
+  // Tailing not allowed under any circumstances or combination of flags.
+  const unsigned long TailForbidden = 1 << 10;
+};