Add comments describing changes introduced in each QUIC version.

net/quic/quic_protocol.h

 // 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.
 
 #ifndef NET_QUIC_QUIC_PROTOCOL_H_
 #define NET_QUIC_QUIC_PROTOCOL_H_
 
 #include <stddef.h>
 #include <limits>
 #include <map>
@@ skipping 260 matching lines @@
 // The available versions of QUIC. Guaranteed that the integer value of the enum
 // will match the version number.
 // When adding a new version to this enum you should add it to
 // kSupportedQuicVersions (if appropriate), and also add a new case to the
 // helper methods QuicVersionToQuicTag, QuicTagToQuicVersion, and
 // QuicVersionToString.
 enum QuicVersion {
   // Special case to indicate unknown/unsupported QUIC version.
   QUIC_VERSION_UNSUPPORTED = 0,
 
-  QUIC_VERSION_15 = 15,
-  QUIC_VERSION_16 = 16,
-  QUIC_VERSION_18 = 18,
-  QUIC_VERSION_19 = 19,
-  QUIC_VERSION_20 = 20,
-  QUIC_VERSION_21 = 21,  // Current version.
+  QUIC_VERSION_15 = 15,  // Revived packets in ReceivedPacketInfo.
+  QUIC_VERSION_16 = 16,  // STOP_WAITING frame.
+  QUIC_VERSION_18 = 18,  // PING frame.
+  QUIC_VERSION_19 = 19,  // Session level flow control.
+  QUIC_VERSION_20 = 20,  // Independent stream/session flow control windows.
+  QUIC_VERSION_21 = 21,  // Headers/crypto streams are flow controlled.
 };
 
 // This vector contains QUIC versions which we currently support.
 // This should be ordered such that the highest supported version is the first
 // element, with subsequent elements in descending order (versions can be
 // skipped as necessary).
 //
 // IMPORTANT: if you are addding to this list, follow the instructions at
 // http://sites/quic/adding-and-removing-versions
 static const QuicVersion kSupportedQuicVersions[] = {QUIC_VERSION_21,
@@ skipping 770 matching lines @@
   // Stores the sequence numbers of all transmissions of this packet.
   // Can never be null.
   SequenceNumberSet* all_transmissions;
   // In flight packets have not been abandoned or lost.
   bool in_flight;
 };
 
 }  // namespace net
 
 #endif  // NET_QUIC_QUIC_PROTOCOL_H_