Gather metrics classifying the cause of the TLS fallback.

net/ssl/ssl_failure_state.h

+// Copyright 2015 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_SSL_SSL_FAILURE_STATE_H_
+#define NET_SSL_SSL_FAILURE_STATE_H_
+
+namespace net {
+
+// Gives more information about the state of the handshake when it failed. This

[Ryan Sleevi, 2015/05/13 22:26:37]

s/the handshake/TLS handshake/

s/when it failed/when it fails/?

Is this documenting too much about the API? That is, the "gives" verb is a
property of the socket, this enum doesn't give anything.

// Describes the most likely cause for the TLS handshake failure. This is an
// approximation used to classify the causes of TLS version fallback.

or something like that?

[davidben, 2015/05/14 23:24:34]

Yeah, your's is much better. DOne.

+// is used to classify on causes of the TLS version fallback. These values are
+// used in histograms, so new values must be appended.
+enum SSLFailureState {
+  // The connection was successful.
+  kSSLFailureNone = 0,
+  // The connection failed for unknown reasons.
+  kSSLFailureUnknown = 1,
+  // The connection failed after sending ClientHello and before receiving
+  // ServerHello.
+  kSSLFailureClientHello = 2,
+  // The connection failed after negotiating TLS_RSA_WITH_AES_128_GCM_SHA256 or
+  // TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and completing the client's second
+  // leg. Some Microsoft IIS servers fail at this point. See
+  // https://crbug.com/433406.
+  kSSLFailureBuggyGCM = 3,
+  // The connection failed after CertificateVerify was sent. Some servers are
+  // known to incorrectly implement TLS 1.2 client auth.
+  kSSLFailureClientAuth = 4,
+  // The connection failed because the server attempted to resume a session at
+  // the wrong version. Some versions of OpenSSL may do this in rare
+  // circumstances. See https://crbug.com/441456
+  kSSLFailureSessionMismatch = 5,
+  // The connection failed after sending the NextProto message. Some F5 servers
+  // fail to parse such messages in TLS 1.1 and TLS 1.2, but not 1.0. See
+  // https://crbug.com/466977.
+  kSSLFailureNextProto = 6,

[Ryan Sleevi, 2015/05/13 22:26:37]

style: Newlines between each value would help (eg line 15+16, line 17+18, line
20+21, etc)

[davidben, 2015/05/14 23:24:34]

Done.

+
+  kSSLFailureMax,
+};
+
+}  // namespace net
+
+#endif  // NET_SSL_SSL_FAILURE_STATE_H_