Created the DtlsSrtpTransport.

webrtc/pc/dtlssrtptransport.h

Index: webrtc/pc/dtlssrtptransport.h
diff --git a/webrtc/pc/dtlssrtptransport.h b/webrtc/pc/dtlssrtptransport.h
new file mode 100644
index 0000000000000000000000000000000000000000..7a2b788c1f8d302488bd71c1b9d86b9f98694d51
--- /dev/null
+++ b/webrtc/pc/dtlssrtptransport.h
@@ -0,0 +1,109 @@
+/*
+ *  Copyright 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_PC_DTLSSRTPTRANSPORT_H_
+#define WEBRTC_PC_DTLSSRTPTRANSPORT_H_
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "webrtc/p2p/base/dtlstransportinternal.h"
+#include "webrtc/pc/srtptransport.h"
+
+namespace webrtc {
+
+// This class exports the keying materials from underneath DtlsTransport and

[pthatcher, 2017/09/12 00:24:44]

"from underneath DtlsTransport" => "from the DtlsTransport underneath"

[Zhi Huang, 2017/09/27 00:46:32]

Done.

+// sets the crypto keys for the wrapped SrtpTransport.
+class DtlsSrtpTransport : public RtpTransportInternal {
+ public:
+  DtlsSrtpTransport(std::unique_ptr<webrtc::SrtpTransport> srtp_transport,
+                    const std::string& content_name);

[pthatcher, 2017/09/12 00:24:44]

Why is the content name needed?

[Zhi Huang, 2017/09/27 00:46:32]

Removed.

+
+  bool SetupDtlsSrtp(bool rtcp);

[pthatcher, 2017/09/12 00:24:44]

Note: this is the kind of place removing support for non-muxed RTCP would be
nice.  RCS doesn't need DTLS support.

[Zhi Huang, 2017/09/27 00:46:33]

Acknowledged.

+
+  // Set a P2P layer DtlsTransport for DtlsSrtpTransport.
+  void SetDtlsTransport(bool rtcp,
+                        cricket::DtlsTransportInternal* dtls_transport);

[pthatcher, 2017/09/12 00:24:44]

The getters have two methods.  I think the setters should have two methods
(SetRtpDtlsTransport and SetRtcpDtlsTransport).

[Zhi Huang, 2017/09/27 00:46:32]

Done.

+
+  // Set the header extension ids that should be encrypted for the given source.
+  // This method doesn't immediately update the SRTP session with the new IDs,
+  // and you need to call SetupDtlsSrtp for that to happen.
+  void SetEncryptedHeaderExtensionIds(cricket::ContentSource source,
+                                      const std::vector<int>& extension_ids);

[pthatcher, 2017/09/12 00:24:44]

Instead of passing in a source, I think it would be better to pass in either
send or receiver extension IDs separately (either separate args or separate
methods).

[Zhi Huang, 2017/09/27 00:46:32]

Agreed. The BaseChannel should take care of the ContentSource. I think we should
change the SrtpTransport too.

+
+  cricket::DtlsTransportInternal* rtp_dtls_transport() {
+    return rtp_dtls_transport_;
+  }
+
+  cricket::DtlsTransportInternal* rtcp_dtls_transport() {
+    return rtcp_dtls_transport_;
+  }
+
+  // RtpTransportInternal overrides.
+  void SetRtcpMuxEnabled(bool enable) override;
+
+  rtc::PacketTransportInternal* rtp_packet_transport() const override;
+
+  void SetRtpPacketTransport(rtc::PacketTransportInternal* rtp) override;
+
+  PacketTransportInterface* GetRtpPacketTransport() const override;
+
+  rtc::PacketTransportInternal* rtcp_packet_transport() const override;
+
+  void SetRtcpPacketTransport(rtc::PacketTransportInternal* rtcp) override;
+
+  PacketTransportInterface* GetRtcpPacketTransport() const override;
+
+  bool IsWritable(bool rtcp) const override;
+
+  bool SendRtpPacket(rtc::CopyOnWriteBuffer* packet,
+                     const rtc::PacketOptions& options,
+                     int flags) override;
+
+  bool SendRtcpPacket(rtc::CopyOnWriteBuffer* packet,
+                      const rtc::PacketOptions& options,
+                      int flags) override;
+
+  bool HandlesPayloadType(int payload_type) const override;
+
+  void AddHandledPayloadType(int payload_type) override;
+
+  bool IsActive() { return active_; }
+
+  void ResetParams();
+
+  RTCError SetParameters(const RtpTransportParameters& parameters) override;
+
+  RtpTransportParameters GetParameters() const override;
+
+  // TODO(zhihuang): Remove this when we remove RtpTransportAdapter.
+  RtpTransportAdapter* GetInternal() override { return nullptr; }
+
+ private:
+  void ConnectToSrtpTransport();
+
+  void OnPacketReceived(bool rtcp,
+                        rtc::CopyOnWriteBuffer* packet,
+                        const rtc::PacketTime& packet_time);
+
+  void OnReadyToSend(bool ready);
+
+  bool active_ = false;

[pthatcher, 2017/09/12 00:24:44]

Can you leave a comment explaining when it becomes active?

[Zhi Huang, 2017/09/27 00:46:32]

Done.

+  std::unique_ptr<SrtpTransport> srtp_transport_;
+  std::string content_name_;
+  // Not owned by this class.

[pthatcher, 2017/09/12 00:24:44]

Who has the responsibility of tracking the lifetime?  The caller of
SetDtlsTransport?  If so, that needs to be documented.

[Zhi Huang, 2017/09/27 00:46:32]

The transport controller should own these.

+  cricket::DtlsTransportInternal* rtp_dtls_transport_ = nullptr;
+  cricket::DtlsTransportInternal* rtcp_dtls_transport_ = nullptr;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_PC_DTLSSRTPTRANSPORT_H_