Android WifiDisplay分析三：RTSP交互以及数据传输

前面我们分析到WifiDisplaySource会调用ANetworkSession的接口去创建一个socket，并在这个socket上监听是否有客户端的连接请求。先来看看Wifi Display规范的一些流程图：

从之前的一篇文章中，当ANetworkSession创建好RTSP的listen socket后，就会把它加入到selelct中等待对方的连接，那我们首先来看ANetworkSession的threadLoop方法：

 void ANetworkSession::threadLoop() {

     int res = select(maxFd + 1, &rs, &ws, NULL, NULL /* tv */);

     {
         Mutex::Autolock autoLock(mLock);

         List > sessionsToAdd;

         for (size_t i = mSessions.size(); res > 0 && i-- > 0;) {
             const sp &session = mSessions.valueAt(i);

             int s = session->socket();

             if (s < 0) {
                 continue;
             }

             if (FD_ISSET(s, &rs) || FD_ISSET(s, &ws)) {
                 --res;
             }

             if (FD_ISSET(s, &rs)) {
                 if (session->isRTSPServer() || session->isTCPDatagramServer()) {
                     struct sockaddr_in remoteAddr;
                     socklen_t remoteAddrLen = sizeof(remoteAddr);

                     int clientSocket = accept(
                             s, (struct sockaddr *)&remoteAddr, &remoteAddrLen);

                     if (clientSocket >= 0) {
                         status_t err = MakeSocketNonBlocking(clientSocket);

                         if (err != OK) {

                         } else {
                             in_addr_t addr = ntohl(remoteAddr.sin_addr.s_addr);

                             ALOGI("incoming connection from %d.%d.%d.%d:%d "
                                   "(socket %d)",
                                   (addr >> 24),
                                   (addr >> 16) & 0xff,
                                   (addr >> 8) & 0xff,
                                   addr & 0xff,
                                   ntohs(remoteAddr.sin_port),
                                   clientSocket);

                             sp clientSession =
                                 new Session(
                                         mNextSessionID++,
                                         Session::CONNECTED,
                                         clientSocket,
                                         session->getNotificationMessage());

                             clientSession->setMode(
                                     session->isRTSPServer()
                                         ? Session::MODE_RTSP
                                         : Session::MODE_DATAGRAM);

                             sessionsToAdd.push_back(clientSession);
                         }
                     } else {
                         ALOGE("accept returned error %d (%s)",
                               errno, strerror(errno));
                     }
                 }
             }

         while (!sessionsToAdd.empty()) {
             sp session = *sessionsToAdd.begin();
             sessionsToAdd.erase(sessionsToAdd.begin());

             mSessions.add(session->sessionID(), session);

             ALOGI("added clientSession %d", session->sessionID());
         }
     }

上面在selelct循环中，首先只有刚创建的RTSP的listen socket，接着如果有客户端的连接请求，就会跳出select语句，然后调用accept去接收对方的连接。接着会去创建一个新的Session会话，我们去看它的构造函数：

 ANetworkSession::Session::Session(
         int32_t sessionID,
         State state,
         int s,
         const sp ¬ify)
     : mSessionID(sessionID),
       mState(state),
       mMode(MODE_DATAGRAM),
       mSocket(s),
       mNotify(notify),
       mSawReceiveFailure(false),
       mSawSendFailure(false),
       mUDPRetries(kMaxUDPRetries),
       mLastStallReportUs(-1ll) {
     if (mState == CONNECTED) {
         struct sockaddr_in localAddr;
         socklen_t localAddrLen = sizeof(localAddr);

         int res = getsockname(
                 mSocket, (struct sockaddr *)&localAddr, &localAddrLen);
         CHECK_GE(res, 0);

         struct sockaddr_in remoteAddr;
         socklen_t remoteAddrLen = sizeof(remoteAddr);

         res = getpeername(
                 mSocket, (struct sockaddr *)&remoteAddr, &remoteAddrLen);
         CHECK_GE(res, 0);

         sp msg = mNotify->dup();
         msg->setInt32("sessionID", mSessionID);
         msg->setInt32("reason", kWhatClientConnected);
         msg->setString("server-ip", localAddrString.c_str());
         msg->setInt32("server-port", ntohs(localAddr.sin_port));
         msg->setString("client-ip", remoteAddrString.c_str());
         msg->setInt32("client-port", ntohs(remoteAddr.sin_port));
         msg->post();
     }
 }

这里的状态mState为CONNECTED，所以会构建一个AMessage并post出去，由前一章的知识，我们知道这里的mNotify是一个kWhatRTSPNotify消息，会在WifiDisplaySource调用createRTSPServer时传进来的一个参数，所以这里创建的AMessage最终还是会被WifiDisplaySource去处理，跳过中间AMessage、ALooperRoster、ALooper的调用关系，我们直接到WifiDisplaySource的onMessageReceived去看如何处理：

 case kWhatRTSPNotify:
 {
     int32_t reason;
     CHECK(msg->findInt32("reason", &reason));

     switch (reason) {
         case ANetworkSession::kWhatError:
         {

             break;
         }

         case ANetworkSession::kWhatClientConnected:
         {
             int32_t sessionID;
             CHECK(msg->findInt32("sessionID", &sessionID));

             if (mClientSessionID > 0) {
                 ALOGW("A client tried to connect, but we already "
                       "have one.");

                 mNetSession->destroySession(sessionID);
                 break;
             }

             CHECK_EQ(mState, AWAITING_CLIENT_CONNECTION);

             CHECK(msg->findString("client-ip", &mClientInfo.mRemoteIP));
             CHECK(msg->findString("server-ip", &mClientInfo.mLocalIP));

             if (mClientInfo.mRemoteIP == mClientInfo.mLocalIP) {
                 // Disallow connections from the local interface
                 // for security reasons.
                 mNetSession->destroySession(sessionID);
                 break;
             }

             CHECK(msg->findInt32(
                         "server-port", &mClientInfo.mLocalPort));
             mClientInfo.mPlaybackSessionID = -1;

             mClientSessionID = sessionID;

             ALOGI("We now have a client (%d) connected.", sessionID);

             mState = AWAITING_CLIENT_SETUP;

             status_t err = sendM1(sessionID);
             CHECK_EQ(err, (status_t)OK);
             break;
         }

         case ANetworkSession::kWhatData:
         {

             break;
         }

         case ANetworkSession::kWhatNetworkStall:
         {
             break;
         }

         default:
             TRESPASS();
     }
     break;
 }

这里的reason是kWhatClientConnected，跳过前面不必要的case语句。如果先前已经连上其它的Sink device，这里就先断开之前的连接；如果没有，将新的SessionID赋予给mClientSessionID，并更改状态为AWAITING_CLIENT_SETUP，接着去看sendM1消息，这时候就要开始WifiDisplay M1~M7消息的发送了。

下面列举M1~M7消息的格式，有兴趣的可以去对照代码分析，我们后面着重分析M6(SetUp)和M7(Play)两个消息。

M1 reqeust:

OPTIONS * RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:24 +0000
Server: stagefright/1.2 (Linux;Android 4.4)
CSeq: 1
Require: org.wfa.wfd1.0

M1 respose:

RTSP/1.0 200 OK
CSeq: 1
Date: Fri, Jan 01 2014 09:02:37 GMT
Public: org.wfa.wfd1.0, GET_PARAMETER, SET_PARAMETER

M2 request:

OPTIONS * RTSP/1.0
CSeq: 2
Require: org.wfa.wfd1.0

M2 response:

RTSP/1.0 200 OK
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 2
Public: org.wfa.wfd1.0, SETUP, TEARDOWN, PLAY, PAUSE, GET_PARAMETER, SET_PARAMETER

M3 request:

GET_PARAMETER rtsp://localhost/wfd1.0 RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 2
Content-Type: text/parameters
Content-Length: 83

wfd_content_protection
wfd_video_formats
wfd_audio_codecs
wfd_client_rtp_ports

M3 response:

RTSP/1.0 200 OK
CSeq: 2
Content-Length: 124
Content-Type: text/parameters

wfd_audio_codecs: LPCM 00000003 00, AAC 00000007 00
wfd_video_formats: 00 00 02 02 0000FFFF 0FFFFFFF 00000FFF 00 0000 0000 01 none none
wfd_content_protection: none
wfd_client_rtp_ports: RTP/AVP/UDP;unicast 19990 0 mode=play

M4 request:

SET_PARAMETER rtsp://localhost/wfd1.0 RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 3
Content-Type: text/parameters
Content-Length: 247
wfd_video_formats: 00 00 02 02 00000020 00000000 00000000 00 0000 0000 00 none none
wfd_audio_codecs: AAC 00000001 00
wfd_presentation_URL: rtsp://192.168.5.200/wfd1.0/streamid=0 none
wfd_client_rtp_ports: RTP/AVP/UDP;unicast 19990 0 mode=play

M4 response:

RTSP/1.0 200 OK
CSeq: 3

M5 request:

SET_PARAMETER rtsp://localhost/wfd1.0 RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 4
Content-Type: text/parameters
Content-Length: 27
wfd_trigger_method: SETUP

M5 response:

RTSP/1.0 200 OK
CSeq: 4

M6 request:

SETUP rtsp://192.168.5.200/wfd1.0/streamid=0 RTSP/1.0
CSeq: 3
Transport: RTP/AVP/UDP;unicast;client_port=19990

M6 response:

RTSP/1.0 200 OK
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 3
Session: 988982966;timeout=30
Transport: RTP/AVP/UDP;unicast;client_port=19990;server_port=22220

我们先来看处理M6 request的方法，代码在onSetupRequest中：

 status_t WifiDisplaySource::onSetupRequest(
         int32_t sessionID,
         int32_t cseq,
         const sp &data) {
     CHECK_EQ(sessionID, mClientSessionID);
     if (mClientInfo.mPlaybackSessionID != -1) {
         sendErrorResponse(sessionID, "400 Bad Request", cseq);
         return ERROR_MALFORMED;
     }

     int32_t playbackSessionID = makeUniquePlaybackSessionID();

     sp notify = new AMessage(kWhatPlaybackSessionNotify, id());
     notify->setInt32("playbackSessionID", playbackSessionID);
     notify->setInt32("sessionID", sessionID);

     sp playbackSession =
         new PlaybackSession(
                 mNetSession, notify, mInterfaceAddr, mHDCP, mMediaPath.c_str());

     looper()->registerHandler(playbackSession);

     AString uri;
     data->getRequestField(1, &uri);

     if (strncasecmp("rtsp://", uri.c_str(), 7)) {
         sendErrorResponse(sessionID, "400 Bad Request", cseq);
         return ERROR_MALFORMED;
     }

     if (!(uri.startsWith("rtsp://") && uri.endsWith("/wfd1.0/streamid=0"))) {
         sendErrorResponse(sessionID, "404 Not found", cseq);
         return ERROR_MALFORMED;
     }

     RTPSender::TransportMode rtcpMode = RTPSender::TRANSPORT_UDP;
     if (clientRtcp < 0) {
         rtcpMode = RTPSender::TRANSPORT_NONE;
     }

     status_t err = playbackSession->init(
             mClientInfo.mRemoteIP.c_str(),
             clientRtp,
             rtpMode,
             clientRtcp,
             rtcpMode,
             mSinkSupportsAudio,
             mUsingPCMAudio,
             mSinkSupportsVideo,
             mChosenVideoResolutionType,
             mChosenVideoResolutionIndex,
             mChosenVideoProfile,
             mChosenVideoLevel);

     if (err != OK) {
         looper()->unregisterHandler(playbackSession->id());
         playbackSession.clear();
     }

     mClientInfo.mPlaybackSessionID = playbackSessionID;
     mClientInfo.mPlaybackSession = playbackSession;

     mState = AWAITING_CLIENT_PLAY;

     scheduleReaper();
     scheduleKeepAlive(sessionID);

     return OK;
 }

跳过前面的关于RTSP回复消息的组织，这里还会创建一个PlaybackSession对象，并调用它的init方法做初始化。

根据前面的背景知识介绍，设备之间的交互将由Session来管理。在代码中，Session的概念由WifiDisplaySource的内部类PlaybackSession来表示。先来看和其相关的类图结构，如下图所示：

由上图可知：

•  PlaybackSession及其内部类Track都从AHandler派生。故它们的工作也依赖于消息循环和处理。Track代表视频流或音频流。
• Track内部通过mMediaPull变量指向一个MediaPull对象。而MediaPull对象则保存了一个MediaSource对象。在PlaybackSession中，此MediaSource的真正类型为SurfaceMediaSource。它表明该Media的源来自Surface。
•  BufferQueue从ISurfaceTexure中派生，根据前面对SurfaceFlinger的介绍，它就是SurfaceFlinger代码示例中代表虚拟设备的State的surface变量。
• 左图中，MediaPull通过kWhatPull消息不断调用MediaSource的read函数。
• 右图中，SurfaceMediaSource的read函数由通过mBufferQueue来读取数据。

• 那么mBufferQueue的数据来自什么地方呢？对，正是来自SurfaceFlinger。

  当然，PlaybackSession拿到这些数据后还需要做编码，然后才能发送给远端设备。由于篇幅关系，本文就不再讨论这些问题了。

  
  

当双方设备准备就绪后，MediaPull会通过kWhatPull消息处理不断调用MediaSource的read函数。在SurfaceMediaSource实现的read函数中，来自SurfaceFlinger的混屏后的数据经由BufferQueue传递到MediaPull中。