Live555不仅实现了RTSP服务器端，还实现了RTSP客户端，我们通过testRTSPClient.cpp这个程序来看一下，Live555的RTSP客户端与服务器端建立RTSP连接的过程。
首先来看一下main函数：
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1 char eventLoopWatchVariable = 0;
2
3 int main(int argc, char** argv) {
4 // Begin by setting up our usage environment:
5 TaskScheduler* scheduler = BasicTaskScheduler::createNew();
6 UsageEnvironment* env = BasicUsageEnvironment::createNew(*scheduler);
7
8 // We need at least one “rtsp://” URL argument:
9 if (argc < 2) {
10 usage(*env, argv[0]);
11 return 1;
12 }
13
14 // There are argc-1 URLs: argv[1] through argv[argc-1]. Open and start streaming each one:
15 for (int i = 1; i <= argc-1; ++i) {
16 openURL(env, argv[0], argv[i]);
17 }
18
19 // All subsequent activity takes place within the event loop:
20 env->taskScheduler().doEventLoop(&eventLoopWatchVariable);
21 // This function call does not return, unless, at some point in time, “eventLoopWatchVariable” gets set to something non-zero.
22
23 return 0;
24
25 // If you choose to continue the application past this point (i.e., if you comment out the “return 0;” statement above),
26 // and if you don’t intend to do anything more with the “TaskScheduler” and “UsageEnvironment” objects,
27 // then you can also reclaim the (small) memory used by these objects by uncommenting the following code:
28 /
29 env->reclaim(); env = NULL;
30 delete scheduler; scheduler = NULL;
31 */
32 }
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和testOnDeamandRTSPServer.cpp一样，首先也是创建TaskScheduler对象和UsageEnvironment对象，然后调用openURL函数去请求某个媒体资源，参数是该媒体资源的RTSP地址，最后使程序进入主循环。
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1 void openURL(UsageEnvironment& env, char const* progName, char const* rtspURL) {
2 // Begin by creating a “RTSPClient” object. Note that there is a separate “RTSPClient” object for each stream that we wish
3 // to receive (even if more than stream uses the same “rtsp://” URL).
4 RTSPClient* rtspClient = ourRTSPClient::createNew(env, rtspURL, RTSP_CLIENT_VERBOSITY_LEVEL, progName);
5 if (rtspClient == NULL) {
6 env << "Failed to create a RTSP client for URL “” << rtspURL << “”: " << env.getResultMsg() << “\n”;
7 return;
8 }
9
10 ++rtspClientCount;
11
12 // Next, send a RTSP “DESCRIBE” command, to get a SDP description for the stream.
13 // Note that this command - like all RTSP commands - is sent asynchronously; we do not block, waiting for a response.
14 // Instead, the following function call returns immediately, and we handle the RTSP response later, from within the event loop:
15 rtspClient->sendDescribeCommand(continueAfterDESCRIBE); //发送DESCRIBE命令，并传入回调函数
16 }
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OpenURL函数很简单，创建一个RTSPClient对象，一个RTSPClient对象代表一个RTSP客户端。然后调用sendDescribeCommand函数发送DESCRIBE命令，回调函数是continueAfterDESCRIBE函数，在收到RTSP服务器端对DESCRIBE命令的回复时调用。
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1 void continueAfterDESCRIBE(RTSPClient* rtspClient, int resultCode, char* resultString) {
2 do {
3 UsageEnvironment& env = rtspClient->envir(); // alias
4 StreamClientState& scs = ((ourRTSPClient*)rtspClient)->scs; // alias
5
6 if (resultCode != 0) { // 返回结果码非0表示出错
7 env << rtspClient << "Failed to get a SDP description: " << resultString << “\n”;
8 delete[] resultString;
9 break;
10 }
11 // resultString即从服务器端返回的SDP信息字符串
12 char const sdpDescription = resultString;
13 env << *rtspClient << “Got a SDP description:\n” << sdpDescription << “\n”;
14
15 // Create a media session object from this SDP description:
16 scs.session = MediaSession::createNew(env, sdpDescription); //根据SDP信息创建一个MediaSession对象
17 delete[] sdpDescription; // because we don’t need it anymore
18 if (scs.session == NULL) {
19 env << *rtspClient << "Failed to create a MediaSession object from the SDP description: " << env.getResultMsg() << “\n”;
20 break;
21 } else if (!scs.session->hasSubsessions()) {
22 env << *rtspClient << “This session has no media subsessions (i.e., no “m=” lines)\n”;
23 break;
24 }
25
26 // Then, create and set up our data source objects for the session. We do this by iterating over the session’s ‘subsessions’,
27 // calling “MediaSubsession::initiate()”, and then sending a RTSP “SETUP” command, on each one.
28 // (Each ‘subsession’ will have its own data source.)
29 scs.iter = new MediaSubsessionIterator(*scs.session);
30 setupNextSubsession(rtspClient); //开始对服务器端的每个ServerMediaSubsession发送SETUP命令请求建立连接
31 return;
32 } while (0);
33
34 // An unrecoverable error occurred with this stream.
35 shutdownStream(rtspClient);
36 }
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客户端收到服务器端对DESCRIBE命令的回复，取得SDP信息后，客户端创建一个MediaSession对象。MediaSession和ServerMediaSession是相对应的概念，MediaSession表示客户端请求服务器端某个媒体资源的会话，类似地，客户端还存在与ServerMediaSubsession相对应的MediaSubsession，表示MediaSession的子会话，创建MediaSession的同时也创建了包含的MediaSubsession对象。然后客户端对服务器端的每个ServerMediaSubsession发送SETUP命令请求建立连接。
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1 void setupNextSubsession(RTSPClient* rtspClient) {
2 UsageEnvironment& env = rtspClient->envir(); // alias
3 StreamClientState& scs = ((ourRTSPClient*)rtspClient)->scs; // alias
4
5 scs.subsession = scs.iter->next();
6 if (scs.subsession != NULL) {
7 if (!scs.subsession->initiate()) { // 调用initiate函数初始化MediaSubsession对象
8 env << *rtspClient << "Failed to initiate the “” << *scs.subsession << “” subsession: " << env.getResultMsg() << “\n”;
9 setupNextSubsession(rtspClient); // give up on this subsession; go to the next one
10 } else {
11 env << *rtspClient << “Initiated the “” << *scs.subsession << “” subsession (”;
12 if (scs.subsession->rtcpIsMuxed()) {
13 env << "client port " << scs.subsession->clientPortNum();
14 } else {
15 env << "client ports " << scs.subsession->clientPortNum() << “-” << scs.subsession->clientPortNum()+1;
16 }
17 env << “)\n”;
18 // 发送SETUP命令
19 // Continue setting up this subsession, by sending a RTSP “SETUP” command:
20 rtspClient->sendSetupCommand(scs.subsession, continueAfterSETUP, False, REQUEST_STREAMING_OVER_TCP);
21 }
22 return;
23 }
24 // 成功与所有的ServerMediaSubsession建立了连接，现在发送PLAY命令
25 // We’ve finished setting up all of the subsessions. Now, send a RTSP “PLAY” command to start the streaming:
26 if (scs.session->absStartTime() != NULL) {
27 // Special case: The stream is indexed by ‘absolute’ time, so send an appropriate “PLAY” command:
28 rtspClient->sendPlayCommand(scs.session, continueAfterPLAY, scs.session->absStartTime(), scs.session->absEndTime());
29 } else {
30 scs.duration = scs.session->playEndTime() - scs.session->playStartTime();
31 rtspClient->sendPlayCommand(scs.session, continueAfterPLAY);
32 }
33 }
34
35 void continueAfterSETUP(RTSPClient rtspClient, int resultCode, char resultString) {
36 do {
37 UsageEnvironment& env = rtspClient->envir(); // alias
38 StreamClientState& scs = ((ourRTSPClient)rtspClient)->scs; // alias
39
40 if (resultCode != 0) {
41 env << *rtspClient << "Failed to set up the “” << *scs.subsession << “” subsession: " << resultString << “\n”;
42 break;
43 }
44
45 env << *rtspClient << “Set up the “” << *scs.subsession << “” subsession (”;
46 if (scs.subsession->rtcpIsMuxed()) {
47 env << "client port " << scs.subsession->clientPortNum();
48 } else {
49 env << "client ports " << scs.subsession->clientPortNum() << “-” << scs.subsession->clientPortNum()+1;
50 }
51 env << “)\n”;
52
53 // Having successfully setup the subsession, create a data sink for it, and call “startPlaying()” on it.
54 // (This will prepare the data sink to receive data; the actual flow of data from the client won’t start happening until later,
55 // after we’ve sent a RTSP “PLAY” command.)
56 //对每个MediaSubsession创建一个MediaSink对象来请求和保存数据
57 scs.subsession->sink = DummySink::createNew(env, *scs.subsession, rtspClient->url());
58 // perhaps use your own custom “MediaSink” subclass instead
59 if (scs.subsession->sink == NULL) {
60 env << *rtspClient << "Failed to create a data sink for the “” << *scs.subsession
61 << “” subsession: " << env.getResultMsg() << “\n”;
62 break;
63 }
64
65 env << *rtspClient << "Created a data sink for the “” << scs.subsession << “” subsession\n";
66 scs.subsession->miscPtr = rtspClient; // a hack to let subsession handle functions get the “RTSPClient” from the subsession
67 scs.subsession->sink->startPlaying((scs.subsession->readSource()),
68 subsessionAfterPlaying, scs.subsession); // 调用MediaSink的startPlaying函数准备播放
69 // Also set a handler to be called if a RTCP “BYE” arrives for this subsession:
70 if (scs.subsession->rtcpInstance() != NULL) {
71 scs.subsession->rtcpInstance()->setByeHandler(subsessionByeHandler, scs.subsession);
72 }
73 } while (0);
74 delete[] resultString;
75
76 // Set up the next subsession, if any: 与下一个ServerMediaSubsession建立连接
77 setupNextSubsession(rtspClient);
78 }
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setupNextSubsession函数中首先调用MediaSubsession的initiate函数初始化MediaSubsession，然后对ServerMediaSubsession发送SETUP命令，收到回复后回调continueAfterSETUP函数。在continueAfterSETUP函数中，为MediaSubsession创建MediaSink对象来请求和保存服务器端发送的数据，然后调用MediaSink::startPlaying函数开始准备播放对应的ServerMediaSubsession，最后调用setupNextSubsession函数与下一个ServerMediaSubsession建立连接，在setupNextSubsession函数中，会检查是否与所有的ServerMediaSubsession都建立了连接，是则发送PLAY命令请求开始传送数据，收到回复则调用continueAfterPLAY函数。
在客户端发送PLAY命令之前，我们先看一下MediaSubsession::initiate函数的内容：
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1 Boolean MediaSubsession::initiate(int useSpecialRTPoffset) {
2 if (fReadSource != NULL) return True; // has already been initiated
3
4 do {
5 if (fCodecName == NULL) {
6 env().setResultMsg(“Codec is unspecified”);
7 break;
8 }
9 //创建客户端socket，包括RTP socket和RTCP socket，准备从服务器端接收数据
10 // Create RTP and RTCP ‘Groupsocks’ on which to receive incoming data.
11 // (Groupsocks will work even for unicast addresses)
12 struct in_addr tempAddr;
13 tempAddr.s_addr = connectionEndpointAddress();
14 // This could get changed later, as a result of a RTSP “SETUP”
15 //使用指定的RTP端口和RTCP端口，RTP端口必须是偶数，而RTCP端口必须是（RTP端口+1）
16 if (fClientPortNum != 0 && (honorSDPPortChoice || IsMulticastAddress(tempAddr.s_addr))) {
17 // The sockets’ port numbers were specified for us. Use these:
18 Boolean const protocolIsRTP = strcmp(fProtocolName, “RTP”) == 0;
19 if (protocolIsRTP && !fMultiplexRTCPWithRTP) {
20 fClientPortNum = fClientPortNum&~1;
21 // use an even-numbered port for RTP, and the next (odd-numbered) port for RTCP
22 }
23 if (isSSM()) {
24 fRTPSocket = new Groupsock(env(), tempAddr, fSourceFilterAddr, fClientPortNum);
25 } else {
26 fRTPSocket = new Groupsock(env(), tempAddr, fClientPortNum, 255);
27 }
28 if (fRTPSocket == NULL) {
29 env().setResultMsg(“Failed to create RTP socket”);
30 break;
31 }
32
33 if (protocolIsRTP) {
34 if (fMultiplexRTCPWithRTP) {
35 // Use the RTP ‘groupsock’ object for RTCP as well:
36 fRTCPSocket = fRTPSocket;
37 } else {
38 // Set our RTCP port to be the RTP port + 1:
39 portNumBits const rtcpPortNum = fClientPortNum|1;
40 if (isSSM()) {
41 fRTCPSocket = new Groupsock(env(), tempAddr, fSourceFilterAddr, rtcpPortNum);
42 } else {
43 fRTCPSocket = new Groupsock(env(), tempAddr, rtcpPortNum, 255);
44 }
45 }
46 }
47 } else {
// 选取随机的RTP端口和RTCP端口
48 // Port numbers were not specified in advance, so we use ephemeral port numbers.
49 // Create sockets until we get a port-number pair (even: RTP; even+1: RTCP).
50 // (However, if we’re multiplexing RTCP with RTP, then we create only one socket,
51 // and the port number can be even or odd.)
52 // We need to make sure that we don’t keep trying to use the same bad port numbers over
53 // and over again, so we store bad sockets in a table, and delete them all when we’re done.
54 HashTable* socketHashTable = HashTable::create(ONE_WORD_HASH_KEYS);
55 if (socketHashTable == NULL) break;
56 Boolean success = False;
57 NoReuse dummy(env());
58 // ensures that our new ephemeral port number won’t be one that’s already in use
59
60 while (1) {
61 // Create a new socket:
62 if (isSSM()) {
63 fRTPSocket = new Groupsock(env(), tempAddr, fSourceFilterAddr, 0);
64 } else {
65 fRTPSocket = new Groupsock(env(), tempAddr, 0, 255);
66 }
67 if (fRTPSocket == NULL) {
68 env().setResultMsg(“MediaSession::initiate(): unable to create RTP and RTCP sockets”);
69 break;
70 }
71
72 // Get the client port number:
73 Port clientPort(0);
74 if (!getSourcePort(env(), fRTPSocket->socketNum(), clientPort)) {
75 break;
76 }
77 fClientPortNum = ntohs(clientPort.num());
78
79 if (fMultiplexRTCPWithRTP) {
80 // Use this RTP ‘groupsock’ object for RTCP as well:
81 fRTCPSocket = fRTPSocket;
82 success = True;
83 break;
84 }
85
86 // To be usable for RTP, the client port number must be even:
87 if ((fClientPortNum&1) != 0) { // it’s odd
88 // Record this socket in our table, and keep trying:
89 unsigned key = (unsigned)fClientPortNum;
90 Groupsock* existing = (Groupsock*)socketHashTable->Add((char const*)key, fRTPSocket);
91 delete existing; // in case it wasn’t NULL
92 continue;
93 }
94
95 // Make sure we can use the next (i.e., odd) port number, for RTCP:
96 portNumBits rtcpPortNum = fClientPortNum|1;
97 if (isSSM()) {
98 fRTCPSocket = new Groupsock(env(), tempAddr, fSourceFilterAddr, rtcpPortNum);
99 } else {
100 fRTCPSocket = new Groupsock(env(), tempAddr, rtcpPortNum, 255);
101 }
102 if (fRTCPSocket != NULL && fRTCPSocket->socketNum() >= 0) {
103 // Success! Use these two sockets.
104 success = True;
105 break;
106 } else {
107 // We couldn’t create the RTCP socket (perhaps that port number’s already in use elsewhere?).
108 delete fRTCPSocket; fRTCPSocket = NULL;
109
110 // Record the first socket in our table, and keep trying:
111 unsigned key = (unsigned)fClientPortNum;
112 Groupsock* existing = (Groupsock*)socketHashTable->Add((char const*)key, fRTPSocket);
113 delete existing; // in case it wasn’t NULL
114 continue;
115 }
116 }
117
118 // Clean up the socket hash table (and contents):
119 Groupsock* oldGS;
120 while ((oldGS = (Groupsock*)socketHashTable->RemoveNext()) != NULL) {
121 delete oldGS;
122 }
123 delete socketHashTable;
124
125 if (!success) break; // a fatal error occurred trying to create the RTP and RTCP sockets; we can’t continue
126 }
127
128 // Try to use a big receive buffer for RTP - at least 0.1 second of
129 // specified bandwidth and at least 50 KB
130 unsigned rtpBufSize = fBandwidth * 25 / 2; // 1 kbps * 0.1 s = 12.5 bytes
131 if (rtpBufSize < 50 * 1024)
132 rtpBufSize = 50 * 1024;
133 increaseReceiveBufferTo(env(), fRTPSocket->socketNum(), rtpBufSize);
134
135 if (isSSM() && fRTCPSocket != NULL) {
136 // Special case for RTCP SSM: Send RTCP packets back to the source via unicast:
137 fRTCPSocket->changeDestinationParameters(fSourceFilterAddr,0,~0);
138 }
139 //创建FramedSource对象来请求数据
140 // Create “fRTPSource” and “fReadSource”:
141 if (!createSourceObjects(useSpecialRTPoffset)) break;
142
143 if (fReadSource == NULL) {
144 env().setResultMsg(“Failed to create read source”);
145 break;
146 }
147 // 创建RTCPInstance对象
148 // Finally, create our RTCP instance. (It starts running automatically)
149 if (fRTPSource != NULL && fRTCPSocket != NULL) {
150 // If bandwidth is specified, use it and add 5% for RTCP overhead.
151 // Otherwise make a guess at 500 kbps.
152 unsigned totSessionBandwidth
153 = fBandwidth ? fBandwidth + fBandwidth / 20 : 500;
154 fRTCPInstance = RTCPInstance::createNew(env(), fRTCPSocket,
155 totSessionBandwidth,
156 (unsigned char const*)
157 fParent.CNAME(),
158 NULL /* we’re a client */,
159 fRTPSource);
160 if (fRTCPInstance == NULL) {
161 env().setResultMsg(“Failed to create RTCP instance”);
162 break;
163 }
164 }
165
166 return True;
167 } while (0);
168
169 deInitiate();
170 fClientPortNum = 0;
171 return False;
172 }
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在MediaSubsession::initiate函数中，首先创建了两个客户端socket分别用于接收RTP数据和RTCP数据；然后创建FramedSource对象用来从服务器端请求数据，FramedSource对象在createSourceObjects函数中被创建，createSourceObjects根据ServerMediaSubsession资源的不同格式创建不同的FramedSource，我们还是以H264视频为例，则创建的是H264VideoRTPSource对象；最后还创建了RTCPInstance对象。
接下来，我们继续看客户端收到PLAY命令回复后调用continueAfterPLAY函数：
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1 void continueAfterPLAY(RTSPClient* rtspClient, int resultCode, char* resultString) {
2 Boolean success = False;
3
4 do {
5 UsageEnvironment& env = rtspClient->envir(); // alias
6 StreamClientState& scs = ((ourRTSPClient*)rtspClient)->scs; // alias
7
8 if (resultCode != 0) {
9 env << rtspClient << "Failed to start playing session: " << resultString << “\n”;
10 break;
11 }
12
13 // Set a timer to be handled at the end of the stream’s expected duration (if the stream does not already signal its end
14 // using a RTCP “BYE”). This is optional. If, instead, you want to keep the stream active - e.g., so you can later
15 // ‘seek’ back within it and do another RTSP “PLAY” - then you can omit this code.
16 // (Alternatively, if you don’t want to receive the entire stream, you could set this timer for some shorter value.)
17 if (scs.duration > 0) {
18 unsigned const delaySlop = 2; // number of seconds extra to delay, after the stream’s expected duration. (This is optional.)
19 scs.duration += delaySlop;
20 unsigned uSecsToDelay = (unsigned)(scs.duration1000000);
21 scs.streamTimerTask = env.taskScheduler().scheduleDelayedTask(uSecsToDelay, (TaskFunc*)streamTimerHandler, rtspClient);
22 }
23
24 env << *rtspClient << “Started playing session”;
25 if (scs.duration > 0) {
26 env << " (for up to " << scs.duration << " seconds)";
27 }
28 env << “…\n”;
29
30 success = True;
31 } while (0);
32 delete[] resultString;
33
34 if (!success) {
35 // An unrecoverable error occurred with this stream.
36 shutdownStream(rtspClient);
37 }
38 }
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continueAfterPLAY函数的内容很简单，只是简单地打印出“Started playing session”。在服务器端收到PLAY命令后，就开始向客户端发送RTP数据包和RTCP数据包，而客户端在MediaSink::startPlaying函数中就开始等待接收来自服务器端的视频数据。
在continueAfterSETUP函数中创建的MediaSink是DummySink对象，DummySink是MediaSink的子类，这个例子中客户端没有利用收到的视频数据，所以叫做DummySink。
客户端调用MediaSink::startPlaying函数开始接收服务器端的数据，这个函数和之前介绍服务器端建立RTSP连接过程时是同一个函数
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1 Boolean MediaSink::startPlaying(MediaSource& source,
2 afterPlayingFunc* afterFunc,
3 void* afterClientData) {
4 // Make sure we’re not already being played:
5 if (fSource != NULL) {
6 envir().setResultMsg(“This sink is already being played”);
7 return False;
8 }
9
10 // Make sure our source is compatible:
11 if (!sourceIsCompatibleWithUs(source)) {
12 envir().setResultMsg(“MediaSink::startPlaying(): source is not compatible!”);
13 return False;
14 }
15 fSource = (FramedSource*)&source; //此处的fSource是之前创立的H264VideoRTPSource对象
16
17 fAfterFunc = afterFunc;
18 fAfterClientData = afterClientData;
19 return continuePlaying();
20 }
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在MediaSink::startPlaying函数中又调用DummySink::continuePlaying函数
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1 Boolean DummySink::continuePlaying() {
2 if (fSource == NULL) return False; // sanity check (should not happen)
3
4 // Request the next frame of data from our input source. “afterGettingFrame()” will get called later, when it arrives:
5 fSource->getNextFrame(fReceiveBuffer, DUMMY_SINK_RECEIVE_BUFFER_SIZE,
6 afterGettingFrame, this,
7 onSourceClosure, this);
8 return True;
9 }
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在DummySink::continuePlaying函数中通过H264VideoRTPSource对象请求服务器端的数据，H264VideoRTPSource是MultiFramedRTPSource的子类，请求成功后回调DummySink::afterGettingFrame函数。在FramedSource::getNextFrame函数中，调用了MultiFramedRTPSource::doGetNextFrame函数：
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1 void MultiFramedRTPSource::doGetNextFrame() {
2 if (!fAreDoingNetworkReads) {
3 // Turn on background read handling of incoming packets:
4 fAreDoingNetworkReads = True;
5 TaskScheduler::BackgroundHandlerProc* handler
6 = (TaskScheduler::BackgroundHandlerProc*)&networkReadHandler;
7 fRTPInterface.startNetworkReading(handler); //通过RTPInterface对象读取网络数据，在服务器端是通过RTPInterface对象发送网络数据
//读到数据后回调networkReadHandler函数来处理
8 }
9
10 fSavedTo = fTo; //读到的数据保存在fTo中
11 fSavedMaxSize = fMaxSize;
12 fFrameSize = 0; // for now
13 fNeedDelivery = True;
14 doGetNextFrame1();
15 }
16
17 void MultiFramedRTPSource::doGetNextFrame1() {
18 while (fNeedDelivery) { 19 // If we already have packet data available, then deliver it now.
20 Boolean packetLossPrecededThis;
21 BufferedPacket* nextPacket
22 = fReorderingBuffer->getNextCompletedPacket(packetLossPrecededThis);
23 if (nextPacket == NULL) break;
24
25 fNeedDelivery = False;
26
27 if (nextPacket->useCount() == 0) {
28 // Before using the packet, check whether it has a special header
29 // that needs to be processed:
30 unsigned specialHeaderSize;
31 if (!processSpecialHeader(nextPacket, specialHeaderSize)) {
32 // Something’s wrong with the header; reject the packet:
33 fReorderingBuffer->releaseUsedPacket(nextPacket);
34 fNeedDelivery = True;
35 break;
36 }
37 nextPacket->skip(specialHeaderSize);
38 }
39
40 // Check whether we’re part of a multi-packet frame, and whether
41 // there was packet loss that would render this packet unusable:
42 if (fCurrentPacketBeginsFrame) {
43 if (packetLossPrecededThis || fPacketLossInFragmentedFrame) {
44 // We didn’t get all of the previous frame.
45 // Forget any data that we used from it:
46 fTo = fSavedTo; fMaxSize = fSavedMaxSize;
47 fFrameSize = 0;
48 }
49 fPacketLossInFragmentedFrame = False;
50 } else if (packetLossPrecededThis) {
51 // We’re in a multi-packet frame, with preceding packet loss
52 fPacketLossInFragmentedFrame = True;
53 }
54 if (fPacketLossInFragmentedFrame) {
55 // This packet is unusable; reject it:
56 fReorderingBuffer->releaseUsedPacket(nextPacket);
57 fNeedDelivery = True;
58 break;
59 }
60
61 // The packet is usable. Deliver all or part of it to our caller:
62 unsigned frameSize;
63 nextPacket->use(fTo, fMaxSize, frameSize, fNumTruncatedBytes,
64 fCurPacketRTPSeqNum, fCurPacketRTPTimestamp,
65 fPresentationTime, fCurPacketHasBeenSynchronizedUsingRTCP,
66 fCurPacketMarkerBit);
67 fFrameSize += frameSize;
68
69 if (!nextPacket->hasUsableData()) {
70 // We’re completely done with this packet now
71 fReorderingBuffer->releaseUsedPacket(nextPacket);
72 }
73
74 if (fCurrentPacketCompletesFrame) { // 成功读到一帧数据
75 // We have all the data that the client wants.
76 if (fNumTruncatedBytes > 0) {
77 envir() << “MultiFramedRTPSource::doGetNextFrame1(): The total received frame size exceeds the client’s buffer size (”
78 << fSavedMaxSize << “). "
79 << fNumTruncatedBytes << " bytes of trailing data will be dropped!\n”;
80 }
81 // Call our own ‘after getting’ function, so that the downstream object can consume the data:
82 if (fReorderingBuffer->isEmpty()) {
83 // Common case optimization: There are no more queued incoming packets, so this code will not get
84 // executed again without having first returned to the event loop. Call our ‘after getting’ function
85 // directly, because there’s no risk of a long chain of recursion (and thus stack overflow):
86 afterGetting(this);
87 } else {
88 // Special case: Call our ‘after getting’ function via the event loop.
89 nextTask() = envir().taskScheduler().scheduleDelayedTask(0,
90 (TaskFunc*)FramedSource::afterGetting, this);
91 }
92 } else {
93 // This packet contained fragmented data, and does not complete
94 // the data that the client wants. Keep getting data:
95 fTo += frameSize; fMaxSize -= frameSize;
96 fNeedDelivery = True;
97 }
98 }
99 }
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在doGetNextFrame1函数中，若成功读取到一个完整的帧，则调用Framed::afterGetting函数，进一步回调了DummySink::afterGettingFrame函数
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1 void DummySink::afterGettingFrame(void* clientData, unsigned frameSize, unsigned numTruncatedBytes,
2 struct timeval presentationTime, unsigned durationInMicroseconds) {
3 DummySink* sink = (DummySink*)clientData;
4 sink->afterGettingFrame(frameSize, numTruncatedBytes, presentationTime, durationInMicroseconds);
5 }
6
7 // If you don’t want to see debugging output for each received frame, then comment out the following line:
8 #define DEBUG_PRINT_EACH_RECEIVED_FRAME 1
9
10 void DummySink::afterGettingFrame(unsigned frameSize, unsigned numTruncatedBytes,
11 struct timeval presentationTime, unsigned /durationInMicroseconds/) {
12 // We’ve just received a frame of data. (Optionally) print out information about it:
13 #ifdef DEBUG_PRINT_EACH_RECEIVED_FRAME
14 if (fStreamId != NULL) envir() << "Stream “” << fStreamId << “”; “;
15 envir() << fSubsession.mediumName() << “/” << fSubsession.codecName() << “:\tReceived " << frameSize << " bytes”;
16 if (numTruncatedBytes > 0) envir() << " (with " << numTruncatedBytes << " bytes truncated)”;
17 char uSecsStr[6+1]; // used to output the ‘microseconds’ part of the presentation time
18 sprintf(uSecsStr, “%06u”, (unsigned)presentationTime.tv_usec);
19 envir() << ".\tPresentation time: " << (int)presentationTime.tv_sec << “.” << uSecsStr;
20 if (fSubsession.rtpSource() != NULL && !fSubsession.rtpSource()->hasBeenSynchronizedUsingRTCP()) {
21 envir() << “!”; // mark the debugging output to indicate that this presentation time is not RTCP-synchronized
22 }
23 #ifdef DEBUG_PRINT_NPT
24 envir() << "\tNPT: " << fSubsession.getNormalPlayTime(presentationTime);
25 #endif
26 envir() << “\n”;
27 #endif
28
29 // Then continue, to request the next frame of data:
30 continuePlaying();
31 }
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在DummySink::afterGettingFrame函数中只是简单地打印出了某个MediaSubsession接收到了多少字节的数据，然后接着利用FramedSource去读取数据。可以看出，在RTSP客户端，Live555也是在MediaSink和FramedSource之间形成了一个循环，不停地从服务器端读取数据。