九 h264 RTP传输详解(1)
前几章对Server端的介绍中有个比较重要的问题没有仔细探究：如何打开文件并获得其SDP信息。我们就从这里入手吧。当RTSPServer收到对某个媒体的DESCRIBE请求时，它会找到对应的ServerMediasession,调用ServerMediaSession::generateSDPDescription()。generateSDPDescription()中会遍历调用ServerMediaSession中所有的调用ServerMediaSubsession，通过subsession->sdpLines()取得每个Subsession的sdp，合并成一个完整的SDP返回之。我们几乎可以断定，文件的打开和分析应该是在每个Subsession的sdpLines()函数中完成的，看看这个函数：
[cpp] view plain copychar const* OnDemandServerMediaSubsession::sdpLines() { if (fSDPLines == NULL) { // We need to construct a set of SDP lines that describe this // subsession (as a unicast stream). To do so, we first create // dummy (unused) source and "RTPSink" objects, // whose parameters we use for the SDP lines: unsigned estBitrate; FramedSource* inputSource = createNewStreamSource(0, estBitrate); if (inputSource == NULL) return NULL; // file not found struct in_addr dummyAddr; dummyAddr.s_addr = 0; Groupsock dummyGroupsock(envir(), dummyAddr, 0, 0); unsigned char rtpPayloadType = 96 + trackNumber() - 1; // if dynamic RTPSink* dummyRTPSink = createNewRTPSink(&dummyGroupsock, rtpPayloadType, inputSource); setSDPLinesFromRTPSink(dummyRTPSink, inputSource, estBitrate); Medium::close(dummyRTPSink); closeStreamSource(inputSource); } return fSDPLines; } 其所为如是：Subsession中直接保存了对应媒体文件的SDP，但是在第一次获取时fSDPLines为NULL，所以需先获取fSDPLines。其做法比较费事，竟然是建了临时的Source和RTPSink，把它们连接成一个StreamToken，Playing一段时间之后才取得了fSDPLines。createNewStreamSource()和createNewRTPSink()都是虚函数，所以此处创建的source和sink都是继承类指定的，我们分析的是H264，也就是H264VideoFileServerMediaSubsession所指定的，来看一下这两个函数：[cpp] view plain copyFramedSource* H264VideoFileServerMediaSubsession::createNewStreamSource( unsigned /*clientSessionId*/, unsigned& estBitrate) { estBitrate = 500; // kbps, estimate // Create the video source: ByteStreamFileSource* fileSource = ByteStreamFileSource::createNew(envir(), fFileName); if (fileSource == NULL) return NULL; fFileSize = fileSource->fileSize(); // Create a framer for the Video Elementary Stream: return H264VideoStreamFramer::createNew(envir(), fileSource); } RTPSink* H264VideoFileServerMediaSubsession::createNewRTPSink( Groupsock* rtpGroupsock, unsigned char rtpPayloadTypeIfDynamic, FramedSource* /*inputSource*/) { return H264VideoRTPSink::createNew(envir(), rtpGroupsock, rtpPayloadTypeIfDynamic); } 可以看到，分别创建了H264VideoStreamFramer和H264VideoRTPSink。可以肯定H264VideoStreamFramer也是一个Source，但它内部又利用了另一个source--ByteStreamFileSource。后面会分析为什么要这样做，这里先不要管它。还没有看到真正打开文件的代码，继续探索：[cpp] view plain copyvoid OnDemandServerMediaSubsession::setSDPLinesFromRTPSink( RTPSink* rtpSink, FramedSource* inputSource, unsigned estBitrate) { if (rtpSink == NULL) return; char const* mediaType = rtpSink->sdpMediaType(); unsigned char rtpPayloadType = rtpSink->rtpPayloadType(); struct in_addr serverAddrForSDP; serverAddrForSDP.s_addr = fServerAddressForSDP; char* const ipAddressStr = strDup(our_inet_ntoa(serverAddrForSDP)); char* rtpmapLine = rtpSink->rtpmapLine(); char const* rangeLine = rangeSDPLine(); char const* auxSDPLine = getAuxSDPLine(rtpSink, inputSource); if (auxSDPLine == NULL) auxSDPLine = ""; char const* const sdpFmt = "m=%s %u RTP/AVP %d/r/n" "c=IN IP4 %s/r/n" "b=AS:%u/r/n" "%s" "%s" "%s" "a=control:%s/r/n"; unsigned sdpFmtSize = strlen(sdpFmt) + strlen(mediaType) + 5 /* max short len */ + 3 /* max char len */ + strlen(ipAddressStr) + 20 /* max int len */ + strlen(rtpmapLine) + strlen(rangeLine) + strlen(auxSDPLine) + strlen(trackId()); char* sdpLines = new char[sdpFmtSize]; sPRintf(sdpLines, sdpFmt, mediaType, // m= <media> fPortNumForSDP, // m= <port> rtpPayloadType, // m= <fmt list> ipAddressStr, // c= address estBitrate, // b=AS:<bandwidth> rtpmapLine, // a=rtpmap:... (if present) rangeLine, // a=range:... (if present) auxSDPLine, // optional extra SDP line trackId()); // a=control:<track-id> delete[] (char*) rangeLine; delete[] rtpmapLine; delete[] ipAddressStr; fSDPLines = strDup(sdpLines); delete[] sdpLines; } 此函数中取得Subsession的sdp并保存到fSDPLines。打开文件应在rtpSink->rtpmapLine()甚至是Source创建时已经做了。我们不防先把它放一放，而是先把SDP的获取过程搞个通透。所以把焦点集中到getAuxSDPLine()上。[cpp] view plain copychar const* OnDemandServerMediaSubsession::getAuxSDPLine( RTPSink* rtpSink, FramedSource* /*inputSource*/) { // Default implementation: return rtpSink == NULL ? NULL : rtpSink->auxSDPLine(); } 很简单，调用了rtpSink->auxSDPLine()那么我们要看H264VideoRTPSink::auxSDPLine()：不用看了，很简单，取得source 中保存的PPS,SPS等形成a=fmpt行。但事实上并没有这么简单，H264VideoFileServerMediaSubsession重写了getAuxSDPLine()！如果不重写，则说明auxSDPLine已经在前面分析文件时获得了，那么既然重写，就说明前面没有获取到，只能在这个函数中重写。look H264VideoFileServerMediaSubsession中这个函数:[cpp] view plain copychar const* H264VideoFileServerMediaSubsession::getAuxSDPLine( RTPSink* rtpSink, FramedSource* inputSource) { if (fAuxSDPLine != NULL) return fAuxSDPLine; // it's already been set up (for a previous client) if (fDummyRTPSink == NULL) { // we're not already setting it up for another, concurrent stream // Note: For H264 video files, the 'config' information ("profile-level-id" and "sprop-parameter-sets") isn't known // until we start reading the file. This means that "rtpSink"s "auxSDPLine()" will be NULL initially, // and we need to start reading data from our file until this changes. fDummyRTPSink = rtpSink; // Start reading the file: fDummyRTPSink->startPlaying(*inputSource, afterPlayingDummy, this); // Check whether the sink's 'auxSDPLine()' is ready: checkForAuxSDPLine(this); } envir().taskScheduler().doEventLoop(&fDoneFlag); return fAuxSDPLine; } 注释里面解释得很清楚，H264不能在文件头中取得PPS/SPS，必须在播放一下后（当然，它是一个原始流文件，没有文件头）才行。也就是说不能从rtpSink中取得了。为了保证在函数退出前能取得AuxSDP，把大循环搬到这里来了。afterPlayingDummy()是在播放结束也就是取得aux sdp之后执行。在大循环之前的checkForAuxSDPLine()做了什么呢？[cpp] view plain copyvoid H264VideoFileServerMediaSubsession::checkForAuxSDPLine1() { char const* dasl; if (fAuxSDPLine != NULL) { // Signal the event loop that we're done: setDoneFlag(); } else if (fDummyRTPSink != NULL && (dasl = fDummyRTPSink->auxSDPLine()) != NULL) { fAuxSDPLine = strDup(dasl); fDummyRTPSink = NULL; // Signal the event loop that we're done: setDoneFlag(); } else { // try again after a brief delay: int uSecsToDelay = 100000; // 100 ms nextTask() = envir().taskScheduler().scheduleDelayedTask(uSecsToDelay, (TaskFunc*) checkForAuxSDPLine, this); } } 它检查是否已取得Aux sdp，如果取得了，设置结束标志，直接返回。如果没有，就检查是否sink中已取得了aux sdp，如果是，也设置结束标志，返回。如果还没有取得，则把这个检查函数做为delay task加入计划任务中。每100毫秒检查一次，每检查一次主要就是调用一次fDummyRTPSink->auxSDPLine()。大循环在检测到fDoneFlag改变时停止，此时已取得了aux sdp。但是如果直到文件结束也没有得到aux sdp，则afterPlayingDummy（）被执行，在其中停止掉这个大循环。然后在父Subsession类中关掉这些临时的source和sink。在直正播放时重新创建。新闻热点
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