如我們在前文 live555 源碼分析：ServerMediaSession 中看到的，H264VideoFileServerMediaSubsession 的繼承層次體系如下圖：

在這個繼承層次體系中，ServerMediaSubsession 定義了可以對流媒體的單個子會話執行的操作，它有著如下這樣的生命周期：

對于這些操作的實現則是由 OnDemandServerMediaSubsession 完成的，本文分析這個類的定義和實現。

我們再來看一下產生 SDP 消息行的 sdpLines()，它在處理 RTSP DESCRIBE 消息時會用到：

char 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 foundstruct in_addr dummyAddr;dummyAddr.s_addr = 0;Groupsock* dummyGroupsock = createGroupsock(dummyAddr, 0);unsigned char rtpPayloadType = 96 + trackNumber()-1; // if dynamicRTPSink* dummyRTPSink = createNewRTPSink(dummyGroupsock, rtpPayloadType, inputSource);if (dummyRTPSink != NULL && dummyRTPSink->estimatedBitrate() > 0) estBitrate = dummyRTPSink->estimatedBitrate();setSDPLinesFromRTPSink(dummyRTPSink, inputSource, estBitrate);Medium::close(dummyRTPSink);delete dummyGroupsock;closeStreamSource(inputSource);}fprintf(stderr, "OnDemandServerMediaSubsession::sdpLines(): %s\n", fSDPLines);return fSDPLines; }

這個函數執行的步驟如下：

第 1 步，調用 createNewStreamSource() 函數，創建 FramedSource。createNewStreamSource() 函數在 H264VideoFileServerMediaSubsession 中定義：

FramedSource* 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); }

返回的 FramedSource 為 ByteStreamFileSource。FramedSource 用于獲取視頻流中的單個幀，它有著如下這樣的繼承層次結構：

第 2 步，調用 createGroupsock() 函數創建 Groupsock。

Groupsock* OnDemandServerMediaSubsession ::createGroupsock(struct in_addr const& addr, Port port) {fprintf(stderr, "OnDemandServerMediaSubsession::createGroupsock().\n");// Default implementation; may be redefined by subclasses:return new Groupsock(envir(), addr, port, 255); }

第 3 步，計算 RTP 載荷類型。

第 4 步，調用 createNewRTPSink() 創建 RTPSink，createNewRTPSink() 同樣在 H264VideoFileServerMediaSubsession 中實現：

RTPSink* H264VideoFileServerMediaSubsession ::createNewRTPSink(Groupsock* rtpGroupsock,unsigned char rtpPayloadTypeIfDynamic,FramedSource* /*inputSource*/) {return H264VideoRTPSink::createNew(envir(), rtpGroupsock, rtpPayloadTypeIfDynamic); }

傳進來的 FramedSource 并沒有被用到。返回的 RTPSink 為 H264VideoRTPSink。H264VideoRTPSink 的繼承層次結構如下圖所示：

第 5 步，調用 setSDPLinesFromRTPSink() 生成 SDP 行。前面的幾步都是在為這一步的執行做準備。這個函數實現如下：

void OnDemandServerMediaSubsession ::setSDPLinesFromRTPSink(RTPSink* rtpSink, FramedSource* inputSource, unsigned estBitrate) {fprintf(stderr, "OnDemandServerMediaSubsession::setSDPLinesFromRTPSink().\n");if (rtpSink == NULL) return;char const* mediaType = rtpSink->sdpMediaType();unsigned char rtpPayloadType = rtpSink->rtpPayloadType();AddressString ipAddressStr(fServerAddressForSDP);char* rtpmapLine = rtpSink->rtpmapLine();char const* rtcpmuxLine = fMultiplexRTCPWithRTP ? "a=rtcp-mux\r\n" : "";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""%s""a=control:%s\r\n";unsigned sdpFmtSize = strlen(sdpFmt)+ strlen(mediaType) + 5 /* max short len */ + 3 /* max char len */+ strlen(ipAddressStr.val())+ 20 /* max int len */+ strlen(rtpmapLine)+ strlen(rtcpmuxLine)+ 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.val(), // c= addressestBitrate, // b=AS:<bandwidth>rtpmapLine, // a=rtpmap:... (if present)rtcpmuxLine, // a=rtcp-mux:... (if present)rangeLine, // a=range:... (if present)auxSDPLine, // optional extra SDP linetrackId()); // a=control:<track-id>delete[] (char*) rangeLine; delete[] rtpmapLine;fSDPLines = strDup(sdpLines);delete[] sdpLines; }

在這個函數中，獲取生成 SDP 消息所需的數據，然后生成 SDP 消息。mediaType 來自于 VideoRTPSink：

char const* VideoRTPSink::sdpMediaType() const {return "video"; }

這也是 VideoRTPSink 提供的僅有的功能了。

rtpPayloadType 來自于 RTPSink：

unsigned char rtpPayloadType() const { return fRTPPayloadType; }

前面在 sdpLines() 中計算得到 RTP 載荷類型，在創建 RTPSink 時計算的值會被傳遞給它。這里返回的正是前面計算得到的值。對于 H.264 視頻流，這個值為 96。

map 行來自于 RTPSink：

char* RTPSink::rtpmapLine() const {if (rtpPayloadType() >= 96) { // the payload format type is dynamicchar* encodingParamsPart;if (numChannels() != 1) {encodingParamsPart = new char[1 + 20 /* max int len */];sprintf(encodingParamsPart, "/%d", numChannels());} else {encodingParamsPart = strDup("");}char const* const rtpmapFmt = "a=rtpmap:%d %s/%d%s\r\n";unsigned rtpmapFmtSize = strlen(rtpmapFmt)+ 3 /* max char len */ + strlen(rtpPayloadFormatName())+ 20 /* max int len */ + strlen(encodingParamsPart);char* rtpmapLine = new char[rtpmapFmtSize];sprintf(rtpmapLine, rtpmapFmt,rtpPayloadType(), rtpPayloadFormatName(),rtpTimestampFrequency(), encodingParamsPart);delete[] encodingParamsPart;return rtpmapLine;} else {// The payload format is staic, so there's no "a=rtpmap:" line:return strDup("");} }

對于 RFC 3551 分配了載荷類型的媒體類型，無需 map 行。對于動態的載荷類型，則需要該行，其中包含了載荷類型、載荷格式名稱，時間戳頻率等，當通道個數大于 1 時，還包含通道個數。

通道個數來自于 MultiFramedRTPSink：

MultiFramedRTPSink(UsageEnvironment& env,Groupsock* rtpgs, unsigned char rtpPayloadType,unsigned rtpTimestampFrequency,char const* rtpPayloadFormatName,unsigned numChannels = 1);

載荷格式名稱和時間戳頻率來自于 H264or5VideoRTPSink，對于 H.264 的視頻流而言，載荷格式名稱為 “H264”，時間戳頻率為 90000：

H264or5VideoRTPSink ::H264or5VideoRTPSink(int hNumber,UsageEnvironment& env, Groupsock* RTPgs, unsigned char rtpPayloadFormat,u_int8_t const* vps, unsigned vpsSize,u_int8_t const* sps, unsigned spsSize,u_int8_t const* pps, unsigned ppsSize): VideoRTPSink(env, RTPgs, rtpPayloadFormat, 90000, hNumber == 264 ? "H264" : "H265"),fHNumber(hNumber), fOurFragmenter(NULL), fFmtpSDPLine(NULL) {

回到 setSDPLinesFromRTPSink()。然后是 range SDP 行，它來自于 ServerMediaSubsession：

char const* ServerMediaSubsession::rangeSDPLine() const {// First, check for the special case where we support seeking by 'absolute' time:char* absStart = NULL; char* absEnd = NULL;getAbsoluteTimeRange(absStart, absEnd);if (absStart != NULL) {char buf[100];if (absEnd != NULL) {sprintf(buf, "a=range:clock=%s-%s\r\n", absStart, absEnd);} else {sprintf(buf, "a=range:clock=%s-\r\n", absStart);}return strDup(buf);}if (fParentSession == NULL) return NULL;// If all of our parent's subsessions have the same duration// (as indicated by "fParentSession->duration() >= 0"), there's no "a=range:" line:if (fParentSession->duration() >= 0.0) return strDup("");// Use our own duration for a "a=range:" line:float ourDuration = duration();if (ourDuration == 0.0) {return strDup("a=range:npt=0-\r\n");} else {char buf[100];sprintf(buf, "a=range:npt=0-%.3f\r\n", ourDuration);return strDup(buf);} }

所需獲取的最后的 SDP 信息是 aux SDP 行，這些信息需要從視頻流數據中解析獲得，通過 H264VideoFileServerMediaSubsession 的 getAuxSDPLine() 函數獲得：

char 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; }

這個函數在首次調用時，會通過 RTPSink 啟動對流媒體文件幀數據的讀取，即調用 RTPSink 的 startPlaying() 函數。

startPlaying() 函數執行一個長長的調用鏈，最終調用 ByteStreamFileSource 的 doGetNextFrame() 函數，這個調用棧如下：

#0 ByteStreamFileSource::doGetNextFrame (this=0x6e4f10) at ByteStreamFileSource.cpp:96 #1 0x00000000004308b4 in FramedSource::getNextFrame (this=0x6e4f10, to=0x7ffff7fd1010 "", maxSize=150000, afterGettingFunc=0x4740c2 <StreamParser::afterGettingBytes(void*, unsigned int, unsigned int, timeval, unsigned int)>, afterGettingClientData=0x6e5020, onCloseFunc=0x47424c <StreamParser::onInputClosure(void*)>, onCloseClientData=0x6e5020) at FramedSource.cpp:78 #2 0x0000000000474096 in StreamParser::ensureValidBytes1 (this=0x6e5020, numBytesNeeded=4) at StreamParser.cpp:159 #3 0x0000000000434fe1 in StreamParser::ensureValidBytes (this=0x6e5020, numBytesNeeded=4) at StreamParser.hh:118 #4 0x0000000000434d75 in StreamParser::test4Bytes (this=0x6e5020) at StreamParser.hh:54 #5 0x000000000047918e in H264or5VideoStreamParser::parse (this=0x6e5020) at H264or5VideoStreamFramer.cpp:951 #6 0x0000000000435de9 in MPEGVideoStreamFramer::continueReadProcessing (this=0x6e4710) at MPEGVideoStreamFramer.cpp:159 #7 0x0000000000435d51 in MPEGVideoStreamFramer::doGetNextFrame (this=0x6e4710) at MPEGVideoStreamFramer.cpp:142 #8 0x00000000004308b4 in FramedSource::getNextFrame (this=0x6e4710, to=0x6ff241 "", maxSize=100000, afterGettingFunc=0x47c788 <H264or5Fragmenter::afterGettingFrame(void*, unsigned int, unsigned int, timeval, unsigned int)>, afterGettingClientData=0x6e5540, onCloseFunc=0x43092e <FramedSource::handleClosure(void*)>, onCloseClientData=0x6e5540) at FramedSource.cpp:78 #9 0x000000000047c2c0 in H264or5Fragmenter::doGetNextFrame (this=0x6e5540) at H264or5VideoRTPSink.cpp:181 #10 0x00000000004308b4 in FramedSource::getNextFrame (this=0x6e5540, to=0x6e69cc "", maxSize=100452, afterGettingFunc=0x45d09c <MultiFramedRTPSink::afterGettingFrame(void*, unsigned int, unsigned int, timeval, unsigned int)>, afterGettingClientData=0x6e5220, onCloseFunc=0x45da86 <MultiFramedRTPSink::ourHandleClosure(void*)>, onCloseClientData=0x6e5220) at FramedSource.cpp:78 #11 0x000000000045d07b in MultiFramedRTPSink::packFrame (this=0x6e5220) at MultiFramedRTPSink.cpp:224 #12 0x000000000045cec8 in MultiFramedRTPSink::buildAndSendPacket (this=0x6e5220, isFirstPacket=1 '\001') at MultiFramedRTPSink.cpp:199 #13 0x000000000045cd07 in MultiFramedRTPSink::continuePlaying (this=0x6e5220) at MultiFramedRTPSink.cpp:159 #14 0x000000000047bfe0 in H264or5VideoRTPSink::continuePlaying (this=0x6e5220) at H264or5VideoRTPSink.cpp:127 #15 0x0000000000405d2a in MediaSink::startPlaying (this=0x6e5220, source=..., afterFunc=0x422a12 <afterPlayingDummy(void*)>, afterClientData=0x6e4aa0)at MediaSink.cpp:78 #16 0x0000000000422c5f in H264VideoFileServerMediaSubsession::getAuxSDPLine (this=0x6e4aa0, rtpSink=0x6e5220, inputSource=0x6e4710)at H264VideoFileServerMediaSubsession.cpp:92 #17 0x0000000000464cd4 in OnDemandServerMediaSubsession::setSDPLinesFromRTPSink (this=0x6e4aa0, rtpSink=0x6e5220, inputSource=0x6e4710, estBitrate=500)at OnDemandServerMediaSubsession.cpp:456 #18 0x000000000046378b in OnDemandServerMediaSubsession::sdpLines (this=0x6e4aa0) at OnDemandServerMediaSubsession.cpp:76 #19 0x000000000042198b in ServerMediaSession::generateSDPDescription (this=0x6e4900) at ServerMediaSession.cpp:240

ByteStreamFileSource 的 doGetNextFrame() 啟動對流媒體數據的讀取，這個函數的實現如下：

void ByteStreamFileSource::doGetNextFrame() {if (feof(fFid) || ferror(fFid) || (fLimitNumBytesToStream && fNumBytesToStream == 0)) {handleClosure();return;}#ifdef READ_FROM_FILES_SYNCHRONOUSLYdoReadFromFile(); #elseif (!fHaveStartedReading) {// Await readable data from the file:envir().taskScheduler().turnOnBackgroundReadHandling(fileno(fFid),(TaskScheduler::BackgroundHandlerProc*)&fileReadableHandler, this);fHaveStartedReading = True;} #endif }

對于異步讀取來說，是注冊一個文件讀取處理程序 fileReadableHandler，對于同步讀取，則是直接通過 doReadFromFile() 讀取文件數據。

回到 H264VideoFileServerMediaSubsession 的 getAuxSDPLine()。執行了 RTPSink 的 startPlaying() 之后，就會通過 checkForAuxSDPLine(void* clientData) 檢查一下視頻流元數據的讀取是否結束：

static void checkForAuxSDPLine(void* clientData) {H264VideoFileServerMediaSubsession* subsess = (H264VideoFileServerMediaSubsession*)clientData;subsess->checkForAuxSDPLine1(); }void H264VideoFileServerMediaSubsession::checkForAuxSDPLine1() {nextTask() = NULL;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 if (!fDoneFlag) {// try again after a brief delay:int uSecsToDelay = 100000; // 100 msnextTask() = envir().taskScheduler().scheduleDelayedTask(uSecsToDelay,(TaskFunc*) checkForAuxSDPLine, this);} }

這個函數在執行的時候，若發現 aux SDP 行還沒有獲取到，就會注冊一個定時器任務過一段時間繼續檢查。

aux SDP 行通過 RTPSink 的 auxSDPLine() 獲取，該函數實際的實現位于 H264VideoRTPSink 中：

char const* H264VideoRTPSink::auxSDPLine() {// Generate a new "a=fmtp:" line each time, using our SPS and PPS (if we have them),// otherwise parameters from our framer source (in case they've changed since the last time that// we were called):H264or5VideoStreamFramer* framerSource = NULL;u_int8_t* vpsDummy = NULL; unsigned vpsDummySize = 0;u_int8_t* sps = fSPS; unsigned spsSize = fSPSSize;u_int8_t* pps = fPPS; unsigned ppsSize = fPPSSize;if (sps == NULL || pps == NULL) {// We need to get SPS and PPS from our framer source:if (fOurFragmenter == NULL) return NULL; // we don't yet have a fragmenter (and therefore not a source)framerSource = (H264or5VideoStreamFramer*)(fOurFragmenter->inputSource());if (framerSource == NULL) return NULL; // we don't yet have a sourceframerSource->getVPSandSPSandPPS(vpsDummy, vpsDummySize, sps, spsSize, pps, ppsSize);if (sps == NULL || pps == NULL) return NULL; // our source isn't ready}// Set up the "a=fmtp:" SDP line for this stream:u_int8_t* spsWEB = new u_int8_t[spsSize]; // "WEB" means "Without Emulation Bytes"unsigned spsWEBSize = removeH264or5EmulationBytes(spsWEB, spsSize, sps, spsSize);if (spsWEBSize < 4) { // Bad SPS size => assume our source isn't readydelete[] spsWEB;return NULL;}u_int32_t profileLevelId = (spsWEB[1]<<16) | (spsWEB[2]<<8) | spsWEB[3];delete[] spsWEB;char* sps_base64 = base64Encode((char*)sps, spsSize);char* pps_base64 = base64Encode((char*)pps, ppsSize);char const* fmtpFmt ="a=fmtp:%d packetization-mode=1"";profile-level-id=%06X"";sprop-parameter-sets=%s,%s\r\n";unsigned fmtpFmtSize = strlen(fmtpFmt)+ 3 /* max char len */+ 6 /* 3 bytes in hex */+ strlen(sps_base64) + strlen(pps_base64);char* fmtp = new char[fmtpFmtSize];sprintf(fmtp, fmtpFmt,rtpPayloadType(),profileLevelId,sps_base64, pps_base64);delete[] sps_base64;delete[] pps_base64;delete[] fFmtpSDPLine; fFmtpSDPLine = fmtp;return fFmtpSDPLine; }

要產生 SDP 行，需要用到視頻流數據的 SPS 幀和 PPS 幀，這兩幀數據有兩種來源，一種是在對象構造時，由調用者傳入：

H264or5VideoRTPSink ::H264or5VideoRTPSink(int hNumber,UsageEnvironment& env, Groupsock* RTPgs, unsigned char rtpPayloadFormat,u_int8_t const* vps, unsigned vpsSize,u_int8_t const* sps, unsigned spsSize,u_int8_t const* pps, unsigned ppsSize): VideoRTPSink(env, RTPgs, rtpPayloadFormat, 90000, hNumber == 264 ? "H264" : "H265"),fHNumber(hNumber), fOurFragmenter(NULL), fFmtpSDPLine(NULL) {if (vps != NULL) {fVPSSize = vpsSize;fVPS = new u_int8_t[fVPSSize];memmove(fVPS, vps, fVPSSize);} else {fVPSSize = 0;fVPS = NULL;}if (sps != NULL) {fSPSSize = spsSize;fSPS = new u_int8_t[fSPSSize];memmove(fSPS, sps, fSPSSize);} else {fSPSSize = 0;fSPS = NULL;}if (pps != NULL) {fPPSSize = ppsSize;fPPS = new u_int8_t[fPPSSize];memmove(fPPS, pps, fPPSSize);} else {fPPSSize = 0;fPPS = NULL;} }

另外一種則是從數據源中獲取：

class H264or5VideoStreamFramer: public MPEGVideoStreamFramer { public:void getVPSandSPSandPPS(u_int8_t*& vps, unsigned& vpsSize,u_int8_t*& sps, unsigned& spsSize,u_int8_t*& pps, unsigned& ppsSize) const {// Returns pointers to copies of the most recently seen VPS (video parameter set)// SPS (sequence parameter set) and PPS (picture parameter set) NAL units.// (NULL pointers are returned if the NAL units have not yet been seen.)vps = fLastSeenVPS; vpsSize = fLastSeenVPSSize;sps = fLastSeenSPS; spsSize = fLastSeenSPSSize;pps = fLastSeenPPS; ppsSize = fLastSeenPPSSize;}

對于從數據源獲取 SPS 幀和 PPS 幀的情況，在從文件中獲取數據之后，這些數據會被推給 framerSource，推 SPS 的過程如下：

#0 H264or5VideoStreamFramer::saveCopyOfSPS (this=0x6e4710, from=0x6ff241 "gB\200*\332\001\020\017\036^R\n\f\n\r\241Bj", size=18)at H264or5VideoStreamFramer.cpp:110 #1 0x00000000004798c4 in H264or5VideoStreamParser::parse (this=0x6e5020) at H264or5VideoStreamFramer.cpp:1069 #2 0x0000000000435de9 in MPEGVideoStreamFramer::continueReadProcessing (this=0x6e4710) at MPEGVideoStreamFramer.cpp:159 #3 0x0000000000435db4 in MPEGVideoStreamFramer::continueReadProcessing (clientData=0x6e4710) at MPEGVideoStreamFramer.cpp:155 #4 0x0000000000474249 in StreamParser::afterGettingBytes1 (this=0x6e5020, numBytesRead=150000, presentationTime=...) at StreamParser.cpp:191 #5 0x0000000000474112 in StreamParser::afterGettingBytes (clientData=0x6e5020, numBytesRead=150000, presentationTime=...) at StreamParser.cpp:170 #6 0x000000000043092a in FramedSource::afterGetting (source=0x6e4f10) at FramedSource.cpp:92 #7 0x0000000000431494 in ByteStreamFileSource::doReadFromFile (this=0x6e4f10) at ByteStreamFileSource.cpp:182 #8 0x0000000000431233 in ByteStreamFileSource::fileReadableHandler (source=0x6e4f10) at ByteStreamFileSource.cpp:126 #9 0x0000000000483a49 in BasicTaskScheduler::SingleStep (this=0x6d9c20, maxDelayTime=0) at BasicTaskScheduler.cpp:171 #10 0x00000000004864b4 in BasicTaskScheduler0::doEventLoop (this=0x6d9c20, watchVariable=0x6e4bb0 "") at BasicTaskScheduler0.cpp:80 #11 0x0000000000422c9c in H264VideoFileServerMediaSubsession::getAuxSDPLine (this=0x6e4aa0, rtpSink=0x6e5220, inputSource=0x6e4710)at H264VideoFileServerMediaSubsession.cpp:98 #12 0x0000000000464cd4 in OnDemandServerMediaSubsession::setSDPLinesFromRTPSink (this=0x6e4aa0, rtpSink=0x6e5220, inputSource=0x6e4710, estBitrate=500)at OnDemandServerMediaSubsession.cpp:456 #13 0x000000000046378b in OnDemandServerMediaSubsession::sdpLines (this=0x6e4aa0) at OnDemandServerMediaSubsession.cpp:76 #14 0x000000000042198b in ServerMediaSession::generateSDPDescription (this=0x6e4900) at ServerMediaSession.cpp:240

推 PPS 的過程如下：

#0 H264or5VideoStreamFramer::saveCopyOfPPS (this=0x6e4710, from=0x6ff241 "h\316\006\342\332\001\020\017\036^R\n\f\n\r\241Bj", size=4)at H264or5VideoStreamFramer.cpp:119 #1 0x0000000000479ad9 in H264or5VideoStreamParser::parse (this=0x6e5020) at H264or5VideoStreamFramer.cpp:1090 #2 0x0000000000435de9 in MPEGVideoStreamFramer::continueReadProcessing (this=0x6e4710) at MPEGVideoStreamFramer.cpp:159 #3 0x0000000000435d51 in MPEGVideoStreamFramer::doGetNextFrame (this=0x6e4710) at MPEGVideoStreamFramer.cpp:142 #4 0x00000000004308b4 in FramedSource::getNextFrame (this=0x6e4710, to=0x6ff241 "h\316\006\342\332\001\020\017\036^R\n\f\n\r\241Bj", maxSize=100000, afterGettingFunc=0x47c788 <H264or5Fragmenter::afterGettingFrame(void*, unsigned int, unsigned int, timeval, unsigned int)>, afterGettingClientData=0x6e5540, onCloseFunc=0x43092e <FramedSource::handleClosure(void*)>, onCloseClientData=0x6e5540) at FramedSource.cpp:78 #5 0x000000000047c2c0 in H264or5Fragmenter::doGetNextFrame (this=0x6e5540) at H264or5VideoRTPSink.cpp:181 #6 0x00000000004308b4 in FramedSource::getNextFrame (this=0x6e5540, to=0x6e69cc "gB\200*\332\001\020\017\036^R\n\f\n\r\241Bj", maxSize=100452, afterGettingFunc=0x45d09c <MultiFramedRTPSink::afterGettingFrame(void*, unsigned int, unsigned int, timeval, unsigned int)>, afterGettingClientData=0x6e5220, onCloseFunc=0x45da86 <MultiFramedRTPSink::ourHandleClosure(void*)>, onCloseClientData=0x6e5220) at FramedSource.cpp:78 #7 0x000000000045d07b in MultiFramedRTPSink::packFrame (this=0x6e5220) at MultiFramedRTPSink.cpp:224 #8 0x000000000045cec8 in MultiFramedRTPSink::buildAndSendPacket (this=0x6e5220, isFirstPacket=0 '\000') at MultiFramedRTPSink.cpp:199 #9 0x000000000045da83 in MultiFramedRTPSink::sendNext (firstArg=0x6e5220) at MultiFramedRTPSink.cpp:422 #10 0x0000000000486c1b in AlarmHandler::handleTimeout (this=0x6e5760) at BasicTaskScheduler0.cpp:34 #11 0x0000000000484d1e in DelayQueue::handleAlarm (this=0x6d9c28) at DelayQueue.cpp:187 #12 0x0000000000483c4c in BasicTaskScheduler::SingleStep (this=0x6d9c20, maxDelayTime=0) at BasicTaskScheduler.cpp:212 #13 0x00000000004864b4 in BasicTaskScheduler0::doEventLoop (this=0x6d9c20, watchVariable=0x6e4bb0 "") at BasicTaskScheduler0.cpp:80 #14 0x0000000000422c9c in H264VideoFileServerMediaSubsession::getAuxSDPLine (this=0x6e4aa0, rtpSink=0x6e5220, inputSource=0x6e4710)at H264VideoFileServerMediaSubsession.cpp:98 #15 0x0000000000464cd4 in OnDemandServerMediaSubsession::setSDPLinesFromRTPSink (this=0x6e4aa0, rtpSink=0x6e5220, inputSource=0x6e4710, estBitrate=500)at OnDemandServerMediaSubsession.cpp:456 #16 0x000000000046378b in OnDemandServerMediaSubsession::sdpLines (this=0x6e4aa0) at OnDemandServerMediaSubsession.cpp:76 #17 0x000000000042198b in ServerMediaSession::generateSDPDescription (this=0x6e4900) at ServerMediaSession.cpp:240

SPS 和 PPS 幀數據要都拿到才能產生
SDP 行。H264VideoRTPSink::auxSDPLine() 在拿不到 SDP 行數據時，會返回 null 給調用者。

對于 H264VideoFileServerMediaSubsession 的 checkForAuxSDPLine() 而言，在拿不到 SDP 的情況下，則是再次調度一個定時器任務執行，等待下一次檢查的到來。

在 H264VideoRTPSink::auxSDPLine() 中，如果已經有了 SPS 和 PPS 數據的話則會先移除 SPS 中的 Emulation 字節，然后計算 SPS 幀數據和 PPS 幀數據的 Base64 編碼，并最終產生 SDP 行。

移除 SPS 中的 Emulation 字節的過程就像下面這樣：

unsigned removeH264or5EmulationBytes(u_int8_t* to, unsigned toMaxSize,u_int8_t const* from, unsigned fromSize) {unsigned toSize = 0;unsigned i = 0;while (i < fromSize && toSize+1 < toMaxSize) {if (i+2 < fromSize && from[i] == 0 && from[i+1] == 0 && from[i+2] == 3) {to[toSize] = to[toSize+1] = 0;toSize += 2;i += 3;} else {to[toSize] = from[i];toSize += 1;i += 1;}} return toSize; }

所謂的 Emulation 字節，是由于 H.264 流數據中，通過 0x00000001 和 0x000001 字節序列作為幀數據的分割序列，為了避免解碼器將編碼的幀數據中的這樣的序列誤認為是幀之間的分割序列，而強制在出現兩個 0x00 字節的幀數據時，插入一個 0x03 字節。而在計算 SPS 的 Base64 時需要先移除這些字節。

H264VideoFileServerMediaSubsession 的 getAuxSDPLine() 在拿到 aux SDP 行之后，就把它返回給調用者 OnDemandServerMediaSubsession::setSDPLinesFromRTPSink()。aux SDP 行通常像下面這樣：

a=rtpmap:96 H264/90000 a=fmtp:96 packetization-mode=1;profile-level-id=42802A;sprop-parameter-sets=Z0KAKtoBEA8eXlIKDAoNoUJq,aM4G4g==

OnDemandServerMediaSubsession::setSDPLinesFromRTPSink() 利用所有的信息，生成最終的子會話 SDP 行。最終的子會話 SDP 行通常像下面這樣：

m=video 0 RTP/AVP 96 c=IN IP4 0.0.0.0 b=AS:500 a=rtpmap:96 H264/90000 a=fmtp:96 packetization-mode=1;profile-level-id=42802A;sprop-parameter-sets=Z0KAKtoBEA8eXlIKDAoNoUJq,aM4G4g== a=control:track1

Done.

live555 源碼分析系列文章

live555 源碼分析：簡介
live555 源碼分析：基礎設施
live555 源碼分析：MediaSever
Wireshark 抓包分析 RTSP/RTP/RTCP 基本工作過程
live555 源碼分析：RTSPServer
live555 源碼分析：DESCRIBE 的處理
live555 源碼分析：SETUP 的處理
live555 源碼分析：PLAY 的處理
live555 源碼分析：RTSPServer 組件結構
live555 源碼分析：ServerMediaSession
live555 源碼分析：子會話 SDP 行生成

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