证据一：向所有客户端发出数据：
[cpp] view plain copyBoolean RTPInterface::sendPacket(unsigned char* packet, unsigned packetSize) { Boolean success = True; // we'll return False instead if any of the sends fail // Normal case: Send as a UDP packet: if (!fGS->output(envir(), fGS->ttl(), packet, packetSize)) success = False; // Also, send over each of our TCP sockets: for (tcpStreamRecord* streams = fTCPStreams; streams != NULL; streams = streams->fNext) { if (!sendRTPOverTCP(packet, packetSize, streams->fStreamSocketNum, streams->fStreamChannelId)) { success = False; } } return success; } 很明显啊，先发送udp数据，一对多的问题在GroupSocket中解决。再发送tcp数据，一对多的问题本地解决。证据二：从所有客户端读取数据：我现在找不到直接的证据，所以我就憶想一下吧：当udp端口或tcp端口收到数据时，分析后，是哪个客户端的数据就发给对应这个客户端的RTPSink或RTCPInstance。好像已经把最开始的问题解答完了。下面让我们来分析一下RTPInterface吧。[cpp] view plain copyvoid RTPInterface::setStreamSocket(int sockNum, unsigned char streamChannelId) { fGS->removeAllDestinations(); addStreamSocket(sockNum, streamChannelId); } void RTPInterface::addStreamSocket(int sockNum, unsigned char streamChannelId) { if (sockNum < 0) return; for (tcpStreamRecord* streams = fTCPStreams; streams != NULL; streams = streams->fNext) { if (streams->fStreamSocketNum == sockNum && streams->fStreamChannelId == streamChannelId) { return; // we already have it } } fTCPStreams = new tcpStreamRecord(sockNum, streamChannelId, fTCPStreams); } setStreamSocket()没必要说了吧，看一下addStreamSocke()。从字面意思应能了解，添加一个流式Socket，也就是添加tcp socket了。循环中查找是否已经存在了，最后如果不存在，就创建之，在tcpStreamRecord的构造函数中己经把自己加入了链表。对于参数，sockNum很易理解，就是socket()返回的那个SOCKET型数据呗，streamChannelId是什么呢？我们不防再猜测一下（很奇怪，我每次都能猜对，嘿嘿...）：rtp over tcp时，这个tcp连接是直接利用了RTSP所用的那个tcp连接，如果同时有很多rtp session，再加上rtsp session，大家都用这一个socket通信，怎么区分你的还是我的?我想这个channel id就是用于解决这个问题。给每个session分配一个唯一的id，在发送自己的包时为包再加上个头部，头部中需要有session的标记－－也就是这个channel id，包的长度等等字段。这样大家就可以穿一条裤子了，术语叫多路复用，但要注意只有tcp才进行多路复用，udp是不用的，因为udp是一个session对应一个socket（加上RTCP是两个）。想像一下，服务端要从这个tcp socket读写数据，必须把一个handler加入TaskScheduler中，这个handler在可读数据时进行读，在可写数据时进行写。在读数据时，对读出的数据进行分析，取得数据包的长度，以及其channel id，跟据channel id找到相应的处handler和对象，交给它们去处理自己的数据。试想两个建立在tcp上的rtp session，这个两个tcp socket既担负着rtsp通讯，又担负着rtp通讯。如果这两个rtp session共用一个stream，那么最终负责这两个session通信的就只有一个RTPInterface，那么这个RTPInterface中的fTCPStreams这个链表中就会有两项，分别对应这两个session。tcpStreamRecord主要用于socket number与channel id的对应。这些tcpStreamRecord是通过addStreamSocket()添加的。处理数据的handler是通过startNetworkReading（）添加的，看一下下：[cpp] view plain copyvoid RTPInterface::startNetworkReading(TaskScheduler::BackgroundHandlerPRoc* handlerProc) { // Normal case: Arrange to read UDP packets: envir().taskScheduler().turnOnBackgroundReadHandling(fGS->socketNum(),handlerProc, fOwner); // Also, receive RTP over TCP, on each of our TCP connections: fReadHandlerProc = handlerProc; for (tcpStreamRecord* streams = fTCPStreams; streams != NULL; streams = streams->fNext) { // Get a socket descriptor for "streams->fStreamSocketNum": SocketDescriptor* socketDescriptor = lookupSocketDescriptor(envir(), streams->fStreamSocketNum); // Tell it about our subChannel: socketDescriptor->registerRTPInterface(streams->fStreamChannelId, this); } } 用UDP时很简单，直接把处理函数做为handler加入taskScheduler即可。而TCP时，需向所有的session的socket都注册自己。可以想像，socketDescriptor代表一个tcp socket，并且它有一个链表之类的东西，其中保存了所有的对这个socket感兴趣的RTPInterface，同时也记录了RTPInterface对应的channal id。只有向socketDescriptor注册了自己，socketDescriptor在读取数据时，才能跟据分析出的channel id找到对应的RTPInterface，才能调用RTPInterface中的数据处理handler，当然，这个函数也不是RTPInteface自己的，而是从startNetworkReading()这个函数接收到的调用者的。上述主要讲的是一个RTPInterface对应多个客户端tcp socket的情形。现在又发现一个问题：SocketDescriptor为什么需要对应多个RTPInterface呢？上面已经讲了，是为了多路复用，因为这个socket即负担rtsp通信又负担rtp通信还负担RTCP通信。SocketDescriptor记录多路复用数据（也就是RTPInterface与channel id）用了一个Hash table：HashTable* fSubChannelHashTable。SocketDescriptor读数据使用函数：static void tcpReadHandler(SocketDescriptor*, int mask)。证据如下：[cpp] view plain copyvoid SocketDescriptor::registerRTPInterface( unsigned char streamChannelId, RTPInterface* rtpInterface) { Boolean isFirstRegistration = fSubChannelHashTable->IsEmpty(); fSubChannelHashTable->Add((char const*) (long) streamChannelId, rtpInterface); if (isFirstRegistration) { // Arrange to handle reads on this TCP socket: TaskScheduler::BackgroundHandlerProc* handler = (TaskScheduler::BackgroundHandlerProc*) &tcpReadHandler; fEnv.taskScheduler().turnOnBackgroundReadHandling(fOurSocketNum, handler, this); } } 可见在注册第一个多路复用对象时启动reand handler。看一下函数主体：[cpp] view plain copyvoid SocketDescriptor::tcpReadHandler1(int mask) { // We expect the following data over the TCP channel: // optional RTSP command or response bytes (before the first '$' character) // a '$' character // a 1-byte channel id // a 2-byte packet size (in network byte order) // the packet data. // However, because the socket is being read asynchronously, this data might arrive in pieces. u_int8_t c; struct sockaddr_in fromAddress; if (fTCPReadingState != AWAITING_PACKET_DATA) { int result = readSocket(fEnv, fOurSocketNum, &c, 1, fromAddress); if (result != 1) { // error reading TCP socket, or no more data available if (result < 0) { // error fEnv.taskScheduler().turnOffBackgroundReadHandling( fOurSocketNum); // stops further calls to us } return; } } switch (fTCPReadingState) { case AWAITING_DOLLAR: { if (c == '$') { fTCPReadingState = AWAITING_STREAM_CHANNEL_ID; } else { // This character is part of a RTSP request or command, which is handled separately: if (fServerRequestAlternativeByteHandler != NULL) { (*fServerRequestAlternativeByteHandler)( fServerRequestAlternativeByteHandlerClientData, c); } } break; } case AWAITING_STREAM_CHANNEL_ID: { // The byte that we read is the stream channel id. if (lookupRTPInterface(c) != NULL) { // sanity check fStreamChannelId = c; fTCPReadingState = AWAITING_SIZE1; } else { // This wasn't a stream channel id that we expected. We're (somehow) in a strange state. Try to recover: fTCPReadingState = AWAITING_DOLLAR; } break; } case AWAITING_SIZE1: { // The byte that we read is the first (high) byte of the 16-bit RTP or RTCP packet 'size'. fSizeByte1 = c; fTCPReadingState = AWAITING_SIZE2; break; } case AWAITING_SIZE2: { // The byte that we read is the second (low) byte of the 16-bit RTP or RTCP packet 'size'. unsigned short size = (fSizeByte1 << 8) | c; // Record the information about the packet data that will be read next: RTPInterface* rtpInterface = lookupRTPInterface(fStreamChannelId); if (rtpInterface != NULL) { rtpInterface->fNextTCPReadSize = size; rtpInterface->fNextTCPReadStreamSocketNum = fOurSocketNum; rtpInterface->fNextTCPReadStreamChannelId = fStreamChannelId; } fTCPReadingState = AWAITING_PACKET_DATA; break; } case AWAITING_PACKET_DATA: { // Call the appropriate read handler to get the packet data from the TCP stream: RTPInterface* rtpInterface = lookupRTPInterface(fStreamChannelId); if (rtpInterface != NULL) { if (rtpInterface->fNextTCPReadSize == 0) { // We've already read all the data for this packet. fTCPReadingState = AWAITING_DOLLAR; break; } if (rtpInterface->fReadHandlerProc != NULL) { rtpInterface->fReadHandlerProc(rtpInterface->fOwner, mask); } } return; } } } 最开始的注释中解释了多路复用头的格式。这一段引起了我的兴趣：[cpp] view plain copycase AWAITING_DOLLAR: { if (c == $) { fTCPReadingState = AWAITING_STREAM_CHANNEL_ID; } else { // This character is part of a RTSP request or command, which is handled separately: if (fServerRequestAlternativeByteHandler != NULL) { (*fServerRequestAlternativeByteHandler)( fServerRequestAlternativeByteHandlerClientData, c); } } break; } 啊！原来ServerRequestAlternativeByteHandler是用于处理RTSP数据的。也就是从这个socket收到RTSP数据时，调用ServerRequestAlternativeByteHandler。如果收到RTP/RTCP数据时，先查看其channel id，跟据id找到RTPInterface(RTCP也是用了RTPIterface进行通信)，设置RTPInterface中与读缓冲有关的变量，然后当读到包数据的开始位置时，调用rtpInterface中保存的数据处理handler。还记得吧，rtpInterface中的这个数据处理handler在UDP时也被使用，在这个函数中要做的是读取一个包的数据，然后处理这个包。而SocketDescriptor把读取位置置于包数据开始的位置再交给数据处理handler，正好可以使用与UDP相同的数据处理handler！还有，socketDescriptor们并不属于任何RTPInterface，而是单独保存在一个Hash table中，这样多个RTPInterface都可以注册到一个socketDescriptor中，以实现多路复用。总结一下通过RTPInterface，live555不仅实现了rtp over udp，还实现了rtp over tcp，而且还实现了同时即有rtp over tcp，又有rtp over udp！最后，channel id是从哪里来的呢？是在RTSP请求中指定的。在哪个请求中呢？自己找去吧。新闻热点
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