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继续说webrtc和rtp

上次文章一已经说明了webrtc 和 rtp 协议的关系，说明了 而sctp 协议 Stream Control Transmission Protocol(SCTP)协议 在chrome中打开如下地址： chrome://webrtc-internals/，可以看到很多信息，里面channel-data-1 是数据通道 可以看到google peer 2 peer 是使用的libjingle 时间戳像是一个服务一样，一直在更改。 这不得不引起我们的注意，webrtc不断更改，技术迭代更新太快了，为什么？基础好，这三个字分量太重，中国有句俗话：勿以恶小而为之，勿以善小而不为，这句话并非只适用在我们定义的善恶之上，也适用在程序员身上，程序员的恶习之一就是不求甚解，如果基础打好，我们的代码便是一片江山。

为什么如此说

zoom公司大家其实都知道，会议的体验很好，因为他们善用了datachannel，有兴趣大家可以仔细研究，推敲，到了今天，技术和核心技术得不到大家的理解，很多人依然认为技术可以获取，缺的是钱，事实上，两个都缺。sctp协议可以适用tcp，也可以使用udp，zoom公司在一个版本中使用datachannel传输RTP数据，原因是什么？为了体验和效率。

web示例

    
     
Realtime communication with WebRTC
      
   
    
    Start    
    Send    
    Stop  
   
    

'use strict';var localConnection;var remoteConnection;var sendChannel;var receiveChannel;var pcConstraint;var dataConstraint;var dataChannelSend = document.querySelector('textarea#dataChannelSend');var dataChannelReceive = document.querySelector('textarea#dataChannelReceive');var startButton = document.querySelector('button#startButton');var sendButton = document.querySelector('button#sendButton');var closeButton = document.querySelector('button#closeButton');startButton.onclick = createConnection;sendButton.onclick = sendData;closeButton.onclick = closeDataChannels;function enableStartButton() {     startButton.disabled = false;}function disableSendButton() {     sendButton.disabled = true;}function createConnection() {     dataChannelSend.placeholder = '';  var servers = null;  pcConstraint = null;  dataConstraint = null;  trace('Using SCTP based data channels');  // For SCTP, reliable and ordered delivery is true by default.  // Add localConnection to global scope to make it visible  // from the browser console.  window.localConnection = localConnection =      new RTCPeerConnection(servers, pcConstraint);  trace('Created local peer connection object localConnection');  sendChannel = localConnection.createDataChannel('sendDataChannel',      dataConstraint);  trace('Created send data channel');  localConnection.onicecandidate = iceCallback1;  sendChannel.onopen = onSendChannelStateChange;  sendChannel.onclose = onSendChannelStateChange;  // Add remoteConnection to global scope to make it visible  // from the browser console.  window.remoteConnection = remoteConnection =      new RTCPeerConnection(servers, pcConstraint);  trace('Created remote peer connection object remoteConnection');  remoteConnection.onicecandidate = iceCallback2;  remoteConnection.ondatachannel = receiveChannelCallback;  localConnection.createOffer().then(    gotDescription1,    onCreateSessionDescriptionError  );  startButton.disabled = true;  closeButton.disabled = false;}function onCreateSessionDescriptionError(error) {     trace('Failed to create session description: ' + error.toString());}function sendData() {     var data = dataChannelSend.value;  sendChannel.send(data);  trace('Sent Data: ' + data);}function closeDataChannels() {     trace('Closing data channels');  sendChannel.close();  trace('Closed data channel with label: ' + sendChannel.label);  receiveChannel.close();  trace('Closed data channel with label: ' + receiveChannel.label);  localConnection.close();  remoteConnection.close();  localConnection = null;  remoteConnection = null;  trace('Closed peer connections');  startButton.disabled = false;  sendButton.disabled = true;  closeButton.disabled = true;  dataChannelSend.value = '';  dataChannelReceive.value = '';  dataChannelSend.disabled = true;  disableSendButton();  enableStartButton();}function gotDescription1(desc) {     localConnection.setLocalDescription(desc);  trace('Offer from localConnection \n' + desc.sdp);  remoteConnection.setRemoteDescription(desc);  remoteConnection.createAnswer().then(    gotDescription2,    onCreateSessionDescriptionError  );}function gotDescription2(desc) {     remoteConnection.setLocalDescription(desc);  trace('Answer from remoteConnection \n' + desc.sdp);  localConnection.setRemoteDescription(desc);}function iceCallback1(event) {     trace('local ice callback');  if (event.candidate) {       remoteConnection.addIceCandidate(      event.candidate    ).then(      onAddIceCandidateSuccess,      onAddIceCandidateError    );    trace('Local ICE candidate: \n' + event.candidate.candidate);  }}function iceCallback2(event) {     trace('remote ice callback');  if (event.candidate) {       localConnection.addIceCandidate(      event.candidate    ).then(      onAddIceCandidateSuccess,      onAddIceCandidateError    );    trace('Remote ICE candidate: \n ' + event.candidate.candidate);  }}function onAddIceCandidateSuccess() {     trace('AddIceCandidate success.');}function onAddIceCandidateError(error) {     trace('Failed to add Ice Candidate: ' + error.toString());}function receiveChannelCallback(event) {     trace('Receive Channel Callback');  receiveChannel = event.channel;  receiveChannel.onmessage = onReceiveMessageCallback;  receiveChannel.onopen = onReceiveChannelStateChange;  receiveChannel.onclose = onReceiveChannelStateChange;}function onReceiveMessageCallback(event) {     trace('Received Message');  dataChannelReceive.value = event.data;}function onSendChannelStateChange() {     var readyState = sendChannel.readyState;  trace('Send channel state is: ' + readyState);  if (readyState === 'open') {       dataChannelSend.disabled = false;    dataChannelSend.focus();    sendButton.disabled = false;    closeButton.disabled = false;  } else {       dataChannelSend.disabled = true;    sendButton.disabled = true;    closeButton.disabled = true;  }}function onReceiveChannelStateChange() {     var readyState = receiveChannel.readyState;  trace('Receive channel state is: ' + readyState);}function trace(text) {     if (text[text.length - 1] === '\n') {       text = text.substring(0, text.length - 1);  }  if (window.performance) {       var now = (window.performance.now() / 1000).toFixed(3);    console.log(now + ': ' + text);  } else {       console.log(text);  }}

上面一段代码是可以运行的，不过大意义没有，他无法产品化，接下去的第三课，我们会大刀去修改他，让他真正实用起来，读者可以先研究以下这一段示例代码。

后面我们会使用rtp协议和信令服务继续改写，让他实用并且产品化。

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