webrtc 和 rtp 协议 之sctp

继续说webrtc和rtp

webrtc和rtp 一 pion的datachannel示例

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
  


  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。