WebRTC | Failed to execute 'setLocalDescription' on 'RTCPeerConnection': Failed to parse SessionDescription. a=msid: Missing track ID in msid attribute.

1.問題回放

　　使用如下代碼獲取局域網IP報錯

　　（代碼來源：https://github.com/diafygi/webrtc-ips 日期：2019-02-16）

Uncaught (in promise) DOMException: Failed to execute 'setLocalDescription' on 'RTCPeerConnection': Failed to parse SessionDescription. a=msid:  Missing track ID in msid attribute.

//get the IP addresses associated with an account
function getIPs(callback){
    var ip_dups = {};

    //compatibility for firefox and chrome
    var RTCPeerConnection = window.RTCPeerConnection
        || window.mozRTCPeerConnection
        || window.webkitRTCPeerConnection;
    var useWebKit = !!window.webkitRTCPeerConnection;

    //bypass naive webrtc blocking using an iframe
    if(!RTCPeerConnection){
        //NOTE: you need to have an iframe in the page right above the script tag
        //
        //<iframe id="iframe" sandbox="allow-same-origin" style="display: none"></iframe>
        //<script>...getIPs called in here...
        //
        var win = iframe.contentWindow;
        RTCPeerConnection = win.RTCPeerConnection
            || win.mozRTCPeerConnection
            || win.webkitRTCPeerConnection;
        useWebKit = !!win.webkitRTCPeerConnection;
    }

    //minimal requirements for data connection
    var mediaConstraints = {
        optional: [{RtpDataChannels: true}]
    };

    var servers = {iceServers: [{urls: "stun:stun.services.mozilla.com"}]};

    //construct a new RTCPeerConnection
    var pc = new RTCPeerConnection(servers, mediaConstraints);

    function handleCandidate(candidate){
        //match just the IP address
        var ip_regex = /([0-9]{1,3}(\.[0-9]{1,3}){3}|[a-f0-9]{1,4}(:[a-f0-9]{1,4}){7})/
        var ip_addr = ip_regex.exec(candidate)[1];

        //remove duplicates
        if(ip_dups[ip_addr] === undefined)
            callback(ip_addr);

        ip_dups[ip_addr] = true;
    }

    //listen for candidate events
    pc.onicecandidate = function(ice){

        //skip non-candidate events
        if(ice.candidate)
            handleCandidate(ice.candidate.candidate);
    };

    //create a bogus data channel
    pc.createDataChannel("");

    //create an offer sdp
    pc.createOffer(function(result){

        //trigger the stun server request
        pc.setLocalDescription(result, function(){}, function(){});

    }, function(){});

    //wait for a while to let everything done
    setTimeout(function(){
        //read candidate info from local description
        var lines = pc.localDescription.sdp.split('\n');

        lines.forEach(function(line){
            if(line.indexOf('a=candidate:') === 0)
                handleCandidate(line);
        });
    }, 1000);
}

//Test: Print the IP addresses into the console
getIPs(function(ip){console.log(ip);});

 

2.分析

　　代碼在此處報錯：pc.setLocalDescription(result, function(){}, function(){})

　　即此API報錯：RTCPeerConnection.setLocalDescription()

　　源代碼在Firefox和Chrome71正常執行，Chrome升級到72穩定版后代碼報錯，瀏覽器debug調試發現Chrome71和Chrome72的offer（代碼中是result）的SDP格式/內容發生了變化，因此可以判斷是新版本Chrome的WebRTC相關API發生了變化，於是在https://www.chromestatus.com/features找到了解釋：

 

　

 

　　文中提供了此鏈接：https://docs.google.com/document/d/1-ZfikoUtoJa9k-GZG1daN0BU3IjIanQ_JSscHxQesvU/edit，划重點：

　　　　WebRTC規范仍然在演進，Chrome上目前SDP格式有兩種：規范定義的SDP格式（Unified Plan）和Chrome自己定義的SDP格式（Plan B），火狐支持前者，如果不指定SDP格式Chrome72默認使用Unified Plan，這和Chrome71的行為不同，那么如何指定SDP格式呢？

　　

Controlling Which SDP Format Is Used

Specifying the sdpSemantics in Your Application

You can override the default behavior by explicitly specifying the sdpSemantics in the RTCPeerConnection constructor, allowing your application to control the SDP format of the client. This flag is Chrome-only; if the browser does not recognize the parameter it is simply ignored.

 

// Use Unified Plan or Plan B regardless of what the default browser behavior is.

new RTCPeerConnection({sdpSemantics:'unified-plan'});

new RTCPeerConnection({sdpSemantics:'plan-b'});

Unless your application is prepared for both cases, it is recommended that you explicitly set the sdpSemantics to avoid surprises when Chrome’s default behavior changes.

 

3.解決

　　將源代碼此行

　　　　var servers = {iceServers: [{urls: "stun:stun.services.mozilla.com"}]};

　　更改為　　　

　　　　var servers = {iceServers: [{urls: "stun:stun.services.mozilla.com"}], sdpSemantics:'plan-b'};

 

4.其他

　　如果Public IP無法解析，請更換stun服務器：

　　　　var servers = {iceServers: [{urls: "stun:stun.l.google.com:19302"}], sdpSemantics:'plan-b'};