WebRTC对Simulcast的支持

最近在调研基于WebRTC的Simulcast的方案，发现WebRTC 1.0的草案里面已经Simulcast的相关定义，并且举出了相关的例子。

EXAMPLE 15
var signalingChannel = new SignalingChannel();
var cOnfiguration= { "iceServers": [{ "urls": "stuns:stun.example.org" }] };
var pc;

// call start() to initiate
function start() {
    pc = new RTCPeerConnection(configuration);

    // let the "negotiationneeded" event trigger offer generation
    pc.Onnegotiationneeded= function () {
        pc.createOffer().then(function (offer) {
            return pc.setLocalDescription(offer);
        })
        .then(function () {
            // send the offer to the other peer
            signalingChannel.send(JSON.stringify({ "desc": pc.localDescription }));
        })
        .catch(logError);
    };

    // get a local stream, show it in a self-view and add it to be sent
    navigator.mediaDevices.getUserMedia({ "audio": true, "video": true })
        .then(function (stream) {
            selfView.srcObject = stream;
            pc.addTransceiver(stream.getAudioTracks()[0], {direction: "sendonly"});
            pc.addTransceiver(stream.getVideoTracks()[0], {
                direction: "sendonly",
                sendEncodings: [
                    {
                      rid: "f",
                    },
                    {
                      rid: "h",
                      scaleDownResolutionBy: 2.0
                    },
                    {
                      rid: "q",
                      scaleDownResolutionBy: 4.0
                    }
                ]
            });
        })
        .catch(logError);
}

signalingChannel.Onmessage= function (evt) {
    var message = JSON.parse(evt.data);
    if (message.desc)
        pc.setRemoteDescription(message.desc).catch(logError);
    else
        pc.addIceCandidate(message.candidate).catch(logError);
};

function logError(error) {
    log(error.name + ": " + error.message);
}

既然标准里面已经有了Simulcast的相关接口，那么各个浏览器里面的WebRTC实现情况如何呢？通过一番搜索之后，发现目前能支持Simulcast的浏览器貌似只有Chrome和Firefox两款，而且只能使用VP8。

Chrome里面其实早就已经支持Simulcast，Google Hangouts里面就已经在使用。不过目前Chrome还不能使用WebRTC 1.0里面方法来启用，只能通过修改Local SDP的方式来开启，实现一个“SIM”的 ssrc group。SDP样式如下：

a=ssrc-group:SIM 2178216979 2295769805 636898760
a=ssrc-group:FID 2178216979 2643517961
a=ssrc-group:FID 2295769805 1534787773
a=ssrc-group:FID 636898760 1305495962
a=ssrc:2178216979 cname:localCname
a=ssrc:2178216979 msid:R6WoG3JzZEw8BVEBuy1XpTuUzzfSGIlr08Ro 541b12bc-754c-4c06-8367-481ec12e3192
a=ssrc:2295769805 cname:localCname
a=ssrc:2295769805 msid:R6WoG3JzZEw8BVEBuy1XpTuUzzfSGIlr08Ro 541b12bc-754c-4c06-8367-481ec12e3192
a=ssrc:636898760 cname:localCname
a=ssrc:636898760 msid:R6WoG3JzZEw8BVEBuy1XpTuUzzfSGIlr08Ro 541b12bc-754c-4c06-8367-481ec12e3192
a=ssrc:2643517961 cname:localCname
a=ssrc:2643517961 msid:R6WoG3JzZEw8BVEBuy1XpTuUzzfSGIlr08Ro 541b12bc-754c-4c06-8367-481ec12e3192
a=ssrc:1534787773 cname:localCname
a=ssrc:1534787773 msid:R6WoG3JzZEw8BVEBuy1XpTuUzzfSGIlr08Ro 541b12bc-754c-4c06-8367-481ec12e3192
a=ssrc:1305495962 cname:localCname
a=ssrc:1305495962 msid:R6WoG3JzZEw8BVEBuy1XpTuUzzfSGIlr08Ro 541b12bc-754c-4c06-8367-481ec12e3192

Firefox里面的实现看上去就是按照WebRTC 1.0定义的接口来实现的，与举例Example 15中类似。SDP样式如下：

a=simulcast: send rid=high;medium;low
a=ssrc:4279128183 cname:{7cc599d1-10d9-4d41-9e53-eb3e74ad54fa}
a=ssrc:1635836235 cname:{7cc599d1-10d9-4d41-9e53-eb3e74ad54fa}
a=ssrc:921425862 cname:{7cc599d1-10d9-4d41-9e53-eb3e74ad54fa}

看来目前要支持多浏览器的Simulcast的互通，WebRTC的服务器必须实现两种SDP的格式转换。Janus的Video Call Demo里面已经有了实现，可以参考：

https://janus.conf.meetecho.com/videocalltest.html?simulcast=true