sdpMung.js

	// Helper method to munge an SDP to enable simulcasting (Chrome only)
	function mungeSdpForSimulcasting(sdp) {
		// Let's munge the SDP to add the attributes for enabling simulcasting
		// (based on https://gist.github.com/ggarber/a19b4c33510028b9c657)
		var lines = sdp.split("\r\n");
		var video = false;
		var ssrc = [-1],
		    ssrc_fid = -1;
		var cname = null,
		    msid = null,
		    mslabel = null,
		    label = null;
		var insertAt = -1;
		for (var i = 0; i < lines.length; i++) {
			var mline = lines[i].match(/m=(\w+) */);
			if (mline) {
				var medium = mline[1];
				if (medium === "video") {
					// New video m-line: make sure it's the first one
					if (ssrc[0] < 0) {
						video = true;
					} else {
						// We're done, let's add the new attributes here
						insertAt = i;
						break;
					}
				} else {
					// New non-video m-line: do we have what we were looking for?
					if (ssrc[0] > -1) {
						// We're done, let's add the new attributes here
						insertAt = i;
						break;
					}
				}
				continue;
			}
			if (!video) continue;
			var fid = lines[i].match(/a=ssrc-group:FID (\d+) (\d+)/);
			if (fid) {
				ssrc[0] = fid[1];
				ssrc_fid = fid[2];
				lines.splice(i, 1);i--;
				continue;
			}
			if (ssrc[0]) {
				var match = lines[i].match('a=ssrc:' + ssrc[0] + ' cname:(.+)');
				if (match) {
					cname = match[1];
				}
				match = lines[i].match('a=ssrc:' + ssrc[0] + ' msid:(.+)');
				if (match) {
					msid = match[1];
				}
				match = lines[i].match('a=ssrc:' + ssrc[0] + ' mslabel:(.+)');
				if (match) {
					mslabel = match[1];
				}
				match = lines[i].match('a=ssrc:' + ssrc + ' label:(.+)');
				if (match) {
					label = match[1];
				}
				if (lines[i].indexOf('a=ssrc:' + ssrc_fid) === 0) {
					lines.splice(i, 1);i--;
					continue;
				}
				if (lines[i].indexOf('a=ssrc:' + ssrc[0]) === 0) {
					lines.splice(i, 1);i--;
					continue;
				}
			}
			if (lines[i].length == 0) {
				lines.splice(i, 1);i--;
				continue;
			}
		}
		if (ssrc[0] < 0) {
			// Couldn't find a FID attribute, let's just take the first video SSRC we find
			insertAt = -1;
			video = false;
			for (var i = 0; i < lines.length; i++) {
				var mline = lines[i].match(/m=(\w+) */);
				if (mline) {
					var medium = mline[1];
					if (medium === "video") {
						// New video m-line: make sure it's the first one
						if (ssrc[0] < 0) {
							video = true;
						} else {
							// We're done, let's add the new attributes here
							insertAt = i;
							break;
						}
					} else {
						// New non-video m-line: do we have what we were looking for?
						if (ssrc[0] > -1) {
							// We're done, let's add the new attributes here
							insertAt = i;
							break;
						}
					}
					continue;
				}
				if (!video) continue;
				if (ssrc[0] < 0) {
					var value = lines[i].match(/a=ssrc:(\d+)/);
					if (value) {
						ssrc[0] = value[1];
						lines.splice(i, 1);i--;
						continue;
					}
				} else {
					var match = lines[i].match('a=ssrc:' + ssrc[0] + ' cname:(.+)');
					if (match) {
						cname = match[1];
					}
					match = lines[i].match('a=ssrc:' + ssrc[0] + ' msid:(.+)');
					if (match) {
						msid = match[1];
					}
					match = lines[i].match('a=ssrc:' + ssrc[0] + ' mslabel:(.+)');
					if (match) {
						mslabel = match[1];
					}
					match = lines[i].match('a=ssrc:' + ssrc + ' label:(.+)');
					if (match) {
						label = match[1];
					}
					if (lines[i].indexOf('a=ssrc:' + ssrc_fid) === 0) {
						lines.splice(i, 1);i--;
						continue;
					}
					if (lines[i].indexOf('a=ssrc:' + ssrc[0]) === 0) {
						lines.splice(i, 1);i--;
						continue;
					}
				}
				if (lines[i].length == 0) {
					lines.splice(i, 1);i--;
					continue;
				}
			}
		}
		if (ssrc[0] < 0) {
			// Still nothing, let's just return the SDP we were asked to munge
			Janus.warn("Couldn't find the video SSRC, simulcasting NOT enabled");
			return sdp;
		}
		if (insertAt < 0) {
			// Append at the end
			insertAt = lines.length;
		}
		// Generate a couple of SSRCs
		ssrc[1] = Math.floor(Math.random() * 0xFFFFFFFF);
		ssrc[2] = Math.floor(Math.random() * 0xFFFFFFFF);
		// Add attributes to the SDP
		for (var i = 0; i < ssrc.length; i++) {
			if (cname) {
				lines.splice(insertAt, 0, 'a=ssrc:' + ssrc[i] + ' cname:' + cname);
				insertAt++;
			}
			if (msid) {
				lines.splice(insertAt, 0, 'a=ssrc:' + ssrc[i] + ' msid:' + msid);
				insertAt++;
			}
			if (mslabel) {
				lines.splice(insertAt, 0, 'a=ssrc:' + ssrc[i] + ' mslabel:' + msid);
				insertAt++;
			}
			if (label) {
				lines.splice(insertAt, 0, 'a=ssrc:' + ssrc[i] + ' label:' + msid);
				insertAt++;
			}
		}
		lines.splice(insertAt, 0, 'a=ssrc-group:SIM ' + ssrc[0] + ' ' + ssrc[1] + ' ' + ssrc[2]);
		sdp = lines.join("\r\n");
		if (!sdp.endsWith("\r\n")) sdp += "\r\n";
		return sdp;
	}