PTPv2 client and RTP relay with timestamping for AES67. Also includes SAP/SDP.

ptp.js

var sdp = require('./sdp');

//SDP Stuff
var samplerate = 48000;
var channels = 2;
var multicastAddr = '239.69.0.111';
var addr = '192.168.1.1';
var encoding = 'L24';
var name = 'RPi 4';
var sessID = Math.floor(Date.now() / 1000);
var sessVersion = sessID;
var ptpMaster;
var sync = false;

//ptp settings
var ptp_domain = 0;

var startSDP = function(){
	sdp.start(addr, multicastAddr, samplerate, channels, encoding, name, sessID, sessVersion, ptpMaster);
}

//RTP Server
var dgram = require('dgram');
var server = dgram.createSocket('udp4');

server.on('listening', function() {
	server.setMulticastInterface(addr);
	console.log('[RTP] relay server started');
});

server.on('message', function(message, remote) {
	if(!sync)
		return;

	message.writeInt32BE(Math.ceil(rtp_timestamp() / 48)*48 + 48, 4);
	server.send(message, 5004, multicastAddr);
});

server.bind(5003);


//PTPv2
var ptpClientEvent = dgram.createSocket('udp4');
var ptpClientGeneral = dgram.createSocket('udp4');
var ptpMulticastAddrs = ['224.0.1.129', '224.0.1.130', '224.0.1.131', '224.0.1.132'];

//vars
var t1, ts1, t2, ts2;
var offset = [0, 0];
var sync_seq;
var req_seq = 0;

//functions
//creates ptp delay_req buffer
var ptp_delay_req = function(){
	var length = 52;
	var buffer = Buffer.alloc(length);
	buffer.writeUInt8(1, 0);
	buffer.writeUInt8(2, 1);
	buffer.writeUInt16BE(length, 2);
	buffer.writeUInt16BE(++req_seq, 30);
	
	return buffer;
}

//convert PTP timestamp to rtp timestamp
var rtp_timestamp = function(){
	var time = ptp_time();
	return ((time[0] * samplerate) + Math.round((time[1] * samplerate) / 1000000000)) & 0xffffffff;
}

//calculated ptp time
var ptp_time = function(){
	var time = process.hrtime();
	var timeS = time[0] - offset[0];
	var timeNS = time[1] - offset[1];

	return [timeS, timeNS];
}

//event msg client
ptpClientEvent.on('listening', function() {
	console.log('[PTP] event msg client started');
	ptpClientEvent.addMembership(ptpMulticastAddrs[ptp_domain], addr);
});

ptpClientEvent.on('message', function(buffer, remote) {
	var recv_ts = process.hrtime();//safe timestamp for ts1

	//read values from buffer
	var type = buffer.readUInt8(0) & 0x0f;
	var version = buffer.readUInt8(1);
	var length = buffer.readUInt16BE(2);
	var domain = buffer.readUInt8(4);
	var flags = buffer.readUInt16BE(6);
	var source = buffer.toString('hex', 20, 28).match(/.{1,2}/g).join('-')+':0';
	var sourceAlt = buffer.toString('hex', 20, 28).match(/.{1,2}/g).join(':');
	var sequence = buffer.readUInt16BE(30);

	if(version != 2 || domain != ptp_domain)//check for version 2 and domain 0
		return;

	if(type != 0)//only process sync messages
		return;

	//do we have a new ptp master?
	if(source != ptpMaster){
		ptpMaster = source;
		console.log('[PTP] New master', sourceAlt, 'on domain', ptp_domain);
		sync = false;
	}

	//save sequence number
	sync_seq = sequence;

	//check if master is two step or not
	if((flags & 0x0200) == 0x0200){
		//two step, wait for follow_up msg for accurate t1
		ts1 = recv_ts;
	}else{
		//got accurate t1 (no follow_up msg)
		ts1 = recv_ts;

		//read t1 timestamp
		var tsS = (buffer.readUInt16BE(34) << 4) + buffer.readUInt32BE(36);
		var tsNS = buffer.readUInt32BE(40);
		t1 = [tsS, tsNS];

		//send delay_req
		ptpClientEvent.send(ptp_delay_req(), 319, ptpMulticastAddrs[ptp_domain], function(err){
			t2 = process.hrtime();
		});
	}
});

ptpClientEvent.bind(319, ptpMulticastAddrs[ptp_domain]);

//general msg Client
ptpClientGeneral.on('listening', function() {
	console.log('[PTP] general msg client started');
	ptpClientGeneral.addMembership(ptpMulticastAddrs[ptp_domain], addr);
});

ptpClientGeneral.on('message', function(buffer, remote) {
	//safe timestamp for ts2
	var recv_ts = process.hrtime();

	//read values from buffer
	var type = buffer.readUInt8(0) & 0x0f;
	var version = buffer.readUInt8(1);
	var length = buffer.readUInt16BE(2);
	var domain = buffer.readUInt8(4);
	var flags = buffer.readUInt16BE(6);
	var source = buffer.toString('hex', 20, 28).match(/.{1,2}/g).join('-')+':0';
	var sequence = buffer.readUInt16BE(30);

	//check for version 2 and domain
	if(version != 2 || domain != ptp_domain)
		return;

	if(type == 0x08 && sync_seq == sequence){ //follow up msg with current seq
		//read t1 timestamp
		var tsS = (buffer.readUInt16BE(34) << 4) + buffer.readUInt32BE(36);
		var tsNS = buffer.readUInt32BE(40);
		t1 = [tsS, tsNS];

		//send delay_req
		ptpClientEvent.send(ptp_delay_req(), 319, ptpMulticastAddrs[ptp_domain], function(err){
			t2 = process.hrtime();
		});
	}else if(type == 0x09 && req_seq == sequence){ //delay_rsp msg
		//read ts2 timestamp
		var tsS = (buffer.readUInt16BE(34) << 4) + buffer.readUInt32BE(36);
		var tsNS = buffer.readUInt32BE(40);
		ts2 = [tsS, tsNS];

		//calc offset
		offset[0] = 0.5 * (ts1[0] - t1[0] - ts2[0] + t2[0]);
		offset[1] = 0.5 * (ts1[1] - t1[1] - ts2[1] + t2[1]);

		//check if the clock was synced before
		if(!sync){
			console.log('[PTP] Synced to Master');
			sync = true;
			
			//start sdp
			startSDP();
		}
	}
});

ptpClientGeneral.bind(320, ptpMulticastAddrs[ptp_domain]);

sdp.js

var dgram = require('dgram');
var socket = dgram.createSocket({ type: 'udp4', reuseAddr: true });

var constructSDPMsg = function(addr, multicastAddr, samplerate, channels, encoding, name, sessID, sessVersion, ptpMaster){
	var sapHeader = Buffer.alloc(8);
	var sapContentType = Buffer.from('application/sdp\0');
	var ip = addr.split('.');

	//write version/options
	sapHeader.writeUInt8(0x20);

	//write hash
	sapHeader.writeUInt16LE(0xefef, 2);

	//write ip
	sapHeader.writeUInt8(parseInt(ip[0]), 4);
	sapHeader.writeUInt8(parseInt(ip[1]), 5);
	sapHeader.writeUInt8(parseInt(ip[2]), 6);
	sapHeader.writeUInt8(parseInt(ip[3]), 7);

	var sdpConfig = [
		'v=0',
		'o=- '+sessID+' '+sessVersion+' IN IP4 '+addr,
		's='+name,
		'c=IN IP4 '+multicastAddr+'/32',
		't=0 0',
		'a=clock-domain:PTPv2 0',
		'm=audio 5004 RTP/AVP 96',
		'a=rtpmap:96 '+encoding+'/'+samplerate+'/'+channels,
		'a=sync-time:0',
		'a=framecount:48',
		'a=ptime:1',
		'a=mediaclk:direct=0',
		'a=ts-refclk:ptp=IEEE1588-2008:'+ptpMaster,
		'a=recvonly',
		''
	];

	var sdpBody = Buffer.from(sdpConfig.join('\r\n'));

	return Buffer.concat([sapHeader, sapContentType, sdpBody]);
}

exports.start = function(addr, multicastAddr, samplerate, channels, encoding, name, sessID, sessVersion, ptpMaster){
	sdpMSG = constructSDPMsg(addr, multicastAddr, samplerate, channels, encoding, name, sessID, sessVersion, ptpMaster);

	socket.bind(9875, function(){
		socket.setMulticastInterface(addr);
		socket.send(sdpMSG, 9875, '239.255.255.255', function(err){});
	});
	
	setInterval(function(){
		socket.send(sdpMSG, 9875, '239.255.255.255', function(err){});
	}, 30*1000);
}

src.sh

#!/bin/sh
gst-launch-1.0 alsasrc device=hw:1 buffer-time=1000 !\
	audioconvert !\
	audio/x-raw, format=S24BE, channels=2, rate=48000 !\
	rtpL24pay min-ptime=1000000 max-ptime=1000000 !\
	application/x-rtp, clock-rate=48000, channels=2 !\
	udpsink host=192.168.1.1 port=5003