timesync

Time synchronization between peers.

Usage scenarios:

- master/slave: Clients synchronize their time to that of a single server,
via either HTTP requests or WebSockets.
- peer-to-peer: Clients are connected in a (dynamic) peer-to-peer network
using WebRTC or WebSockets and must converge to a single, common time in the
network.

Install

Install via npm:

``
npm install timesync
`

Usage

A timesync client can basically connect to one server or multiple peers,
and will synchronize it's time. The synchronized time can be retrieved via
the method now(), and the client can subscribe to events like 'change'
and 'sync'.

`js
// create a timesync instance
var ts = timesync({
server: '...', // either a single server,
peers: [...] // or multiple peers
});

// get notified on changes in the offset
ts.on('change', function (offset) {
console.log('offset from system time:', offset, 'ms');
}

// get the synchronized time
console.log('now:', new Date(ts.now()));
`

Example

Here a full usage example with express.js, showing both server and client side.
timesync has build-in support for requests over http and can be used with
express, a default http server, or other solutions. timesync can also be
used over other transports than http, for example using websockets or webrtc.
This is demonstrated in the advanced examples.

More examples are available in the /examples folder.
Some of the examples use libraries like express or socket.io.
Before you can run these examples you will have to install these dependencies.

server.js

`js
var express = require('express');
var timesyncServer = require('timesync/server');

// create an express app
var port = 8081;
var app = express();
app.listen(port);
console.log('Server listening at http://localhost:' + port);

// serve static index.html
app.get('/', express.static(__dirname));

// handle timesync requests
app.use('/timesync', timesyncServer.requestHandler);
`

index.html

`html











`

API

Client

$3

An instance of timesync is created as:

`js
var ts = timesync(options);
`

#### Options

The following options are available:

Name | Type | Default | Description
---------- | ---------------------- | ---------- | ----------------------------------------
delay | number | 1000 | Delay in milliseconds between every request sent.
interval | number or null | 3600000 | Interval in milliseconds for running a synchronization. Defaults to 1 hour. Set to null to disable automatically running synchronizations (synchronize by calling sync()).
now | function | Date.now | Function returning the local system time.
peers | string[] or string | [] | Array or comma separated string with uri's or id's of the peers to synchronize with. Cannot be used in conjunction with option server.
repeat | number | 5 | Number of times to do a request to every peer.
server | string | none | Url of a single server in case of a master/slave configuration. Cannot be used in conjunction with option peers.
timeout | number | 10000 | Timeout in milliseconds for requests to fail.

$3

Name | Return type | Description
--------------------- | ----------- | ----------------------------------
destroy() | none | Destroy the timesync instance. Stops automatic synchronization. If timesync is currently executing a synchronization, this synchronization will be finished first.
now() | number | Get the synchronized time. Returns a timestamp. To create a Date, call new Date(time.now()).
on(event, callback) | Object | Register a callback handler for an event. Returns the timesync instance. See section Events for more information.
off(event [, callback]) | Object | Unregister a callback handler for an event. If no callback is provided, all callbacks of this event will be removed. Returns the timesync instance. See section Events for more information.
sync() | none | Do a synchronization with all peers now.

To be able to send and receive messages from peers, timesync needs a transport. To hook up a transport like a websocket or http requests, one has to override the send(id, data) method of the timesync instance, and has to call ts.receive(id, data) on incoming messages.

Name | Return type | Description
----------------------------------- | ----------- | ----------------------------------
send(to, data, timeout) : Promise | none | Send a message to a peer. to is the id of the peer, and data a JSON object containing the message. Must return a Promise which resolves when the message has been sent, or rejects when sending failed or a timeout occurred.
receive(from, data) | none | Receive a message from a peer. from is the id of the sender, and data a JSON object containing the message.

timesync sends messages using the JSON-RPC protocol, as described in the section Protocol.

$3

timesync emits events when starting and finishing a synchronization, and when the time offset changes. To listen for events:

`js
ts.on('change', function (offset) {
console.log('offset changed:', offset);
});
`

Available events:

Name | Description
---------| ----------
change | Emitted when the offset is changed. This can only happen during a synchronization. Callbacks are called with the new offset (a number) as argument.
error | Emitted when an error occurred. Callbacks are called with the error as argument.
sync | Emitted when a synchronization is started or finished. Callback are called with a value 'start' or 'end' as argument.

$3

Name | Type | Description
--------- | -------- | --------------------------------------------
offset | number | The offset from system time in milliseconds.
options | Object | An object holding all options of the timesync instance. One can safely adjust options like peers at any time. Not all options can be changed after construction, for example a changed interval value will not be applied.

Server

timesync comes with a build in server to serve as a master for time synchronization. Clients can adjust their time to that of the server. The server basically just implements a POST request responding with its current time, and serves the static files timesync.js and timesync.min.js from the /dist folder. It's quite easy to implement this request handler yourself, as is demonstrated in the advanced examples.

The protocol used by the server is described in the section Protocol.

$3

The server can be loaded in node.js as:

`js
var timesyncServer = require('timesync/server');
`

$3

Name | Return type | Description
----------------------------- | ------------ | ----------------------------------
createServer() | http.Server| Create a new, dedicated http Server. This is just a shortcut for doing http.createServer( timesyncServer.requestHandler ).
attachServer(server, [path])| http.Server| Attach a request handler for time synchronization requests to an existing http Server. Argument server must be an instance of http.Server. Argument path is optional, and is /timesync by default.

$3

Name | Type | Description
----------------- | ---------- | --------------------------------------------
requestHandler | function | A default request handler, handling requests for the timesync server. Signature is requestHandler(request, response). This handler can be used to attach to an expressjs server, or to create a plain http server by doing http.createServer( timesyncServer.requestHandler ).

Protocol

timesync sends messages using the JSON-RPC protocol. A peer sends a message:

`json
{"jsonrpc": "2.0", "id": "12345", "method": "timesync"}
`

The receiving peer replies with the same id and its current time:

`json
{"jsonrpc": "2.0", "id": "12345", "result": 1423151204595}
`

The sending peer matches the returned message by id and uses the result to adjust it's offset.

Algorithm

timesync uses a simple synchronization protocol aimed at the gaming industry, and extends this for peer-to-peer networks. The algorithm is described here:

> A simple algorithm with these properties is as follows:
>
> 1. Client stamps current local time on a "time request" packet and sends to server
> 2. Upon receipt by server, server stamps server-time and returns
> 3. Upon receipt by client, client subtracts current time from sent time and divides by two to compute latency. It subtracts current time from server time to determine client-server time delta and adds in the half-latency to get the correct clock delta. (So far this algorithm is very similar to SNTP)
> 4. The first result should immediately be used to update the clock since it will get the local clock into at least the right ballpark (at least the right timezone!)
> 5. The client repeats steps 1 through 3 five or more times, pausing a few seconds each time. Other traffic may be allowed in the interim, but should be minimized for best results
> 6. The results of the packet receipts are accumulated and sorted in lowest-latency to highest-latency order. The median latency is determined by picking the mid-point sample from this ordered list.
> 7. All samples above approximately 1 standard-deviation from the median are discarded and the remaining samples are averaged using an arithmetic mean.

This algorithm assumes multiple clients synchronizing with a single server. In case of multiple peers, timesync will take the average offset of all peers (excluding itself) as offset.

Tutorials

- Using the timesync library in Android applications

Resources

- A Stream-based Time Synchronization Technique For Networked Computer Games
- Network Time Protocol

Build

To build the library:

npm install
npm run build

This will generate the files timesync.js and timesync.min.js in the folder /dist`.

To automatically build on changes, run:

npm run watch