Websocket

The WebSocket Protocol
joseph@艾鍗學院
2016/09/11

HTTP Issue
Depends on Request / Response architecture
• Only client can send Requests
• Server can only Respond to Requests
• Can’t send another Request before Response
Too much traffic overhead & header on every
Request / Response
HTTP is Good For Docs Not Apps

HTTP Header (too much traffic overhead!!)
HTTP Client
Server

HTTP 改善方法
避免一來一回: 使用AJAX 達到非同步通訊
(但依然需要發出請求, 因為這還是polling)
避免過多的polling traffic ,使用Long Polling
(這也會消耗伺服器頻寬和資源,且還是half-duplex)
固定http Header size且改成binary協定,
且要做成 Full-duplex

Long Polling技術
An attempt to offer realtime server interaction, using a
persistent or long-lasting HTTP connection between the
server and the client.
The browser makes an Ajax-style request to the server, which
is kept open until the server has new data to send to the
browser, which is sent to the browser in a complete response.
The browser initiates a new long polling request in order to
obtain subsequent events.
Specific technologies:

WebSockets - Sockets on the Web
● Part of HTML5
● W3C API and IETF Protocol (RFC 6455 )
● Full-duplex, bidirectional communication
● Unsecured (TCP) and secured (SSL) modes
● Traverses firewalls, proxies and routers
● Text (UTF-8) and binary data
● Ping/Pong messages for keep-alive
● Share ports 80 and 443 with HTTP/HTTPS

WebSocket
A WebSocket is a standard bidirectional TCP socket between
the client and the server.
The socket starts out as a HTTP connection and then
"Upgrades" to a TCP socket after a HTTP handshake.
After the handshake, either side can send data with full
duplex mode
WebSocket Client WebSocket Server

WebSocket優點
• 傳送的HTTP Request Header僅2bytes
• 伺服器可主動Push資料給用戶端,而不是總是傳統HTTP
Request/Response的Cycle(half-duplex),

WebSocket Connection Process
The client requesting a webSocket connection, sends an HTTP request to
the server on port 80.
That HTTP request is a perfectly legal HTTP request, but it has a header
included on it Upgrade: websocket.
If the server supports the webSocket protocol, then it responds with a
legal HTTP response with a 101 status code that includes a
header Connection: Upgrade.
At that point, both sides then switch protocols to the webSocket protocol
and all future communication on that socket is done using the data format
for the webSocket frame.
Any other incoming HTTP requests that do not contain
the upgrade request header are treated as normal HTTP requests

透過HTTP請求升級
成WebSocket通訊
HTTP "101 Switching
Protocols"
同意接受升級

Packet Opcodes (Types)
0 - continuation frame
1 - text frame (UTF-8)
2 - binary frame
3-7 - reserved (data)
8 - connection close
9 - ping
10 - pong
11-15 - reserved (control)

WebSocket frame
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
F
I
N
R
S
V
1
R
S
V
2
R
S
V
3
opcode(4)
M
A
S
K
payload len(7) extended payload len(16/64)
extended payload len continued(16/64)
masking key(0/32)
masking key continued payload ...

socket.io
socket.io : Cross browser way to do JavaScript-based real time
communication is
socket.io uses WebSockets if the client supports it, and falls
back on other things if it’s not, and even has AJAX polling and
multi-part streaming

Sending and receiving events
Socket.IO allows you to emit and receive custom events.
Besides connect, message and disconnect, you can emit
custom events
Socket.IO allows you to emit and receive custom events. Besides connect, message and disconnect, you can emit custom events:

Node.js Socket.IO (Web Socket Server)
npm install socket.io
var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);
app.get('/', function(req, res){
res.sendFile(__dirname + '/index.html');
});
io.on('connection', function(socket){
socket.on('chat message', function(msg){
io.emit('chat message', msg);
});
});
http.listen(3000, function(){
console.log('listening on *:3000');
});
io代表所有sockets
socket 代表單一連入的socket

Node.js Socket.IO (Web Socket Client)
<body>
<ul id="messages"></ul>
<form action="">
<input id="m" autocomplete="off" /><button>Send</button>
</form>
<script src="https://cdn.socket.io/socket.io-1.2.0.js"></script>
<script src="http://code.jquery.com/jquery-1.11.1.js"></script>
<script>
var socket = io();
$('form').submit(function(){
socket.emit('chat message', $('#m').val());
$('#m').val('');
return false;
});
socket.on('chat message', function(msg){
$('#messages').append($('<li>').text(msg));
});
</script>
</body>

探討:
使用AJAX由於Client不斷發出HTTP Request給Server確認是
否有資料回傳，而HTTP Request Header占了大量的Bytes，
傳送時不僅占了大量網路頻寬，對Server資源的消耗量也
會變大。

實驗: Polling SetInterval() 與SetTimeout() 差異
輪詢（polling）是讓瀏覽器每隔一段時間就自動送出一個
HTTP 請求給伺服器

使用setInterval() 可能會有這個問題!
a=1 a=2a=0 a=1 a=2
a=1
Time

使用setTimeout() 可以避免錯亂!
Using setTimeout() doesn’t guarantee execution on a fixed
interval per se. But, it does guarantee that the previous
interval has completed before the next interval is called
a=1 a=2a=0 a=1 a=2
Time

Read More
• Socket.IO : http://socket.io
• RFC 6455 - The WebSocket Protocol - IETF Tools

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