Escribir servidores WebSocket

Introducción

Un servidor WebSocket es simplemente una aplicación TCP que escucha en cualquier puerto de una servidor que sigue un protocolo específico. La tarea de crear un servidor propio personalizado suele asustar a los desarrolladores, sin embargo puede resultar muy fácil implementar un servidor WebSocket en la plataforma que elijas.

Un servidor WebSocket puede ser escrito en cualquier lenguaje de programación Server-Side que sea soporte Berkeley Sockets, como por ejemplo C++ o Python o inclusive PHP y JavaScript para servidores. Este no va a ser un tutorial para ningún lenguaje espefícamente sino que te ayudará a escribir tu propio servidor.

Necesitarás conocer como trabaja el protocolo HTTP y una experiencia intermedia en programación. Dependiendo de las capacidades de tu lenguaje puede ser necesario tener conocimientos en sockets TCP. Esta guía te dará el conocimiento necesario para escribir un servidor con WebSocket.

Un servidor WebSocket es explicado a un muy bajo nivel aquí. Los servidores WebSocket usualmente estan separados y especializados (por una cuestión de balance de cargas y otra razones prácticas), por lo tanto deberías usar un Reverse Proxy (semejante a un servidor HTTP común) casi siempre para detectar los Handshakes de WebSocket, preprocesarlos, y reenviar los datos de los clientes al servidor WebSocket real.
 

Paso 1: El Handshake del WebSocket

Antes que nada, el servidor debe escuchar las conexiones entrantes usando un socket TCP estandar. Dependiendo de tu plataforma, esto puede ser manejado por tí. Por ejemplo asumamos que tu servidor esta escuchando la dirección example.com en el puerto 8000, y tu socket en el servidor responde a la petición GET con /chat.

El Handshake es el puente desde HTTP a WS. En el Handshake se negocian los detalles de la conexión y cualquier de las partes pueden abandonar el proceso antes de completar dicha conexión si los términos no son favorables. El servidor debe ser cuidadoso al analizar lo que el cliente pide, de lo contrario podrían introducirse problemas de seguridad.

Petición de Handshake en el cliente

A pesar de que estamos creando un servidor, un cliente es quien tiene que comenzar el proceso de Handshake de WebSocket. Entonces tú tienes que saber cómo interpretar la petición del cliente. El cliente enviará una linda petición HTTP estandar que lucirá algo asi (la versión del HTTP debe ser 1.1 o mayor y el método debe ser GET):

GET /chat HTTP/1.1
Host: example.com:8000
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

El cliente puede solicitar aquí extensiones y/o sub protocolos; vea Misceláneos para más detalles. También, cabeceras comunes como User-Agent, Referer, Cookie, or cabeceras de autenticación podrían ser incluidos. Haz lo que quieras con ellos; no pertencen a WebSocket. También puedes ignorarlos. En muchas configuraciones comunes, un proxy inverso ya ha tratado con ellos.

Si alguna cabecera no se entiende o posee un valor incorrecto, el servidor debe responder "400 Bad Request" e inmediatamente cerrar la conexión. Normalmente, también puede dar la razón porque falló el handshake en el cuerpo de la respuesta HTTP, pero el mensaje podría no ser mostrado (el browser no lo muestra). Si el servidor no comprende que la versión del WebSockets, debería enviar una cabecera Sec-WebSocket-Version que contenga la(s) versión(es) no entendidas. (Esta guía explica v13, la más nueva). Ahora, vamos a ver la cabecera más curiosa, Sec-WebSocket-Key.

Respuesta de Handshake del servidor

Después de la petición, el servidor debería enviar una linda respuesta (aunque todavía en formato HTTP) que se verá asi (hay que recordar que la cabecera termina con \r \n y agrega un \r \n extra después del último):

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

Adicionalmente, el servidor puede decidir respecto de las solicitudes "extension/subprotocol" en este punto (ver Miscelláneos para más detalles). La cabecera Sec-WebSocket-Accept es interesante. El servidor deve derivarla a partir de la cabecera Sec-WebSocket-Key enviada anteriormente por el cliente. Para lograr esto se deben concatenar la cabecera del cliente Sec-WebSocket-Key y el string "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" (es un "magic string"), calcular el hash SHA-1 del resultado y devolver el string codificado en base64 de este hash.

Así, si la cabecera Sec-WebSocket-Key era "dGhlIHNhbXBsZSBub25jZQ==", la correspondiente respuesta Sec-WebSocket-Accept será "s3pPLMBiTxaQ9kYGzzhZRbK+xOo=". Una vez que el servidor envía estas cabeceras, el "handshake" se considera completo y puedes comenzar a intercambiar datos.

Llevando registro de los clientes

Esto no está directamente relacionado con el protocolo WebSocket, pero no está de más mencionarlo: tu servidor debe llevar el registro de los sockets de los clientes, de manera de no realizar handshakes constantemente con los clientes que ya han completado este proceso. La misma dirección IP cliente puede intentar conectarse múltiples veces (pero el servidor puede denegar la conexión si se intentan demasiadas conexiones con el objetivo de evitar ataques ataques DoS).

Step 2: Exchanging Data Frames

Either the client or the server can choose to send a message at any time — that's the magic of WebSockets. However, extracting information from these so-called "frames" of data is a not-so-magical experience. Although all frames follow the same specific format, data going from the client to the server is masked using XOR encryption (with a 32-bit key). Section 5 of the specification describes this in detail.

Format

Each data frame (from the client to the server or vice-versa) follows this same format:

 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|R|R|R| opcode|M| Payload len |    Extended payload length    |
|I|S|S|S|  (4)  |A|     (7)     |             (16/64)           |
|N|V|V|V|       |S|             |   (if payload len==126/127)   |
| |1|2|3|       |K|             |                               |
+-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - +
|     Extended payload length continued, if payload len == 127  |
+ - - - - - - - - - - - - - - - +-------------------------------+
|                               |Masking-key, if MASK set to 1  |
+-------------------------------+-------------------------------+
| Masking-key (continued)       |          Payload Data         |
+-------------------------------- - - - - - - - - - - - - - - - +
:                     Payload Data continued ...                :
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
|                     Payload Data continued ...                |
+---------------------------------------------------------------+

RSV1-3 can be ignored, they are for extensions.

The MASK bit simply tells whether the message is encoded. Messages from the client must be masked, so your server should expect this to be 1. (In fact, section 5.1 of the spec says that your server must disconnect from a client if that client sends an unmasked message.) When sending a frame back to the client, do not mask it and do not set the mask bit. We'll explain masking later. Note: You have to mask messages even when using a secure socket.

The opcode field defines how to interpret the payload data: 0x0 for continuation, 0x1 for text (which is always encoded in UTF-8), 0x2 for binary, and other so-called "control codes" that will be discussed later. In this version of WebSockets, 0x3 to 0x7 and 0xB to 0xF have no meaning.

The FIN bit tells whether this is the last message in a series. If it's 0, then the server will keep listening for more parts of the message; otherwise, the server should consider the message delivered. More on this later.

Decoding Payload Length

To read the payload data, you must know when to stop reading. That's why the payload length is important to know. Unfortunately, this is somewhat complicated. To read it, follow these steps:

1. Read bits 9-15 (inclusive) and interpret that as an unsigned integer. If it's 125 or less, then that's the length; you're done. If it's 126, go to step 2. If it's 127, go to step 3.
2. Read the next 16 bits and interpret those as an unsigned integer. You're done.
3. Read the next 64 bits and interpret those as an unsigned integer. You're done.

Reading and Unmasking the Data

If the MASK bit was set (and it should be, for client-to-server messages), read the next 4 octets (32 bits); this is the masking key. Once the payload length and masking key is decoded, you can go ahead and read that number of bytes from the socket. Let's call the data ENCODED, and the key MASK. To get DECODED, loop through the octets (bytes a.k.a. characters for text data) of ENCODED and XOR the octet with the (i modulo 4)th octet of MASK. In pseudo-code (that happens to be valid JavaScript):

var DECODED = "";
for (var i = 0; i < ENCODED.length; i++) {
    DECODED[i] = ENCODED[i] ^ MASK[i % 4];
}

Now you can figure out what DECODED means depending on your application.

Message Fragmentation

The FIN and opcode fields work together to send a message split up into separate frames.  This is called message fragmentation. Fragmentation is only available on opcodes 0x0 to 0x2.

Recall that the opcode tells what a frame is meant to do. If it's 0x1, the payload is text. If it's 0x2, the payload is binary data. However, if it's 0x0, the frame is a continuation frame. This means the server should concatenate the frame's payload to the last frame it received from that client. Here is a rough sketch, in which a server reacts to a client sending text messages. The first message is sent in a single frame, while the second message is sent across three frames. FIN and opcode details are shown only for the client:

Client: FIN=1, opcode=0x1, msg="hello"
Server: (process complete message immediately) Hi.
Client: FIN=0, opcode=0x1, msg="and a"
Server: (listening, new message containing text started)
Client: FIN=0, opcode=0x0, msg="happy new"
Server: (listening, payload concatenated to previous message)
Client: FIN=1, opcode=0x0, msg="year!"
Server: (process complete message) Happy new year to you too!

Notice the first frame contains an entire message (has FIN=1 and opcode!=0x0), so the server can process or respond as it sees fit. The second frame sent by the client has a text payload (opcode=0x1), but the entire message has not arrived yet (FIN=0). All remaining parts of that message are sent with continuation frames (opcode=0x0), and the final frame of the message is marked by FIN=1. Section 5.4 of the spec describes message fragmentation.

Pings and Pongs: The Heartbeat of WebSockets

At any point after the handshake, either the client or the server can choose to send a ping to the other party. When the ping is received, the recipient must send back a pong as soon as possible. You can use this to make sure that the client is still connected, for example.

A ping or pong is just a regular frame, but it's a control frame. Pings have an opcode of 0x9, and pongs have an opcode of 0xA. When you get a ping, send back a pong with the exact same Payload Data as the ping (for pings and pongs, the max payload length is 125). You might also get a pong without ever sending a ping; ignore this if it happens.

Step 4: Closing the connection

To close a connection either the client or server can send a control frame with data containing a specified control sequence to begin the closing handshake (detailed in Section 5.5.1). Upon receiving such a frame, the other peer sends a Close frame in response. The first peer then closes the connection. Any further data received after closing of connection is then discarded. 

Miscellaneous

WebSocket extensions and subprotocols are negotiated via headers during the handshake. Sometimes extensions and subprotocols seem too similar to be different things, but there is a clear distinction. Extensions control the WebSocket frame and modify the payload, while subprotocols structure the WebSocket payload and never modify anything. Extensions are optional and generalized (like compression); subprotocols are mandatory and localized (like ones for chat and for MMORPG games).

Extensions

Think of an extension as compressing a file before e-mailing it to someone. Whatever you do, you're sending the same data in different forms. The recipient will eventually be able to get the same data as your local copy, but it is sent differently. That's what an extension does. WebSockets defines a protocol and a simple way to send data, but an extension such as compression could allow sending the same data but in a shorter format.

TODO

Subprotocols

Think of a subprotocol as a custom XML schema or doctype declaration. You're still using XML and its syntax, but you're additionally restricted by a structure you agreed on. WebSocket subprotocols are just like that. They do not introduce anything fancy, they just establish structure. Like a doctype or schema, both parties must agree on the subprotocol; unlike a doctype or schema, the subprotocol is implemented on the server and cannot be externally refered to by the client.

A client has to ask for a specific subprotocol. To do so, it will send something like this as part of the original handshake:

GET /chat HTTP/1.1
...
Sec-WebSocket-Protocol: soap, wamp

or, equivalently:

...
Sec-WebSocket-Protocol: soap
Sec-WebSocket-Protocol: wamp

Now the server must pick one of the protocols that the client suggested and it supports. If there are more than one, send the first one the client sent. Imagine our server can use both soap and wamp. Then, in the response handshake, it'll send:

Sec-WebSocket-Protocol: soap

If you want your server to obey certain subprotocols, then naturally you'll need extra code on the server. Let's imagine we're using a subprotocol json. In this subprotocol, all data is passed as JSON. If the client solicits this protocol and the server wants to use it, the server will need to have a JSON parser. Practically speaking, this will be part of a library, but the server will need to pass the data around.

Related

• Tutorial: Websocket server in C#
• Writing WebSocket client applications
• Tutorial: Websocket server in VB.NET