332 lines
13 KiB
Markdown
332 lines
13 KiB
Markdown
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# websocket-extensions [](http://travis-ci.org/faye/websocket-extensions-node)
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A minimal framework that supports the implementation of WebSocket extensions in
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a way that's decoupled from the main protocol. This library aims to allow a
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WebSocket extension to be written and used with any protocol library, by
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defining abstract representations of frames and messages that allow modules to
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co-operate.
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`websocket-extensions` provides a container for registering extension plugins,
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and provides all the functions required to negotiate which extensions to use
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during a session via the `Sec-WebSocket-Extensions` header. By implementing the
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APIs defined in this document, an extension may be used by any WebSocket library
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based on this framework.
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## Installation
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```
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$ npm install websocket-extensions
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```
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## Usage
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There are two main audiences for this library: authors implementing the
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WebSocket protocol, and authors implementing extensions. End users of a
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WebSocket library or an extension should be able to use any extension by passing
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it as an argument to their chosen protocol library, without needing to know how
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either of them work, or how the `websocket-extensions` framework operates.
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The library is designed with the aim that any protocol implementation and any
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extension can be used together, so long as they support the same abstract
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representation of frames and messages.
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### Data types
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The APIs provided by the framework rely on two data types; extensions will
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expect to be given data and to be able to return data in these formats:
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#### *Frame*
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*Frame* is a structure representing a single WebSocket frame of any type. Frames
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are simple objects that must have at least the following properties, which
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represent the data encoded in the frame:
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| property | description |
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| ------------ | ------------------------------------------------------------------ |
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| `final` | `true` if the `FIN` bit is set, `false` otherwise |
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| `rsv1` | `true` if the `RSV1` bit is set, `false` otherwise |
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| `rsv2` | `true` if the `RSV2` bit is set, `false` otherwise |
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| `rsv3` | `true` if the `RSV3` bit is set, `false` otherwise |
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| `opcode` | the numeric opcode (`0`, `1`, `2`, `8`, `9`, or `10`) of the frame |
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| `masked` | `true` if the `MASK` bit is set, `false` otherwise |
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| `maskingKey` | a 4-byte `Buffer` if `masked` is `true`, otherwise `null` |
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| `payload` | a `Buffer` containing the (unmasked) application data |
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#### *Message*
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A *Message* represents a complete application message, which can be formed from
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text, binary and continuation frames. It has the following properties:
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| property | description |
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| -------- | ----------------------------------------------------------------- |
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| `rsv1` | `true` if the first frame of the message has the `RSV1` bit set |
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| `rsv2` | `true` if the first frame of the message has the `RSV2` bit set |
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| `rsv3` | `true` if the first frame of the message has the `RSV3` bit set |
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| `opcode` | the numeric opcode (`1` or `2`) of the first frame of the message |
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| `data` | the concatenation of all the frame payloads in the message |
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### For driver authors
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A driver author is someone implementing the WebSocket protocol proper, and who
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wishes end users to be able to use WebSocket extensions with their library.
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At the start of a WebSocket session, on both the client and the server side,
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they should begin by creating an extension container and adding whichever
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extensions they want to use.
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```js
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var Extensions = require('websocket-extensions'),
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deflate = require('permessage-deflate');
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var exts = new Extensions();
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exts.add(deflate);
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```
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In the following examples, `exts` refers to this `Extensions` instance.
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#### Client sessions
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Clients will use the methods `generateOffer()` and `activate(header)`.
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As part of the handshake process, the client must send a
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`Sec-WebSocket-Extensions` header to advertise that it supports the registered
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extensions. This header should be generated using:
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```js
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request.headers['sec-websocket-extensions'] = exts.generateOffer();
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```
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This returns a string, for example `"permessage-deflate;
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client_max_window_bits"`, that represents all the extensions the client is
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offering to use, and their parameters. This string may contain multiple offers
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for the same extension.
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When the client receives the handshake response from the server, it should pass
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the incoming `Sec-WebSocket-Extensions` header in to `exts` to activate the
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extensions the server has accepted:
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```js
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exts.activate(response.headers['sec-websocket-extensions']);
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```
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If the server has sent any extension responses that the client does not
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recognize, or are in conflict with one another for use of RSV bits, or that use
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invalid parameters for the named extensions, then `exts.activate()` will
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`throw`. In this event, the client driver should fail the connection with
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closing code `1010`.
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#### Server sessions
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Servers will use the method `generateResponse(header)`.
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A server session needs to generate a `Sec-WebSocket-Extensions` header to send
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in its handshake response:
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```js
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var clientOffer = request.headers['sec-websocket-extensions'],
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extResponse = exts.generateResponse(clientOffer);
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response.headers['sec-websocket-extensions'] = extResponse;
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```
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Calling `exts.generateResponse(header)` activates those extensions the client
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has asked to use, if they are registered, asks each extension for a set of
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response parameters, and returns a string containing the response parameters for
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all accepted extensions.
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#### In both directions
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Both clients and servers will use the methods `validFrameRsv(frame)`,
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`processIncomingMessage(message)` and `processOutgoingMessage(message)`.
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The WebSocket protocol requires that frames do not have any of the `RSV` bits
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set unless there is an extension in use that allows otherwise. When processing
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an incoming frame, sessions should pass a *Frame* object to:
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```js
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exts.validFrameRsv(frame)
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```
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If this method returns `false`, the session should fail the WebSocket connection
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with closing code `1002`.
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To pass incoming messages through the extension stack, a session should
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construct a *Message* object according to the above datatype definitions, and
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call:
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```js
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exts.processIncomingMessage(message, function(error, msg) {
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// hand the message off to the application
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});
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```
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If any extensions fail to process the message, then the callback will yield an
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error and the session should fail the WebSocket connection with closing code
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`1010`. If `error` is `null`, then `msg` should be passed on to the application.
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To pass outgoing messages through the extension stack, a session should
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construct a *Message* as before, and call:
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```js
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exts.processOutgoingMessage(message, function(error, msg) {
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// write message to the transport
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});
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```
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If any extensions fail to process the message, then the callback will yield an
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error and the session should fail the WebSocket connection with closing code
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`1010`. If `error` is `null`, then `message` should be converted into frames
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(with the message's `rsv1`, `rsv2`, `rsv3` and `opcode` set on the first frame)
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and written to the transport.
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At the end of the WebSocket session (either when the protocol is explicitly
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ended or the transport connection disconnects), the driver should call:
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```js
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exts.close(function() {})
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```
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The callback is invoked when all extensions have finished processing any
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messages in the pipeline and it's safe to close the socket.
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### For extension authors
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An extension author is someone implementing an extension that transforms
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WebSocket messages passing between the client and server. They would like to
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implement their extension once and have it work with any protocol library.
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Extension authors will not install `websocket-extensions` or call it directly.
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Instead, they should implement the following API to allow their extension to
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plug into the `websocket-extensions` framework.
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An `Extension` is any object that has the following properties:
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| property | description |
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| -------- | ---------------------------------------------------------------------------- |
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| `name` | a string containing the name of the extension as used in negotiation headers |
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| `type` | a string, must be `"permessage"` |
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| `rsv1` | either `true` if the extension uses the RSV1 bit, `false` otherwise |
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| `rsv2` | either `true` if the extension uses the RSV2 bit, `false` otherwise |
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| `rsv3` | either `true` if the extension uses the RSV3 bit, `false` otherwise |
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It must also implement the following methods:
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```js
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ext.createClientSession()
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```
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This returns a *ClientSession*, whose interface is defined below.
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```js
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ext.createServerSession(offers)
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```
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This takes an array of offer params and returns a *ServerSession*, whose
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interface is defined below. For example, if the client handshake contains the
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offer header:
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```
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Sec-WebSocket-Extensions: permessage-deflate; server_no_context_takeover; server_max_window_bits=8, \
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permessage-deflate; server_max_window_bits=15
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```
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then the `permessage-deflate` extension will receive the call:
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```js
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ext.createServerSession([
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{ server_no_context_takeover: true, server_max_window_bits: 8 },
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{ server_max_window_bits: 15 }
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]);
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```
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The extension must decide which set of parameters it wants to accept, if any,
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and return a *ServerSession* if it wants to accept the parameters and `null`
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otherwise.
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#### *ClientSession*
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A *ClientSession* is the type returned by `ext.createClientSession()`. It must
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implement the following methods, as well as the *Session* API listed below.
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```js
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clientSession.generateOffer()
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// e.g. -> [
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// { server_no_context_takeover: true, server_max_window_bits: 8 },
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// { server_max_window_bits: 15 }
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// ]
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```
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This must return a set of parameters to include in the client's
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`Sec-WebSocket-Extensions` offer header. If the session wants to offer multiple
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configurations, it can return an array of sets of parameters as shown above.
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```js
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clientSession.activate(params) // -> true
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```
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This must take a single set of parameters from the server's handshake response
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and use them to configure the client session. If the client accepts the given
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parameters, then this method must return `true`. If it returns any other value,
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the framework will interpret this as the client rejecting the response, and will
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`throw`.
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#### *ServerSession*
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A *ServerSession* is the type returned by `ext.createServerSession(offers)`. It
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must implement the following methods, as well as the *Session* API listed below.
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```js
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serverSession.generateResponse()
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// e.g. -> { server_max_window_bits: 8 }
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```
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This returns the set of parameters the server session wants to send in its
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`Sec-WebSocket-Extensions` response header. Only one set of parameters is
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returned to the client per extension. Server sessions that would confict on
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their use of RSV bits are not activated.
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#### *Session*
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The *Session* API must be implemented by both client and server sessions. It
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contains two methods, `processIncomingMessage(message)` and
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`processOutgoingMessage(message)`.
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```js
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session.processIncomingMessage(message, function(error, msg) { ... })
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```
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The session must implement this method to take an incoming *Message* as defined
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above, transform it in any way it needs, then return it via the callback. If
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there is an error processing the message, this method should yield an error as
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the first argument.
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```js
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session.processOutgoingMessage(message, function(error, msg) { ... })
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```
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The session must implement this method to take an outgoing *Message* as defined
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above, transform it in any way it needs, then return it via the callback. If
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there is an error processing the message, this method should yield an error as
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the first argument.
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Note that both `processIncomingMessage()` and `processOutgoingMessage()` can
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perform their logic asynchronously, are allowed to process multiple messages
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concurrently, and are not required to complete working on messages in the same
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order the messages arrive. `websocket-extensions` will reorder messages as your
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extension emits them and will make sure every extension is given messages in the
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order they arrive from the driver. This allows extensions to maintain state that
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depends on the messages' wire order, for example keeping a DEFLATE compression
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context between messages.
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```js
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session.close()
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```
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The framework will call this method when the WebSocket session ends, allowing
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the session to release any resources it's using.
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## Examples
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- Consumer: [websocket-driver](https://github.com/faye/websocket-driver-node)
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- Provider: [permessage-deflate](https://github.com/faye/permessage-deflate-node)
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