Per-message DEFLATE compression extension for WebSocket connections
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lib Use windowBits:9 for deflate and inflate streams if the session is co… Sep 8, 2017
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README.md

permessage-deflate Build status

Implements the permessage-deflate WebSocket protocol extension as a plugin for websocket-extensions.

Installation

$ npm install permessage-deflate

Usage

Add the plugin to your extensions:

var Extensions = require('websocket-extensions'),
    deflate    = require('permessage-deflate');

var exts = new Extensions();
exts.add(deflate);

The extension can be configured, for example:

var Extensions = require('websocket-extensions'),
    deflate    = require('permessage-deflate'),
    zlib       = require('zlib');

deflate = deflate.configure({
  level: zlib.Z_BEST_COMPRESSION,
  maxWindowBits: 13
});

var exts = new Extensions();
exts.add(deflate);

The set of available options can be split into two sets: those that control the session's compressor for outgoing messages and do not need to be communicated to the peer, and those that are negotiated as part of the protocol. The settings only affecting the compressor are described fully in the zlib documentation:

  • level: sets the compression level, can be an integer from 0 to 9, or one of the constants zlib.Z_NO_COMPRESSION, zlib.Z_BEST_SPEED, zlib.Z_BEST_COMPRESSION, or zlib.Z_DEFAULT_COMPRESSION
  • memLevel: sets how much memory the compressor allocates, can be an integer from 1 to 9, or one of the constants zlib.Z_MIN_MEMLEVEL, zlib.Z_MAX_MEMLEVEL, or zlib.Z_DEFAULT_MEMLEVEL
  • strategy: can be one of the constants zlib.Z_FILTERED, zlib.Z_HUFFMAN_ONLY, zlib.Z_RLE, zlib.Z_FIXED, or zlib.Z_DEFAULT_STRATEGY

The other options relate to settings that are negotiated via the protocol and can be used to set the local session's behaviour and control that of the peer:

  • noContextTakeover: if true, stops the session reusing a deflate context between messages
  • requestNoContextTakeover: if true, makes the session tell the other peer not to reuse a deflate context between messages
  • maxWindowBits: an integer from 8 to 15 inclusive that sets the maximum size of the session's sliding window; a lower window size will be used if requested by the peer
  • requestMaxWindowBits: an integer from 8 to 15 inclusive to ask the other peer to use to set its maximum sliding window size, if supported