WebSocket Per-frame Compression
draft-ietf-hybi-websocket-perframe-compression-01

HyBi Working Group                                            T. Yoshino
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                             May 2, 2012
Expires: November 3, 2012

                    WebSocket Per-frame Compression
           draft-ietf-hybi-websocket-perframe-compression-01

Abstract

   This specification defines a WebSocket extension that adds per-frame
   compression functionality to the WebSocket Protocol.  It compresses
   the "Application data" portion of WebSocket data frames using
   specified compression algorithm.  One reserved bit RSV1 in the
   WebSocket frame header is allocated to control application of
   compression for each frame.  This specification provides one
   compression method available for the extension using DEFLATE.

   Please send feedback to the hybi@ietf.org mailing list.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 3, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conformance Requirements . . . . . . . . . . . . . . . . . . .  4
   3.  Extension Negotiation  . . . . . . . . . . . . . . . . . . . .  5
   4.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Sending  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Receiving  . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  DEFLATE method . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Method Parameters  . . . . . . . . . . . . . . . . . . . .  7
     5.2.  Application Data Transformation  . . . . . . . . . . . . .  7
       5.2.1.  Compression  . . . . . . . . . . . . . . . . . . . . .  7
       5.2.2.  Decompression  . . . . . . . . . . . . . . . . . . . .  8
       5.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . .  8
     5.3.  Intermediaries . . . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Implementation Note  . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Registration of the "perframe-compress" WebSocket
           Extension Name . . . . . . . . . . . . . . . . . . . . . . 11
     7.2.  Registration of the "Per-frame Compressed" WebSocket
           Framing Header Bit . . . . . . . . . . . . . . . . . . . . 11
     7.3.  WebSocket Per-frame Compression Method Name Registry . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Introduction

   _This section is non-normative._

   As well as other communication protocols, the WebSocket Protocol
   [RFC6455] can benefit from compression technology.  This
   specification defines a WebSocket extension that applies a
   compression algorithm to octets exchanged over the WebSocket Protocol
   using its extension framework.  This extension negotiates what
   compression method to use on opening handshake, and then compresses
   to the octets in the "Application data" portion of data frames using
   the method.  We can apply this extension to various compression
   algorithms by specifying how to negotiate parameters and transform
   "Application data".  A client may offer multiple compression methods
   on opening handshake, and then the server chooses one from them.
   This extension uses the RSV1 bit of the WebSocket frame header to
   indicate whether the frame is compressed or not, so that we can
   choose to skip frames with incompressible contents without applying
   extra compression.

   This specification provides one specific compression method for this
   extension "deflate" which is based on DEFLATE [RFC1951].  We chose
   DEFLATE since it's widely available as library on various platforms
   and the overhead it adds for each chunk is small.  To align the end
   of compressed data to octet boundary, this method uses the algorithm
   described in the Section 2.1 of the PPP Deflate Protocol [RFC1979].
   Endpoints can take over the LZ77 sliding window [LZ77] used to build
   previous frames to get better compression ratio.  For resource-
   limited devices, method parameters to limit the usage of memory for
   compression context are provided.

   The simplest "Sec-WebSocket-Extensions" header in the client's
   opening handshake to request DEFLATE based per-frame compression is
   the following:

       Sec-WebSocket-Extensions: perframe-compress: method=deflate

   The simplest header from the server to accept this extension is the
   same.

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2.  Conformance Requirements

   Everything in this specification except for sections explicitly
   marked non-normative is normative.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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3.  Extension Negotiation

   The registered extension token for this extension is
   "perframe-compress".

   To request use of the Per-frame Compression Extension, a client MUST
   include an element with the "perframe-compress" extension tokenas its
   extension identifier in the "Sec-WebSocket-Extensions" header in its
   opening handshake.  The element MUST contain exactly one extension
   parameter named "method".  The value of the "method" extension
   parameter is an ordered list of compression method descriptions.
   Each compression method description has a method name and optional
   method parameters.  The grammar of the list is
   "requested-method-list" defined in the following ABNFs.

       requested-method-list = 1#method-desc
       method-desc = method-name *(";" method-param)
       method-name = token
       method-param = token ["=" (token | quoted-string)]

   To accept use of the Per-frame Compression Extension, a server MUST
   include an element with the "perframe-comrpess" extension token as
   its extension identifier in the "Sec-WebSocket-Extensions" header in
   its opening handshake.  The element MUST contain exactly one
   extension parameter named "method".  The value of the "method"
   extension parameter MUST be a compression method description.  The
   compression method description MUST be a valid response for any of
   the methods listed by the client in its opening handshake.  Its
   grammar is "accepted-method-list" defined in the following ABNF.

       accepted-method = method-desc

   The value of the "method" parameter MUST be quoted by using
   "quoted-string" syntax if it doesn't conform to token syntax.

   Once the extension is accepted, both endpoints MUST use the algorithm
   described in Section 4 to exchange frames.

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4.  Framing

   This section describes how to apply the negotiated compression method
   to the contents of WebSocket frames.

   This extension allocates one bit field at the RSV1 bit of the
   WebSocket header and names it "Per-frame Compressed" bit.  Any
   extension specifies use of RSV1 bit is incompatible with this
   extension.  This bit indicates whether the compression is applied to
   the frame or not.  Frames with the "Per-frame Compressed" bit set are
   called "per-frame compressed frame".  They have compressed data in
   its "Application data" portion.  Frames with the bit unset are called
   "per-frame uncompressed frame".  They have uncompressed data in its
   "Application data" portion.

   This extension operates only on data frames.  This extension doesn't
   modify the "Extension data" portion.

4.1.  Sending

   To send a frame as a per-frame compressed frame, an endpoint MUST use
   the following algorithm.

   1.  Compress the octets in "Application data" portion of the frame
       using the compression method.

   2.  Build a frame by putting the resulting octets in the "Application
       data" portion instead of the original octets.  The payload length
       field of the frame MUST be the sum of the size of the "Extension
       data" portion and the size of the resulting octets.

   3.  Set the "Per-frame Compressed" bit of the frame to 1.

   To send a frame as a per-frame uncompressed frame, an endpoint MUST
   set "Per-frame Compressed Bit" of the frame to 0.  "Application data"
   portion MUST be sent as-is without applying the compression method.

4.2.  Receiving

   To receive a per-frame compressed frame, an endpoint MUST decompress
   the octets in the "Application data" portion.

   An endpoint MUST receive a per-frame uncompressed frame as-is without
   decompression.

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5.  DEFLATE method

   This section defines a method named "deflate" for this extension that
   compresses "Application data" using DEFLATE [RFC1951] and byte
   boundary alignment method introduced in [RFC1979].

5.1.  Method Parameters

   An endpoint MAY include one or more method parameters in the method
   description as defined below.

   Maximum LZ77 sliding window size
      An endpoint MAY attach "max_window_bits" method parameter to limit
      the LZ77 sliding window size that the other peer uses to build
      frames.  This parameter MUST have an integer value in the range
      between 8 to 15 indicating the base-2 logarithm of the LZ77
      sliding window size.  An endpoint that received this parameter
      MUST NOT use LZ77 sliding window size greater than the size
      specified by this parameter to build frames.

   Disallow compression context takeover
      An endpoint MAY attach "no_context_takeover" method parameter to
      disallow the other peer to take over the LZ77 sliding window used
      to build previous frames.  This parameter has no value.  An
      endpoint that received this parameter MUST reset its LZ77 sliding
      window for sending to empty for each frame.

   A server MUST ignore any unknown method parameter in the "deflate"
   method description in the client's opening handshake.

   A client MUST _Fail the WebSocket Connection_ if there is any unknown
   method parameter in the "deflate" method description in the server's
   opening handshake.

5.2.  Application Data Transformation

5.2.1.  Compression

   An endpoint MUST use the following algorithm to compress the
   "Application data" portion.

   1.  Compress all the octets in the "Application data" portion using
       DEFLATE.  Multiple blocks MAY be used.  Any type of block MAY be
       used.  Both block with "BFINAL" set to 0 and 1 MAY be used.

   2.  If the resulting data does not end with an empty block with no
       compression ("BTYPE" set to 0), append an empty block with no
       compression to the tail.

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   3.  Remove 4 octets (that are 0x00 0x00 0xff 0xff) from the tail.

   An endpoint MUST NOT use an LZ77 sliding window greater than 32,768
   byte to build frames to send.

   If an endpoint received the "max_window_bits" method parameter, the
   endpoint MUST NOT use an LZ77 sliding window greater than the
   "max_window_bits"-th power of 2 byte to build frames to send.

   Unless an endpoint received the "no_context_takeover" method
   parameter, the endpoint MAY take over the LZ77 sliding window used to
   build the last per-frame compressed frame.

   If an endpoint received the "no_context_takeover" method parameter,
   the endpoint MUST reset its LZ77 sliding window for sending to empty
   for each frame.

5.2.2.  Decompression

   An endpoint MUST use the following algorithm to decompress the
   "Application data" portion.

   1.  Append 4 octets of 0x00 0x00 0xff 0xff to the tail.

   2.  Decompress the resulting octets using DEFLATE.

   Unless an endpoint sent the "max_window_bits" method parameter, the
   endpoint MUST use 32,768 byte LZ77 sliding window to decompress
   received frames.

   If an endpoint sent the "max_window_bits" method parameter, the
   endpoint MAY reduce the size of the LZ77 sliding window to decompress
   received frames down to the "max_window_bits"-th power of 2 byte.

   Unless an endpoint sent the "no_context_takeover" method parameter,
   the endpoint MUST take over the LZ77 sliding window used to parse the
   last per-frame compressed frame.

   If an endpoint sent the "no_context_takeover" method parameter, the
   endpoint MAY reset its LZ77 sliding window for receiving to empty for
   each frame.

5.2.3.  Examples

   _This section is non-normative._

   These are examples of resulting data after applying the algorithm
   above.

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   o  "Hello" in one compressed block

      *  0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00

      "Hello" in one compressed block in the next frame

      *  0xf2 0x00 0x11 0x00 0x00

   o  "Hello" in one block with no compression

      *  0x00 0x05 0x00 0xfa 0xff 0x48 0x65 0x6c 0x6c 0x6f 0x00

   o  "Hello" in one block with "BFINAL" set to 1

      *  0xf3 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00

   o  "He" and "llo" in separate blocks

      *  0xf2 0x48 0x05 0x00 0x00 0x00 0xff 0xff 0xca 0xc9 0xc9 0x07
         0x00

5.3.  Intermediaries

   When intermediaries forward frames, they MAY decompress and/or
   compress the frames under constraints negotiated on opening handshake
   of the connection(s).

5.4.  Implementation Note

   _This section is non-normative._

   On most of common software development platforms, the operation of
   aligning compressed data to byte boundary using an empty block with
   no compression is available as library.  For example, Zlib [Zlib]
   does this when "Z_SYNC_FLUSH" is passed to deflate function.

   To get sufficient compression ratio, LZ77 sliding window size of
   1,024 or more is recommended.

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6.  Security Considerations

   There's no security concern for now.

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7.  IANA Considerations

7.1.  Registration of the "perframe-compress" WebSocket Extension Name

   This section describes a WebSocket extension name registration in the
   WebSocket Extension Name Registry [RFC6455].

   Extension Identifier
      perframe-compress

   Extension Common Name
      WebSocket Per-frame Compression

   Extension Definition
      This document.

   Known Incompatible Extensions
      None

   The "perframe-compress" token is used in the
   "Sec-WebSocket-Extensions" header in the WebSocket opening handshake
   to negotiate use of the Per-frame Compression Extension.

7.2.  Registration of the "Per-frame Compressed" WebSocket Framing
      Header Bit

   This section describes a WebSocket framing header bit registration in
   the WebSocket Framing Header Bits Registry [RFC6455].

   Header Bit
      RSV1

   Common Name
      Per-frame Compressed

   Meaning
      The frame is compressed or not.

   Reference
      Section 4 of this document.

   The "Per-frame Compressed" framing header bit is used to indicate
   whether the "Application data" portion of the frame is compressed by
   the Per-frame Compression Extension or not.

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7.3.  WebSocket Per-frame Compression Method Name Registry

   This specification creates a new IANA registry for names of
   compression methods to be used with the WebSocket Per-frame
   Compression Extension in accordance with the principles set out in
   [RFC5226].

   As part of this registry, IANA maintains the following information:

   Method Identifier
      The identifier of the method, as will be used in the method
      description as defined Section 3 of this specification.  The value
      must conform to the method-name ABNF as defined in Section 3 of
      this specification.

   Method Common Name
      The name of the method, as the method is generally referred to.

   Method Definition
      A reference to the document in which the method being used with
      this extension is defined.

   WebSocket Per-frame Compression method names are to be subject to the
   "First Come First Served" IANA registration policy [RFC5226].

   IANA has added initial values to the registry as follows.

     +------------+-------------+---------------+
     | Identifier | Common Name | Definition    |
     +------------+-------------+---------------+
     | deflate    | DEFLATE     | This document |
     +------------+-------------+---------------+

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8.  Acknowledgements

   Special thanks to Patrick McManus who wrote up the initial
   specification of DEFLATE based compression extension for the
   WebSocket Protocol which I referred to write this specification.

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9.  References

9.1.  Normative References

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, December 2011.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [LZ77]     Ziv, J. and A. Lempel, "A Universal Algorithm for
              Sequential Data Compression", IEEE Transactions on
              Information Theory, Vol. 23, No. 3, pp. 337-343.

9.2.  Informative References

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996.

   [RFC1979]  Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.

   [Zlib]     Gailly, J. and M. Adler, "Zlib", <http://zlib.net/>.

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Author's Address

   Takeshi Yoshino
   Google, Inc.

   Email: tyoshino@google.com

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