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Versions: (draft-tyoshino-hybi-websocket-perframe-deflate) 00 01 02 03 04 draft-tyoshino-hybi-permessage-compression

HyBi Working Group                                            T. Yoshino
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                           April 2, 2012
Expires: October 4, 2012


                    WebSocket Per-frame Compression
           draft-ietf-hybi-websocket-perframe-compression-00

Abstract

   This specification defines a general scheme to add per-frame
   compression functionality to the WebSocket Protocol using its
   extension mechanism, and one specific compression extension using
   DEFLATE.  In this scheme, the "Application data" part of WebSocket
   data frames is compressed using specified compression algorithm, and
   one reserved bit in the WebSocket frame header is allocated to
   control application of compression for each frame.

   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 4, 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.  General Per-frame Compression Scheme . . . . . . . . . . . . .  5
     3.1.  Sending  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Receiving  . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Per-frame DEFLATE Extension  . . . . . . . . . . . . . . . . .  6
     4.1.  Extension Negotiation  . . . . . . . . . . . . . . . . . .  6
     4.2.  Application Data Transformation  . . . . . . . . . . . . .  7
       4.2.1.  Compression  . . . . . . . . . . . . . . . . . . . . .  7
       4.2.2.  Decompression  . . . . . . . . . . . . . . . . . . . .  7
       4.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Intermediaries . . . . . . . . . . . . . . . . . . . . . .  8
     4.4.  Implementation Note  . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Registration of the "deflate-frame" WebSocket
           Extension Name . . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  Registration of the "Per-frame compressed" WebSocket
           Framing Header Bit . . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14



















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1.  Introduction

   _This section is non-normative._

   As well as other protocols, the WebSocket Protocol [RFC6455] can
   benefit from compression technology.  This specification defines a
   scheme to apply compression algorithms to octets exchanged over the
   WebSocket Protocol using its extension framework, and then defines
   one specific compression extension using DEFLATE [RFC1951].

   The per-frame compression scheme applies the specified compression
   algorithm to the octets in the "Application data" part of data
   frames.  It also specifies the use of the RSV1 bit of the WebSocket
   frame header to indicate whether any compression is applied to the
   frame or not, so that we can choose to skip frames with
   incompressible contents without applying extra compression.  By
   specifying extension negotiation and how to transform octets in
   "Application data", we can define per-frame compression extensions
   for various compression algorithms based on this scheme.

   We also introduce one specific extension in this specification by
   applying DEFLATE to the scheme.  It is called "Per-frame DEFLATE
   extension".  DEFLATE algorithm is widely available as library on
   various platforms.  Overhead it adds for each chunk is small.  So,
   it's chosen for the first example of the compression extension for
   the WebSocket Protocol.  To align the end of compressed data to octet
   boundary, the extension 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.

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

       Sec-WebSocket-Extensions: deflate-frame

   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.  General Per-frame Compression Scheme

   This section describes a general scheme to apply a compression
   algorithm to the contents of WebSocket frames.

   This scheme allocates one bit field called "Per-frame compressed" at
   the RSV1 bit.  This bit indicates whether any kind of per-frame
   compression is applied to the frame or not.  Because of this,
   compression extensions based on this scheme are incompatible with
   each other.

   This scheme operates only on data frames, and only on the
   "Application data" therein (it does not affect the "Extension data"
   portion of the "Payload data").

3.1.  Sending

   To send a frame with the compression applied, an endpoint MUST use
   the following algorithm.

   1.  Apply the compression to the "Application data" portion of the
       frame.

   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 one of these resulting octets.  "Per-frame
       compressed" bit MUST be set to 1.

   To send a frame with the compression not applied, an endpoint MUST
   set "Per-frame compressed" bit of the frame to 0 and send the
   "Application data" portion as-is without applying any compression.

3.2.  Receiving

   To receive a frame with "Per-frame compressed" bit set to 1, an
   endpoint MUST decompress the octets in the "Application data" portion
   based on the compression algorithm.

   An endpoint MUST receive a frame with "Per-frame compressed" bit set
   to 0 as-is without any compression processing.










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4.  Per-frame DEFLATE Extension

   This section defines one specific compression extension by applying
   DEFLATE to the scheme described in Section 3.

4.1.  Extension Negotiation

   The registered extension token for this extension is "deflate-frame".

   To request use of per-frame DEFLATE extension, a client MUST include
   the "deflate-frame" extension token in the "Sec-WebSocket-Extensions"
   header in its opening handshake.

   To accept use of per-frame DEFLATE extension requested by the client,
   a server MUST include the "deflate-frame" extension token in the
   "Sec-WebSocket-Extensions" header in its opening handshake.

   An endpoint MAY attach one or more extension parameters as defined
   below to the extension token.

   Maximum LZ77 sliding window size
      An endpoint MAY attach "max_window_bits" extension 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 this value to
      build frames.

   Disallow compression context takeover
      An endpoint MAY attach "no_context_takeover" extension 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 use an empty LZ77
      sliding window to build every frame.

   A server MUST ignore any unknown extension parameter attached to
   "deflate-frame" extension token in the client's opening handshake.

   A client MUST _Fail the WebSocket Connection_ if any unknown
   extension parameter is attached to "deflate-frame" extension token in
   the server's opening handshake.

   Once per-frame DEFLATE extension is accepted, both endpoints MUST use
   the algorithm described in Section 4.2 to exchange frames.






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4.2.  Application Data Transformation

   This extension transforms the "Application data" portion by using the
   scheme described in Section 3 with DEFLATE as follows.

4.2.1.  Compression

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

   1.  Apply DEFLATE [RFC1951] to all the octets.  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.

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

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

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

   Unless it's prohibited by the other peer by the "no_context_takeover"
   extension parameter on opening handshake, an endpoint MAY take over
   the LZ77 sliding window used to build the last frame to send with
   DEFLATE applied.

4.2.2.  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" extension parameter on
   opening handshake, the endpoint MUST use 32,768 byte LZ77 sliding
   window to decode received frames.

   If an endpoint sent the "max_window_bits" extension parameter on
   opening handshake, it MAY reduce the size of LZ77 sliding window to
   decode received frames down to the "max_window_bits"-th power of 2



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   byte.

   Unless the endpoint sent the "no_context_takeover" extension
   parameter on opening handshake, an endpoint MUST take over the LZ77
   sliding window used to decode the last received frame with DEFLATE
   applied.

4.2.3.  Examples

   _This section is non-normative._

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

   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

4.3.  Intermediaries

   Intermediaries MAY decompress and/or compress frames when they
   forward them under constraints negotiated on opening handshake as
   described in Section 4.2.

4.4.  Implementation Note

   _This section is non-normative._

   On common software development platforms, the operation of aligning
   compressed data to octet boundary using an empty block with no
   compression is available as library.  For example, Zlib [Zlib] does



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   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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5.  Security Considerations

   There's no security concern for now.
















































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6.  IANA Considerations

6.1.  Registration of the "deflate-frame" WebSocket Extension Name

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

   Extension Identifier
      deflate-frame

   Extension Common Name
      WebSocket Per-frame DEFLATE

   Extension Definition
      Section 4.1 and Section 4.2 of this document.

   Known Incompatible Extensions
      None

   The "deflate-frame" token is used in the "Sec-WebSocket-Extensions"
   header in the WebSocket opening handshake to negotiate use of per-
   frame DEFLATE compression extension.

6.2.  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
      Compression is applied to the frame or not.

   Reference
      Section 3 of this document.

   The "Per-frame compressed" framing header bit is used to indicate
   whether any negotiated per-frame compression extension applied
   compression to the "Application data" portion of the frame or not.







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7.  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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8.  References

8.1.  Normative References

   [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.

8.2.  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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