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Versions: (draft-ietf-hybi-websocket-perframe-compression) 00 draft-ietf-hybi-permessage-compression

HyBi Working Group                                            T. Yoshino
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                            July 4, 2012
Expires: January 5, 2013


                   WebSocket Per-message Compression
             draft-tyoshino-hybi-permessage-compression-00

Abstract

   This specification defines a WebSocket extension that adds
   compression functionality to the WebSocket Protocol.  It compresses
   the payload of non-control WebSocket messages using specified
   compression algorithm.  One reserved bit RSV1 in the WebSocket frame
   header is allocated to control application of compression for each
   message.  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
   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 January 5, 2013.

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
     3.1.  Negotiation Example  . . . . . . . . . . . . . . . . . . .  6
   4.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Sending  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Receiving  . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  DEFLATE method . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Method Parameters  . . . . . . . . . . . . . . . . . . . .  8
     5.2.  Application Data Transformation  . . . . . . . . . . . . .  9
       5.2.1.  Compression  . . . . . . . . . . . . . . . . . . . . .  9
       5.2.2.  Decompression  . . . . . . . . . . . . . . . . . . . . 10
       5.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . 10
     5.3.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . 11
     5.4.  Implementation Notes . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     7.1.  Registration of the "permessage-compress" WebSocket
           Extension Name . . . . . . . . . . . . . . . . . . . . . . 13
     7.2.  Registration of the "Per-message Compressed" WebSocket
           Framing Header Bit . . . . . . . . . . . . . . . . . . . . 13
     7.3.  WebSocket Per-message Compression Method Name Registry . . 14
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
















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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
   the octets in non-control messages using the method.  We can apply
   this extension to various compression algorithms by specifying how to
   negotiate parameters and transform payload.  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 message is
   compressed or not, so that we can choose to skip messages with
   incompressible contents without applying extra compression.

   This specification provides one specific compression method "deflate"
   which is based on DEFLATE [RFC1951] for this extension.  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 messages 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-message compression is
   the following:

       Sec-WebSocket-Extensions: permessage-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
   "permessage-compress".

   To request use of the Per-message Compression Extension, a client
   MUST include an element with the "permessage-compress" 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 is a list of compression method descriptions,
   ordered by preference.  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)]

   The list MAY contain multiple method descriptions with the same
   method name.

   To accept use of the Per-message Compression Extension, a server MUST
   choose one compression method description to accept from ones listed
   by the client, and include an element with the "permessage-compress"
   extension token in the "Sec-WebSocket-Extensions" header in its
   opening handshake.  The chosen description is called "accepted
   request".  The element MUST contain exactly one extension parameter
   named "method".  The value of the "method" extension parameter MUST
   be a compression method description.  This description is called
   "method agreement".  The method name in the "method agreement" MUST
   be one of the accepted request.  The "method agreement" MUST conform
   the "accepted request".  Its grammar is "method-agreement" defined in
   the following ABNF.

       method-agreement = method-desc

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

   If a client doesn't support the method and its configuration
   specified by the "method agreement", the client MUST _Fail the
   WebSocket Connection_.  Otherwise, both endpoints MUST use the
   algorithm described in Section 4 to exchange messages.






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3.1.  Negotiation Example

   _This section is non-normative._

   These are "Sec-WebSocket-Extensions" header value examples that
   negotiate the Per-message Compression Extension.

   o  Request foo method.  Since foo matches token syntax, it doesn't
      need to be quoted.

          permessage-compress; method=foo

   o  Request foo method with a parameter x with 10 as its value.  Since
      the method parameter value contains a semicolon, it doesn't match
      token syntax.  Quotation is needed.

          permessage-compress; method="foo; x=10"

   o  Request foo method and bar method.  Since the method parameter
      value contains a comma, it doesn't match token syntax.  Quotation
      is needed.

          permessage-compress; method="foo, bar"

   o  Request foo method with parameter x with "Hello World" (quotation
      for clarification) as its value and bar method.  Since "Hello
      World" contains a space, it needs to be quoted.  Since quoted
      "Hello World" contains double quotations and a space, it needs to
      be quoted again.

          permessage-compress; method="foo; x=\"Hello World\", bar"




















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4.  Framing

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

   This extension allocates the RSV1 bit of the WebSocket header and
   names it the "Per-message Compressed" bit.  Any extension requiring
   the use of the RSV1 bit is incompatible with this extension.  This
   bit MAY be set only on the first fragment of a message.  This bit
   indicates whether the compression method is applied to the message or
   not.  Messages with the "Per-message Compressed" bit set (on its
   first fragment) are called "compressed messages".  They have
   compressed data in their payload.  Messages with the bit unset are
   called "uncompressed messages".  They have uncompressed data in their
   payload.

   This extension MUST NOT be used after any extension for which frame
   boundary needs to be preserved.  This extension MUST NOT be used
   after any extension that uses "Extension data" field or any of the
   reserved bits on the WebSocket header as per-frame attribute.

   This extension operates only on data frames.

4.1.  Sending

   To send a compressed message, an endpoint MUST use the following
   algorithm.

   1.  Compress the payload of the message using the compression method.

   2.  Build frame(s) for the message by putting the resulting octets
       instead of the original octets.

   3.  Set the "Per-message Compressed" bit of the first fragment to 1.

   To send an uncompressed message, an endpoint MUST set the
   "Per-message Compressed" bit of the first fragment of the message to
   0.  The payload of the message MUST be sent as-is without applying
   the compression method.

4.2.  Receiving

   To receive a compressed message, an endpoint MUST decompress its
   payload.

   An endpoint MUST receive an uncompressed message as-is without
   decompression.




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5.  DEFLATE method

   This section defines a method named "deflate" for this extension that
   compresses the payload of messages 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

      A client MAY attach the "s2c_max_window_bits" method parameter to
      limit the LZ77 sliding window size that the server uses to build
      messages.  If the "accepted request" has this method parameter,
      the server MUST NOT use LZ77 sliding window size greater than the
      size specified by this parameter to build messages.  If the
      "accepted request" has this method parameter, the server MUST
      attach this method parameter with the same value as one of the
      "accepted request".

      A server MAY attach the "c2s_max_window_bits" method parameter to
      limit the LZ77 sliding window size that the client uses to build
      messages.  A client that received this parameter MUST NOT use LZ77
      sliding window size greater than the size specified by this
      parameter to build messages.

      These parameters MUST have an integer value in the range between 8
      to 15 indicating the base-2 logarithm of the LZ77 sliding window
      size.

   Disallow compression context takeover

      A client MAY attach the "s2c_no_context_takeover" method parameter
      to disallow the server to take over the LZ77 sliding window used
      to build previous messages.  If the "accepted request" has this
      method parameter, the server MUST reset its LZ77 sliding window
      for sending to empty for each message.  If the "accepted request"
      has this method parameter, the server MUST attach this method
      parameter.

      A server MAY attach the "c2s_no_context_takeover" method parameter
      to disallow the client to take over the LZ77 sliding window used
      to build previous messages.  A client that received this parameter
      MUST reset its LZ77 sliding window for sending to empty for each
      message.




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      These parameters have no value.

   A server MUST ignore any method parameter other than
   "s2c_max_window_bits" and "s2c_no_context_takeover" in the received
   "deflate" method description.

   A client MUST _Fail the WebSocket Connection_ if there is any method
   parameter other than the "s2c_max_window_bits",
   "c2s_max_window_bits", "s2c_no_context_takeover" and
   "c2s_no_context_takeover" in the received "deflate" method
   description.  A client MUST _Fail the WebSocket Connection_ if it
   doesn't support the method and its configuration specified by the
   received "deflate" method description.

5.2.  Application Data Transformation

5.2.1.  Compression

   An endpoint MUST use the following algorithm to compress a message.

   1.  Compress all the octets of the payload of the message 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.

   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
   bytes to build messages to send.

   If the "method agreement" has the "s2c_max_window_bits" method
   parameter and its value is w, the server MUST NOT use an LZ77 sliding
   window greater than w-th power of 2 bytes to build messages to send.
   If the "method agreement" has the "c2s_max_window_bits" method
   parameter and its value is w, the client MUST NOT use an LZ77 sliding
   window greater than w-th power of 2 bytes to build messages to send.

   If the "method agreement" has the "s2c_no_context_takeover" method
   parameter, the server MUST reset its LZ77 sliding window for sending
   to empty for each message.  Otherwise, the server MAY take over the
   LZ77 sliding window used to build the last compressed message.  If
   the "method agreement" has the "c2s_no_context_takeover" method
   parameter, the client MUST reset its LZ77 sliding window for sending
   to empty for each message.  Otherwise, the client MAY take over the
   LZ77 sliding window used to build the last compressed message.



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5.2.2.  Decompression

   An endpoint MUST use the following algorithm to decompress a message.

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

   2.  Decompress the resulting octets using DEFLATE.

   If the "method agreement" has the "s2c_max_window_bits" method
   parameter and its value is w, the client MAY reduce the size of the
   LZ77 sliding window to decompress received messages down to the w-th
   power of 2 bytes.  Otherwise, the client MUST use a 32,768 byte LZ77
   sliding window to decompress received messages.  If the "method
   agreement" has the "c2s_max_window_bits" method parameter and its
   value is w, the server MAY reduce the size of the LZ77 sliding window
   to decompress received messages down to the w-th power of 2 bytes.
   Otherwise, the server MUST use a 32,768 byte LZ77 sliding window to
   decompress received messages.

   If the "method agreement" has the "s2c_no_context_takeover" method
   parameter, the client MAY reset its LZ77 sliding window for receiving
   to empty for each message.  Otherwise, the client MUST take over the
   LZ77 sliding window used to parse the last compressed message.  If
   the "method agreement" has the "c2s_no_context_takeover" method
   parameter, the server MAY reset its LZ77 sliding window for receiving
   to empty for each message.  Otherwise, the server MUST take over the
   LZ77 sliding window used to parse the last compressed message.

5.2.3.  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




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   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 messages, they MAY decompress and/or
   compress the messages according to the constraints negotiated during
   the opening handshake of the connection(s).

5.4.  Implementation Notes

   _This section is non-normative._

   On most common software development platforms, the operation of
   aligning compressed data to byte boundaries using an empty block with
   no compression is available as a 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 are no security concerns for now.
















































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7.  IANA Considerations

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

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

   Extension Identifier
      permessage-compress

   Extension Common Name
      WebSocket Per-message Compression

   Extension Definition
      This document.

   Known Incompatible Extensions
      None

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

7.2.  Registration of the "Per-message 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-message Compressed

   Meaning
      The message is compressed or not.

   Reference
      Section 4 of this document.

   The "Per-message Compressed" framing header bit is used on the first
   fragment of non-control messages to indicate whether the payload of
   the message is compressed by the Per-message Compression Extension or
   not.






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7.3.  WebSocket Per-message Compression Method Name Registry

   This specification creates a new IANA registry for names of
   compression methods to be used with the WebSocket Per-message
   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-message 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 to 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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