HyBi Working Group                                            T. Yoshino
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                        October 19, 2012                        January 24, 2013
Expires: April 22, July 28, 2013

                   WebSocket Per-message Compression
               draft-ietf-hybi-permessage-compression-04
               draft-ietf-hybi-permessage-compression-05

Abstract

   This specification defines document specifies a framework for creating WebSocket extension extensions
   that adds add compression functionality to the WebSocket Protocol.  It compresses
   Extensions based on this framework compress the payload of non-control non-
   control WebSocket messages using a 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 document
   also specifies one specific 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 April 22, July 28, 2013.

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   Copyright (c) 2012 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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  5
   4.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Sending  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Receiving  . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  DEFLATE method . . . . . . .  permessage-deflate extension . . . . . . . . . . . . . . . . .  8
     5.1.  Method Parameters  . . . . . . . . . . . . . . . . . . . .  8
       5.1.1.  Disallow compression context takeover  . . . . . . . .  8
       5.1.2.  Limit maximum LZ77 sliding window size . . . . . . . .  9
       5.1.3.  Example  . . . . . . . . . . . . . . . . . . . . . . .  9 10
     5.2.  Application Data Transformation  . . . . . . . . . . . . . 10
       5.2.1.  Compression  . . . . . . . . . . . . . . . . . . . . . 10
       5.2.2.  Decompression  . . . . . . . . . . . . . . . . . . . . 11
       5.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . 12
     5.3.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . 14
     5.4.  Implementation Notes . . . . . . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
     7.1.  Registration of the "permessage-compress" "permessage-deflate" WebSocket
           Extension Name . . . . . . . . . . . . . . . . . . . . . . 17
     7.2.  Registration of the "Per-message Compressed" WebSocket
           Framing Header Bit . . . . . . . . . . . . . . . . . . . . 17
     7.3.  WebSocket Per-message Compression Method Name Registry . . 18
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 18
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21 20

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 document
   specifies a framework for creating WebSocket extension extensions that applies apply a
   compression algorithm to octets exchanged over the WebSocket Protocol
   using its extension framework.  This extension negotiates what
   compression method to use  Extensions based on this framework
   negotiate compression parameters during the opening handshake, and
   then compresses compress the octets in non-control messages using the method.  We can apply
   this extension to messages.  Extensions for
   various compression algorithms can be specified by specifying describing how to
   negotiate parameters and transform payload. data in payloads.  A client may
   offer multiple compression methods on opening handshake, and then algorithms during the opening handshake by
   listing multiple compression extensions.  The server chooses may choose
   preferred one from them.  This extension uses  Extensions based on this framework share
   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 avoiding extra compression.

   This specification provides document also specifies one specific compression method "deflate" extension "permessage-
   deflate" which is based on DEFLATE [RFC1951] for this extension. algorithm.  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 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 messages to get better compression ratio.  For resource-
   limited
   resource-limited devices, method this extension provides parameters to limit the usage of
   memory usage for compression context are provided. context.

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

       Sec-WebSocket-Extensions: permessage-compress; method=deflate permessage-deflate

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

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].

3.  Extension Negotiation

   The registered extension token

   Extension names and negotiation methods are specified individually
   for this extension each compression algorithm.  There is
   "permessage-compress". no additional rule for
   extension naming.  Extensions build based on this framework are
   collectively called "Per-message Compression Extensions".

   To request use of the a Per-message Compression Extension, a client MUST
   include an element with the "permessage-compress" extension
   token as its extension identifier token 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" contains extension parameter is a list of compression method descriptions,
   ordered parameters as specified 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
   specification of 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 extension.  A client MAY contain list multiple method descriptions Per-
   message Compression Extensions with the same
   method name. name to offer use of the
   same algorithm with different configurations.

   To accept use of the a 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" its extension token in the
   "Sec-WebSocket-Extensions" header in its opening handshake.  The chosen description is called "accepted
   request".  The
   element in the server's "Sec-WebSocket-Extensions"
   MUST contain exactly one extension parameter named "method".  The
   value of the "method" contains extension parameter MUST be a compression
   method description.  This description is called "method agreement".
   The method name in parameters as specified by the "method agreement" MUST be one
   specification of the accepted
   request. extension.  The "method agreement" parameters MUST be derived from
   the "accepted
   request" parameters sent by the client and the server's capability.  If the server doesn't support
   any of the descriptions listed by the client, the server MUST  To
   reject use of the a Per-message Compression Extension.  Its grammar is
   "method-agreement" defined Extension, a server MUST
   simply ignore the element in the following ABNF.

       method-agreement = method-desc

   The value of "Sec-WebSocket-Extensions" header in
   the "method" parameter MUST be quoted by using
   "quoted-string" syntax if it doesn't conform to token syntax. client's opening handshake.

   If a client doesn't support the method extension and its configuration
   specified by parameters replied
   from the "method agreement", server, the client MUST _Fail the WebSocket Connection_.
   Otherwise, once _the WebSocket Connection is established_, both
   endpoints MUST use the algorithm described in Section 4 to exchange
   messages.

3.1.  Negotiation Example

   _This section is non-normative._

   These are "Sec-WebSocket-Extensions" header value examples that
   negotiate the Per-message Compression Extension. permessage-foo and
   permessage-bar in the examples are extension names of Per-message
   Compression Extensions for hypothetical compression algorithm foo and
   bar.

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

          permessage-compress; method=foo foo.

          permessage-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"

          permessage-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 a feature x but also allowing fallback to
      one without the feature.

          permessage-compress; method="foo; use_x, foo"

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

          permessage-foo; z="Hello World" contains double quotations

   o  Request foo and bar.

          permessage-foo, permessage-bar

   o  Request foo with a space, it needs to
      be quoted again.

          permessage-compress; method="foo; x=\"Hello World\", bar" parameter use_y which enables a feature y as
      first choice, and also list one without the parameter as a
      fallback plan.

          permessage-foo; use_y, permessage-foo

4.  Framing

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

   This extension document allocates the RSV1 bit of the WebSocket header for
   extensions based on this framework, and names it the "Per-message
   Compressed" bit.  Any other extension requiring the use of the RSV1
   bit is incompatible with this extension. these extensions.  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

   Per-message Compression Extensions MUST NOT be used after any
   extension for which frame boundary needs to be preserved.  This extension  Per-
   message Compression Extensions 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

   Per-message Compression Extensions 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.

5.  DEFLATE method  permessage-deflate extension

   This section defines specifies a method named "deflate" for this specific extension called
   "permessage-deflate" that compresses the payload of messages using
   DEFLATE [RFC1951] and byte boundary alignment method introduced in
   [RFC1979].

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

5.1.  Method Parameters

   The following 4 method parameters are defined for "deflate" method in the following subsections. subsections
   for this extension.

   o  "s2c_no_context_takeover"

   o  "c2s_no_context_takeover"

   o  "s2c_max_window_bits"

   o  "c2s_max_window_bits"

   A server MUST ignore "deflate" method descriptions that: a "permessage-deflate" extension entry if any of
   the following is true:

   o  have  It has any method parameter unknown to the server

   o  have  It has any method parameter with an invalid value

   o  It is not supported by the server

   A client MUST _Fail the WebSocket Connection_ if any of the following
   is true about the "method
   agreement": received "permessage-deflate" extension entry:

   o  It has any method parameter unknown to the client

   o  It has any method parameter with an invalid value

   o  It is not supported by the client

5.1.1.  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.  Servers SHOULD be able to accept the
   "s2c_no_context_takeover" method parameter.  If the "accepted
   request" has  To accept a request with this method
   parameter, the a server:

   o  MUST attach this parameter to its response

   o  MUST reset its LZ77 sliding window for sending to empty for each
      message

   o  MUST attach this method parameter to its "method agreement"

   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.  Clients SHOULD be able to accept the
   "c2s_no_context_takeover" method parameter.  A client that received this
   parameter MUST reset its LZ77 sliding window for sending to empty for
   each message.

   These parameters have no value.

5.1.2.  Limit 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.
   This parameter MUST have a decimal integer value in the range between
   8 to 15 indicating the base-2 logarithm of the LZ77 sliding window
   size.  The ABNF [RFC5234] for the value of this parameter is 1*DIGIT.
   Servers MAY be able to accept the "s2c_max_window_bits" method parameter.  If the "accepted request"
   has
   To accept a request with this method parameter, the server:

   o  MUST attach this method parameter with the same value as one of the
      "accepted request" to its "method agreement" response

   o  MUST NOT use LZ77 sliding window size greater than the size
      specified by this parameter to build messages

   A client MAY attach the "c2s_max_window_bits" method parameter if the client
   can adjust LZ77 sliding window size based on the
   "c2s_max_window_bits" sent by the server.  This parameter has no
   value.

   If the "accepted request" received request has the "c2s_max_window_bits" method parameter, the
   server MAY attach respond to the request with the "c2s_max_window_bits" method
   parameter to limit the LZ77 sliding window size that the client uses
   to build messages.  Otherwise, the server MUST NOT attach accept the request
   with a response with the parameter.  This parameter sent by the
   server MUST have a decimal integer value in the range between 8 to 15
   indicating the base-2 logarithm of the LZ77 sliding window size.  The
   ABNF for the value of this parameter is 1*DIGIT.  A client that
   received this parameter MUST NOT use LZ77 sliding window size greater
   than the size specified by this parameter to build messages.

5.1.3.  Example

   _This section is non-normative._

   This example sent by a client is asking the server to use LZ77
   sliding window size of 1,024 bytes or less and declaring that the
   client can accept the "c2s_max_window_bits" parameter.

       Sec-WebSocket-Extensions: permessage-compress;
           method="deflate; permessage-deflate;
           c2s_max_window_bits;
           s2c_max_window_bits=10"
           s2c_max_window_bits=10

   This request might be rejected by the server because it doesn't
   support the "s2c_max_window_bits" parameter.  Since there's only one
   method description
   compression extension listed in the header, the server need to give
   up use of the Per-message Compression Extension entirely.  If
   reduction of LZ77 sliding window size by the server is mandatory for
   the client, this is fine.

   The next example lists two configurations so that the server can
   accept permessage-compress permessage-deflate by picking supported one of the configurations it
   supports. from them.

       Sec-WebSocket-Extensions: permessage-compress;
           method="deflate;
           permessage-deflate; s2c_max_window_bits=10, deflate"
           permessage-deflate

   The server can choose to accept the second description extension entry by sending
   back this for example:

       Sec-WebSocket-Extensions: permessage-compress;
           method=deflate permessage-deflate

   Since the "c2s_max_window_bits" parameter was not specified for both
   of the method descriptions, extensions, the server cannot use the "c2s_max_window_bits"
   parameter.

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.

   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.
       After this step, the last octet of the compressed data contains
       the (part of) header bits with "BTYPE" set to 0.

   In the first step:

   o  Multiple blocks MAY be used.

   o  Any type of block MAY be used.

   o  Both block with "BFINAL" set to 0 and 1 MAY be used.

   o  When any block with "BFINAL" set to 1 doesn't end at byte
      boundary, minimal padding bits of 0 MUST be added to make it end
      at byte boundary, and then the next block MUST start at the byte
      boundary if any.

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

   If the "method agreement" has server specified 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 server specified 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.

   If the "method agreement" has server specified 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 server specified 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.

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 server specified 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 server specified 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.

   If the "method agreement" has server specified 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 server specified 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.

5.2.3.  Examples

   _This section is non-normative._

   This section introduces examples of how the DEFLATE method permessage-deflate
   transforms messages.

5.2.3.1.  A message compressed using 1 compressed block

   Suppose that a text message "Hello" is sent using the DEFLATE method. sent.  When 1 compressed block
   (compressed with fixed Huffman code, "BFINAL" is not set) is used,
   compressed data to be sent in payload is obtained as follows.

   Compress "Hello" into 1 compressed block and flush it into a byte
   array using an empty block with no compression:

       0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00 0x00 0xff 0xff

   Strip 0x00 0x00 0xff 0xff from the tail:

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

   To send it without fragmentation, just build a frame putting the
   whole data in payload data:

       0xc1 0x07 0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00

   The first 2 octets are the WebSocket protocol's overhead (FIN=1,
   RSV1=1, RSV2=0, RSV3=0, opcode=text, MASK=0, Payload length=7).

   To send it after fragmentation, split the compressed payload and
   build frames for each of split data as well as fragmentation process
   done when the compression extension is not used.  For example, the
   first fragment may contain 3 octets of the payload:

       0x41 0x03 0xf2 0x48 0xcd

   and the second (last) fragment contain 4 octets of the payload:

       0x80 0x04 0xc9 0xc9 0x07 0x00

   Note that RSV1 is set only on the first fragment.

5.2.3.2.  Sharing LZ77 Sliding Window

   Suppose that the next message to send is also "Hello".  If it's
   disallowed by the other peer (using some extension parameter) to take
   over the LZ77 sliding window used for the last message, the next
   message is compressed into the same byte array (if the same "BTYPE"
   and "BFINAL" value are used).  If it's allowed, the next message can
   be compressed into shorter payload:

       0xf2 0x00 0x11 0x00 0x00

   instead of:

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

   Note that even if any uncompressed message is inserted between the
   two "Hello" messages, it doesn't affect context sharing between the
   two "Hello" messages.

5.2.3.3.  Using a Block with No Compression

   Blocks with no compression can be also used.  A block with no
   compression containing "Hello" flushed into a byte array using an
   empty block with no compression is:

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

   So, payload of a message containing "Hello" converted into a DEFLATE
   block with no compression is:

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

   If it's not fragmented, the frame for this message is:

       0xc1 0x0b 0x00 0x05 0x00 0xfa 0xff 0x48 0x65 0x6c 0x6c 0x6f 0x00

   The first 2 octets are the WebSocket protocol's overhead (FIN=1,
   RSV1=1, RSV2=0, RSV3=0, opcode=text, MASK=0, Payload length=7).  Note
   that RSV1 must be set for this message (only on the first fragment of
   it) because RSV1 indicates whether DEFLATE is applied to the message
   including use of blocks with no compression or not.

5.2.3.4.  Using a Block with BFINAL Set to 1

   On platform where the flush method based on an empty block with no
   compression is not avaiable, implementors can choose to flush data
   using blocks with "BFINAL" set to 1.  Using a block with "BFINAL" set
   to 1 and "BTYPE" set to 1, "Hello" is compressed into:

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

   So, payload of a message containing "Hello" compressed using this
   parameter setting is:

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

   The last 1 octet contains the header bits with "BFINAL" set to 0 and
   "BTYPE" set to 0, and 7 padding bits of 0.  It's necessary to make
   the payload able to be processed by the same manner as messages
   flushed using blocks with BFINAL unset.

5.2.3.5.  Two Blocks in 1 Message

   Two or more blocks may be used in 1 message.

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

   The first 3 octets and the least significant two bits of the 4th
   octet consist one block with "BFINAL" set to 0 and "BTYPE" set to 1
   containing "He".  The rest of the 4th octet contains the header bits
   with "BFINAL" set to 0 and "BTYPE" set to 0, and the 3 padding bits
   of 0.  Together with the following 4 octets (0x00 0x00 0xff 0xff),
   the header bits consist an empty block with no compression.  Then, a
   block containing "llo" follows.

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.

6.  Security Considerations

   There are no security concerns for now.

7.  IANA Considerations

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

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

   Extension Identifier
      permessage-compress
      permessage-deflate

   Extension Common Name
      WebSocket Per-message Compression Deflate

   Extension Definition
      This document.

   Known Incompatible Extensions
      None

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

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 |
     +------------+-------------+---------------+

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.

   Thank you to the following people who participated in discussions on
   the HyBi WG and contributed ideas and/or provided detailed reviews
   (the list is likely to be incomplete): Alexey Melnikov, Arman
   Djusupov, Bjoern Hoehrmann, Brian McKelvey, Greg Wilkins, Inaki Baz
   Castillo, Jamie Lokier, Joakim Erdfelt, John A. Tamplin, Julian
   Reschke, Kenichi Ishibashi, Mark Nottingham, Peter Thorson, Roberto
   Peon and Simone Bordet.  Note that people listed above didn't
   necessarily endorse the end result of this work.

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.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 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/>.

Author's Address

   Takeshi Yoshino
   Google, Inc.

   Email: tyoshino@google.com