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Versions: (draft-tyoshino-hybi-permessage-compression) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18

HyBi Working Group                                            T. Yoshino
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                            May 13, 2014
Expires: November 14, 2014


                  Compression Extensions for WebSocket
               draft-ietf-hybi-permessage-compression-18

Abstract

   This document specifies a framework for creating WebSocket extensions
   that add compression functionality to the WebSocket Protocol.  An
   extension based on this framework compresses the payload data portion
   of non-control WebSocket messages on a per-message basis using
   parameters negotiated during the opening handshake.  This framework
   provides a general method to apply a compression algorithm to the
   contents of WebSocket messages.  For each compression algorithm, an
   extension is defined by specifying parameter negotiation and payload
   transformation algorithm in detail.  This document also specifies one
   specific compression extension using the DEFLATE algorithm.

   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 November 14, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   (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
   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 and Terminology . . . . . . . . . . .  4
   3.  Complementary Terminology  . . . . . . . . . . . . . . . . . .  5
   4.  WebSocket Per-message Compression Extension  . . . . . . . . .  6
   5.  Extension Negotiation  . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Negotiation Examples . . . . . . . . . . . . . . . . . . .  9
   6.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Compression  . . . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  Decompression  . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  permessage-deflate extension . . . . . . . . . . . . . . . . . 15
     8.1.  Method Parameters  . . . . . . . . . . . . . . . . . . . . 16
       8.1.1.  Context Takeover Control . . . . . . . . . . . . . . . 16
       8.1.2.  Limiting the LZ77 sliding window size  . . . . . . . . 18
       8.1.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 20
     8.2.  Message Payload Transformation . . . . . . . . . . . . . . 21
       8.2.1.  Compression  . . . . . . . . . . . . . . . . . . . . . 21
       8.2.2.  Decompression  . . . . . . . . . . . . . . . . . . . . 22
       8.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . 23
     8.3.  Implementation Notes . . . . . . . . . . . . . . . . . . . 26
     8.4.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . 27
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
     10.1. Registration of the "permessage-deflate" WebSocket
           Extension Name . . . . . . . . . . . . . . . . . . . . . . 29
     10.2. Registration of the "Per-message Compressed" WebSocket
           Framing Header Bit . . . . . . . . . . . . . . . . . . . . 29
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
     12.2. Informative References . . . . . . . . . . . . . . . . . . 31
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32








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1.  Introduction

   This document specifies a framework to add compression functionality
   to the WebSocket Protocol [RFC6455].  This framework specifies how to
   define WebSocket Per-message Compression Extensions (PMCEs)
   individually for various compression algorithms based on the
   extension concept of the WebSocket Protocol specified in Section 9 of
   [RFC6455].  A WebSocket client and a peer WebSocket server negotiate
   use of a PMCE and determine parameters to configure the compression
   algorithm during the WebSocket opening handshake.  The client and
   server can then exchange non-control messages using frames with
   compressed data in the payload data portion.

   This framework only specifies a general method to apply a compression
   algorithm to the contents of WebSocket messages.  A document
   specifying an individual PMCE describes how to negotiate
   configuration parameters for the compression algorithm and how to
   transform (compress and decompress) data in the payload data portion
   in detail.

   A WebSocket client may offer multiple PMCEs during the WebSocket
   opening handshake.  A peer WebSocket server received those offers may
   choose and accept preferred one or decline all of them.  PMCEs use
   the RSV1 bit of the WebSocket frame header to indicate whether a
   message is compressed or not, so that an endpoint can choose not to
   compress messages with incompressible contents.

   This document also specifies one specific PMCE based on the DEFLATE
   [RFC1951] algorithm.  The extension name of the PMCE is "permessage-
   deflate".  We chose DEFLATE since it's widely available as a library
   on various platforms and the overhead is small.  To align the end of
   compressed data to an octet boundary, this extension uses the
   algorithm described in Section 2.1 of [RFC1979].  Endpoints can take
   over the LZ77 sliding window [LZ77] used to build frames for previous
   messages to get better compression ratio.  For resource-limited
   devices, this extension provides parameters to limit memory usage for
   compression context.














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2.  Conformance Requirements and Terminology

   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].

   Requirements phrased in the imperative as part of algorithms (such as
   "strip any leading space characters" or "return false and abort these
   steps") are to be interpreted with the meaning of the key word
   ("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.

   Conformance requirements phrased as algorithms or specific steps can
   be implemented in any manner, so long as the end result is
   equivalent.  In particular, the algorithms defined in this
   specification are intended to be easy to understand and are not
   intended to be performant.

   This document references the procedure to _Fail the WebSocket
   Connection_.  This procedure is defined in Section 7.1.7 of
   [RFC6455].

   This document references the event that _The WebSocket Connection is
   Established_ and the event that _A WebSocket Message Has Been
   Received_.  These events are defined in Section 4.1 and Section 6.2,
   respectively, of [RFC6455].

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [RFC5234].  The DIGIT (decimal 0-9) rule is included by reference, as
   defined in the Appendix B.1 of [RFC5234].






















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3.  Complementary Terminology

   This document defines some terms about WebSocket and WebSocket
   Extension mechanism that are underspecified or not defined at all in
   [RFC6455].  This terminology is effective only in this document and
   any other documents that refer to this section.

   "A non-control message" means a message that consists of non-control
   frames as defined in Section 5.6 of [RFC6455].

   "A message payload (or payload of a message)" means concatenation of
   the payload data portion of all frames representing a single message,
   as well as how /data/ is formed from in Section 6.2 of [RFC6455].

   "An extension in use next to extension X" means the extension listed
   next to X in the "Sec-WebSocket-Extensions" header in the server's
   opening handshake as defined in Section 9.1 of [RFC6455].  Such an
   extension is applied to outgoing data from the application right
   after X on sender side but applied right before X to incoming data
   from the underlying transport.

   "An extension in use preceding extension X" means the extension
   listed right before X in the "Sec-WebSocket-Extensions" header in the
   server's opening handshake.  Such an extension is applied to outgoing
   data from the application right before X on sender side but applied
   right after X to incoming data from the underlying transport.

   "An extension negotiation offer" means each element in the
   "Sec-WebSocket-Extensions" header in the client's opening handshake.

   "An extension negotiation response" means each element in the
   "Sec-WebSocket-Extensions" header in the server's opening handshake.

   "A corresponding extension negotiation response for an extension
   negotiation offer" means an extension negotiation response a server
   sends back to the peer client that contains the same extension name
   as the offer and meets the requirements represented by the offer.

   "Accepting an extension negotiation offer" means including a
   corresponding extension negotiation response for the offer in the
   "Sec-WebSocket-Extensions" header in the server's opening handshake.

   "Declining an extension negotiation offer" means not including a
   corresponding extension negotiation response for the offer in the
   "Sec-WebSocket-Extensions" header in the server's opening handshake.






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4.  WebSocket Per-message Compression Extension

   WebSocket Per-message Compression Extensions (PMCEs) are extensions
   to the WebSocket Protocol enabling compression functionality.  PMCEs
   are built based on the extension concept of the WebSocket Protocol
   specified in Section 9 of [RFC6455].  PMCEs are individually defined
   for each compression algorithm to be implemented, and are registered
   in the WebSocket Extension Name Registry created in Section 11.4 of
   [RFC6455].  Each PMCE refers to this framework and defines the
   following:

   o  The contents of its extension negotiation offer/response to
      include in the "Sec-WebSocket-Extensions" header.  The contents
      include the extension name of the PMCE and any applicable
      extension parameters.

   o  How to interpret the extension parameters exchanged during the
      opening handshake

   o  How to transform the payload of a message.

   One such extension is defined in Section 8 of this document and is
   registered in Section 10.  Other PMCEs may be defined in other
   documents.

   Section 5 describes the basic extension negotiation process.
   Section 6 describes how to apply the compression algorithm with
   negotiated parameters to the contents of WebSocket messages.























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5.  Extension Negotiation

   To offer use of a PMCE, a client includes a
   "Sec-WebSocket-Extensions" header element with the extension name of
   the PMCE in the "Sec-WebSocket-Extensions" header in the client's
   opening handshake of the WebSocket connection.  Extension parameters
   in the element represent the PMCE offer in detail.  For example, a
   client lists preferred configuration parameter values for the
   compression algorithm of the PMCE.  A client offers multiple PMCE
   choices to the server by including multiple elements in the
   "Sec-WebSocket-Extensions" header, one for each PMCE offered.  The
   set of elements MAY include multiple PMCEs with the same extension
   name to offer use of the same algorithm with different configuration
   parameters.  The order of elements means the client's preference.  An
   element precedes another element has higher preference.  It is
   RECOMMENDED that a server accepts PMCEs with higher preference if the
   server supports it.

   A PMCE negotiation offer informs requests and/or hints to the server.
   A request in a PMCE negotiation offer indicates constraints on the
   server's behavior that must be satisfied if the server accepts the
   offer.  A hint in a PMCE negotiation offer indicates information
   about the client's behavior that the server may either safely ignore
   or refer to when the server decides its behavior.

   To accept use of an offered PMCE, a server includes a
   "Sec-WebSocket-Extensions" header element with the extension name of
   the PMCE in the "Sec-WebSocket-Extensions" header in the server's
   opening handshake of the WebSocket connection.  Extension parameters
   in the element represent the configuration parameters of the PMCE to
   use in detail.  We call these extension parameters and their values
   "agreed parameters".  The element MUST represent a PMCE that is fully
   supported by the server.  The contents of the element doesn't need to
   be exactly the same as one of the received extension negotiation
   offers.  For example, an extension negotiation offer with an
   extension parameter "X" indicating availability of the feature X may
   be accepted with an element without the extension parameter meaning
   that the server declined use of the feature.

   "Agreed parameters" MUST represent how the requests and hints in the
   client's extension negotiation offer have been handled in addition to
   the server's requests and hints on the client's behavior, so that the
   client can configure its behavior without identifying which PMCE
   extension negotiation offer has been accepted.

   For example, if a client sends an extension negotiation offer
   including a parameter "enable_compression" and another without the
   parameter, the server accepts the former and tell that to the client



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   by sending back an element including a parameter that acknowledges
   "enable_compression".  The name of the acknowledging parameter
   doesn't need to be the same as the offer.

   General negotiation flow will be like the following.  How to handle
   parameters in detail will be specified in the specifications for each
   PMCE.

   A client makes an offer including parameters identifying the
   following:

   o  Hints about how the client is planning to compress data

   o  Requests about how the server compresses data

   o  Limitation of the client's compression functionality

   The peer server makes a determination of its behavior based on these
   parameters if it can and wants to proceed with this PMCE enabled, and
   responds to the client with parameters identifying the following:

   o  Requests about how the client compresses data

   o  How the server will compress data

   The client makes a determination of its behavior based on these
   parameters from the server if it can and wants to proceed with this
   PMCE enabled.  Otherwise, the client starts closing handshake with
   close code 1010.

   There can be compression features that can be applied separately for
   each direction.  For such features, the acknowledging parameter and
   the parameter for the reverse direction must be chosen to be
   distinguishable from each other.  For example, we can add "server_"
   prefix to parameters affecting data sent from a server and "client_"
   prefix to ones affecting data sent from a client to make them
   distinguishable.

   A server MUST NOT accept a PMCE extension negotiation offer together
   with another extension if the PMCE will conflict with the extension
   on use of the RSV1 bit.  A client that received a response accepting
   a PMCE extension negotiation offer together with such an extension
   MUST _Fail the WebSocket Connection_.

   A server MUST NOT accept a PMCE extension negotiation offer together
   with another extension if the PMCE will be applied to output of the
   extension and any of the following conditions applies to the
   extension:



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   o  The extension requires boundary of fragments to be preserved
      between output from the extension at the sender and input to the
      extension at the receiver.

   o  The extension uses the "Extension data" field or any of the
      reserved bits on the WebSocket header as a per-frame attribute.

   A client that received a response accepting a PMCE extension
   negotiation offer together with such an extension MUST _Fail the
   WebSocket Connection_.

   A server declines all offered PMCEs by not including any element with
   PMCE names.  If a server responds with no PMCE element in the
   "Sec-WebSocket-Extensions" header, both endpoints proceed without
   Per-message Compression once _the WebSocket Connection is
   established_.

   If a server gives an invalid response, such as accepting a PMCE that
   the client did not offer, the client MUST _Fail the WebSocket
   Connection_.

   If a server responds with a valid PMCE element in the
   "Sec-WebSocket-Extensions" header and _the WebSocket Connection is
   established_, both endpoints MUST use the algorithm described in
   Section 6 and the message payload transformation (compressing and
   decompressing) procedure of the PMCE configured with the "agreed
   parameters" returned by the server to exchange messages.

5.1.  Negotiation Examples

   The following are example values for the "Sec-WebSocket-Extensions"
   header offering PMCEs. permessage-foo and permessage-bar in the
   examples are hypothetical extension names of PMCEs for compression
   algorithm foo and bar.

   o  Offer the permessage-foo.

          permessage-foo

   o  Offer the permessage-foo with a parameter x with a value of 10.

          permessage-foo; x=10

      The value MAY be quoted.

          permessage-foo; x="10"





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   o  Offer the permessage-foo as first choice and the permessage-bar as
      a fallback plan.

          permessage-foo, permessage-bar

   o  Offer the permessage-foo with a parameter use_y which enables a
      feature y as first choice, and the permessage-foo without the
      use_y parameter as a fallback plan.

          permessage-foo; use_y, permessage-foo









































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6.  Framing

   PMCEs operate only on non-control messages.

   This document allocates the RSV1 bit of the WebSocket header for
   PMCEs, and calls the bit the "Per-message Compressed" bit.  On a
   WebSocket connection where a PMCE is in use, this bit indicates
   whether a message is compressed or not.

   A message with the "Per-message Compressed" bit set on the first
   fragment of the message is called a "compressed message".  Frames of
   a compressed message have compressed data in the payload data
   portion.  An endpoint received a compressed message decompresses the
   concatenation of the compressed data of the frames of the message by
   following the decompression procedure specified by the PMCE in use.
   The endpoint uses the bytes corresponding to the application data
   portion in this decompressed data for the _A WebSocket Message Has
   Been Received_ event instead of the received data as-is.

   A message with the "Per-message Compressed" bit unset on the first
   fragment of the message is called an "uncompressed message".  Frames
   of an uncompressed message have uncompressed original data as-is in
   the payload data portion.  An endpoint received an uncompressed
   message uses the concatenation of the application data portion of the
   frames of the message as-is for the _A WebSocket Message Has Been
   Received_ event.

6.1.  Compression

   An endpoint MUST use the following algorithm to send a message in the
   form of a compressed message.

   1.  Compress the message payload of the original message by following
       the compression procedure of the PMCE.  The original message may
       be input from the application layer or output of another
       WebSocket extension depending on what extensions were negotiated.

   2.  If this PMCE is the last extension to process outgoing messages,
       build frame(s) by using the compressed data instead of the
       original data for the message payload, and setting the
       "Per-message Compressed" bit of the first frame, then send the
       frame(s) as described in Section 6.1 of RFC6455.  Otherwise, pass
       the transformed message payload and modified header values
       including "Per-message Compressed" bit value set to 1 to the
       extension next to the PMCE.  If the extension expects frames for
       input, build a frame for the message and pass it.

   An endpoint MUST use the following algorithm to send a message in the



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   form of an uncompressed message.  If this PMCE is the last extension
   to process outgoing messages, build frame(s) by using the original
   data for the payload data portion as-is and unsetting the
   "Per-message Compressed" bit of the first frame, then send the
   frame(s) as described in Section 6.1 of RFC6455.  Otherwise, pass the
   message payload and header values to the extension next to the PMCE
   as-is.  If the extension expects frames for input, build a frame for
   the message and pass it.

   An endpoint MUST NOT set the "Per-message Compressed" bit of control
   frames and non-first fragments of a data message.  An endpoint
   received such a frame MUST _Fail the WebSocket Connection_.

   PMCEs don't change the opcode field.  The opcode of the first frame
   of a compress message indicates the opcode of the original message.

   The payload data portion in frames generated by a PMCE is not subject
   to the constraints for the original data type.  For example, the
   concatenation of the output data corresponding to the application
   data portion of frames of a compressed text message is not required
   to be valid UTF-8.  At the receiver, the payload data portion after
   decompression is subject to the constraints for the original data
   type again.

6.2.  Decompression

   An endpoint MUST use the following algorithm to receive a message in
   the form of a compressed message.

   1.  Concatenate the payload data portion of the received frames of
       the compressed message.  The received frames may be direct input
       from the underlying transport or output of another WebSocket
       extension depending on what extensions were negotiated.

   2.  Decompress the concatenation by following the decompression
       procedure of the PMCE.

   3.  If this is the last extension to process incoming messages,
       deliver the _A WebSocket Message Has Been Received_ event to the
       application layer with the decompressed message payload and
       header values including the "Per-message Compressed" bit unset to
       0.  Otherwise, pass the decompressed message payload and header
       values including the "Per-message Compressed" bit unset to 0 to
       the extension preceding the PMCE.  If the extension expects
       frames for input, build a frame for the message and pass it.

   An endpoint MUST use the following algorithm to receive a message in
   the form of an uncompressed message.  If this PMCE is the last



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   extension to process incoming messages, deliver the _A WebSocket
   Message Has Been Received_ event to the application layer with the
   received message payload and header values as-is.  Otherwise, pass
   the message payload and header values to the extension preceding the
   PMCE as-is.  If the extension expects frames for input, build a frame
   for the message and pass it.













































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7.  Intermediaries

   When an intermediary proxies a WebSocket connection, the intermediary
   MAY add, change or remove Per-message Compression on proxied messages
   if the intermediary meets all of the following requirements:

   o  The intermediary understands that Per-message Compression.

   o  The intermediary can read all data of the proxied WebSocket
      connection including the opening handshake request, opening
      handshake response, and messages.

   o  The intermediary can alter the proxied data before forwarding them
      accordingly to conform to the constraints of the new combination
      of extensions.  For example, if Per-message Compression is removed
      from messages, the corresponding element in the
      "Sec-WebSocket-Extensions" in the opening handshake response which
      enabled the Per-message Compression must also be removed.

   Otherwise, the intermediary MUST NOT add, change or remove Per-
   message Compression on proxied messages.






























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8.  permessage-deflate extension

   This section specifies a specific PMCE called "permessage-deflate".
   It compresses the payload of a message using the DEFLATE algorithm
   [RFC1951] and the byte boundary aligning method introduced in
   [RFC1979].

   This section uses the term "byte" with the same meaning as RFC1951,
   i.e. 8 bits stored or transmitted as a unit (same as an octet).

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

   Four extension parameters are defined for permessage-deflate to help
   endpoints manage per-connection resource usage.

   o  "server_no_context_takeover"

   o  "client_no_context_takeover"

   o  "server_max_window_bits"

   o  "client_max_window_bits"

   These parameters enable two methods (no_context_takeover and
   max_window_bits) of constraining memory usage that may be applied
   independently to either direction of WebSocket traffic.  The
   extension parameters with the "client_" prefix are used to negotiate
   DEFLATE parameters to control compression on messages sent by a
   client and received by a server.  The client refers to parameters
   with the "client_" prefix to configure its compressor, while the
   server refers to them to configure its decompressor.  The extension
   parameters with the "server_" prefix are used to negotiate DEFLATE
   parameters to control compression on messages sent by a server and
   received by a client.  The server refers to parameters with the
   "server_" prefix to configure its compressor, while the client refers
   to them to configure its decompressor.  All of these four parameters
   are defined for both a client's extension negotiation offer and a
   server's extension negotiation response.

   A server MUST decline an extension negotiation offer for this
   extension if any of the following conditions is met:

   o  The offer has any extension parameter not defined for use in an
      offer.

   o  The offer has any extension parameter with an invalid value.




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   o  The offer has multiple extension parameters with the same name.

   o  The server doesn't support the offered configuration.

   A client MUST _Fail the WebSocket Connection_ if the peer server
   accepted an extension negotiation offer for this extension with an
   extension negotiation response meeting any of the following
   condition:

   o  The response has any extension parameter not defined for use in a
      response.

   o  The response has any extension parameter with an invalid value.

   o  The response has multiple extension parameters with the same name.

   o  The client doesn't support the configuration that the response
      represents.

   The term "LZ77 sliding window" used in this section means the buffer
   used by the DEFLATE algorithm to store recently processed input.  The
   DEFLATE compression algorithm searches the buffer for match with the
   next input.

   The term "use context take over" used in this section means to use
   the same LZ77 sliding window the endpoint used to build frames of the
   last sent message to build frames of the next message.

8.1.  Method Parameters

8.1.1.  Context Takeover Control

8.1.1.1.  server_no_context_takeover

   A client MAY include the "server_no_context_takeover" extension
   parameter in an extension negotiation offer.  This extension
   parameter has no value.  By including this extension parameter in an
   extension negotiation offer, a client prevents the peer server from
   using context take over.  If the peer server doesn't use context take
   over, the client doesn't need to reserve memory to retain the LZ77
   sliding window in between messages.

   Absence of this extension parameter in an extension negotiation offer
   indicates that the client can receive a message which the server
   built using context take over.

   A server accepts an extension negotiation offer including the
   "server_no_context_takeover" extension parameter by including the



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   "server_no_context_takeover" extension parameter in the corresponding
   extension negotiation response to send back to the client.  The
   "server_no_context_takeover" extension parameter in an extension
   negotiation response has no value.

   It is RECOMMENDED that a server supports the
   "server_no_context_takeover" extension parameter in an extension
   negotiation offer.

   A server MAY include the "server_no_context_takeover" extension
   parameter in an extension negotiation response even if the extension
   negotiation offer being accepted by the extension negotiation
   response didn't have the "server_no_context_takeover" extension
   parameter.

8.1.1.2.  client_no_context_takeover

   A client MAY include the "client_no_context_takeover" extension
   parameter in an extension negotiation offer.  This extension
   parameter has no value.  By including this extension parameter in an
   extension negotiation offer, a client informs the peer server of a
   hint that even if the server won't include the
   "client_no_context_takeover" extension parameter in the corresponding
   extension negotiation response to the offer, the client is not going
   to use context take over.

   A server MAY include the "client_no_context_takeover" extension
   parameter in an extension negotiation response.  If the received
   extension negotiation offer includes the "client_no_context_takeover"
   extension parameter, the server may either ignore the parameter or
   use the parameter to avoid taking over an LZ77 sliding window
   unnecessarily by including "client_no_context_takeover" extension
   parameter in the corresponding extension negotiation response to the
   offer.  The "client_no_context_takeover" extension parameter in an
   extension negotiation response has no value.  By including the
   "client_no_context_takeover" extension parameter in an extension
   negotiation response, a server prevents the peer client from using
   context take over.  This reduces the amount of memory that the server
   has to reserve for the connection.

   Absence of this extension parameter in an extension negotiation
   response indicates that the server can receive messages built by the
   client using context take over.

   A client MUST support the "client_no_context_takeover" extension
   parameter in an extension negotiation response.





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8.1.2.  Limiting the LZ77 sliding window size

8.1.2.1.  server_max_window_bits

   A client MAY include the "server_max_window_bits" extension parameter
   in an extension negotiation offer.  This parameter has a decimal
   integer value without leading zeroes between 8 to 15 inclusive
   indicating the base-2 logarithm of the LZ77 sliding window size and
   MUST conform to the ABNF below.

       server_max_window_bits = 1*DIGIT

   By including this parameter in an extension negotiation offer, a
   client limits the LZ77 sliding window size that the server uses to
   compress messages.  If the peer server uses small LZ77 sliding window
   to compress messages, the client can reduce the memory for the LZ77
   sliding window.

   A server declines an extension negotiation offer with this parameter
   if the server doesn't support it.

   Absence of this parameter in an extension negotiation offer indicates
   that the client can receive messages compressed using an LZ77 sliding
   window of up to 32,768 bytes.

   A server accepts an extension negotiation offer with this parameter
   by including the "server_max_window_bits" extension parameter in the
   extension negotiation response to send back to the client with the
   same or smaller value as the offer.  The "server_max_window_bits"
   extension parameter in an extension negotiation response has a
   decimal integer value without leading zeroes between 8 to 15
   inclusive indicating the base-2 logarithm of the LZ77 sliding window
   size and MUST conform to the ABNF below.

       server_max_window_bits = 1*DIGIT

   A server MAY include the "server_max_window_bits" extension parameter
   in an extension negotiation response even if the extension
   negotiation offer being accepted by the response didn't include the
   "server_max_window_bits" extension parameter.

8.1.2.2.  client_max_window_bits

   A client MAY include the "client_max_window_bits" extension parameter
   in an extension negotiation offer.  This parameter has no value or a
   decimal integer value without leading zeroes between 8 to 15
   inclusive indicating the base-2 logarithm of the LZ77 sliding window
   size.  If a value is specified for this parameter, the value MUST



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   conform to the ABNF below.

       client_max_window_bits = 1*DIGIT

   By including this parameter in an offer, a client informs the peer
   server of that the client supports the "client_max_window_bits"
   extension parameter in an extension negotiation response, and
   optionally a hint by attaching a value to the parameter.  If the
   "client_max_window_bits" extension parameter in an extension
   negotiation offer has a value, the parameter also informs the peer
   server of a hint that even if the server won't include the
   "client_max_window_bits" extension parameter in the corresponding
   extension negotiation response with a value greater than one in the
   extension negotiation offer or the server doesn't include the
   extension parameter at all, the client is not going to use LZ77
   sliding window size greater than the size specified by the value in
   the extension negotiation offer to compress messages.

   If a received extension negotiation offer has the
   "client_max_window_bits" extension parameter, the server MAY include
   the "client_max_window_bits" extension parameter in the corresponding
   extension negotiation response to the offer.  If the
   "client_max_window_bits" extension parameter in a received extension
   negotiation offer has a value, the server may either ignore this
   value or use this value to avoid allocating an unnecessarily big LZ77
   sliding window by including the "client_max_window_bits" extension
   parameter in the corresponding extension negotiation response to the
   offer with a value equal to or smaller than the received value.  The
   "client_max_window_bits" extension parameter in an extension
   negotiation response has a decimal integer value without leading
   zeroes between 8 to 15 inclusive indicating the base-2 logarithm of
   the LZ77 sliding window size and MUST conform to the ABNF below.

       client_max_window_bits = 1*DIGIT

   By including this extension parameter in an extension negotiation
   response, a server limits the LZ77 sliding window size that the
   client uses to compress messages.  This reduces the amount of memory
   for decompression context that the server has to reserve for the
   connection.

   If a received extension negotiation offer doesn't have the
   "client_max_window_bits" extension parameter, the corresponding
   extension negotiation response to the offer MUST NOT include the
   "client_max_window_bits" extension parameter.

   Absence of this extension parameter in an extension negotiation
   response indicates that the server can receive messages compressed



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   using an LZ77 sliding window of up to 32,768 bytes.

8.1.3.  Example

   The simplest "Sec-WebSocket-Extensions" header in a client's opening
   handshake to offer use of the permessage-deflate is as follows:

       Sec-WebSocket-Extensions: permessage-deflate

   Since the "client_max_window_bits" extension parameter is not
   included in this extension negotiation offer, the server must not
   accept the offer with an extension negotiation response including the
   "client_max_window_bits" extension parameter.  The simplest
   "Sec-WebSocket-Extensions" header in a server's opening handshake to
   accept use of the permessage-deflate is the same.

   The following extension negotiation offer sent by a client is asking
   the server to use the LZ77 sliding window size of 1,024 bytes or less
   and declaring that the client supports the "client_max_window_bits"
   extension parameter in an extension negotiation response.

       Sec-WebSocket-Extensions:
           permessage-deflate;
           client_max_window_bits; server_max_window_bits=10

   This extension negotiation offer might be rejected by the server
   because the server doesn't support the "server_max_window_bits"
   extension parameter in an extension negotiation offer.  This is fine
   if the client cannot receive messages compressed using a larger
   sliding window size, but if the client just prefers using a small
   window but wants to fallback to the "permessage-deflate" without the
   "server_max_window_bits" extension parameter, the client can make an
   offer with the fallback option like this:

       Sec-WebSocket-Extensions:
           permessage-deflate;
           client_max_window_bits; server_max_window_bits=10,
           permessage-deflate;
           client_max_window_bits

   The server can accept permessage-deflate by picking the supported one
   from the listed offers.  To accept the first option, for example, the
   server may send back a response as follows:

       Sec-WebSocket-Extensions:
           permessage-deflate; server_max_window_bits=10

   To accept the second option, for example, the server may send back a



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   response as follows:

       Sec-WebSocket-Extensions: permessage-deflate

8.2.  Message Payload Transformation

8.2.1.  Compression

   An endpoint uses 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 DEFLATE block
       with no compression (the "BTYPE" bits are set to 00), append an
       empty DEFLATE block with no compression to the tail end.

   3.  Remove 4 octets (that are 0x00 0x00 0xff 0xff) from the tail end.
       After this step, the last octet of the compressed data contains
       (possibly part of) the DEFLATE header bits with the "BTYPE" bits
       set to 00.

   When using DEFLATE in the first step above:

   o  An endpoint MAY use multiple DEFLATE blocks to compress one
      message.

   o  An endpoint MAY use DEFLATE blocks of any type.

   o  An endpoint MAY use both DEFLATE blocks with the "BFINAL" bit set
      to 0 and DEFLATE blocks with the "BFINAL" bit set to 1.

   o  When any DEFLATE block with the "BFINAL" bit set to 1 doesn't end
      at a byte boundary, an endpoint MUST add minimal padding bits of 0
      to make it end at a byte boundary.  The next DEFLATE block follows
      the padded data if any.

   An endpoint fragments a compressed message by splitting the result of
   running this algorithm.  Even when only a part of payload is
   available, a fragment can be built by compressing the available data
   and choosing block type appropriately so that the end of the
   resulting compressed data is aligned at a byte boundary.  Note that
   for non-final fragments, the removal of 0x00 0x00 0xff 0xff must not
   be done.

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




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   If the "agreed parameters" contain the "client_no_context_takeover"
   extension parameter, the client MUST start compressing each new
   message with an empty LZ77 sliding window.  Otherwise, the client MAY
   take over the LZ77 sliding window used to build the last compressed
   message.  Note that even if the client has included the
   "client_no_context_takeover" extension parameter in its offer, the
   client MAY take over the LZ77 sliding window used to build the last
   compressed message if the "agreed parameters" don't contain the
   "client_no_context_takeover" extension parameter.  The client-to-
   server "client_no_context_takeover" extension parameter is just a
   hint for the server to build an extension negotiation response.

   If the "agreed parameters" contain the "server_no_context_takeover"
   extension parameter, the server MUST start compressing each new
   message with an empty LZ77 sliding window.  Otherwise, the server MAY
   take over the LZ77 sliding window used to build the last compressed
   message.

   If the "agreed parameters" contain the "client_max_window_bits"
   extension parameter with a value of w, the client MUST NOT use an
   LZ77 sliding window longer than the w-th power of 2 bytes to compress
   messages to send.  Note that even if the client has included in its
   offer the "client_max_window_bits" extension parameter with a value
   smaller than one in the "agreed parameters", the client MAY use an
   LZ77 sliding window with any size to compress messages to send as
   long as the size conforms to the "agreed parameters".  The client-to-
   server "client_max_window_bits" extension parameter is just a hint
   for the server to build an extension negotiation response.

   If the "agreed parameters" contain the "server_max_window_bits"
   extension parameter with a value of w, the server MUST NOT use an
   LZ77 sliding window longer than the w-th power of 2 bytes to compress
   messages to send.

8.2.2.  Decompression

   An endpoint uses the following algorithm to decompress a message.

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

   2.  Decompress the resulting data using DEFLATE.

   If the "agreed parameters" contain the "server_no_context_takeover"
   extension parameter, the client MAY decompress each new message with
   an empty LZ77 sliding window.  Otherwise, the client MUST decompress
   each new message using the LZ77 sliding window used to process the
   last compressed message.



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   If the "agreed parameters" contain the "client_no_context_takeover"
   extension parameter, the server MAY decompress each new message with
   an empty LZ77 sliding window.  Otherwise, the server MUST decompress
   each new message using the LZ77 sliding window used to process the
   last compressed message.  Note that even if the client has included
   the "client_no_context_takeover" extension parameter in its offer,
   the server MUST decompress each new message using the LZ77 sliding
   window used to process the last compressed message if the "agreed
   parameters" don't contain the "client_no_context_takeover" extension
   parameter.  The client-to-server "client_no_context_takeover"
   extension parameter is just a hint for the server to build an
   extension negotiation response.

   If the "agreed parameters" contain the "server_max_window_bits"
   extension parameter with a value of w, the client MAY reduce the size
   of its 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 "agreed parameters" contain the "client_max_window_bits"
   extension parameter with a value of w, the server MAY reduce the size
   of its 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.  Note that
   even if the client has included in its offer the
   "client_max_window_bits" extension parameter with a value smaller
   than one in the "agreed parameters", the client MUST use an LZ77
   sliding window of a size that conforms the "agreed parameters" to
   compress messages to send.  The client-to-server
   "client_max_window_bits" extension parameter is just a hint for the
   server to build an extension negotiation response.

8.2.3.  Examples

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

8.2.3.1.  A message compressed using 1 compressed DEFLATE block

   Suppose that an endpoint sends a text message "Hello".  If the
   endpoint uses 1 compressed DEFLATE block (compressed with fixed
   Huffman code and the "BFINAL" bit is not set) to compress the
   message, the endpoint obtains the compressed data to use for the
   message payload as follows.

   The endpoint compresses "Hello" into 1 compressed DEFLATE block and
   flushes the resulting data into a byte array using an empty DEFLATE
   block with no compression:



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

   By stripping 0x00 0x00 0xff 0xff from the tail end, the endpoint gets
   the data to use for the message payload:

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

   Suppose that the endpoint sends this compressed message without
   fragmentation.  The endpoint builds one frame by putting the whole
   compressed data in the payload data portion of the frame:

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

   The first 2 octets (0xc1 0x07) are the WebSocket frame header (FIN=1,
   RSV1=1, RSV2=0, RSV3=0, opcode=text, MASK=0, Payload length=7).  The
   following figure shows what value is set in each field of the
   WebSocket frame header.

        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-------+-+-------------+
       |F|R|R|R| opcode|M| Payload len |
       |I|S|S|S|       |A|             |
       |N|V|V|V|       |S|             |
       | |1|2|3|       |K|             |
       +-+-+-+-+-------+-+-------------+
       |1|1|0|0|   1   |0|      7      |
       +-+-+-+-+-------+-+-------------+

   Suppose that the endpoint sends the compressed message with
   fragmentation.  The endpoint splits the compressed data into
   fragments and builds frames for each fragment.  For example, if the
   fragments are 3 and 4 octet, the first frame is:

       0x41 0x03 0xf2 0x48 0xcd

   and the second frame is:

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

   Note that the RSV1 bit is set only on the first frame.

8.2.3.2.  Sharing LZ77 Sliding Window

   Suppose that a client has sent a message "Hello" as a compressed
   message and will send the same message "Hello" again as a compressed
   message.




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

   This is the payload of the first message the client has sent.  If the
   "agreed parameters" contain the "client_no_context_takeover"
   extension parameter, the client compresses the payload of the next
   message into the same bytes (if the client uses the same "BTYPE"
   value and "BFINAL" value).  So, the payload of the second message
   will be:

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

   If the "agreed parameters" did not contain the
   "client_no_context_takeover" extension parameter, the client can
   compress the payload of the next message into shorter bytes by
   referencing the history in the LZ77 sliding window.  So, the payload
   of the second message will be:

       0xf2 0x00 0x11 0x00 0x00

   So, 2 bytes are saved in total.

   Note that even if some uncompressed messages (with the RSV1 bit
   unset) are inserted between the two "Hello" messages, they don't
   affect the LZ77 sliding window.

8.2.3.3.  Using a DEFLATE Block with No Compression


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

   This is a frame constituting a text message "Hello" built using a
   DEFLATE block with no compression.  The first 2 octets (0xc1 0x0b)
   are the WebSocket frame header (FIN=1, RSV1=1, RSV2=0, RSV3=0,
   opcode=text, MASK=0, Payload length=7).  Note that the RSV1 bit is
   set for this message (only on the first fragment if the message is
   fragmented) because the RSV1 bit is set when DEFLATE is applied to
   the message, including the case when only DEFLATE blocks with no
   compression are used.  The third to 13th octet consists a payload
   data containing "Hello" compressed using a DEFLATE block with no
   compression.

8.2.3.4.  Using a DEFLATE Block with BFINAL Set to 1

   On platform where the flush method using an empty DEFLATE block with
   no compression is not available, implementors can choose to flush
   data using DEFLATE blocks with "BFINAL" set to 1.

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



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   This is a payload of a message containing "Hello" compressed using a
   DEFLATE block with "BFINAL" set to 1.  The first 7 octets constitute
   a DEFLATE block with "BFINAL" set to 1 and "BTYPE" set to 01
   containing "Hello".  The last 1 octet (0x00) contains the header bits
   with "BFINAL" set to 0 and "BTYPE" set to 00, and 5 padding bits of
   0.  This octet is necessary to allow the payload to be decompressed
   in the same manner as messages flushed using DEFLATE blocks with
   BFINAL unset.

8.2.3.5.  Two DEFLATE Blocks in 1 Message

   Two or more DEFLATE blocks may be used in 1 message.

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

   The first 3 octets (0xf2 0x48 0x05) and the least significant two
   bits of the 4th octet (0x00) constitute one DEFLATE block with
   "BFINAL" set to 0 and "BTYPE" set to 01 containing "He".  The rest of
   the 4th octet contains the header bits with "BFINAL" set to 0 and
   "BTYPE" set to 00, and the 3 padding bits of 0.  Together with the
   following 4 octets (0x00 0x00 0xff 0xff), the header bits constitute
   an empty DEFLATE block with no compression.  A DEFLATE block
   containing "llo" follows the empty DEFLATE block.

8.2.3.6.  Generating an Empty Fragment Manually

   Suppose that an endpoint is sending data with unknown size.  The
   endpoint may encounter the end of data signal from the data source
   when its buffer for uncompressed data is empty.  In such a case, the
   endpoint just needs to send the last fragment with FIN bit set to 1
   and payload set to DEFLATE block(s) which contains 0 byte data.  If
   the compression library being used doesn't generate any data when its
   buffer is empty, an empty uncompressed DEFLATE block can be built
   manually and used for this purpose as follows:

       0x00

   The only octet 0x00 contains the header bits with "BFINAL" set to 0
   and "BTYPE" set to 00, and 5 padding bits of 0.

8.3.  Implementation Notes

   On most common software development platforms, their DEFLATE
   compression library provides a method to align compressed data to
   byte boundaries using an empty DEFLATE block with no compression.
   For example, Zlib [Zlib] does this when "Z_SYNC_FLUSH" is passed to
   the deflate function.




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   Some platforms may provide only methods to output and process
   compressed data with ZLIB header and Adler-32 checksum.  On such
   platforms, developers need to write stub code to remove and
   complement them manually.

   To obtain a useful compression ratio, an LZ77 sliding window size of
   1,024 or more is RECOMMENDED.

   On the direction where context takeover is disallowed, an endpoint
   can easily figure out whether a certain message will be shorter if
   compressed or not..  Otherwise, it's not easy to know whether future
   messages will benefit from having a certain message compressed.
   Implementor may employ some heuristics to determine this.

8.4.  Intermediaries

   When an intermediary forwards a message, the intermediary MAY change
   compression on the messages as far as the resulting sequence of
   messages conforms to the constraints based on the "agreed
   parameters".  For example, an intermediary may decompress a received
   message, unset the "Per-message Compressed" bit and forward it to the
   other peer.  Since such a compression change may affect the LZ77
   sliding window, the intermediary may need to parse and transform the
   following messages, too.



























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9.  Security Considerations

   There is a known exploit for combination of a secure transport
   protocol and history-based compression [CRIME].  Implementors should
   give attention to this point when integrating this extension with
   other extensions or protocols.













































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10.  IANA Considerations

10.1.  Registration of the "permessage-deflate" WebSocket Extension Name

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

   Extension Identifier
      permessage-deflate

   Extension Common Name
      WebSocket Per-message Deflate

   Extension Definition
      This document.

   Known Incompatible Extensions
      None

   The "permessage-deflate" extension name is used in the
   "Sec-WebSocket-Extensions" header in the WebSocket opening handshake
   to negotiate use of the permessage-deflate extension.

10.2.  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 6 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 PMCE or not.







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11.  Acknowledgements

   Special thanks to Patrick McManus who wrote up the initial
   specification of a 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): Adam Rice, Alexey Melnikov,
   Arman Djusupov, Bjoern Hoehrmann, Brian McKelvey, Dario Crivelli,
   Greg Wilkins, Inaki Baz Castillo, Jamie Lokier, Joakim Erdfelt, John
   A. Tamplin, Julian Reschke, Kenichi Ishibashi, Mark Nottingham, Peter
   Thorson, Roberto Peon, Simone Bordet, Tobias Oberstein and Yutaka
   Hirano.  Note that people listed above didn't necessarily endorse the
   end result of this work.




































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12.  References

12.1.  Normative References

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

   [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.

12.2.  Informative References

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

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

   [CRIME]    Rizzo, J. and T. Duong, "The CRIME attack", Ekoparty 2012,
              September 2012.























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Internet-Draft    Compression Extensions for WebSocket          May 2014


Author's Address

   Takeshi Yoshino
   Google, Inc.

   Email: tyoshino@google.com













































Yoshino                 Expires November 14, 2014              [Page 32]


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