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rfc:rfc7692

Internet Engineering Task Force (IETF) T. Yoshino Request for Comments: 7692 Google, Inc. Category: Standards Track December 2015 ISSN: 2070-1721

                Compression Extensions for WebSocket

Abstract

 This document defines 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 WebSocket data messages on a per-message basis using parameters
 negotiated during the opening handshake.  This framework provides a
 general method for applying a compression algorithm to the contents
 of WebSocket messages.  Each compression algorithm has to be defined
 in a document defining the extension by specifying the parameter
 negotiation and the payload transformation algorithm in detail.  This
 document also specifies one specific compression extension using the
 DEFLATE algorithm.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7692.

Yoshino Standards Track [Page 1] RFC 7692 Compression Extensions for WebSocket December 2015

Copyright Notice

 Copyright (c) 2015 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
 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 ........................3
 3. Complementary Terminology .......................................4
 4. WebSocket Per-Message Compression Extension .....................5
 5. Extension Negotiation ...........................................5
    5.1. General Negotiation Flow ...................................9
    5.2. Negotiation Examples .......................................9
 6. Framing ........................................................10
    6.1. Compression ...............................................10
    6.2. Decompression .............................................12
 7. The "permessage-deflate" Extension .............................12
    7.1. Extension Parameters ......................................14
         7.1.1. Context Takeover Control ...........................14
         7.1.2. Limiting the LZ77 Sliding Window Size ..............16
         7.1.3. Examples ...........................................18
    7.2. Message Payload Transformation ............................19
         7.2.1. Compression ........................................19
         7.2.2. Decompression ......................................21
         7.2.3. Examples ...........................................22
    7.3. Implementation Notes ......................................25
 8. Security Considerations ........................................25
 9. IANA Considerations ............................................26
    9.1. Registration of the "permessage-deflate" WebSocket
         Extension Name ............................................26
    9.2. Registration of the "Per-Message Compressed"
         WebSocket Framing Header Bit ..............................26
 10. References ....................................................27
    10.1. Normative References .....................................27
    10.2. Informative References ...................................27
 Acknowledgements ..................................................28
 Author's Address ..................................................28

Yoshino Standards Track [Page 2] RFC 7692 Compression Extensions for WebSocket December 2015

1. Introduction

 This document specifies a framework for adding compression
 functionality to the WebSocket Protocol [RFC6455].  The framework
 specifies how to define WebSocket Per-Message Compression Extensions
 (PMCEs) for a compression algorithm based on the extension concept of
 the WebSocket Protocol specified in Section 9 of [RFC6455].  A
 WebSocket client and a peer WebSocket server negotiate the use of a
 PMCE and determine the parameters required to configure the
 compression algorithm during the WebSocket opening handshake.  The
 client and server can then exchange data messages whose frames
 contain compressed data in the payload data portion.
 This framework only specifies a general method for applying a
 compression algorithm to the contents of WebSocket messages.  Each
 individual PMCE has to be specified in a document describing in
 detail how to negotiate the configuration parameters for the specific
 compression algorithm used by that PMCE and how to transform
 (compress and decompress) data in the payload data portion.
 A WebSocket client may offer multiple PMCEs during the WebSocket
 opening handshake.  A peer WebSocket server receiving the offer may
 choose to accept the preferred PMCE 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 DEFLATE algorithm is widely available on
 various platforms, and its overhead is small.  The extension name of
 this PMCE is "permessage-deflate".  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 achieve a better compression ratio.  For resource-limited devices,
 this extension provides parameters to limit memory usage for
 compression context.

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.

Yoshino Standards Track [Page 3] RFC 7692 Compression Extensions for WebSocket December 2015

 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 Sections 4.1 and 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].

3. Complementary Terminology

 This document defines some terms about WebSocket and WebSocket
 extension mechanisms that are underspecified or not defined at all in
 [RFC6455].
 data message - a message consisting of data frames as defined in
 Section 5.6 of [RFC6455].
 message payload (or payload of a message) - the concatenation of the
 payload data portion of all data frames (see Section 6.2 of
 [RFC6455]) representing a single message.
 next extension in use after extension X - the next extension listed
 after 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 the sender side but is applied right before X to incoming
 data from the underlying transport.
 extension in use preceding extension X - 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 the sender side but is applied
 right after X to incoming data from the underlying transport.
 extension negotiation offer - each element in the "Sec-WebSocket-
 Extensions" header in the client's opening handshake.

Yoshino Standards Track [Page 4] RFC 7692 Compression Extensions for WebSocket December 2015

 extension negotiation response - each element in the "Sec-WebSocket-
 Extensions" header in the server's opening handshake.
 corresponding extension negotiation response for an extension
 negotiation offer - an extension negotiation response that a server
 sends back to the peer client containing the same extension name as
 the offer and meeting the requirements represented by the offer.
 Accepting an extension negotiation offer - 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 - not including a
 corresponding extension negotiation response for the offer in the
 "Sec-WebSocket-Extensions" header in the server's opening handshake.

4. WebSocket Per-Message Compression Extension

 WebSocket 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 referring to this framework MUST define the following:
 o  The extension name of the PMCE and any applicable extension
    parameters that MUST be included in the "Sec-WebSocket-Extensions"
    header during the extension negotiation offer/response.
 o  How to interpret the extension parameters exchanged during the
    opening handshake.
 o  How to transform the payload of a message.
 One PMCE is defined in Section 7 of this document and is registered
 in Section 9.  Other PMCEs may be defined in the future 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.

5. Extension Negotiation

 To offer use of a PMCE, a client MUST include the extension name of
 the PMCE in the "Sec-WebSocket-Extensions" header field of its
 opening handshake of the WebSocket connection.  Extension parameters

Yoshino Standards Track [Page 5] RFC 7692 Compression Extensions for WebSocket December 2015

 are used to specify the PMCE offer in detail.  For example, a client
 lists its preferred configuration parameter values for the
 compression algorithm of the PMCE.  A client may also offer multiple
 PMCE choices to the server by including multiple elements in the
 "Sec-WebSocket-Extensions" header, one for each PMCE offered.  This
 set of elements MAY include multiple PMCEs with the same extension
 name to offer the possibility to use the same algorithm with
 different configuration parameters.  The order of elements is
 important as it specifies the client's preference.  An element
 preceding another element has higher preference.  It is recommended
 that a server accepts PMCEs with higher preference if the server
 supports them.
 A PMCE negotiation offer provides 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.  For example, suppose that a server sends data compressed with
 the DEFLATE algorithm to a client.  The server must keep the original
 bytes of data that it recently compressed and sent to the client.
 The client must keep the result of decompressing the bytes of data
 that it recently received from the server.  The amount of bytes of
 data kept is called the LZ77 window size.  The LZ77 window size of
 the client must not be less than the LZ77 window size of the server.
 In a PMCE negotiation offer, the client MUST inform the server of its
 LZ77 window size so that the server uses an LZ77 window size that is
 not greater than the LZ77 window size of the client.  This
 restriction on the LZ77 window size is an example of a request in a
 PMCE negotiation offer.
 A hint in a PMCE negotiation offer provides information about the
 client's behavior that the server may either safely ignore or refer
 to when the server decides its behavior.  For example, suppose that a
 client sends data compressed with the DEFLATE algorithm to a server.
 The client must keep the original bytes of data that it recently
 compressed and sent to the server.  The server must keep the result
 of decompressing the bytes of data that it recently received from the
 client.  The LZ77 window size of the server must not be less than the
 LZ77 window size of the client.  In a PMCE negotiation offer, the
 client MAY inform the server of the maximum LZ77 window size the
 client can afford so that the server can choose to use an LZ77 window
 size that is not greater than the maximum size of the client.  This
 information is an example of a hint in a PMCE negotiation offer.
 It's waste of memory to use an LZ77 window size greater than the LZ77
 window size the client actually uses.  Using the hint, the server can
 avoid the waste of memory.  Since the hint itself doesn't specify the

Yoshino Standards Track [Page 6] RFC 7692 Compression Extensions for WebSocket December 2015

 constraints on the endpoints, the server must use the "agreed
 parameters" (defined below) to explicitly ask the client not to use
 an LZ77 window size greater than the LZ77 window size of the server.
 To accept the use of an offered PMCE, a server MUST include the
 extension name of the PMCE in the "Sec-WebSocket-Extensions" header
 field of its opening handshake of the WebSocket connection.
 Extension parameters represent the detailed configuration parameters
 for the PMCE to use.  These extension parameters and their values are
 called "agreed parameters".  The element MUST represent a PMCE that
 is fully supported by the server.  The contents of the element don't
 need to be exactly the same as those of the received extension
 negotiation offers.  For example, suppose that a server received a
 PMCE extension negotiation offer with an extension parameter "X"
 indicating that the client can enable an optional feature named X.
 The server may accept the PMCE offer with an element without the
 extension parameter "X", meaning that the server chose not to enable
 the feature X.  In this case, the offer contains the extension
 parameter "X", but the "agreed parameters" don't contain the
 extension parameter "X".
 "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 exactly which
 PMCE extension negotiation offer has been accepted.
 For example, if a client sends an extension negotiation offer that
 includes a parameter "enable_compression" and another without this
 parameter, the server accepts the former and informs the client by
 sending back an element that includes parameter(s) acknowledging
 "enable_compression".  The name of the acknowledging parameter
 doesn't need to be exactly the same as the offer.  For example, two
 parameters, "enable_strong_compression" and
 "enable_weak_compression", may be defined as acknowledging parameters
 for "enable_compression".
 Compression features can be applied differently for each direction.
 For such features, the acknowledging parameter and the parameter in
 the reverse direction must be chosen to distinguish them.  For
 example, in order to make parameters distinguishable, a "server_"
 prefix can be added to parameters affecting data sent from a server,
 and a "client_" prefix can be added to parameters affecting data sent
 from a client.

Yoshino Standards Track [Page 7] RFC 7692 Compression Extensions for WebSocket December 2015

 A server MUST NOT accept a PMCE extension negotiation offer together
 with another extension if the PMCE will conflict with the extension
 on their 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 the output of
 the extension and any of the following conditions applies to the
 extension:
 o  The extension requires the boundary of frames to be preserved
    between the output from the extension at the sender and the 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 receives a response accepting a PMCE extension
 negotiation offer together with such an extension MUST _Fail the
 WebSocket Connection_.
 A server declining all offered PMCEs MUST NOT include 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.

Yoshino Standards Track [Page 8] RFC 7692 Compression Extensions for WebSocket December 2015

5.1. General Negotiation Flow

 This section describes a general negotiation flow.  How to handle
 parameters in detail must be specified in the document specifying the
 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  Limitations concerning the client's compression functionality
 The peer server makes a determination of its behavior based on these
 parameters.  If the server can and wants to proceed with this PMCE
 enabled, the server responds to the client with parameters
 identifying the following:
 o  Requests about how the client compresses data
 o  How the server will compress data
 Based on these parameters received from the server, the client
 determines its behavior and if it can and wants to proceed with this
 PMCE enabled.  Otherwise, the client starts the closing handshake
 with close code 1010.

5.2. 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 the
 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"

Yoshino Standards Track [Page 9] RFC 7692 Compression Extensions for WebSocket December 2015

 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

6. Framing

 PMCEs operate only on data 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 receiving 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 receiving 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 which extensions were
     negotiated.

Yoshino Standards Track [Page 10] RFC 7692 Compression Extensions for WebSocket December 2015

 2.  Process the compressed data as follows:
  • If this PMCE is the last extension to process outgoing

messages, build frame(s) using the compressed data instead of

        the original data for the message payload, set the "Per-
        Message Compressed" bit of the first frame, and then send the
        frame(s) as described in Section 6.1 of [RFC6455].
  • Otherwise, pass the transformed message payload and modified

header values, including the "Per-Message Compressed" bit

        value set to 1, to the next extension after 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
 form of an uncompressed message.
 1.  Process the original data as follows:
  • If this PMCE is the last extension to process outgoing

messages, build frame(s) using the original data for the

        payload data portion as is, unset the "Per-Message Compressed"
        bit of the first frame, and then send the frame(s) as
        described in Section 6.1 of [RFC6455].
  • Otherwise, pass the message payload and header values to the

next extension after 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
 receiving such a frame MUST _Fail the WebSocket Connection_.
 PMCEs do not change the opcode field.  The opcode of the first frame
 of a compressed 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.

Yoshino Standards Track [Page 11] RFC 7692 Compression Extensions for WebSocket December 2015

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 which extensions were negotiated.
 2.  Decompress the concatenation by following the decompression
     procedure of the PMCE.
 3.  Process the decompressed message as follows:
  • 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.
 1.  Process the received message as follows:
  • If this PMCE is the last 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.

7. The "permessage-deflate" Extension

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

Yoshino Standards Track [Page 12] RFC 7692 Compression Extensions for WebSocket December 2015

 This section uses the term "byte" with the same meaning as used in
 [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 by the client
 to configure its compressor and by the server to configure its
 decompressor.  The extension parameters with the "server_" prefix are
 used by the server to configure its compressor and by the client to
 configure its decompressor.  All 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 are met:
 o  The negotiation offer contains an extension parameter not defined
    for use in an offer.
 o  The negotiation offer contains an extension parameter with an
    invalid value.
 o  The negotiation offer contains 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
 conditions:

Yoshino Standards Track [Page 13] RFC 7692 Compression Extensions for WebSocket December 2015

 o  The negotiation response contains an extension parameter not
    defined for use in a response.
 o  The negotiation response contains an extension parameter with an
    invalid value.
 o  The negotiation response contains multiple extension parameters
    with the same name.
 o  The client does not support the configuration that the response
    represents.
 The term "LZ77 sliding window" [LZ77] 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 a
 match with the following input.
 The term "use context takeover" used in this section means that the
 same LZ77 sliding window used by the endpoint to build frames of the
 previous sent message is reused to build frames of the next message
 to be sent.

7.1. Extension Parameters

7.1.1. Context Takeover Control

7.1.1.1. The "server_no_context_takeover" Extension Parameter

 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 takeover.  If the peer server doesn't use context
 takeover, the client doesn't need to reserve memory to retain the
 LZ77 sliding window between messages.
 Absence of this extension parameter in an extension negotiation offer
 indicates that the client can decompress a message that the server
 built using context takeover.
 A server accepts an extension negotiation offer that includes the
 "server_no_context_takeover" extension parameter by including the
 "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.

Yoshino Standards Track [Page 14] RFC 7692 Compression Extensions for WebSocket December 2015

 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 include the "server_no_context_takeover" extension
 parameter.

7.1.1.2. The "client_no_context_takeover" Extension Parameter

 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 doesn'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 takeover.
 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 the LZ77 sliding window
 unnecessarily by including the "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 takeover.  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 decompress messages built by
 the client using context takeover.
 A client MUST support the "client_no_context_takeover" extension
 parameter in an extension negotiation response.

Yoshino Standards Track [Page 15] RFC 7692 Compression Extensions for WebSocket December 2015

7.1.2. Limiting the LZ77 Sliding Window Size

7.1.2.1. The "server_max_window_bits" Extension Parameter

 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 will use
 to compress messages.  If the peer server uses a small LZ77 sliding
 window to compress messages, the client can reduce the memory needed
 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.

7.1.2.2. The "client_max_window_bits" Extension Parameter

 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

Yoshino Standards Track [Page 16] RFC 7692 Compression Extensions for WebSocket December 2015

 inclusive indicating the base-2 logarithm of the LZ77 sliding window
 size.  If a value is specified for this parameter, the value MUST
 conform to the ABNF below.
     client-max-window-bits = 1*DIGIT
 By including this parameter in an offer, a client informs the peer
 server 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 doesn't include the
 "client_max_window_bits" extension parameter in the corresponding
 extension negotiation response with a value greater than the one in
 the extension negotiation offer or if the server doesn't include the
 extension parameter at all, the client is not going to use an 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 the 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.

Yoshino Standards Track [Page 17] RFC 7692 Compression Extensions for WebSocket December 2015

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

7.1.3. Examples

 The simplest "Sec-WebSocket-Extensions" header in a client's opening
 handshake to offer use of the "permessage-deflate" extension looks
 like this:
     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 that includes
 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" extension is the same:
     Sec-WebSocket-Extensions: permessage-deflate
 The following extension negotiation offer sent by a client is asking
 the server to use an LZ77 sliding window with a 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 fall back 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

Yoshino Standards Track [Page 18] RFC 7692 Compression Extensions for WebSocket December 2015

 The server can accept "permessage-deflate" by picking any 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
 response as follows:
     Sec-WebSocket-Extensions: permessage-deflate

7.2. Message Payload Transformation

7.2.1. 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 part of the payload is
 available, a fragment can be built by compressing the available data

Yoshino Standards Track [Page 19] RFC 7692 Compression Extensions for WebSocket December 2015

 and choosing the 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.
 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.

Yoshino Standards Track [Page 20] RFC 7692 Compression Extensions for WebSocket December 2015

7.2.2. 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.
 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.

Yoshino Standards Track [Page 21] RFC 7692 Compression Extensions for WebSocket December 2015

7.2.3. Examples

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

7.2.3.1. A Message Compressed Using One Compressed DEFLATE Block

 Suppose that an endpoint sends a text message "Hello".  If the
 endpoint uses one compressed DEFLATE block (compressed with fixed
 Huffman code and the "BFINAL" bit 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 one compressed DEFLATE block and
 flushes the resulting data into a byte array using an empty DEFLATE
 block with no compression:
     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 all of the
 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      |
     +-+-+-+-+-------+-+-------------+

Yoshino Standards Track [Page 22] RFC 7692 Compression Extensions for WebSocket December 2015

 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 octets, 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.

7.2.3.2. 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.
     0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
 The above is the payload of the first message that 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 fewer 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.

Yoshino Standards Track [Page 23] RFC 7692 Compression Extensions for WebSocket December 2015

7.2.3.3. Using a DEFLATE Block with No Compression

 A DEFLATE block with no compression may be used.
     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 3rd to 13th octets consist of the payload
 data containing "Hello" compressed using a DEFLATE block with no
 compression.

7.2.3.4. Using a DEFLATE Block with "BFINAL" Set to 1

 On platforms on which 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
 This is the 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.

7.2.3.5. Two DEFLATE Blocks in One Message

 Two or more DEFLATE blocks may be used in one 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.

Yoshino Standards Track [Page 24] RFC 7692 Compression Extensions for WebSocket December 2015

7.2.3.6. Generating an Empty Fragment

 Suppose that an endpoint is sending data of 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 the FIN bit set to
 1 and the payload set to the DEFLATE block(s), which contains 0 bytes
 of 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 and used for this purpose as follows:
     0x00
 The single octet 0x00 contains the header bits with "BFINAL" set to 0
 and "BTYPE" set to 00, and 5 padding bits of 0.

7.3. Implementation Notes

 On most common software development platforms, the DEFLATE
 compression library provides a method for aligning 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.
 Some platforms may only provide methods to output and process
 compressed data with a zlib header and an Adler-32 checksum.  On such
 platforms, developers need to write stub code to remove and
 complement the zlib and Adler-32 checksum by themselves.
 To obtain a useful compression ratio, an LZ77 sliding window size of
 1,024 or more is RECOMMENDED.
 If a side disallows context takeover, its endpoint can easily figure
 out whether or not a certain message will be shorter if compressed.
 Otherwise, it's not easy to know whether future messages will benefit
 from having a certain message compressed.  Implementors may employ
 some heuristics to determine this.

8. Security Considerations

 There is a known exploit when history-based compression is combined
 with a secure transport [CRIME].  Implementors should pay attention
 to this point when integrating this extension with other extensions
 or protocols.

Yoshino Standards Track [Page 25] RFC 7692 Compression Extensions for WebSocket December 2015

9. IANA Considerations

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

 IANA has registered the following WebSocket extension name in the
 "WebSocket Extension Name Registry" defined in [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.

9.2. Registration of the "Per-Message Compressed" WebSocket Framing

    Header Bit
 IANA has registered the following WebSocket framing header bit in the
 "WebSocket Framing Header Bits Registry" defined in [RFC6455].
 Value
    RSV1
 Description
    The "Per-Message Compressed" bit, which indicates whether or not
    the message is compressed.  RSV1 is set for compressed messages
    and unset for uncompressed messages.
 Reference
    Section 6 of this document.
 The "Per-Message Compressed" framing header bit is used on the first
 fragment of data messages to indicate whether the payload of the
 message is compressed by the PMCE or not.

Yoshino Standards Track [Page 26] RFC 7692 Compression Extensions for WebSocket December 2015

10. References

10.1. Normative References

 [CRIME]    Rizzo, J. and T. Duong, "The CRIME attack", EKOparty
            Security Conference, September 2012.
 [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,
            DOI 10.1109/TIT.1977.1055714, May 1977,
            <https://www.cs.duke.edu/courses/spring03/cps296.5/papers/
            ziv_lempel_1977_universal_algorithm.pdf>.
 [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
            version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
            <http://www.rfc-editor.org/info/rfc1951>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.
 [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
            RFC 6455, DOI 10.17487/RFC6455, December 2011,
            <http://www.rfc-editor.org/info/rfc6455>.

10.2. Informative References

 [RFC1979]  Woods, J., "PPP Deflate Protocol", RFC 1979,
            DOI 10.17487/RFC1979, August 1996,
            <http://www.rfc-editor.org/info/rfc1979>.
 [zlib]     Gailly, J. and M. Adler, "zlib", <http://www.zlib.net/>.

Yoshino Standards Track [Page 27] RFC 7692 Compression Extensions for WebSocket December 2015

Acknowledgements

 Special thanks to Patrick McManus who wrote up the initial
 specification of a DEFLATE-based compression extension for the
 WebSocket Protocol, which I referred to when writing this
 specification.
 Thanks to the following people who participated in discussions on the
 HyBi WG and contributed ideas and/or provided detailed reviews (the
 list is likely incomplete): Adam Rice, Alexander Philippou, 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, Salvatore Loreto, Simone
 Bordet, Tobias Oberstein, and Yutaka Hirano.  Note that the people
 listed above didn't necessarily endorse the end result of this work.

Author's Address

 Takeshi Yoshino
 Google, Inc.
 Email: tyoshino@google.com

Yoshino Standards Track [Page 28]

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