Compression Extensions for WebSocket
draft-ietf-hybi-permessage-compression-09

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HyBi Working Group                                            T. Yoshino
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                          April 25, 2013
Expires: October 27, 2013

                  Compression Extensions for WebSocket
               draft-ietf-hybi-permessage-compression-09

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
   compression 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
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 27, 2013.

Copyright Notice

   Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . .  8
   6.  Framing  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Compression  . . . . . . . . . . . . . . . . . . . . . . .  9
     6.2.  Decompression  . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  permessage-deflate extension . . . . . . . . . . . . . . . . . 13
     8.1.  Method Parameters  . . . . . . . . . . . . . . . . . . . . 14
       8.1.1.  Context Takeover Control . . . . . . . . . . . . . . . 14
       8.1.2.  Limiting the LZ77 sliding window size  . . . . . . . . 15
       8.1.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 15
     8.2.  Message Payload Transformation . . . . . . . . . . . . . . 16
       8.2.1.  Compression  . . . . . . . . . . . . . . . . . . . . . 16
       8.2.2.  Decompression  . . . . . . . . . . . . . . . . . . . . 18
       8.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . 18
     8.3.  Implementation Notes . . . . . . . . . . . . . . . . . . . 21
     8.4.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     10.1. Registration of the "permessage-deflate" WebSocket
           Extension Name . . . . . . . . . . . . . . . . . . . . . . 24
     10.2. Registration of the "Per-message Compressed" WebSocket
           Framing Header Bit . . . . . . . . . . . . . . . . . . . . 24
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     12.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27

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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
   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 the PPP Deflate Protocol
   [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 Argumented 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.

   Non-control message means a message consists of non-control frames.

   Message payload (or payload of a message) means concatenation of the
   payload data portion of all frames consisting the message.

   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.  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.

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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 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 content to put in the "Sec-WebSocket-Extensions" header.  The
      content includes the extension name of the PMCE and any applicable
      extension parameters.

   o  How to interpret 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.

   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 offers.  For example, an
   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.

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

   A server MUST NOT accept a PMCE 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:

   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 per-frame attribute.

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

   A server declines all offered PMCEs by not including any element with

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   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 to exchange messages, using the message payload
   transformation (compressing and decompressing) procedure of the PMCE
   returned by the server.

5.1.  Negotiation Examples

   The followings 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"

   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 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 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 as
       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 as 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 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 direct input
       from 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 next to the PMCE.  If the extension expects frames
       as 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 next to the
   PMCE as-is.  If the extension expects frames as 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 the messages.
   Such a change must not be made if the new combination of extensions
   after the change doesn't conform to the constraints of the
   extensions.  The elements in the "Sec-WebSocket-Extensions" for the
   PMCE in the opening handshakes with the connected client and server
   must be altered by the intermediary accordingly to match the new
   combination of extensions.

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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".

   For an offer for this extension, the following 3 extension parameters
   are defined.

   o  "s2c_no_context_takeover"

   o  "s2c_max_window_bits"

   o  "c2s_max_window_bits"

   For a response for this extension, the following 4 extension
   parameters are defined.

   o  "s2c_no_context_takeover"

   o  "c2s_no_context_takeover"

   o  "s2c_max_window_bits"

   o  "c2s_max_window_bits"

   A server MUST decline a "permessage-deflate" offer 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.

   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 server accepted
   a "permessage-deflate" offer with a response meeting any of the
   following condition:

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   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 the response
      represents.

   The term "LZ77 sliding window" used in this section means the buffer
   storing recently processed input.  The LZ77 algorithm searches the
   buffer for match with the next input.

8.1.  Method Parameters

8.1.1.  Context Takeover Control

   A client MAY attach the "s2c_no_context_takeover" extension
   parameter.  The "s2c_no_context_takeover" extension parameter has no
   value.  Using this extension parameter, a client can prevent the peer
   server from using the same LZ77 sliding window it used to build
   frames of the last sent message to build frames of the next message.
   When the peer server doesn't use the same LZ77 sliding window to
   compress multiple messages, the client doesn't need to reserve memory
   to retain the LZ77 sliding window in between messages.  A server
   accepts an offer with this extension parameter by including the
   "s2c_no_context_takeover" extension parameter in the response.  A
   server which accepted an offer with this extension parameter MUST
   start the compression of each message with an empty LZ77 sliding
   window.

   It is RECOMMENDED that servers implement the
   "s2c_no_context_takeover" parameter.

   A server MAY attach the "c2s_no_context_takeover" extension
   parameter.  The "c2s_no_context_takeover" extension parameter has no
   value.  Using this extension parameter, a server can prevent the peer
   client from using the same LZ77 sliding window it used to build
   frames of the last sent message to build frames for the next message.
   This can reduce the amount of memory that the server has to reserve
   for the connection, in the same way the "s2c_no_context_takeover"
   extension does for the client.  A client that received this parameter
   MUST start the compression of each message with an empty LZ77 sliding
   window.

   It is RECOMMENDED that clients implement the
   "c2s_no_context_takeover" parameter.

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8.1.2.  Limiting the LZ77 sliding window size

   A client MAY attach the "s2c_max_window_bits" extension parameter to
   limit the LZ77 sliding window size that the server uses to compress
   messages.  This extension parameter MUST have a decimal integer value
   without leading zeroes between 8 to 15 inclusive indicating the
   base-2 logarithm of the LZ77 sliding window size.

       s2c_max_window_bits = 1*DIGIT

   A server will decline an offer with this extension parameter if the
   server doesn't support the extension parameter.  A server accepts an
   offer with this extension parameter by including the extension
   parameter with the same value as the offer in the response.  If a
   server accepts an offer with this extension parameter, the server
   MUST NOT use LZ77 sliding window size greater than the size specified
   by the extension parameter to compress messages.

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

   If a server receives an offer with the "c2s_max_window_bits"
   extension parameter, the server MAY include the "c2s_max_window_bits"
   parameter in the response to the offer to limit the LZ77 sliding
   window size that the client uses to build messages.  If a server
   received and accepts an offer without the "c2s_max_window_bits"
   extension parameter, the server MUST NOT include the
   "c2s_max_window_bits" extension parameter in the response to the
   offer.  The "c2s_max_window_bits" extension parameter in the server's
   opening handshake MUST have a decimal integer value without leading
   zeroes between 8 to 15 inclusive indicating the base-2 logarithm of
   the LZ77 sliding window size.

       c2s_max_window_bits = 1*DIGIT

   If a client received the "c2s_max_window_bits" extension parameter,
   the client MUST NOT use an LZ77 sliding window size greater than the
   size specified by the extension parameter to build messages.

8.1.3.  Example

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

       Sec-WebSocket-Extensions: permessage-deflate

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   Since the "c2s_max_window_bits" extension parameter is not specified,
   the server may not accept the offer with the "c2s_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 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 can accept the "c2s_max_window_bits" extension parameter.

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

   This offer might be rejected by the server because the server doesn't
   support the "s2c_max_window_bits" extension parameter.  This is fine
   if the client cannot support a larger sliding window size, but if the
   client wants to fallback to the "permessage-deflate" without the
   "s2c_max_window_bits" option, the client should offer the fallback
   option explicitly like this:

       Sec-WebSocket-Extensions:
           permessage-deflate;
           c2s_max_window_bits; s2c_max_window_bits=10,
           permessage-deflate;
           c2s_max_window_bits

   This example offers two configurations so that the server can accept
   permessage-deflate by picking a supported one.  To accept the first
   option, the server might send back, for example:

       Sec-WebSocket-Extensions:
           permessage-deflate; s2c_max_window_bits=10

   And to accept the second option, the server might send back, for
   example:

       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.

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   2.  If the resulting data does not end with an empty DEFLATE block
       with no compression (the "BTYPE" bits is 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.

   In 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 byte boundary, an endpoint adds minimal padding bits of 0 to
      make it end at byte boundary.  The next DEFLATE block follows the
      padded data if any.

   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 "c2s_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.

   If the "agreed parameters" contain the "s2c_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 "c2s_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.

   If the "agreed parameters" contain the "s2c_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.

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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 "s2c_no_context_takeover"
   extension parameter, the client MAY start decompressing each new
   message with an empty LZ77 sliding window.  Otherwise, the client
   MUST take over the LZ77 sliding window used to process the last
   compressed message.

   If the "agreed parameters" contain the "c2s_no_context_takeover"
   extension parameter, the server MAY start decompressing each new
   message with an empty LZ77 sliding window.  Otherwise, the server
   MUST take over the LZ77 sliding window used to process the last
   compressed message.

   If the "agreed parameters" contain the "s2c_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 "c2s_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.

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

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   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 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

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   message.

       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 "c2s_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
   "c2s_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

   Note that even if some uncompressed messages (with the RSV1 bit
   unset) are inserted between the two "Hello" messages, they will make
   no difference to 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" compressed 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 avaiable, 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.

8.3.  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.

   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 conform 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.

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   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 and Simone Bordet.  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

   [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

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

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

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

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

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Author's Address

   Takeshi Yoshino
   Google, Inc.

   Email: tyoshino@google.com

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