HyBi Working Group T. Yoshino
Internet-Draft Google, Inc.
Intended status: Standards Track January 14, 2014
Expires: July 18, 2014
Compression Extensions for WebSocket
draft-ietf-hybi-permessage-compression-16
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
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 18, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conformance Requirements and Terminology . . . . . . . . . . . 4
3. Complementary Terminology . . . . . . . . . . . . . . . . . . 5
4. WebSocket Per-message Compression Extension . . . . . . . . . 6
5. Extension Negotiation . . . . . . . . . . . . . . . . . . . . 7
5.1. Negotiation Examples . . . . . . . . . . . . . . . . . . . 9
6. Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Compression . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Decompression . . . . . . . . . . . . . . . . . . . . . . 12
7. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 14
8. permessage-deflate extension . . . . . . . . . . . . . . . . . 15
8.1. Method Parameters . . . . . . . . . . . . . . . . . . . . 16
8.1.1. Context Takeover Control . . . . . . . . . . . . . . . 16
8.1.2. Limiting the LZ77 sliding window size . . . . . . . . 18
8.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Message Payload Transformation . . . . . . . . . . . . . . 21
8.2.1. Compression . . . . . . . . . . . . . . . . . . . . . 21
8.2.2. Decompression . . . . . . . . . . . . . . . . . . . . 22
8.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 23
8.3. Implementation Notes . . . . . . . . . . . . . . . . . . . 26
8.4. Intermediaries . . . . . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10.1. Registration of the "permessage-deflate" WebSocket
Extension Name . . . . . . . . . . . . . . . . . . . . . . 29
10.2. Registration of the "Per-message Compressed" WebSocket
Framing Header Bit . . . . . . . . . . . . . . . . . . . . 29
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
12.2. Informative References . . . . . . . . . . . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32
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1. Introduction
This document specifies a framework to add compression functionality
to the WebSocket Protocol [RFC6455]. This framework specifies how to
define WebSocket Per-message Compression Extensions (PMCEs)
individually for various compression algorithms based on the
extension concept of the WebSocket Protocol specified in Section 9 of
[RFC6455]. A WebSocket client and a peer WebSocket server negotiate
use of a PMCE and determine parameters to configure the compression
algorithm during the WebSocket opening handshake. The client and
server can then exchange non-control messages using frames with
compressed data in the payload data portion. This framework
specifies a general method to apply a compression algorithm to the
contents of WebSocket messages. A document specifying an individual
PMCE describes how to negotiate configuration parameters for the
compression algorithm and how to transform (compress and decompress)
data in the payload data portion in detail. A WebSocket client may
offer multiple PMCEs during the WebSocket opening handshake. A peer
WebSocket server received those offers may choose and accept
preferred one or decline all of them. PMCEs use the RSV1 bit of the
WebSocket frame header to indicate whether a message is compressed or
not, so that an endpoint can choose not to compress messages with
incompressible contents.
This document also specifies one specific PMCE based on the DEFLATE
[RFC1951] algorithm. The extension name of the PMCE is "permessage-
deflate". We chose DEFLATE since it's widely available as a library
on various platforms and the overhead is small. To align the end of
compressed data to an octet boundary, this extension uses the
algorithm described in Section 2.1 of [RFC1979]. Endpoints can take
over the LZ77 sliding window [LZ77] used to build frames for previous
messages to get better compression ratio. For resource-limited
devices, this extension provides parameters to limit memory usage for
compression context.
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2. Conformance Requirements and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these
steps") are to be interpreted with the meaning of the key word
("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.
Conformance requirements phrased as algorithms or specific steps can
be implemented in any manner, so long as the end result is
equivalent. In particular, the algorithms defined in this
specification are intended to be easy to understand and are not
intended to be performant.
This document references the procedure to _Fail the WebSocket
Connection_. This procedure is defined in Section 7.1.7 of
[RFC6455].
This document references the event that _The WebSocket Connection is
Established_ and the event that _A WebSocket Message Has Been
Received_. These events are defined in Section 4.1 and Section 6.2,
respectively, of [RFC6455].
This document uses the 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 that consists of non-control
frames as defined in Section 5.6 of [RFC6455].
Message payload (or payload of a message) means concatenation of the
payload data portion of all frames representing a single message, as
well as how /data/ is formed from in Section 6.2 of [RFC6455].
Extension in use next to extension X means the extension listed next
to X in the "Sec-WebSocket-Extensions" header in the server's opening
handshake as defined in Section 9.1 of [RFC6455]. Such an extension
is applied to outgoing data from the application right after X on
sender side but applied right before X to incoming data from the
underlying transport.
Extension negotiation offer means each element in the
"Sec-WebSocket-Extensions" header in the client's opening handshake.
Extension negotiation response means each element in the
"Sec-WebSocket-Extensions" header in the server's opening handshake.
Accept an extension negotiation offer means including a corresponding
extension negotiation response in the "Sec-WebSocket-Extensions"
header in the server's opening handshake.
Decline an extension negotiation offer means not including a
corresponding extension negotiation response in the
"Sec-WebSocket-Extensions" header in the server's opening handshake.
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4. WebSocket Per-message Compression Extension
WebSocket Per-message Compression Extensions (PMCEs) are extensions
to the WebSocket Protocol enabling compression functionality. PMCEs
are built based on 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. The order of elements means the client's preference. An
element precedes another element has higher preference. It is
RECOMMENDED that a server accepts PMCEs with higher preference if the
server supports it.
A PMCE negotiation offer informs requests and/or hints to the server.
A request in a PMCE negotiation offer indicates constraints on the
server's behavior that must be satisfied if the server accepts the
offer. A hint in a PMCE negotiation offer indicates information
about the client's behavior that the server may either safely ignore
or refer to when the server decides its behavior.
To accept use of an offered PMCE, a server includes a
"Sec-WebSocket-Extensions" header element with the extension name of
the PMCE in the "Sec-WebSocket-Extensions" header in the server's
opening handshake of the WebSocket connection. Extension parameters
in the element represent the configuration parameters of the PMCE to
use in detail. We call these extension parameters and their values
"agreed parameters". The element MUST represent a PMCE that is fully
supported by the server. The contents of the element doesn't need to
be exactly the same as one of the received extension negotiation
offers. For example, an extension negotiation offer with an
extension parameter "X" indicating availability of the feature X may
be accepted with an element without the extension parameter meaning
that the server declined use of the feature.
"Agreed parameters" MUST represent how the requests and hints in the
client's extension negotiation offer have been handled in addition to
the server's requests and hints on the client's behavior, so that the
client can configure its behavior without identifying which PMCE
extension negotiation offer has been accepted.
For example, if a client sends an extension negotiation offer
including a parameter "enable_compression" and another without the
parameter, the server accepts the former by sending back an element
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including a parameter that acknowledges "enable_compression". The
name of the acknowledging parameter doesn't need to be the same as
the offer.
General negotiation flow will be like the followings. How to handle
parameters in detail will be specified in the specifications for each
PMCEs.
A client makes an offer including parameters identifying the
followings:
o Hints about how the client is planning to compress data
o Requests about how the server compresses data
o Limitation of the client's compression functionality
The peer server uses these parameters, makes a determination of its
behavior based on them if it can and wants to proceed with this PMCE
enabled, and responds to the client with parameters identifying the
followings:
o Requests about how the client compresses data
o How the server will compress data
The client uses these parameters from the server make a determination
of its behavior based on them if it can and wants to proceed.
Otherwise, the client starts closing handshake with close code 1010.
There can be compression features that can be applied separately for
each direction. For such features, the acknowledging parameter and
the parameter for the reverse direction must be chosen to be
distinguishable from each other. For example, we can add "server_"
prefix to parameters affecting data sent from a server and "client_"
prefix to ones affecting data sent from a client to make them
distinguishable.
A server MUST NOT accept a PMCE extension negotiation offer together
with another extension if the PMCE will conflict with the extension
on use of the RSV1 bit. A client that receives a response accepting
a PMCE extension negotiation offer together with such an extension
MUST _Fail the WebSocket Connection_.
A server MUST NOT accept a PMCE extension negotiation offer together
with another extension if the PMCE will be applied to output of the
extension and any of the following conditions applies to the
extension:
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o The extension requires boundary of fragments to be preserved
between output from the extension at the sender and input to the
extension at the receiver.
o The extension uses the "Extension data" field or any of the
reserved bits on the WebSocket header as per-frame attribute.
A client receiving a response accepting a PMCE extension negotiation
offer together with such an extension MUST _Fail the WebSocket
Connection_.
A server declines all offered PMCEs by not including any element with
PMCE names. If a server responds with no PMCE element in the
"Sec-WebSocket-Extensions" header, both endpoints proceed without
Per-message Compression once _the WebSocket Connection is
established_.
If a server gives an invalid response, such as accepting a PMCE that
the client did not offer, the client MUST _Fail the WebSocket
Connection_.
If a server responds with a valid PMCE element in the
"Sec-WebSocket-Extensions" header and _the WebSocket Connection is
established_, both endpoints MUST use the algorithm described in
Section 6 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"
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o Offer the permessage-foo as first choice and the permessage-bar as
a fallback plan.
permessage-foo, permessage-bar
o Offer the permessage-foo with a parameter use_y which enables a
feature y as first choice, and the permessage-foo without the
use_y parameter as a fallback plan.
permessage-foo; use_y, permessage-foo
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6. Framing
PMCEs operate only on non-control messages.
This document allocates the RSV1 bit of the WebSocket header for
PMCEs, and calls the bit the "Per-message Compressed" bit. On a
WebSocket connection where a PMCE is in use, this bit indicates
whether a message is compressed or not.
A message with the "Per-message Compressed" bit set on the first
fragment of the message is called a "compressed message". Frames of
a compressed message have compressed data in the payload data
portion. An endpoint 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 if
the intermediary meets all of the following conditions:
o The intermediary understands that Per-message Compression.
o The intermediary can read all of proxied data including the
opening handshake request, opening handshake response, and
messages.
o The intermediary can alter the proxied data before forwarding them
accordingly to conform to the constraints of the new combination
of extensions. For example, if a PMCE is removed from messages,
the corresponding element in the "Sec-WebSocket-Extensions" in the
opening handshake response must also be removed.
Otherwise, the intermediary MUST NOT make any change on the proxied
data.
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8. permessage-deflate extension
This section specifies a specific PMCE called "permessage-deflate".
It compresses the payload of a message using the DEFLATE algorithm
[RFC1951] and the byte boundary aligning method introduced in
[RFC1979].
This section uses the term "byte" with the same meaning as RFC1951,
i.e. 8 bits stored or transmitted as a unit (same as an octet).
The registered extension name for this extension is
"permessage-deflate".
Four extension parameters are defined for permessage-deflate to help
endpoints manage per-connection resource usage.
o "server_no_context_takeover"
o "client_no_context_takeover"
o "server_max_window_bits"
o "client_max_window_bits"
These represent two methods (no_context_takeover and max_window_bits)
of constraining memory usage that may be applied independently to
either direction of WebSocket traffic. The extension parameters with
the client_ prefix are used to negotiate DEFLATE parameters to
control compression on messages sent by a client and received by a
server. The client refers to parameters with the client_ prefix to
configure its compressor, while the server refers to them to
configure its decompressor. The extension parameters with the
server_ prefix are used to negotiate DEFLATE parameters to control
compression on messages sent by a server and received by a client.
The server refers to parameters with the server_ prefix to configure
its compressor, while the client refers to them to configure its
decompressor. All four parameters are defined for both a client's
extension negotiation offer and a server's extension negotiation
response.
A server MUST decline an extension negotiation offer for this
extension if any of the following conditions is met:
o The offer has any extension parameter not defined for use in an
offer.
o The offer has any extension parameter with an invalid value.
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o The offer has multiple extension parameters with the same name.
o The server doesn't support the offered configuration.
A client MUST _Fail the WebSocket Connection_ if the peer server
accepted an extension negotiation offer for this extension with an
extension negotiation response meeting any of the following
condition:
o The response has any extension parameter not defined for use in a
response.
o The response has any extension parameter with an invalid value.
o The response has multiple extension parameters with the same name.
o The client doesn't support the configuration 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
8.1.1.1. server_no_context_takeover
A client MAY include the "server_no_context_takeover" extension
parameter in an extension negotiation offer. This extension
parameter has no value. By including this extension parameter in an
extension negotiation offer, a client prevents the peer server from
using the same LZ77 sliding window it used to build frames of the
last sent message to build frames of the next message. If 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.
Absence of this extension parameter in an extension negotiation offer
indicates that the client can receive a message which the server has
built using the same LZ77 sliding window the server used to build the
last sent message.
A server accepts an extension negotiation offer including the
"server_no_context_takeover" extension parameter by including the
"server_no_context_takeover" extension parameter in an extension
negotiation response. The "server_no_context_takeover" extension
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parameter in an extension negotiation response has no value.
It is RECOMMENDED that a server supports the
"server_no_context_takeover" extension parameter in an extension
negotiation offer.
A server MAY include the "server_no_context_takeover" extension
parameter in an extension negotiation response even if the extension
negotiation offer being accepted by the extension negotiation
response didn't have the "server_no_context_takeover" extension
parameter.
8.1.1.2. client_no_context_takeover
A client MAY include the "client_no_context_takeover" extension
parameter in an extension negotiation offer. This extension
parameter has no value. By including this extension parameter in an
extension negotiation offer, a client informs the peer server a hint
that even if the server won't include the
"client_no_context_takeover" extension parameter in the extension
negotiation response to the offer, the client is not going to use the
same LZ77 sliding window it used to build frames of the last sent
message to build frames of the next message.
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 it or use it to
avoid taking over an LZ77 sliding window unnecessarily by including
"client_no_context_takeover" extension parameter in the 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 the same LZ77 sliding window it used to build
frames of the last sent message to build frames for the next message.
This reduces the amount of memory that the server has to reserve for
the connection.
Absence of this extension parameter in an extension negotiation
response indicates that the server can receive a message which the
client has built using the same LZ77 sliding window the client used
to build the last sent message.
A client MUST support the "client_no_context_takeover" extension
parameter in an extension negotiation response.
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8.1.2. Limiting the LZ77 sliding window size
8.1.2.1. server_max_window_bits
A client MAY include the "server_max_window_bits" extension parameter
in an extension negotiation offer. This extension parameter has a
decimal integer value without leading zeroes between 8 to 15
inclusive indicating the base-2 logarithm of the LZ77 sliding window
size and MUST conform to the ABNF below.
server_max_window_bits = 1*DIGIT
By including this parameter in an extension negotiation offer, a
client limits the LZ77 sliding window size that the server uses to
compress messages. If the peer server uses small LZ77 sliding window
to compress messages, the client can reduce the memory for the LZ77
sliding window.
A server declines an extension negotiation offer with this extension
parameter if the server doesn't support it.
Absence of this extension 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 extension
parameter by including the "server_max_window_bits" extension
parameter in the extension negotiation response with the same or
smaller value as the extension negotiation 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 extension negotiation
response didn't include the "server_max_window_bits" extension
parameter.
8.1.2.2. client_max_window_bits
A client MAY include the "client_max_window_bits" extension parameter
in an extension negotiation offer. This extension parameter has no
value or a decimal integer value without leading zeroes between 8 to
15 inclusive indicating the base-2 logarithm of the LZ77 sliding
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window size. If a value is specified for this extension parameter,
the value MUST conform to the ABNF below.
client_max_window_bits = 1*DIGIT
By including this extension parameter in an extension negotiation
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 including an extension
parameter value. If the "client_max_window_bits" extension parameter
in an extension negotiation offer has a value, the extension
parameter also informs the peer server a hint that even if the server
won't includes the "client_max_window_bits" extension parameter in an
extension negotiation response with a value greater than one in the
extension negotiation offer or the server doesn't include the
extension parameter at all, the client is not going to use LZ77
sliding window size greater than the size specified by the value in
the extension negotiation offer to compress messages.
If a received extension negotiation offer has the
"client_max_window_bits" extension parameter, the server MAY include
the "client_max_window_bits" extension parameter in the 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
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
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 server MUST NOT
include the "client_max_window_bits" extension parameter in an
extension negotiation response to the offer.
Absence of this extension parameter in an extension negotiation
response indicates that the server can receive messages compressed
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using an LZ77 sliding window of up to 32,768 bytes.
8.1.3. Example
The simplest "Sec-WebSocket-Extensions" header in a client's opening
handshake to offer use of the permessage-deflate is the following:
Sec-WebSocket-Extensions: permessage-deflate
Since the "client_max_window_bits" extension parameter is not
included in this extension negotiation offer, the server MUST NOT
accept the offer with an extension negotiation response including the
"client_max_window_bits" extension parameter. The simplest
"Sec-WebSocket-Extensions" header in a server's opening handshake to
accept use of the permessage-deflate is the same.
The following extension negotiation offer sent by a client is asking
the server to use the LZ77 sliding window size of 1,024 bytes or less
and declaring that the client supports the "client_max_window_bits"
extension parameter in an extension negotiation response.
Sec-WebSocket-Extensions:
permessage-deflate;
client_max_window_bits; server_max_window_bits=10
This extension negotiation offer might be rejected by the server
because the server doesn't support the "server_max_window_bits"
extension parameter in an extension negotiation offer. This is fine
if the client cannot receive messages compressed using a larger
sliding window size, but if the client wants to fallback to the
"permessage-deflate" without the "server_max_window_bits" extension
parameter, the client can offer the fallback option explicitly like
this:
Sec-WebSocket-Extensions:
permessage-deflate;
client_max_window_bits; server_max_window_bits=10,
permessage-deflate;
client_max_window_bits
This extension negotiation offer lists 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; server_max_window_bits=10
And to accept the second option, the server might send back, for
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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.
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 a byte boundary, an endpoint adds minimal padding bits of 0 to
make it end at a byte boundary. The next DEFLATE block follows
the padded data if any.
When the message is going to be fragmented, the concatenation of all
payload of the fragments must be the result of running this
algorithm. Even when only a part of payload is available, a fragment
can be built by compressing the available data and aligning the end
of compressed data at a byte boundary. Note that for non-final
fragments, 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"
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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" doesn'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 a 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 a response.
If the "agreed parameters" contain the "server_max_window_bits"
extension parameter with a value of w, the server MUST NOT use an
LZ77 sliding window longer than the w-th power of 2 bytes to compress
messages to send.
8.2.2. Decompression
An endpoint uses the following algorithm to decompress a message.
1. Append 4 octets of 0x00 0x00 0xff 0xff to the tail end of the
payload of the message.
2. Decompress the resulting data using DEFLATE.
If the "agreed parameters" contain the "server_no_context_takeover"
extension parameter, the client MAY 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.
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If the "agreed parameters" contain the "client_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. Note that even if the client has included the
"client_no_context_takeover" extension parameter in its offer, the
server MUST take over the LZ77 sliding window used to process the
last compressed message if the "agreed parameters" doesn'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 a 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
that 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 a response.
8.2.3. Examples
This section introduces examples of how the permessage-deflate
transforms messages.
8.2.3.1. A message compressed using 1 compressed DEFLATE block
Suppose that an endpoint sends a text message "Hello". If the
endpoint uses 1 compressed DEFLATE block (compressed with fixed
Huffman code and the "BFINAL" bit is not set) to compress the
message, the endpoint obtains the compressed data to use for the
message payload as follows.
The endpoint compresses "Hello" into 1 compressed DEFLATE block and
flushes the resulting data into a byte array using an empty DEFLATE
block with no compression:
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0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00 0x00 0xff 0xff
By stripping 0x00 0x00 0xff 0xff from the tail end, the endpoint gets
the data to use for the message payload:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
Suppose that the endpoint sends this compressed message without
fragmentation. The endpoint builds one frame by putting the whole
compressed data in the payload data portion of the frame:
0xc1 0x07 0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
The first 2 octets (0xc1 0x07) are the WebSocket frame header (FIN=1,
RSV1=1, RSV2=0, RSV3=0, opcode=text, MASK=0, Payload length=7). The
following figure shows what value is set in each field of the
WebSocket frame header.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-------+-+-------------+
|F|R|R|R| opcode|M| Payload len |
|I|S|S|S| |A| |
|N|V|V|V| |S| |
| |1|2|3| |K| |
+-+-+-+-+-------+-+-------------+
|1|1|0|0| 1 |0| 7 |
+-+-+-+-+-------+-+-------------+
Suppose that the endpoint sends the compressed message with
fragmentation. The endpoint splits the compressed data into
fragments and builds frames for each fragment. For example, if the
fragments are 3 and 4 octet, the first frame is:
0x41 0x03 0xf2 0x48 0xcd
and the second frame is:
0x80 0x04 0xc9 0xc9 0x07 0x00
Note that the RSV1 bit is set only on the first frame.
8.2.3.2. Sharing LZ77 Sliding Window
Suppose that a client has sent a message "Hello" as a compressed
message and will send the same message "Hello" again as a compressed
message.
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0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
This is the payload of the first message the client has sent. If the
"agreed parameters" contain the "client_no_context_takeover"
extension parameter, the client compresses the payload of the next
message into the same bytes (if the client uses the same "BTYPE"
value and "BFINAL" value). So, the payload of the second message
will be:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
When concatenated with the first message:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00 0xf2 0x48 0xcd 0xc9 0xc9
0x07 0x00
If the "agreed parameters" did not contain the
"client_no_context_takeover" extension parameter, the client can
compress the payload of the next message into shorter bytes by
referencing the history in the LZ77 sliding window. So, the payload
of the second message will be:
0xf2 0x00 0x11 0x00 0x00
When concatenated with the first message:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00 0xf2 0x00 0x11 0x00 0x00
So, 2 bytes were saved in total.
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
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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
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.
Some platforms may provide only methods to output and process
compressed data with ZLIB header and Adler-32 checksum. On such
platforms, developers need to write stub code to remove and
complement them manually.
To obtain a useful compression ratio, an LZ77 sliding window size of
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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.
Since such a compression change may affect the LZ77 sliding window,
the intermediary may need to parse and transform the following
messages, too.
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9. Security Considerations
There is a known exploit for combination of a secure transport
protocol and history-based compression [CRIME]. Implementors should
give attention to this point when integrating this extension with
other extensions or protocols.
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10. IANA Considerations
10.1. Registration of the "permessage-deflate" WebSocket Extension Name
This section describes a WebSocket extension name registration in the
WebSocket Extension Name Registry [RFC6455].
Extension Identifier
permessage-deflate
Extension Common Name
WebSocket Per-message Deflate
Extension Definition
This document.
Known Incompatible Extensions
None
The "permessage-deflate" extension name is used in the
"Sec-WebSocket-Extensions" header in the WebSocket opening handshake
to negotiate use of the permessage-deflate extension.
10.2. Registration of the "Per-message Compressed" WebSocket Framing
Header Bit
This section describes a WebSocket framing header bit registration in
the WebSocket Framing Header Bits Registry [RFC6455].
Header Bit
RSV1
Common Name
Per-message Compressed
Meaning
The message is compressed or not.
Reference
Section 6 of this document.
The "Per-message Compressed" framing header bit is used on the first
fragment of non-control messages to indicate whether the payload of
the message is compressed by the PMCE or not.
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11. Acknowledgements
Special thanks to Patrick McManus who wrote up the initial
specification of a DEFLATE-based compression extension for the
WebSocket Protocol to which I referred to write this specification.
Thank you to the following people who participated in discussions on
the HyBi WG and contributed ideas and/or provided detailed reviews
(the list is likely to be incomplete): Adam Rice, Alexey Melnikov,
Arman Djusupov, Bjoern Hoehrmann, Brian McKelvey, Dario Crivelli,
Greg Wilkins, Inaki Baz Castillo, Jamie Lokier, Joakim Erdfelt, John
A. Tamplin, Julian Reschke, Kenichi Ishibashi, Mark Nottingham, Peter
Thorson, Roberto Peon, Simone Bordet and Tobias Oberstein. Note that
people listed above didn't necessarily endorse the end result of this
work.
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12. References
12.1. Normative References
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LZ77] Ziv, J. and A. Lempel, "A Universal Algorithm for
Sequential Data Compression", IEEE Transactions on
Information Theory, Vol. 23, No. 3, pp. 337-343.
12.2. Informative References
[RFC1979] Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.
[Zlib] Gailly, J. and M. Adler, "Zlib", <http://zlib.net/>.
[CRIME] Rizzo, J. and T. Duong, "The CRIME attack", Ekoparty 2012,
September 2012.
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Author's Address
Takeshi Yoshino
Google, Inc.
Email: tyoshino@google.com
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