HyBi Working Group T. Yoshino
Internet-Draft Google, Inc.
Intended status: Standards Track November 12, 2014
Expires: May 16, 2015
Compression Extensions for WebSocket
draft-ietf-hybi-permessage-compression-19
Abstract
This document specifies a framework for creating WebSocket extensions
that add compression functionality to the WebSocket Protocol. An
extension based on this framework compresses the payload data portion
of non-control WebSocket messages on a per-message basis using
parameters negotiated during the opening handshake. This framework
provides a general method to apply a compression algorithm to the
contents of WebSocket messages. For each compression algorithm, an
extension is defined by specifying parameter negotiation and payload
transformation algorithm in detail. This document also specifies one
specific compression extension using the DEFLATE algorithm.
Please send feedback to the hybi@ietf.org mailing list.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 16, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conformance Requirements and Terminology . . . . . . . . . . . 4
3. Complementary Terminology . . . . . . . . . . . . . . . . . . 5
4. WebSocket Per-message Compression Extension . . . . . . . . . 6
5. Extension Negotiation . . . . . . . . . . . . . . . . . . . . 7
5.1. Negotiation Examples . . . . . . . . . . . . . . . . . . . 9
6. Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Compression . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Decompression . . . . . . . . . . . . . . . . . . . . . . 12
7. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 14
8. permessage-deflate extension . . . . . . . . . . . . . . . . . 15
8.1. Method Parameters . . . . . . . . . . . . . . . . . . . . 16
8.1.1. Context Takeover Control . . . . . . . . . . . . . . . 16
8.1.2. Limiting the LZ77 sliding window size . . . . . . . . 18
8.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Message Payload Transformation . . . . . . . . . . . . . . 21
8.2.1. Compression . . . . . . . . . . . . . . . . . . . . . 21
8.2.2. Decompression . . . . . . . . . . . . . . . . . . . . 22
8.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 23
8.3. Implementation Notes . . . . . . . . . . . . . . . . . . . 26
8.4. Intermediaries . . . . . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10.1. Registration of the "permessage-deflate" WebSocket
Extension Name . . . . . . . . . . . . . . . . . . . . . . 29
10.2. Registration of the "Per-message Compressed" WebSocket
Framing Header Bit . . . . . . . . . . . . . . . . . . . . 29
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
12.2. Informative References . . . . . . . . . . . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32
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1. Introduction
This document specifies a framework to add compression functionality
to the WebSocket Protocol [RFC6455]. This framework specifies how to
define WebSocket Per-message Compression Extensions (PMCEs)
individually for various compression algorithms based on the
extension concept of the WebSocket Protocol specified in Section 9 of
[RFC6455]. A WebSocket client and a peer WebSocket server negotiate
use of a PMCE and determine parameters to configure the compression
algorithm during the WebSocket opening handshake. The client and
server can then exchange non-control messages using frames with
compressed data in the payload data portion.
This framework only specifies a general method to apply a compression
algorithm to the contents of WebSocket messages. A document
specifying an individual PMCE describes how to negotiate
configuration parameters for the compression algorithm and how to
transform (compress and decompress) data in the payload data portion
in detail.
A WebSocket client may offer multiple PMCEs during the WebSocket
opening handshake. A peer WebSocket server received those offers may
choose and accept preferred one or decline all of them. PMCEs use
the RSV1 bit of the WebSocket frame header to indicate whether a
message is compressed or not, so that an endpoint can choose not to
compress messages with incompressible contents.
This document also specifies one specific PMCE based on the DEFLATE
[RFC1951] algorithm. The extension name of the PMCE is "permessage-
deflate". We chose DEFLATE since it's widely available as a library
on various platforms and the overhead is small. To align the end of
compressed data to an octet boundary, this extension uses the
algorithm described in Section 2.1 of [RFC1979]. Endpoints can take
over the LZ77 sliding window [LZ77] used to build frames for previous
messages to get better compression ratio. For resource-limited
devices, this extension provides parameters to limit memory usage for
compression context.
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2. Conformance Requirements and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these
steps") are to be interpreted with the meaning of the key word
("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.
Conformance requirements phrased as algorithms or specific steps can
be implemented in any manner, so long as the end result is
equivalent. In particular, the algorithms defined in this
specification are intended to be easy to understand and are not
intended to be performant.
This document references the procedure to _Fail the WebSocket
Connection_. This procedure is defined in Section 7.1.7 of
[RFC6455].
This document references the event that _The WebSocket Connection is
Established_ and the event that _A WebSocket Message Has Been
Received_. These events are defined in Section 4.1 and Section 6.2,
respectively, of [RFC6455].
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234]. The DIGIT (decimal 0-9) rule is included by reference, as
defined in the Appendix B.1 of [RFC5234].
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3. Complementary Terminology
This document defines some terms about WebSocket and WebSocket
Extension mechanism that are underspecified or not defined at all in
[RFC6455]. This terminology is effective only in this document and
any other documents that refer to this section.
"A non-control message" means a message that consists of non-control
frames as defined in Section 5.6 of [RFC6455].
"A message payload (or payload of a message)" means concatenation of
the payload data portion of all frames representing a single message,
as well as how /data/ is formed from in Section 6.2 of [RFC6455].
"An extension in use next to extension X" means the extension listed
next to X in the "Sec-WebSocket-Extensions" header in the server's
opening handshake as defined in Section 9.1 of [RFC6455]. Such an
extension is applied to outgoing data from the application right
after X on sender side but applied right before X to incoming data
from the underlying transport.
"An extension in use preceding extension X" means the extension
listed right before X in the "Sec-WebSocket-Extensions" header in the
server's opening handshake. Such an extension is applied to outgoing
data from the application right before X on sender side but applied
right after X to incoming data from the underlying transport.
"An extension negotiation offer" means each element in the
"Sec-WebSocket-Extensions" header in the client's opening handshake.
"An extension negotiation response" means each element in the
"Sec-WebSocket-Extensions" header in the server's opening handshake.
"A corresponding extension negotiation response for an extension
negotiation offer" means an extension negotiation response a server
sends back to the peer client that contains the same extension name
as the offer and meets the requirements represented by the offer.
"Accepting an extension negotiation offer" means including a
corresponding extension negotiation response for the offer in the
"Sec-WebSocket-Extensions" header in the server's opening handshake.
"Declining an extension negotiation offer" means not including a
corresponding extension negotiation response for the offer in the
"Sec-WebSocket-Extensions" header in the server's opening handshake.
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4. WebSocket Per-message Compression Extension
WebSocket Per-message Compression Extensions (PMCEs) are extensions
to the WebSocket Protocol enabling compression functionality. PMCEs
are built based on the extension concept of the WebSocket Protocol
specified in Section 9 of [RFC6455]. PMCEs are individually defined
for each compression algorithm to be implemented, and are registered
in the WebSocket Extension Name Registry created in Section 11.4 of
[RFC6455]. Each PMCE refers to this framework and defines the
following:
o The contents of its extension negotiation offer/response to
include in the "Sec-WebSocket-Extensions" header. The contents
include the extension name of the PMCE and any applicable
extension parameters.
o How to interpret the extension parameters exchanged during the
opening handshake
o How to transform the payload of a message.
One such extension is defined in Section 8 of this document and is
registered in Section 10. Other PMCEs may be defined in other
documents.
Section 5 describes the basic extension negotiation process.
Section 6 describes how to apply the compression algorithm with
negotiated parameters to the contents of WebSocket messages.
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5. Extension Negotiation
To offer use of a PMCE, a client includes a
"Sec-WebSocket-Extensions" header element with the extension name of
the PMCE in the "Sec-WebSocket-Extensions" header in the client's
opening handshake of the WebSocket connection. Extension parameters
in the element represent the PMCE offer in detail. For example, a
client lists preferred configuration parameter values for the
compression algorithm of the PMCE. A client offers multiple PMCE
choices to the server by including multiple elements in the
"Sec-WebSocket-Extensions" header, one for each PMCE offered. The
set of elements MAY include multiple PMCEs with the same extension
name to offer use of the same algorithm with different configuration
parameters. The order of elements means the client's preference. An
element precedes another element has higher preference. It is
RECOMMENDED that a server accepts PMCEs with higher preference if the
server supports it.
A PMCE negotiation offer informs requests and/or hints to the server.
A request in a PMCE negotiation offer indicates constraints on the
server's behavior that must be satisfied if the server accepts the
offer. A hint in a PMCE negotiation offer indicates information
about the client's behavior that the server may either safely ignore
or refer to when the server decides its behavior.
To accept use of an offered PMCE, a server includes a
"Sec-WebSocket-Extensions" header element with the extension name of
the PMCE in the "Sec-WebSocket-Extensions" header in the server's
opening handshake of the WebSocket connection. Extension parameters
in the element represent the configuration parameters of the PMCE to
use in detail. We call these extension parameters and their values
"agreed parameters". The element MUST represent a PMCE that is fully
supported by the server. The contents of the element doesn't need to
be exactly the same as one of the received extension negotiation
offers. For example, an extension negotiation offer with an
extension parameter "X" indicating availability of the feature X may
be accepted with an element without the extension parameter meaning
that the server declined use of the feature.
"Agreed parameters" MUST represent how the requests and hints in the
client's extension negotiation offer have been handled in addition to
the server's requests and hints on the client's behavior, so that the
client can configure its behavior without identifying which PMCE
extension negotiation offer has been accepted.
For example, if a client sends an extension negotiation offer
including a parameter "enable_compression" and another without the
parameter, the server accepts the former and tell that to the client
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by sending back an element including a parameter that acknowledges
"enable_compression". The name of the acknowledging parameter
doesn't need to be the same as the offer.
General negotiation flow will be like the following. How to handle
parameters in detail will be specified in the specifications for each
PMCE.
A client makes an offer including parameters identifying the
following:
o Hints about how the client is planning to compress data
o Requests about how the server compresses data
o Limitation of the client's compression functionality
The peer server makes a determination of its behavior based on these
parameters if it can and wants to proceed with this PMCE enabled, and
responds to the client with parameters identifying the following:
o Requests about how the client compresses data
o How the server will compress data
The client makes a determination of its behavior based on these
parameters from the server if it can and wants to proceed with this
PMCE enabled. Otherwise, the client starts closing handshake with
close code 1010.
There can be compression features that can be applied separately for
each direction. For such features, the acknowledging parameter and
the parameter for the reverse direction must be chosen to be
distinguishable from each other. For example, we can add "server_"
prefix to parameters affecting data sent from a server and "client_"
prefix to ones affecting data sent from a client to make them
distinguishable.
A server MUST NOT accept a PMCE extension negotiation offer together
with another extension if the PMCE will conflict with the extension
on use of the RSV1 bit. A client that received a response accepting
a PMCE extension negotiation offer together with such an extension
MUST _Fail the WebSocket Connection_.
A server MUST NOT accept a PMCE extension negotiation offer together
with another extension if the PMCE will be applied to output of the
extension and any of the following conditions applies to the
extension:
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o The extension requires boundary of fragments to be preserved
between output from the extension at the sender and input to the
extension at the receiver.
o The extension uses the "Extension data" field or any of the
reserved bits on the WebSocket header as a per-frame attribute.
A client that received a response accepting a PMCE extension
negotiation offer together with such an extension MUST _Fail the
WebSocket Connection_.
A server declines all offered PMCEs by not including any element with
PMCE names. If a server responds with no PMCE element in the
"Sec-WebSocket-Extensions" header, both endpoints proceed without
Per-message Compression once _the WebSocket Connection is
established_.
If a server gives an invalid response, such as accepting a PMCE that
the client did not offer, the client MUST _Fail the WebSocket
Connection_.
If a server responds with a valid PMCE element in the
"Sec-WebSocket-Extensions" header and _the WebSocket Connection is
established_, both endpoints MUST use the algorithm described in
Section 6 and the message payload transformation (compressing and
decompressing) procedure of the PMCE configured with the "agreed
parameters" returned by the server to exchange messages.
5.1. Negotiation Examples
The following are example values for the "Sec-WebSocket-Extensions"
header offering PMCEs. permessage-foo and permessage-bar in the
examples are hypothetical extension names of PMCEs for compression
algorithm foo and bar.
o Offer the permessage-foo.
permessage-foo
o Offer the permessage-foo with a parameter x with a value of 10.
permessage-foo; x=10
The value MAY be quoted.
permessage-foo; x="10"
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o Offer the permessage-foo as first choice and the permessage-bar as
a fallback plan.
permessage-foo, permessage-bar
o Offer the permessage-foo with a parameter use_y which enables a
feature y as first choice, and the permessage-foo without the
use_y parameter as a fallback plan.
permessage-foo; use_y, permessage-foo
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6. Framing
PMCEs operate only on non-control messages.
This document allocates the RSV1 bit of the WebSocket header for
PMCEs, and calls the bit the "Per-message Compressed" bit. On a
WebSocket connection where a PMCE is in use, this bit indicates
whether a message is compressed or not.
A message with the "Per-message Compressed" bit set on the first
fragment of the message is called a "compressed message". Frames of
a compressed message have compressed data in the payload data
portion. An endpoint received a compressed message decompresses the
concatenation of the compressed data of the frames of the message by
following the decompression procedure specified by the PMCE in use.
The endpoint uses the bytes corresponding to the application data
portion in this decompressed data for the _A WebSocket Message Has
Been Received_ event instead of the received data as-is.
A message with the "Per-message Compressed" bit unset on the first
fragment of the message is called an "uncompressed message". Frames
of an uncompressed message have uncompressed original data as-is in
the payload data portion. An endpoint received an uncompressed
message uses the concatenation of the application data portion of the
frames of the message as-is for the _A WebSocket Message Has Been
Received_ event.
6.1. Compression
An endpoint MUST use the following algorithm to send a message in the
form of a compressed message.
1. Compress the message payload of the original message by following
the compression procedure of the PMCE. The original message may
be input from the application layer or output of another
WebSocket extension depending on what extensions were negotiated.
2. If this PMCE is the last extension to process outgoing messages,
build frame(s) by using the compressed data instead of the
original data for the message payload, and setting the
"Per-message Compressed" bit of the first frame, then send the
frame(s) as described in Section 6.1 of RFC6455. Otherwise, pass
the transformed message payload and modified header values
including "Per-message Compressed" bit value set to 1 to the
extension next to the PMCE. If the extension expects frames for
input, build a frame for the message and pass it.
An endpoint MUST use the following algorithm to send a message in the
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form of an uncompressed message. If this PMCE is the last extension
to process outgoing messages, build frame(s) by using the original
data for the payload data portion as-is and unsetting the
"Per-message Compressed" bit of the first frame, then send the
frame(s) as described in Section 6.1 of RFC6455. Otherwise, pass the
message payload and header values to the extension next to the PMCE
as-is. If the extension expects frames for input, build a frame for
the message and pass it.
An endpoint MUST NOT set the "Per-message Compressed" bit of control
frames and non-first fragments of a data message. An endpoint
received such a frame MUST _Fail the WebSocket Connection_.
PMCEs don't change the opcode field. The opcode of the first frame
of a compress message indicates the opcode of the original message.
The payload data portion in frames generated by a PMCE is not subject
to the constraints for the original data type. For example, the
concatenation of the output data corresponding to the application
data portion of frames of a compressed text message is not required
to be valid UTF-8. At the receiver, the payload data portion after
decompression is subject to the constraints for the original data
type again.
6.2. Decompression
An endpoint MUST use the following algorithm to receive a message in
the form of a compressed message.
1. Concatenate the payload data portion of the received frames of
the compressed message. The received frames may be direct input
from the underlying transport or output of another WebSocket
extension depending on what extensions were negotiated.
2. Decompress the concatenation by following the decompression
procedure of the PMCE.
3. If this is the last extension to process incoming messages,
deliver the _A WebSocket Message Has Been Received_ event to the
application layer with the decompressed message payload and
header values including the "Per-message Compressed" bit unset to
0. Otherwise, pass the decompressed message payload and header
values including the "Per-message Compressed" bit unset to 0 to
the extension preceding the PMCE. If the extension expects
frames for input, build a frame for the message and pass it.
An endpoint MUST use the following algorithm to receive a message in
the form of an uncompressed message. If this PMCE is the last
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extension to process incoming messages, deliver the _A WebSocket
Message Has Been Received_ event to the application layer with the
received message payload and header values as-is. Otherwise, pass
the message payload and header values to the extension preceding the
PMCE as-is. If the extension expects frames for input, build a frame
for the message and pass it.
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7. Intermediaries
When an intermediary proxies a WebSocket connection, the intermediary
MAY add, change or remove Per-message Compression on proxied messages
if the intermediary meets all of the following requirements:
o The intermediary understands that Per-message Compression.
o The intermediary can read all data of the proxied WebSocket
connection including the opening handshake request, opening
handshake response, and messages.
o The intermediary can alter the proxied data before forwarding them
accordingly to conform to the constraints of the new combination
of extensions. For example, if Per-message Compression is removed
from messages, the corresponding element in the
"Sec-WebSocket-Extensions" in the opening handshake response which
enabled the Per-message Compression must also be removed.
Otherwise, the intermediary MUST NOT add, change or remove Per-
message Compression on proxied messages.
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8. permessage-deflate extension
This section specifies a specific PMCE called "permessage-deflate".
It compresses the payload of a message using the DEFLATE algorithm
[RFC1951] and the byte boundary aligning method introduced in
[RFC1979].
This section uses the term "byte" with the same meaning as RFC1951,
i.e. 8 bits stored or transmitted as a unit (same as an octet).
The registered extension name for this extension is
"permessage-deflate".
Four extension parameters are defined for permessage-deflate to help
endpoints manage per-connection resource usage.
o "server_no_context_takeover"
o "client_no_context_takeover"
o "server_max_window_bits"
o "client_max_window_bits"
These parameters enable two methods (no_context_takeover and
max_window_bits) of constraining memory usage that may be applied
independently to either direction of WebSocket traffic. The
extension parameters with the "client_" prefix are used to negotiate
DEFLATE parameters to control compression on messages sent by a
client and received by a server. The client refers to parameters
with the "client_" prefix to configure its compressor, while the
server refers to them to configure its decompressor. The extension
parameters with the "server_" prefix are used to negotiate DEFLATE
parameters to control compression on messages sent by a server and
received by a client. The server refers to parameters with the
"server_" prefix to configure its compressor, while the client refers
to them to configure its decompressor. All of these four parameters
are defined for both a client's extension negotiation offer and a
server's extension negotiation response.
A server MUST decline an extension negotiation offer for this
extension if any of the following conditions is met:
o The offer has any extension parameter not defined for use in an
offer.
o The offer has any extension parameter with an invalid value.
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o The offer has multiple extension parameters with the same name.
o The server doesn't support the offered configuration.
A client MUST _Fail the WebSocket Connection_ if the peer server
accepted an extension negotiation offer for this extension with an
extension negotiation response meeting any of the following
condition:
o The response has any extension parameter not defined for use in a
response.
o The response has any extension parameter with an invalid value.
o The response has multiple extension parameters with the same name.
o The client doesn't support the configuration that the response
represents.
The term "LZ77 sliding window" used in this section means the buffer
used by the DEFLATE algorithm to store recently processed input. The
DEFLATE compression algorithm searches the buffer for match with the
next input.
The term "use context take over" used in this section means to use
the same LZ77 sliding window the endpoint used to build frames of the
last sent message to build frames of the next message.
8.1. Method Parameters
8.1.1. Context Takeover Control
8.1.1.1. server_no_context_takeover
A client MAY include the "server_no_context_takeover" extension
parameter in an extension negotiation offer. This extension
parameter has no value. By including this extension parameter in an
extension negotiation offer, a client prevents the peer server from
using context take over. If the peer server doesn't use context take
over, the client doesn't need to reserve memory to retain the LZ77
sliding window in between messages.
Absence of this extension parameter in an extension negotiation offer
indicates that the client can receive a message which the server
built using context take over.
A server accepts an extension negotiation offer including the
"server_no_context_takeover" extension parameter by including the
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"server_no_context_takeover" extension parameter in the corresponding
extension negotiation response to send back to the client. The
"server_no_context_takeover" extension parameter in an extension
negotiation response has no value.
It is RECOMMENDED that a server supports the
"server_no_context_takeover" extension parameter in an extension
negotiation offer.
A server MAY include the "server_no_context_takeover" extension
parameter in an extension negotiation response even if the extension
negotiation offer being accepted by the extension negotiation
response didn't have the "server_no_context_takeover" extension
parameter.
8.1.1.2. client_no_context_takeover
A client MAY include the "client_no_context_takeover" extension
parameter in an extension negotiation offer. This extension
parameter has no value. By including this extension parameter in an
extension negotiation offer, a client informs the peer server of a
hint that even if the server won't include the
"client_no_context_takeover" extension parameter in the corresponding
extension negotiation response to the offer, the client is not going
to use context take over.
A server MAY include the "client_no_context_takeover" extension
parameter in an extension negotiation response. If the received
extension negotiation offer includes the "client_no_context_takeover"
extension parameter, the server may either ignore the parameter or
use the parameter to avoid taking over an LZ77 sliding window
unnecessarily by including "client_no_context_takeover" extension
parameter in the corresponding extension negotiation response to the
offer. The "client_no_context_takeover" extension parameter in an
extension negotiation response has no value. By including the
"client_no_context_takeover" extension parameter in an extension
negotiation response, a server prevents the peer client from using
context take over. This reduces the amount of memory that the server
has to reserve for the connection.
Absence of this extension parameter in an extension negotiation
response indicates that the server can receive messages built by the
client using context take over.
A client MUST support the "client_no_context_takeover" extension
parameter in an extension negotiation response.
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8.1.2. Limiting the LZ77 sliding window size
8.1.2.1. server_max_window_bits
A client MAY include the "server_max_window_bits" extension parameter
in an extension negotiation offer. This parameter has a decimal
integer value without leading zeroes between 8 to 15 inclusive
indicating the base-2 logarithm of the LZ77 sliding window size and
MUST conform to the ABNF below.
server_max_window_bits = 1*DIGIT
By including this parameter in an extension negotiation offer, a
client limits the LZ77 sliding window size that the server uses to
compress messages. If the peer server uses small LZ77 sliding window
to compress messages, the client can reduce the memory for the LZ77
sliding window.
A server declines an extension negotiation offer with this parameter
if the server doesn't support it.
Absence of this parameter in an extension negotiation offer indicates
that the client can receive messages compressed using an LZ77 sliding
window of up to 32,768 bytes.
A server accepts an extension negotiation offer with this parameter
by including the "server_max_window_bits" extension parameter in the
extension negotiation response to send back to the client with the
same or smaller value as the offer. The "server_max_window_bits"
extension parameter in an extension negotiation response has a
decimal integer value without leading zeroes between 8 to 15
inclusive indicating the base-2 logarithm of the LZ77 sliding window
size and MUST conform to the ABNF below.
server_max_window_bits = 1*DIGIT
A server MAY include the "server_max_window_bits" extension parameter
in an extension negotiation response even if the extension
negotiation offer being accepted by the response didn't include the
"server_max_window_bits" extension parameter.
8.1.2.2. client_max_window_bits
A client MAY include the "client_max_window_bits" extension parameter
in an extension negotiation offer. This parameter has no value or a
decimal integer value without leading zeroes between 8 to 15
inclusive indicating the base-2 logarithm of the LZ77 sliding window
size. If a value is specified for this parameter, the value MUST
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conform to the ABNF below.
client_max_window_bits = 1*DIGIT
By including this parameter in an offer, a client informs the peer
server of that the client supports the "client_max_window_bits"
extension parameter in an extension negotiation response, and
optionally a hint by attaching a value to the parameter. If the
"client_max_window_bits" extension parameter in an extension
negotiation offer has a value, the parameter also informs the peer
server of a hint that even if the server won't include the
"client_max_window_bits" extension parameter in the corresponding
extension negotiation response with a value greater than one in the
extension negotiation offer or the server doesn't include the
extension parameter at all, the client is not going to use LZ77
sliding window size greater than the size specified by the value in
the extension negotiation offer to compress messages.
If a received extension negotiation offer has the
"client_max_window_bits" extension parameter, the server MAY include
the "client_max_window_bits" extension parameter in the corresponding
extension negotiation response to the offer. If the
"client_max_window_bits" extension parameter in a received extension
negotiation offer has a value, the server may either ignore this
value or use this value to avoid allocating an unnecessarily big LZ77
sliding window by including the "client_max_window_bits" extension
parameter in the corresponding extension negotiation response to the
offer with a value equal to or smaller than the received value. The
"client_max_window_bits" extension parameter in an extension
negotiation response has a decimal integer value without leading
zeroes between 8 to 15 inclusive indicating the base-2 logarithm of
the LZ77 sliding window size and MUST conform to the ABNF below.
client_max_window_bits = 1*DIGIT
By including this extension parameter in an extension negotiation
response, a server limits the LZ77 sliding window size that the
client uses to compress messages. This reduces the amount of memory
for decompression context that the server has to reserve for the
connection.
If a received extension negotiation offer doesn't have the
"client_max_window_bits" extension parameter, the corresponding
extension negotiation response to the offer MUST NOT include the
"client_max_window_bits" extension parameter.
Absence of this extension parameter in an extension negotiation
response indicates that the server can receive messages compressed
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using an LZ77 sliding window of up to 32,768 bytes.
8.1.3. Example
The simplest "Sec-WebSocket-Extensions" header in a client's opening
handshake to offer use of the permessage-deflate is as follows:
Sec-WebSocket-Extensions: permessage-deflate
Since the "client_max_window_bits" extension parameter is not
included in this extension negotiation offer, the server must not
accept the offer with an extension negotiation response including the
"client_max_window_bits" extension parameter. The simplest
"Sec-WebSocket-Extensions" header in a server's opening handshake to
accept use of the permessage-deflate is the same.
The following extension negotiation offer sent by a client is asking
the server to use the LZ77 sliding window size of 1,024 bytes or less
and declaring that the client supports the "client_max_window_bits"
extension parameter in an extension negotiation response.
Sec-WebSocket-Extensions:
permessage-deflate;
client_max_window_bits; server_max_window_bits=10
This extension negotiation offer might be rejected by the server
because the server doesn't support the "server_max_window_bits"
extension parameter in an extension negotiation offer. This is fine
if the client cannot receive messages compressed using a larger
sliding window size, but if the client just prefers using a small
window but wants to fallback to the "permessage-deflate" without the
"server_max_window_bits" extension parameter, the client can make an
offer with the fallback option like this:
Sec-WebSocket-Extensions:
permessage-deflate;
client_max_window_bits; server_max_window_bits=10,
permessage-deflate;
client_max_window_bits
The server can accept permessage-deflate by picking the supported one
from the listed offers. To accept the first option, for example, the
server may send back a response as follows:
Sec-WebSocket-Extensions:
permessage-deflate; server_max_window_bits=10
To accept the second option, for example, the server may send back a
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response as follows:
Sec-WebSocket-Extensions: permessage-deflate
8.2. Message Payload Transformation
8.2.1. Compression
An endpoint uses the following algorithm to compress a message.
1. Compress all the octets of the payload of the message using
DEFLATE.
2. If the resulting data does not end with an empty DEFLATE block
with no compression (the "BTYPE" bits are set to 00), append an
empty DEFLATE block with no compression to the tail end.
3. Remove 4 octets (that are 0x00 0x00 0xff 0xff) from the tail end.
After this step, the last octet of the compressed data contains
(possibly part of) the DEFLATE header bits with the "BTYPE" bits
set to 00.
When using DEFLATE in the first step above:
o An endpoint MAY use multiple DEFLATE blocks to compress one
message.
o An endpoint MAY use DEFLATE blocks of any type.
o An endpoint MAY use both DEFLATE blocks with the "BFINAL" bit set
to 0 and DEFLATE blocks with the "BFINAL" bit set to 1.
o When any DEFLATE block with the "BFINAL" bit set to 1 doesn't end
at a byte boundary, an endpoint MUST add minimal padding bits of 0
to make it end at a byte boundary. The next DEFLATE block follows
the padded data if any.
An endpoint fragments a compressed message by splitting the result of
running this algorithm. Even when only a part of payload is
available, a fragment can be built by compressing the available data
and choosing block type appropriately so that the end of the
resulting compressed data is aligned at a byte boundary. Note that
for non-final fragments, the removal of 0x00 0x00 0xff 0xff must not
be done.
An endpoint MUST NOT use an LZ77 sliding window longer than 32,768
bytes to compress messages to send.
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If the "agreed parameters" contain the "client_no_context_takeover"
extension parameter, the client MUST start compressing each new
message with an empty LZ77 sliding window. Otherwise, the client MAY
take over the LZ77 sliding window used to build the last compressed
message. Note that even if the client has included the
"client_no_context_takeover" extension parameter in its offer, the
client MAY take over the LZ77 sliding window used to build the last
compressed message if the "agreed parameters" don't contain the
"client_no_context_takeover" extension parameter. The client-to-
server "client_no_context_takeover" extension parameter is just a
hint for the server to build an extension negotiation response.
If the "agreed parameters" contain the "server_no_context_takeover"
extension parameter, the server MUST start compressing each new
message with an empty LZ77 sliding window. Otherwise, the server MAY
take over the LZ77 sliding window used to build the last compressed
message.
If the "agreed parameters" contain the "client_max_window_bits"
extension parameter with a value of w, the client MUST NOT use an
LZ77 sliding window longer than the w-th power of 2 bytes to compress
messages to send. Note that even if the client has included in its
offer the "client_max_window_bits" extension parameter with a value
smaller than one in the "agreed parameters", the client MAY use an
LZ77 sliding window with any size to compress messages to send as
long as the size conforms to the "agreed parameters". The client-to-
server "client_max_window_bits" extension parameter is just a hint
for the server to build an extension negotiation response.
If the "agreed parameters" contain the "server_max_window_bits"
extension parameter with a value of w, the server MUST NOT use an
LZ77 sliding window longer than the w-th power of 2 bytes to compress
messages to send.
8.2.2. Decompression
An endpoint uses the following algorithm to decompress a message.
1. Append 4 octets of 0x00 0x00 0xff 0xff to the tail end of the
payload of the message.
2. Decompress the resulting data using DEFLATE.
If the "agreed parameters" contain the "server_no_context_takeover"
extension parameter, the client MAY decompress each new message with
an empty LZ77 sliding window. Otherwise, the client MUST decompress
each new message using the LZ77 sliding window used to process the
last compressed message.
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If the "agreed parameters" contain the "client_no_context_takeover"
extension parameter, the server MAY decompress each new message with
an empty LZ77 sliding window. Otherwise, the server MUST decompress
each new message using the LZ77 sliding window used to process the
last compressed message. Note that even if the client has included
the "client_no_context_takeover" extension parameter in its offer,
the server MUST decompress each new message using the LZ77 sliding
window used to process the last compressed message if the "agreed
parameters" don't contain the "client_no_context_takeover" extension
parameter. The client-to-server "client_no_context_takeover"
extension parameter is just a hint for the server to build an
extension negotiation response.
If the "agreed parameters" contain the "server_max_window_bits"
extension parameter with a value of w, the client MAY reduce the size
of its LZ77 sliding window to decompress received messages down to
the w-th power of 2 bytes. Otherwise, the client MUST use a 32,768
byte LZ77 sliding window to decompress received messages.
If the "agreed parameters" contain the "client_max_window_bits"
extension parameter with a value of w, the server MAY reduce the size
of its LZ77 sliding window to decompress received messages down to
the w-th power of 2 bytes. Otherwise, the server MUST use a 32,768
byte LZ77 sliding window to decompress received messages. Note that
even if the client has included in its offer the
"client_max_window_bits" extension parameter with a value smaller
than one in the "agreed parameters", the client MUST use an LZ77
sliding window of a size that conforms the "agreed parameters" to
compress messages to send. The client-to-server
"client_max_window_bits" extension parameter is just a hint for the
server to build an extension negotiation response.
8.2.3. Examples
This section introduces examples of how the permessage-deflate
transforms messages.
8.2.3.1. A message compressed using 1 compressed DEFLATE block
Suppose that an endpoint sends a text message "Hello". If the
endpoint uses 1 compressed DEFLATE block (compressed with fixed
Huffman code and the "BFINAL" bit is not set) to compress the
message, the endpoint obtains the compressed data to use for the
message payload as follows.
The endpoint compresses "Hello" into 1 compressed DEFLATE block and
flushes the resulting data into a byte array using an empty DEFLATE
block with no compression:
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0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00 0x00 0xff 0xff
By stripping 0x00 0x00 0xff 0xff from the tail end, the endpoint gets
the data to use for the message payload:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
Suppose that the endpoint sends this compressed message without
fragmentation. The endpoint builds one frame by putting the whole
compressed data in the payload data portion of the frame:
0xc1 0x07 0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
The first 2 octets (0xc1 0x07) are the WebSocket frame header (FIN=1,
RSV1=1, RSV2=0, RSV3=0, opcode=text, MASK=0, Payload length=7). The
following figure shows what value is set in each field of the
WebSocket frame header.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-------+-+-------------+
|F|R|R|R| opcode|M| Payload len |
|I|S|S|S| |A| |
|N|V|V|V| |S| |
| |1|2|3| |K| |
+-+-+-+-+-------+-+-------------+
|1|1|0|0| 1 |0| 7 |
+-+-+-+-+-------+-+-------------+
Suppose that the endpoint sends the compressed message with
fragmentation. The endpoint splits the compressed data into
fragments and builds frames for each fragment. For example, if the
fragments are 3 and 4 octet, the first frame is:
0x41 0x03 0xf2 0x48 0xcd
and the second frame is:
0x80 0x04 0xc9 0xc9 0x07 0x00
Note that the RSV1 bit is set only on the first frame.
8.2.3.2. Sharing LZ77 Sliding Window
Suppose that a client has sent a message "Hello" as a compressed
message and will send the same message "Hello" again as a compressed
message.
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0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
This is the payload of the first message the client has sent. If the
"agreed parameters" contain the "client_no_context_takeover"
extension parameter, the client compresses the payload of the next
message into the same bytes (if the client uses the same "BTYPE"
value and "BFINAL" value). So, the payload of the second message
will be:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
If the "agreed parameters" did not contain the
"client_no_context_takeover" extension parameter, the client can
compress the payload of the next message into shorter bytes by
referencing the history in the LZ77 sliding window. So, the payload
of the second message will be:
0xf2 0x00 0x11 0x00 0x00
So, 2 bytes are saved in total.
Note that even if some uncompressed messages (with the RSV1 bit
unset) are inserted between the two "Hello" messages, they don't
affect the LZ77 sliding window.
8.2.3.3. Using a DEFLATE Block with No Compression
0xc1 0x0b 0x00 0x05 0x00 0xfa 0xff 0x48 0x65 0x6c 0x6c 0x6f 0x00
This is a frame constituting a text message "Hello" built using a
DEFLATE block with no compression. The first 2 octets (0xc1 0x0b)
are the WebSocket frame header (FIN=1, RSV1=1, RSV2=0, RSV3=0,
opcode=text, MASK=0, Payload length=7). Note that the RSV1 bit is
set for this message (only on the first fragment if the message is
fragmented) because the RSV1 bit is set when DEFLATE is applied to
the message, including the case when only DEFLATE blocks with no
compression are used. The third to 13th octet consists a payload
data containing "Hello" compressed using a DEFLATE block with no
compression.
8.2.3.4. Using a DEFLATE Block with BFINAL Set to 1
On platform where the flush method using an empty DEFLATE block with
no compression is not available, implementors can choose to flush
data using DEFLATE blocks with "BFINAL" set to 1.
0xf3 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00
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This is a payload of a message containing "Hello" compressed using a
DEFLATE block with "BFINAL" set to 1. The first 7 octets constitute
a DEFLATE block with "BFINAL" set to 1 and "BTYPE" set to 01
containing "Hello". The last 1 octet (0x00) contains the header bits
with "BFINAL" set to 0 and "BTYPE" set to 00, and 5 padding bits of
0. This octet is necessary to allow the payload to be decompressed
in the same manner as messages flushed using DEFLATE blocks with
BFINAL unset.
8.2.3.5. Two DEFLATE Blocks in 1 Message
Two or more DEFLATE blocks may be used in 1 message.
0xf2 0x48 0x05 0x00 0x00 0x00 0xff 0xff 0xca 0xc9 0xc9 0x07 0x00
The first 3 octets (0xf2 0x48 0x05) and the least significant two
bits of the 4th octet (0x00) constitute one DEFLATE block with
"BFINAL" set to 0 and "BTYPE" set to 01 containing "He". The rest of
the 4th octet contains the header bits with "BFINAL" set to 0 and
"BTYPE" set to 00, and the 3 padding bits of 0. Together with the
following 4 octets (0x00 0x00 0xff 0xff), the header bits constitute
an empty DEFLATE block with no compression. A DEFLATE block
containing "llo" follows the empty DEFLATE block.
8.2.3.6. Generating an Empty Fragment Manually
Suppose that an endpoint is sending data with unknown size. The
endpoint may encounter the end of data signal from the data source
when its buffer for uncompressed data is empty. In such a case, the
endpoint just needs to send the last fragment with FIN bit set to 1
and payload set to DEFLATE block(s) which contains 0 byte data. If
the compression library being used doesn't generate any data when its
buffer is empty, an empty uncompressed DEFLATE block can be built
manually and used for this purpose as follows:
0x00
The only octet 0x00 contains the header bits with "BFINAL" set to 0
and "BTYPE" set to 00, and 5 padding bits of 0.
8.3. Implementation Notes
On most common software development platforms, their DEFLATE
compression library provides a method to align compressed data to
byte boundaries using an empty DEFLATE block with no compression.
For example, Zlib [Zlib] does this when "Z_SYNC_FLUSH" is passed to
the deflate function.
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Some platforms may provide only methods to output and process
compressed data with ZLIB header and Adler-32 checksum. On such
platforms, developers need to write stub code to remove and
complement them manually.
To obtain a useful compression ratio, an LZ77 sliding window size of
1,024 or more is RECOMMENDED.
On the direction where context takeover is disallowed, an endpoint
can easily figure out whether a certain message will be shorter if
compressed or not.. Otherwise, it's not easy to know whether future
messages will benefit from having a certain message compressed.
Implementor may employ some heuristics to determine this.
8.4. Intermediaries
When an intermediary forwards a message, the intermediary MAY change
compression on the messages as far as the resulting sequence of
messages conforms to the constraints based on the "agreed
parameters". For example, an intermediary may decompress a received
message, unset the "Per-message Compressed" bit and forward it to the
other peer. Since such a compression change may affect the LZ77
sliding window, the intermediary may need to parse and transform the
following messages, too.
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9. Security Considerations
There is a known exploit for combination of a secure transport
protocol and history-based compression [CRIME]. Implementors should
give attention to this point when integrating this extension with
other extensions or protocols.
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10. IANA Considerations
10.1. Registration of the "permessage-deflate" WebSocket Extension Name
This section describes a WebSocket extension name registration in the
WebSocket Extension Name Registry [RFC6455].
Extension Identifier
permessage-deflate
Extension Common Name
WebSocket Per-message Deflate
Extension Definition
This document.
Known Incompatible Extensions
None
The "permessage-deflate" extension name is used in the
"Sec-WebSocket-Extensions" header in the WebSocket opening handshake
to negotiate use of the permessage-deflate extension.
10.2. Registration of the "Per-message Compressed" WebSocket Framing
Header Bit
This section describes a WebSocket framing header bit registration in
the WebSocket Framing Header Bits Registry [RFC6455].
Header Bit
RSV1
Common Name
Per-message Compressed
Meaning
The message is compressed or not.
Reference
Section 6 of this document.
The "Per-message Compressed" framing header bit is used on the first
fragment of non-control messages to indicate whether the payload of
the message is compressed by the PMCE or not.
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11. Acknowledgements
Special thanks to Patrick McManus who wrote up the initial
specification of a DEFLATE-based compression extension for the
WebSocket Protocol to which I referred to write this specification.
Thank you to the following people who participated in discussions on
the HyBi WG and contributed ideas and/or provided detailed reviews
(the list is likely to be incomplete): Adam Rice, Alexey Melnikov,
Arman Djusupov, Bjoern Hoehrmann, Brian McKelvey, Dario Crivelli,
Greg Wilkins, Inaki Baz Castillo, Jamie Lokier, Joakim Erdfelt, John
A. Tamplin, Julian Reschke, Kenichi Ishibashi, Mark Nottingham, Peter
Thorson, Roberto Peon, Simone Bordet, Tobias Oberstein and Yutaka
Hirano. Note that people listed above didn't necessarily endorse the
end result of this work.
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12. References
12.1. Normative References
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LZ77] Ziv, J. and A. Lempel, "A Universal Algorithm for
Sequential Data Compression", IEEE Transactions on
Information Theory, Vol. 23, No. 3, pp. 337-343.
12.2. Informative References
[RFC1979] Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.
[Zlib] Gailly, J. and M. Adler, "Zlib", <http://zlib.net/>.
[CRIME] Rizzo, J. and T. Duong, "The CRIME attack", Ekoparty 2012,
September 2012.
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Author's Address
Takeshi Yoshino
Google, Inc.
Email: tyoshino@google.com
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