HyBi Working Group T. Yoshino
Internet-Draft Google, Inc.
Intended status: Standards Track April 1, 2013
Expires: October 3, 2013
Compression Extensions for WebSocket
draft-ietf-hybi-permessage-compression-08
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 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 October 3, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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. WebSocket Per-message Compression Extension . . . . . . . . . 5
4. Extension Negotiation . . . . . . . . . . . . . . . . . . . . 6
4.1. Negotiation Examples . . . . . . . . . . . . . . . . . . . 7
5. Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Compression . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Decompression . . . . . . . . . . . . . . . . . . . . . . 9
6. permessage-deflate extension . . . . . . . . . . . . . . . . . 10
6.1. Method Parameters . . . . . . . . . . . . . . . . . . . . 11
6.1.1. Context Takeover Control . . . . . . . . . . . . . . . 11
6.1.2. Limiting the LZ77 sliding window size . . . . . . . . 11
6.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Payload Data Transformation . . . . . . . . . . . . . . . 13
6.2.1. Compression . . . . . . . . . . . . . . . . . . . . . 13
6.2.2. Decompression . . . . . . . . . . . . . . . . . . . . 14
6.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 15
6.3. Intermediaries . . . . . . . . . . . . . . . . . . . . . . 18
6.4. Implementation Notes . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8.1. Registration of the "permessage-deflate" WebSocket
Extension Name . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Registration of the "Per-message Compressed" WebSocket
Framing Header Bit . . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23
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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 determines parameters to configure the compression
algorithm during the WebSocket opening handshake. The client and
server then can 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 the DEFLATE since it's widely available as a
library on various platforms and the overhead of the DEFLATE is
small. To align the end of compressed data to octet boundary, this
extension uses the algorithm described in Section 2.1 of the PPP
Deflate Protocol [RFC1979]. Endpoints can take over the LZ77 sliding
window [LZ77] used to build frames for previous messages to get
better compression ratio. For resource-limited devices, this
extension provides parameters to limit memory usage for compression
context.
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2. Conformance Requirements and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these
steps") are to be interpreted with the meaning of the key word
("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.
Conformance requirements phrased as algorithms or specific steps can
be implemented in any manner, so long as the end result is
equivalent. In particular, the algorithms defined in this
specification are intended to be easy to understand and are not
intended to be performant.
This document references the procedure to _Fail the WebSocket
Connection_. This procedure is defined in Section 7.1.7 of
[RFC6455].
This document references the event that _the WebSocket Connection is
established_ and the event that _A WebSocket Message Has Been
Received_. This event is defined in Section 4.1 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. WebSocket Per-message Compression Extension
WebSocket Per-message Compression Extensions (PMCEs) are extensions
to the WebSocket Protocol enabling compression feature. PMCEs are
built based on Section 9 of [RFC6455]. PMCEs are individually
defined for various compression algorithms, 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
followings:
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 data portion.
One such extension is defined in Section 6 of this document and is
registered in Section 8. Other PMCEs may be defined in other
documents.
Section 4 describes basic extension negotiation process. Section 5
describes how to apply the compression algorithm with negotiated
parameters to the contents of WebSocket messages.
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4. Extension Negotiation
To offer use of a PMCE, a client includes a
"Sec-WebSocket-Extensions" header element with the extension name of
the PMCE in the "Sec-WebSocket-Extensions" header in the client's
opening handshake of the WebSocket connection. Extension parameters
in the element represent the PMCE offer in detail. For example, a
client lists preferred configuration parameter values for the
compression algorithm of the PMCE. A client offers multiple PMCE
choices to the server by including multiple elements in the
"Sec-WebSocket-Extensions" header, one for each PMCE offered. The
set of elements MAY include multiple PMCEs with the same extension
name to offer use of the same algorithm with different configuration
parameters.
To accept use of an offered PMCE, a server includes a
"Sec-WebSocket-Extensions" header element with the extension name of
the PMCE in the "Sec-WebSocket-Extensions" header in the server's
opening handshake of the WebSocket connection. Extension parameters
in the element represent the configuration parameters of the PMCE to
use in detail. We call these extension parameters and their values
"agreed parameters". The element MUST represent a PMCE that is fully
supported by the server. The contents of the element doesn't need to
exactly the same as one of the received offers. For example, an
offer with an extension parameter "X" indicating availability of the
feature X may be accepted with an element without the extension
parameter meaning that the server declined use of the feature.
A server MUST NOT accept a PMCE offer together with any extension if
the PMCE will conflict with the extension on use of the RSV1 bit. A
client received a response accepting a PMCE offer together with such
an extension MUST _Fail the WebSocket Connection_.
A server MUST NOT accept a PMCE offer together with any extension if
the PMCE will be applied to output of the extension and any of the
following conditions is met about the extension:
o The extension requires boundary of fragments to be preserved
between output from the extension at the sender and input to the
extension at the receiver.
o The extension uses the "Extension data" field or any of the
reserved bits on the WebSocket header as per-frame attribute.
A client received a response accepting a PMCE offer together with
such an extension MUST _Fail the WebSocket Connection_.
A server declines all offered PMCEs by not including any element with
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PMCE names. If a server responds with no PMCE element in the
"Sec-WebSocket-Extensions" header, both endpoints proceed without
Per-message Compression once _the WebSocket Connection is
established_.
If a server gives an invalid response, such as accepting a PMCE that
the client did not offer, the client MUST _Fail the WebSocket
Connection_.
If a server responds with a valid PMCE element in the
"Sec-WebSocket-Extensions" header and _the WebSocket Connection is
established_, both endpoints MUST use the algorithm described in
Section 5 to exchange messages, using the payload data transformation
(compressing and decompressing) procedure of the PMCE returned by the
server.
4.1. Negotiation Examples
The followings are example values for the "Sec-WebSocket-Extensions"
header offering PMCEs. permessage-foo and permessage-bar in the
examples are hypothetical extension names of PMCEs for compression
algorithm foo and bar.
o Offer the permessage-foo.
permessage-foo
o Offer the permessage-foo with a parameter x with a value of 10.
permessage-foo; x=10
The value MAY be quoted.
permessage-foo; x="10"
o Offer the permessage-foo as first choice and the permessage-bar as
a fallback plan.
permessage-foo, permessage-bar
o Offer the permessage-foo with a parameter use_y which enables a
feature y as first choice, and the permessage-foo without the
use_y parameter as a fallback plan.
permessage-foo; use_y, permessage-foo
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5. Framing
PMCEs operate only on non-control messages. PMCEs operate only on
the payload data portion and the "Per-message Compressed" bit.
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 "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 decompressing 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 "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.
5.1. Compression
An endpoint MUST use the following algorithm to send a message in the
form of a compressed message.
1. Compress the payload data portion of the original message by
following the compression procedure of the PMCE. The original
message may input from application layer or output of another
WebSocket extension depending on what extensions are negotiated.
2. If this PMCE is the last extension to process outgoing messages,
build frame(s) by putting the compressed data instead of the
original data for the payload data portion, set the "Per-message
Compressed" bit of the first frame, and send the frame(s).
Otherwise, pass the transformed payload data and modified header
values including "Per-message Compressed" bit value set to 1 to
the next extension.
An endpoint MUST use the following algorithm to send a message in the
form of an uncompressed message. If this PMCE is the last extension
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to process outgoing messages, build frame(s) by putting the original
data for payload data portion as-is, unset the "Per-message
Compressed" bit of the first frame, and send the frame(s).
Otherwise, pass the payload data and header values to the next
extension as-is.
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 may be not valid
UTF-8. At the receiver, the payload data portion after decompression
is subject to the constraints for the original data type again.
5.2. 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 are 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 payload data and header
values including the "Per-message Compressed" bit unset to 0.
Otherwise, pass the decompressed payload data and header values
including the "Per-message Compressed" bit unset to 0 to the next
extension.
An endpoint MUST use the following algorithm to receive a message in
the form of an uncompressed message. If this PMCE is the last
extension to process incoming messages, deliver the _A WebSocket
Message Has Been Received_ event to the application layer with the
received payload data and header values as-is. Otherwise, pass the
payload data and header values to the next extension as-is.
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6. permessage-deflate extension
This section specifies a specific PMCE called "permessage-deflate".
It compresses the payload data portion of messages using the DEFLATE
[RFC1951] and the byte boundary aligning method introduced in
[RFC1979].
The registered extension name for this extension is
"permessage-deflate".
For an offer for this extension, the following 3 extension parameters
are defined.
o "s2c_no_context_takeover"
o "s2c_max_window_bits"
o "c2s_max_window_bits"
For a response for this extension, the following 4 extension
parameters are defined.
o "s2c_no_context_takeover"
o "c2s_no_context_takeover"
o "s2c_max_window_bits"
o "c2s_max_window_bits"
A server MUST decline a "permessage-deflate" offer if any of the
following conditions is met:
o The offer has any extension parameter not defined for use in an
offer.
o The offer has any extension parameter with an invalid value.
o The offer has multiple extension parameters with the same name.
o The server doesn't support the offered configuration.
A client MUST _Fail the WebSocket Connection_ if the server accepted
a "permessage-deflate" offer with a response meeting any of the
following condition:
o The response has any extension parameter not defined for use in a
response.
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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.
6.1. Method Parameters
6.1.1. Context Takeover Control
A client MAY attach the "s2c_no_context_takeover" extension
parameter. The "s2c_no_context_takeover" extension parameter has no
value. Using this extension parameter, a client can disallow the
peer server to use the same LZ77 sliding window to build frames of
the last sent message to build frames of the next message to send.
If the peer server doesn't use the same LZ77 sliding window to
compress two or more messages, the client can reduce the amount of
memory for the LZ77 sliding window to decompress received messages.
A server accepts an offer with this extension parameter by including
the "s2c_no_context_takeover" extension parameter in the response. A
server accepted an offer with this extension parameter MUST empty its
LZ77 sliding window to compress messages to send each time the server
builds a new message.
It is RECOMMENDED to make a server be able to accept the
"s2c_no_context_takeover" parameter.
A server MAY attach the "c2s_no_context_takeover" extension
parameter. The "c2s_no_context_takeover" extension parameter has no
value. Using this extension parameter, a server can disallow the
peer client to use the LZ77 sliding window used to build frames of
the last sent message to build frames for the next message to send.
If the peer client doesn't use the same LZ77 sliding window to
compress two or more messages, the server can reduce the amount of
memory for the LZ77 sliding window to decompress received messages.
A client that received this parameter MUST empty its LZ77 sliding
window to compress messages to send each time the client builds a new
message.
It is RECOMMENDED to make a client be able to accept the
"c2s_no_context_takeover" parameter.
6.1.2. Limiting the LZ77 sliding window size
A client MAY attach the "s2c_max_window_bits" extension parameter to
limit the LZ77 sliding window size that the server uses to build
messages. This extension parameter MUST have a decimal integer value
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in the range between 8 to 15 indicating the base-2 logarithm of the
LZ77 sliding window size.
s2c_max_window_bits = 1*DIGIT
A server declines an offer with this extension parameter if the
server doesn't support the extension parameter. A server accepts an
offer with this extension parameter by including the extension
parameter with the same value as the offer in the response. If a
server accepts an offer with this extension parameter, the server
MUST NOT use LZ77 sliding window size greater than the size specified
by the extension parameter to compress messages
A client MAY attach the "c2s_max_window_bits" extension parameter if
the client can adjust LZ77 sliding window size based on the
"c2s_max_window_bits" sent by the server. This parameter has no
value.
If a server received and accepts an offer with the
"c2s_max_window_bits" extension parameter, the server MAY include the
"c2s_max_window_bits" parameter in the response to the offer to limit
the LZ77 sliding window size that the client uses to build messages.
If a server received and accepts an offer without the
"c2s_max_window_bits" extension parameter, the server MUST NOT
include the "c2s_max_window_bits" extension parameter in the response
to the offer. The "c2s_max_window_bits" extension parameter in the
server's opening handshake MUST have a decimal integer value in the
range between 8 to 15 indicating the base-2 logarithm of the LZ77
sliding window size.
c2s_max_window_bits = 1*DIGIT
If a client received the "c2s_max_window_bits" extension parameter,
the client MUST NOT use LZ77 sliding window size greater than the
size specified by the extension parameter to build messages.
6.1.3. 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 "c2s_max_window_bits" extension parameter is not specified,
the server may not accept the offer with the "c2s_max_window_bits"
extension parameter. The simplest "Sec-WebSocket-Extensions" header
in a server's opening handshake to accept use of the permessage-
deflate is the same.
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The following offer sent by a client is asking the server to use the
LZ77 sliding window size of 1,024 bytes or less and declaring that
the client can accept the "c2s_max_window_bits" extension parameter.
Sec-WebSocket-Extensions:
permessage-deflate;
c2s_max_window_bits; s2c_max_window_bits=10
This offer might be rejected by the server because the server doesn't
support the "s2c_max_window_bits" extension parameter. This is fine
if the "s2c_max_window_bits" is mandatory for the client, but if the
client want to fallback to the "permessage-deflate" without the
"s2c_max_window_bits", the client should offer the fallback option in
addition like this:
Sec-WebSocket-Extensions:
permessage-deflate;
c2s_max_window_bits; s2c_max_window_bits=10,
permessage-deflate;
c2s_max_window_bits
This example offers two configurations so that the server can accept
permessage-deflate by picking supported one from them. To accept the
first option, the server sends back this for example:
Sec-WebSocket-Extensions:
permessage-deflate; s2c_max_window_bits=10
And to accept the second option, the server sends back this for
example:
Sec-WebSocket-Extensions: permessage-deflate
6.2. Payload Data Transformation
6.2.1. Compression
An endpoint uses the following algorithm to compress a message.
1. Compress all the octets of the payload data portion of the
message using the DEFLATE.
2. If the resulting data does not end with an empty DEFLATE block
with no compression (the "BTYPE" bit is set to 0), 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
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(possibly part of) the DEFLATE header bits with the "BTYPE" bit
set to 0.
In using the DEFLATE in the first step above:
o An endpoints MAY use multiple DEFLATE blocks to compress one
message.
o An endpoints MAY use DEFLATE blocks of any type.
o An endpoints MAY use both DEFLATE blocks with the "BFINAL" bit set
to 0 and DEFLATE blocks with the "BFINAL" bit set to 1.
o When any DEFLATE block with the "BFINAL" bit set to 1 doesn't end
at byte boundary, an endpoint adds minimal padding bits of 0 to
make it end at byte boundary. The next DEFLATE block follows the
padded data if any.
An endpoint MUST NOT use an LZ77 sliding window longer than 32,768
bytes to compress messages to send.
If the "agreed parameters" contain the "c2s_no_context_takeover"
extension parameter, the client MUST empty its LZ77 sliding window to
compress messages to send each time the client compresses a new
message to send. Otherwise, the client MAY take over the LZ77
sliding window used to build the last compressed message.
If the "agreed parameters" contain the "s2c_no_context_takeover"
extension parameter, the server MUST empty its LZ77 sliding window to
compress messages to send each time the server compresses a new
message to send. Otherwise, the server MAY take over the LZ77
sliding window used to build the last compressed message.
If the "agreed parameters" contain the "c2s_max_window_bits"
extension parameter with a value of w, the client MUST NOT use an
LZ77 sliding window longer than w-th power of 2 bytes to compress
messages to send.
If the "agreed parameters" contain the "s2c_max_window_bits"
extension parameter with a value of w, the server MUST NOT use an
LZ77 sliding window longer than w-th power of 2 bytes to compress
messages to send.
6.2.2. Decompression
An endpoint uses the following algorithm to decompress a message.
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1. Append 4 octets of 0x00 0x00 0xff 0xff to the tail end of the
payload data portion of the message.
2. Decompress the resulting data using the DEFLATE.
If the "agreed parameters" contain the "s2c_no_context_takeover"
extension parameter, the client MAY empty its LZ77 sliding window to
decompress received messages each time the client decompresses a new
received message. Otherwise, the client MUST take over the LZ77
sliding window used to process the last compressed message.
If the "agreed parameters" contain the "c2s_no_context_takeover"
extension parameter, the server MAY empty its LZ77 sliding window to
decompress received messages each time the server decompresses a new
received message. Otherwise, the server MUST take over the LZ77
sliding window used to process the last compressed message.
If the "agreed parameters" contain the "s2c_max_window_bits"
extension parameter with a value of w, the client MAY reduce the size
of its LZ77 sliding window to decompress received messages down to
the w-th power of 2 bytes. Otherwise, the client MUST use a 32,768
byte LZ77 sliding window to decompress received messages.
If the "agreed parameters" contain the "c2s_max_window_bits"
extension parameter with a value of w, the server MAY reduce the size
of its LZ77 sliding window to decompress received messages down to
the w-th power of 2 bytes. Otherwise, the server MUST use a 32,768
byte LZ77 sliding window to decompress received messages.
6.2.3. Examples
This section introduces examples of how the permessage-deflate
transforms messages.
6.2.3.1. 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 put in the
payload data portion 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:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00 0x00 0xff 0xff
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By stripping 0x00 0x00 0xff 0xff from the tail end, the endpoint gets
the data to put in the payload data portion:
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.
6.2.3.2. Sharing LZ77 Sliding Window
Suppose that a client has sent a message "Hello" as a compressed
message and will send the same message "Hello" again as a compressed
message.
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
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This is the payload of the first message the client has sent. If the
"agreed parameters" contain the "c2s_no_context_takeover" extension
parameter, the client compresses the payload data portion of the next
message into the same bytes (if the client uses the same "BTYPE"
value and "BFINAL" value). So, the payload will be:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
If the "agreed parameters" contain the "c2s_no_context_takeover"
extension parameter, the client can compress the payload data portion
of the next message into shorter bytes utilizing the history in the
LZ77 sliding window. So, the payload will be:
0xf2 0x00 0x11 0x00 0x00
Note that even if any uncompressed message (any message with the RSV1
bit unset) is inserted between the two "Hello" messages, such a
message doesn't make any change on the LZ77 sliding window.
6.2.3.3. Using a DEFLATE Block with No Compression
0xc1 0x0b 0x00 0x05 0x00 0xfa 0xff 0x48 0x65 0x6c 0x6c 0x6f 0x00
This is a frame consisting 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 the DEFLATE is applied
to the message and it includes the case only DEFLATE blocks with no
compression are used. The third to 13th octet consists a payload
containing "Hello" compressed using a DEFLATE block with no
compression.
6.2.3.4. 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 octet consist a
DEFLATE block with "BFINAL" set to 1 and "BTYPE" set to 1 containing
"Hello". The last 1 octet (0x00) contains the header bits with
"BFINAL" set to 0 and "BTYPE" set to 0, and 7 padding bits of 0.
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This octet is necessary to allow the payload to be decompressed in
the same manner as messages flushed using DEFLATE blocks with BFINAL
unset.
6.2.3.5. 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) consist one DEFLATE block with "BFINAL"
set to 0 and "BTYPE" set to 1 containing "He";. The rest of the 4th
octet contains the header bits with "BFINAL" set to 0 and "BTYPE" set
to 0, and the 3 padding bits of 0. Together with the following 4
octets (0x00 0x00 0xff 0xff), the header bits consist an empty
DEFLATE block with no compression. A DEFLATE block containing "llo"
follows the empty DEFLATE block.
6.3. Intermediaries
When an intermediary forwards messages, the intermediary MAY add,
change or remove Per-message Compression on the messages. The
elements in the "Sec-WebSocket-Extensions" for the PMCE in the
opening handshakes with the connected client and server must be
altered by the intermediary accordingly to match the new framing.
6.4. Implementation Notes
On most common software development platforms, their DEFLATE
compression library provide a method to align compressed data to byte
boundaries using an empty DEFLATE block with no compression. For
example, Zlib [Zlib] does this when "Z_SYNC_FLUSH" is passed to the
deflate function.
To attain sufficient compression ratio, the LZ77 sliding window size
of 1,024 or more is RECOMMENDED.
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7. Security Considerations
There is a known exploit for combination of a secure transport
protocol and a dictionary based compression [CRIME]. Implementors
should give attention to this point when integrating this extension
with other extensions or protocols.
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8. IANA Considerations
8.1. 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.
8.2. 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 5 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 data
portion of the message is compressed by the PMCE or not.
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9. 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): Alexey Melnikov, Arman
Djusupov, Bjoern Hoehrmann, Brian McKelvey, Dario Crivelli, Greg
Wilkins, Inaki Baz Castillo, Jamie Lokier, Joakim Erdfelt, John A.
Tamplin, Julian Reschke, Kenichi Ishibashi, Mark Nottingham, Peter
Thorson, Roberto Peon and Simone Bordet. Note that people listed
above didn't necessarily endorse the end result of this work.
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10. References
10.1. Normative References
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LZ77] Ziv, J. and A. Lempel, "A Universal Algorithm for
Sequential Data Compression", IEEE Transactions on
Information Theory, Vol. 23, No. 3, pp. 337-343.
10.2. Informative References
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC1979] Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.
[Zlib] Gailly, J. and M. Adler, "Zlib", <http://zlib.net/>.
[CRIME] Rizzo, J. and T. Duong, "The CRIME attack", Ekoparty 2012,
September 2012.
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Author's Address
Takeshi Yoshino
Google, Inc.
Email: tyoshino@google.com
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