draft-ietf-hybi-permessage-compression-17.txt   draft-ietf-hybi-permessage-compression-18.txt 
HyBi Working Group T. Yoshino HyBi Working Group T. Yoshino
Internet-Draft Google, Inc. Internet-Draft Google, Inc.
Intended status: Standards Track January 22, 2014 Intended status: Standards Track May 13, 2014
Expires: July 26, 2014 Expires: November 14, 2014
Compression Extensions for WebSocket Compression Extensions for WebSocket
draft-ietf-hybi-permessage-compression-17 draft-ietf-hybi-permessage-compression-18
Abstract Abstract
This document specifies a framework for creating WebSocket extensions This document specifies a framework for creating WebSocket extensions
that add compression functionality to the WebSocket Protocol. An that add compression functionality to the WebSocket Protocol. An
extension based on this framework compresses the payload data portion extension based on this framework compresses the payload data portion
of non-control WebSocket messages on a per-message basis using of non-control WebSocket messages on a per-message basis using
parameters negotiated during the opening handshake. This framework parameters negotiated during the opening handshake. This framework
provides a general method to apply a compression algorithm to the provides a general method to apply a compression algorithm to the
contents of WebSocket messages. For each compression algorithm, an contents of WebSocket messages. For each compression algorithm, an
extension is defined by specifying parameter negotiation and extension is defined by specifying parameter negotiation and payload
compression algorithm in detail. This document also specifies one transformation algorithm in detail. This document also specifies one
specific compression extension using the DEFLATE algorithm. specific compression extension using the DEFLATE algorithm.
Please send feedback to the hybi@ietf.org mailing list. Please send feedback to the hybi@ietf.org mailing list.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current. Drafts is at http://datatracker.ietf.org/drafts/current.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 26, 2014. This Internet-Draft will expire on November 14, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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7. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 14 7. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 14
8. permessage-deflate extension . . . . . . . . . . . . . . . . . 15 8. permessage-deflate extension . . . . . . . . . . . . . . . . . 15
8.1. Method Parameters . . . . . . . . . . . . . . . . . . . . 16 8.1. Method Parameters . . . . . . . . . . . . . . . . . . . . 16
8.1.1. Context Takeover Control . . . . . . . . . . . . . . . 16 8.1.1. Context Takeover Control . . . . . . . . . . . . . . . 16
8.1.2. Limiting the LZ77 sliding window size . . . . . . . . 18 8.1.2. Limiting the LZ77 sliding window size . . . . . . . . 18
8.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 20 8.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Message Payload Transformation . . . . . . . . . . . . . . 21 8.2. Message Payload Transformation . . . . . . . . . . . . . . 21
8.2.1. Compression . . . . . . . . . . . . . . . . . . . . . 21 8.2.1. Compression . . . . . . . . . . . . . . . . . . . . . 21
8.2.2. Decompression . . . . . . . . . . . . . . . . . . . . 22 8.2.2. Decompression . . . . . . . . . . . . . . . . . . . . 22
8.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 23 8.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 23
8.3. Implementation Notes . . . . . . . . . . . . . . . . . . . 27 8.3. Implementation Notes . . . . . . . . . . . . . . . . . . . 26
8.4. Intermediaries . . . . . . . . . . . . . . . . . . . . . . 27 8.4. Intermediaries . . . . . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28 9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10.1. Registration of the "permessage-deflate" WebSocket 10.1. Registration of the "permessage-deflate" WebSocket
Extension Name . . . . . . . . . . . . . . . . . . . . . . 29 Extension Name . . . . . . . . . . . . . . . . . . . . . . 29
10.2. Registration of the "Per-message Compressed" WebSocket 10.2. Registration of the "Per-message Compressed" WebSocket
Framing Header Bit . . . . . . . . . . . . . . . . . . . . 29 Framing Header Bit . . . . . . . . . . . . . . . . . . . . 29
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . . 31 12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
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This document specifies a framework to add compression functionality This document specifies a framework to add compression functionality
to the WebSocket Protocol [RFC6455]. This framework specifies how to to the WebSocket Protocol [RFC6455]. This framework specifies how to
define WebSocket Per-message Compression Extensions (PMCEs) define WebSocket Per-message Compression Extensions (PMCEs)
individually for various compression algorithms based on the individually for various compression algorithms based on the
extension concept of the WebSocket Protocol specified in Section 9 of extension concept of the WebSocket Protocol specified in Section 9 of
[RFC6455]. A WebSocket client and a peer WebSocket server negotiate [RFC6455]. A WebSocket client and a peer WebSocket server negotiate
use of a PMCE and determine parameters to configure the compression use of a PMCE and determine parameters to configure the compression
algorithm during the WebSocket opening handshake. The client and algorithm during the WebSocket opening handshake. The client and
server can then exchange non-control messages using frames with server can then exchange non-control messages using frames with
compressed data in the payload data portion. This framework compressed data in the payload data portion.
specifies a general method to apply a compression algorithm to the
contents of WebSocket messages. A document specifying an individual This framework only specifies a general method to apply a compression
PMCE describes how to negotiate configuration parameters for the algorithm to the contents of WebSocket messages. A document
compression algorithm and how to transform (compress and decompress) specifying an individual PMCE describes how to negotiate
data in the payload data portion in detail. A WebSocket client may configuration parameters for the compression algorithm and how to
offer multiple PMCEs during the WebSocket opening handshake. A peer transform (compress and decompress) data in the payload data portion
WebSocket server received those offers may choose and accept in detail.
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 A WebSocket client may offer multiple PMCEs during the WebSocket
not, so that an endpoint can choose not to compress messages with opening handshake. A peer WebSocket server received those offers may
incompressible contents. 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 This document also specifies one specific PMCE based on the DEFLATE
[RFC1951] algorithm. The extension name of the PMCE is "permessage- [RFC1951] algorithm. The extension name of the PMCE is "permessage-
deflate". We chose DEFLATE since it's widely available as a library 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 on various platforms and the overhead is small. To align the end of
compressed data to an octet boundary, this extension uses the compressed data to an octet boundary, this extension uses the
algorithm described in Section 2.1 of [RFC1979]. Endpoints can take algorithm described in Section 2.1 of [RFC1979]. Endpoints can take
over the LZ77 sliding window [LZ77] used to build frames for previous over the LZ77 sliding window [LZ77] used to build frames for previous
messages to get better compression ratio. For resource-limited messages to get better compression ratio. For resource-limited
devices, this extension provides parameters to limit memory usage for devices, this extension provides parameters to limit memory usage for
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This document references the procedure to _Fail the WebSocket This document references the procedure to _Fail the WebSocket
Connection_. This procedure is defined in Section 7.1.7 of Connection_. This procedure is defined in Section 7.1.7 of
[RFC6455]. [RFC6455].
This document references the event that _The WebSocket Connection is This document references the event that _The WebSocket Connection is
Established_ and the event that _A WebSocket Message Has Been Established_ and the event that _A WebSocket Message Has Been
Received_. These events are defined in Section 4.1 and Section 6.2, Received_. These events are defined in Section 4.1 and Section 6.2,
respectively, of [RFC6455]. respectively, of [RFC6455].
This document uses the Argumented Backus-Naur Form (ABNF) notation of This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234]. The DIGIT (decimal 0-9) rule is included by reference, as [RFC5234]. The DIGIT (decimal 0-9) rule is included by reference, as
defined in the Appendix B.1 of [RFC5234]. defined in the Appendix B.1 of [RFC5234].
3. Complementary Terminology 3. Complementary Terminology
This document defines some terms about WebSocket and WebSocket This document defines some terms about WebSocket and WebSocket
Extension mechanism that are underspecified or not defined at all in Extension mechanism that are underspecified or not defined at all in
[RFC6455]. This terminology is effective only in this document and [RFC6455]. This terminology is effective only in this document and
any other documents that refer to this section. any other documents that refer to this section.
Non-control message means a message that consists of non-control "A non-control message" means a message that consists of non-control
frames as defined in Section 5.6 of [RFC6455]. frames as defined in Section 5.6 of [RFC6455].
Message payload (or payload of a message) means concatenation of the "A message payload (or payload of a message)" means concatenation of
payload data portion of all frames representing a single message, as the payload data portion of all frames representing a single message,
well as how /data/ is formed from in Section 6.2 of [RFC6455]. 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 "An extension in use next to extension X" means the extension listed
to X in the "Sec-WebSocket-Extensions" header in the server's opening next to X in the "Sec-WebSocket-Extensions" header in the server's
handshake as defined in Section 9.1 of [RFC6455]. Such an extension opening handshake as defined in Section 9.1 of [RFC6455]. Such an
is applied to outgoing data from the application right after X on extension is applied to outgoing data from the application right
sender side but applied right before X to incoming data from the after X on sender side but applied right before X to incoming data
underlying transport. from the underlying transport.
Extension negotiation offer means each element in the "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. "Sec-WebSocket-Extensions" header in the client's opening handshake.
Extension negotiation response means each element in the "An extension negotiation response" means each element in the
"Sec-WebSocket-Extensions" header in the server's opening handshake. "Sec-WebSocket-Extensions" header in the server's opening handshake.
Accept an extension negotiation offer means including a corresponding "A corresponding extension negotiation response for an extension
extension negotiation response in the "Sec-WebSocket-Extensions" negotiation offer" means an extension negotiation response a server
header in the server's opening handshake. sends back to the peer client that contains the same extension name
as the offer and meets the requirements represented by the offer.
Decline an extension negotiation offer means not including a "Accepting an extension negotiation offer" means including a
corresponding extension negotiation response in the 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. "Sec-WebSocket-Extensions" header in the server's opening handshake.
4. WebSocket Per-message Compression Extension 4. WebSocket Per-message Compression Extension
WebSocket Per-message Compression Extensions (PMCEs) are extensions WebSocket Per-message Compression Extensions (PMCEs) are extensions
to the WebSocket Protocol enabling compression functionality. PMCEs to the WebSocket Protocol enabling compression functionality. PMCEs
are built based on Section 9 of [RFC6455]. PMCEs are individually are built based on the extension concept of the WebSocket Protocol
defined for each compression algorithm to be implemented, and are specified in Section 9 of [RFC6455]. PMCEs are individually defined
registered in the WebSocket Extension Name Registry created in for each compression algorithm to be implemented, and are registered
Section 11.4 of [RFC6455]. Each PMCE refers to this framework and in the WebSocket Extension Name Registry created in Section 11.4 of
defines the following: [RFC6455]. Each PMCE refers to this framework and defines the
following:
o The content to put in the "Sec-WebSocket-Extensions" header. The o The contents of its extension negotiation offer/response to
content includes the extension name of the PMCE and any applicable include in the "Sec-WebSocket-Extensions" header. The contents
include the extension name of the PMCE and any applicable
extension parameters. extension parameters.
o How to interpret extension parameters exchanged during the opening o How to interpret the extension parameters exchanged during the
handshake opening handshake
o How to transform the payload of a message. o How to transform the payload of a message.
One such extension is defined in Section 8 of this document and is One such extension is defined in Section 8 of this document and is
registered in Section 10. Other PMCEs may be defined in other registered in Section 10. Other PMCEs may be defined in other
documents. documents.
Section 5 describes the basic extension negotiation process. Section 5 describes the basic extension negotiation process.
Section 6 describes how to apply the compression algorithm with Section 6 describes how to apply the compression algorithm with
negotiated parameters to the contents of WebSocket messages. negotiated parameters to the contents of WebSocket messages.
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that the server declined use of the feature. that the server declined use of the feature.
"Agreed parameters" MUST represent how the requests and hints in the "Agreed parameters" MUST represent how the requests and hints in the
client's extension negotiation offer have been handled in addition to 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 the server's requests and hints on the client's behavior, so that the
client can configure its behavior without identifying which PMCE client can configure its behavior without identifying which PMCE
extension negotiation offer has been accepted. extension negotiation offer has been accepted.
For example, if a client sends an extension negotiation offer For example, if a client sends an extension negotiation offer
including a parameter "enable_compression" and another without the including a parameter "enable_compression" and another without the
parameter, the server accepts the former by sending back an element parameter, the server accepts the former and tell that to the client
including a parameter that acknowledges "enable_compression". The by sending back an element including a parameter that acknowledges
name of the acknowledging parameter doesn't need to be the same as "enable_compression". The name of the acknowledging parameter
the offer. doesn't need to be the same as the offer.
General negotiation flow will be like the followings. How to handle General negotiation flow will be like the following. How to handle
parameters in detail will be specified in the specifications for each parameters in detail will be specified in the specifications for each
PMCEs. PMCE.
A client makes an offer including parameters identifying the A client makes an offer including parameters identifying the
followings: following:
o Hints about how the client is planning to compress data o Hints about how the client is planning to compress data
o Requests about how the server compresses data o Requests about how the server compresses data
o Limitation of the client's compression functionality o Limitation of the client's compression functionality
The peer server uses these parameters, makes a determination of its The peer server makes a determination of its behavior based on these
behavior based on them if it can and wants to proceed with this PMCE parameters if it can and wants to proceed with this PMCE enabled, and
enabled, and responds to the client with parameters identifying the responds to the client with parameters identifying the following:
followings:
o Requests about how the client compresses data o Requests about how the client compresses data
o How the server will compress data o How the server will compress data
The client uses these parameters from the server make a determination The client makes a determination of its behavior based on these
of its behavior based on them if it can and wants to proceed. parameters from the server if it can and wants to proceed with this
Otherwise, the client starts closing handshake with close code 1010. PMCE enabled. Otherwise, the client starts closing handshake with
close code 1010.
There can be compression features that can be applied separately for There can be compression features that can be applied separately for
each direction. For such features, the acknowledging parameter and each direction. For such features, the acknowledging parameter and
the parameter for the reverse direction must be chosen to be the parameter for the reverse direction must be chosen to be
distinguishable from each other. For example, we can add "server_" distinguishable from each other. For example, we can add "server_"
prefix to parameters affecting data sent from a server and "client_" prefix to parameters affecting data sent from a server and "client_"
prefix to ones affecting data sent from a client to make them prefix to ones affecting data sent from a client to make them
distinguishable. distinguishable.
A server MUST NOT accept a PMCE extension negotiation offer together A server MUST NOT accept a PMCE extension negotiation offer together
with another extension if the PMCE will conflict with the extension with another extension if the PMCE will conflict with the extension
on use of the RSV1 bit. A client that receives a response accepting on use of the RSV1 bit. A client that received a response accepting
a PMCE extension negotiation offer together with such an extension a PMCE extension negotiation offer together with such an extension
MUST _Fail the WebSocket Connection_. MUST _Fail the WebSocket Connection_.
A server MUST NOT accept a PMCE extension negotiation offer together A server MUST NOT accept a PMCE extension negotiation offer together
with another extension if the PMCE will be applied to output of the with another extension if the PMCE will be applied to output of the
extension and any of the following conditions applies to the extension and any of the following conditions applies to the
extension: extension:
o The extension requires boundary of fragments to be preserved o The extension requires boundary of fragments to be preserved
between output from the extension at the sender and input to the between output from the extension at the sender and input to the
extension at the receiver. extension at the receiver.
o The extension uses the "Extension data" field or any of the o The extension uses the "Extension data" field or any of the
reserved bits on the WebSocket header as per-frame attribute. reserved bits on the WebSocket header as a per-frame attribute.
A client receiving a response accepting a PMCE extension negotiation A client that received a response accepting a PMCE extension
offer together with such an extension MUST _Fail the WebSocket negotiation offer together with such an extension MUST _Fail the
Connection_. WebSocket Connection_.
A server declines all offered PMCEs by not including any element with A server declines all offered PMCEs by not including any element with
PMCE names. If a server responds with no PMCE element in the PMCE names. If a server responds with no PMCE element in the
"Sec-WebSocket-Extensions" header, both endpoints proceed without "Sec-WebSocket-Extensions" header, both endpoints proceed without
Per-message Compression once _the WebSocket Connection is Per-message Compression once _the WebSocket Connection is
established_. established_.
If a server gives an invalid response, such as accepting a PMCE that If a server gives an invalid response, such as accepting a PMCE that
the client did not offer, the client MUST _Fail the WebSocket the client did not offer, the client MUST _Fail the WebSocket
Connection_. Connection_.
If a server responds with a valid PMCE element in the If a server responds with a valid PMCE element in the
"Sec-WebSocket-Extensions" header and _the WebSocket Connection is "Sec-WebSocket-Extensions" header and _the WebSocket Connection is
established_, both endpoints MUST use the algorithm described in established_, both endpoints MUST use the algorithm described in
Section 6 to exchange messages, using the message payload Section 6 and the message payload transformation (compressing and
transformation (compressing and decompressing) procedure of the PMCE decompressing) procedure of the PMCE configured with the "agreed
returned by the server. parameters" returned by the server to exchange messages.
5.1. Negotiation Examples 5.1. Negotiation Examples
The followings are example values for the "Sec-WebSocket-Extensions" The following are example values for the "Sec-WebSocket-Extensions"
header offering PMCEs. permessage-foo and permessage-bar in the header offering PMCEs. permessage-foo and permessage-bar in the
examples are hypothetical extension names of PMCEs for compression examples are hypothetical extension names of PMCEs for compression
algorithm foo and bar. algorithm foo and bar.
o Offer the permessage-foo. o Offer the permessage-foo.
permessage-foo permessage-foo
o Offer the permessage-foo with a parameter x with a value of 10. o Offer the permessage-foo with a parameter x with a value of 10.
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PMCEs operate only on non-control messages. PMCEs operate only on non-control messages.
This document allocates the RSV1 bit of the WebSocket header for This document allocates the RSV1 bit of the WebSocket header for
PMCEs, and calls the bit the "Per-message Compressed" bit. On a PMCEs, and calls the bit the "Per-message Compressed" bit. On a
WebSocket connection where a PMCE is in use, this bit indicates WebSocket connection where a PMCE is in use, this bit indicates
whether a message is compressed or not. whether a message is compressed or not.
A message with the "Per-message Compressed" bit set on the first A message with the "Per-message Compressed" bit set on the first
fragment of the message is called a "compressed message". Frames of fragment of the message is called a "compressed message". Frames of
a compressed message have compressed data in the payload data a compressed message have compressed data in the payload data
portion. An endpoint receiving a compressed message decompresses the portion. An endpoint received a compressed message decompresses the
concatenation of the compressed data of the frames of the message by concatenation of the compressed data of the frames of the message by
following the decompression procedure specified by the PMCE in use. following the decompression procedure specified by the PMCE in use.
The endpoint uses the bytes corresponding to the application data The endpoint uses the bytes corresponding to the application data
portion in this decompressed data for the _A WebSocket Message Has portion in this decompressed data for the _A WebSocket Message Has
Been Received_ event instead of the received data as-is. Been Received_ event instead of the received data as-is.
A message with the "Per-message Compressed" bit unset on the first A message with the "Per-message Compressed" bit unset on the first
fragment of the message is called an "uncompressed message". Frames fragment of the message is called an "uncompressed message". Frames
of an uncompressed message have uncompressed original data as-is in of an uncompressed message have uncompressed original data as-is in
the payload data portion. An endpoint received an uncompressed the payload data portion. An endpoint received an uncompressed
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be input from the application layer or output of another be input from the application layer or output of another
WebSocket extension depending on what extensions were negotiated. WebSocket extension depending on what extensions were negotiated.
2. If this PMCE is the last extension to process outgoing messages, 2. If this PMCE is the last extension to process outgoing messages,
build frame(s) by using the compressed data instead of the build frame(s) by using the compressed data instead of the
original data for the message payload, and setting the original data for the message payload, and setting the
"Per-message Compressed" bit of the first frame, then send the "Per-message Compressed" bit of the first frame, then send the
frame(s) as described in Section 6.1 of RFC6455. Otherwise, pass frame(s) as described in Section 6.1 of RFC6455. Otherwise, pass
the transformed message payload and modified header values the transformed message payload and modified header values
including "Per-message Compressed" bit value set to 1 to the including "Per-message Compressed" bit value set to 1 to the
extension next to the PMCE. If the extension expects frames as extension next to the PMCE. If the extension expects frames for
input, build a frame for the message and pass it. input, build a frame for the message and pass it.
An endpoint MUST use the following algorithm to send a message in the 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 form of an uncompressed message. If this PMCE is the last extension
to process outgoing messages, build frame(s) by using the original to process outgoing messages, build frame(s) by using the original
data for the payload data portion as-is and unsetting the data for the payload data portion as-is and unsetting the
"Per-message Compressed" bit of the first frame, then send 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 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 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 as-is. If the extension expects frames for input, build a frame for
the message and pass it. the message and pass it.
An endpoint MUST NOT set the "Per-message Compressed" bit of control An endpoint MUST NOT set the "Per-message Compressed" bit of control
frames and non-first fragments of a data message. An endpoint frames and non-first fragments of a data message. An endpoint
received such a frame MUST _Fail the WebSocket Connection_. received such a frame MUST _Fail the WebSocket Connection_.
PMCEs don't change the opcode field. The opcode of the first frame PMCEs don't change the opcode field. The opcode of the first frame
of a compress message indicates the opcode of the original message. of a compress message indicates the opcode of the original message.
The payload data portion in frames generated by a PMCE is not subject The payload data portion in frames generated by a PMCE is not subject
to the constraints for the original data type. For example, the to the constraints for the original data type. For example, the
concatenation of the data corresponding to the application data concatenation of the output data corresponding to the application
portion of frames of a compressed text message is not required to be data portion of frames of a compressed text message is not required
valid UTF-8. At the receiver, the payload data portion after to be valid UTF-8. At the receiver, the payload data portion after
decompression is subject to the constraints for the original data decompression is subject to the constraints for the original data
type again. type again.
6.2. Decompression 6.2. Decompression
An endpoint MUST use the following algorithm to receive a message in An endpoint MUST use the following algorithm to receive a message in
the form of a compressed message. the form of a compressed message.
1. Concatenate the payload data portion of the received frames of 1. Concatenate the payload data portion of the received frames of
the compressed message. The received frames may direct input the compressed message. The received frames may be direct input
from underlying transport or output of another WebSocket from the underlying transport or output of another WebSocket
extension depending on what extensions were negotiated. extension depending on what extensions were negotiated.
2. Decompress the concatenation by following the decompression 2. Decompress the concatenation by following the decompression
procedure of the PMCE. procedure of the PMCE.
3. If this is the last extension to process incoming messages, 3. If this is the last extension to process incoming messages,
deliver the _A WebSocket Message Has Been Received_ event to the deliver the _A WebSocket Message Has Been Received_ event to the
application layer with the decompressed message payload and application layer with the decompressed message payload and
header values including the "Per-message Compressed" bit unset to header values including the "Per-message Compressed" bit unset to
0. Otherwise, pass the decompressed message payload and header 0. Otherwise, pass the decompressed message payload and header
values including the "Per-message Compressed" bit unset to 0 to values including the "Per-message Compressed" bit unset to 0 to
the extension next to the PMCE. If the extension expects frames the extension preceding the PMCE. If the extension expects
as input, build a frame for the message and pass it. frames for input, build a frame for the message and pass it.
An endpoint MUST use the following algorithm to receive a message in An endpoint MUST use the following algorithm to receive a message in
the form of an uncompressed message. If this PMCE is the last the form of an uncompressed message. If this PMCE is the last
extension to process incoming messages, deliver the _A WebSocket extension to process incoming messages, deliver the _A WebSocket
Message Has Been Received_ event to the application layer with the Message Has Been Received_ event to the application layer with the
received message payload and header values as-is. Otherwise, pass received message payload and header values as-is. Otherwise, pass
the message payload and header values to the extension next to the the message payload and header values to the extension preceding the
PMCE as-is. If the extension expects frames as input, build a frame PMCE as-is. If the extension expects frames for input, build a frame
for the message and pass it. for the message and pass it.
7. Intermediaries 7. Intermediaries
When an intermediary proxies a WebSocket connection, the intermediary When an intermediary proxies a WebSocket connection, the intermediary
MAY add, change or remove Per-message Compression on the messages if MAY add, change or remove Per-message Compression on proxied messages
the intermediary meets all of the following conditions: if the intermediary meets all of the following requirements:
o The intermediary understands that Per-message Compression. o The intermediary understands that Per-message Compression.
o The intermediary can read all of proxied data including the o The intermediary can read all data of the proxied WebSocket
opening handshake request, opening handshake response, and connection including the opening handshake request, opening
messages. handshake response, and messages.
o The intermediary can alter the proxied data before forwarding them o The intermediary can alter the proxied data before forwarding them
accordingly to conform to the constraints of the new combination accordingly to conform to the constraints of the new combination
of extensions. For example, if a PMCE is removed from messages, of extensions. For example, if Per-message Compression is removed
the corresponding element in the "Sec-WebSocket-Extensions" in the from messages, the corresponding element in the
opening handshake response must also be removed. "Sec-WebSocket-Extensions" in the opening handshake response which
enabled the Per-message Compression must also be removed.
Otherwise, the intermediary MUST NOT make any change on the proxied Otherwise, the intermediary MUST NOT add, change or remove Per-
data. message Compression on proxied messages.
8. permessage-deflate extension 8. permessage-deflate extension
This section specifies a specific PMCE called "permessage-deflate". This section specifies a specific PMCE called "permessage-deflate".
It compresses the payload of a message using the DEFLATE algorithm It compresses the payload of a message using the DEFLATE algorithm
[RFC1951] and the byte boundary aligning method introduced in [RFC1951] and the byte boundary aligning method introduced in
[RFC1979]. [RFC1979].
This section uses the term "byte" with the same meaning as RFC1951, 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). i.e. 8 bits stored or transmitted as a unit (same as an octet).
skipping to change at page 15, line 29 skipping to change at page 15, line 29
endpoints manage per-connection resource usage. endpoints manage per-connection resource usage.
o "server_no_context_takeover" o "server_no_context_takeover"
o "client_no_context_takeover" o "client_no_context_takeover"
o "server_max_window_bits" o "server_max_window_bits"
o "client_max_window_bits" o "client_max_window_bits"
These represent two methods (no_context_takeover and max_window_bits) These parameters enable two methods (no_context_takeover and
of constraining memory usage that may be applied independently to max_window_bits) of constraining memory usage that may be applied
either direction of WebSocket traffic. The extension parameters with independently to either direction of WebSocket traffic. The
the client_ prefix are used to negotiate DEFLATE parameters to extension parameters with the "client_" prefix are used to negotiate
control compression on messages sent by a client and received by a DEFLATE parameters to control compression on messages sent by a
server. The client refers to parameters with the client_ prefix to client and received by a server. The client refers to parameters
configure its compressor, while the server refers to them to with the "client_" prefix to configure its compressor, while the
configure its decompressor. The extension parameters with the server refers to them to configure its decompressor. The extension
server_ prefix are used to negotiate DEFLATE parameters to control parameters with the "server_" prefix are used to negotiate DEFLATE
compression on messages sent by a server and received by a client. parameters to control compression on messages sent by a server and
The server refers to parameters with the server_ prefix to configure received by a client. The server refers to parameters with the
its compressor, while the client refers to them to configure its "server_" prefix to configure its compressor, while the client refers
decompressor. All four parameters are defined for both a client's to them to configure its decompressor. All of these four parameters
extension negotiation offer and a server's extension negotiation are defined for both a client's extension negotiation offer and a
response. server's extension negotiation response.
A server MUST decline an extension negotiation offer for this A server MUST decline an extension negotiation offer for this
extension if any of the following conditions is met: extension if any of the following conditions is met:
o The offer has any extension parameter not defined for use in an o The offer has any extension parameter not defined for use in an
offer. offer.
o The offer has any extension parameter with an invalid value. o The offer has any extension parameter with an invalid value.
o The offer has multiple extension parameters with the same name. o The offer has multiple extension parameters with the same name.
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extension negotiation response meeting any of the following extension negotiation response meeting any of the following
condition: condition:
o The response has any extension parameter not defined for use in a o The response has any extension parameter not defined for use in a
response. response.
o The response has any extension parameter with an invalid value. o The response has any extension parameter with an invalid value.
o The response has multiple extension parameters with the same name. o The response has multiple extension parameters with the same name.
o The client doesn't support the configuration the response o The client doesn't support the configuration that the response
represents. represents.
The term "LZ77 sliding window" used in this section means the buffer The term "LZ77 sliding window" used in this section means the buffer
storing recently processed input. The LZ77 algorithm searches the used by the DEFLATE algorithm to store recently processed input. The
buffer for match with the next input. 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. Method Parameters
8.1.1. Context Takeover Control 8.1.1. Context Takeover Control
8.1.1.1. server_no_context_takeover 8.1.1.1. server_no_context_takeover
A client MAY include the "server_no_context_takeover" extension A client MAY include the "server_no_context_takeover" extension
parameter in an extension negotiation offer. This extension parameter in an extension negotiation offer. This extension
parameter has no value. By including this extension parameter in an parameter has no value. By including this extension parameter in an
extension negotiation offer, a client prevents the peer server from extension negotiation offer, a client prevents the peer server from
using the same LZ77 sliding window it used to build frames of the using context take over. If the peer server doesn't use context take
last sent message to build frames of the next message. If the peer over, the client doesn't need to reserve memory to retain the LZ77
server doesn't use the same LZ77 sliding window to compress multiple sliding window in between messages.
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 Absence of this extension parameter in an extension negotiation offer
indicates that the client can receive a message which the server has indicates that the client can receive a message which the server
built using the same LZ77 sliding window the server used to build the built using context take over.
last sent message.
A server accepts an extension negotiation offer including the A server accepts an extension negotiation offer including the
"server_no_context_takeover" extension parameter by including the "server_no_context_takeover" extension parameter by including the
"server_no_context_takeover" extension parameter in the corresponding
extension negotiation response to send back to the client. The
"server_no_context_takeover" extension parameter in an extension "server_no_context_takeover" extension parameter in an extension
negotiation response. The "server_no_context_takeover" extension negotiation response has no value.
parameter in an extension negotiation response has no value.
It is RECOMMENDED that a server supports the It is RECOMMENDED that a server supports the
"server_no_context_takeover" extension parameter in an extension "server_no_context_takeover" extension parameter in an extension
negotiation offer. negotiation offer.
A server MAY include the "server_no_context_takeover" extension A server MAY include the "server_no_context_takeover" extension
parameter in an extension negotiation response even if the extension parameter in an extension negotiation response even if the extension
negotiation offer being accepted by the extension negotiation negotiation offer being accepted by the extension negotiation
response didn't have the "server_no_context_takeover" extension response didn't have the "server_no_context_takeover" extension
parameter. parameter.
8.1.1.2. client_no_context_takeover 8.1.1.2. client_no_context_takeover
A client MAY include the "client_no_context_takeover" extension A client MAY include the "client_no_context_takeover" extension
parameter in an extension negotiation offer. This extension parameter in an extension negotiation offer. This extension
parameter has no value. By including this extension parameter in an parameter has no value. By including this extension parameter in an
extension negotiation offer, a client informs the peer server a hint extension negotiation offer, a client informs the peer server of a
that even if the server won't include the hint that even if the server won't include the
"client_no_context_takeover" extension parameter in the extension "client_no_context_takeover" extension parameter in the corresponding
negotiation response to the offer, the client is not going to use the extension negotiation response to the offer, the client is not going
same LZ77 sliding window it used to build frames of the last sent to use context take over.
message to build frames of the next message.
A server MAY include the "client_no_context_takeover" extension A server MAY include the "client_no_context_takeover" extension
parameter in an extension negotiation response. If the received parameter in an extension negotiation response. If the received
extension negotiation offer includes the "client_no_context_takeover" extension negotiation offer includes the "client_no_context_takeover"
extension parameter, the server may either ignore it or use it to extension parameter, the server may either ignore the parameter or
avoid taking over an LZ77 sliding window unnecessarily by including use the parameter to avoid taking over an LZ77 sliding window
"client_no_context_takeover" extension parameter in the extension unnecessarily by including "client_no_context_takeover" extension
negotiation response to the offer. The "client_no_context_takeover" parameter in the corresponding extension negotiation response to the
extension parameter in an extension negotiation response has no offer. The "client_no_context_takeover" extension parameter in an
value. By including the "client_no_context_takeover" extension extension negotiation response has no value. By including the
parameter in an extension negotiation response, a server prevents the "client_no_context_takeover" extension parameter in an extension
peer client from using the same LZ77 sliding window it used to build negotiation response, a server prevents the peer client from using
frames of the last sent message to build frames for the next message. context take over. This reduces the amount of memory that the server
This reduces the amount of memory that the server has to reserve for has to reserve for the connection.
the connection.
Absence of this extension parameter in an extension negotiation Absence of this extension parameter in an extension negotiation
response indicates that the server can receive a message which the response indicates that the server can receive messages built by the
client has built using the same LZ77 sliding window the client used client using context take over.
to build the last sent message.
A client MUST support the "client_no_context_takeover" extension A client MUST support the "client_no_context_takeover" extension
parameter in an extension negotiation response. parameter in an extension negotiation response.
8.1.2. Limiting the LZ77 sliding window size 8.1.2. Limiting the LZ77 sliding window size
8.1.2.1. server_max_window_bits 8.1.2.1. server_max_window_bits
A client MAY include the "server_max_window_bits" extension parameter A client MAY include the "server_max_window_bits" extension parameter
in an extension negotiation offer. This extension parameter has a in an extension negotiation offer. This parameter has a decimal
decimal integer value without leading zeroes between 8 to 15 integer value without leading zeroes between 8 to 15 inclusive
inclusive indicating the base-2 logarithm of the LZ77 sliding window indicating the base-2 logarithm of the LZ77 sliding window size and
size and MUST conform to the ABNF below. MUST conform to the ABNF below.
server_max_window_bits = 1*DIGIT server_max_window_bits = 1*DIGIT
By including this parameter in an extension negotiation offer, a By including this parameter in an extension negotiation offer, a
client limits the LZ77 sliding window size that the server uses to client limits the LZ77 sliding window size that the server uses to
compress messages. If the peer server uses small LZ77 sliding window compress messages. If the peer server uses small LZ77 sliding window
to compress messages, the client can reduce the memory for the LZ77 to compress messages, the client can reduce the memory for the LZ77
sliding window. sliding window.
A server declines an extension negotiation offer with this extension A server declines an extension negotiation offer with this parameter
parameter if the server doesn't support it. if the server doesn't support it.
Absence of this extension parameter in an extension negotiation offer Absence of this parameter in an extension negotiation offer indicates
indicates that the client can receive messages compressed using an that the client can receive messages compressed using an LZ77 sliding
LZ77 sliding window of up to 32,768 bytes. window of up to 32,768 bytes.
A server accepts an extension negotiation offer with this extension A server accepts an extension negotiation offer with this parameter
parameter by including the "server_max_window_bits" extension by including the "server_max_window_bits" extension parameter in the
parameter in the extension negotiation response with the same or extension negotiation response to send back to the client with the
smaller value as the extension negotiation offer. The same or smaller value as the offer. The "server_max_window_bits"
"server_max_window_bits" extension parameter in an extension extension parameter in an extension negotiation response has a
negotiation response has a decimal integer value without leading decimal integer value without leading zeroes between 8 to 15
zeroes between 8 to 15 inclusive indicating the base-2 logarithm of inclusive indicating the base-2 logarithm of the LZ77 sliding window
the LZ77 sliding window size and MUST conform to the ABNF below. size and MUST conform to the ABNF below.
server_max_window_bits = 1*DIGIT server_max_window_bits = 1*DIGIT
A server MAY include the "server_max_window_bits" extension parameter A server MAY include the "server_max_window_bits" extension parameter
in an extension negotiation response even if the extension in an extension negotiation response even if the extension
negotiation offer being accepted by the extension negotiation negotiation offer being accepted by the response didn't include the
response didn't include the "server_max_window_bits" extension "server_max_window_bits" extension parameter.
parameter.
8.1.2.2. client_max_window_bits 8.1.2.2. client_max_window_bits
A client MAY include the "client_max_window_bits" extension parameter A client MAY include the "client_max_window_bits" extension parameter
in an extension negotiation offer. This extension parameter has no in an extension negotiation offer. This parameter has no value or a
value or a decimal integer value without leading zeroes between 8 to decimal integer value without leading zeroes between 8 to 15
15 inclusive indicating the base-2 logarithm of the LZ77 sliding inclusive indicating the base-2 logarithm of the LZ77 sliding window
window size. If a value is specified for this extension parameter, size. If a value is specified for this parameter, the value MUST
the value MUST conform to the ABNF below. conform to the ABNF below.
client_max_window_bits = 1*DIGIT client_max_window_bits = 1*DIGIT
By including this extension parameter in an extension negotiation By including this parameter in an offer, a client informs the peer
offer, a client informs the peer server that the client supports the 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 "client_max_window_bits" extension parameter in an extension
negotiation response, and optionally a hint by including an extension negotiation offer has a value, the parameter also informs the peer
parameter value. If the "client_max_window_bits" extension parameter server of a hint that even if the server won't include the
in an extension negotiation offer has a value, the extension "client_max_window_bits" extension parameter in the corresponding
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 response with a value greater than one in the
extension negotiation offer or the server doesn't include the extension negotiation offer or the server doesn't include the
extension parameter at all, the client is not going to use LZ77 extension parameter at all, the client is not going to use LZ77
sliding window size greater than the size specified by the value in sliding window size greater than the size specified by the value in
the extension negotiation offer to compress messages. the extension negotiation offer to compress messages.
If a received extension negotiation offer has the If a received extension negotiation offer has the
"client_max_window_bits" extension parameter, the server MAY include "client_max_window_bits" extension parameter, the server MAY include
the "client_max_window_bits" extension parameter in the extension the "client_max_window_bits" extension parameter in the corresponding
negotiation response to the offer. If the "client_max_window_bits" extension negotiation response to the offer. If the
extension parameter in a received extension negotiation offer has a "client_max_window_bits" extension parameter in a received extension
value, the server may either ignore this value or use this value to negotiation offer has a value, the server may either ignore this
avoid allocating an unnecessarily big LZ77 sliding window by value or use this value to avoid allocating an unnecessarily big LZ77
including the "client_max_window_bits" extension parameter in the sliding window by including the "client_max_window_bits" extension
extension negotiation response to the offer with a value equal to or parameter in the corresponding extension negotiation response to the
smaller than the received value. The "client_max_window_bits" offer with a value equal to or smaller than the received value. The
extension parameter in an extension negotiation response has a "client_max_window_bits" extension parameter in an extension
decimal integer value without leading zeroes between 8 to 15 negotiation response has a decimal integer value without leading
inclusive indicating the base-2 logarithm of the LZ77 sliding window zeroes between 8 to 15 inclusive indicating the base-2 logarithm of
size and MUST conform to the ABNF below. the LZ77 sliding window size and MUST conform to the ABNF below.
client_max_window_bits = 1*DIGIT client_max_window_bits = 1*DIGIT
By including this extension parameter in an extension negotiation By including this extension parameter in an extension negotiation
response, a server limits the LZ77 sliding window size that the response, a server limits the LZ77 sliding window size that the
client uses to compress messages. This reduces the amount of memory client uses to compress messages. This reduces the amount of memory
that the server has to reserve for the connection. for decompression context that the server has to reserve for the
connection.
If a received extension negotiation offer doesn't have the If a received extension negotiation offer doesn't have the
"client_max_window_bits" extension parameter, the server MUST NOT "client_max_window_bits" extension parameter, the corresponding
include the "client_max_window_bits" extension parameter in an extension negotiation response to the offer MUST NOT include the
extension negotiation response to the offer. "client_max_window_bits" extension parameter.
Absence of this extension parameter in an extension negotiation Absence of this extension parameter in an extension negotiation
response indicates that the server can receive messages compressed response indicates that the server can receive messages compressed
using an LZ77 sliding window of up to 32,768 bytes. using an LZ77 sliding window of up to 32,768 bytes.
8.1.3. Example 8.1.3. Example
The simplest "Sec-WebSocket-Extensions" header in a client's opening The simplest "Sec-WebSocket-Extensions" header in a client's opening
handshake to offer use of the permessage-deflate is the following: handshake to offer use of the permessage-deflate is as follows:
Sec-WebSocket-Extensions: permessage-deflate Sec-WebSocket-Extensions: permessage-deflate
Since the "client_max_window_bits" extension parameter is not Since the "client_max_window_bits" extension parameter is not
included in this extension negotiation offer, the server MUST NOT included in this extension negotiation offer, the server must not
accept the offer with an extension negotiation response including the accept the offer with an extension negotiation response including the
"client_max_window_bits" extension parameter. The simplest "client_max_window_bits" extension parameter. The simplest
"Sec-WebSocket-Extensions" header in a server's opening handshake to "Sec-WebSocket-Extensions" header in a server's opening handshake to
accept use of the permessage-deflate is the same. accept use of the permessage-deflate is the same.
The following extension negotiation offer sent by a client is asking 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 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" and declaring that the client supports the "client_max_window_bits"
extension parameter in an extension negotiation response. extension parameter in an extension negotiation response.
Sec-WebSocket-Extensions: Sec-WebSocket-Extensions:
permessage-deflate; permessage-deflate;
client_max_window_bits; server_max_window_bits=10 client_max_window_bits; server_max_window_bits=10
This extension negotiation offer might be rejected by the server This extension negotiation offer might be rejected by the server
because the server doesn't support the "server_max_window_bits" because the server doesn't support the "server_max_window_bits"
extension parameter in an extension negotiation offer. This is fine extension parameter in an extension negotiation offer. This is fine
if the client cannot receive messages compressed using a larger if the client cannot receive messages compressed using a larger
sliding window size, but if the client wants to fallback to the sliding window size, but if the client just prefers using a small
"permessage-deflate" without the "server_max_window_bits" extension window but wants to fallback to the "permessage-deflate" without the
parameter, the client can offer the fallback option explicitly like "server_max_window_bits" extension parameter, the client can make an
this: offer with the fallback option like this:
Sec-WebSocket-Extensions: Sec-WebSocket-Extensions:
permessage-deflate; permessage-deflate;
client_max_window_bits; server_max_window_bits=10, client_max_window_bits; server_max_window_bits=10,
permessage-deflate; permessage-deflate;
client_max_window_bits client_max_window_bits
This extension negotiation offer lists two configurations so that the The server can accept permessage-deflate by picking the supported one
server can accept permessage-deflate by picking a supported one. To from the listed offers. To accept the first option, for example, the
accept the first option, the server might send back, for example: server may send back a response as follows:
Sec-WebSocket-Extensions: Sec-WebSocket-Extensions:
permessage-deflate; server_max_window_bits=10 permessage-deflate; server_max_window_bits=10
And to accept the second option, the server might send back, for To accept the second option, for example, the server may send back a
example: response as follows:
Sec-WebSocket-Extensions: permessage-deflate Sec-WebSocket-Extensions: permessage-deflate
8.2. Message Payload Transformation 8.2. Message Payload Transformation
8.2.1. Compression 8.2.1. Compression
An endpoint uses the following algorithm to compress a message. An endpoint uses the following algorithm to compress a message.
1. Compress all the octets of the payload of the message using 1. Compress all the octets of the payload of the message using
DEFLATE. DEFLATE.
2. If the resulting data does not end with an empty DEFLATE block 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 with no compression (the "BTYPE" bits are set to 00), append an
empty DEFLATE block with no compression to the tail end. empty DEFLATE block with no compression to the tail end.
3. Remove 4 octets (that are 0x00 0x00 0xff 0xff) from 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 After this step, the last octet of the compressed data contains
(possibly part of) the DEFLATE header bits with the "BTYPE" bits (possibly part of) the DEFLATE header bits with the "BTYPE" bits
set to 00. set to 00.
In using DEFLATE in the first step above: When using DEFLATE in the first step above:
o An endpoint MAY use multiple DEFLATE blocks to compress one o An endpoint MAY use multiple DEFLATE blocks to compress one
message. message.
o An endpoint MAY use DEFLATE blocks of any type. o An endpoint MAY use DEFLATE blocks of any type.
o An endpoint MAY use both DEFLATE blocks with the "BFINAL" bit set 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. 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 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 at a byte boundary, an endpoint MUST add minimal padding bits of 0
make it end at a byte boundary. The next DEFLATE block follows to make it end at a byte boundary. The next DEFLATE block follows
the padded data if any. the padded data if any.
When the message is going to be fragmented, the concatenation of all An endpoint fragments a compressed message by splitting the result of
payload of the fragments must be the result of running this running this algorithm. Even when only a part of payload is
algorithm. Even when only a part of payload is available, a fragment available, a fragment can be built by compressing the available data
can be built by compressing the available data and aligning the end and choosing block type appropriately so that the end of the
of compressed data at a byte boundary. Note that for non-final resulting compressed data is aligned at a byte boundary. Note that
fragments, removal of 0x00 0x00 0xff 0xff must not be done. 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 An endpoint MUST NOT use an LZ77 sliding window longer than 32,768
bytes to compress messages to send. bytes to compress messages to send.
If the "agreed parameters" contain the "client_no_context_takeover" If the "agreed parameters" contain the "client_no_context_takeover"
extension parameter, the client MUST start compressing each new extension parameter, the client MUST start compressing each new
message with an empty LZ77 sliding window. Otherwise, the client MAY message with an empty LZ77 sliding window. Otherwise, the client MAY
take over the LZ77 sliding window used to build the last compressed take over the LZ77 sliding window used to build the last compressed
message. Note that even if the client has included the message. Note that even if the client has included the
"client_no_context_takeover" extension parameter in its offer, the "client_no_context_takeover" extension parameter in its offer, the
client MAY take over the LZ77 sliding window used to build the last client MAY take over the LZ77 sliding window used to build the last
compressed message if the "agreed parameters" doesn't contain the compressed message if the "agreed parameters" don't contain the
"client_no_context_takeover" extension parameter. The client-to- "client_no_context_takeover" extension parameter. The client-to-
server "client_no_context_takeover" extension parameter is just a server "client_no_context_takeover" extension parameter is just a
hint for the server to build a response. hint for the server to build an extension negotiation response.
If the "agreed parameters" contain the "server_no_context_takeover" If the "agreed parameters" contain the "server_no_context_takeover"
extension parameter, the server MUST start compressing each new extension parameter, the server MUST start compressing each new
message with an empty LZ77 sliding window. Otherwise, the server MAY message with an empty LZ77 sliding window. Otherwise, the server MAY
take over the LZ77 sliding window used to build the last compressed take over the LZ77 sliding window used to build the last compressed
message. message.
If the "agreed parameters" contain the "client_max_window_bits" If the "agreed parameters" contain the "client_max_window_bits"
extension parameter with a value of w, the client MUST NOT use an 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 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 messages to send. Note that even if the client has included in its
offer the "client_max_window_bits" extension parameter with a value offer the "client_max_window_bits" extension parameter with a value
smaller than one in the "agreed parameters", the client MAY use an smaller than one in the "agreed parameters", the client MAY use an
LZ77 sliding window with any size to compress messages to send as LZ77 sliding window with any size to compress messages to send as
long as the size conforms to the "agreed parameters". The client-to- long as the size conforms to the "agreed parameters". The client-to-
server "client_max_window_bits" extension parameter is just a hint server "client_max_window_bits" extension parameter is just a hint
for the server to build a response. for the server to build an extension negotiation response.
If the "agreed parameters" contain the "server_max_window_bits" If the "agreed parameters" contain the "server_max_window_bits"
extension parameter with a value of w, the server MUST NOT use an 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 LZ77 sliding window longer than the w-th power of 2 bytes to compress
messages to send. messages to send.
8.2.2. Decompression 8.2.2. Decompression
An endpoint uses the following algorithm to decompress a message. 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 1. Append 4 octets of 0x00 0x00 0xff 0xff to the tail end of the
payload of the message. payload of the message.
2. Decompress the resulting data using DEFLATE. 2. Decompress the resulting data using DEFLATE.
If the "agreed parameters" contain the "server_no_context_takeover" If the "agreed parameters" contain the "server_no_context_takeover"
extension parameter, the client MAY start decompressing each new extension parameter, the client MAY decompress each new message with
message with an empty LZ77 sliding window. Otherwise, the client an empty LZ77 sliding window. Otherwise, the client MUST decompress
MUST take over the LZ77 sliding window used to process the last each new message using the LZ77 sliding window used to process the
compressed message. last compressed message.
If the "agreed parameters" contain the "client_no_context_takeover" If the "agreed parameters" contain the "client_no_context_takeover"
extension parameter, the server MAY start decompressing each new extension parameter, the server MAY decompress each new message with
message with an empty LZ77 sliding window. Otherwise, the server an empty LZ77 sliding window. Otherwise, the server MUST decompress
MUST take over the LZ77 sliding window used to process the last each new message using the LZ77 sliding window used to process the
compressed message. Note that even if the client has included the last compressed message. Note that even if the client has included
"client_no_context_takeover" extension parameter in its offer, the the "client_no_context_takeover" extension parameter in its offer,
server MUST take over the LZ77 sliding window used to process the the server MUST decompress each new message using the LZ77 sliding
last compressed message if the "agreed parameters" doesn't contain window used to process the last compressed message if the "agreed
the "client_no_context_takeover" extension parameter. The client-to- parameters" don't contain the "client_no_context_takeover" extension
server "client_no_context_takeover" extension parameter is just a parameter. The client-to-server "client_no_context_takeover"
hint for the server to build a response. 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" If the "agreed parameters" contain the "server_max_window_bits"
extension parameter with a value of w, the client MAY reduce the size extension parameter with a value of w, the client MAY reduce the size
of its LZ77 sliding window to decompress received messages down to 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 the w-th power of 2 bytes. Otherwise, the client MUST use a 32,768
byte LZ77 sliding window to decompress received messages. byte LZ77 sliding window to decompress received messages.
If the "agreed parameters" contain the "client_max_window_bits" If the "agreed parameters" contain the "client_max_window_bits"
extension parameter with a value of w, the server MAY reduce the size extension parameter with a value of w, the server MAY reduce the size
of its LZ77 sliding window to decompress received messages down to 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 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 byte LZ77 sliding window to decompress received messages. Note that
even if the client has included in its offer the even if the client has included in its offer the
"client_max_window_bits" extension parameter with a value smaller "client_max_window_bits" extension parameter with a value smaller
that one in the "agreed parameters", the client MUST use an LZ77 than one in the "agreed parameters", the client MUST use an LZ77
sliding window of a size that conforms the "agreed parameters" to sliding window of a size that conforms the "agreed parameters" to
compress messages to send. The client-to-server compress messages to send. The client-to-server
"client_max_window_bits" extension parameter is just a hint for the "client_max_window_bits" extension parameter is just a hint for the
server to build a response. server to build an extension negotiation response.
8.2.3. Examples 8.2.3. Examples
This section introduces examples of how the permessage-deflate This section introduces examples of how the permessage-deflate
transforms messages. transforms messages.
8.2.3.1. A message compressed using 1 compressed DEFLATE block 8.2.3.1. A message compressed using 1 compressed DEFLATE block
Suppose that an endpoint sends a text message "Hello". If the Suppose that an endpoint sends a text message "Hello". If the
endpoint uses 1 compressed DEFLATE block (compressed with fixed endpoint uses 1 compressed DEFLATE block (compressed with fixed
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This is the payload of the first message the client has sent. If the This is the payload of the first message the client has sent. If the
"agreed parameters" contain the "client_no_context_takeover" "agreed parameters" contain the "client_no_context_takeover"
extension parameter, the client compresses the payload of the next extension parameter, the client compresses the payload of the next
message into the same bytes (if the client uses the same "BTYPE" message into the same bytes (if the client uses the same "BTYPE"
value and "BFINAL" value). So, the payload of the second message value and "BFINAL" value). So, the payload of the second message
will be: will be:
0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00 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 If the "agreed parameters" did not contain the
"client_no_context_takeover" extension parameter, the client can "client_no_context_takeover" extension parameter, the client can
compress the payload of the next message into shorter bytes by compress the payload of the next message into shorter bytes by
referencing the history in the LZ77 sliding window. So, the payload referencing the history in the LZ77 sliding window. So, the payload
of the second message will be: of the second message will be:
0xf2 0x00 0x11 0x00 0x00 0xf2 0x00 0x11 0x00 0x00
When concatenated with the first message: So, 2 bytes are saved in total.
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 Note that even if some uncompressed messages (with the RSV1 bit
unset) are inserted between the two "Hello" messages, they will make unset) are inserted between the two "Hello" messages, they don't
no difference to the LZ77 sliding window. affect the LZ77 sliding window.
8.2.3.3. Using a DEFLATE Block with No Compression 8.2.3.3. Using a DEFLATE Block with No Compression
0xc1 0x0b 0x00 0x05 0x00 0xfa 0xff 0x48 0x65 0x6c 0x6c 0x6f 0x00 0xc1 0x0b 0x00 0x05 0x00 0xfa 0xff 0x48 0x65 0x6c 0x6c 0x6f 0x00
This is a frame constituting a text message "Hello" compressed using This is a frame constituting a text message "Hello" built using a
a DEFLATE block with no compression. The first 2 octets (0xc1 0x0b) DEFLATE block with no compression. The first 2 octets (0xc1 0x0b)
are the WebSocket frame header (FIN=1, RSV1=1, RSV2=0, RSV3=0, 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 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 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 fragmented) because the RSV1 bit is set when DEFLATE is applied to
the message, including the case when only DEFLATE blocks with no the message, including the case when only DEFLATE blocks with no
compression are used. The third to 13th octet consists a payload compression are used. The third to 13th octet consists a payload
data containing "Hello" compressed using a DEFLATE block with no data containing "Hello" compressed using a DEFLATE block with no
compression. compression.
8.2.3.4. Using a DEFLATE Block with BFINAL Set to 1 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 On platform where the flush method using an empty DEFLATE block with
no compression is not avaiable, implementors can choose to flush data no compression is not available, implementors can choose to flush
using DEFLATE blocks with "BFINAL" set to 1. data using DEFLATE blocks with "BFINAL" set to 1.
0xf3 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00 0xf3 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00
This is a payload of a message containing "Hello" compressed using a This is a payload of a message containing "Hello" compressed using a
DEFLATE block with "BFINAL" set to 1. The first 7 octets constitute 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 a DEFLATE block with "BFINAL" set to 1 and "BTYPE" set to 01
containing "Hello". The last 1 octet (0x00) contains the header bits 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 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 0. This octet is necessary to allow the payload to be decompressed
in the same manner as messages flushed using DEFLATE blocks with in the same manner as messages flushed using DEFLATE blocks with
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On the direction where context takeover is disallowed, an endpoint On the direction where context takeover is disallowed, an endpoint
can easily figure out whether a certain message will be shorter if can easily figure out whether a certain message will be shorter if
compressed or not.. Otherwise, it's not easy to know whether future compressed or not.. Otherwise, it's not easy to know whether future
messages will benefit from having a certain message compressed. messages will benefit from having a certain message compressed.
Implementor may employ some heuristics to determine this. Implementor may employ some heuristics to determine this.
8.4. Intermediaries 8.4. Intermediaries
When an intermediary forwards a message, the intermediary MAY change When an intermediary forwards a message, the intermediary MAY change
compression on the messages as far as the resulting sequence of compression on the messages as far as the resulting sequence of
messages conform to the constraints based on the "agreed parameters". messages conforms to the constraints based on the "agreed
For example, an intermediary may decompress a received message, unset parameters". For example, an intermediary may decompress a received
the "Per-message Compressed" bit and forward it to the other peer. message, unset the "Per-message Compressed" bit and forward it to the
Since such a compression change may affect the LZ77 sliding window, other peer. Since such a compression change may affect the LZ77
the intermediary may need to parse and transform the following sliding window, the intermediary may need to parse and transform the
messages, too. following messages, too.
9. Security Considerations 9. Security Considerations
There is a known exploit for combination of a secure transport There is a known exploit for combination of a secure transport
protocol and history-based compression [CRIME]. Implementors should protocol and history-based compression [CRIME]. Implementors should
give attention to this point when integrating this extension with give attention to this point when integrating this extension with
other extensions or protocols. other extensions or protocols.
10. IANA Considerations 10. IANA Considerations
skipping to change at page 30, line 17 skipping to change at page 30, line 17
Special thanks to Patrick McManus who wrote up the initial Special thanks to Patrick McManus who wrote up the initial
specification of a DEFLATE-based compression extension for the specification of a DEFLATE-based compression extension for the
WebSocket Protocol to which I referred to write this specification. WebSocket Protocol to which I referred to write this specification.
Thank you to the following people who participated in discussions on Thank you to the following people who participated in discussions on
the HyBi WG and contributed ideas and/or provided detailed reviews the HyBi WG and contributed ideas and/or provided detailed reviews
(the list is likely to be incomplete): Adam Rice, Alexey Melnikov, (the list is likely to be incomplete): Adam Rice, Alexey Melnikov,
Arman Djusupov, Bjoern Hoehrmann, Brian McKelvey, Dario Crivelli, Arman Djusupov, Bjoern Hoehrmann, Brian McKelvey, Dario Crivelli,
Greg Wilkins, Inaki Baz Castillo, Jamie Lokier, Joakim Erdfelt, John Greg Wilkins, Inaki Baz Castillo, Jamie Lokier, Joakim Erdfelt, John
A. Tamplin, Julian Reschke, Kenichi Ishibashi, Mark Nottingham, Peter A. Tamplin, Julian Reschke, Kenichi Ishibashi, Mark Nottingham, Peter
Thorson, Roberto Peon, Simone Bordet and Tobias Oberstein. Note that Thorson, Roberto Peon, Simone Bordet, Tobias Oberstein and Yutaka
people listed above didn't necessarily endorse the end result of this Hirano. Note that people listed above didn't necessarily endorse the
work. end result of this work.
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996. version 1.3", RFC 1951, May 1996.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
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