HyBi Working Group T. Yoshino
Internet-Draft Google, Inc.
Intended status: Standards Track April 27, 2011
Expires: October 29, 2011
WebSocket Per-frame DEFLATE Extension
draft-tyoshino-hybi-websocket-perframe-deflate-00
Abstract
This specification defines a per-frame DEFLATE compression extension
for the WebSocket protocol. This extension enables compressing only
the Application Data part of WebSocket frames using DEFLATE
compression while leaving the other parts such as opcode, payload
length, the Extension Data part untouched.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Extension Overview . . . . . . . . . . . . . . . . . . . . 3
2. Conformance Requirements . . . . . . . . . . . . . . . . . . . 4
3. Extension Negotiation . . . . . . . . . . . . . . . . . . . . 5
4. Detailed Specification . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. Registration of the "deflate-application-data"
WebSocket Extension Keyword . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
1.1. Background
_This section is non-normative._
The WebSocket protocol is designed to allow for extensions
[I-D.ietf-hybi-thewebsocketprotocol]. This enables use of
compression extension to reduce the amount of data to be sent over
network.
The WebSocket protocol specification contains built-in compression
extension "deflate-stream" that applies DEFLATE to everything
including header octets such as opcode and payload length. It's
simple, however it requires intermediaries that are not interested in
the contents of the Application Data part to decompress received
octets using DEFLATE to see opcode, etc.
1.2. Extension Overview
_This section is non-normative._
Per-frame DEFLATE extension adds another DEFLATE [RFC1951] based
compression option to the WebSocket protocol. Per-frame DEFLATE
extension compresses only the octets in the Application Data part of
a frame. Header octets are unchanged by this extension.
To align the end of compressed data to octet boundary, this extension
uses the algorithm described in the Section 2.1 of the PPP Deflate
Protocol [RFC1979]. The frame sender keeps using the same LZ77
sliding window across multiple frames [LZ77].
This extension doesn't use any of reserved bits, opcodes or Extension
Data part.
The simplest "Sec-WebSocket-Extensions" header in the opening
handshake from the client that negotiates only per-frame DEFLATE
extension is the following:
Sec-WebSocket-Extensions: deflate-application-data
The server also sends the same header to accept the use of this
compression extension.
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2. Conformance Requirements
Everything in this specification except for sections explicitly
marked non-normative is normative.
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].
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3. Extension Negotiation
The registered extension token for this extension is
"deflate-application-data".
To use per-frame DEFLATE extension, a user-agent MUST send an opening
handshake request with the "deflate-application-data" extension token
included in its "Sec-WebSocket-Extensions" header.
To accept use of per-frame DEFLATE extension requested by the user-
agent, a server MUST send an opening handshake response with the
"deflate-application-data" extension token included in its
"Sec-WebSocket-Extensions" header.
Any extension parameter MUST NOT be attached to
"deflate-application-data" extension token.
Once per-frame DEFLATE extension is accepted, both endpoints MUST use
the algorithm described in Section 4 in sending frames.
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4. Detailed Specification
This extension does not use any of reserved bits, opcodes or the
Extension Data part in a WebSocket frame.
This extension operates only on Data Frames, and only on the
Application Data therein (it does not affect the Extension Data
portion of the Payload Data).
A sender using this extension MUST compress all octets of the
Application Data part using DEFLATE [RFC1951]. The sender MAY use
multiple blocks to compress data for one frame. The sender MAY use
blocks with any effective "BTYPE" defined in [RFC1951] for one frame.
"BFINAL" bit MUST NOT be set for any block. The sender MUST use the
algorithm described in the Data paragraph of the Section 2.1 of the
PPP Deflate Protocol [RFC1979] to align the end of the compressed
data to octet boundary. That is that the sender MUST append an empty
uncompressed block to the tail of the compressed data, and then
remove 4 octets from the tail of the concatenated data. The sender
MUST put this data (the octets left after removing 4 octets) in the
Application Data part instead of the original data. The payload
length field of the frame MUST be the number of octets in the
Extension Data part and the Application Data part after this process.
The senders MAY keep using the same LZ77 sliding window across
multiple frames [LZ77].
A receiver MUST append 4 octets of 0x00 0x00 0xff 0xff to the tail of
the received octets in the Application Data part and decompress the
concatenated octets using DEFLATE to get the original octets for the
Application Data part. The receiver MUST keep using the same LZ77
sliding window for all frames on the same WebSocket connection. The
receiver MUST use 32768 byte LZ77 sliding window.
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5. Security Considerations
There's no security concern for now.
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6. IANA Considerations
6.1. Registration of the "deflate-application-data" WebSocket Extension
Keyword
Name of token.
deflate-application-data
Author/Change controller
IETF <iesg@ietf.org>
Contact
HYBi <hybi@ietf.org>
References
RFC XXX
The "deflate-application-data" token is used in the
"Sec-WebSocket-Extensions" header in the WebSocket opening handshake
to negotiate use of per-frame DEFLATE compression extension.
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7. Acknowledgements
Special thanks to Patrick McManus who wrote up the initial version of
DEFLATE-based compression extension specification for the WebSocket
protocol which I referred when I write this specification.
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8. References
8.1. Normative References
[I-D.ietf-hybi-thewebsocketprotocol]
Fette, I., "The WebSocket protocol",
draft-ietf-hybi-thewebsocketprotocol (work in progress),
April 2011.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[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.
8.2. Informative References
[RFC1979] Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.
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Author's Address
Takeshi Yoshino
Google, Inc.
Email: tyoshino@google.com
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