HyBi Working Group T. Yoshino
Internet-Draft Google, Inc.
Intended status: Standards Track August 2, 2011
Expires: February 3, 2012
WebSocket Per-frame DEFLATE Extension
draft-tyoshino-hybi-websocket-perframe-deflate-02
Abstract
This specification defines a per-frame DEFLATE compression extension
for the WebSocket protocol. This extension compresses the
application data part of WebSocket data frames using DEFLATE.
Status of this Memo
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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 . . . . . . . . . . . . . . . . . . . 4
3. Extension Negotiation . . . . . . . . . . . . . . . . . . . . 5
4. Application Data Transformation . . . . . . . . . . . . . . . 6
4.1. Sending . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Receiving . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Registration of the "deflate-frame" WebSocket
Extension Keyword . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
_This section is non-normative._
As well as other protocols, octets exchanged over the WebSocket
protocol [I-D.ietf-hybi-thewebsocketprotocol] can benefit from
compression. This specification adds DEFLATE [RFC1951] based
compression functionality to the WebSocket protocol using its
extension framework.
Per-frame DEFLATE extension applies DEFLATE to the octets in the
application data part of data frames. 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]. Endpoints
can take over LZ77 sliding window [LZ77] from previous frames to get
better compression ratio.
The simplest "Sec-WebSocket-Extensions" header in the client opening
handshake to request per-frame DEFLATE extension is the following:
Sec-WebSocket-Extensions: deflate-frame
The simplest header from the server to accept this extension is the
same.
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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-frame".
To request use of per-frame DEFLATE extension, a client MUST include
the "deflate-frame" extension token in the "Sec-WebSocket-Extensions"
header in its opening handshake request.
To accept use of per-frame DEFLATE extension requested by the client,
a server MUST include the "deflate-frame" extension token in the
"Sec-WebSocket-Extensions" header in its opening handshake response.
An endpoint MAY attach one or more extension parameters as defined
below to the extension token.
Maximum LZ77 sliding window size
An endpoint MAY attach "max_window_bits" extension parameter to
limit the LZ77 sliding window size that the other peer uses. This
parameter MUST have an integer value in the range between 8 to 15
indicating the base-2 logarithm of the LZ77 sliding window size.
An endpoint that received this parameter MUST NOT use LZ77 sliding
window size greater than this value to compress a frame.
Disallow compression context takeover
An endpoint MAY attach "no_context_takeover" extension parameter
to disallow the other peer to take over LZ77 sliding window
fromprevious frames. This parameter has no value. An endpoint
that received this parameter MUST NOT take over LZ77 sliding
window from previous frames to compress a frame.
An endpoint MUST _Fail the WebSocket Connection_ if any unknown
extension parameter is attached to "deflate-frame" extension token in
the opening handshake request or response.
Once per-frame DEFLATE extension is accepted, both endpoints MUST use
the algorithm described in Section 4 to exchange frames.
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4. Application Data Transformation
This extension uses one reserved bit to indicate DEFLATE is applied
to the frame or not. We call this "COMP" bit.
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).
4.1. Sending
To send a frame with DEFLATE applied, an endpoint MUST use the
following algorithm.
1. Apply DEFLATE [RFC1951] to all the octets in the application data
part of the frame. Multiple blocks MAY be used. Any type of
block MAY be used. Both block with "BFINAL" set to 0 and 1 MAY
be used.
2. If the resulting data does not end with a block with no
compression ("BTYPE" set to 0), append an empty block with no
compression to the tail.
3. Remove 4 octets (that is 0x00 0x00 0xff 0xff) from the tail.
4. Build a frame by putting the resulting octets in the application
data part instead of the original octets. The payload length
field of the frame MUST be the sum of the size of the extension
data part and this resulting octets. "COMP" bit MUST be set to
1.
To send a frame with DEFLATE not applied, an endpoint MUST set "COMP"
bit of the frame to 0.
An endpoint MUST NOT use LZ77 sliding window size greater than
32,768.
If an endpoint received the "max_window_bits" extension parameter on
opening handshake, it MUST NOT use LZ77 sliding window size greater
than the "max_window_bits"-th power of 2.
An endpoint MAY take over LZ77 sliding window from the last frame it
applied DEFLATE unless it's prohibited by the other peer by
"no_context_takeover" extension parameter.
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4.2. Receiving
To receive a frame with "COMP" bit set to 1, an endpoint MUST use the
following algorithm.
1. Append 4 octets of 0x00 0x00 0xff 0xff to the tail of the
application data part.
2. Decode the resulting octets using DEFLATE.
An endpoint MUST take over LZ77 sliding window from the last frame it
decoded by applying DEFLATE unless it sent the "no_context_takeover"
extension parameter.
An endpoint MUST use 32,768 byte LZ77 sliding window unless it sent
"max_window_bits" extension parameter.
4.3. Examples
_This section is non-normative._
These are examples of resulting data after applying the algorithm
above.
o "Hello" in one compressed block
* 0xf2 0x48 0xcd 0xc9 0xc9 0x07 0x00
"Hello" in one compressed block in the next frame
* 0xf2 0x00 0x11 0x00 0x00
o "Hello" in one block with no compression
* 0x00 0x05 0x00 0xfa 0xff 0x48 0x65 0x6c 0x6c 0x6f 0x00
o "Hello" in one block with "BFINAL" set to 1
* 0xf3 0x48 0xcd 0xc9 0xc9 0x07 0x00 0x00
o "He" and "llo" in separate blocks
* 0xf2 0x48 0x05 0x00 0x00 0x00 0xff 0xff 0xca 0xc9 0xc9 0x07
0x00
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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-frame" WebSocket Extension Keyword
Name of token.
deflate-frame
Author/Change controller
IETF <iesg@ietf.org>
Contact
HYBi <hybi@ietf.org>
References
RFC XXX
The "deflate-frame" 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
specification of DEFLATE based compression extension for the
WebSocket protocol which I referred to 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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