HyBi Working Group T. Yoshino
Internet-Draft Google, Inc.
Intended status: Standards Track March 10, 2012
Expires: September 11, 2012
WebSocket Per-frame Compression
draft-tyoshino-hybi-websocket-perframe-deflate-06
Abstract
This specification defines a general per-frame compression scheme for
the WebSocket Protocol and one specific compression extension using
DEFLATE. This scheme compresses the "Application data" part of
WebSocket data frames using specified compression algorithm.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conformance Requirements . . . . . . . . . . . . . . . . . . . 4
3. General Per-frame Compression Scheme . . . . . . . . . . . . . 5
3.1. Sending . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Receiving . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Per-frame DEFLATE Extension . . . . . . . . . . . . . . . . . 6
4.1. Extension Negotiation . . . . . . . . . . . . . . . . . . 6
4.2. Application Data Transformation . . . . . . . . . . . . . 6
4.2.1. Compression . . . . . . . . . . . . . . . . . . . . . 7
4.2.2. Decompression . . . . . . . . . . . . . . . . . . . . 7
4.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Implementation Note . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Registration of the "deflate-frame" WebSocket
Extension Name . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Registration of the "Per-frame compressed" WebSocket
Framing Header Bit . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
_This section is non-normative._
As well as other protocols, octets exchanged over the WebSocket
Protocol [RFC6455] can benefit from compression technology. This
specification introduces a scheme to apply compression algorithms to
the WebSocket Protocol, and then adds DEFLATE [RFC1951] based
compression functionality to the WebSocket Protocol using its
extension framework.
The per-frame compression scheme applies the specified compression
algorithm to the octets in the "Application data" part of data
frames. It specifies the use of the RSV1 bit of the WebSocket
framing to indicate whether any compression is applied to the frame
or not, so that we can choose to skip compression for frames with
incompressible contents. By specifying extension negotiation and how
to transform octets in "Application data", we can define per-frame
compression extensions for various compression algorithms based on
this scheme.
The specific extension we introduce in this specification by applying
DEFLATE to the scheme is called "Per-frame DEFLATE extension". To
align the end of compressed data to octet boundary, the extension
uses the algorithm described in the Section 2.1 of the PPP Deflate
Protocol [RFC1979]. Endpoints can take over the LZ77 sliding window
[LZ77] used to build previous frames to get better compression ratio.
The simplest "Sec-WebSocket-Extensions" header in the client's
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. General Per-frame Compression Scheme
This section describes a general scheme to apply a compression
algorithm to the contents of WebSocket frames. Any other compression
extension specifications MAY apply their compression algorithm to
this scheme to define extensions.
This scheme allocates one bit field called "Per-frame compressed" at
the RSV1 bit. This bit indicates whether any kind of per-frame
compression is applied to the frame or not.
This scheme operates only on data frames, and only on the
"Application data" therein (it does not affect the "Extension data"
portion of the "Payload data").
3.1. Sending
To send a frame with the compression applied, an endpoint MUST use
the following algorithm.
1. Apply the compression to the "Application data" portion of the
frame.
2. Build a frame by putting the resulting octets in the "Application
data" portion instead of the original octets. The payload length
field of the frame MUST be the sum of the size of the "Extension
data" portion and one of these resulting octets. "Per-frame
compressed" bit MUST be set to 1.
To send a frame with the compression not applied, an endpoint MUST
set "Per-frame compressed" bit of the frame to 0.
3.2. Receiving
To receive a frame with "Per-frame compressed" bit set to 1, an
endpoint MUST decompress the octets in the "Application data" portion
based on the compression algorithm.
An endpoint MUST receive a frame with "Per-frame compressed" bit set
to 0 as-is without any compression processing.
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4. Per-frame DEFLATE Extension
4.1. 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.
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.
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 to
build frames. 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
build frames.
Disallow compression context takeover
An endpoint MAY attach "no_context_takeover" extension parameter
to disallow the other peer to take over the LZ77 sliding window
used to build previous frames. This parameter has no value. An
endpoint that received this parameter MUST use an empty LZ77
sliding window to build every frame.
A server MUST ignore any unknown extension parameter attached to
"deflate-frame" extension token in the client's opening handshake.
A client MUST _Fail the WebSocket Connection_ if any unknown
extension parameter is attached to "deflate-frame" extension token in
the server's opening handshake.
Once per-frame DEFLATE extension is accepted, both endpoints MUST use
the algorithm described in Section 4.2 to exchange frames.
4.2. Application Data Transformation
This extension transforms the "Application data" portion by using the
scheme described in Section 3 with DEFLATE as follows.
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4.2.1. Compression
An endpoint MUST use the following algorithm to compress the
"Application data" portion.
1. Apply DEFLATE [RFC1951] to all the octets. 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 an empty block with no
compression ("BTYPE" set to 0), append an empty block with no
compression to the tail.
3. Remove 4 octets (that are 0x00 0x00 0xff 0xff) from the tail.
An endpoint MUST NOT use LZ77 sliding window size greater than 32,768
to build frames.
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 to build frames.
Unless it's prohibited by the other peer by the "no_context_takeover"
extension parameter on opening handshake, an endpoint MAY take over
the LZ77 sliding window used to build the last frame with DEFLATE
applied.
4.2.2. Decompression
An endpoint MUST use the following algorithm to decompress the
"Application data" portion.
1. Append 4 octets of 0x00 0x00 0xff 0xff to the tail.
2. Decompress the resulting octets using DEFLATE.
Unless an endpoint sent the "max_window_bits" extension parameter on
opening handshake, the endpoint MUST use 32,768 byte LZ77 sliding
window to decode frames.
If an endpoint sent the "max_window_bits" extension parameter on
opening handshake, it MAY reduce LZ77 sliding window size down to the
"max_window_bits"-th power of 2 to decode frames.
Unless the endpoint sent the "no_context_takeover" extension
parameter on opening handshake, an endpoint MUST take over the LZ77
sliding window used to decode the last frame with DEFLATE applied.
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4.2.3. 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
4.3. Implementation Note
_This section is non-normative._
On common software development platforms, the operation of aligning
compressed data to octet boundary using an empty block with no
compression is available as library. For example, Zlib [Zlib] does
this when "Z_SYNC_FLUSH" is passed to deflate function.
To get sufficient compression ratio, LZ77 sliding window size of
1,024 or more is recommended.
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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 Name
This section describes a WebSocket extension name registration in the
WebSocket Extension Name Registry. [RFC6455].
Extension Identifier
deflate-frame
Extension Common Name
WebSocket Per-frame DEFLATE
Extension Definition
Section 4.1 and Section 4.2 of this document.
Known Incompatible Extensions
None
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.
6.2. Registration of the "Per-frame compressed" WebSocket Framing
Header Bit
This section describes a WebSocket framing header bit registration in
the WebSocket Framing Header Bits Registry. [RFC6455]
Header Bit
RSV1
Common Name
Per-frame compressed
Meaning
Compression is applied to the frame or not.
Reference
Section 3 of this document.
The "Per-frame compressed" framing header bit is used to indicate
whether any negotiated per-frame compression extension applied
compression to the "Application data" portion of the frame or not.
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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
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LZ77] Ziv, J. and A. Lempel, "A Universal Algorithm for
Sequential Data Compression", IEEE Transactions on
Information Theory, Vol. 23, No. 3, pp. 337-343.
8.2. Informative References
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC1979] Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.
[Zlib] Gailly, J. and M. Adler, "Zlib", <http://zlib.net/>.
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Author's Address
Takeshi Yoshino
Google, Inc.
Email: tyoshino@google.com
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