Internet DRAFT - draft-tyoshino-hybi-permessage-compression
draft-tyoshino-hybi-permessage-compression
HyBi Working Group T. Yoshino
Internet-Draft Google, Inc.
Intended status: Standards Track July 4, 2012
Expires: January 5, 2013
WebSocket Per-message Compression
draft-tyoshino-hybi-permessage-compression-00
Abstract
This specification defines a WebSocket extension that adds
compression functionality to the WebSocket Protocol. It compresses
the payload of non-control WebSocket messages using specified
compression algorithm. One reserved bit RSV1 in the WebSocket frame
header is allocated to control application of compression for each
message. This specification provides one compression method
available for the extension using DEFLATE.
Please send feedback to the hybi@ietf.org mailing list.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 5, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conformance Requirements . . . . . . . . . . . . . . . . . . . 4
3. Extension Negotiation . . . . . . . . . . . . . . . . . . . . 5
3.1. Negotiation Example . . . . . . . . . . . . . . . . . . . 6
4. Framing . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Sending . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Receiving . . . . . . . . . . . . . . . . . . . . . . . . 7
5. DEFLATE method . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Method Parameters . . . . . . . . . . . . . . . . . . . . 8
5.2. Application Data Transformation . . . . . . . . . . . . . 9
5.2.1. Compression . . . . . . . . . . . . . . . . . . . . . 9
5.2.2. Decompression . . . . . . . . . . . . . . . . . . . . 10
5.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . 10
5.3. Intermediaries . . . . . . . . . . . . . . . . . . . . . . 11
5.4. Implementation Notes . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7.1. Registration of the "permessage-compress" WebSocket
Extension Name . . . . . . . . . . . . . . . . . . . . . . 13
7.2. Registration of the "Per-message Compressed" WebSocket
Framing Header Bit . . . . . . . . . . . . . . . . . . . . 13
7.3. WebSocket Per-message Compression Method Name Registry . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
_This section is non-normative._
As well as other communication protocols, the WebSocket Protocol
[RFC6455] can benefit from compression technology. This
specification defines a WebSocket extension that applies a
compression algorithm to octets exchanged over the WebSocket Protocol
using its extension framework. This extension negotiates what
compression method to use on opening handshake, and then compresses
the octets in non-control messages using the method. We can apply
this extension to various compression algorithms by specifying how to
negotiate parameters and transform payload. A client may offer
multiple compression methods on opening handshake, and then the
server chooses one from them. This extension uses the RSV1 bit of
the WebSocket frame header to indicate whether the message is
compressed or not, so that we can choose to skip messages with
incompressible contents without applying extra compression.
This specification provides one specific compression method "deflate"
which is based on DEFLATE [RFC1951] for this extension. We chose
DEFLATE since it's widely available as library on various platforms
and the overhead it adds for each chunk is small. To align the end
of compressed data to octet boundary, this method 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 messages to get better compression ratio. For resource-
limited devices, method parameters to limit the usage of memory for
compression context are provided.
The simplest "Sec-WebSocket-Extensions" header in the client's
opening handshake to request DEFLATE based per-message compression is
the following:
Sec-WebSocket-Extensions: permessage-compress; method=deflate
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
"permessage-compress".
To request use of the Per-message Compression Extension, a client
MUST include an element with the "permessage-compress" extension
token as its extension identifier in the "Sec-WebSocket-Extensions"
header in its opening handshake. The element MUST contain exactly
one extension parameter named "method". The value of the "method"
extension parameter is a list of compression method descriptions,
ordered by preference. Each compression method description has a
method name and optional method parameters. The grammar of the list
is "requested-method-list" defined in the following ABNFs.
requested-method-list = 1#method-desc
method-desc = method-name *(";" method-param)
method-name = token
method-param = token ["=" (token | quoted-string)]
The list MAY contain multiple method descriptions with the same
method name.
To accept use of the Per-message Compression Extension, a server MUST
choose one compression method description to accept from ones listed
by the client, and include an element with the "permessage-compress"
extension token in the "Sec-WebSocket-Extensions" header in its
opening handshake. The chosen description is called "accepted
request". The element MUST contain exactly one extension parameter
named "method". The value of the "method" extension parameter MUST
be a compression method description. This description is called
"method agreement". The method name in the "method agreement" MUST
be one of the accepted request. The "method agreement" MUST conform
the "accepted request". Its grammar is "method-agreement" defined in
the following ABNF.
method-agreement = method-desc
The value of the "method" parameter MUST be quoted by using
"quoted-string" syntax if it doesn't conform to token syntax.
If a client doesn't support the method and its configuration
specified by the "method agreement", the client MUST _Fail the
WebSocket Connection_. Otherwise, both endpoints MUST use the
algorithm described in Section 4 to exchange messages.
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3.1. Negotiation Example
_This section is non-normative._
These are "Sec-WebSocket-Extensions" header value examples that
negotiate the Per-message Compression Extension.
o Request foo method. Since foo matches token syntax, it doesn't
need to be quoted.
permessage-compress; method=foo
o Request foo method with a parameter x with 10 as its value. Since
the method parameter value contains a semicolon, it doesn't match
token syntax. Quotation is needed.
permessage-compress; method="foo; x=10"
o Request foo method and bar method. Since the method parameter
value contains a comma, it doesn't match token syntax. Quotation
is needed.
permessage-compress; method="foo, bar"
o Request foo method with parameter x with "Hello World" (quotation
for clarification) as its value and bar method. Since "Hello
World" contains a space, it needs to be quoted. Since quoted
"Hello World" contains double quotations and a space, it needs to
be quoted again.
permessage-compress; method="foo; x=\"Hello World\", bar"
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4. Framing
This section describes how to apply the negotiated compression method
to the contents of WebSocket messages.
This extension allocates the RSV1 bit of the WebSocket header and
names it the "Per-message Compressed" bit. Any extension requiring
the use of the RSV1 bit is incompatible with this extension. This
bit MAY be set only on the first fragment of a message. This bit
indicates whether the compression method is applied to the message or
not. Messages with the "Per-message Compressed" bit set (on its
first fragment) are called "compressed messages". They have
compressed data in their payload. Messages with the bit unset are
called "uncompressed messages". They have uncompressed data in their
payload.
This extension MUST NOT be used after any extension for which frame
boundary needs to be preserved. This extension MUST NOT be used
after any extension that uses "Extension data" field or any of the
reserved bits on the WebSocket header as per-frame attribute.
This extension operates only on data frames.
4.1. Sending
To send a compressed message, an endpoint MUST use the following
algorithm.
1. Compress the payload of the message using the compression method.
2. Build frame(s) for the message by putting the resulting octets
instead of the original octets.
3. Set the "Per-message Compressed" bit of the first fragment to 1.
To send an uncompressed message, an endpoint MUST set the
"Per-message Compressed" bit of the first fragment of the message to
0. The payload of the message MUST be sent as-is without applying
the compression method.
4.2. Receiving
To receive a compressed message, an endpoint MUST decompress its
payload.
An endpoint MUST receive an uncompressed message as-is without
decompression.
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5. DEFLATE method
This section defines a method named "deflate" for this extension that
compresses the payload of messages using DEFLATE [RFC1951] and byte
boundary alignment method introduced in [RFC1979].
5.1. Method Parameters
An endpoint MAY include one or more method parameters in the method
description as defined below.
Maximum LZ77 sliding window size
A client MAY attach the "s2c_max_window_bits" method parameter to
limit the LZ77 sliding window size that the server uses to build
messages. If the "accepted request" has this method parameter,
the server MUST NOT use LZ77 sliding window size greater than the
size specified by this parameter to build messages. If the
"accepted request" has this method parameter, the server MUST
attach this method parameter with the same value as one of the
"accepted request".
A server MAY attach the "c2s_max_window_bits" method parameter to
limit the LZ77 sliding window size that the client uses to build
messages. A client that received this parameter MUST NOT use LZ77
sliding window size greater than the size specified by this
parameter to build messages.
These parameters MUST have an integer value in the range between 8
to 15 indicating the base-2 logarithm of the LZ77 sliding window
size.
Disallow compression context takeover
A client MAY attach the "s2c_no_context_takeover" method parameter
to disallow the server to take over the LZ77 sliding window used
to build previous messages. If the "accepted request" has this
method parameter, the server MUST reset its LZ77 sliding window
for sending to empty for each message. If the "accepted request"
has this method parameter, the server MUST attach this method
parameter.
A server MAY attach the "c2s_no_context_takeover" method parameter
to disallow the client to take over the LZ77 sliding window used
to build previous messages. A client that received this parameter
MUST reset its LZ77 sliding window for sending to empty for each
message.
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These parameters have no value.
A server MUST ignore any method parameter other than
"s2c_max_window_bits" and "s2c_no_context_takeover" in the received
"deflate" method description.
A client MUST _Fail the WebSocket Connection_ if there is any method
parameter other than the "s2c_max_window_bits",
"c2s_max_window_bits", "s2c_no_context_takeover" and
"c2s_no_context_takeover" in the received "deflate" method
description. A client MUST _Fail the WebSocket Connection_ if it
doesn't support the method and its configuration specified by the
received "deflate" method description.
5.2. Application Data Transformation
5.2.1. Compression
An endpoint MUST use the following algorithm to compress a message.
1. Compress all the octets of the payload of the message using
DEFLATE. 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 an LZ77 sliding window greater than 32,768
bytes to build messages to send.
If the "method agreement" has the "s2c_max_window_bits" method
parameter and its value is w, the server MUST NOT use an LZ77 sliding
window greater than w-th power of 2 bytes to build messages to send.
If the "method agreement" has the "c2s_max_window_bits" method
parameter and its value is w, the client MUST NOT use an LZ77 sliding
window greater than w-th power of 2 bytes to build messages to send.
If the "method agreement" has the "s2c_no_context_takeover" method
parameter, the server MUST reset its LZ77 sliding window for sending
to empty for each message. Otherwise, the server MAY take over the
LZ77 sliding window used to build the last compressed message. If
the "method agreement" has the "c2s_no_context_takeover" method
parameter, the client MUST reset its LZ77 sliding window for sending
to empty for each message. Otherwise, the client MAY take over the
LZ77 sliding window used to build the last compressed message.
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5.2.2. Decompression
An endpoint MUST use the following algorithm to decompress a message.
1. Append 4 octets of 0x00 0x00 0xff 0xff to the tail of the payload
of the message.
2. Decompress the resulting octets using DEFLATE.
If the "method agreement" has the "s2c_max_window_bits" method
parameter and its value is w, the client MAY reduce the size of the
LZ77 sliding window to decompress received messages down to the w-th
power of 2 bytes. Otherwise, the client MUST use a 32,768 byte LZ77
sliding window to decompress received messages. If the "method
agreement" has the "c2s_max_window_bits" method parameter and its
value is w, the server MAY reduce the size of the LZ77 sliding window
to decompress received messages down to the w-th power of 2 bytes.
Otherwise, the server MUST use a 32,768 byte LZ77 sliding window to
decompress received messages.
If the "method agreement" has the "s2c_no_context_takeover" method
parameter, the client MAY reset its LZ77 sliding window for receiving
to empty for each message. Otherwise, the client MUST take over the
LZ77 sliding window used to parse the last compressed message. If
the "method agreement" has the "c2s_no_context_takeover" method
parameter, the server MAY reset its LZ77 sliding window for receiving
to empty for each message. Otherwise, the server MUST take over the
LZ77 sliding window used to parse the last compressed message.
5.2.3. 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
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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
5.3. Intermediaries
When intermediaries forward messages, they MAY decompress and/or
compress the messages according to the constraints negotiated during
the opening handshake of the connection(s).
5.4. Implementation Notes
_This section is non-normative._
On most common software development platforms, the operation of
aligning compressed data to byte boundaries using an empty block with
no compression is available as a 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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6. Security Considerations
There are no security concerns for now.
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7. IANA Considerations
7.1. Registration of the "permessage-compress" WebSocket Extension Name
This section describes a WebSocket extension name registration in the
WebSocket Extension Name Registry [RFC6455].
Extension Identifier
permessage-compress
Extension Common Name
WebSocket Per-message Compression
Extension Definition
This document.
Known Incompatible Extensions
None
The "permessage-compress" token is used in the
"Sec-WebSocket-Extensions" header in the WebSocket opening handshake
to negotiate use of the Per-message Compression Extension.
7.2. Registration of the "Per-message Compressed" WebSocket Framing
Header Bit
This section describes a WebSocket framing header bit registration in
the WebSocket Framing Header Bits Registry [RFC6455].
Header Bit
RSV1
Common Name
Per-message Compressed
Meaning
The message is compressed or not.
Reference
Section 4 of this document.
The "Per-message Compressed" framing header bit is used on the first
fragment of non-control messages to indicate whether the payload of
the message is compressed by the Per-message Compression Extension or
not.
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7.3. WebSocket Per-message Compression Method Name Registry
This specification creates a new IANA registry for names of
compression methods to be used with the WebSocket Per-message
Compression Extension in accordance with the principles set out in
[RFC5226].
As part of this registry, IANA maintains the following information:
Method Identifier
The identifier of the method, as will be used in the method
description as defined Section 3 of this specification. The value
must conform to the method-name ABNF as defined in Section 3 of
this specification.
Method Common Name
The name of the method, as the method is generally referred to.
Method Definition
A reference to the document in which the method being used with
this extension is defined.
WebSocket Per-message Compression method names are to be subject to
the "First Come First Served" IANA registration policy [RFC5226].
IANA has added initial values to the registry as follows.
+------------+-------------+---------------+
| Identifier | Common Name | Definition |
+------------+-------------+---------------+
| deflate | DEFLATE | This document |
+------------+-------------+---------------+
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8. Acknowledgements
Special thanks to Patrick McManus who wrote up the initial
specification of DEFLATE based compression extension for the
WebSocket Protocol to which I referred to write this specification.
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9. References
9.1. Normative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LZ77] Ziv, J. and A. Lempel, "A Universal Algorithm for
Sequential Data Compression", IEEE Transactions on
Information Theory, Vol. 23, No. 3, pp. 337-343.
9.2. 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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