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Re: SPDY Header Frames

From: James M Snell <jasnell@gmail.com>
Date: Mon, 16 Jul 2012 23:51:30 -0700
Message-ID: <CABP7RbdZPYYqtDFWUprrAmGQ8GehOTh-LvXq9gizXGUJg4=68g@mail.gmail.com>
To: ietf-http-wg@w3.org
Ok... spent some time this evening playing around with the header frame
syntax a bit more to see what further optimizations could be made and to
see if the binary encoded header id's made any noticeable difference in
size and ease of processing.

Here's the revised structure I played around with...

1. Within a HEADER block, I assume two possible types of headers,
REGISTERED and EXTENSION. A REGISTERED header is one that would be known to
the registrar and assigned a numeric id and a codepage. If the codepage is
0, the implication is that the header is MUST UNDERSTAND and is considered
one of the core headers for the basic operation of the protocol. Codepages
1-9 are MUST-IGNORE... that is, if a user-agent or server comes across a
header on these code pages that is not understood, the header can simply be
ignored. Codepages 10-14 are PRIVATE USE, with Codepage 10 being reserved
for MUST UNDERSTAND PRIVATE USE headers. EXTENSION headers are simple name
value pairs essentially as they exist today.

2. Within extension headers, the name portion MUST be ASCII and MUST NOT be
longer than 255 bytes (quite generous really).

3. Values may be binary or character based, as indicated by a flags field.
Values may be up to max(int32) in length.

4. Registered HTTP Methods can be identified by numeric value. Extension
Methods can be identified by character value.

5. The structure for REGISTERED HEADERS is...

  +------------------------------+
  |0| id (15-bit)| flags(8-bit)  |
  +------------------------------+
  | len (32-bit) |     value     |
  +------------------------------+

6. The structure for EXTENSION HEADERS is...

  +------------------------------+
  |1| flags(7-bit) | namelen (8) |
  +------------------------------+
  | name | val len (32) | value  |
  +------------------------------+

Assuming the following registered headers...

  public static final short VERSION = 1;
  public static final short METHOD = 2;
  public static final short HOST = 3;
  public static final short SESSION = 4;
  public static final short CHARSET = 5;
  public static final short REQUEST_URI = 6;
  public static final short ACCEPT_LANG = 4097;

And the following registered methods...

  public static final byte GET = 1;
  public static final byte POST = 2;
  public static final byte PUT = 3;
  public static final byte DELETE = 4;
  public static final byte PATCH = 5;
  public static final byte HEAD = 6;
  public static final byte OPTIONS = 7;

Let's assume that what we want to to encode a HTTP GET for resource:
  http://www.example.org/this/is/the/request?is=it&not=beautiful

With a session identifier of "session_key", ACCEPT_LANG = en-US and default
charset encoding for all character based header values is "US-ASCII"...
Let's also add an extension header "ext" with value "foo"...

The Version header can be encoded as:
  {0, 1, 0, 0, 0, 0, 2, 2, 0}

The GET Method header can be encoded as:
  {0, 2, 0, 0, 0, 0, 1, 1}

The Host header would be encoded as:
  {  0,   3,   1,  0,   0,   0,  15, 119, 119, 119,
    46, 101, 120, 97, 109, 112, 108, 101,  46, 111,
   114, 103}

The Accept-Lang header would be encoded as:
  {16, 1, 1, 0, 0, 0, 5, 'e', 'n', '-', 'U', 'S'}

The Extension header ext: foo would be encoded as:
  {-128, 1, 3, 101, 120, 116, 0, 0, 0, 3, 102, 111, 111}

The entire header block is encoded into a structure of 145 bytes in length;

[8, 0, 1, 0, 0, 0, 0, 2, 2, 0, 0, 2, 0, 0, 0, 0, 1, 1, 0, 3, 1, 0, 0, 0,
15, 119, 119, 119, 46, 101, 120, 97, 109, 112, 108, 101, 46, 111, 114, 103,
0, 6, 1, 0, 0, 0, 40, 47, 116, 104, 105, 115, 47, 105, 115, 47, 116, 104,
101, 47, 114, 101, 113, 117, 101, 115, 116, 63, 105, 115, 61, 105, 116, 38,
110, 111, 116, 61, 98, 101, 97, 117, 116, 105, 102, 117, 108, 0, 5, 1, 0,
0, 0, 8, 117, 115, 45, 97, 115, 99, 105, 105, 0, 4, 1, 0, 0, 0, 11, 115,
101, 115, 115, 105, 111, 110, 95, 107, 101, 121, 16, 1, 1, 0, 0, 0, 5, 101,
110, 45, 85, 83, -128, 1, 3, 101, 120, 116, 0, 0, 0, 3, 102, 111, 111]

By comparison, the same structure encoded using the existing SPDY HEADER
block would require 208 bytes sans compression.

After applying compression of the block using the SPDY dictionary, the
block compresses into 6 compact bytes.

[120, 63, -29, -58, -89, -62]
Assuming this structure was used within a SPDY_STREAM message, unencrypted,
a proxy/router that is scanning the headers to determine where to route the
SYN_STREAM too would need only to look at the first two bytes of each
header to determine if the header is either the HOST, METHOD, REQUEST_URI,
VERSION or SESSION identifier. This scheme should prove to be significantly
faster to scan and perform operations on than the current all-text-key-pair
model. As always, tho, your mileage may vary.

/end-experiment

- James

On Fri, Jul 13, 2012 at 3:16 PM, James M Snell <jasnell@gmail.com> wrote:

> This note is intended to provide some additional thoughts for discussion
> around the design and use of SPDY as the possible basis for HTTP/2.0. The
> intent is to provide fuel for discussion... comments are definitely welcome.
>
> As discussed within draft-tarreau-httpbis-network-friendly-00, and as has
> been mentioned several times in discussion on list, handling of headers
> within the current SPDY framing, and in particular the layering of HTTP/1.1
> messages into SPDY frames is less than optimal. There is significant wasted
> space, duplication, etc that -- strictly speaking -- really isn't
> necessary. While I recognize that the following increases the basic
> complexity of the protocol, it allows fairly significant optimization
> following the same basic lines of reasoning expressed in
> draft-tarreau-httpbis-network-friendly-00.
>
> Section 2.6.1 of the SPDY draft defines header blocks using the following
> format:
>
>    +------------------------------------+
>    | Number of Name/Value pairs (int32) |
>    +------------------------------------+
>    |     Length of name (int32)         |
>    +------------------------------------+
>    |           Name (string)            |
>    +------------------------------------+
>    |     Length of value  (int32)       |
>    +------------------------------------+
>    |          Value   (string)          |
>    +------------------------------------+
>    |           (repeats)                |
>
> This structure is used within SYN_STREAM and HEADERS frames.
>
> What I propose is the following revised structure:
>
>    +------------------------------------+
>    |     Number of Headers (int32)      |
>    +------------------------------------+
>    |T| Flags (7) |     Length (24)      |
>    +------------------------------------+
>    |              Data                  |
>    +------------------------------------+
>    |T| Flags (7) |     Length (24)      |
>    +-------------------------------------
>    |              Data                  |
>    +-------------------------------------
>    |             (repeats)              |
>
> T is a single bit identifying the Header Type. There are two types..
> REGISTERED (0) and EXTENSION (1)
>
> Flags provides flags for the specific header field. The flag 0x1 indicates
> that the header value contains Character Data. If not set, the value is
> assumed to consist of raw octets. 0x2 indicates that the value is
> compressed.
>
> Length is an unsigned 24-bit value specifying the number of octets after
> the length field.
>
> When the T bit is NOT set, the Header field is a REGISTERED Header, the
> structure of which is:
>
>    +------------------------------------+
>    |0| Flags (7) |     Length (24)      |
>    +------------------------------------+
>    | ID | Value Length (int32) |Value...|
>    +------------------------------------+
>
> The ID is a 32-bit number uniquely identifying the registered field. Each
> is assigned by the registrar. For instance, the "Host" field could have a
> registered value of "1", the "Accept-Lang" field could have a registered
> value of "6", and so forth.
>
> The Value Length is a 32-bit value indicating the length of the value.
>
> If Flag 0x1 is set, the value is assumed to contain character data. When
> set, the value MUST be preceded by a single unsigned 8-bit integer
> identifying the character encoding utilized. The values are assigned by the
> registrar. For instance, US-ASCII could have a registered value of "1",
> while "UTF-8" could have a registered value of "2".
>
> For example:
>
>    +------------------------------------+
>    |0| 0000001 |     24                 |
>    +------------------------------------+
>    | 1 | 16 | 1 |    www.example.org    |
>    +------------------------------------+
>
> This Header record indicates a REGISTERED header containing character
> content, the header ID = 1, the charset used is US-ASCII and the value is "
> www.example.org". The header is expressed with a total of 28 bytes.
>
> When the T bit IS set, the Header field is an EXTENSION Header, the
> structure of which is:
>
>    +------------------------------------+
>    |0| Flags (7) |     Length (24)      |
>    +------------------------------------+
>    |      Length of name (int32)        |
>    +------------------------------------+
>    |           Name (string)            |
>    +------------------------------------+
>    |      Length of value (int32)       |
>    +------------------------------------+
>    |              Value                 |
>    +------------------------------------+
>
> For example.. an extension header that contains raw binary data...
>
>    +------------------------------------+
>    |0| 0000000 |       Length (24)      |
>    +------------------------------------+
>    |                5                   |
>    +------------------------------------+
>    |              x-foo                 |
>    +------------------------------------+
>    |                4                   |
>    +------------------------------------+
>    |           {raw bytes}              |
>    +------------------------------------+
>
> The header is expressed with a total of 21 bytes.
>
> The same flags apply. 0x1 indicates that the value is character data. If
> 0x1 is not set, the value contains raw octets. The key difference is that
> there is a 32-bit name length and variable length name field in place of
> the 32-bit ID field in the REGISTERED header. All other details remain the
> same.
>
> As is currently the case in SPDY, if a single header value contains
> multiple values, each can be separated using a single NUL (0) byte.
>
> There are several advantages to this approach:
>
> 1. Commonly used header names are omitted in favor of registered, known
> numeric IDs, saving space and making it more efficient to scan over
> commonly used headers. For instance, intermediaries that route requests
> based on common headers such as Host etc could choose to ignore EXTENSION
> header fields entirely, and scan only for the ID's of the fields they are
> interested in, rather than having to parse the entire bag of header names.
>
> 2. Header values can be expressed as raw octets or character data.
> Currently, mechanisms within HTTP require developers to muck around with
> Base64 encoding or other encodings when including detail within a header.
> This approach would eliminate that extra step. For instance, if I wanted to
> have a Content-Integrity header whose value is an hmac digest, I would be
> able to drop the raw bytes of the digest into the header value rather than
> base64 or hex encoding it into an ASCII string, saving CPU cycles and
> reducing the amount of data that must be transmitted.
>
> 3. Header values that contain character data would not be limited to
> US-ASCII. Multiple charset encodings would be allowed... obviously this has
> a whole slew of issues associated with it that need to be carefully
> considered. The charset encoding flag could be dropped, if necessary, from
> this proposal.
>
> For HTTP/1.1 Compatibility, each REGISTERED Header would be mapped to a
> known, registered HTTP/1.1 header, allowing one to one translation from the
> optimized form to the HTTP/1.1 form. Binary values would be base64-encoded.
> If a particular header does not allow for Base64 encoded values under
> HTTP/1.1, the down-level recipient would have the option of responding with
> an appropriate 404 response.
>
> That's it for now. There are additional considerations to be given to the
> specific selection of header fields to include within the SYN_STREAM vs.
> follow-on HEADERS frames but that's a separate conversation. As always,
> feedback is welcome...
>
> - James
>
>
Received on Tuesday, 17 July 2012 06:52:22 GMT

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