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Re: SPDY Header Frames

From: Jonathan Ballard <dzonatas@gmail.com>
Date: Fri, 13 Jul 2012 17:12:09 -0700
Message-ID: <CAAPAK-76qiRiAOg_wYBQYAb1yMUuR-A4JVcT+OjwgyM5nuS6qg@mail.gmail.com>
To: Roland Zink <roland@zinks.de>
Cc: ietf-http-wg@w3.org
Maybe it's not quite intuitive enough, especially when we cross-reference
the similarities of that WAP binary encode with JPEG2000's binary encode,
the MIME types specifically: http://www.faqs.org/rfcs/rfc3745.html (and
more)

I haven't seen any patent that extends those MIME types with either the
static index or dynamic index value, which accomplishes the compressed
header desired in HTTP2. Sorry, I have no sample for this post.

On Fri, Jul 13, 2012 at 4:36 PM, Roland Zink <roland@zinks.de> wrote:

>  A similar binary encoding for HTTP headers is used in the WAP (Wireless
> Application Protocol). The goal there was to reduce the number of bytes so
> there are more optimization on size but skipping of values is a bit more
> difficult. If anybody is interested the full specification can be found at
> http://www.openmobilealliance.org/tech/affiliates/wap/wap-230-wsp-20010705-a.pdf.
> The header encoding is in section 8.4.
>
> Regards,
> Roland
>
>
>
> On 14.07.2012 00:16, James M Snell 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 Saturday, 14 July 2012 00:12:37 GMT

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