- From: Jonathan Ballard <dzonatas@gmail.com>
- Date: Fri, 13 Jul 2012 17:12:09 -0700
- To: Roland Zink <roland@zinks.de>
- Cc: ietf-http-wg@w3.org
- Message-ID: <CAAPAK-76qiRiAOg_wYBQYAb1yMUuR-A4JVcT+OjwgyM5nuS6qg@mail.gmail.com>
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 UTC