- From: James M Snell <jasnell@gmail.com>
- Date: Tue, 17 Jul 2012 00:21:11 -0700
- To: Mike Belshe <mike@belshe.com>
- Cc: ietf-http-wg@w3.org
- Message-ID: <CABP7RbcxomO3oZFpNNthj2if89LtkbKeR6D3_pYvf3drCKVtfA@mail.gmail.com>
On Tue, Jul 17, 2012 at 12:18 AM, Mike Belshe <mike@belshe.com> wrote: > i like the direction of this. a good blend of a registry and extension > headers. > > i wasn't quite sure how you got to the 6 byte compressed header, tho. > > Heh.. chalk that up to temporary idiocy on my part... and a complete lack of either caffeine or alcohol in my system. > mike > > > On Mon, Jul 16, 2012 at 11:51 PM, James M Snell <jasnell@gmail.com> wrote: > >> 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¬=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 07:22:05 UTC