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Re: Suggestion for simplified HTTP 2.0 header compression algorithm

From: Michael Sweet <msweet@apple.com>
Date: Wed, 06 Nov 2013 10:07:47 -0500
Cc: Hans Spaak <hans.spaak@ericsson.com>, "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>, Salvatore Loreto <salvatore.loreto@ericsson.com>, Robert Skog <robert.skog@ericsson.com>, Thorsten Herber <thorsten.herber@ericsson.com>
Message-id: <B1FA8BCF-F954-4BE1-B714-0F9AE025CBAF@apple.com>
To: James M Snell <jasnell@gmail.com>
James,

I think I’ve commented in the past that I prefer your proposed encoding - my only nit is the timestamp encoding due to the requirement to support large integers…

In my mind HTTP/2.0’s big killer feature is support for multiple, parallel requests over a single connection.  This eliminates the largest source of latency for web clients - connection setup time, which can be significant for encrypted connections - and it potentially allows web servers and clients to do more work with less resources.

Header compression is only important for getting the initial request into a single packet to avoid the RTT, but once you have reached that goal any further compression is, IMHO, not (as) important.  If we keep adding more and more state that needs to be maintained by both ends, we risk losing all of the resource savings (above) and making it less likely to be implemented at all.


On Nov 6, 2013, at 8:54 AM, James M Snell <jasnell@gmail.com> wrote:

> Throughout this process I've gone through several iterations of a
> decidedly less complicated header encoding mechanism [1] and have
> consistently come up against the point of view that achieving the best
> possible compression ratio appears to be the single most important
> deciding factor. Whenever I have raised concerns about the level of
> complexity, I've been told that the tooling will handle it so it's not
> a problem.
> 
> [1] http://tools.ietf.org/html/draft-snell-httpbis-bohe-13
> 
> My personal opinion is that achieving the best possible compression
> ratio should not be the sole deciding factor and that the currently
> proposed mechanism is far into the "trying to be way too clever" range
> and introduces far too much unjustified additional complexity. In
> other words, I'm very -1 on it but, thus far, I appear to be in the
> minority. (and yes, FWIW, I have taken the time to implement the
> current scheme so I'm not just speaking out of some theoretical
> concern).
> 
> - James
> 
> 
> 
> On Wed, Nov 6, 2013 at 5:02 AM, Michael Sweet <msweet@apple.com> wrote:
>> Hans,
>> 
>> I am all for simplifying the current header compression algorithm, which is
>> growing needlessly complicated IMHO as people try to achieve ever greater
>> compression ratios without regard to implementation complexity or
>> interoperability concerns.
>> 
>> That said, I don’t like the idea of putting headers in the settings frame
>> since it doesn’t play well with proxies - clients need to be able to direct
>> requests to multiple origin servers and proxies need to be able to
>> consolidate multiple client connections onto a single connection to the
>> origin server.  (it will be hard enough for the current header compression
>> scheme to work for that, but at least it is possible)
>> 
>> 
>> On Nov 6, 2013, at 4:04 AM, Hans Spaak <hans.spaak@ericsson.com> wrote:
>> 
>> Hello,
>> 
>> 
>> 
>> The proposed header compression algorithm is rather complicated and we
>> foresee that  there will be at least initially a lot of interworking
>> problems
>> when different compressor and de-compressor implementations need to work
>> together.
>> We experienced those problems when the equally complicated WAP1.x header
>> compression was implemented.
>> 
>> 
>> 
>> Here is our proposal for simplifying how headers are compressed in HTTP/2.0.
>> 
>> It does not compress as hard as the currently proposed solution (if the
>> encoder is written in a very smart way),
>> 
>> but it is much easier to implement.
>> 
>> 
>> 
>> Please have a look at it and comment.
>> 
>> 
>> 
>> Exchange of static headers, which are the same for one TCP connection:
>> 
>> 
>> 
>> The SETTINGS frame is used to transmit the static request and response
>> headers.
>> 
>> The HEADERS frame is used to transmit the dynamic request and response
>> headers.
>> 
>> Static headers and dynamic headers are combined in the following way:
>> 
>> 1.       Emit the dynamic headers.
>> 
>> 2.       Emit the static headers whose header names are not present in the
>> dynamic headers.
>> 
>> 
>> 
>>                         Client                        Server
>> 
>>                           |                             |
>> 
>>                           |SETTINGS                     |
>>                           | - static request headers    |
>>                           |   (:method, :host,          |
>> 
>>                           |    User-Agent, Accept, …)   |
>> 
>>                           |---------------------------->| Store static
>> request
>>                           |                             | headers for
>> connection
>> 
>>                           |SETTINGS                     |
>> 
>>                           | - static response headers   |
>> 
>>                           |   (:status 200, Server, …)  |
>> 
>>                           |                             |
>> 
>>     Store static response |<----------------------------|
>> 
>>    headers for connection |                             |
>> 
>>                           |HEADERS                      |
>> 
>>                           | - dynamic request headers   |
>> 
>>                           |   (:path, …)                |
>> 
>>                           |---------------------------->| Combine static
>> and
>> 
>>                           |                             | dynamic request
>> 
>>                           |                             | headers.
>>                           |                             | Dynamic headers
>> have
>> 
>>                           |                             | precedence over
>> static
>> 
>>                           |                             | headers.
>> 
>>                           |HEADERS                      |
>> 
>>                           | - dynamic response headers  |
>> 
>>                           |   (Content-Type, Age, …)    |
>> 
>>        Combine static and |<----------------------------|
>> 
>> dynamic response headers. |                             |
>>      Dynamic headers have |                             |
>> 
>>    precedence over static |                             |
>> 
>>                  headers. |                             |
>> 
>> 
>> 
>> Coding of header names and values:
>> 
>> 
>> 
>> Standard headers are coded in the following way:
>> 
>>  0   1   2   3   4   5   6   7
>> 
>> +---+---+---+---+---+---+---+---+
>> 
>> | 1 | Index of standard header  |
>> 
>> +---+---------------------------+
>> 
>> |  Value length (8+)            |
>> 
>> +-------------------------------+
>> 
>> | Value String (Length octets)  |
>> 
>> +-------------------------------+
>> 
>> 
>> 
>> 
>> 
>> Non-standard headers are coded in the following way:
>> 
>>  0   1   2   3   4   5   6   7
>> 
>> +---+---+---+---+---+---+---+---+
>> 
>> | 0 | Name Length (7+)          |
>> 
>> +---+---------------------------+
>> 
>> | Name String (Length octets)   |
>> 
>> +-------------------------------+
>> 
>> | Value length (8+)             |
>> 
>> +-------------------------------+
>> 
>> | Value String (Length octets)  |
>> 
>> +-------------------------------+
>> 
>> 
>> 
>> Indices for standard headers (example only):
>> 
>> 
>> 
>> 0 :scheme
>> 
>> 1 :host
>> 
>> 2 :path
>> 
>> 3 :method
>> 
>> 4 :status
>> 
>> 5 accept
>> 
>> 6 accept-charset
>> 
>> 7 accept-encoding
>> 
>> 8 accept-language
>> 
>> 9 age
>> 
>> 10 cache-control
>> 
>> 11 content-length
>> 
>> …
>> 
>> 
>> 
>> Br,
>> 
>> 
>> 
>> Hans Spaak
>> 
>> Ericsson AB
>> 
>> E-mail: hans.spaak@ericsson.com
>> 
>> Web: www.ericsson.com
>> 
>> 
>> 
>> 
>> 
>> 
>> _______________________________________________________________
>> Michael Sweet, Senior Printing System Engineer, PWG Chair
>> 

_______________________________________________________________
Michael Sweet, Senior Printing System Engineer, PWG Chair
Received on Wednesday, 6 November 2013 15:08:40 UTC

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