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RE: delta encoding and state management

From: RUELLAN Herve <Herve.Ruellan@crf.canon.fr>
Date: Fri, 18 Jan 2013 13:50:02 +0000
To: James M Snell <jasnell@gmail.com>, Nico Williams <nico@cryptonector.com>
CC: Roberto Peon <grmocg@gmail.com>, "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>
Message-ID: <6C71876BDCCD01488E70A2399529D5E52E13CE@ADELE.crf.canon.fr>
I agree that finding optimized binary encodings for headers will help us reducing the size of the data transmitted.

At the same time, stateful information is also very useful when transmitting a set of successive messages. It allows encoding a header as a reference to another header present in a previous message.

In my experiments, I tried to devise a binary encoding for the Accept header. However, I found that I was not able to reach the compression ratio obtained by using references to previous messages. Currently, in a set of requests to get a full web page, the Accept header will take 4 or 5 different values. This allows for the stateful compression to be very efficient.

The drawback of stateful compression is that this state must be stored. I understand that this can be a critical problem for intermediaries. I think that we should work for minimizing the amount of state an intermediary has to store for each connection. I was also wondering if anyone had some rough figure of what would be acceptable by an intermediary.

Hervé.

> -----Original Message-----
> From: James M Snell [mailto:jasnell@gmail.com]
> Sent: vendredi 18 janvier 2013 00:32
> To: Nico Williams
> Cc: Roberto Peon; ietf-http-wg@w3.org
> Subject: Re: delta encoding and state management
> 
> Agreed on all points. At this point I'm turning my attention towards
> identifying all of the specific headers we can safely and successfully provide
> optimized binary encodings for. The rest will be left as is. The existing bohe
> draft defines an encoding structure for the list of headers themselves, I will
> likely drop that and focus solely on the representation of the header values
> for now. My goal is to have an updated draft done in time for the upcoming
> interim meeting.
> 
> 
> On Thu, Jan 17, 2013 at 2:16 PM, Nico Williams <nico@cryptonector.com>
> wrote:
> 
> 
> 	On Thu, Jan 17, 2013 at 3:44 PM, James M Snell <jasnell@gmail.com>
> wrote:
> 
> 	> We certainly cannot come up with optimized binary encodings for
> everything
> 	> but we can get a good ways down the road optimizing the parts we
> do know
> 	> about. We've already seen, for instance, that date headers can be
> optimized
> 	> significantly; and the separation of individual cookie crumbs allows
> us to
> 	> keep from having to resend the entire cookie whenever just one
> small part
> 	> changes. I'm certain there are other optimizations we can make
> without
> 	> feeling like we have to find encodings for everything.
> 
> 
> 	The only way cookie compression can work is by having connection
> 	state.  But often the whole point of cookies is to not store state on
> 	the server but on the client.
> 
> 	The more state we associate with connections the more pressure
> there
> 	will be to close connections sooner and then we'll have to establish
> 	new connections, build new compression state, and then have it torn
> 	down again.  Fast TCP can help w.r.t. reconnect overhead, but that's
> 	about it.
> 
> 	We need to do more than measure compression ratios.  We need to
> 	measure state size and performance impact on fully-loaded
> middleboxes.
> 	 We need to measure the full impact of compression on the user
> 	experience.  A fabulous compression ratio might nonetheless spell
> doom
> 	for the user experience and thence the protocol.  If we take the
> wrong
> 	measures we risk failure for the new protocol, and we may not try
> 	again for a long time.
> 
> 	Also, with respect to some of those items we cannot encode
> minimally
> 	(cookies, URIs, ...): their size is really in the hands of the
> 	entities that create them -- let *them* worry about compression.
> That
> 	might cause some pressure to create shorter, less-meaningful URIs,
> 	but... we're already there anyways.
> 
> 	Nico
> 	--
> 
> 

Received on Friday, 18 January 2013 13:50:36 GMT

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