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Re: HTTP router point-of-view concerns

From: Sam Pullara <spullara@gmail.com>
Date: Thu, 11 Jul 2013 10:21:00 -0700
Cc: Martin Thomson <martin.thomson@gmail.com>, Amos Jeffries <squid3@treenet.co.nz>, HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <092D65A8-8CB7-419D-B6A4-77CAE40A0026@gmail.com>
To: James M Snell <jasnell@gmail.com>
How sure are we that the entire idea of header compression isn't a bad idea? I implemented something similar in the WebLogic T3 protocol (BubblingAbbrevTable, probably still in there) and it was mostly just a pain. If I were to go back I would just use gzip with some agreed upon seed dictionary. Thought I would bring this up since it seems like it is a very controversial feature to begin with.

Sam

On Jul 11, 2013, at 10:14 AM, James M Snell <jasnell@gmail.com> wrote:

> Yes, the ability to set compression context size to 0 is very useful.
> My fears around this area are:
> 
> 1. In order to achieve maximum throughput, Intermediaries may opt to
> *always* set compression context to 0, forcing the headers to always
> be passed as Literals, killing the utility of having the header
> compression mechanism there in the first place.
> 
> 2. The assumption of a non-zero default compression context size when
> the connection is established opens a race condition that a malicious
> sender could exploit in a denial of service attack. Yes, the receiver
> could opt to terminate the connection once it detects bad behavior,
> but there is still a potential window of time there where the receiver
> could be forced to do significant additional work.
> 
>  (This is particularly bad given that header continuations are unbounded.)
> 
> 3. Setting the compression context size to 0 does not stop the sender
> from sending the Indexed Literal instructions anyway. The receiving
> endpoint would still be required to process those instructions even if
> the data is not actually being indexed, causing CPU cycles to be
> consumed. For any individual block of headers it may not be a
> significant load, but it's something that needs to be addressed.
> 
>  (This can be fixed in the spec by stating that any attempt to Index
> any individual (name,value) whose size is greater than the available
> header table size results in a Compression Error. Making this change
> would mean that when Compression Context size is 0, the only operation
> that would not result in an error is Literal without Indexing. This
> was discussed on the list but as far as I can tell it's not yet
> captured in the spec).
> 
> 4. The fact that header continuations can be unbounded is deeply
> troubling, especially given that the endpoint is required to buffer
> and process the complete header block (well.. that's only half true,
> the encoding does allow for incremental processing of the HEADERS
> frame payloads but the spec requires that the complete header block is
> always processed). Sure, the recipient is free to terminate the
> connection as soon as it detects bad behavior, but the sender could
> end up forcing the recipient to do a significant amount of extra
> processing with a never ending sequence of HEADERS frames. Smart
> implementations will know how to deal with this, yes, but overall it
> adds to the already growing list of "New Complex Things" that an
> HTTP/2 implementer needs to know about.
> 
>  (In the implementation I've done, I provide a configuration
> parameter that allows a developer to cap the number of the
> continuations and the total size of the header block)
> 
> I know that we're in "implementation" phase right now and that
> everyone is busy getting their code ready for testing in August, but
> after updating my implementation to the latest version of the draft,
> my concerns with regards to stateful header compression definitely
> remain.
> 
> On Thu, Jul 11, 2013 at 9:36 AM, Martin Thomson
> <martin.thomson@gmail.com> wrote:
>> On 10 July 2013 21:20, Amos Jeffries <squid3@treenet.co.nz> wrote:
>>> It seems not to be negotiable from the recipients side.
>> 
>> Compression context size = 0 is entirely negotiable from the recipient
>> end, with a small wrinkle, that I know some folks are working on.
>> Which is, a client can start using a default compression context size
>> prior to learning that a server has no space (substitute intermediary
>> as appropriate there).
>> 
> 
Received on Thursday, 11 July 2013 19:20:56 UTC

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