Re: HTTP router point-of-view concerns

Header compression on the upstream path is more or less required to enable
effective  prioritized mux of multiple transactions due to interactions
with TCP congestion control. If you don't have it you cannot effectively
achieve the parallelism needed to leverage the most important HTTP/2
feature with out an RTT penalty. and RTT's are the enemy.

Without compression you can "pipeline" between 1 and 10 requests depending
on your cookie size. Probably closer to 3 or 4. With compression, the sky
is more or less the limit.



On Thu, Jul 11, 2013 at 1:21 PM, Sam Pullara <spullara@gmail.com> wrote:

> 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).
> >>
> >
>
>
>