Re: http/2 prioritization/fairness bug with proxies

[Martin Thomson <[e-mail]>]

I'm thinking a simple mechanism.  The proxy can feed requests back to
clients based on its own prioritization and use flow control windows
on the back-end connections to ensure that it doesn't have to buffer
infinitely.  Basically, the same sorts of logic that would be needed
by a proxy that serves multiple clients.

Yes, this is sub-optimal because it leads to either poor bandwidth
utilization, lots of buffering, or worse.

Sure, it's very easy to get this wrong.  I don't believe it
unreasonable to consider this sort of thing to be marked "hard hat
area".  In both directions.

On 11 February 2013 14:46, William Chan (陈智昌) <[e-mail]> wrote:
> Theoretically possible is one thing. But the moment we get into the game of
> trying to carve up portions of BDP via per-stream flow control windows for
> prioritization purposes in normal operation (as opposed to just trying to
> make reasonable progress under excessive load), I think we're in trouble,
> and likely to get into performance issues due to poor implementations. As
> I've stated before, I hope most implementations (and believe we should add
> recommendations for this behavior) only use flow control (if they use it at
> all, which hopefully they don't because it's hard) for maintaining
> reasonable buffer sizes.
>
>
> On Mon, Feb 11, 2013 at 2:39 PM, Martin Thomson <[e-mail]>
> wrote:
>>
>> Keep in mind that it is always possible for the intermediary to apply
>> flow control to the infinite length stream so that, regardless of
>> priority, it doesn't consume more than its fair share.
>>
>> On 10 February 2013 14:40, William Chan (陈智昌) <[e-mail]>
>> wrote:
>> > First, I totally agree SPDY/4 prioritization changes are far more
>> > reaching.
>> > Let's not talk about them yet. I share your complexity concern and agree
>> > we
>> > need data before proceeding with many of those features, and I plan on
>> > getting data.
>> >
>> > As far as grouping, I think it's definitely a bug for the case where you
>> > have a forward proxy with users sharing the same HTTP/2.0 connection to
>> > an
>> > origin server. You can have many high priority short-lived streams which
>> > can
>> > starve out others, unless we implement the vague notion of "don't starve
>> > out
>> > streams", which is difficult when it might be a sustained rate of high
>> > priority short-lived streams. It's easier in your latter case of
>> > infinite,
>> > high-priority streams.
>> >
>> > You're right that the high priority, infinite stream can starve streams
>> > within the same group. I don't think this means that grouping is not
>> > required, but that grouping is potentially insufficient. For intragroup
>> > starvation, I think it's debatable about whether the server should be
>> > smart
>> > and not allow streams to _completely_ starve other streams within a
>> > group,
>> > or that clients should have a reprioritization facility. I think this is
>> > a
>> > discussion worth having, but I'd personally classify it as separate from
>> > whether or not the grouping feature is necessary.
>> >
>> > On Sun, Feb 10, 2013 at 2:06 PM, Mike Belshe <[e-mail]> wrote:
>> >>
>> >>
>> >>
>> >>
>> >> On Mon, Feb 4, 2013 at 5:47 PM, William Chan (陈智昌)
>> >> <[e-mail]>
>> >> wrote:
>> >>>
>> >>> I'm sorry if I am unclear in any way. Please continue to
>> >>> challenge/question my comments/assertions so I can clarify my position
>> >>> as appropriate.
>> >>>
>> >>> Just to be clear here, I stand by that it's a protocol bug currently.
>> >>
>> >>
>> >> +0.5.  :-)
>> >>
>> >> Mark - I think we could open a issue ticket with the current HTTP/2.0
>> >> draft that this is a bug which will present itself for servers that
>> >> implement naively.  This isn't strictly a "bug", since the spec says
>> >> the
>> >> server can do whatever it wants with priorities, but this is subtle
>> >> enough
>> >> (surfacing primarily in http/2 -> http/2 proxy situations), that many
>> >> server
>> >> implementors won't think of it unless we mention it in the spec.
>> >>
>> >> The bare minimum would be to simply document it and tell servers not to
>> >> starve out streams.  However, this is probably a wimpy approach and I
>> >> think
>> >> we can do better.
>> >>
>> >> The SPDY/4 prioritization changes are far more reaching than just
>> >> fixing
>> >> this bug.  While I like grouping as a feature, I don't believe it
>> >> actually
>> >> fixes this bug:  a browser could open a high priority, infinite stream,
>> >> which is competing across a shared proxy backend with other streams;
>> >> unless
>> >> the user manually switches tabs (or does something to force changed
>> >> groups),
>> >> the starvation can still occur.
>> >>
>> >> Mike
>> >>
>> >>
>> >>
>> >>
>> >>>
>> >>> I agree with adding more hooks to convey advisory priority semantics.
>> >>> That said, "advisory" is open to interpretation. I agree that the
>> >>> sender should ultimately be in control of how it orders responses, and
>> >>> indeed there are of course many situations where it's best for the
>> >>> sender to ignore the advisory priority. Yet, if the advisory priority
>> >>> semantics are generally not respected, then clients will not be able
>> >>> to rely on them, and will be forced to implement prioritization at a
>> >>> higher layer, which suffers from the link underutilization vs
>> >>> contention tradeoff I highlighted earlier.
>> >>>
>> >>> I appreciate the concern that we're adding complexity by introducing
>> >>> new semantics. I am arguing that because the existing mechanisms for
>> >>> addressing starvation are suboptimal, we should treat this as a
>> >>> protocol bug and thus change the protocol in such a way as to fix this
>> >>> problem. My suggestion for doing so was adding new priority "grouping"
>> >>> semantics. I am hopeful that these new semantics will not introduce an
>> >>> inordinate amount of specification, as the primary idea is that the
>> >>> current SPDY priority levels would apply within a "group". I think we
>> >>> can come up with a way to define a group that will be relatively easy
>> >>> to spec.
>> >>>
>> >>> SPDY/4 introduces other prioritization semantics beyond just grouping,
>> >>> but I wanted to focus on this one first, as I believe this is a bug
>> >>> that we *need* to fix. The other SPDY/4 priority changes are of a
>> >>> performance optimization nature, and I believe they will need to be
>> >>> justified by data. I have no plans to raise them up in this group
>> >>> until we have said data.
>> >>>
>> >>> On Tue, Feb 5, 2013 at 5:34 AM, Poul-Henning Kamp <[e-mail]>
>> >>> wrote:
>> >>> > Content-Type: text/plain; charset=ISO-8859-1
>> >>> > --------
>> >>> > In message
>> >>> > <[e-mail]>,
>> >>> > Ben Nive
>> >>> > n-Jenkins writes:
>> >>> >
>> >>> >>So the idea is the protocol contains enough 'hooks' to sufficiently
>> >>> >>express the different priorities between & within groups that folks
>> >>> >>would like to express but isn't prescriptive about how anyone uses
>> >>> >> or
>> >>> >>implements different prioritisation, scheduling, etc schemes.
>> >>> >
>> >>> > That was clearly not how the original poster presented it:
>> >>> >
>> >>> >         "I consider all those options as suboptimal, and thus
>> >>> >         consider this issue to be a protocol bug. Our SPDY/4
>> >>> >         prioritization proposal addresses this by [...]"
>> >>> >
>> >>> > --
>> >>> > Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
>> >>> > [e-mail]         | TCP/IP since RFC 956
>> >>> > FreeBSD committer       | BSD since 4.3-tahoe
>> >>> > Never attribute to malice what can adequately be explained by
>> >>> > incompetence.
>> >>>
>> >>
>> >
>
>