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Re: multiplexing -- don't do it

From: Mike Belshe <mike@belshe.com>
Date: Wed, 4 Apr 2012 10:10:05 +0200
Message-ID: <CABaLYCtLsPVssQ2WUdnxW4c-OOVaf5CCkHWB-9zxmanAGOzWuA@mail.gmail.com>
To: Willy Tarreau <w@1wt.eu>
Cc: "William Chan (?????????)" <willchan@chromium.org>, "Roy T. Fielding" <fielding@gbiv.com>, "ietf-http-wg@w3.org Group" <ietf-http-wg@w3.org>
On Wed, Apr 4, 2012 at 7:49 AM, Willy Tarreau <w@1wt.eu> wrote:

> Hi William,
>
> On Wed, Apr 04, 2012 at 01:46:29AM +0200, William Chan (?????????) wrote:
> > > It works just fine.  The data shows only that a general-purpose
> browser,
> > > that doesn't even bother to report the nature of network protocol
> errors,
> > > encounters a small percentage of network problems that exceed its
> users'
> > > tolerance for failure conditions because its users have no control over
> > > their network.  That might indicate that the browser cannot deploy it,
> or
> > > it might indicate that there was a protocol bug on the browser that
> failed
> > > on edge cases (just like Netscape 1-3 had a buffer reading bug that
> would
> > > only trigger if the blank line CRLF occurred on a 256 byte buffer
> > > boundary).
> > >
> >
> > I'm starting to get data back, but not in a state that I'd reliably
> > release. That said, there are very clear indicators of intermediaries
> > causing problems, especially when the pipeline depth exceeds 3 requests.
>
> Personally I'm still thinking that if we only pipeline on http/2 and not
> on http/1, we'll avoid all the associated risks. The real issue I can
> imagine with broken intermediaries is related to those still parsing
> requests in *packets* without reassembling them as a stream. Once your
> requests exceeds 1 packet (or the product's limit), you can run into
> trouble. Seen that on Alteon years ago (was an excellent L3/L4 LB but
> marketing made it L7 and caused issues everywhere).
>
> As we suggested in our draft, once the requests are reduced, it's possible
> to send the next ones along with the first request in a header field, this
> protects us against 1.1-only intermediaries which can't see them.
>

No - you can't rely on packet boundaries at the TCP layer to solve race
conditions at the app layer.  Example, at the time of the first request,
you may not have a second request to bundle with it, but 2ms later, you
might.  So, you'd end up with packet splitting again. And I haven't even
mentioned that the app layer has no idea how big the packet payload is
anyway....



> > As for networks that control their own deployments of intermediaries, are
> > these entirely private networks? If you go over the public internet at
> any
> > point, I'd expect to encounter some form of intermediary not controlled
> by
> > administrators.
>
> We still need to keep in mind that broken intermediaries are deployed at
> two
> places :
>  - inside the users' ISP network (eg: caches, LB, compressors)
>  - on the server side
>

Or at the user's school or corporation.  Or in load balancing equipment in
some shared data center.  Or.... anywhere.  There are myriads of
configurations possible.


>
> When users complain they can't access site X or Y, either they complain on
> the ISP side and the ISP has to find a quick fix (last time I experienced
> this was just two weeks ago). And when it's on the server side, the fix has
> to be applied too otherwise the site loses a lot of visitors (experienced
> this at the same time, the fight was whether it was the ISP or the site
> which was broken ; the breakage was unrelated to HTTP, it was TCP which
> failed due to use of CGN).
>
> What must absolutely be avoided is the random hang that cannot be diagnosed
> and the issues which cause random delays. We need a clean fallback to a
> working behaviour.


You're going through all the same thought processes that browser vendors
went through years ago.  It's great, it really is :-)  But, I don't think
you're going to find a pot of gold at the end of this rainbow.

If you really want to try, the firefox bug database is a treasure trove.
 Just go search for pipelining and check out some of the still-open bugs.

145392 <https://bugzilla.mozilla.org/show_bug.cgi?id=145392>nor--Allnobody
NEW---some images do not load [apache CGI pipelining
problem]<https://bugzilla.mozilla.org/show_bug.cgi?id=145392>
465814 <https://bugzilla.mozilla.org/show_bug.cgi?id=465814>maj--Macnobody
UNCO---Not all images load on a page with pipelining
enabled<https://bugzilla.mozilla.org/show_bug.cgi?id=465814>
716840 <https://bugzilla.mozilla.org/show_bug.cgi?id=716840>nor--Windnobody
UNCO---Images get the wrong content with pipelining
enabled<https://bugzilla.mozilla.org/show_bug.cgi?id=716840>

Some of the comments in here confirm my earlier speculation that mobile
browsers using pipelining have simply been letting errors go unchecked.

To quote, "

André Roaldseth 2012-01-10 01:58:28 PST

One of the earliest posts about this problem is
http://forums.mozillazine.org/viewtopic.php?f=38&t=284489 which dates
from 2004.

I have enabled network.http.pipelining and not touched any of the
other pipelining settings.

We became aware of the issue
(http://tech.vg.no/2011/12/14/safari-on-ios-5-randomly-switches-images/)
first with iOS 5 which has pipelining enabled by default. Now we've
reproduced it with Firefox 9 and we have some unconfirmed reports
about it happening with Opera. Several other news pages(like
www.db.no, www.ap.no) has also experienced the problem.

We have lots of tcpdumps from debugging the problem on iOS 5, but
everything looks to be correct in them."


Searching for iOS5 pipelining errors yields lots of other hits from google
on the subject.


Mike





> > > pipelining deploy it within environments wherein they do control the
> > > network
> > > and can rubbish the stupid intermediaries that fail to implement it
> > > correctly.
> > >
> >
> > What are these environments? Are they private networks? In these cases,
> is
> > HTTP pipelining that big a win? Do these networks operate on a global
> > scale? Or are they more local? If local, I'd expect the RTTs to be much
> > lower, and pipelining to be less of a win.
>
> It depends. Some corporate users are forced to browse on their laptop via
> their corporate VPN and proxies. When you run a VPN on top of 3G in which
> you use a proxy, you're exactly like a smartphone user (except you don't
> suffer from the DNS round trip).
>
> > Also, I'm going to take the opportunity to ask a dumb question (sorry, I
> > lack your guys' experience with all the uses of HTTP). To what extent do
> > these other environments matter? If you don't run over the public
> internet,
> > instead running over private networks, can't you run whatever protocol
> you
> > want anyway? Is it more about saving time and not having to write more
> > code? Can they just use HTTP/1.1 and forget HTTP/2.0?
>
> In my opinion we must not fragment 1.1 and 2.0. If 2.0 cannot replace 1.1
> everywhere I will consider it a total failure. It's not acceptable to
> design a new protocol that some people will say "I won't use it because X
> or Y". I prefer by large "I'm not compatible right now but am planning to
> upgrade".
>





>
> > It's not clear to me which intermediaries are causing the problems. Your
> > statement here seems to be predicated on the problematic intermediaries
> > being located closer to the client. Do we have any data to support this?
>
> I don't have data either but from what I have *observed*, server-side tends
> to be a lot more careful at what they deploy so that all their
> infrastructure
> is compatible with their own needs. ISPs tend to deploy something which
> seems
> OK for most usages (and it's not easy for them to catch all corner cases).
> So
> this practice tends to put more breakage at the ISP's than at the content
> provider's. Also, when an ISP deploys a transparent cache, economics are
> much
> more involved than when a content provider deploys a load balancer, so the
> ability to accept tradeoffs is easy to understand in the former case.
>
> > What features do they need beyond what's offered in HTTP/1.1? Or is the
> > assumption that we want to completely kill off HTTP/1.1? What about
> Mike's
> > point in his httpbis presentation that we may want different protocols
> for
> > the "backoffice" and the general internet?
>
> I do think that what's in the backoffice must converge to 2.0 too and that
> what's outside may be a superset of this. Once again, personal opinion
> only.
> I would hate to say users "hey we released 2.0 and only browsers will be
> able to use it - if you have a web server that's 2.0 and want to put a LB
> in front of it, you must degrade it to 1.1 first". That does not make much
> sense in my opinion.
>
> > And falls back to HTTP/1.1 in a reasonably fast manner that does not
> > significantly degrade user experience.
>
> This is extremely important (reason why I do like the Upgrade).
>
> Regards,
> Willy
>
>
Received on Wednesday, 4 April 2012 08:10:40 GMT

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