Re: multiplexing -- don't do it

On Sat, Mar 31, 2012 at 3:25 PM, Peter L <bizzbyster@...> wrote:

Multiplexing inefficiencies:

I don't have data on this but I can share my experience developing a web accelerator for satellite networks that does prefetching (similar to Server Push but prefetched objects are buffered in the modem as opposed to the browser itself) and has supported multiplexing similar to SPDY.

Bad performance of hi priority objects (browser requested JavaScript say) when preceded by low priority objects (Images or prefetched objects) is a serious problem when objects are multiplexed, especially for congested networks. The ideal solution is for the optimizer to send in priority order but also mark the priority of packets, maintain original streams, as well as the visibility of headers so that COTS networking equipment can be used to differentially shape the streams based on priority. 

If we are on a transport such as SCTP, we can do some of these things and avoid some HOL blocking cases that can occur with multiplexing over TCP. I could see defining a mapping for HTTP2 onto such pre-multiplexed protocols, but Mark would know best if that is something we can do. It'd make sense to me, though you'd end up with two dufferent

  

Hypothetical Example: Assuming SPDY's Server Push feature is widely adopted, ISPs will want to be able to treat prefetched data with a lower priority than interactive or user requested data when their networks are congested.

Bleh! You have to consider the second-order consequences of doing things like that.  If you do that, then server push is effectively optional and people will just inline again, and the likely outcome is that you'll neither have an effective cache, nor decrease overall bandwidth. Anything that forces people to inline has strongly negative effects on efficiency and latency as compared to server push.

I am, however, all in support of the browser declaring what it will or will not support w.r.t. server push. The client-side's incentives align with the user, and so putting the policy decision there makes sense so long as it doesn't incur an extra RT from negotiation.

 

They already have the gear in place to do this type of L7 differential shaping. It's good for end users and good for site owners as it allows for good page load times even during peak busy hours. Do you agree HTTP 2.0 should not break this type of functionality?

Traffic shaping is an awesome technology, but the interests of the middleware providers rarely aligns with the user. Look at the manoeuvring that AT&T is doing these days w.r.t. bandwidth on mobile devices, or the debacle of putting the full set of HTTP over port 80 (even when the endpoints agree and are tested to speak the full set) as examples.

I don't want to be in a world where they look at which site (as opposed to port) you're going to and potentially raise or lower the priority of your packets because someone pays them more? Screw that!

In any case, do we believe that even an elightened and altruistic middleware provider can separate and distinguish between the hi-pri things within the one protocol effectively? I think that alone is a difficult problem... and thankfully probably out of the purvue of the WG.

-=R

 

Thanks,

Peter

On Mar 30, 2012, at 9:22 AM, Mike Belshe <mike@...> wrote:

On Fri, Mar 30, 2012 at 4:07 AM, Peter L <bizzbyster@...> wrote:

I'm new to this list but have been studying web performance over high latency networks for many years and multiplexing seems to me like the wrong way to go. The main benefit of multiplexing is to work around the 6 connections per domain limit but it reduces transparency on the network, decreases the granularity/modularity of load balancing and increases object processing latency in general on the back end as everything has to pass through the same multiplexer, and introduces its own intractable inefficiencies.

The CPU processing at the server is one thing we could optimize for.  Or we could optimize for user's getting their pages faster.

Data suggests that your claims of inefficiency are simply incorrect.  But if you have a benchmark to report upon, we could discuss that.

 

In particular the handling of a low priority in flight object ahead of a high priority object when packet loss is present is a step backwards from what we have today for sites that get beyond the 6 connections per domain limit via domain sharding. Why not just introduce an option in HTTP 2.0 that allows clients and servers to negotiate max concurrent connections per domain?

As you can see from data, websites are not having any trouble getting around the 6 connection limit already.  

We could do this, but it would do nothing to make pages load faster or be lighter weight on the network.

 

When web sites shard domains, aren't they essentially telling the browser that they will happily accept lots more connections? I'm sure this suggestion has long since been shot  down but browsing around on the web I'm not finding it.

As for header compression, again this is a trade-off between transparency/multiple streams and bandwidth savings. But I'd think this group could come up with ways to reduce the bytes in the protocol (including cookies) without requiring the use of a single compression history, resulting in an order-sensitive multiplexed stream.

I'm not sure why you are opposed to compression.  We could reduce the bytes as well, and nobody is against that.

What is "transparency on the wire"?  You mean an ascii protocol that you can read?  I don't think this is a very interesting goal, as most people don't look at the wire.  Further, if we make it a secure protocol, its a moot point, since the wire is clearly not human readable.

mike

 

Thanks,

Peter

On Thu, Mar 29, 2012 at 9:26 AM, Mike Belshe <mike@...> wrote:

I thought the goal was to figure out HTTP/2.0; I hope that the goals of SPDY are in-line with the goals of HTTP/2.0, and that ultimately SPDY just goes away.

Mike

On Thu, Mar 29, 2012 at 2:22 PM, Willy Tarreau <w@...> wrote:

Hello,

after seeing all the disagreements that were expressed on the list these
days (including from me) about what feature from SPDY we'd like to have
mandatory or not in HTTP, I'm thinking that part of the issue comes from
the fact that there are a number of different usages of HTTP right now,
all of them fairly legitimate.

First I think that everyone here agrees that something needs to be done
to improve end user experience especially in the mobile networks. And
this is reflected by all proposals, including the http-ng draft from
14 years ago!

Second, the privacy issues are a mess because we try to address a social
problem by technical means. It's impossible to decide on a protocol if
we all give an example of what we'd like to protect and what we'd prefer
not to protect because it is useless and possibly counter-productive.

And precisely, some of the disagreement comes from the fact that we're
trying to see these impacts on the infrastructure we know today, which
would obviously be a total breakage. As PHK said it, a number of sites
will not want to afford crypto for privacy. I too know some sites which
would significantly increase their operating costs by doing so. But
what we're designing is not for now but for tomorrow.

What I think is that anyway we need a smooth upgrade path from current
HTTP/1.1 infrastructure and what will constitute the web tomorrow without
making any bigbang.

SPDY specifically addresses issues observed between the browser and the
server-side infrastructure. Some of its mandatory features are probably
not desirable past the server-side frontend *right now* (basically
whatever addresses latency and privacy concerns). Still, it would be
too bad not to make the server side infrastructure benefit from a good
lifting by progressively migrating from 1.1 to 2.0.

What does this mean ? Simply that we have to consider HTTP/2.0 as a
subset of SPDY or that SPDY should be an add-on to HTTP. And that
makes a lot of sense. First, SPDY already is an optimized messaging
alternative to HTTP. It carries HTTP/1.1, it can as well carry HTTP/2.0
since we're supposed to maintain compatible semantics.

We could then get to a point where :
 - an http:// scheme indicates a connection to HTTP/1.x or 2.x server
 - an https:// scheme indicates a connection to HTTP/1.x or 2.x server
   via an SSL/TLS layer
 - a spdy:// scheme indicates a connection to HTTP/1.x or 2.x server
   via a SPDY layer

By having HTTP/2.0 upgradable from 1.1, this split is natural :

       +----------------------------+
       |       Application          |
       +----+-----------------------+
       | WS |     HTTP/2.0          |
       +----+--------------+        |
       |      HTTP/1.1     |        |
       |         +-----+---+--------+
       |         | TLS | SPDY       |
       +---------+-----+------------+   server-side
           ^        ^        ^
           |        |        |
           |        |        |
           |        |        |
       +---------+-----+------------+  user-agent
       |         | TLS | SPDY       |
       |         +-----+-------+----+
       |  HTTP/1.1, 2.0        |    |
       +-------------------+---+    |
       |                   |   WS   |
       |  Applications     +--------+
       |                            |
       +----------------------------+

The upgrade path would then be much easier :

 1) have browsers, intermediaries and servers progressively
    adopt HTTP/2.0 and support a seamless upgrade

 2) have browsers, some intermediaries and some servers
    progressively adopt SPDY for the front-line

 3) have a lot of web sites offer URLs as spdy:// instead of http://,
    and implement mandatory redirects from http:// to spdy:// like a
    few sites are currently doing (eg: twitter)

 4) have browsers at some point use the SPDY as the default scheme
    for any domain name typed on the URL bar.

 5) have browsers at one point disable by default transparent support
    for the old http:// scheme (eg: put a warning or have to tweak
    some settings for this). This will probably 10-20 years from now.

Before we get to point 5, we'd have a number of sites running on the
new protocol, with an efficient HTTP/2.0 deployed at many places
including the backoffice, and with SPDY used by web browsers for
improved performance/privacy. That will not prevent specific agents
from still only using a simpler HTTP/2.0 for some uses.

So I think that what we should do is to distinguish between what is
really desirable to have in HTTP and what is contentious. Everything
which increases costs or causes trouble for *some* use cases should
not be mandatory in HTTP but would be in the SPDY layer (as it is
today BTW).

I think that the current SPDY+HTTP mix has shown that the two protocols
are complementary and can be efficient together. Still we can significantly
improve HTTP to make both benefit from this, starting with the backoffice
infrastructure where most of the requests lie.

Willy