Re: multiplexing -- don't do it

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. 

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. 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?

Thanks,

Peter


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

> 
> 
> On Fri, Mar 30, 2012 at 4:07 AM, Peter L <bizzbyster@gmail.com> 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@belshe.com> 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@1wt.eu> 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
> 
> 
> 
> 
>