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Re: Backwards compatibility

From: Mark Watson <watsonm@netflix.com>
Date: Sat, 31 Mar 2012 01:03:16 +0000
To: Mike Belshe <mike@belshe.com>
CC: William Chan (陈智昌) <willchan@chromium.org>, "<ietf-http-wg@w3.org>" <ietf-http-wg@w3.org>
Message-ID: <CB94B4CB-67A2-441F-AAF8-C060A23E6228@netflix.com>

On Mar 30, 2012, at 4:46 PM, Mike Belshe wrote:

On Fri, Mar 30, 2012 at 6:53 PM, Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:

On Mar 30, 2012, at 9:29 AM, William Chan (陈智昌) wrote:

On Fri, Mar 30, 2012 at 6:13 PM, Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:

I'd like to make a plea/request/suggestion that wherever possible new features be added incrementally to HTTP1.1, in a backwards compatible way, in preference to a "new protocol" approach. A "new protocol" is required only if it is not technically possible (or especially awkward) to add the feature in a backwards compatible way.

The object should be to enable incremental implementation and deployment on a feature by feature basis, rather than all-or-nothing. HTTP1.1 has been rather successful and there is an immense quantity of code and systems - including intermediaries of various sorts - that work well with HTTP1.1. It should be possible to add features to that code and those systems without forklifting substantial amounts of it. It is better if intermediaries that do not support the new features cause fallback to HTTP1.1 vs the alternative of just blocking the new protocol. In particular, it should not cost a round trip to fall back to HTTP1.1. It is often lamented that the Internet is now the "port-80 network", but at least it is that.

Don't forget port 443. And I agree, it should not cost a round trip to fallback to HTTP/1.1.

Many of the features contemplated as solutions to the problems of HTTP1.1 can be implemented this way: avoiding head-of-line blocking of responses just requires a request id that is dropped by intermediaries that don't support it and echoed on responses. Request and response header compression can be negotiated - again with a request flag that is just dropped by unsupporting intermediaries. Pipelined requests could be canceled with a new method. These things are responsible for most of the speed improvements of SPDY, I believe.

It's unclear to me how this would work. Are you suggesting waiting a HTTP request/response pair to figure out if the id gets echoed, before trying to multiplex requests? Or would you rely on HTTP pipelining as a fallback if the ids don't get echoed?

Send the requests (yes, pipelined). If they come back without ids, then they are coming back in the order they were sent. If they come back with ids, then that tells you which response is which.

You can't do this until you've got confirmation that the server is going to give you a HTTP/1.1 response.  It could come back HTTP/1.0.

So do we first have to do a 1.1 request successfully (with 1.1 response) before we can ever attempt to do a pipelined upgrade?

For each server, yes. Servers don't often get downgraded from 1.1 to 1.0, so you could cache that result for quite a while.

The former incurs a large latency cost. The latter depends very much on how deployable you view pipelining on the overall internet.

It's certainly widely deployed in servers and non-transparent proxies. Non-supporting non-transparent proxies are easily detected. Yes, broken transparent proxies are a (small) problem, but you can also detect these.

I am skeptical it is sufficiently deployable and we on Chromium are gathering numbers to answer this question (http://crbug.com/110794).

Our internal figures suggest that more than 95% of users can successfully use pipelining. That's an average. On some ISPs the figure is much lower.

Do you a research result to cite here?  Sounds interesting.  5% failures is pretty high.

No, these are just internal figures right now. Yes, it does seem high, I've a feeling many of those are false negatives where we avoid pipelining unnecessarily.

Also, pipelining is clearly inferior to multiplexing.

Yes, but perhaps in practice not by much. To render a page you need all the objects, so from a time-to-page-load perspective it makes no difference how you multiplex them, as long as the link remains fully utilized. To see some difference you need some notion of object importance and some metric for 'page loaded except for the unimportant bits'. You send the most important requests first. Even then it's not clear that multiplexing within objects will perform significantly better than object by object sending.

Don't forget that pipelining does *not* apply to all resources.  Even when pipelining works end-to-end, browsers need to take great care not to accidentally pipeline a critical resource behind a slow one (like a hanging GET).  This leads to browsers doing tricks like "only pipeline images together" or other subsets of pipelining.

I was assuming you could avoid the head-of-line blocking with an extension that allows out-of-order responses.

But when we consider pipelining a fallback for SPDY, this all falls apart.  SPDY did not have these restrictions.  So now, SPDY would need to run in some sort of degraded mode for what types of requests are pipelined just so fallback to a HTTP/1.1 protocol that the server might not support (because it could be HTTP/1.0) or which the user might not support because he's one of the unlucky 5% (according to Mark's data) where pipelining just breaks altogether.

All in all, we've now compounded 3 unique restrictions on the initial set of requests in order to work around past bugs in order to support use of the Upgrade header.

Realistically, you're going to get one request on the upgrade, and you'll have to wait to open up the parallel requests.  This is a significant restriction of the Upgrade process - it requires a round trip to kick into the real protocol at full gear.

This is highly annoying, but for initial web page loads, probably won't be a significant burden because the browser initially only has one URL.  For page reloads, or validations, or subsequent pages on reconnect, it will be a performance hit.

Interleaving within responses does require some kind of framing layer, but I'd like to learn why anything more complex than interleaving the existing chunked-transfer chunks is needed (this is also especially easy to undo).

Sorry, I'm not sure I understand what you mean by interleaving existing chunked-transfer chunks. Are these being interleaved across different responses (that requires framing, right?).

Interleaving data from multiple responses requires some kind of framing, yes. Chunked transfer encoding is a kind of framing that is already supported by HTTP. Allowing chunks to be associated with different responses would be a simple change. Maybe it feels like a hack ? That was my question: why isn't a small enhancement to the existing framing sufficient ?

Even if you could hack it into a chunk, thats a real jumbled mess.  Why do you want to do this?  It doesn't give you backward compatibility in any way (existing browsers won't know what to do with these nonstandard chunks anyway), its just a mess for the sake of a mess?

So, your answer to my question is fairly clear ;-)

It doesn't feel like such a 'mess' to me - we're talking about negotiating use of new protocol elements. They're only used if both ends support them so, yes, the only kind of backwards compatibility is that the use of framing is negotiated, rather than assumed from the start. My point was that you don't need a whole shim layer to do this, because HTTP already has framing. Perhaps it makes little difference, but it means you can develop and deploy functionality incrementally, rather than all-or-nothing.

Putting my question another way, what is the desired new feature that really *requires* that we break backwards compatibility with the extremely successful HTTP1.1 ?


See my question above

header compression,

Easily negotiated: an indicator in the first request indicates that the client supports it. If that indicator survives to the server, the server can start compressing response headers right away. If the client receives a compressed response it can start compressing future requests on that connection. It's important that this indicator be one which is dropped by intermediaries that don't support compression.


I think you mean "re-priortization". I can send requests in priority order - what I can't do is change that order to response to user actions. How big a deal is this, vs closing the connection and re-issuing outstanding requests in the new order ?

Its the difference between web pages rendering faster or slower.    Load up 100 image requests on your twitter page, and then fetch the images before the JS.  The page loads slower unless you lower the priority of the images.  But you still don't want to add serialization delays that HTTP has.

BTW - the effects of priorities has been measured, and you're welcome to use the existing benchmarking harness to verify yourself that these things are true in real code rather than just theory.  (see dev.chromium.org/spdy<http://dev.chromium.org/spdy>).  I wish I had published the tests when I did this long ago - spent a lot of time on it.

Again, I don't think you need anything more than the basic possibility to return responses out-of-order to get most of the gains. Send the requests in priority order and have the server return them in priority order, unless a response is not available in which case other responses can push ahead. The absence of interleaving within responses just reduces the granularity. Request the JS first, then the 100 images. With interleaving, if the JS is available half way through sending image 3, we can start sending the JS right there. Without interleaving you have to wait until the end of image 3.

What you don't have is, as I said, "re-prioritization", where the client can change its mind about the priority order after sending the requests - you'd have to close the connection and send the requests again.

Not perfect, but I feel you could get a good chunk of the gains, with out-of-order responses and negotiated compression.

Set aside that the significant advantages of small incremental changes to a well-understood, widely deployed, very successful protocol vs invention and all-at-once deployment of a new one.





Received on Saturday, 31 March 2012 01:03:48 UTC

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