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Re: HTTP/3 Prioritization Proposal

From: Mark Nottingham <mnot@mnot.net>
Date: Sat, 4 May 2019 16:32:19 +1000
Cc: Patrick McManus <mcmanus@ducksong.com>, Patrick Meenan <patmeenan@gmail.com>, Lucas Pardue <lucaspardue.24.7@gmail.com>, Amos Jeffries <squid3@treenet.co.nz>, HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <F8B9A0EE-EA51-4704-BA2E-61C69A0E26B7@mnot.net>
To: Ian Swett <ianswett@google.com>
> On 4 May 2019, at 6:26 am, Ian Swett <ianswett@google.com> wrote:
>> This is a much more important property in an aggregator like a CDN who might be bringing different front end connections into a single backend connection.. the priority expressed by the client should exist in some ways e2e (css before imgs!), but in other ways hop to hop (you don't want every css to stall every browser's images).. the tree allows that.
> This statement concerns me for a few reasons.  One is I doubt any CDNs can pull this off at scale, so I don't think it's practical.  

Any particular reason why? Most CDNs have a hierarchy of some kind, to protect the origin from having to serve too many CDN nodes directly. 

This came up at the latest HTTP Workshop; no one with an intermediary was using priorities in this way -- but that's because most intermediaries aren't using H2 on the back-end connection. Once they do, the next hurdle they'll hit is that H2's use of TCP means that using a single connection to origin is unattractive, so they'll use a number in parallel. In theory, H3 should address that; in practice, we'll see.

Regardless, it's not unreasonable to think that an intermediary will multiplex more than one client's traffic onto a connection (either to a parent node, or to the origin). 

> Someone should correct me if I'm wrong.  Another is that to pull this off, you'd need reliable ways to know that a single user was the owner of two different connections, which seems potentially concerning from a privacy perspective?

Why would that be? 

>  Lastly, I don't think it would result in optimal loading.  If one could do this, strict numerical priorities would likely work better, because they'd preserve most of the clients original intent instead of equally sharing bandwidth between blocking resources(ie: HTML, CSS) and non-blocking ones(ie: images).

I'm not sure what you mean here. With strict numerical priorities, an intermediary would either have to pass through the priorities unchanged (in which case a misbehaving client could claim everything is e.g., priority 255 and hog resources), or it'd have to normalise the priorities in some fashion, which practically (if not completely) precludes preserving the relative priority of a given client's requests.


Mark Nottingham   https://www.mnot.net/
Received on Saturday, 4 May 2019 06:32:53 UTC

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