Re: Why should caches and intermediaries ignore If-Match?

[Tom Bergan <[e-mail]>]

On Fri, Mar 3, 2017 at 2:58 PM, Mark Nottingham <[e-mail]> wrote:

>
> > On 4 Mar 2017, at 9:30 am, Roy T. Fielding <[e-mail]> wrote:
> >
> > On Mar 1, 2017, at 5:49 PM, Tom Bergan <[e-mail]> wrote:
> >>
> >> Here is the use case:
> >>
> >> We have a content-optimization (compression) proxy sitting between the
> browser and origin server. Among other things, the proxy can compress
> videos. When the browser starts playing a video, it makes an initial HTTP
> request to fetch (part of) the video, then builds an in-memory
> representation of the video and uses additional HTTP range requests as
> needed to fetch the rest of the video. For example, range requests are used
> to implement seeking.
> >>
> >> The challenge is that we now have multiple representations of every
> video: the original representation (from the origin server) and one or more
> compressed representations served by the proxy. When the browser makes an
> initial request for a video, it gets one of these representations. When it
> makes a subsequent range request, we want to ensure that it receives the
> *same* representation that it received on the initial request. Otherwise
> the browser cannot combine the second response with the first response and
> video playback will fail.
> >>
> >> An additional challenge is that the browser and proxy both have a
> cache. In theory, we control the entire connection and could add custom
> code to the browser, proxy, and caches to implement any protocol that we
> invent. In practice, both caches are intended to be HTTP-compliant caches
> and we'd rather not add custom hacks for use cases like this if we can
> avoid it.
> >>
> >> The browser needs to label each range request with the ETag it expects
> to receive. If-Match originally seemed like the perfect solution: The
> browser adds `If-Match: ETag` to every range request. If a cache has a copy
> of the video with a *different* ETag, the cache forwards the request to the
> next server in the chain rather than returning its cached copy (as would
> happen if we used If-Range instead of If-Match). Similarly, the proxy knows
> if the browser is requesting a compressed video or the original video, so
> it can respond accordingly. However, as discussed previously in this
> thread, If-Match doesn't work like this.
> >>
> >> Note that I agree it doesn't make sense for a cache to return 412 and
> we don't need that behavior. The semantics I'm looking for is: "Send me
> this representation if you have it, otherwise forward to the next server. A
> 4xx means that this representation is not current in the origin or in any
> intermediate cache or proxy."
> >>
> >> Hope that makes sense.
> >
> > You have several choices:
> >
> > 1) implement this using transfer encodings because they don't change
> range offsets;
> >     presumably, these would be added/removed by the protocol handlers
> before
> >     the caches ever see them.
>
> Ew.


Can you expand what you mean by this? I'm not sure I followed.

In case I wasn't clear, the proxy actually produces a completely different
transcode of the original video, possibly in a different container format
or codec. The "compressed" video is actually a completely different file
than the original video; this is not just compression via Content-Encoding.

> 2) use If-Range and configure your proxy to forward the request when no
> match;
> >     yes, that's legitimate HTTP (a server is free to ignore partial
> requests and a proxy
> >     can forward any request it likes).
>
> Nod.


This doesn't help with the caches, which return 200 when there is no match
on the If-Range etag rather than forwarding the request. If we didn't have
any HTTP caches in the middle, we would have already done this :)

> 3) use If-Match and deal with the extra round-trip after a 412.
>
> Why doesn't the logic in #2 apply here as well? Intermediary servers
> aren't required to 412.
>
>
> > I don't see any reason here to change the handling of If-Match.  Partial
> requests are
> > why we have If-Range.
> >
> > ....Roy
>
> --
> Mark Nottingham   https://www.mnot.net/
>
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