Re: [quicwg/base-drafts] NUM_PLACEHOLDERS MUST NOT be zero (#2753)

Kazuho Oku <notifications@github.com> Wed, 05 June 2019 11:03 UTC

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Subject: Re: [quicwg/base-drafts] NUM_PLACEHOLDERS MUST NOT be zero (#2753)
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@rmarx 
> Do you mean placeholders are no longer needed if we bring back the exclusive bit __AND allow streams to depend on other streams__?

Correct. IIUC, the context is if the browsers need to use a placeholder under current scheme (that already allows a stream to depend on another) with the exclusive bit being revived.

Let me be precise.

I think there is a rough consensus that a ideal tree looks like:

```
root
  +- JS1 (blocking)
      +- CSS1 (blocking)
        +- CSS2 (blocking)
            +- image1 (weight=X)
            +- image2 (weight=X)
            +- image3 (weight=X)
            +- image4 (weight=X)
```

HTML could be either within the root and JS1, or within CSS2 and image1.

The other thing that is missing in the chart above are the the asynchronously loaded scripts and CSS files. However, we could argue that prioritization of them is not important, as long as it receives comparable or less bandwidth than the images.

If we agree on that, there is no "need" for browsers to use placeholders (or idle streams as placeholders), neither in the HTTP/2 prioritization tree nor the current HTTP/3 scheme with the exclusive bit being revived.

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