HTTP Layer rework and PUSH_PROMISE contents

In draft-ietf-httpbis-http2.xml:

>

> +        <t>

> +          The server can choose to send one or more push promises

> +          associated with the response. These notify the client that

> +          the server intends to deliver additional resources to the client

> +          as specified in <xref target="PushResources" />. If the server

> +          sends PUSH_PROMISE frames, those MUST be sent prior to sending

> +          any header blocks or DATA frames that reference the promised resources.

> +          For instance, if the server receives a request for a document

> +          containing embedded links to multiple image files, and the

> +          server chooses to push those additional images to the client,

> +          all of the push promises MUST be sent prior to sending the DATA frames

> +          that contain the image links. Likewise, if the server pushes

> +          resources referenced by the header block (i.e. using Link headers),

> +          the server MUST send the push promises before sending the header

> +          block.

You're upgrading a SHOULD in the current spec to a MUST. Has this been discussed on-list?

Speaking as the http.sys owner for Windows, this concerns me. We don't know the content of the entity body fragments or response headers an application hands us, only the order. The only way we can definitively comply with this MUST is to make all PUSH_PROMISE frames precede all DATA or HEADERS frames. As a SHOULD, we're free to leave proper behavior to the app using our APIs, while maintaining non-optional protocol compliance at our layer.