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I think, but could be completely wrong, that browsers can reject a pushed resource at the start. Since the "connection" cost for each resource is minimal, this wouldn't be too much of a problem. Someone confirm or deny this.

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with http2 server push the server first sends a frame advertising it's going to push some resource (denoted by it's uri). the client can reject this resource before it's sent. this means the server has to delay sending to give the client the opportunity, however

The server does not delay sending until the advertising frame is acknowledged. So, for a large push the client might cancel the push, but for small pushes the client will already have received the data before the cancel can reach the server.

In general, if you are only pushing something that is both small and required that isn't a problem, but when using push for things like MPEG-DASH this becomes a point to think about.

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