> Maybe I'm being dense, but wouldn't swapping these chunks serve that purpose? eg:

Sorry - I could probably have been more clear. `payload->stream->write()` doesn't necessarily write the bytes to wire. For instance, I believe the WinHttp implementation caches the bytes and only writes them to the wire when the number of bytes has passed a certain threshold. As it is up to the individual implementation of the subtransport as to when the bytes are actually written to the wire, we would need to plumb the callback down into the subtransport (or, expose this knowledge in some other fashion).

I will swap those calls around though, and it will be slightly more accurate.