As discussed in #220 and https://github.com/quicwg/base-drafts/issues/3265, mid-stream trailers (i.e., those that arrive before the end of the payload body) are interesting.

A few things to discuss:

• What are some potential use cases?

• We recently stopped automatically combining trailers into headers, as that created a lot of issues. What about these -- are they yet again distinct from trailers as well as headers? Or can they be "downgraded" into trailers automatically? In other words, when transitioning from a hop that supports these from one that doesn't, are they just dropped onto the floor?

• Related - is the expectation that implementations will create separate APIs for these, or would it be that trailers are just available / writable early?

• What relationship does one of these have to the payload body? E.g., could I write one that "knows" exactly where in the body it occurs, so that it could sign a chunk of the body relative to it? Or could a handling implementation change its ordering in relation to body bytes?