I'm wondering if we should invest some effort to tighten the bounds on max size of H2 connection output buffers. I think that the current H2 codec implementation eagerly moves data frames from the per-stream buffers to the output buffer as long as connection window is available, even in cases where the output buffer is already over the configured high watermark.

In the nghttp2 API documentation, I found mentions that nghttp2_send_data_callback can return NGHTTP2_ERR_WOULDBLOCK to stop data frame generation in cases where the generated frames wouldn't fit in the output buffer. It would be good to explore how this API works and how use of NGHTTP2_ERR_WOULDBLOCK affects the relative ordering of generated control and data frames when asked to resume.

Possible behavior strawman:

• Prioritize control frames over data frames when serializing
• Serializing control frames when output buffer is above high-watermark is acceptable.
• Avoid generating data frames when above high watermark
• Continue requiring last data frame serialized before trailers
• Trigger sendPendingFrames when transitioning below low-watermark

Also, let's reach out to nghttp2 if there are some missing APIs that prevent us from getting the behavior we want.