Code change:
method, and trailer fields for metadata collected while sending the payload. <del> A server responds to a client's request by sending one or more HTTP </del> <ins> A server responds to a client's request by sending one or more </ins> <del> messages, each including a success or error code (status.codes).  The response might also contain header fields for server information, resource metadata, and representation metadata (header.and.trailer.fields), a payload body (payload.body) to be interpreted in accordance with the status code, and trailer fields for metadata collected while sending the payload. </del> <ins> messages, each including a status code (status.codes).  The response might also contain header fields for server information, resource metadata, and representation metadata (header.and.trailer.fields), a payload body (payload.body) to be interpreted in accordance with the status code, and trailer fields for metadata collected while sending the payload. </ins> <del> One of the functions of the message framing mechanism is to assure that messages are </del> <ins> One of the functions of message framing is to assure that messages are </ins> . A message is considered complete when all of the octets indicated by its framing are available.  Note that, when no explicit framing is

Related discussion:
text: long-lived "uploading/downloading" in-band signaling support such as load reporting, app-level flow-control, authn update etc Semantics trailers are a special case of mid-stream metadata, e.g. only trailers are able to convey the status of the entire transaction or the entire resource (body) our experience is that in-order delivery is useful and often assumed, since the underlying transport (for HTTP body) is in-order and reliable. Out-of-order metadata is likely a use case for connection-level metadata, IMO with the above assumption on in-order delivery, you really only need one API for delivering in-stream or trailer metadata Multi-part without in-stream metadata, you would have to use multipart (with all the overhead) for structured data, the original content-type can be preserved, e.g. JSON stream; and for media data, the content-type may have to be application/octet-stream
Copying over the questions from NAME over in URL, as they are good ones:
The semantic model of trailers is that they provide additional metadata that may (or may not) be useful to recipients but was not available prior to the initial body data sent. How the are useful and what they might contain would be defined by the field name. A recipient is free to drop the trailer metadata. A mid-stream trailer frame would have the same role in flow control as a HEADERS frame, which is none at the current time. The proposed frames would not have any specific position, but the values inside trailer fields might identify specific positions (on their own, regardless of framing). A cache would either drop the values or store them separately as trailer fields in the order received.
HTTP/2's flow control only cares about DATA frames. But all frames are subject to TCP's flow control. In HTTP/3, all frames are sent on streams which is subject to QUIC's flow control. Responding to the original question, I don't know what a "new form of flow control" would look like and I don't see how one would improve things.
What's the minimal change that would need to happen to semantics to enable this in a future version of HTTP (or an extension to an existing one)?
I think you would need to make "trailer part" less special, and simply say that a version of HTTP can define a way to carry additional field sections during or after the body.