Code change:
connection (===) between the user agent (UA) and the origin server (O). <del> Each major version of HTTP defines its own syntax for the inclusion of information in messages.  Nevertheless, a common abstraction is that a message includes some form of envelope/framing, a potential set of named fields up front (a header section), a potential body, and a potential following set of named fields (a trailer section). </del> <ins> Each major version of HTTP defines its own syntax for the communication of messages.  Nevertheless, a common abstraction is that each message contains some form of envelope/framing with self- descriptive control data that indicates its semantics and routing, a potential set of named fields up front (a header section), a potential body, and potential fields sent after the body begins (trailer sections). </ins> <del> A client sends an HTTP request to a server in the form of a </del> <ins> A client sends requests to a server in the form of a </ins> message with a method (methods) and request target.  The request might also contain header fields for request modifiers, client

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.