Code change:
5.6.3. <ins> Like header fields, trailer fields with the same name are processed in the order received; multiple trailer field lines with the same name have the equivalent semantics as appending the multiple values as a list of members, even when the field lines are received in separate trailer sections.  Trailer fields that might be generated more than once during a message MUST be defined as a list value even if each member value is only processed once per field line received. Trailer fields are expected (but not required) to be processed one trailer section at a time.  That is, for each trailer section received, a recipient that is looking for trailer fields will parse the received section into fields, invoke any associated processing for those fields at that point in the message processing, and then append those fields to the set of trailer fields received for the overall message. This behavior allows for iterative processing of trailer fields that contain incremental signatures or mid-stream status information, and fields that might refer to each other's values within the same section.  However, there is no guarantee that trailer sections won't shift in relation to the message body stream, or won't be recombined (or dropped) in transit, so trailer fields that refer to data outside the present trailer section need to use self-descriptive references (i.e., refer to the data by name, ordinal position, or an octet range) rather than assume it is the data most recently received. Likewise, at the end of a message, a recipient MAY treat the entire set of received trailer fields as one data structure to be considered as the message concludes.  Additional processing expectations, if any, can be defined within the field specification for a field intended for use in trailers. 5.6.4. </ins> The "Trailer" header field provides a list of field names that the sender anticipates sending as trailer fields within that message. This allows a recipient to prepare for receipt of the indicated

Related discussion:
text: Does this mean it's now valid to interleave HTTP/2 DATA frames and HEADER frames?
Does this mean we cannot implement this on HTTP/2?
The feature was removed during WGLC, but no -- it would have required a new frame type.
Okay, so chunk extensions remains a unique feature of HTTP/1 chunked encoding and there is no plan to expand that further?
Yes.
As discussed in and URL, 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?
Some preliminary considerations. Incremental checksums eg. Service management (eg. convey the ETA of the transfer, ...) Sending signatures of partial body (in mid-stream) has some complexities respect to sending a signature of the whole message in the header/trailer section, and requires a security URL trailers, when used for integrity can't be dropped without informing the user agent They probably need to be defined as 1#field-item because mid-stream trailers will be sent multiple times (eg. one per chunk) Whenever they are tied to the partial transmitted representation, they will eventually be unuseful when merged, unless they have some associative propertyThey will be especially useful when tied to a portion of the body They will be significantly less useful when not tied to a portion of the body
Potential use cases mostly limited to long-lived streaming traffic, i.e.