Sending responses with unknown body size in HTTP/2

Hello,

I've been thinking about how a server can approach sending a body of unknown total size.

In HTTP/1.1, we had the chunked transfer coding. From RFC 7230 section 4.1:

   The chunked transfer coding wraps the payload body in order to
   transfer it as a series of chunks, each with its own size indicator,
   followed by an OPTIONAL trailer containing header fields.  Chunked
   enables content streams of unknown size to be transferred as a
   sequence of length-delimited buffers, which enables the sender to
   retain connection persistence and the recipient to know when it has
   received the entire message.

In HTTP/2, RFC 7540 section 8.1 says


   HTTP/2 uses DATA frames to carry message payloads.  The "chunked"

   transfer encoding defined in Section 4.1 of [RFC7230]<https://tools.ietf.org/html/rfc7230#section-4.1> MUST NOT be

   used in HTTP/2.

RFC 7540 section 8.1.3 has an example where it shows how an HTTP/1.1 chunked response could look in HTTP/2 frame form.


HTTP/1.1 200 OK                  HEADERS

     Content-Type: image/jpeg   ==>     - END_STREAM

     Transfer-Encoding: chunked         + END_HEADERS

     Trailer: Foo                         :status = 200

                                          content-length = 123

     123                                  content-type = image/jpeg

     {binary data}                        trailer = Foo

     0

     Foo: bar                         DATA

                                        - END_STREAM

                                      {binary data}



                                      HEADERS

                                        + END_STREAM

                                        + END_HEADERS

                                          foo = bar

The HTTP/1.1 response does not contain a Content-Length header field (presumably because the length is unknown). The HTTP/2 format does include a content-length but I'd argue that the example now implies the length is now known before the payload is sent.

Would it be valid here, to omit the content-length in the HTTP/2 HEADERS?

When thinking through this problem a while back, I had thought that in the HTTP/2 case it would be possible to append the content-length as a trailer but I convinced myself that was also invalid due to a number of other HTTP semantic rules.

Kind regards
Lucas