 HTTP trailer field

An HTTP trailer field is a message field carried in a trailer section after the content. It can provide metadata whose final value becomes available while the content is being generated or processed. Its placement lets a sender stream content before that metadata is complete.^[ref]

For a concrete HTTP/1.1 example, define an illustrative extension field Example-Count to report the number of content octets and explicitly permit it in trailers. This is a definition for the example, not a standard field. A response can announce Transfer-Encoding: chunked and Trailer: Example-Count in its header section. Its following transfer-coded body, using visible escape notation, is 3\r\nabc\r\n0\r\nExample-Count: 3\r\n\r\n.

Here \r\n denotes a carriage-return/line-feed pair in the actual stream. The three-octet chunk supplies abc. The zero-sized chunk ends the chunks, and Example-Count: 3 then supplies the trailer metadata before the final empty line. The content is just abc; the trailer field is not appended to the representation data.^[ref]

The initial Trailer field names metadata the sender anticipates sending. A recipient can use this announcement to prepare its processing, but it does not guarantee that every announced field will actually arrive. The field value itself remains in the later trailer section.^[ref]

Placement is constrained by semantics. A sender must know that a field's definition permits trailer use; fields needed earlier for HTTP message framing, routing or content interpretation generally cannot simply be moved to the end. A recipient must not merge trailer fields into the header section unless their definitions explicitly permit and define the merge.^[ref]

Trailers can also be discarded in transit, so an application cannot assume that arbitrary metadata placed there will reach or be acted upon by every recipient. Keeping HTTP trailer fields distinct from HTTP header fields preserves the distinction between information available before content processing and information learned afterwards.^[ref]