Allows the sender to include additional fields at the end of chunked responses.

Fastly reads this header from responses and writes it into responses. It is defined by an external standard.

PROTECTED:  This header cannot be modified in VCL.

When a message includes a message body encoded with the chunked transfer coding and the sender desires to send metadata in the form of trailer fields at the end of the message, the sender SHOULD generate a Trailer header field before the message body to indicate which fields will be present in the trailers. This allows the recipient to prepare for receipt of that metadata before it starts processing the body, which is useful if the message is being streamed and the recipient wishes to confirm an integrity check on the fly.