The HTTPTrailerrequest andresponse header allows the sender to include additional fields at the end of chunked messages in order to supply metadata that might be dynamically generated while the message body is sent.

Syntax

Trailer: header-names

Directives

header-names

HTTP header fields which will be present in the trailer part of chunked messages.The following header names aredisallowed:

• Content-Encoding,Content-Type,Content-Range, andTrailer
• Authentication headers (e.g.,Authorization orSet-Cookie)
• Message framing headers (e.g.,Transfer-Encoding andContent-Length)
• Routing headers (e.g.,Host)
• Request modifiers (e.g., controls and conditionals, likeCache-Control,Max-Forwards, orTE)

Examples

Server-Timing as HTTP trailer

Some browsers support showing server timing data in developer tools when theServer-Timing header is sent as a trailer.In the following response, theTrailer header is used to indicate that aServer-Timing header will follow the response body.A metriccustom-metric with a duration of123.4 milliseconds is sent:

HTTP/1.1 200 OKTransfer-Encoding: chunkedTrailer: Server-Timing--- response body ---Server-Timing: custom-metric;dur=123.4

Specifications

Browser compatibility

See also

• Server-Timing
• Transfer-Encoding
• TE
• Chunked transfer encoding