Appendix B. HTTP Header Fields
General Header Fields
Trailer General Header Field
The Trailer field indicates that a number of header fields are present in the trailer of a chunked transfer-coding encoded message. This is partly because HTTP/1.1 messages must include a Trailer field when using chunked transfer coding. This tells the recipient which fields to find in the trailer.
If there is no Trailer field in the header, the trailer should not include any header fields other than the Transfer-Encoding, Content-Length, and Trailer ...