TE

https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

    The TE request-header field indicates what extension transfer-codings it is willing to accept in the response and whether or not it is willing to accept trailer fields in a chunked transfer-coding. Its value may consist of the keyword "trailers" and/or a comma-separated list of extension transfer-coding names with optional accept parameters (as described in section 3.6).

       TE        = "TE" ":" #( t-codings )
       t-codings = "trailers" | ( transfer-extension [ accept-params ] )

The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer-coding, as defined in section 3.6.1. This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding.

Examples of its use are:

       TE: deflate
       TE:
       TE: trailers, deflate;q=0.5

The TE header field only applies to the immediate connection. Therefore, the keyword MUST be supplied within a Connection header field (section 14.10) whenever TE is present in an HTTP/1.1 message.

A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules:

      1. The "chunked" transfer-coding is always acceptable. If the
         keyword "trailers" is listed, the client indicates that it is
         willing to accept trailer fields in the chunked response on
         behalf of itself and any downstream clients. The implication is
         that, if given, the client is stating that either all
         downstream clients are willing to accept trailer fields in the
         forwarded response, or that it will attempt to buffer the
         response on behalf of downstream recipients.

         Note: HTTP/1.1 does not define any means to limit the size of a
         chunked response such that a client can be assured of buffering
         the entire response.

      2. If the transfer-coding being tested is one of the transfer-
         codings listed in the TE field, then it is acceptable unless it
         is accompanied by a qvalue of 0. (As defined in section 3.9, a
         qvalue of 0 means "not acceptable.")

      3. If multiple transfer-codings are acceptable, then the
         acceptable transfer-coding with the highest non-zero qvalue is
         preferred.  The "chunked" transfer-coding always has a qvalue
         of 1.

If the TE field-value is empty or if no TE field is present, the only transfer-coding is "chunked". A message with no transfer-coding is always acceptable.