*
     * The following protocols are currently supported:
     *
     * * [http/1.1][rfc_2616]
     * * [h2][rfc_7540]
     * * [h2 with prior knowledge(cleartext only)][rfc_7540_34]
     *
     * **This is an evolving set.** Future releases include support for transitional
     * protocols. The http/1.1 transport will never be dropped.
     *