H2_PRIOR_KNOWLEDGE.protocol -> H2_PRIOR_KNOWLEDGE
        HTTP_2.protocol -> HTTP_2
        SPDY_3.protocol -> SPDY_3
        QUIC.protocol -> QUIC
        else -> {
          // Support HTTP3 draft like h3-29
          if (protocol.startsWith(HTTP_3.protocol)) HTTP_3 else throw IOException("Unexpected protocol: $protocol")
        }
      }
    }
  }