set(value) {
      val protocolList = value.toImmutableList()
      require(Protocol.H2_PRIOR_KNOWLEDGE !in protocolList || protocolList.size == 1) {
        "protocols containing h2_prior_knowledge cannot use other protocols: $protocolList"
      }
      require(Protocol.HTTP_1_1 in protocolList || Protocol.H2_PRIOR_KNOWLEDGE in protocolList) {
        "protocols doesn't contain http/1.1: $protocolList"
      }