code: public final OkHttpChannelBuilder connectionSpec(
        com.squareup.okhttp.ConnectionSpec connectionSpec) {
      Preconditions.checkArgument(connectionSpec.isTls(), "plaintext ConnectionSpec is not accepted");
      this.connectionSpec = Utils.convertSpec(connectionSpec);
      return this;
    }
docs: For secure connection, provides a ConnectionSpec to specify Cipher suite and
  TLS versions.
  
  <p>By default a modern, HTTP/2-compatible spec will be used.
  
  <p>This method is only used when building a secure connection. For plaintext
  connection, use {@link #usePlaintext()} instead.
  
  @throws IllegalArgumentException
  If {@code connectionSpec} is not with TLS