code: public static Message buildAckEpoch(long epoch, Zxid lastZxid) {
      ZabMessage.Zxid zxid = toProtoZxid(lastZxid);
  
      AckEpoch ackEpoch = AckEpoch.newBuilder()
                          .setAcknowledgedEpoch(epoch)
                          .setLastZxid(zxid)
                          .build();
  
      return Message.newBuilder().setType(MessageType.ACK_EPOCH)
                                 .setAckEpoch(ackEpoch)
                                 .build();
    }
docs: Creates a ACK_EPOCH message.
  
  @param epoch the last leader proposal the follower has acknowledged.
  @param lastZxid the last zxid of the follower.
  @return the protobuf message.