How to connect to various NVR without sdk?

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Hello guys, sorry for my bad english. I have three brands NVR: Hikvision, Tiandy and Dahau and about several hundred camers. I write three programs for remote management using three various sdk. Two using c/c++ sdk: dahua and tiandy, win7. And one using web sdk: its hikvision, with Activex, only Internet Explorer. I thought about how can i do it all in one way, without sdk, with XML or Onvif or something methods. I have seen similar program like tecstar, who connecting to various NVR or something devices. Thanks for any help!!!