[Mp4-tech][H.264] questions about picture order count

[minmin]

Hi Experts!
  I am not very clear about the relationship among PicOrderCntMsb,
pic_order_cnt_lsb and PicOrderCnt described in ISO/IEC 14496-10 
clause 8.2.1.1. 
And what is poc used for?
  If I want to extract only I frames(include IDR and non-IDR) from the
bitstream, and form these frames into a new bitstream,is it necesary for
each frame to reset some fields(such as "frame_num","pic_order_cnt_lsb"
etc.) associated with POC?
And is there any other fields needed to reset?
And how to reset?
Hope for your reply!
Thanks a lot!
-- 
mealimin <minm necas.nec.com.cn>