Hi, My job is developing program, now i need do some Bitcoin Core research, such as P2P, BlockChain, POW and so on.
I have clone Bitcoin Core code from GitHub to learn, but i come across difficulties, such as:
when one node(N1) creates peer-two-peer relationship with another node(N2):
    1)N1 send VERSION message(msg) to N2;
    2)N2 reply VERACK msg and VERSION msg
    3)N1 reply VERACK msg
when N1 send INV msg to N2(question: N1 knows what hash value send?), what N2 need do before reply GETDATA or GETHEADERS msg?
I refer to Bitcoin wiki and official site, but cannot find some useful materials, could you give me some suggestions?

Thank you.