I have no idea how to handle an in-game packet when the resend field is something else than 0.

The seq1 and seq2 of the resend packet determine which one to resend and thats all i know.
Sometime the packet to resend doesnt even exist. And the resend field happen to be more than 1, with 1 byte in the datas .. wtf ??

and another question. that resend thing only happen in lag/packet doesnt reach right ??

Have you read this? (mainly, status description in the 1st post)