http://blog.163.com/modingfa_002/blog/static/1109254662013111510358109/
http://baike.baidu.com/link?url=Z6FpaB_q4GCj5CkKNUNdRjNK2GgwTxNUfxeHwg1EAM5uPyJrn_We2QMXGWx5S1TP-bBNsJq61Wlh_pVnoTlSqq
https://www.ibm.com/developerworks/cn/linux/l-cn-socketftp/