1. tcp_syn_retries - INTEGER
    2.         Number of times initial SYNs for an active TCP connection attempt
    3.         will be retransmitted. Should not be higher than 127. Default value
    4.         is 6, which corresponds to 63seconds till the last retransmission
    5.         with the current initial RTO of 1second. With this the final timeout
    6.         for an active TCP connection attempt will happen after 127seconds
    8. # 设置为 2？
    1. tcp_keepalive_time - INTEGER
    2.         How often TCP sends out keepalive messages when keepalive is enabled.
    3.         Default: 2hours.
    5. # 300 s
    1. tcp_orphan_retries - INTEGER
    2.         This value influences the timeout of a locally closed TCP connection,
    3.         when RTO retransmissions remain unacknowledged.
    4.         See tcp_retries2 for more details.
    6.         The default value is 8.
    7.         If your machine is a loaded WEB server,
    8.         you should think about lowering this value, such sockets
    9.         may consume significant resources. Cf. tcp_max_orphans.
    11. # 普通 web 服务器设置为 1
    1. tcp_max_syn_backlog - INTEGER
    2.         Maximal number of remembered connection requests, which have not
    3.         received an acknowledgment from connecting client.
    4.         The minimal value is 128 for low memory machines, and it will
    5.         increase in proportion to the memory of machine.
    6.         If server suffers from overload, try increasing this number.
    8. # 默认值为1024，加大队列长度为8192
    1. tcp_synack_retries - INTEGER
    2.         Number of times SYNACKs for a passive TCP connection attempt will
    3.         be retransmitted. Should not be higher than 255. Default value
    4.         is 5, which corresponds to 31seconds till the last retransmission
    5.         with the current initial RTO of 1second. With this the final timeout
    6.         for a passive TCP connection will happen after 63seconds.
    1. tcp_tw_recycle - BOOLEAN
    2.         Enable fast recycling TIME-WAIT sockets. Default value is 0.
    3.         It should not be changed without advice/request of technical
    4.         experts.
    1. tcp_tw_reuse - BOOLEAN
    2.         Allow to reuse TIME-WAIT sockets for new connections when it is
    3.         safe from protocol viewpoint. Default value is 0.
    4.         It should not be changed without advice/request of technical
    5.         experts.
    1. tcp_fin_timeout - INTEGER
    2.         The length of time an orphaned (no longer referenced by any
    3.         application) connection will remain in the FIN_WAIT_2 state
    4.         before it is aborted at the local end.  While a perfectly
    5.         valid "receive only" state for an un-orphaned connection, an
    6.         orphaned connection in FIN_WAIT_2 state could otherwise wait
    7.         forever for the remote to close its end of the connection.
    8.         Cf. tcp_max_orphans
    9.         Default: 60 seconds
    1. tcp_keepalive_probes - INTEGER
    2.         How many keepalive probes TCP sends out, until it decides that the
    3.         connection is broken. Default value: 9.