What does rfc 1337 say ?

From RFC 1337.
state removes the hazard of old duplicates for “fast” or “long” connections, in which clock-driven ISN selection is unable to prevent overlap of the old and new sequence spaces. The TIME-WAIT delay allows all old duplicate segments time enough to die in the Internet before the connection is reopened.

echo 1 > /proc/sys/net/ipv4/tcp_rfc1337

 /* FIN arrived, enter true time-wait state. */
                tw->tw_substate   = TCP_TIME_WAIT;
                tcptw->tw_rcv_nxt = TCP_SKB_CB(skb)->end_seq;
                if (tmp_opt.saw_tstamp) {
                        tcptw->tw_ts_recent_stamp = get_seconds();
                        tcptw->tw_ts_recent       = tmp_opt.rcv_tsval;