TCP KEEPALIVE and TIME_WAIT

client.py

import socket
import time

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
	s.connect(('xxxxxxxx', 8080))
	s.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
        s.setsockopt(socket.SOL_TCP, socket.TCP_KEEPIDLE, 35)
	s.send(b'GET / HTTP/1.1\r\nHost: localhost')
	data = s.recv(1024)
	print(data)
	time.sleep(180)

$ cat /proc/sys/net/ipv4/tcp_keepalive_time 
30
$ cat /proc/sys/net/ipv4/tcp_keepalive_intvl 
10

• With SO_KEEPALIVE option on client side, it should send keepalive packets when tcp_keepalive_time elapses after the last packet is sent.
• It would repeatedly send keepalive packet every tcp_keepalive_intvl until it reaches to tcp_keepalive_probes in case no ACK confirmed.
• tcp_keepalive_time can be overwritten after the socket is created. It will be effective for the socket right after the value is changed.
• TCP_KEEPIDLE option overwrites the value taken from tcp_keepalive_time.
• With SO_KEEPALIVE option on server side, no effect for sending keepalive packets at all.
• As an other note, with tcp_tw_reuse enabled (1), TIME_WAIT socket will be reused immediately instead of being blocked during tcp_fin_timeout.
• We can find the time when TIME_WAIT socket is released with netstat -o option as follows.

   -o, --timers
       Include information related to networking timers.

State     Timer
TIME_WAIT     timewait (53.32/0/0)