如果没有开启keepalive 默认发送等超时

一个TCP连接上,如果通信双方都不向对方发送数据,那么TCP

连接就不会有任何数据交换,这就是我们今天要讲的TCP keepalive机制的由来

永远记住TCP 不是轮训的协议

客户端发出三次握手以后故障宕机或被踢掉网线，对于 web 服务器而已，下一个数据包将永远无法到来，

但是它一无所知。TCP 不会采用类似于轮询的方式来询问：

小老弟你有什么东西要发给我吗？


TCP 的 half open



node1:/root/test#cat t200.py 
# coding = utf-8
# -*- coding:utf-8 -*-
import socket
import time
ip_port = ('192.168.137.2',8080)
BUFSIZE = 9000
SEND_BUF_SIZE =  2
RECV_BUF_SIZE = 1
s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)#创建套接字
print("Buffer size [Before]: %d" % s.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF))
print("Buffer size [Before]: %d" % s.getsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF))
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.setsockopt( socket.SOL_SOCKET, socket.SO_SNDBUF, SEND_BUF_SIZE)
s.setsockopt( socket.SOL_SOCKET, socket.SO_RCVBUF, RECV_BUF_SIZE)
print 'alter----------------';
print s.getsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF)
print s.getsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF)
#s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 0)
s.bind(ip_port)#绑定地址
s.listen(5)#监听链接
print('server listening8080...')
while True: #无限等待连接
    conn,addr = s.accept() #接受客户端连接
    print(conn)
    print(addr)
    print('接到来自%s的链接'%addr[0])
    while True: #通信循环，无限接受客户端信息
        #print conn.getpeername()
        msg = conn.recv(BUFSIZE) #接受消息的内容
        print 'xxx--------------------xxx'
        print msg
        print 'xxx--------------------xxx'
        if len(msg)==0:break  #如果 不加，已连接的客户端突然断开，recv不再阻塞，发生死循环
        print '111-----------------------111'
        print msg
        print '111-----------------------111'
        #time.sleep(13.6)
        conn.send('response:'+str(len(msg)))
        #conn.send(msg.upper())#服务端发送消息
    conn.close()#关闭链接
s.close()#关闭套接字








node1:/root/test#netstat -na | grep 8080
tcp        0      0 192.168.137.2:8080          0.0.0.0:*                   LISTEN      
tcp        0      0 192.168.137.2:8080          192.168.137.3:52175         ESTABLISHED 
node1:/root/test#


node2:/root#netstat -na | grep 8080
tcp        0      0 192.168.137.3:52175         192.168.137.2:8080          ESTABLISHED 
node2:/root#


此时断开node1的网络:

发现TCP长连接还在:

node2:/root#netstat -na | grep 8080
tcp        0     15 192.168.137.3:52175         192.168.137.2:8080          ESTABLISHED 

node2:/root/test#python connect1.py 
Traceback (most recent call last):
  File "connect1.py", line 10, in <module>
    print s.recv(90)
socket.timeout: timed out
You have mail in /var/spool/mail/root
node2:/root/test#vim connect1.py 
node2:/root/test#time python connect1.py 
Traceback (most recent call last):
  File "connect1.py", line 10, in <module>
    print s.recv(90)
socket.timeout: timed out

real	1m50.085s
user	0m0.021s
sys	0m0.005s


node2:/root/test#cat connect1.py 
# -*- coding: utf-8 -*-
import socket
import time
s = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
s.settimeout(20)
s.connect(("192.168.137.2",8080))
time.sleep(90)
s.send('aaaaaaaaaaaaaaa')

print s.recv(90)

node2:/root#netstat -na | grep 8080
tcp        0     15 192.168.137.3:52175         192.168.137.2:8080          ESTABLISHED 
You have new mail in /var/spool/mail/root
node2:/root#netstat -na | grep 8080
tcp        0     16 192.168.137.3:52175         192.168.137.2:8080          FIN_WAIT1   
node2:/root#
node2:/root#
node2:/root#netstat -na | grep 8080
tcp        0     16 192.168.137.3:52175         192.168.137.2:8080          FIN_WAIT1   
node2:/root#netstat -na | grep 8080


node1:/root#cat 8080.txt 
tcp        0      0 192.168.137.2:8080          0.0.0.0:*                   LISTEN      
tcp        0      0 192.168.137.2:8080          192.168.137.3:52176         ESTABLISHED 
tcp        0      0 192.168.137.2:8080          192.168.137.3:52175         ESTABLISHED