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TCP Recycle

TCP connection tear down is one part which is least documented in the internet. In an ideal web server client architecture, once the server sends FIN, client will send FIN+ACK, server sends an ACK and enters time wait state. Look at this diagram


Now lets consider our problem. We have deployed new servers which receive huge traffic with less processing time(100 ms). Now all these connections enter into time wait state to tear down connection. Time Wait state is useful for two reasons
  1. If the last ack sent by the server is missed half way, client will retransmit Fin+Ack. If the socket was reused by some other connection, there would be confusions in the network
  2. If  a router malfunctioned and any of the data from client to server was lost. Client would have retransmitted it after RTO (retransmission timeout) , There is a high probability that router might reinject the old packet if its segment lifetime is less than MSL. Now this packet could also cause conflict if the same tcp connection tuple is used on server side
In our case the incoming connection rate is constant(almost) say lambda. These packets reach timewait after 100ms. The timewait is constant in linux 60s. In kernel code it is defined as a macro

#define TIME_WAIT_LENGTH 60*HZ

After receiving live traffic for some 20 minutes, number of connections in timewait shoots upto 1.4 million. The bottle neck is not in the application, its in the timewait state. But time wait is needed for gracefully closing TCP.

There are lot of links in internet which says changing tcp_fin_timeout in /etc/sysctl.conf will reduce time wait length. This is seriously wrong, tcp_fin_timeout reduces FIN_WAIT 2 state time(Refer diagram above).

Now there came a new kernel parameter in TCP called tcp_tw_recycle in /etc/sysctl.conf. In this case the server waits only for retransmission timeout (around 1-3s) and reuses the socket for new connection.
The case 1 of timewait, failed ACK will be handled in this scenario as it waits for RTO. But the case 2, where a packet stays alive for a MSL(Maximum Segment Length), is not handled gracefully. To avoid conflicts TCP stores the recent timestamp from the client and accepts packets only in increasing order of time stamps from clients. This seems well and good and time spent on time_wait state is reduced. We implemented this on our systems and connections in time wait drastically reduced. Then why didnt TCP use this by default? Lets consider a scenario, 

Some 4 systems are trying to connect to a server which uses recycle parameter. The clients are sharing a wifi. So they are behind a NAT and  they use same public ip. The clocks in 4 systems need not be in sync. So if the system which has a higher timestamp (say t1) has succesfully established a connection with server and closed it. Other clients cant establish connection with server until their timestamp is greater than t1.  Abnormal behaviour will be seen for devices behind NAT and firewall. So this parameter is not encouraged in most of the cases.



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