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ss -o shows TCP timer in the following format:

timer:(<timer_name>,<expire_time>,<retrans>)

What do <expire_time> and <retrans> mean?

I found <expire_time> counts down to zero and then restart counting from some number again. Its starting value differs from TCP socket to TCP socket.

<retrans> seems always zero for all the TCP sockets.

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expire_time is the time left until the timer expires. The TCP stack in the Linux kernel supports a number of timers, and they all have an expiration time.

retrans is the number of retransmissions which have occurred. TCP implementations retransmit packets they believe have been lost; they counts these retransmissions so that they will know when to give up. You shouldn’t see this too often; one way to force it is to try to open a connection on a port which isn’t rejected immediately, e.g. (based on an example in one of your previous questions):

curl http://www.google.com:9000

If you run that, you’ll see curl sitting there for a while, and ss -o will show a SYN-SENT entry with an increasing retransmission count. You’ll also see the back-off applied in such circumstances: the initial expiration time will increase every time the packet is retransmitted.

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  • Is the expire_time reset when something is transmitted (whether retransmitted or not)?
    – Tim
    Commented Feb 7, 2019 at 15:58
  • Yes, it resets when a packet is transmitted.
    – Stephen Kitt
    Commented Feb 7, 2019 at 16:48

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