how to display tcp rto

Calculating the TCP Retransmission Timeout always makes my head explode, yet it can be useful when troubleshooting TCP algorithms like FRTO or TIME_WAIT timeout when TCP Timestamps are in use.

This excellent post by sgros gives a very thorough overview of the RTO algorithms in the kernel and how RTO is actually calculated:

• http://sgros.blogspot.com/2012/02/calculating-tcp-rto.html

However, I don't want to do that, and I certainly don't want to do it for every socket in realtime. Is there an easy way to display the RTO for every socket?

There sure is:

ss -i

The -i option of ss is the flag to "Show internal TCP information", check man ss for more options.

Here's an example of a socket on my system including its RTO:

State   Recv-Q   Send-Q    Local Address:Port      Peer Address:Port
ESTAB   0        0           10.99.0.123:51308   192.168.34.111:http
cubic wscale:6,7 rto:660 rtt:396/38.75 ato:40 cwnd:10 send 272.3Kbps rcv_space:14600
                 ^^RTO^^

ss is one of the "next generation" iproute2 commands to replace the older netstat. Much like ip addr has replaced ifconfig, ip route has replaced route, and ip neigh has replaced arp.