Extract TCP round trip time (RTT) estimations on linux

Question

I have apache server running on Ubuntu. Client connects and downloads an image. I need to extract RTT estimations for the underlying TCP connection. Is there a way to do this? Maybe something like running my tcp stack in debug mode to have it log this info somewhere?

Note that I don't want to run tcpdump and extract RTTs from the recorded trace! I need the TCP stack's RTT estimations (apparently this is part of the info you can get with TCP_INFO socket option). Basically need something like tcpprob (kprobe) to insert a hook and record the estimated RTT of the TCP connection on every incoming packet (or on every change).

UPDATE:

I found a solution. rtt, congestion window and more can be logged using tcpprobe. I posted an answer below.

Answer 1

This can be done using tcpprobe, which is a module that inserts a hook into the tcp_recv processing path using kprobe records the state of a TCP connection in response to incoming packets.

Let's say you want to probe tcp connection on port 443, you need to do the following:

sudo modprobe tcp_probe port=443 full=1
sudo chmod 444 /proc/net/tcpprobe
cat /proc/net/tcpprobe > /tmp/output.out &
pid=$!

full=1: log on every ack packet received

full=0: log on only condo changes (if you use this your output might be empty)

Now pid is the process which is logging the probe. To stop, simply kill this process:

kill $pid

The format of output.out (according to the source at line 198):

[time][src][dst][length][snd_nxt][snd_una][snd_cwnd][ssthresh][snd_wnd][srtt][rcv_wnd]

Answer 2

This can be done without the need for any additional kernel modules using the ss command (part of the iproute package), which can provide detailed info on open sockets. It won't show it for every packet but most of this info is calculated over a number of packets. E.g. To list the currently open TCP (t option) sockets and associated internal TCP info (i) information - including congestion control algorithm, rtt, cwnd etc:

ss -ti

Here's some example output:

State       Recv-Q Send-Q        Local Address:Port        Peer Address:Port
ESTAB       0      0             192.168.56.102:ssh        192.168.56.1:46327
  cubic wscale:6,7 rto:201 rtt:0.242/0.055 ato:40 mss:1448 rcvmss:1392
advmss:1448 cwnd:10 bytes_acked:33169 bytes_received:6069 segs_out:134
segs_in:214 send 478.7Mbps lastsnd:5 lastrcv:6 lastack:5 
pacing_rate 955.4Mbps rcv_rtt:3 rcv_space:28960