问题
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.
回答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]
回答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 a 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
来源：https://stackoverflow.com/questions/34907684/extract-tcp-round-trip-time-rtt-estimations-on-linux