What's the meaning of Nginx variables of "$tcpinfo_rtt, $tcpinfo_rttvar, $tcpinfo_snd_cwnd, $tcpinfo_rcv_space"

[yanqun sun]

Hi, all:
  I want to get the network latency between the users and my Nginx servers
on tcp layer. I searched about this and found several Nginx variables
bellow:

$tcpinfo_rtt, $tcpinfo_rttvar, $tcpinfo_snd_cwnd, $tcpinfo_rcv_space
information about the client TCP connection; available on systems that
support the TCP_INFO socket option


  So does anybody can make an explanation for those variables?
  I think *$tcpinfo_rtt* probably means the round trip time from the client
to the server on tcp layer.
  and what about the rttvar, snd_cwnd and rcv_space?

  Thanks!
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