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

[Ruslan Ermilov]

On Fri, Dec 16, 2016 at 03:09:14PM +0800, yanqun sun wrote:
> 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?

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