Netfilter nf_tables connlimit module

modulename: nft_connlimit.ko

configname: CONFIG_NFT_CONNLIMIT

Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>Network packet filtering framework (Netfilter)
└─>Core Netfilter Configuration
└─>Netfilter nf_tables connlimit module
In linux kernel since version 4.2 (release Date: 2015-08-30)  
This option adds the "connlimit" expression that you can use to
ratelimit rule matchings per connections.

source code:
net/netfilter/nft_connlimit.c