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一个TCP连接由一个四元组所确定
源IP源端口目的IP目的端口
任意一个元素改变，都代表一个新的连接，以Nginx为例，它的端口是固定使用80。另外服务器的IP也是固定的，那么理论上最多可以建立 2 ^ 32 (ip数）
× 2 ^ 16 (端口数) 个连接。
<>文件描述符限制
实际上，能建立的连接远小于这个数字。我们每打开一个连接都对应一个文件描述符fd（linux系统下一切皆文件）。出于安全角度的考虑，linux系统在多个位置都限制了可打开的文件描述符的数量，包括系统级、用户级、进程级。
* 系统级：当前系统可打开的最大数量，通过fs.file-max参数可修改
* 用户级：指定用户可打开的最大数量，修改/etc/security/limits.conf
* 进程级：单个进程可打开的最大数量，通过fs.nr_open参数可修改
<>内存限制
每一条TCP连接都有file、socket等内核对象，这样一个空TCP连接需要占用3.3KB内存，考虑到实际收发还要配置缓存区，这样内存占用更高了。
通过sysctl命令查看接收缓存区大小
$ sysctl -a | grep rmem net.ipv4.tcp_rmem = 4096 87380 8388608 net.core.
rmem_default = 212992 net.core.rmem_max = 8388608
tcp_rmem中的第一个值是TCP连接所需分配的最少字节数。该值默认是4K，最大为8MB。所以需要接收数据时，需要起码再分配4K内存。
通过sysctl命令查看发送缓存区大小
$ sysctl -a | grep wmem net.ipv4.tcp_wmem = 4096 65536 8388608 net.core.
wmem_default = 212992 net.core.wmem_max = 8388608
tcp_wmem中的第一个值是发送缓存区的最小值，默认也是4K。如果数据很大的话，该缓存区实际分配的也会比默认值大。
