socket.setMulticastTTL(ttl)
ttl<integer>
设置 IP_MULTICAST_TTL 套接字选项。
一般来说，TTL 表示"生存时间"。
这里特指一个 IP 数据包传输时允许的最大跳步数，尤其是对多播传输。
当 IP 数据包每向前经过一个路由或网关时，TTL 值减 1，若经过某个路由时，TTL 值被减至 0，便不再继续向前传输。
ttl 参数可以是 0 到 255 之间。
在大多数系统上，默认值是 1。
This method throws EBADF if called on an unbound socket.
ttl<integer>
Sets the IP_MULTICAST_TTL socket option. While TTL generally stands for
"Time to Live", in this context it specifies the number of IP hops that a
packet is allowed to travel through, specifically for multicast traffic. Each
router or gateway that forwards a packet decrements the TTL. If the TTL is
decremented to 0 by a router, it will not be forwarded.
The ttl argument may be between 0 and 255. The default on most systems is 1.
This method throws EBADF if called on an unbound socket.