Router Solicitation Message
Source Address An IP address assigned to the sending interface, or the unspecified address if no address is assigned to the sending interface.
Destination Address Typically the all-routers multicast address.
源IPv6地址：在lwip中，要么是链路本地地址，要么是任意地址IP6_ADDR_ANY6，也就是全零地址。
目的IPv6地址：所有路由器多播地址，FF02::2（多播地址FF::/8，第一个02表示scope是链路本地，第二个02是所有路由器）
Router Advertisement Message
Source Address MUST be the link-local address assigned to the interface from which this message is sent.
Destination Address Typically the Source Address of an invoking Router Solicitation or the all-nodes multicast address.
源IPv6地址：必须是链路本地地址
目的IPv6地址：rs包的源地址或者所有节点多播地址，FF02::1（第二个02是所有节点）