dhcpv6 reply消息中，IANA有T1和T2两个定时器，IAPD也有。
T1是一个定时器，通常是preferred lifetime的一半，当T1超时时，client发起renew消息。
T2是一个定时器，通常是preferred lifetime的0.8，当T1发起的renew没有收到回复时，启动T2定时器，T2定时器超时，client发起rebind消息。
参考
https://techhub.hpe.com/eginfolib/networking/docs/switches/5710/5200-4994_l3-ip-svcs_cg/content/517701955.htm
Address/prefix lease renewal
An IPv6 address/prefix assigned by a DHCPv6 server has a valid lifetime. After the valid lifetime expires, the DHCPv6 client cannot use the IPv6 address/prefix. To use the IPv6 address/prefix, the DHCPv6 client must renew the lease time.
Figure 70: Using the Renew message for address/prefix lease renewal
As shown in Figure 70, at T1, the DHCPv6 client sends a Renew message to the DHCPv6 server. The recommended value of T1 is half the preferred lifetime. The DHCPv6 server responds with a Reply message, informing the client whether the lease is renewed.
Figure 71: Using the Rebind message for address/prefix lease renewal
As shown in Figure 71:
If the DHCPv6 client does not receive a response from the DHCPv6 server after sending a Renew message at T1, it multicasts a Rebind message to all DHCPv6 servers at T2. Typically, the value of T2 is 0.8 times the preferred lifetime.
The DHCPv6 server responds with a Reply message, informing the client whether the lease is renewed.
If the DHCPv6 client does not receive a response from any DHCPv6 server before the valid lifetime expires, the client stops using the address/prefix.