DHCP versus DHCPv6 Release behaviour

[Ross Chandler]

Hi All,

I’ve an issue with different DHCP client behaviour for IPv4 versus IPv6. 

When my dual-stack (residential broadband) CPE router's WAN interface settings are changed in any way its DHCP client does not send a DHCP Release but its DHCPv6 client always sends a Release. Consequently the DHCP lease remains at the server but the DHCPv6 lease is deleted. 

The end user finds that their dynamic IPv4 address changes less than their IPv6 delegated prefix. As long as the CPE remains connected and renewing they get the same IPv4 address. They can mess with the WAN settings or reboot the CPE and the DHCP client will still get the same IPv4 address. For DHCPv6 the server appears to increment to the next prefix rather than immediately give the previously Released prefix back out again even when the next Solicit it receives is from the same client.

The sequence for DHCP after saving changes is

Client DHCP does not send Release
Client DHCP Discover
Server DHCP Offer
Client DHCP Request
Server DHCP ACK

and for DHCPv6 it is

Client DHCPv6 Release
Server DHCPv6 Reply
Client DHCPv6 Solicit
Server DHCPv6 Reply

The DHCPv6 client does not send a Release if the CPE is given a soft or hard reboot. When the CPE comes back up it sends a DHCPv6 Solicit and gets the same delegated prefix.

The DHCPv6 server has the capability to ignore Releases and remember a lease beyond the lease's preferred lifetime which could be used to make the  DHCP & DHCPv6 behaviour the same for the end user.

The choices seem to be

1. Do nothing and accept dynamic IPv6 delegated prefixes changing more often than the IPv4 address
2. Make the DHCP client send Releases when DHCPv6 sends them
3. Make the DHCPv6 client not send Releases under the same conditions DHCP doesn’t send them
4. Remember the DHCPv6 lease for some duration after a Release comes in so that it may be given out again

I’m leaning towards option 4 and with that the lease's stickiness could also be further increased to make it a pseudo static assignment. Although on the other hand some users have got used to dynamic IPv4 addresses and actually want their addresses to change. I think option 2 makes IPv4 behaviour worse so probably isn’t acceptable. Option 3 seems worse too as it is better to inform the server and let it decide what to do with the lease. 

Regards,
Ross