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








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