DHCPv6 gets a welcome

DHCPv6 became a proposed IETF standard four years ago. It has all the features of DHCPv4, along with some capabilities that make it easier for network managers to renumber networks; that in turn should make it easier to merge networks or switch service providers.

Nominum's Lemon says network managers have a love/hate relationship with DHCP in IPv4 networks. They love it because it gives them control over their networks, but they hate having another network service to manage.

"Network managers like the fact that they can assign IP addresses with DHCP, but it's more important that they can see what's on their network. It gives them information about how many devices are on the network and how much usage of the network there is," Lemon says. "It's almost like they have a finger on the pulse of their network if they use DHCP."

To complicate the situation further, there's also a stateless version of DHCPv6. With this feature, network managers can use stateless address-configuration to let clients get their own IPv6 addresses, but they can follow up with a DHCPv6 information request to gather network configuration information and to configure DNS or other servers.

"In low-overhead, low-management deployments, you might want to use this lightweight configuration of DHCPv6," says Ralph Droms, principal engineer with Cisco and one of the chairs of the IETF's Dynamic Host Configuration working group.

Nonetheless, many IPv6 experts now believe that corporations will stick with stateful address-allocation through DHCPv6. "For corporate users, stateless autoconfiguration is not a good selling point for IPv6," Lemon admits. "I think it's a valid selling point for ISPs. If you're a large ISP, stateless autoconfiguration on the customer network is a really good thing because it means you're going to get fewer phone calls. But I think corporate IT departments are going to want the level of information they get out of DHCP."

What early adopters are doing

This debate matters to corporate network managers, who are going to have to choose whether they want to migrate to IPv6 with autoconfiguration or DHCPv6.

This is especially true for US federal IT departments, which are under a mandate to support IPv6 on their backbone networks by 2008. The federal mandate doesn't specify DHCPv6.

Federal agencies "are still trying to determine what the best route is going to be to transition to IPv6," Cisco's West says. "There is a lot of discussion about DHCPv6.... I absolutely believe that DHCPv6 will happen in the federal market based on the need for information assurance. We think it's going to be critical for transition."

One early IPv6 adopter that's interested in DHCPv6 is Bechtel. The engineering services giant has 9,900 of the 18,000 computers on its network running IPv6. Bechtel has been using the IPv6 autoconfiguration feature because DHCPv6 wasn't available in Windows XP. "One of the things we like about Vista is that it does support DHCPv6,"says Fred Wettling, Bechtel's manager of IT standards and strategies.

The company uses DHCPv4 for its IPv4 clients and autoconfiguration for its IPv6 clients, he says. "Over time, having one place where we control everything will be handy," he adds. "We'll start implementing Windows Server 2008 [which supports DHCPv6], but we're still going to be in a mixed mode for awhile."

Wettling says IPv6 autoconfiguration has "served us very, very well. The performance is good and predictable."

In the meantime, the debate between DHCPv6 and autoconfiguration continues. "There's been an ongoing discussion for years," Cisco's Droms says. "There's still a constituency with a loud voice that says we ought to get rid of DHCP altogether. Some network administrators don't see the need for that kind of control and visibility into their network addressing assignments, and they see running a DHCP service as a significant operational expense."

Droms predicts most enterprises will spend the money to support DHCPv6, however. "In a network environment where a network administrator wants to know what hosts and what devices are connected to the network and what addresses those devices are using on the network, the network administrator will want to use DHCPv6," he says. "It's very useful for debugging purposes, it's useful for accounting and useful for controlling access to the network."