DHCPv6 Prefix Delegation in Windows Vista
In a previous post, I had discussed the DHCPv6 client behaviour in Windows Vista. A few customers who're evaluating IPv6 in Windows Vista responded asking about the support for DHCPv6 Prefix Delegation and/or asked why the Vista DHCPv6 client doesn't request an IPv6 subnet prefix by default. In most cases, this question came from Japan where users tried to test IPv6 broadband network connections with Windows Vista and their ISP was allocating a /64 IPv6 prefix to each customer. I'll address this query here.
A host running Windows Vista is by default a network end-point. Network traffic is initiated and terminated on this machine, but this machine does not actively participating in the routing of network traffic for other hosts. DHCPv6 Prefix Delegation is intended for network elements that behave as a router. The terminology used in RFC 3633 ["IPv6 Prefix Options for DHCPv6"] indicates this: the client requesting an IPv6 prefix is called the "requesting router", and the server which leases the IPv6 prefix is termed the "delegating router". Since the host running Windows Vista doesn't behave as a router by default, the DHCPv6 client doesn't request an IPv6 prefix by default from the server.
Windows Vista can be used to route network traffic when specific features are used. DHCPv6 Prefix Delegation support in Vista is hence initiated only when these features are enabled. An example of such features is Internet Connection Sharing (ICS). When ICS is enabled on a network interface, Windows Vista can route traffic between other hosts on the same link and the network to which that interface is attached. So when ICS is enabled in Windows Vista, it will act as a requesting router and send a DHCPv6 Solicit message on that network interface in order to obtain an IPv6 subnet prefix for its link. The IPv6 subnet prefix obtained through this means is then included in the Router Advertisements sent out to the other machines on that link, in order to allow them to configure an IPv6 address based on the prefix obtained.
To support 3rd party applications from other ISVs that want to enable similar routing scenarios on Windows Vista and in Windows Server 2008, we provide DHCPv6 APIs for Prefix Delegation. See the DHCP client API documentation for Windows Vista under http://msdn2.microsoft.com/en-us/library/aa363344.aspx for further details - specifically, the APIs Dhcpv6RequestPrefix, Dhcpv6RenewPrefix and Dhcpv6ReleasePrefix.
I hope you find this information useful for understanding the DHCPv6 behaviour and for developing with IPv6 for Windows Vista and Windows Server 2008!!
Santosh Chandwani
Windows Enterprise Networking
[This posting is provided "AS IS" with no warranties, and confers no rights.]
