mation changes on the network. In order to get hosts with stateless clients to refresh the information, a DHCPv6 server may include the Information Refresh Time option in the INFORMATION-REPLY message, if the client had indicated support for the option in the request message [84]. The Information Refresh Time option provides a maximum time that a client should wait before refreshing the configuration information, and hence setting the time with suitable value makes it possible to update configuration information on clients. DHCPv6 Options At the time of writing, more than 70 DHCPv6 options have been defined. In this section we have chosen to describe only the options that are discussed in this book or well- known to be useful in 3GPP access networks. Obviously, as only a subset of options is listed here, deployments, even some 3GPP networks, will make use of other options as well. A full and up-to-date option code list is available on the IANA web page: http://www.iana.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xm DNS Recursive Name Server option: 'DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)' describes options that hosts can use to obtain Recursive DNS Server (RDNSS) addresses and domain search lists [85]. RDNSS Selection option: A new DHCPv6 option for RDNSS selection is specified in RFC 6731 [86]. The RDNSS Selection option configures nodes with information which domains and networks a specific RDNSS has special knowledge of. The client can then use the information when selecting to which RDNSS a DNS query should be sent. Rapid Commit option: The Rapid Commit option can be used to optimize DHCPv6 signaling into two messages: DHCPv6 SOLICIT and reply a for it. The optimizations save an extra RTT that would otherwise be spent on signaling. The client sending a Rapid Commit option commits to accept the information sent by a DHCPv6 server that also accepts the commitment with a single message exchange [54]. Introduction to IPv6 Session Initiation Protocol (SIP) Serve