DHCPv4 and DHCPv6 Options for Access Network Discovery and Selection Function (ANDSF) Discovery
Extracted elements (15)
ANDSF discovery via DHCP is introduced because 3GPP access-network level solutions are insufficient for mobile nodes to discover and select non-3GPP networks; DHCP provides a lightweight, existing infrastructure mechanism for distributing ANDSF server addresses alongside DNS-based discovery.
ANDSF is defined in 3GPP TS 23.402 and TS 24.302 and operates over the S14 reference point; the DHCP options defined here serve as one discovery mechanism alongside DNS-based discovery, and implementations must be consistent with those 3GPP specifications.
ANDSF servers MUST be listed in order of preference within the DHCPv6 ANDSF IPv6 Address Option.
The DHCP client MAY initiate a new DHCP exchange or piggyback on other DHCP message exchanges when requesting ANDSF server addresses.
The DHCP client SHOULD process ANDSF servers in decreasing order of preference as listed in the DHCPv6 option.
The mobile node MAY request the IP address of an ANDSF server during initial association with a network, when policy/access-network information is required, when network information is outdated, or when the MN has no ANDSF information.
The requesting and sending of the proposed DHCPv4 ANDSF options MUST follow the rules for DHCP options in RFC 2131.
The requesting and sending of the proposed DHCPv6 ANDSF options MUST follow the rules for DHCP options in RFC 3315.
To discover an ANDSF server address via DHCPv6, the mobile node MUST include an ANDSF IPv6 Address Option in the Option Request Option (ORO) in the respective DHCP messages as defined in RFC 3315.
To request an ANDSF server address via DHCPv4, the mobile node MUST include an ANDSF IPv4 Address Option in the Parameter Request List (PRL) in the respective DHCP messages as defined in RFC 2131 and RFC 2132.
This document registers two new DHCP option codes: OPTION-IPv4_Address-ANDSF (142) for DHCPv4 and OPTION-IPv6_Address-ANDSF (143) for DHCPv6, to enable mobile nodes to discover ANDSF servers.
If an adversary modifies a DHCP server response or inserts its own response, a mobile node could be directed to contact a rogue ANDSF server; a modified response could also be used to mount an amplification attack. DHCP authentication (RFC 3118 / RFC 3315) MAY be used to mitigate these attacks.
In deployments where DHCP authentication is not available, 3GPP-specific lower-layer security services (3GPP TS 33.402) can protect DHCP messages, and the 3GPP ANDSF framework provides additional mechanisms to protect message exchanges between ANDSF client and server at the higher layer.
The ANDSF IPv4 Address Option for DHCPv4 (option code 142) carries one or more IPv4 addresses of ANDSF servers. The Length field is set to 4N bytes where N is the number of IPv4 addresses; it excludes the Option Code and Length fields themselves.
The ANDSF IPv6 Address Option for DHCPv6 (option code 143) carries one or more IPv6 addresses of ANDSF servers. The Length field is set to 16N bytes where N is the number of IPv6 addresses; all values are in network byte order.