DHCPv6 Prefix Delegation for Network Mobility (NEMO)
Extracted elements (20)
DHCPv6 client-server unicast (via Server Unicast option) is not recommended for this scenario because it requires a prior Solicit/Advertise exchange, which itself requires the relay agent to be present—providing little gain over the relay-based approach already specified.
When the mobile router is away from home, it assumes the role of a DHCPv6 relay agent co-located with the requesting router function, in addition to its normal requesting router role. This dual-role design is necessary because DHCPv6 messages must be unicast to the home agent rather than multicast on the visited link.
The DHCPv6 messages exchanged between the mobile router and the home agent MAY also be used for other DHCPv6 functions such as global address assignment and DNS configuration, alongside DHCPv6PD. The home agent MAY also act as a DHCPv6 relay agent for mobile nodes while acting as delegating router for mobile routers.
The Rapid Commit option can be combined with DHCPv6PD in a mobile network to provide prefix delegation with only a two-message exchange between the mobile router and the delegating router, minimizing signaling overhead.
A DHCPv6 relay agent function MUST be used at the mobile router when it is away from home. This relay agent is co-located with the DHCPv6 client function, and the client MUST hand any outbound DHCPv6 messages to the co-located relay agent.
DHCPv6 messages from the mobile router to the home agent MUST use the Home Address destination option (unicast from home address to home agent's global unicast address). DHCPv6 replies from the home agent to the mobile router MUST be sent using Routing Header type 2.
If the mobile router does not have any active delegated prefixes with unexpired leases, the mobile router MUST initiate a DHCPv6 message exchange with a DHCPv6 Solicit message. If the mobile router has one or more active delegated prefixes, it MUST initiate exchange with a DHCPv6 Rebind message.
It is RECOMMENDED that the mobile router use sequential probing of home agents for DHCPv6PD service rather than probing multiple home agents simultaneously.
The home agent, upon reception of the implicit Binding Update from the mobile router, MUST select the prefixes to be delegated via DHCPv6PD. Once DHCPv6 signaling is completed, the home agent MUST add an entry in its binding cache including the delegated prefixes.
The mobile router MUST assign its home address from the prefix on its Home Link, since it cannot derive its home address from its mobile network prefixes (which are not yet known before registering with the home agent).
The mobile router MUST register at the home agent (by sending a Binding Update) before initiating a DHCPv6 message exchange for prefix delegation. The mobile router MUST use implicit BU signaling, since the mobile router may not have yet requested any prefixes.
The relay agent MUST be configured with the address of the DHCPv6 server. The mobile router MUST configure the DHCPv6 relay agent to forward DHCPv6 messages to the home agent, which is assumed to host the DHCPv6 server.
Upon receipt of a Relay-reply message, the relay agent function MUST extract the message for the client from the Relay Message option and hand the message to the DHCPv6 client in the mobile router.
When querying a home agent to determine if it provides DHCPv6PD service, it is RECOMMENDED that MRD and MRC be set to non-zero values so the mobile router discontinues sending Solicit messages after 6 attempts, concluding the home agent will not provide DHCPv6PD service.
When selecting a home agent, a mobile router that requires DHCPv6PD service MUST identify a home agent that will provide the service, determined by initiating a DHCPv6 Solicit message exchange and checking for prefix delegation in the response.
When the mobile router is on its home link, the home agent MUST use the home link to exchange DHCPv6PD messages with the mobile router, and the DHCPv6 co-located relay function MUST be disabled.
When sending a Relay-forward message, the relay agent sets msg-type to RELAY-FORW, hop-count to 1, link-address to the home address of the mobile router, peer-address to the home address of the mobile router, and options MUST include a Relay Message option.
The home agent MUST only add prefixes to a mobile router's Binding Cache Entry if the mobile router has a valid DHCPv6 Prefix Delegation lease for those prefixes, preventing a mobile router from claiming mobile network prefixes belonging to another mobile router.
While the mobile router is away from home, the IPsec security mechanism mandated by Mobile IPv6 (RFC3776) MUST be used to secure the DHCPv6 signaling. Example SPD and SAD entries using ESP in transport mode are provided for both mobile router and home agent.
The mobile router transitions between two operational states: (1) at-home, where it acts only as DHCPv6 requesting router communicating directly over the home link with relay disabled; and (2) away-from-home, where it additionally acts as a co-located DHCPv6 relay agent forwarding messages via the Mobile IPv6 tunnel to the home agent.