Registering Self-generated IPv6 Addresses using DHCPv6

DHCPv6 Address Registration Procedure

DHCPv6 Address Registration Request The DHCPv6 client sends an ADDR-REG-INFORM message to inform that an IPv6 address is in use. The format of the ADDR-REG-INFORM message is described as follows:

DHCPv6 ADDR-REG-INFORM message The client MUST generate a transaction ID as described in and insert this value in the "transaction-id" field. The client MUST include a Client Identifier option in the ADDR-REG-INFORM message. The ADDR-REG-INFORM message MUST NOT contain the Server Identifier option and MUST contain exactly one IA Address option containing the address being registered. The valid-lifetime and preferred-lifetime fields in the option MUST match the current Valid Lifetime and Preferred Lifetime of the address being registered. The ADDR-REG-INFORM message is dedicated for clients to initiate an address registration request toward an address registration server. Consequently, clients MUST NOT put any Option Request Option(s) in the ADDR-REG-INFORM message. Clients MAY include other options, such as the Client FQDN Option . The client sends the DHCPv6 ADDR-REG-INFORM message to the All_DHCP_Relay_Agents_and_Servers multicast address (ff02::1:2). The client MUST send separate messages for each address being registered. Unlike other types of messages, which are sent from the link-local address of the client, the ADDR-REG-INFORM message MUST be sent from the address being registered. This is primarily for "fate sharing" purposes - for example, if the network implements some form of L2 security to prevent a client from spoofing other clients' addresses this prevents an attacker from spoofing ADDR-REG-INFORM messages. On clients with multiple interfaces, the client MUST only send the packet on the network interface that has the address being registered, even if it has multiple interfaces with different addresses. If the same address is configured on multiple interfaces, then the client MUST send ADDR-REG-INFORM each time the address is configured on an interface that did not previously have it, and refresh each registration independently from the others. The client MUST only send the ADDR-REG-INFORM message for valid () addresses of global scope (). This includes ULA addresses, which are defined in to have global scope. The client MUST NOT send the ADDR-REG-INFORM message for addresses configured by DHCPv6. The client SHOULD NOT send the ADDR-REG-INFORM message if it has not received any Router Advertisement message with either M or O flags set to 1. Clients MUST discard any received ADDR-REG-INFORM messages.

Server message processing Servers MUST discard any ADDR-REG-INFORM messages that meet any of the following conditions:

the message does not include a Client Identifier option;
the message includes a Server Identifier option;
the message does not include the IA Address option, or the IP address in the IA Address option does not match the source address of the original ADDR-REG-INFORM message sent by the client. The source address of the original message is the source IP address of the packet if it is not relayed, or the Peer-Address field of the innermost Relay-Forward message if it is relayed.
the message includes an Option Request Option.

If the message is not discarded, the address registration server SHOULD verify that the address being registered is "appropriate to the link" as defined by . If the server believes that the address being registered is not appropriate to the link, it MUST drop the message, and SHOULD log this fact. Otherwise, the server:

SHOULD register or update a binding between the provided Client Identifier and IPv6 address in its database. The lifetime of the binding is equal to the Valid Lifetime of the address reported by the client. If there is already a binding between the registered address and another another client, the server SHOULD log the fact and update the binding.
SHOULD log the address registration information (as is done normally for clients to which it has assigned an address), unless configured not to do so.
SHOULD mark the address as unavailable for use and not include it in future ADVERTISE messages.
MUST send back an ADDR-REG-REPLY message to ensure the client does not retransmit.

Although a client "MUST NOT send the ADDR-REG-INFORM message for addresses configured by DHCPv6", if a server does receive such a message, it should log and discard it. DHCPv6 relay agents and switches that relay address registration messages directly from clients SHOULD include the client's link-layer address in the relayed message using the Client Link-Layer Address option ().

DHCPv6 Address Registration Acknowledgement The server MUST acknowledge receipt of a valid ADDR-REG-INFORM message by sending back an ADDR-REG-REPLY message. The format of the ADDR-REG-REPLY message is described as follows:

DHCPv6 ADDR-REG-REPLY message If the ADDR-REG-INFORM message that the server is replying to was not relayed, then the IPv6 destination address of the message MUST be the address being registered. If the ADDR-REG-INFORM message was relayed, then the server MUST construct the Relay-reply message as specified in section 19.3. The server MUST copy the transaction-id from the ADDR-REG-INFORM message to the transaction-id field of the ADDR-REG-REPLY. The ADDR-REG-REPLY message MUST contain an IA Address option for the address being registered. The option MUST be identical to the one in the ADDR-REG-INFORM message that the server is replying to. Servers MUST ignore any received ADDR-REG-REPLY messages. Clients MUST discard any ADDR-REG-REPLY messages that meet any of the following conditions:

The IPv6 destination address does not match the address being registered.
The IA-Address option does not match the address being registered.
The address being registered is not assigned to the interface receiving the message.
The transaction-id does not match the transaction-id the client used in the corresponding ADDR-REG-INFORM message.

The ADDR-REG-REPLY message only indicates that the ADDR-REG-INFORM message has been received and that the client should not retansmit it. The ADDR-REG-REPLY message MUST NOT be considered as any indication of the address validity and MUST NOT be required for the address to be usable. DHCPv6 relays, or other devices that snoop ADDR-REG-REPLY messages, MUST NOT add or alter any forwarding or security state based on the ADDR-REG-REPLY message.

Retransmission To reduce the effects of packet loss on registration, the client SHOULD retransmit the registration message. Retransmissions SHOULD follow the standard retransmission logic specified by section 15 of with the following default parameters:

IRT 1 sec
MRC 3

The client SHOULD allow these parameters to be configured by the administrator. To comply with section 16.1 of , the client MUST leave the transaction ID unchanged in retransmissions of an ADDR-REG-INFORM message. When the client retranmits the registration message, the lifetimes in the packet MUST be updated so that they match the current lifetimes of the address. If an ADDR-REG-REPLY message is received for the address being registered, the client MUST stop retransmission. However, the client cannot rely on the server acknowledging receipt of the registration message, because the server might not support address registration.

Registration Expiry and Refresh The client MUST refresh addresses as described below. Each refresh is scheduled after AddrRegRefresh seconds, where AddrRegRefresh is min(4 hours, 80% of the address's current Valid Lifetime). Refreshes SHOULD be jittered by +/- 10% to avoid synchronization causing a large number of registration messages to arrive at the same time. Whenever the client creates an address or receives a PIO which changes the Valid Lifetime of an existing address by more than 1%, then:

If no refresh is currently scheduled, it MUST register immediately and schedule a refresh.
If a refresh is currently scheduled, it MUST reschedule the existing refresh if this would result in the refresh being sooner than currently scheduled.

Discussion: this algorithm ensures that refreshes are not sent too frequently, while ensuring that the server never believes that the address has expired when it has not. Specifically:

If the network never changes the lifetime, or stops refreshing the lifetime, then only one refresh ever occurs. The address expires.
Point #1 ensures that any time the network changes the lifetime when no refresh is scheduled, the server will be informed of the correct lifetime. If the network does not change the address's lifetime, then the server already knows the correct lifetime and no refresh needs to be sent.
Point #2 ensures that if the network reduces the lifetime of the address, then the server will be informed of the new lifetime. If the network increases the lifetime of the address, the refresh will be sent at the previously scheduled time, and the server will be informed of the correct lifetime. From this point on, either the address expires (and the server is informed of when this will happen) or an RA increases the lifetime, in which case a refresh will be sent.
The 1% tolerance ensures that the client will not refresh or reschedule refreshes if the Valid Lifetime experiences minor changes due to transmission delays or clock skew between the client and the router(s) sending the Router Advertisement.

Registration refresh packets SHOULD be retransmitted using the same logic as described in the 'Retransmission' section above. The client MUST generate a new transaction ID when refreshing the registration. When the Client-Identifier-to-IPv6-address binding has expired, the server SHOULD remove it and consider the address as available for use. The client MAY choose to notify the server when an address is no longer being used (e.g., if the client is disconnecting from the network, the address lifetime expired, or the address is being removed from the interface). To indicate that the address is not being used anymore the client MUST set the preferred-lifetime and valid-lifetime fields of the IA Address option to zero. If the server receives a message with a valid-lifetime of zero, it SHOULD act as if the address has expired.