Registering Self-Generated IPv6 Addresses Using DHCPv6

DHCPv6 Address Registration Procedure

DHCPv6 Address Registration Option The Address Registration option (OPTION_ADDR_REG_ENABLE) indicates that the server supports the mechanism described in this document. The format of the Address Registration option is described as follows:

DHCPv6 Address Registration Option

option-code:: OPTION_ADDR_REG_ENABLE (148)
option-len:: 0

If a client has the address registration mechanism enabled, it MUST include this option in all Option Request options that it sends. A server that is configured to support the address registration mechanism MUST include this option in Advertise and Reply messages if the client message it is replying to contained this option in the Option Request option.

DHCPv6 Address Registration Request Message The DHCPv6 client sends an ADDR-REG-INFORM message to inform that an IPv6 address is assigned to the client's interface. The format of the ADDR-REG-INFORM message is described as follows:

DHCPv6 ADDR-REG-INFORM Message

msg-type:: Identifies the DHCPv6 message type; set to ADDR-REG-INFORM (36).
transaction-id:: The transaction ID for this message exchange.
options:: The options carried in this message.

The client MUST generate a transaction ID as described in and insert this value in the transaction-id field. The client MUST include the 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 Layer 2 security to prevent a client from spoofing other clients' MAC 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 the ADDR-REG-INFORM message 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 Unique Local Addresses (ULAs), which are defined in to have global scope. This also includes statically assigned addresses of global scope (such addresses are considered to be valid indefinitely). 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 unless it has received a Router Advertisement (RA) message with either the 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 is the peer-address field of the innermost Relay-forward message if it is relayed; or
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 or within a prefix delegated to the client via DHCPv6 for Prefix Delegation (DHCPv6-PD) (see ). If the address being registered fails this verification, the server MUST drop the message and SHOULD log this fact. If the message passes the verification, the server:

MUST log the address registration information (as is done normally for clients to which it has assigned an address), unless it is configured not to do so. The server SHOULD log the client DHCP Unique Identifier (DUID) and the link-layer address, if available. The server MAY log any other information.
SHOULD register a binding between the provided Client Identifier and IPv6 address in its database, if no binding exists. 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 the same client, the server MUST update its lifetime. If there is already a binding between the registered address and another client, the server SHOULD log the fact and update the binding.
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.

If a client is multihomed (i.e., connected to multiple administrative domains, each operating its own DHCPv6 infrastructure), the requirement to verify that the registered address is appropriate for the link or belongs to a delegated prefix ensures that each DHCPv6 server only registers bindings for addresses from the given administrative domain. As mentioned in , 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 MUST include the client's link-layer address in the relayed message using the Client Link-Layer Address option if they would do so for other DHCPv6 client messages such as Solicit, Request, and Rebind.

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

msg-type:: Identifies the DHCPv6 message type; set to ADDR-REG-REPLY (37).
transaction-id:: The transaction ID for this message exchange.
options:: The options carried in this 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 . 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; or
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 retransmit it. The ADDR-REG-REPLY message MUST NOT be considered to be 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.

Signaling Address Registration Support To avoid undesired multicast traffic, the client MUST NOT register addresses using this mechanism unless the DHCPv6 infrastructure supports address registration. The client can discover this by including the OPTION_ADDR_REG_ENABLE option in the Option Request options that it sends. If the client receives and processes an Advertise or Reply message with the OPTION_ADDR_REG_ENABLE option, it concludes that the DHCPv6 infrastructure supports address registration. When the client detects address registration support, it MUST start the registration process (unless configured not to do so) and MUST immediately register any addresses that are already in use. Once the client starts the registration process, it MUST NOT stop registering addresses until it disconnects from the link, even if subsequent Advertise or Reply messages do not contain the OPTION_ADDR_REG_ENABLE option. The client MUST discover whether the DHCPv6 infrastructure supports address registration every time it connects to a network or when it detects it has moved to a new link, without utilizing any prior knowledge about address registration support on that network or link. This client behavior allows networks to progressively roll out support for the Address Registration option across the DHCPv6 infrastructure without causing clients to frequently stop and restart address registration if some of the network's DHCPv6 servers support it and some do not. A client with multiple interfaces MUST discover address registration support for each interface independently. The client MUST NOT send address registration messages on a given interface unless the client has discovered that the interface is connected to a network that supports address registration.

Retransmission To reduce the effects of packet loss on registration, the client MUST retransmit the registration message. Retransmissions SHOULD follow the standard retransmission logic specified by with the following default parameters for the initial retransmission time (IRT) and maximum retransmission count (MRC):

IRT 1 sec
MRC 3

The client SHOULD allow these parameters to be configured by the administrator. To comply with , the client MUST leave the transaction ID unchanged in retransmissions of an ADDR-REG-INFORM message. When the client retransmits 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.

Registration Expiry and Refresh The client MUST refresh registrations to ensure that the server is always aware of which addresses are still valid. The client SHOULD perform refreshes as described below.

SLAAC Addresses For an address configured using SLAAC, a function AddrRegRefreshInterval(address) is defined as 80% of the address's current Valid Lifetime. When calculating this value, the client applies a multiplier of AddrRegDesyncMultiplier to avoid synchronization with other clients, which could cause a large number of registration messages to reach the server at the same time. AddrRegDesyncMultiplier is a random value uniformly distributed between 0.9 and 1.1 (inclusive) and is chosen by the client when it starts the registration process, to ensure that refreshes for addresses with the same lifetime are coalesced (see below). Whenever the client registers or refreshes an address, it calculates a NextAddrRegRefreshTime for that address as AddrRegRefreshInterval seconds in the future but does not schedule any refreshes. Whenever the network changes the Valid Lifetime of an existing address by more than 1%, for example, by sending a Prefix Information Option (PIO) with a new Valid Lifetime, the client calculates a new AddrRegRefreshInterval. The client schedules a refresh for min(now + AddrRegRefreshInterval, NextAddrRegRefreshTime). If the refresh would be scheduled in the past, then the refresh occurs immediately. Justification: 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, after every registration:

If the network never changes the lifetime of an address (e.g., if no further PIOs are received, or if all PIO lifetimes decrease in step with the passage of time), then no refreshes occur. Refreshes are not necessary, because the address expires at the time the server expects it to expire.
Any time the network changes the lifetime of an address (i.e., changes the time at which the address will expire), the client ensures that a refresh is scheduled, so that server will be informed of the new expiry.
Because AddrRegDesyncMultiplier is at most 1.1, the refresh never occurs later than a point 88% between the time when the address was registered and the time when the address will expire. This allows the client to retransmit the registration for up to 12% of the original interval before it expires. This may not be possible if the network sends a Router Advertisement (RA) very close to the time when the address would have expired. In this case, the client refreshes immediately, which is the best it can do.
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 RA.
AddrRegRefreshCoalesce () allows battery-powered clients to wake up less often. In particular, it allows the client to coalesce refreshes for multiple addresses formed from the same prefix, such as the stable and privacy addresses. Higher values will result in fewer wakeups but may result in more network traffic, because if a refresh is sent early, then the next RA received will cause the client to immediately send a refresh message.
In typical networks, the lifetimes in periodic RAs either contain constant values or values that decrease over time to match another lifetime, such as the lifetime of a prefix delegated to the network. In both these cases, this algorithm will refresh on the order of once per address lifetime, which is similar to the number of refreshes that are necessary using stateful DHCPv6.
Because refreshes occur at least once per address lifetime, the network administrator can control the address refresh frequency by appropriately setting the Valid Lifetime in the PIO.

Statically Assigned Addresses A statically assigned address has an infinite Valid Lifetime that is not affected by RAs. Therefore, whenever the client registers or refreshes a statically assigned address, the next refresh is scheduled for StaticAddrRegRefreshInterval seconds in the future. The default value of StaticAddrRegRefreshInterval is 4 hours. This ensures static addresses are still refreshed periodically, but refreshes for static addresses do not cause excessive multicast traffic. The StaticAddrRegRefreshInterval interval SHOULD be configurable.

Transmitting Refreshes When a refresh is performed, the client MAY refresh all addresses assigned to the interface that are scheduled to be refreshed within the next AddrRegRefreshCoalesce seconds. The value of AddrRegRefreshCoalesce is implementation dependent, and a suggested default is 60 seconds. Registration refresh packets MUST be retransmitted using the same logic as used for initial registrations (see ). The client MUST generate a new transaction ID when refreshing the registration. When a Client-Identifier-to-IPv6-address binding expires, the server MUST 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 in the ADDR-REG-INFORM message to zero. If the server receives a message with a valid-lifetime of zero, it MUST act as if the address has expired.