0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               3               |    hardware type (16 bits)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .             link-layer address (variable length)              .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

Figure 6: DUID-LL format

The choice of network interface can be completely arbitrary, as long as that interface provides a unique link-layer address and is permanently attached to the device on which the DUID-LL is being generated. The same DUID-LL SHOULD be used in configuring all network interfaces connected to the device, regardless of which interface's link-layer address was used to generate the DUID.

DUID-LL is recommended for devices that have a permanently-connected network interface with a link-layer address, and do not have nonvolatile, writable stable storage. DUID-LL MUST NOT be used by DHCP clients or servers that cannot tell whether or not a network interface is permanently attached to the device on which the DHCP client is running.

10. Identity Association

An "identity-association" (IA) is a construct through which a server and a client can identify, group, and manage a set of related IPv6 addresses. Each IA consists of an IAID and associated configuration information.

A client must associate at least one distinct IA with each of its network interfaces for which it is to request the assignment of IPv6 addresses from a DHCP server. The client uses the IAs assigned to an interface to obtain configuration information from a server for that interface. Each IA must be associated with exactly one interface.

The IAID uniquely identifies the IA and must be chosen to be unique among the IAIDs on the client. The IAID is chosen by the client. For any given use of an IA by the client, the IAID for that IA MUST be consistent across restarts of the DHCP client. The client may maintain consistency either by storing the IAID in non-volatile storage or by using an algorithm that will consistently produce the same IAID as long as the configuration of the client has not changed. There may be no way for a client to maintain consistency of the IAIDs if it does not have non-volatile storage and the client's hardware configuration changes.

The configuration information in an IA consists of one or more IPv6 addresses along with the times T1 and T2 for the IA. See section 22.4 for the representation of an IA in a DHCP message.

Each address in an IA has a preferred lifetime and a valid lifetime, as defined in [RFC2462]. The lifetimes are transmitted from the DHCP server to the client in the IA option. The lifetimes apply to the use of IPv6 addresses, as described in section 5.5.4 of RFC 2462.

11. Selecting Addresses for Assignment to an IA

A server selects addresses to be assigned to an IA according to the address assignment policies determined by the server administrator and the specific information the server determines about the client from some combination of the following sources:

-

The link to which the client is attached. The server determines the link as follows:

*

If the server receives the message directly from the client and the source address in the IP datagram in which the message was received is a link-local address, then the client is on the same link to which the interface over which the message was received is attached.

*

If the server receives the message from a forwarding relay agent, then the client is on the same link as the one to which the interface, identified by the link-address field in the message from the relay agent, is attached.

*

If the server receives the message directly from the client and the source address in the IP datagram in which the message was received is not a link-local address, then the client is on the link identified by the source address in the IP datagram (note that this situation can occur only if the server has enabled the use of unicast message delivery by the client and the client has sent a message for which unicast delivery is allowed).

-

The DUID supplied by the client.

-

Other information in options supplied by the client.

-

Other information in options supplied by the relay agent.

Any address assigned by a server that is based on an EUI-64 identifier MUST include an interface identifier with the "u" (universal/local) and "g" (individual/group) bits of the interface identifier set appropriately, as indicated in section 2.5.1 of [RFC2373].

A server MUST NOT assign an address that is otherwise reserved for some other purpose. For example, a server MUST NOT assign reserved anycast addresses, as defined in RFC 2526, from any subnet.

12. Management of Temporary Addresses

A client may request the assignment of temporary addresses (see RFC 3041 [RFC3041] for the definition of temporary addresses). DHCPv6 handling of address assignment is no different for temporary addresses. DHCPv6 says nothing about details of temporary addresses like lifetimes, how clients use temporary addresses, rules for generating successive temporary addresses, etc.

Clients ask for temporary addresses and servers assign them. Temporary addresses are carried in the Identity Association for Temporary Addresses (IA_TA) option (see section 22.5). Each IA_TA option contains at most one temporary address for each of the prefixes on the link to which the client is attached.

The IAID number space for the IA_TA option IAID number space is separate from the IA_NA option IAID number space.

The server MAY update the DNS for a temporary address, as described in section 4 of RFC 3041.

13. Transmission of Messages by a Client

Unless otherwise specified in this document, or in a document that describes how IPv6 is carried over a specific type of link (for link types that do not support multicast), a client sends DHCP messages to the All_DHCP_Relay_Agents_and_Servers.

A client uses multicast to reach all servers or an individual server. An individual server is indicated by specifying that server's DUID in a Server Identifier option (see section 22.3) in the client's message (all servers will receive this message but only the indicated server will respond). All servers are indicated by not supplying this option.

A client may send some messages directly to a server using unicast, as described in section 22.12.

14. Reliability of Client Initiated Message Exchanges

DHCP clients are responsible for reliable delivery of messages in the client-initiated message exchanges described in sections 17 and 18. If a DHCP client fails to receive an expected response from a server, the client must retransmit its message. This section describes the retransmission strategy to be used by clients in client-initiated message exchanges.

Note that the procedure described in this section is slightly modified when used with the Solicit message. The modified procedure is described in section 17.1.2.

The client begins the message exchange by transmitting a message to the server. The message exchange terminates when either the client successfully receives the appropriate response or responses from a server or servers, or when the message exchange is considered to have failed according to the retransmission mechanism described below.

The client retransmission behavior is controlled and described by the following variables:

RT

Retransmission timeout

IRT

Initial retransmission time

MRC

Maximum retransmission count

MRT

Maximum retransmission time

MRD

Maximum retransmission duration

RAND

Randomization factor

With each message transmission or retransmission, the client sets RT according to the rules given below. If RT expires before the message exchange terminates, the client recomputes RT and retransmits the message.

Each of the computations of a new RT include a randomization factor (RAND), which is a random number chosen with a uniform distribution between -0.1 and +0.1. The randomization factor is included to minimize synchronization of messages transmitted by DHCP clients.

The algorithm for choosing a random number does not need to be cryptographically sound. The algorithm SHOULD produce a different sequence of random numbers from each invocation of the DHCP client.

RT for the first message transmission is based on IRT:

   RT = IRT + RAND*IRT
      

RT for each subsequent message transmission is based on the previous value of RT:

   RT = 2*RTprev + RAND*RTprev
      

MRT specifies an upper bound on the value of RT (disregarding the randomization added by the use of RAND). If MRT has a value of 0, there is no upper limit on the value of RT. Otherwise:

   if (RT > MRT)
      RT = MRT + RAND*MRT
      

MRC specifies an upper bound on the number of times a client may retransmit a message. Unless MRC is zero, the message exchange fails once the client has transmitted the message MRC times.

MRD specifies an upper bound on the length of time a client may retransmit a message. Unless MRD is zero, the message exchange fails once MRD seconds have elapsed since the client first transmitted the message.

If both MRC and MRD are non-zero, the message exchange fails whenever either of the conditions specified in the previous two paragraphs are met.

If both MRC and MRD are zero, the client continues to transmit the message until it receives a response.

15. Message Validation

Clients and servers SHOULD discard any messages that contain options that are not allowed to appear in the received message. For example, an IA option is not allowed to appear in an Information-request message. Clients and servers MAY choose to extract information from such a message if the information is of use to the recipient.

A server MUST discard any Solicit, Confirm, Rebind or Information-request messages it receives with a unicast destination address.

Message validation based on DHCP authentication is discussed in section 21.4.2.

If a server receives a message that contains options it should not contain (such as an Information-request message with an IA option), is missing options that it should contain, or is otherwise not valid, it MAY send a Reply (or Advertise as appropriate) with a Server Identifier option, a Client Identifier option if one was included in the message and a Status Code option with status UnSpecFail.

15.1. Use of Transaction IDs

The "transaction-id" field holds a value used by clients and servers to synchronize server responses to client messages. A client SHOULD generate a random number that cannot easily be guessed or predicted to use as the transaction ID for each new message it sends. Note that if a client generates easily predictable transaction identifiers, it may become more vulnerable to certain kinds of attacks from off-path intruders. A client MUST leave the transaction ID unchanged in retransmissions of a message.

15.2. Solicit Message

Clients MUST discard any received Solicit messages.

Servers MUST discard any Solicit messages that do not include a Client Identifier option or that do include a Server Identifier option.

15.3. Advertise Message

Clients MUST discard any received Advertise messages that meet any of the following conditions:

-

the message does not include a Server Identifier option.

-

the message does not include a Client Identifier option.

-

the contents of the Client Identifier option does not match the client's DUID.

-

the "transaction-id" field value does not match the value the client used in its Solicit message.

Servers and relay agents MUST discard any received Advertise messages.

15.4. Request Message

Clients MUST discard any received Request messages.

Servers MUST discard any received Request message that meet any of the following conditions:

-

the message does not include a Server Identifier option.

-

the contents of the Server Identifier option do not match the server's DUID.

-

the message does not include a Client Identifier option.

15.5. Confirm Message

Clients MUST discard any received Confirm messages.

Servers MUST discard any received Confirm messages that do not include a Client Identifier option or that do include a Server Identifier option.

15.6. Renew Message

Clients MUST discard any received Renew messages.

Servers MUST discard any received Renew message that meets any of the following conditions:

-

the message does not include a Server Identifier option.

-

the contents of the Server Identifier option does not match the server's identifier.

-

the message does not include a Client Identifier option.

15.7. Rebind Message

Clients MUST discard any received Rebind messages.

Servers MUST discard any received Rebind messages that do not include a Client Identifier option or that do include a Server Identifier option.

15.8. Decline Messages

Clients MUST discard any received Decline messages.

Servers MUST discard any received Decline message that meets any of the following conditions:

-

the message does not include a Server Identifier option.

-

the contents of the Server Identifier option does not match the server's identifier.

-

the message does not include a Client Identifier option.

15.9. Release Message

Clients MUST discard any received Release messages.

Servers MUST discard any received Release message that meets any of the following conditions:

-

the message does not include a Server Identifier option.

-

the contents of the Server Identifier option does not match the server's identifier.

-

the message does not include a Client Identifier option.

15.10. Reply Message

Clients MUST discard any received Reply message that meets any of the following conditions:

-

the message does not include a Server Identifier option.

-

the "transaction-id" field in the message does not match the value used in the original message.

If the client included a Client Identifier option in the original message, the Reply message MUST include a Client Identifier option and the contents of the Client Identifier option MUST match the DUID of the client; OR, if the client did not include a Client Identifier option in the original message, the Reply message MUST NOT include a Client Identifier option.

Servers and relay agents MUST discard any received Reply messages.

15.11. Reconfigure Message

Servers and relay agents MUST discard any received Reconfigure messages.

Clients MUST discard any Reconfigure messages that meets any of the following conditions:

-

the message was not unicast to the client.

-

the message does not include a Server Identifier option.

-

the message does not include a Client Identifier option that contains the client's DUID.

-

the message does not contain a Reconfigure Message option and the msg-type must be a valid value.

-

the message includes any IA options and the msg-type in the Reconfigure Message option is INFORMATION-REQUEST.

-

the message does not include DHCP authentication:

*

the message does not contain an authentication option.

*

the message does not pass the authentication validation performed by the client.

15.12. Information-request Message

Clients MUST discard any received Information-request messages.

Servers MUST discard any received Information-request message that meets any of the following conditions:

-

The message includes a Server Identifier option and the DUID in the option does not match the server's DUID.

-

The message includes an IA option.

15.13. Relay-forward Message

Clients MUST discard any received Relay-forward messages.

15.14. Relay-reply Message

Clients and servers MUST discard any received Relay-reply messages.

16. Client Source Address and Interface Selection

When a client sends a DHCP message to the All_DHCP_Relay_Agents_and_Servers address, it SHOULD send the message through the interface for which configuration information is being requested. However, the client MAY send the message through another interface attached to the same link, if and only if the client is certain the two interfaces are attached to the same link. The client MUST use a link-local address assigned to the interface for which it is requesting configuration information as the source address in the header of the IP datagram.

When a client sends a DHCP message directly to a server using unicast (after receiving the Server Unicast option from that server), the source address in the header of the IP datagram MUST be an address assigned to the interface for which the client is interested in obtaining configuration and which is suitable for use by the server in responding to the client.

17. DHCP Server Solicitation

This section describes how a client locates servers that will assign addresses to IAs belonging to the client.

The client is responsible for creating IAs and requesting that a server assign IPv6 addresses to the IA. The client first creates an IA and assigns it an IAID. The client then transmits a Solicit message containing an IA option describing the IA. Servers that can assign addresses to the IA respond to the client with an Advertise message. The client then initiates a configuration exchange as described in section 18.

If the client will accept a Reply message with committed address assignments and other resources in response to the Solicit message, the client includes a Rapid Commit option (see section 22.14) in the Solicit message.

17.1. Client Behavior

A client uses the Solicit message to discover DHCP servers configured to assign addresses or return other configuration parameters on the link to which the client is attached.

17.1.1. Creation of Solicit Messages

The client sets the "msg-type" field to SOLICIT. The client generates a transaction ID and inserts this value in the "transaction-id" field.

The client MUST include a Client Identifier option to identify itself to the server. The client includes IA options for any IAs to which it wants the server to assign addresses. The client MAY include addresses in the IAs as a hint to the server about addresses for which the client has a preference. The client MUST NOT include any other options in the Solicit message, except as specifically allowed in the definition of individual options.

The client uses IA_NA options to request the assignment of non-temporary addresses and uses IA_TA options to request the assignment of temporary addresses. Either IA_NA or IA_TA options, or a combination of both, can be included in DHCP messages.

The client SHOULD include an Option Request option (see section 22.7) to indicate the options the client is interested in receiving. The client MAY additionally include instances of those options that are identified in the Option Request option, with data values as hints to the server about parameter values the client would like to have returned.

The client includes a Reconfigure Accept option (see section 22.20) if the client is willing to accept Reconfigure messages from the server.

17.1.2. Transmission of Solicit Messages

The first Solicit message from the client on the interface MUST be delayed by a random amount of time between 0 and SOL_MAX_DELAY. In the case of a Solicit message transmitted when DHCP is initiated by IPv6 Neighbor Discovery, the delay gives the amount of time to wait after IPv6 Neighbor Discovery causes the client to invoke the stateful address autoconfiguration protocol (see section 5.5.3 of RFC 2462). This random delay desynchronizes clients which start at the same time (for example, after a power outage).

The client transmits the message according to section 14, using the following parameters:

IRT

SOL_TIMEOUT

MRT

SOL_MAX_RT

MRC

0

MRD

0

If the client has included a Rapid Commit option in its Solicit message, the client terminates the waiting process as soon as a Reply message with a Rapid Commit option is received.

If the client is waiting for an Advertise message, the mechanism in section 14 is modified as follows for use in the transmission of Solicit messages. The message exchange is not terminated by the receipt of an Advertise before the first RT has elapsed. Rather, the client collects Advertise messages until the first RT has elapsed. Also, the first RT MUST be selected to be strictly greater than IRT by choosing RAND to be strictly greater than 0.

A client MUST collect Advertise messages for the first RT seconds, unless it receives an Advertise message with a preference value of 255. The preference value is carried in the Preference option (section 22.8). Any Advertise that does not include a Preference option is considered to have a preference value of 0. If the client receives an Advertise message that includes a Preference option with a preference value of 255, the client immediately begins a client-initiated message exchange (as described in section 18) by sending a Request message to the server from which the Advertise message was received. If the client receives an Advertise message that does not include a Preference option with a preference value of 255, the client continues to wait until the first RT elapses. If the first RT elapses and the client has received an Advertise message, the client SHOULD continue with a client-initiated message exchange by sending a Request message.

If the client does not receive any Advertise messages before the first RT has elapsed, it begins the retransmission mechanism described in section 14. The client terminates the retransmission process as soon as it receives any Advertise message, and the client acts on the received Advertise message without waiting for any additional Advertise messages.

A DHCP client SHOULD choose MRC and MRD to be 0. If the DHCP client is configured with either MRC or MRD set to a value other than 0, it MUST stop trying to configure the interface if the message exchange fails. After the DHCP client stops trying to configure the interface, it SHOULD restart the reconfiguration process after some external event, such as user input, system restart, or when the client is attached to a new link.

17.1.3. Receipt of Advertise Messages

The client MUST ignore any Advertise message that includes a Status Code option containing the value NoAddrsAvail, with the exception that the client MAY display the associated status message to the user.

Upon receipt of one or more valid Advertise messages, the client selects one or more Advertise messages based upon the following criteria.

-

Those Advertise messages with the highest server preference value are preferred over all other Advertise messages.

-

Within a group of Advertise messages with the same server preference value, a client MAY select those servers whose Advertise messages advertise information of interest to the client. For example, the client may choose a server that returned an advertisement with configuration options of interest to the client.

-

The client MAY choose a less-preferred server if that server has a better set of advertised parameters, such as the available addresses advertised in IAs.

Once a client has selected Advertise message(s), the client will typically store information about each server, such as server preference value, addresses advertised, when the advertisement was received, and so on.

If the client needs to select an alternate server in the case that a chosen server does not respond, the client chooses the next server according to the criteria given above.

17.1.4. Receipt of Reply Message

If the client includes a Rapid Commit option in the Solicit message, it will expect a Reply message that includes a Rapid Commit option in response. The client discards any Reply messages it receives that do not include a Rapid Commit option. If the client receives a valid Reply message that includes a Rapid Commit option, it processes the message as described in section 18.1.8. If it does not receive such a Reply message and does receive a valid Advertise message, the client processes the Advertise message as described in section 17.1.3.

If the client subsequently receives a valid Reply message that includes a Rapid Commit option, it either:

• processes the Reply message as described in section 18.1.8, and discards any Reply messages received in response to the Request message, or
• processes any Reply messages received in response to the Request message and discards the Reply message that includes the Rapid Commit option.

17.2. Server Behavior

A server sends an Advertise message in response to valid Solicit messages it receives to announce the availability of the server to the client.

17.2.1. Receipt of Solicit Messages

The server determines the information about the client and its location as described in section 11 and checks its administrative policy about responding to the client. If the server is not permitted to respond to the client, the server discards the Solicit message. For example, if the administrative policy for the server is that it may only respond to a client that is willing to accept a Reconfigure message, if the client indicates with a Reconfigure Accept option in the Solicit message that it will not accept a Reconfigure message, the servers discard the Solicit message.

If the client has included a Rapid Commit option in the Solicit message and the server has been configured to respond with committed address assignments and other resources, the server responds to the Solicit with a Reply message as described in section 17.2.3. Otherwise, the server ignores the Rapid Commit option and processes the remainder of the message as if no Rapid Commit option were present.

17.2.2. Creation and Transmission of Advertise Messages

The server sets the "msg-type" field to ADVERTISE and copies the contents of the transaction-id field from the Solicit message received from the client to the Advertise message. The server includes its server identifier in a Server Identifier option and copies the Client Identifier from the Solicit message into the Advertise message.

The server MAY add a Preference option to carry the preference value for the Advertise message. The server implementation SHOULD allow the setting of a server preference value by the administrator. The server preference value MUST default to zero unless otherwise configured by the server administrator.

The server includes a Reconfigure Accept option if the server wants to require that the client accept Reconfigure messages.

The server includes options the server will return to the client in a subsequent Reply message. The information in these options may be used by the client in the selection of a server if the client receives more than one Advertise message. If the client has included an Option Request option in the Solicit message, the server includes options in the Advertise message containing configuration parameters for all of the options identified in the Option Request option that the server has been configured to return to the client. The server MAY return additional options to the client if it has been configured to do so. The server must be aware of the recommendations on packet sizes and the use of fragmentation in section 5 of RFC 2460.

If the Solicit message from the client included one or more IA options, the server MUST include IA options in the Advertise message containing any addresses that would be assigned to IAs contained in the Solicit message from the client. If the client has included addresses in the IAs in the Solicit message, the server uses those addresses as hints about the addresses the client would like to receive.

If the server will not assign any addresses to any IAs in a subsequent Request from the client, the server MUST send an Advertise message to the client that includes only a Status Code option with code NoAddrsAvail and a status message for the user, a Server Identifier option with the server's DUID, and a Client Identifier option with the client's DUID.

If the Solicit message was received directly by the server, the server unicasts the Advertise message directly to the client using the address in the source address field from the IP datagram in which the Solicit message was received. The Advertise message MUST be unicast on the link from which the Solicit message was received.

If the Solicit message was received in a Relay-forward message, the server constructs a Relay-reply message with the Advertise message in the payload of a "relay-message" option. If the Relay-forward messages included an Interface-id option, the server copies that option to the Relay-reply message. The server unicasts the Relay-reply message directly to the relay agent using the address in the source address field from the IP datagram in which the Relay-forward message was received.

17.2.3. Creation and Transmission of Reply Messages

The server MUST commit the assignment of any addresses or other configuration information message before sending a Reply message to a client in response to a Solicit message.

DISCUSSION:

• When using the Solicit-Reply message exchange, the server commits the assignment of any addresses before sending the Reply message. The client can assume it has been assigned the addresses in the Reply message and does not need to send a Request message for those addresses.
• Typically, servers that are configured to use the Solicit-Reply message exchange will be deployed so that only one server will respond to a Solicit message. If more than one server responds, the client will only use the addresses from one of the servers, while the addresses from the other servers will be committed to the client but not used by the client.

The server includes a Rapid Commit option in the Reply message to indicate that the Reply is in response to a Solicit message.

The server includes a Reconfigure Accept option if the server wants to require that the client accept Reconfigure messages.

The server produces the Reply message as though it had received a Request message, as described in section 18.2.1. The server transmits the Reply message as described in section 18.2.8.

18. DHCP Client-Initiated Configuration Exchange

A client initiates a message exchange with a server or servers to acquire or update configuration information of interest. The client may initiate the configuration exchange as part of the operating system configuration process, when requested to do so by the application layer, when required by Stateless Address Autoconfiguration or as required to extend the lifetime of an address (Renew and Rebind messages).

18.1. Client Behavior

A client uses Request, Renew, Rebind, Release and Decline messages during the normal life cycle of addresses. It uses Confirm to validate addresses when it may have moved to a new link. It uses Information-Request messages when it needs configuration information but no addresses.

If the client has a source address of sufficient scope that can be used by the server as a return address, and the client has received a Server Unicast option (section 22.12) from the server, the client SHOULD unicast any Request, Renew, Release and Decline messages to the server.

DISCUSSION:

• Use of unicast may avoid delays due to the relaying of messages by relay agents, as well as avoid overhead and duplicate responses by servers due to the delivery of client messages to multiple servers. Requiring the client to relay all DHCP messages through a relay agent enables the inclusion of relay agent options in all messages sent by the client. The server should enable the use of unicast only when relay agent options will not be used.

18.1.1. Creation and Transmission of Request Messages

The client uses a Request message to populate IAs with addresses and obtain other configuration information. The client includes one or more IA options in the Request message. The server then returns addresses and other information about the IAs to the client in IA options in a Reply message.

The client generates a transaction ID and inserts this value in the "transaction-id" field.

The client places the identifier of the destination server in a Server Identifier option.

The client MUST include a Client Identifier option to identify itself to the server. The client adds any other appropriate options, including one or more IA options (if the client is requesting that the server assign it some network addresses).

The client MUST include an Option Request option (see section 22.7) to indicate the options the client is interested in receiving. The client MAY include options with data values as hints to the server about parameter values the client would like to have returned.

The client includes a Reconfigure Accept option (see section 22.20) indicating whether or not the client is willing to accept Reconfigure messages from the server.

The client transmits the message according to section 14, using the following parameters:

IRT

REQ_TIMEOUT

MRT

REQ_MAX_RT

MRC

REQ_MAX_RC

MRD

0

If the message exchange fails, the client takes an action based on the client's local policy. Examples of actions the client might take include:

-

Select another server from a list of servers known to the client; for example, servers that responded with an Advertise message.

-

Initiate the server discovery process described in section 17.

-

Terminate the configuration process and report failure.

18.1.2. Creation and Transmission of Confirm Messages

Whenever a client may have moved to a new link, the prefixes from the addresses assigned to the interfaces on that link may no longer be appropriate for the link to which the client is attached. Examples of times when a client may have moved to a new link include:

o

The client reboots.

o

The client is physically connected to a wired connection.

o

The client returns from sleep mode.

o

The client using a wireless technology changes access points.

In any situation when a client may have moved to a new link, the client MUST initiate a Confirm/Reply message exchange. The client includes any IAs assigned to the interface that may have moved to a new link, along with the addresses associated with those IAs, in its Confirm message. Any responding servers will indicate whether those addresses are appropriate for the link to which the client is attached with the status in the Reply message it returns to the client.

The client sets the "msg-type" field to CONFIRM. The client generates a transaction ID and inserts this value in the "transaction-id" field.

The client MUST include a Client Identifier option to identify itself to the server. The client includes IA options for all of the IAs assigned to the interface for which the Confirm message is being sent. The IA options include all of the addresses the client currently has associated with those IAs. The client SHOULD set the T1 and T2 fields in any IA_NA options, and the preferred-lifetime and valid-lifetime fields in the IA Address options to 0, as the server will ignore these fields.

The first Confirm message from the client on the interface MUST be delayed by a random amount of time between 0 and CNF_MAX_DELAY. The client transmits the message according to section 14, using the following parameters:

IRT

CNF_TIMEOUT

MRT

CNF_MAX_RT

MRC

0

MRD

CNF_MAX_RD

If the client receives no responses before the message transmission process terminates, as described in section 14, the client SHOULD continue to use any IP addresses, using the last known lifetimes for those addresses, and SHOULD continue to use any other previously obtained configuration parameters.

18.1.3. Creation and Transmission of Renew Messages

To extend the valid and preferred lifetimes for the addresses associated with an IA, the client sends a Renew message to the server from which the client obtained the addresses in the IA containing an IA option for the IA. The client includes IA Address options in the IA option for the addresses associated with the IA. The server determines new lifetimes for the addresses in the IA according to the administrative configuration of the server. The server may also add new addresses to the IA. The server may remove addresses from the IA by setting the preferred and valid lifetimes of those addresses to zero.

The server controls the time at which the client contacts the server to extend the lifetimes on assigned addresses through the T1 and T2 parameters assigned to an IA.

At time T1 for an IA, the client initiates a Renew/Reply message exchange to extend the lifetimes on any addresses in the IA. The client includes an IA option with all addresses currently assigned to the IA in its Renew message.

If T1 or T2 is set to 0 by the server (for an IA_NA) or there are no T1 or T2 times (for an IA_TA), the client may send a Renew or Rebind message, respectively, at the client's discretion.

The client sets the "msg-type" field to RENEW. The client generates a transaction ID and inserts this value in the "transaction-id" field.

The client places the identifier of the destination server in a Server Identifier option.

The client MUST include a Client Identifier option to identify itself to the server. The client adds any appropriate options, including one or more IA options. The client MUST include the list of addresses the client currently has associated with the IAs in the Renew message.

The client MUST include an Option Request option (see section 22.7) to indicate the options the client is interested in receiving. The client MAY include options with data values as hints to the server about parameter values the client would like to have returned.

The client transmits the message according to section 14, using the following parameters:

IRT

REN_TIMEOUT

MRT

REN_MAX_RT

MRC

0

MRD

Remaining time until T2

The message exchange is terminated when time T2 is reached (see section 18.1.4), at which time the client begins a Rebind message exchange.

18.1.4. Creation and Transmission of Rebind Messages

At time T2 for an IA (which will only be reached if the server to which the Renew message was sent at time T1 has not responded), the client initiates a Rebind/Reply message exchange with any available server. The client includes an IA option with all addresses currently assigned to the IA in its Rebind message.

The client sets the "msg-type" field to REBIND. The client generates a transaction ID and inserts this value in the "transaction-id" field.

The client MUST include a Client Identifier option to identify itself to the server. The client adds any appropriate options, including one or more IA options. The client MUST include the list of addresses the client currently has associated with the IAs in the Rebind message.

The client MUST include an Option Request option (see section 22.7) to indicate the options the client is interested in receiving. The client MAY include options with data values as hints to the server about parameter values the client would like to have returned.

The client transmits the message according to section 14, using the following parameters:

IRT

REB_TIMEOUT

MRT

REB_MAX_RT

MRC

0

MRD

Remaining time until valid lifetimes of all addresses have expired

The message exchange is terminated when the valid lifetimes of all the addresses assigned to the IA expire (see section 10), at which time the client has several alternative actions to choose from; for example:

-

The client may choose to use a Solicit message to locate a new DHCP server and send a Request for the expired IA to the new server.

-

The client may have other addresses in other IAs, so the client may choose to discard the expired IA and use the addresses in the other IAs.

18.1.5. Creation and Transmission of Information-request Messages

The client uses an Information-request message to obtain configuration information without having addresses assigned to it.

The client sets the "msg-type" field to INFORMATION-REQUEST. The client generates a transaction ID and inserts this value in the "transaction-id" field.

The client SHOULD include a Client Identifier option to identify itself to the server. If the client does not include a Client Identifier option, the server will not be able to return any client-specific options to the client, or the server may choose not to respond to the message at all. The client MUST include a Client Identifier option if the Information-Request message will be authenticated.

The client MUST include an Option Request option (see section 22.7) to indicate the options the client is interested in receiving. The client MAY include options with data values as hints to the server about parameter values the client would like to have returned.

The first Information-request message from the client on the interface MUST be delayed by a random amount of time between 0 and INF_MAX_DELAY. The client transmits the message according to section 14, using the following parameters:

IRT

INF_TIMEOUT

MRT

INF_MAX_RT

MRC

0

MRD

0

18.1.6. Creation and Transmission of Release Messages

To release one or more addresses, a client sends a Release message to the server.

The client sets the "msg-type" field to RELEASE. The client generates a transaction ID and places this value in the "transaction-id" field.

The client places the identifier of the server that allocated the address(es) in a Server Identifier option.

The client MUST include a Client Identifier option to identify itself to the server. The client includes options containing the IAs for the addresses it is releasing in the "options" field. The addresses to be released MUST be included in the IAs. Any addresses for the IAs the client wishes to continue to use MUST NOT be added to the IAs.

The client MUST NOT use any of the addresses it is releasing as the source address in the Release message or in any subsequently transmitted message.

Because Release messages may be lost, the client should retransmit the Release if no Reply is received. However, there are scenarios where the client may not wish to wait for the normal retransmission timeout before giving up (e.g., on power down). Implementations SHOULD retransmit one or more times, but MAY choose to terminate the retransmission procedure early.

The client transmits the message according to section 14, using the following parameters:

IRT

REL_TIMEOUT

MRT

0

MRC

REL_MAX_RC

MRD

0

The client MUST stop using all of the addresses being released as soon as the client begins the Release message exchange process. If addresses are released but the Reply from a DHCP server is lost, the client will retransmit the Release message, and the server may respond with a Reply indicating a status of NoBinding. Therefore, the client does not treat a Reply message with a status of NoBinding in a Release message exchange as if it indicates an error.

Note that if the client fails to release the addresses, each address assigned to the IA will be reclaimed by the server when the valid lifetime of that address expires.

18.1.7. Creation and Transmission of Decline Messages

If a client detects that one or more addresses assigned to it by a server are already in use by another node, the client sends a Decline message to the server to inform it that the address is suspect.

The client sets the "msg-type" field to DECLINE. The client generates a transaction ID and places this value in the "transaction-id" field.

The client places the identifier of the server that allocated the address(es) in a Server Identifier option.

The client MUST include a Client Identifier option to identify itself to the server. The client includes options containing the IAs for the addresses it is declining in the "options" field. The addresses to be declined MUST be included in the IAs. Any addresses for the IAs the client wishes to continue to use should not be in added to the IAs.

The client MUST NOT use any of the addresses it is declining as the source address in the Decline message or in any subsequently transmitted message.

The client transmits the message according to section 14, using the following parameters:

IRT

DEC_TIMEOUT

MRT

0

MRC

DEC_MAX_RC

MRD

0

If addresses are declined but the Reply from a DHCP server is lost, the client will retransmit the Decline message, and the server may respond with a Reply indicating a status of NoBinding. Therefore, the client does not treat a Reply message with a status of NoBinding in a Decline message exchange as if it indicates an error.

18.1.8. Receipt of Reply Messages

Upon the receipt of a valid Reply message in response to a Solicit (with a Rapid Commit option), Request, Confirm, Renew, Rebind or Information-request message, the client extracts the configuration information contained in the Reply. The client MAY choose to report any status code or message from the status code option in the Reply message.

The client SHOULD perform duplicate address detection [RFC2462] on each of the addresses in any IAs it receives in the Reply message before using that address for traffic. If any of the addresses are found to be in use on the link, the client sends a Decline message to the server as described in section 18.1.7.

If the Reply was received in response to a Solicit (with a Rapid Commit option), Request, Renew or Rebind message, the client updates the information it has recorded about IAs from the IA options contained in the Reply message:

-

Record T1 and T2 times.

-

Add any new addresses in the IA option to the IA as recorded by the client.

-

Update lifetimes for any addresses in the IA option that the client already has recorded in the IA.

-

Discard any addresses from the IA, as recorded by the client, that have a valid lifetime of 0 in the IA Address option.

-

Leave unchanged any information about addresses the client has recorded in the IA but that were not included in the IA from the server.

Management of the specific configuration information is detailed in the definition of each option in section 22.

If the client receives a Reply message with a Status Code containing UnspecFail, the server is indicating that it was unable to process the message due to an unspecified failure condition. If the client retransmits the original message to the same server to retry the desired operation, the client MUST limit the rate at which it retransmits the message and limit the duration of the time during which it retransmits the message.

When the client receives a Reply message with a Status Code option with the value UseMulticast, the client records the receipt of the message and sends subsequent messages to the server through the interface on which the message was received using multicast. The client resends the original message using multicast.

When the client receives a NotOnLink status from the server in response to a Confirm message, the client performs DHCP server solicitation, as described in section 17, and client-initiated configuration as described in section 18. If the client receives any Reply messages that do not indicate a NotOnLink status, the client can use the addresses in the IA and ignore any messages that indicate a NotOnLink status.

When the client receives a NotOnLink status from the server in response to a Request, the client can either re-issue the Request without specifying any addresses or restart the DHCP server discovery process (see section 17).

The client examines the status code in each IA individually. If the status code is NoAddrsAvail, the client has received no usable addresses in the IA and may choose to try obtaining addresses for the IA from another server. The client uses addresses and other information from any IAs that do not contain a Status Code option with the NoAddrsAvail code. If the client receives no addresses in any of the IAs, it may either try another server (perhaps restarting the DHCP server discovery process) or use the Information-request message to obtain other configuration information only.

When the client receives a Reply message in response to a Renew or Rebind message, the client examines each IA independently. For each IA in the original Renew or Rebind message, the client:

-

sends a Request message if the IA contained a Status Code option with the NoBinding status (and does not send any additional Renew/Rebind messages)

-

sends a Renew/Rebind if the IA is not in the Reply message

-

otherwise accepts the information in the IA

When the client receives a valid Reply message in response to a Release message, the client considers the Release event completed, regardless of the Status Code option(s) returned by the server.

When the client receives a valid Reply message in response to a Decline message, the client considers the Decline event completed, regardless of the Status Code option(s) returned by the server.

18.2. Server Behavior

For this discussion, the Server is assumed to have been configured in an implementation specific manner with configuration of interest to clients.

In most instances, the server will send a Reply in response to a client message. This Reply message MUST always contain the Server Identifier option containing the server's DUID and the Client Identifier option from the client message if one was present.

In most Reply messages, the server includes options containing configuration information for the client. The server must be aware of the recommendations on packet sizes and the use of fragmentation in section 5 of RFC 2460. If the client included an Option Request option in its message, the server includes options in the Reply message containing configuration parameters for all of the options identified in the Option Request option that the server has been configured to return to the client. The server MAY return additional options to the client if it has been configured to do so.

18.2.1. Receipt of Request Messages

When the server receives a Request message via unicast from a client to which the server has not sent a unicast option, the server discards the Request message and responds with a Reply message containing a Status Code option with the value UseMulticast, a Server Identifier option containing the server's DUID, the Client Identifier option from the client message, and no other options.

When the server receives a valid Request message, the server creates the bindings for that client according to the server's policy and configuration information and records the IAs and other information requested by the client.

The server constructs a Reply message by setting the "msg-type" field to REPLY, and copying the transaction ID from the Request message into the transaction-id field.

The server MUST include a Server Identifier option containing the server's DUID and the Client Identifier option from the Request message in the Reply message.

If the server finds that the prefix on one or more IP addresses in any IA in the message from the client is not appropriate for the link to which the client is connected, the server MUST return the IA to the client with a Status Code option with the value NotOnLink.

If the server cannot assign any addresses to an IA in the message from the client, the server MUST include the IA in the Reply message with no addresses in the IA and a Status Code option in the IA containing status code NoAddrsAvail.

For any IAs to which the server can assign addresses, the server includes the IA with addresses and other configuration parameters, and records the IA as a new client binding.

The server includes a Reconfigure Accept option if the server wants to require that the client accept Reconfigure messages.

The server includes other options containing configuration information to be returned to the client as described in section 18.2.

If the server finds that the client has included an IA in the Request message for which the server already has a binding that associates the IA with the client, the client has resent a Request message for which it did not receive a Reply message. The server either resends a previously cached Reply message or sends a new Reply message.

18.2.2. Receipt of Confirm Messages

When the server receives a Confirm message, the server determines whether the addresses in the Confirm message are appropriate for the link to which the client is attached. If all of the addresses in the Confirm message pass this test, the server returns a status of Success. If any of the addresses do not pass this test, the server returns a status of NotOnLink. If the server is unable to perform this test (for example, the server does not have information about prefixes on the link to which the client is connected), or there were no addresses in any of the IAs sent by the client, the server MUST NOT send a reply to the client.

The server ignores the T1 and T2 fields in the IA options and the preferred-lifetime and valid-lifetime fields in the IA Address options.

The server constructs a Reply message by setting the "msg-type" field to REPLY, and copying the transaction ID from the Confirm message into the transaction-id field.

The server MUST include a Server Identifier option containing the server's DUID and the Client Identifier option from the Confirm message in the Reply message. The server includes a Status Code option indicating the status of the Confirm message.

18.2.3. Receipt of Renew Messages

When the server receives a Renew message via unicast from a client to which the server has not sent a unicast option, the server discards the Renew message and responds with a Reply message containing a Status Code option with the value UseMulticast, a Server Identifier option containing the server's DUID, the Client Identifier option from the client message, and no other options.

When the server receives a Renew message that contains an IA option from a client, it locates the client's binding and verifies that the information in the IA from the client matches the information stored for that client.

If the server cannot find a client entry for the IA the server returns the IA containing no addresses with a Status Code option set to NoBinding in the Reply message.

If the server finds that any of the addresses are not appropriate for the link to which the client is attached, the server returns the address to the client with lifetimes of 0.

If the server finds the addresses in the IA for the client then the server sends back the IA to the client with new lifetimes and T1/T2 times. The server may choose to change the list of addresses and the lifetimes of addresses in IAs that are returned to the client.

The server constructs a Reply message by setting the "msg-type" field to REPLY, and copying the transaction ID from the Renew message into the transaction-id field.

The server MUST include a Server Identifier option containing the server's DUID and the Client Identifier option from the Renew message in the Reply message.

The server includes other options containing configuration information to be returned to the client as described in section 18.2.

18.2.4. Receipt of Rebind Messages

When the server receives a Rebind message that contains an IA option from a client, it locates the client's binding and verifies that the information in the IA from the client matches the information stored for that client.

If the server cannot find a client entry for the IA and the server determines that the addresses in the IA are not appropriate for the link to which the client's interface is attached according to the server's explicit configuration information, the server MAY send a Reply message to the client containing the client's IA, with the lifetimes for the addresses in the IA set to zero. This Reply constitutes an explicit notification to the client that the addresses in the IA are no longer valid. In this situation, if the server does not send a Reply message it silently discards the Rebind message.

If the server finds that any of the addresses are no longer appropriate for the link to which the client is attached, the server returns the address to the client with lifetimes of 0.

If the server finds the addresses in the IA for the client then the server SHOULD send back the IA to the client with new lifetimes and T1/T2 times.

The server constructs a Reply message by setting the "msg-type" field to REPLY, and copying the transaction ID from the Rebind message into the transaction-id field.

The server MUST include a Server Identifier option containing the server's DUID and the Client Identifier option from the Rebind message in the Reply message.

The server includes other options containing configuration information to be returned to the client as described in section 18.2.

18.2.5. Receipt of Information-request Messages

When the server receives an Information-request message, the client is requesting configuration information that does not include the assignment of any addresses. The server determines all configuration parameters appropriate to the client, based on the server configuration policies known to the server.

The server constructs a Reply message by setting the "msg-type" field to REPLY, and copying the transaction ID from the Information-request message into the transaction-id field.

The server MUST include a Server Identifier option containing the server's DUID in the Reply message. If the client included a Client Identification option in the Information-request message, the server copies that option to the Reply message.

The server includes options containing configuration information to be returned to the client as described in section 18.2.

If the Information-request message received from the client did not include a Client Identifier option, the server SHOULD respond with a Reply message containing any configuration parameters that are not determined by the client's identity. If the server chooses not to respond, the client may continue to retransmit the Information-request message indefinitely.

18.2.6. Receipt of Release Messages

When the server receives a Release message via unicast from a client to which the server has not sent a unicast option, the server discards the Release message and responds with a Reply message containing a Status Code option with value UseMulticast, a Server Identifier option containing the server's DUID, the Client Identifier option from the client message, and no other options.

Upon the receipt of a valid Release message, the server examines the IAs and the addresses in the IAs for validity. If the IAs in the message are in a binding for the client, and the addresses in the IAs have been assigned by the server to those IAs, the server deletes the addresses from the IAs and makes the addresses available for assignment to other clients. The server ignores addresses not assigned to the IA, although it may choose to log an error.

After all the addresses have been processed, the server generates a Reply message and includes a Status Code option with value Success, a Server Identifier option with the server's DUID, and a Client Identifier option with the client's DUID. For each IA in the Release message for which the server has no binding information, the server adds an IA option using the IAID from the Release message, and includes a Status Code option with the value NoBinding in the IA option. No other options are included in the IA option.

A server may choose to retain a record of assigned addresses and IAs after the lifetimes on the addresses have expired to allow the server to reassign the previously assigned addresses to a client.

18.2.7. Receipt of Decline Messages

When the server receives a Decline message via unicast from a client to which the server has not sent a unicast option, the server discards the Decline message and responds with a Reply message containing a Status Code option with the value UseMulticast, a Server Identifier option containing the server's DUID, the Client Identifier option from the client message, and no other options.

Upon the receipt of a valid Decline message, the server examines the IAs and the addresses in the IAs for validity. If the IAs in the message are in a binding for the client, and the addresses in the IAs have been assigned by the server to those IAs, the server deletes the addresses from the IAs. The server ignores addresses not assigned to the IA (though it may choose to log an error if it finds such an address).

The client has found any addresses in the Decline messages to be already in use on its link. Therefore, the server SHOULD mark the addresses declined by the client so that those addresses are not assigned to other clients, and MAY choose to make a notification that addresses were declined. Local policy on the server determines when the addresses identified in a Decline message may be made available for assignment.

After all the addresses have been processed, the server generates a Reply message and includes a Status Code option with the value Success, a Server Identifier option with the server's DUID, and a Client Identifier option with the client's DUID. For each IA in the Decline message for which the server has no binding information, the server adds an IA option using the IAID from the Release message and includes a Status Code option with the value NoBinding in the IA option. No other options are included in the IA option.

18.2.8. Transmission of Reply Messages

If the original message was received directly by the server, the server unicasts the Reply message directly to the client using the address in the source address field from the IP datagram in which the original message was received. The Reply message MUST be unicast through the interface on which the original message was received.

If the original message was received in a Relay-forward message, the server constructs a Relay-reply message with the Reply message in the payload of a Relay Message option (see section 22.10). If the Relay-forward messages included an Interface-id option, the server copies that option to the Relay-reply message. The server unicasts the Relay-reply message directly to the relay agent using the address in the source address field from the IP datagram in which the Relay-forward message was received.

19. DHCP Server-Initiated Configuration Exchange

A server initiates a configuration exchange to cause DHCP clients to obtain new addresses and other configuration information. For example, an administrator may use a server-initiated configuration exchange when links in the DHCP domain are to be renumbered. Other examples include changes in the location of directory servers, addition of new services such as printing, and availability of new software.

19.1. Server Behavior

A server sends a Reconfigure message to cause a client to initiate immediately a Renew/Reply or Information-request/Reply message exchange with the server.

19.1.1. Creation and Transmission of Reconfigure Messages

The server sets the "msg-type" field to RECONFIGURE. The server sets the transaction-id field to 0. The server includes a Server Identifier option containing its DUID and a Client Identifier option containing the client's DUID in the Reconfigure message.

The server MAY include an Option Request option to inform the client of what information has been changed or new information that has been added. In particular, the server specifies the IA option in the Option Request option if the server wants the client to obtain new address information. If the server identifies the IA option in the Option Request option, the server MUST include an IA option that contains no other sub-options to identify each IA that is to be reconfigured on the client.

Because of the risk of denial of service attacks against DHCP clients, the use of a security mechanism is mandated in Reconfigure messages. The server MUST use DHCP authentication in the Reconfigure message.

The server MUST include a Reconfigure Message option (defined in section 22.19) to select whether the client responds with a Renew message or an Information-Request message.

The server MUST NOT include any other options in the Reconfigure except as specifically allowed in the definition of individual options.

A server sends each Reconfigure message to a single DHCP client, using an IPv6 unicast address of sufficient scope belonging to the DHCP client. If the server does not have an address to which it can send the Reconfigure message directly to the client, the server uses a Relay-reply message (as described in section 20.3) to send the Reconfigure message to a relay agent that will relay the message to the client. The server may obtain the address of the client (and the appropriate relay agent, if required) through the information the server has about clients that have been in contact with the server, or through some external agent.

To reconfigure more than one client, the server unicasts a separate message to each client. The server may initiate the reconfiguration of multiple clients concurrently; for example, a server may send a Reconfigure message to additional clients while previous reconfiguration message exchanges are still in progress.

The Reconfigure message causes the client to initiate a Renew/Reply or Information-request/Reply message exchange with the server. The server interprets the receipt of a Renew or Information-request message (whichever was specified in the original Reconfigure message) from the client as satisfying the Reconfigure message request.

19.1.2. Time Out and Retransmission of Reconfigure Messages

If the server does not receive a Renew or Information-request message from the client in REC_TIMEOUT milliseconds, the server retransmits the Reconfigure message, doubles the REC_TIMEOUT value and waits again. The server continues this process until REC_MAX_RC unsuccessful attempts have been made, at which point the server SHOULD abort the reconfigure process for that client.

Default and initial values for REC_TIMEOUT and REC_MAX_RC are documented in section 5.5.

19.2. Receipt of Renew Messages

The server generates and sends a Reply message to the client as described in sections 18.2.3 and 18.2.8, including options for configuration parameters.

The server MAY include options containing the IAs and new values for other configuration parameters in the Reply message, even if those IAs and parameters were not requested in the Renew message from the client.

19.3. Receipt of Information-request Messages

The server generates and sends a Reply message to the client as described in sections 18.2.5 and 18.2.8, including options for configuration parameters.

The server MAY include options containing new values for other configuration parameters in the Reply message, even if those parameters were not requested in the Information-request message from the client.

19.4. Client Behavior

A client receives Reconfigure messages sent to the UDP port 546 on interfaces for which it has acquired configuration information through DHCP. These messages may be sent at any time. Since the results of a reconfiguration event may affect application layer programs, the client SHOULD log these events, and MAY notify these programs of the change through an implementation-specific interface.

19.4.1. Receipt of Reconfigure Messages

Upon receipt of a valid Reconfigure message, the client responds with either a Renew message or an Information-request message as indicated by the Reconfigure Message option (as defined in section 22.19). The client ignores the transaction-id field in the received Reconfigure message. While the transaction is in progress, the client silently discards any Reconfigure messages it receives.

DISCUSSION:

• The Reconfigure message acts as a trigger that signals the client to complete a successful message exchange. Once the client has received a Reconfigure, the client proceeds with the message exchange (retransmitting the Renew or Information-request message if necessary); the client ignores any additional Reconfigure messages until the exchange is complete. Subsequent Reconfigure messages cause the client to initiate a new exchange.
• How does this mechanism work in the face of duplicated or retransmitted Reconfigure messages? Duplicate messages will be ignored because the client will begin the exchange after the receipt of the first Reconfigure. Retransmitted messages will either trigger the exchange (if the first Reconfigure was not received by the client) or will be ignored. The server can discontinue retransmission of Reconfigure messages to the client once the server receives the Renew or Information-request message from the client.
• It might be possible for a duplicate or retransmitted Reconfigure to be sufficiently delayed (and delivered out of order) to arrive at the client after the exchange (initiated by the original Reconfigure) has been completed. In this case, the client would initiate a redundant exchange. The likelihood of delayed and out of order delivery is small enough to be ignored. The consequence of the redundant exchange is inefficiency rather than incorrect operation.

19.4.2. Creation and Transmission of Renew Messages

When responding to a Reconfigure, the client creates and sends the Renew message in exactly the same manner as outlined in section 18.1.3, with the exception that the client copies the Option Request option and any IA options from the Reconfigure message into the Renew message.

19.4.3. Creation and Transmission of Information-request Messages

When responding to a Reconfigure, the client creates and sends the Information-request message in exactly the same manner as outlined in section 18.1.5, with the exception that the client includes a Server Identifier option with the identifier from the Reconfigure message to which the client is responding.

19.4.4. Time Out and Retransmission of Renew or Information-request

Messages

The client uses the same variables and retransmission algorithm as it does with Renew or Information-request messages generated as part of a client-initiated configuration exchange. See sections 18.1.3 and 18.1.5 for details. If the client does not receive a response from the server by the end of the retransmission process, the client ignores and discards the Reconfigure message.

19.4.5. Receipt of Reply Messages

Upon the receipt of a valid Reply message, the client processes the options and sets (or resets) configuration parameters appropriately. The client records and updates the lifetimes for any addresses specified in IAs in the Reply message.

20. Relay Agent Behavior

The relay agent MAY be configured to use a list of destination addresses, which MAY include unicast addresses, the All_DHCP_Servers multicast address, or other addresses selected by the network administrator. If the relay agent has not been explicitly configured, it MUST use the All_DHCP_Servers multicast address as the default.

If the relay agent relays messages to the All_DHCP_Servers multicast address or other multicast addresses, it sets the Hop Limit field to 32.

20.1. Relaying a Client Message or a Relay-forward Message

A relay agent relays both messages from clients and Relay-forward messages from other relay agents. When a relay agent receives a valid message to be relayed, it constructs a new Relay-forward message. The relay agent copies the source address from the header of the IP datagram in which the message was received to the peer-address field of the Relay-forward message. The relay agent copies the received DHCP message (excluding any IP or UDP headers) into a Relay Message option in the new message. The relay agent adds to the Relay-forward message any other options it is configured to include.

20.1.1. Relaying a Message from a Client

If the relay agent received the message to be relayed from a client, the relay agent places a global or site-scoped address with a prefix assigned to the link on which the client should be assigned an address in the link-address field. This address will be used by the server to determine the link from which the client should be assigned an address and other configuration information. The hop-count in the Relay-forward message is set to 0.

If the relay agent cannot use the address in the link-address field to identify the interface through which the response to the client will be relayed, the relay agent MUST include an Interface-id option (see section 22.18) in the Relay-forward message. The server will include the Interface-id option in its Relay-reply message. The relay agent fills in the link-address field as described in the previous paragraph regardless of whether the relay agent includes an Interface-id option in the Relay-forward message.

20.1.2. Relaying a Message from a Relay Agent

If the message received by the relay agent is a Relay-forward message and the hop-count in the message is greater than or equal to HOP_COUNT_LIMIT, the relay agent discards the received message.

The relay agent copies the source address from the IP datagram in which the message was received from the client into the peer-address field in the Relay-forward message and sets the hop-count field to the value of the hop-count field in the received message incremented by 1.

If the source address from the IP datagram header of the received message is a global or site-local address (and the device on which the relay agent is running belongs to only one site), the relay agent sets the link-address field to 0; otherwise the relay agent sets the link-address field to a global or site-local address assigned to the interface on which the message was received, or includes an Interface-ID option to identify the interface on which the message was received.

20.2. Relaying a Relay-reply Message

The relay agent processes any options included in the Relay-reply message in addition to the Relay Message option, and then discards those options.

The relay agent extracts the message from the Relay Message option and relays it to the address contained in the peer-address field of the Relay-reply message.

If the Relay-reply message includes an Interface-id option, the relay agent relays the message from the server to the client on the link identified by the Interface-id option. Otherwise, if the link-address field is not set to zero, the relay agent relays the message on the link identified by the link-address field.

20.3. Construction of Relay-reply Messages

A server uses a Relay-reply message to return a response to a client if the original message from the client was relayed to the server in a Relay-forward message or to send a Reconfigure message to a client if the server does not have an address it can use to send the message directly to the client.

A response to the client MUST be relayed through the same relay agents as the original client message. The server causes this to happen by creating a Relay-reply message that includes a Relay Message option containing the message for the next relay agent in the return path to the client. The contained Relay-reply message contains another Relay Message option to be sent to the next relay agent, and so on. The server must record the contents of the peer-address fields in the received message so it can construct the appropriate Relay-reply message carrying the response from the server.

For example, if client C sent a message that was relayed by relay agent A to relay agent B and then to the server, the server would send the following Relay-Reply message to relay agent B:

          
   msg-type:       RELAY-REPLY
   hop-count:      1
   link-address:   0
   peer-address:   A
   Relay Message option, containing:
     msg-type:     RELAY-REPLY
     hop-count:    0
     link-address: address from link to which C is attached
     peer-address: C
     Relay Message option: <response from server>
            
          

            
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          DHCP realm                           |
   |                      (variable length)                        |
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            key ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           HMAC-MD5                            |
   |                          (128 bits)                           |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              
            

            
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |                 Value (128 bits)              |
   +-+-+-+-+-+-+-+-+                                               |
   .                                                               .
   .                                                               .
   .                                               +-+-+-+-+-+-+-+-+
   |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
            

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          option-code          |           option-len          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          option-data                          |
   |                      (option-len octets)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          
          

          
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        OPTION_CLIENTID        |          option-len           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  .                                                               .
  .                              DUID                             .
  .                        (variable length)                      .
  .                                                               .
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        OPTION_SERVERID        |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                              DUID                             .
   .                        (variable length)                      .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OPTION_IA_NA         |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        IAID (4 octets)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              T1                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              T2                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                         IA_NA-options                         .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OPTION_IA_TA         |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        IAID (4 octets)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                         IA_TA-options                         .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OPTION_IAADDR        |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                         IPv6 address                          |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      preferred-lifetime                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        valid-lifetime                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                        IAaddr-options                         .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           OPTION_ORO          |           option-len          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    requested-option-code-1    |    requested-option-code-2    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       OPTION_PREFERENCE       |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  pref-value   |
   +-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_ELAPSED_TIME      |           option-len          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          elapsed-time         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        OPTION_RELAY_MSG       |           option-len          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                       DHCP-relay-message                      .
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OPTION_AUTH          |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   protocol    |   algorithm   |      RDM      |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |          replay detection (64 bits)           +-+-+-+-+-+-+-+-+
   |                                               |   auth-info   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   .                   authentication information                  .
   .                       (variable length)                       .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OPTION_UNICAST       |        option-len             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                       server-address                          |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       OPTION_STATUS_CODE      |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          status-code          |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   .                                                               .
   .                        status-message                         .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_RAPID_COMMIT      |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       OPTION_USER_CLASS       |          option-len           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                          user-class-data                      .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
   |        user-class-len         |          opaque-data          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_VENDOR_CLASS      |           option-len          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       enterprise-number                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                       vendor-class-data                       .
   .                             . . .                             .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
   |       vendor-class-len        |          opaque-data          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_VENDOR_OPTS       |           option-len          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       enterprise-number                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                          option-data                          .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          opt-code             |             option-len        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                          option-data                          .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_INTERFACE_ID      |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                         interface-id                          .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_RECONF_MSG        |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    msg-type   |
   +-+-+-+-+-+-+-+-+
            
          

          
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     OPTION_RECONF_ACCEPT      |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            
          

        Client Server IA_NA  Option Pref  Time Relay Auth. Server
          ID     ID   IA_TA  Request            Msg.       Unica.
Solicit   *             *      *           *           *
Advert.   *      *      *            *                 *
Request   *      *      *      *           *           *
Confirm   *             *      *           *           *
Renew     *      *      *      *           *           *
Rebind    *             *      *           *           *
Decline   *      *      *      *           *           *
Release   *      *      *      *           *           *
Reply     *      *      *            *                 *     *
Reconf.   *      *             *                       *
Inform.   * (see note)         *           *           *
R-forw.                                          *     *
R-repl.                                          *     *

         Status  Rap. User  Vendor Vendor Inter. Recon. Recon.
          Code  Comm. Class Class  Spec.    ID    Msg.  Accept
Solicit           *     *     *      *                    *
Advert.    *            *     *      *                    *
Request                 *     *      *                    *
Confirm                 *     *      *
Renew                   *     *      *                    *
Rebind                  *     *      *                    *
Decline                 *     *      *
Release                 *     *      *
Reply      *      *     *     *      *                    *
Reconf.                                            *
Inform.                 *     *      *                    *
R-forw.                 *     *      *      *
R-repl.                 *     *      *      *

             Option  IA_NA/ IAADDR Relay  Relay
             Field   IA_TA         Forw.  Reply
Client ID      *
Server ID      *
IA_NA/IA_TA    *
IAADDR                 *
ORO            *
Preference     *
Elapsed Time   *
Relay Message                        *      *
Authentic.     *
Server Uni.    *
Status Code    *       *       *
Rapid Comm.    *
User Class     *
Vendor Class   *
Vendor Info.   *
Interf. ID                           *      *
Reconf. MSG.   *
Reconf. Accept *