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 7: 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.

11. 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 or delegated prefixes. Each IA consists of an IAID and associated configuration information.

The IAID uniquely identifies the IA and must be chosen to be unique among the IAIDs for that IA type 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. If the client uses only one IAID, it can use a well-known value, e.g., zero.

If the client wishes to obtain a distinctly new address or prefix and deprecate the existing one, the client sends a Release message to the server for the IAs using the original IAID. Then the client creates a new IAID, to be used in future Request messages to obtain new IAs.

11.1. Identity Associations for Address Assignment

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 configuration information in an IA consists of one or more IPv6 addresses along with the times T1 and T2 for the IA. See Section 20.4 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 [RFC4862]. 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 [RFC4862].

11.2. Identity Associations for Prefix Delegation

An IA_PD is different from an IA for address assignment, in that it does not need to be associated with exactly one interface. One IA_PD can be associated with the requesting router, with a set of interfaces or with exactly one interface. A requesting router must create at least one distinct IA_PD. It may associate a distinct IA_PD with each of its downstream network interfaces and use that IA_PD to obtain a prefix for that interface from the delegating router.

The configuration information in an IA_PD consists of one or more IPv6 prefixes along with the times T1 and T2 for the IA_PD. See Section 20.21 for the representation of an IA_PD in a DHCP message.

12. Assignment to an IA

12.1. Selecting Addresses for Assignment to an IA_NA

A server selects addresses to be assigned to an IA_NA 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. According to [RFC6221], the server MUST ignore any link-address field whose value is zero. The link address field refers to the link-address field of the Relay-Forward message, and the link-address fields in any Relay-Forward messages that may be nested within the Relay-Forward message.

*

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, e.g. IA Address options that include the client's requests for specific addresses.

-

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 [RFC4291].

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 [RFC2526], from any subnet.

12.2. Assignment of Prefixes for IA_PD

The mechanism through which the delegating router selects prefix(es) for delegation is not specified in this document. Examples of ways in which the server might select prefix(es) for a client include: static assignment based on subscription to an ISP; dynamic assignment from a pool of available prefixes; selection based on an external authority such as a RADIUS server using the Framed-IPv6-Prefix option as described in [RFC3162].

12.3. Assignment of Temporary Addresses

A client may request the assignment of temporary addresses (see [RFC4941] for the definition of temporary addresses). DHCP handling of address assignment is no different for temporary addresses.

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 20.5). 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 lifetime of the assigned temporary address is set in the IA Address Option (see Section 20.6) with in the IA_TA option. It is RECOMMENDED to set short lifetimes, typically shorter than TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME (see Section 5, [RFC4941].

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

A DHCP server implementation MAY generate temporary addresses referring to the algorithm defined in Section 3.2.1, [RFC4941], with additional condition that the new address is not duplicated with any assigned addresses.

The server MAY update the DNS for a temporary address, as described in section 4 of [RFC4941].

On the clients, by default, temporary addresses are preferred in source address selection, according to Rule 7, [RFC6724]. However, this policy is overridable.

One of the most important properties of temporary address is unlinkability of different actions over time. So, it is NOT RECOMMENDED for a client to renew expired temporary addresses, though DHCP provides such possibility (see Section 20.5).

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 20.3) 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 20.12.

13.1. Rate Limiting

In order to avoid prolonged message bursts that may be caused by possible logic loops, a DHCP client MUST limit the rate of DHCP messages it transmits. One example is that a client obtains an address, but does not like the response; it reverts back to Solicit procedure, discovers the same (sole) server, requests an address and gets the same address as before (the server still has the lease that was requested just previously). This loops can repeat infinitely if there is not a quit/stop mechanism. Therefore, a client must not initiate transmissions too frequently.

A recommended method for implementing the rate limiting function is a token bucket, limiting the average rate of transmission to a certain number in a certain time. This method of bounding burstiness also guarantees that the long-term transmission rate will not exceed.

TRT

Transmission Rate Limit

The Transmission Rate Limit parameter (TRT) SHOULD be configurable. A possible default could be 20 packets in 20 seconds.

For a device that has multiple interfaces, the limit MUST be enforced on a per interface basis.

Rate limiting of forwarded DHCP messages and server-side messages are out of scope of this specification.

13.2. Choosing transmission time when T1 / T2 is 0

In certain cases, T1 and/or T2 time may be set to zero. Currently there are three such cases: 1. a client received an IA_NA option with a zero value; 2. a client received an IA_PD option with a zero value; 3. a client received an IA_TA option (which does not contain T1 and T2 fields and are not generally renewed). Additional cases may appear in the future. This is an indication that the transmission times are left at the client's discretion. They are not completely discretionary, though.

When T1 and/or T2 times are set to zero, the client MUST choose a time to avoid packet storms. In particular, it MUST NOT transmit immediately. If the client received multiple IA containers, it SHOULD pick renew and/or rebind transmission times so all IA containers are handled in one exchange, if possible. Client MUST choose the transmission times to not violate rate limiting restrictions, defined in Section 13.1.

14. Reliability of Client Initiated Message Exchanges

DHCP clients are responsible for reliable delivery of messages in the client-initiated message exchanges described in Section 17. 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 17.1.1.

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 MUST update an "elapsed-time" value within an Elapsed Time option (see Section 20.9) in the retransmitted message. In some cases, the client may also need to modify values in the IA Address or IA Prefix options, if a valid lifetime for any of the client's leases expires before retransmission. Thus, whenever this document refers to a "retransmission" of a client's message, it refers to both modifying the original message and sending this new message instance to the server. The retransmitting client MUST NOT resend the original message to the server, i.e. the client MUST update all options in the retransmitted message, for which such update is required by the protocol (e.g. Elapsed Time option).

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.

A client is not expected to listen for a response during the entire period between transmission of Solicit or Information-request messages.

15. Message Validation

Clients and servers might get messages that contain options 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 either to extract information from such a message if the information is of use to the recipient, or to ignore such message completely and just drop it.

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.

Clients, relay agents and servers MUST NOT discard messages that contain unknown options (or instances of vendor options with unknown enterprise-numbers). These should be ignored as if they weren't present.

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

A client or server MUST silently discard any received DHCP messages with an unknown message type.

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 message that meets 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 meets 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 message 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.

-

the Reconfigure Message option msg-type is not 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

Client's behavior is different depending on the purpose of the configuration.

16.1. Address Assignment

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 if the interface is a logical interface without direct link attachment or the client is certain that two interfaces are attached to the same link.

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 IPv6 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.

16.2. Prefix Delegation

Delegated prefixes are not associated with a particular interface in the same way as addresses are for address assignment, and mentioned above.

When a client (acting as requesting router) sends a DHCP message for the purpose of prefix delegation, it SHOULD be sent on the interface associated with the upstream router (ISP network). The upstream interface is typically determined by configuration. This rule applies even in the case where a separate IA_PD is used for each downstream interface.

When a requesting router sends a DHCP message directly to a delegating router using unicast (after receiving the Server Unicast option from that delegating router), the source address SHOULD be an address from the upstream interface and which is suitable for use by the delegating router in responding to the requesting router.

17. DHCP Configuration Exchanges

A client initiates a message exchange with a server or servers to acquire or update configuration information of interest.

A DHCP client that does not need to have a DHCP server assign it IP addresses or delegated prefixes, can obtain configuration information such as a list of available DNS servers [RFC3646] or NTP servers [RFC4075] through a single message and reply exchange with a DHCP server. To obtain configuration information the client first sends an Information-request message (see Section 17.1.6) to the All_DHCP_Relay_Agents_and_Servers multicast address. Servers respond with a Reply message containing the configuration information for the client (see Section 17.2.6).

To request the assignment of one or more IPv6 addresses or delegated prefixes, a client first locates a DHCP server and then requests the assignment of addresses and other configuration information from the server. The client does it by sending the Solicit message (see Section 17.1.1) to the All_DHCP_Relay_Agents_and_Servers multicast address and collecting Advertise messages from the servers which respond to the client's message and selects a server from which it wants to obtain configuration information. This process is referred to as server discovery or server solicitation. When the client has selected the server it sends a Request message to this server as described in the Section 17.1.2.

A client willing to perform the Solicit-Reply message exchange described in Section 17.1.1 includes a Rapid Commit option (see Section 20.14) in its Solicit message.

The client has many reasons to initiate the configuration exchange:

1.

as part of the operating system configuration/bootstrap process,

2.

when requested to do so by the application layer (through an operating system specific API),

3.

when required by Stateless Address Autoconfiguration (as defined in [RFC2462])

4.

or as required to extend the lifetime of address(es) and/or delegated prefix(es), using Renew and Rebind messages.

A server may initiate a message exchange with a client by sending a Reconfigure message to cause the client to send a Renew, Rebind or Information-request message to refresh its configuration information as soon as the Reconfigure message is received by the client.

The client is responsible for creating IAs and requesting that a server assign IPv6 addresses and/or delegated prefixes to the IAs. The client first creates the IAs and assigns IAIDs to them. The client then transmits a Solicit message containing the IA options describing the IAs. The client MUST NOT be using any of the addresses or delegated prefixes for which it tries to obtain the bindings by sending the Solicit message. In particular, if the client had some valid bindings and has chosen to start the server solicitation process to obtain the bindings from a different server, the client MUST stop using the addresses and delegated prefixes for the bindings it had obtained from the previous server, and which it is now trying to obtain from a new server.

If the client will accept a Reply message with committed leases assignments and other resources in response to the Solicit message, the client includes a Rapid Commit option (see Section 20.14) in the Solicit message.

Servers that can assign addresses or delegated prefixes to the IAs respond to the client with an Advertise message or Reply message if the client included a Rapid Commit option and the server is configured to accept it.

If the server responds with an Advertise message, the client initiates a configuration exchange as described in Section 17.1.2.

Requesting router and delegating router use the IA_PD Prefix option to exchange information about prefix(es) in much the same way as IA Address options are used for assigned addresses. Typically, a single DHCP session is used to exchange information about addresses and prefixes, i.e. IA_NA and IA_PD options are carried in the same message.

17.1. Client Behavior

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

A client uses Request, Renew, Rebind, Release and Decline messages during the normal life cycle of addresses and delegated prefixes. When a client detects it may have moved to a new link, it uses Confirm if it only has addresses and Rebind if it has delegated prefixes (and addresses). It uses Information-request messages when it needs configuration information but no addresses and no prefixes.

When a client requests multiple IA option types in a Solicit, Request, Renew, or Rebind, it is possible that the available server(s) may only be configured to offer a subset of them. When possible, the client SHOULD use the best configuration available and continue to request the additional IAs in subsequent messages ([RFC7550]). This allows the client to maintain a single session and state machine. In practice, especially in the case of handling IA_NA and IA_PD requests ([RFC7084]), this situation should be rare or a temporary operational error. Thus, it is more likely for the client to get all configuration if it continues, in each subsequent configuration exchange, to request all the configuration information it is programmed to try to obtain, including any stateful configuration options for which no results were returned in previous message exchanges.

Upon receipt of a Reconfigure message from the server, a client responds with a Renew, Rebind or an Information-request message as indicated by the Reconfigure Message option. The server sends IAs and/or other configuration information to the client in a Reply message.

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 20.12) 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. However, 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.

17.1.1. Creation and Transmission 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 leases.

The client MUST include an Elapsed Time option (see Section 20.9) to indicate how long the client has been trying to complete the current DHCP message exchange.

The client MAY include addresses in the IA_NA and IA_TA options as hints to the server about the addresses for which the client has a preference.

The client MAY include values in the IA Prefix option encapsulated within IA_PD option as hints for the delegated prefix and/or prefix length 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, IA_TA options to request the assignment of temporary addresses and IA_PD options to request prefix delegation. Either IA_NA, IA_TA or IA_PD options, or a combination of all, can be included in DHCP messages. In addition, multiple instances of any IA option type can be included.

The client MUST include an Option Request option (see Section 20.7) to request the SOL_MAX_RT option (see Section 20.23) and any other 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 20.20) if the client is willing to accept Reconfigure messages from the server.

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 [RFC4862]). 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

A client that wishes to use the Rapid Commit 2-message exchange includes a Rapid Commit option in its Solicit message. The client may receive a number of different replies from different servers. The client will make note of any Advertise messages that it receives. The client will discard any Reply messages that do not contain the Rapid Commit option.

Upon receipt of a valid Reply with the Rapid Commit option, the client processes the message as described in Section 17.1.10

At the end of the first RT period, if no suitable Reply messages are received, but the client has valid Advertise messages, then the client processes the Advertise as described in Section 17.1.9.

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 17.1.10, 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.

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 20.8). 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 17.1.2) 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.

17.1.2. Creation and Transmission of Request Messages

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

The client MUST include an Elapsed Time option (see Section 20.9) to indicate how long the client has been trying to complete the current DHCP message exchange.

The client MUST include an Option Request option (see Section 20.7) to request the SOL_MAX_RT option (see Section 20.23) and any other 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 20.20) 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.

17.1.3. Creation and Transmission of Confirm Messages

Whenever a client may have moved to a new link, the prefixes/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 (and has no stable storage or persisted DHCP state).

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 and the client does not have any delegated prefixes obtained from the DHCP server from which it has obtained the addresses, the client SHOULD 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.

If the client has any valid delegated prefixes obtained from the DHCP server from which it has obtained the addresses, the client initiates Rebind/Reply exchange as described in Section 17.1.12 instead of sending the Confirm message.

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 MUST include an Elapsed Time option (see Section 20.9) to indicate how long the client has been trying to complete the current DHCP message exchange.

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.

17.1.4. Creation and Transmission of Renew Messages

To extend the valid and preferred lifetimes for the leases assigned to the IAs and obtain new addresses or delegated prefixes for IAs, the client sends a Renew message to the server from which the leases were obtained, which includes IA options for the IAs whose lease lifetimes are to be extended. The client includes IA Address options within IA_NA and IA_TA options for the addresses assigned to the IAs. The client includes IA Prefix options within IA_PD options for the delegated prefixes assigned to the IAs. The server determines new lifetimes for the leases according to the administrative configuration of the server. The server may also add leases to the IAs. The server can remove leases from the IAs by returning IA Address options (for IA_NA and IA_TA) and IA Prefix options (for IA_PD) with preferred and valid lifetimes set to 0.

The server controls the time at which the client contacts the server to extend the lifetimes on assigned leases through the T1 and T2 parameters assigned to an IA. However, as the client Renews/Rebinds all IAs from the server at the same time, the client MUST select a T1 and T2 time from all IA options, which will guarantee that the client will send Renew/Rebind messages not later than at the T1/T2 times associated with any of the client's bindings (earliest T1/T2).

At time T1, the client initiates a Renew/Reply message exchange to extend the lifetimes on any leases in the IA.

If T1 or T2 had been set to 0 by the server (for an IA_NA or IA_PD) or there are no T1 or T2 times (for an IA_TA) in a previous Reply, the client may send a Renew or Rebind message, respectively, at the client's discretion. The client MUST follow the rules defined in Section 13.2.

If the client is sending a Renew message in response to a Reconfigure message from the server, the client copies IA options from the Reconfigure message into the Renew message.

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 MUST include a Server Identifier option in the Renew message, identifying the server with which the client most recently communicated.

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 an Elapsed Time option (see Section 20.9) to indicate how long the client has been trying to complete the current DHCP message exchange.

For IAs to which leases have been assigned, the client includes a corresponding IA option containing an IA Address option for each address assigned to the IA and IA Prefix option for each prefix assigned to the IA. The client MUST NOT include addresses and prefixes in any IA option that the client did not obtain from the server or that are no longer valid (that have a valid lifetime of 0).

The client MAY include an IA option for each binding it desires but has been unable to obtain. In this case, if the client includes the IA_PD option to request prefix delegation, the client MAY include the IA Prefix option encapsulated within the IA_PD option, with the IPv6 prefix field set to 0 and the "prefix-length" field set to the desired length of the prefix to be delegated. The server MAY use this value as a hint for the prefix length. The client SHOULD NOT include IA Prefix option with the IPv6 prefix field set to 0 unless it is supplying a hint for the prefix length.

When the client is responding to a Reconfigure, the client copies the Option Request option from the Reconfigure message into the Renew message. Otherwise, the client includes Option Request option (see Section 20.7) to request the SOL_MAX_RT option (see Section 20.23) and any other 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 earliest T2

The message exchange is terminated when earliest time T2 is reached. If the client is responding to a Reconfigure, the client ignores and discards the Reconfigure message. In this case, the client continues to operate as if Reconfigure message was not received, i.e. it uses T1/T2 times associated with the client's leases to determine when it should send Renew or Rebind to the server. If the terminated Renew exchange was not initiated as a result of receiving a Reconfigure message, the client begins a Rebind message exchange (see Section 17.1.5) when the earliest time T2 is reached.

17.1.5. Creation and Transmission of Rebind Messages

At time T2 (which will only be reached if the server to which the Renew message was sent starting at time T1 has not responded), the client initiates a Rebind/Reply message exchange with any available server.

The client constructs the Rebind message as described in Section 17.1.4 with the following differences:

-

The client sets the "msg-type" field to REBIND.

-

The client does not include the Server Identifier option in the Rebind message.

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 leases in all IAs have expired

If all leases for an IA have expired, the client may choose to include this IA in subsequent Rebind messages to indicate that the client is interested in assignment of the leases to this IA.

The message exchange is terminated when the valid lifetimes of all leases across all IAs have expired, at which time the client uses the Solicit message to locate a new DHCP server and sends a Request for the expired IAs to the new server. If the terminated Rebind exchange was initiated as a result of receiving a Reconfigure message, the client ignores and discards the Reconfigure message.

17.1.6. Creation and Transmission of Information-request Messages

The client uses an Information-request message to obtain configuration information without having addresses and/or delegated prefixes 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 an Elapsed Time option (see Section 20.9) to indicate how long the client has been trying to complete the current DHCP message exchange.

The client MUST include an Option Request option (see Section 20.7) to request the INF_MAX_RT option (see Section 20.24), the Information Refresh Time option [RFC4242], and any other 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.

When responding to a Reconfigure, the client includes a Server Identifier option with the identifier from the Reconfigure message to which the client is responding.

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

17.1.7. Creation and Transmission of Release Messages

To release one or more leases, 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 lease(s) in a Server Identifier option.

The client MUST include a Client Identifier option to identify itself to the server.

The client MUST include an Elapsed Time option (see Section 20.9) to indicate how long the client has been trying to complete the current DHCP message exchange.

The client includes options containing the IAs for the leases it is releasing in the "options" field. The leases to be released MUST be included in the IAs. Any leases 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 leases being released before the client begins the Release message exchange process. For an address, this means the address MUST have been removed from the interface. For a delegated prefix, this means the prefix MUST have been advertised with a Preferred Lifetime and a Valid Lifetime of zero in a Router Advertisement message as described in Section 5.5.3, (e) of [RFC4862] - also see L-13 in Section 4.3 of [RFC7084].

If leases 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 lease, each lease assigned to the IA will be reclaimed by the server when the valid lifetime of that lease expires.

17.1.8. 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 Decline message is not used in prefix delegation and thus the client MUST NOT include IA_PD options in the Decline message.

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 MUST include an Elapsed Time option (see Section 20.9) to indicate how long the client has been trying to complete the current DHCP message exchange.

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.

The client SHOULD NOT send a Release message for other bindings it may have received just because it sent a Decline message. The client SHOULD retain the non-conflicting bindings. The client SHOULD treat the failure to acquire a binding as a result of the conflict, to be equivalent to not having received the binding, insofar as it behaves when sending Renew and Rebind messages.

17.1.9. Receipt of Advertise Messages

The client MUST process SOL_MAX_RT and INF_MAX_RT options in an Advertise message, even if the message contains a Status Code option indicating a failure, and the Advertise message will be discarded by the client.

The client MUST ignore any Advertise message that contains no addresses (IAADDR options encapsulated in IA_NA or IA_TA options) and no delegated prefixes (IAPREFIX options encapsulated in IA_PD options) with the exception that the client:

-

MUST process an included SOL_MAX_RT option and

-

MUST process an included INF_MAX_RT option.

A client can display any associated status message(s) to the user or activity log.

The client ignoring this Advertise message MUST NOT restart the Solicit retransmission timer.

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.

-

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

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.

In practice, this means that the client will maintain independent per-IA state machines per each selected server.

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.

17.1.10. 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 top-level Status Code option if present.

If the client receives a Reply message with a status code of UnspecFail, the server is indicating that it was unable to process the client's 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 (see Section 13.1).

If the client receives a Reply message with a status code of 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.

Otherwise (no status code or another status code), the client processes the Reply as described below based on the original message for which the Reply was received.

The client MAY choose to report any status code or message from the Status Code option in the Reply message.

17.1.10.1. Reply for Solicit (with Rapid Commit), Request, Renew or Rebind

If the client receives a NotOnLink status from the server in response to a Solicit (with a Rapid Commit option) or a Request, the client can either re-issue the message without specifying any addresses or restart the DHCP server discovery process (see Section 17).

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, if appropriate for the IA type.

-

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

-

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

-

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

-

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

If the client can operate with the addresses and/or prefixes obtained from the server:

-

The client uses the addresses, delegated prefixes, and other information from any IAs that do not contain a Status Code option with the NoAddrsAvail or NoPrefixAvail status code. The client MAY include the IAs for which it received the NoAddrsAvail or NoPrefixAvail status code, with no addresses or prefixes, in subsequent Renew and Rebind messages sent to the server, to retry obtaining the addresses or prefixes for these IAs.

-

The client MUST perform duplicate address detection as per [RFC4862] Section 5.4, which does list some exceptions, on each of the received addresses in any IAs, on which it has not performed duplicate address detection during processing of any of the previous Reply messages from the server. The client performs the duplicate address detection before using the received addresses for the traffic. If any of the addresses are found to be in use on the link, the client sends a Decline message to the server for those addresses as described in Section 17.1.8.

-

For each IAADDR, which does not have any associated reachability information, in order avoid the problems described in [RFC4943], hosts MUST NOT assume that any addresses are reachable on-link as a result of receiving an IA_NA or IA_TA. Addresses obtained from IA_NA or IA_TA MUST NOT be used to form an implicit prefix with a length other than 128.

-

For each IA_PD the requesting router assigns a subnet from each of the delegated prefixes to each of the links to which the associated interfaces are attached, with the following exception: the requesting router MUST NOT assign any delegated prefixes or subnets from the delegated prefix(es) to the link through which it received the DHCP message from the delegating router (see [RFC6603] for exceptions).

When a requesting router subnets a delegated prefix, it must assign additional bits to the prefix to generate unique, longer prefixes. For example, if the requesting router in Figure 1 were delegated 2001:db8:0::/48, it might generate 2001:db8:0:1::/64 and 2001:db8:0:2::/64 for assignment to the two links in the subscriber network. If the requesting router were delegated 2001:db8:0::/48 and 2001:db8:5::/48, it might assign 2001:db8:0:1::/64 and 2001:db8:5:1::/64 to one of the links, and 2001:db8:0:2::/64 and 2001:db8:5:2::/64 for assignment to the other link.

If the requesting router assigns a delegated prefix to a link to which the router is attached, and begins to send router advertisements for the prefix on the link, the requesting router MUST set the valid lifetime in those advertisements to be no later than the valid lifetime specified in the IA_PD Prefix option. A requesting router MAY use the preferred lifetime specified in the IA_PD Prefix option.

Management of the specific configuration information is detailed in the definition of each option in Section 20.

When a client had received a configuration option in earlier Reply, then sent Renew, Rebind or Information-Request and that requested option does not appear in in the Reply any more, it MUST stop using the received configuration information. In other words, client should behave as if it never received this option at all and return to whatever default state regarding that configuration information was. If there is no viable way to stop using the information, the values received/configured from the option MAY persist if there are no other sources for that data and they have no external impact. For example, a client that previously received Client FQDN option and used it to set up its hostname is allowed to continue using it if there is no reasonable way for a host to unset its hostname and it has no external impact. As a counter example, a client that used to receive an NTP server address from the DHCP server and does not receive it any more, MUST stop using the configured NTP server IPv6 address. The client SHOULD be open to other sources of the same configuration information. This behavior does not apply to any IA containers, as their processing is described in detail in other parts of this document.

If the Reply message contains any IAs, but the client finds no usable addresses and/or delegated prefixes in any of these IAs, the client 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:

-

Sends a Request message if any of the IAs in the Reply message contains the NoBinding status code. The client places IA options in this message for only those IAs for which the server returned the NoBinding status code in the Reply message. The client continues to use other bindings for which the server did not return an error.

-

Sends a Renew/Rebind if any of the IAs are not in the Reply message, but in this case the client MUST limit the rate at which it sends these messages, to avoid the Renew/Rebind storm.

-

Otherwise accepts the information in the IA.

Whenever a client restarts the DHCP server discovery process or selects an alternate server, as described in Section 17.1.9, the client SHOULD stop using all the addresses and delegated prefixes for which it has bindings and try to obtain all required leases from the new server. This facilitates the client using a single state machine for all bindings.

17.1.10.2. Reply for Release and Decline

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.

17.1.10.3. Reply for Confirm

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 17. If the client receives any Reply messages that indicate a success status (explicit or implicit), the client can use the addresses in the IA and ignore any messages that indicate a NotOnLink status.

17.1.11. Receipt of Reconfigure Messages

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.

Upon receipt of a valid Reconfigure message, the client responds with either a Renew message, a Rebind message, or an Information-request message as indicated by the Reconfigure Message option (as defined in Section 20.19). The client ignores the transaction-id field in the received Reconfigure message. While the transaction is in progress, the client 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, Rebind 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.

17.1.12. Bindings Verification by Requesting Router

In some circumstances the requesting router may need verification that the delegating router still has a valid binding for the requesting router. Examples of times when a requesting router may ask for such verification include:

• The requesting router reboots.
• The requesting router's upstream link flaps.
• The requesting router is physically disconnected from a wired connection.

If such verification is needed the requesting router MUST initiate a Rebind/Reply message exchange as described in Section 17.1.5, with the exception that the retransmission parameters should be set as for the Confirm message, described in Section 17.1.3. The requesting router includes any IA_PDs, along with prefixes associated with those IA_PDs in its Rebind message.

17.2. Server Behavior

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

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

In most instances, the server will send a Reply in response to a Request, Confirm, Renew, Rebind, Decline and Information-request messages sent by a client. The server will also send a Reply in response to a Solicit with a Rapid Commit option, when the server is configured to respond with committed lease assignments.

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 [RFC2460]. 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.

The server MAY initiate a configuration exchange, by sending Reconfigure messages, to cause DHCP clients to obtain new addresses, prefixes 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.

When a client receives a Reconfigure message from the server, the client sends Renew, Rebind or Information-request message as indicated by the as indicated by the Reconfigure Message option (as defined in Section 20.19). The server sends IAs and/or other configuration information to the client in a Reply message. 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 client's message.

17.2.1. Receipt of Solicit Messages

The server determines the information about the client and its location as described in Section 12 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 does not include a Reconfigure Accept option (see Section 20.20) in the Solicit message, the servers discard the Solicit message.

If the server is permitted to respond to the client, the client has not included a Rapid Commit option in the Solicit message or the server has not been configured to respond with committed assignment of leases and other resources, the server sends an Advertise message to the client as described in Section 17.2.9.

If the client has included a Rapid Commit option in the Solicit message and the server has been configured to respond with committed assignments of leases and other resources, the server responds to the Solicit with a Reply message. The server MUST commit the assignment of any addresses and delegated prefixes or other configuration information before sending a Reply message to a client. In this case the server includes a Rapid Commit option in the Reply message to indicate that the Reply is in response to a Solicit message.

DISCUSSION:

• When using the Solicit-Reply message exchange, the server commits the assignment of any leases before sending the Reply message. The client can assume it has been assigned the leases 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 leases from one of the servers, while the leases from the other servers will be committed to the client but not used by the client.

The server includes a Reconfigure Accept option if the server is configured to use reconfigure mechanism (see Section 17.2.11). Sending this option back to the client may useful using server selection process.

The server produces the Reply message as though it had received a Request message, as described in Section 17.2.2. The server transmits the Reply message as described in Section 17.2.10.

17.2.2. 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 (or is not currently configured to send a unicast option to the client), 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.

The server examines all IAs in the message from the client.

For each IA_NA and IA_TA in the Request message the server checks if the prefixes on included IP addresses are appropriate for the link to which the client is connected. If any of the prefixes on the included IP addresses 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 does not send the NotOnLink status code but it cannot assign any IP addresses to an IA, the server MUST return the IA in the Reply message with no addresses in the IA and a Status Code option containing status code NoAddrsAvail.

For any IA_PD in the Request message, to which the server cannot assign any delegated prefixes, the server MUST return the IA_PD option in the Reply message with the Status Code option containing status code NoPrefixAvail.

The server MAY assign different addresses and/or delegated prefixes to an IA than included in the IA within the Request message sent by the client.

For all IAs to which the server can assign addresses or delegated prefixes, the server includes the IAs with addresses (for IA_NA and IA_TA), prefixes (for IA_PD) and other configuration parameters, and records the IA as a new client binding. The server MUST NOT include any addresses or delegated prefixes in the IA which the server does not assign to the client.

The T1/T2 times set in each applicable IA option for a Reply MUST be the same values across all IAs. The server MUST determine the T1/T2 times across all of the applicable client's bindings in the Reply. This facilitates the client being able to renew all of the bindings at the same time.

The server MAY include a Reconfigure Accept option. New server implementations are discouraged from sending Reconfigure Accept option, as it does not convey any additional information. If the reconfigure mechanism is supported, the server is supposed to send Authentication option with Reconfigure Key (see Section 19.4 for details). However, it is still allowed to send Reconfigure Accept option to maintain backward compatibility with servers that implement [RFC3315].

The server includes other options containing configuration information to be returned to the client as described in Section 17.2.

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 server sends a new Reply message with existing bindings, possibly with updated lifetimes. The server may update the bindings according to its local policies, but the server SHOULD generate the response again and not simply retransmit previously sent information, even if the transaction-id matches previous transmission. The server MUST NOT cache its responses.

17.2.3. 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.

17.2.4. 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 (or is not currently configured to send a unicast option to the client), 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.

For each IA in the Renew message from a client, the server 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 finds the client entry for the IA, the server sends back the IA to the client with new lifetimes and, if applicable, T1/T2 times. If the server is unable to extend the lifetimes of an address or delegated prefix in the IA, the server MAY choose not to include the IA Address or IA Prefix option for this address or delegated prefix. If the server chooses to include the IA Address or IA Prefix option for such an address or delegated prefix, the server SHOULD set T1 and T2 to the valid lifetime for the IA option unless the server also includes other addresses or delegated prefix which the server is able to extend.

The server may choose to change the list of addresses or delegated prefixes and the lifetimes in IAs that are returned to the client.

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

For each IA for which the server cannot find a client entry, the server has the following choices depending on the server's policy and configuration information:

-

If the server is configured to create new bindings as a result of processing Renew messages, the server SHOULD create a binding and return the IA with assigned addresses or delegated prefixes with lifetimes and, if applicable, T1/T2 times and other information requested by the client. If the client included the IA Prefix option within the IA_PD option with zero value in the "IPv6 prefix" field and non-zero value in the "prefix-length" field, the server MAY use the "prefix-length" value as a hint for the length of the prefixes to be assigned (see [I-D.ietf-dhc-dhcpv6-prefix-length-hint-issue] for further details on prefix length hints).

-

If the server is configured to create new bindings as a result of processing Renew messages, but the server will not assign any leases to an IA, the server returns the IA option containing a Status Code option with the NoAddrsAvail or NoPrefixAvail status code and a status message for a user.

-

If the server does not support creation of new bindings for the client sending a Renew message, or if this behavior is disabled according to the server's policy or configuration information, the server returns the IA option containing a Status Code option with the NoBinding status code and a status message for a user.

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 17.2.

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

The T1/T2 times set in each applicable IA option for a Reply MUST be the same values across all IAs. The server MUST determine the T1/T2 times across all of the applicable client's bindings in the Reply. This facilitates the client being able to renew all of the bindings at the same time.

17.2.5. 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 finds the client entry for the IA and the server determines that the addresses or delegated prefixes in the IA are appropriate for the link to which the client's interface is attached according to the server's explicit configuration information, the server SHOULD send back the IA to the client with new lifetimes and, if applicable, T1/T2 times. If the server is unable to extend the lifetimes of an address in the IA, the server MAY choose not to include the IA Address option for this address. If the server is unable to extend the lifetimes of a delegated prefix in the IA, the server MAY choose not to include the IA Prefix option for this prefix.

If the server finds that the client entry for the IA and any of the addresses or delegated prefixes are no longer appropriate for the link to which the client's interface is attached according to the server's explicit configuration information, the server returns the address or delegated prefix to the client with lifetimes of 0.

If the server cannot find a client entry for the IA, the server checks if the IA contains addresses (for IA_NA and IA_TA) or delegated prefixes (for IA_PD). The server checks if the addresses and delegated prefixes are appropriate for the link to which the client's interface is attached according to the server's explicit configuration information. For any address which is not appropriate for the link to which the client's interface is attached, the server MAY include the IA Address option with the lifetimes of 0. For any delegated prefix which is not appropriate for the link to which the client's interface is attached, the server MAY include the IA Prefix option with the lifetimes of 0. The Reply with lifetimes of 0 constitutes an explicit notification to the client that the specific addresses and delegated prefixes are no longer valid and MUST NOT be used by the client. If the server chooses to not include any IAs containing IA Address or IA Prefix options with lifetimes of 0 and the server does not include any other IAs with leases and/or status codes, the server does not send a Reply message. In this situation the server silently discards the Rebind message.

Otherwise, for each IA for which the server cannot find a client entry, the server has the following choices depending on the server's policy and configuration information:

-

If the server is configured to create new bindings as a result of processing Rebind messages (also see the note about the Rapid Commit option below), the server SHOULD create a binding and return the IA with allocated leases with lifetimes and, if applicable, T1/T2 times and other information requested by the client. The server MUST NOT return any addresses or delegated prefixes in the IA which the server does not assign to the client.

-

If the server is configured to create new bindings as a result of processing Rebind messages, but the server will not assign any leases to an IA, the server returns the IA option containing a Status Code option with the NoAddrsAvail or NoPrefixAvail status code and a status message for a user.

-

If the server does not support creation of new bindings for the client sending a Rebind message, or if this behavior is disabled according to the server's policy or configuration information, the server returns the IA option containing a Status Code option with the NoBinding status code and a status message for a user.

When the server creates new bindings for the IA, it is possible that other servers also create bindings as a result of receiving the same Rebind message. This is the same issue as in the Discussion under "Rapid Commit Option"; see Section 20.14. Therefore, the server SHOULD only create new bindings during processing of a Rebind message if the server is configured to respond with a Reply message to a Solicit message containing the Rapid Commit option.

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 17.2.

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

The T1/T2 times set in each applicable IA option for a Reply MUST be the same values across all IAs. The server MUST determine the T1/T2 times across all of the applicable client's bindings in the Reply. This facilitates the client being able to renew all of the bindings at the same time.

17.2.6. 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 leases. 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 17.2. The server MAY include additional options that were not requested by the client in the Information-request message.

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.

17.2.7. Receipt of Release Messages

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

When the server receives a Release message via unicast from a client to which the server has not sent a unicast option (or is not currently configured to send a unicast option to the client), 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 leases in the IAs for validity. If the IAs in the message are in a binding for the client, and the leases in the IAs have been assigned by the server to those IAs, the server deletes the leases from the IAs and makes the leases available for assignment to other clients. The server ignores leases not assigned to the IA, although it may choose to log an error.

After all the leases 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 leases and IAs after the lifetimes on the leases have expired to allow the server to reassign the previously assigned leases to a client.

17.2.8. 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 (or is not currently configured to send a unicast option to the client), 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 by setting the "msg-type" field to REPLY, and copying the transaction ID from the Decline message into the transaction-id field. The client 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 Decline message and includes a Status Code option with the value NoBinding in the IA option. No other options are included in the IA option.

17.2.9. 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 option 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 indicate it supports Reconfigure mechanism. This information may be used by the client during server selection process.

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. The server MUST include options in the Advertise message containing configuration parameters for all of the options identified in the Option Request option in the Solicit message that the server has been configured to return to the client. If the Option Request option includes a container option the server MUST include all the options that are eligible to be encapsulated in the container. The Option Request option MAY be used to signal support for a feature even when that option is encapsulated as in the case of the Prefix Excluded option [RFC6603]. In this case, special processing is required by the server. 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 [RFC2460].

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 and/or delegated prefixes that would be assigned to IAs contained in the Solicit message from the client. If the client has included addresses in the IA in the Solicit message, the server MAY use those addresses as hints about the addresses that the client would like to receive. If the client has included IA Prefix option in the IA_PD, the server MAY use the prefix contained in the IPv6 prefix field and/or the prefix length contained in the "prefix-length" field as a hints about the prefixes the client would like to receive. If the server is not going to assign an address or delegated prefix received as a hint in the Solicit message, the server MUST NOT include this address or delegated prefix in the Advertise message

If the server will not assign any addresses to an IA (IA_NA or IA_IA) in subsequent Request from the client, the server MUST include the IA in the Advertise message with no addresses in the IA and a Status Code option encapsulated in the IA containing status code NoAddrsAvail.

If the server will not assign any prefixes to an IA_PD in subsequent Request from the client, the server MUST include the IA_PD in the Advertise message with no prefixes in the IA and a Status Code option encapsulated in the IA_PD containing status code NoPrefixAvail.

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.

17.2.10. 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 20.10). 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.

17.2.11. 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 to identify each IA that is to be reconfigured on the client. The IA options included by the server MUST NOT contain any options.

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 20.19) to select whether the client responds with a Renew message, a Rebind 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 18.3) 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, a Rebind/Reply, or Information-request/Reply message exchange with the server. The server interprets the receipt of a Renew, a Rebind, or Information-request message (whichever was specified in the original Reconfigure message) from the client as satisfying the Reconfigure message request.

If the server does not receive a Renew, Rebind, 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 6.5.

18. 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.

If the relay agent receives a message other than Relay-forward and Relay-reply and the relay agent does not recognize its message type, it MUST forward them as described in Section 18.1.1.

18.1. 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 (for a definition of a valid message, see Section 4.1 of [RFC7283]) 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.

[RFC6221] defines a Lightweight DHCPv6 Relay Agent (LDRA) that allows Relay Agent Information to be inserted by an access node that performs a link- layer bridging (i.e., non-routing) function.

18.1.1. 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, ULA [RFC4193] 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. If not addresses of other scopes are available the relay agent may fill in the link-address field with a link-local address from the interface the original message was received on. That is not recommended as it requires additional information to be provided in the server configuration. See Section 3.2 of [I-D.ietf-dhc-topo-conf] for detailed discussion.

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 20.18) 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.

18.1.2. 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 relay agent 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-scoped 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-scoped 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.

18.1.3. Relay Agent Behavior with Prefix Delegation

A relay agent forwards messages containing Prefix Delegation options in the same way as described earlier in this section.

If a delegating router communicates with a requesting router through a relay agent, the delegating router may need a protocol or other out-of-band communication to configure routing information for delegated prefixes on any router through which the requesting router may forward traffic.

18.2. 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. Relay agents MUST NOT modify the 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.

If the relay agent receives a Relay-reply message, it MUST process the message as defined above, regardless of the type of message encapsulated in the Relay Message option.

18.3. 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>

          

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   |                      (option-len octets)                      |
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  .                                                               .
  .                              DUID                             .
  .                        (variable length)                      .
  .                                                               .
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   .                                                               .
   .                              DUID                             .
   .                        (variable length)                      .
   .                                                               .
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   |                                                               |
   .                         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               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          

    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_PD          |         option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         IAID (4 octets)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              T1                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              T2                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                          IA_PD-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_IAPREFIX        |         option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      preferred-lifetime                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        valid-lifetime                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | prefix-length |                                               |
   +-+-+-+-+-+-+-+-+          IPv6 prefix                          |
   |                           (16 octets)                         |
   |                                                               |
   |                                                               |
   |                                                               |
   |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               .
   +-+-+-+-+-+-+-+-+                                               .
   .                       IAprefix-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-code          |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       SOL_MAX_RT 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-code          |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       INF_MAX_RT value                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          

        Client Server IA_NA/                  Elap. Relay Auth. Server
          ID     ID   IA_TA IA_PD  ORO   Pref Time   Msg.       Unicast
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. SOL_MAX_RT
         Code  Comm. Class Class  Spec.    ID    Msg.  Accept INF_MAX_RT
Solicit          *     *     *      *                    *
Advert.   *            *     *      *                    *        *
Request                *     *      *                    *
Confirm                *     *      *
Renew                  *     *      *                    *
Rebind                 *     *      *                    *
Decline                *     *      *
Release                *     *      *
Reply     *      *     *     *      *                    *        *
Reconf.                                           *
Inform.                *     *      *                    *
R-forw.                *     *      *      *
R-repl.                *     *      *      *

             Option  IA_NA/                        Relay  Relay
             Field   IA_TA  IAADDR IA_PD  IAPREFIX Forw.  Reply
Client ID      *
Server ID      *
IA_NA/IA_TA    *
IAADDR                 *
IA_PD          *
IAPREFIX                             *
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 *
SOL_MAX_RT     *
INF_MAX_RT     *