18. DHCP Client-Initiated Configuration Exchange

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

18.1. Client Behavior

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

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

   DISCUSSION:

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

18.1.1. Creation and Transmission of Request Messages

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

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

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

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

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

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

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

      IRT   REQ_TIMEOUT

      MRT   REQ_MAX_RT

      MRC   REQ_MAX_RC

      MRD   0

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

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

   -  Initiate the server discovery process described in section 17.

   -  Terminate the configuration process and report failure.

18.1.2. Creation and Transmission of Confirm Messages

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

   o  The client reboots.

   o  The client is physically connected to a wired connection.

   o  The client returns from sleep mode.

   o  The client using a wireless technology changes access points.

   In any situation when a client may have moved to a new link, the
   client MUST initiate a Confirm/Reply message exchange.  The client
   includes any IAs assigned to the interface that may have moved to a
   new link, along with the addresses associated with those IAs, in its

   Confirm message.  Any responding servers will indicate whether those
   addresses are appropriate for the link to which the client is
   attached with the status in the Reply message it returns to the
   client.

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

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

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

      IRT   CNF_TIMEOUT

      MRT   CNF_MAX_RT

      MRC   0

      MRD   CNF_MAX_RD

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

18.1.3. Creation and Transmission of Renew Messages

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

   new addresses to the IA.  The server may remove addresses from the IA
   by setting the preferred and valid lifetimes of those addresses to
   zero.

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

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

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

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

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

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

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

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

      IRT   REN_TIMEOUT

      MRT   REN_MAX_RT

      MRC   0

      MRD   Remaining time until T2

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

18.1.4. Creation and Transmission of Rebind Messages

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

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

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

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

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

      IRT   REB_TIMEOUT

      MRT   REB_MAX_RT

      MRC   0

      MRD   Remaining time until valid lifetimes of all addresses have
            expired

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

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

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

18.1.5. Creation and Transmission of Information-request Messages

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

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

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

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

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

      IRT   INF_TIMEOUT

      MRT   INF_MAX_RT

      MRC   0

      MRD   0

18.1.6. Creation and Transmission of Release Messages

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

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

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

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

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

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

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

      IRT   REL_TIMEOUT

      MRT   0

      MRC   REL_MAX_RC

      MRD   0

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

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

18.1.7. Creation and Transmission of Decline Messages

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

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

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

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

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

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

      IRT   DEC_TIMEOUT

      MRT   0

      MRC   DEC_MAX_RC

      MRD   0

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

18.1.8. Receipt of Reply Messages

   Upon the receipt of a valid Reply message in response to a Solicit
   (with a Rapid Commit option), Request, Confirm, Renew, Rebind or
   Information-request message, the client extracts the configuration

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

   The client SHOULD perform duplicate address detection [17] on each of
   the addresses in any IAs it receives in the Reply message before
   using that address for traffic.  If any of the addresses are found to
   be in use on the link, the client sends a Decline message to the
   server as described in section 18.1.7.

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

   -  Record T1 and T2 times.

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

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

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

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

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

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

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

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

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

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

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

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

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

   -  otherwise accepts the information in the IA

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

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

18.2. Server Behavior

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

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

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

18.2.1. Receipt of Request Messages

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

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

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

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

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

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

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

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

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

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

18.2.2. Receipt of Confirm Messages

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

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

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

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

18.2.3. Receipt of Renew Messages

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

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

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

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

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

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

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

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

18.2.4. Receipt of Rebind Messages

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

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

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

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

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

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

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

18.2.5. Receipt of Information-request Messages

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

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

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

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

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

18.2.6. Receipt of Release Messages

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

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

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

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

18.2.7. Receipt of Decline Messages

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

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

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

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

18.2.8. Transmission of Reply Messages

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

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

19. DHCP Server-Initiated Configuration Exchange

   A server initiates a configuration exchange to cause DHCP clients to
   obtain new addresses and other configuration information.  For
   example, an administrator may use a server-initiated configuration
   exchange when links in the DHCP domain are to be renumbered.  Other

   examples include changes in the location of directory servers,
   addition of new services such as printing, and availability of new
   software.

19.1. Server Behavior

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

19.1.1. Creation and Transmission of Reconfigure Messages

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

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

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

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

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

   A server sends each Reconfigure message to a single DHCP client,
   using an IPv6 unicast address of sufficient scope belonging to the
   DHCP client.  If the server does not have an address to which it can
   send the Reconfigure message directly to the client, the server uses
   a Relay-reply message (as described in section 20.3) to send the
   Reconfigure message to a relay agent that will relay the message to
   the client.  The server may obtain the address of the client (and the

   appropriate relay agent, if required) through the information the
   server has about clients that have been in contact with the server,
   or through some external agent.

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

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

19.1.2. Time Out and Retransmission of Reconfigure Messages

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

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

19.2. Receipt of Renew Messages

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

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

19.3. Receipt of Information-request Messages

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

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

19.4. Client Behavior

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

19.4.1. Receipt of Reconfigure Messages

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

   DISCUSSION:

      The Reconfigure message acts as a trigger that signals the client
      to complete a successful message exchange.  Once the client has
      received a Reconfigure, the client proceeds with the message
      exchange (retransmitting the Renew or Information-request message
      if necessary); the client ignores any additional Reconfigure
      messages until the exchange is complete.  Subsequent Reconfigure
      messages cause the client to initiate a new exchange.

      How does this mechanism work in the face of duplicated or
      retransmitted Reconfigure messages?  Duplicate messages will be
      ignored because the client will begin the exchange after the
      receipt of the first Reconfigure.  Retransmitted messages will
      either trigger the exchange (if the first Reconfigure was not
      received by the client) or will be ignored.  The server can
      discontinue retransmission of Reconfigure messages to the client
      once the server receives the Renew or Information-request message
      from the client.

      It might be possible for a duplicate or retransmitted Reconfigure
      to be sufficiently delayed (and delivered out of order) to arrive
      at the client after the exchange (initiated by the original
      Reconfigure) has been completed.  In this case, the client would
      initiate a redundant exchange.  The likelihood of delayed and out

      of order delivery is small enough to be ignored.  The consequence
      of the redundant exchange is inefficiency rather than incorrect
      operation.

19.4.2. Creation and Transmission of Renew Messages

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

19.4.3. Creation and Transmission of Information-request Messages

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

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

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

19.4.5. Receipt of Reply Messages

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

20. Relay Agent Behavior

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

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

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

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

20.1.1. Relaying a Message from a Client

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

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

20.1.2. Relaying a Message from a Relay Agent

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

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

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

20.2. Relaying a Relay-reply Message

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

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

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

20.3. Construction of Relay-reply Messages

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

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

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

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

   When sending a Reconfigure message to a client through a relay agent,
   the server creates a Relay-reply message that includes a Relay
   Message option containing the Reconfigure message for the next relay
   agent in the return path to the client.  The server sets the
   peer-address field in the Relay-reply message header to the address
   of the client, and sets the link-address field as required by the
   relay agent to relay the Reconfigure message to the client.  The
   server obtains the addresses of the client and the relay agent
   through prior interaction with the client or through some external
   mechanism.