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:

The client reboots.
The client is physically connected to a wired connection.
The client returns from sleep mode.
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 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 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 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.