RFC 3315
Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
Pages: 101
Obsoleted by: 8415
Updated by: 4361 5494 6221 6422 6644 7083 7227 7283 7550
Obsoleted by: 8415
Updated by: 4361 5494 6221 6422 6644 7083 7227 7283 7550
Part 2 of 5 – Pages 16 to 38
6. Client/Server Message Formats
All DHCP messages sent between clients and servers share an identical fixed format header and a variable format area for options. All values in the message header and in options are in network byte order.
Options are stored serially in the options field, with no padding
between the options. Options are byte-aligned but are not aligned in
any other way such as on 2 or 4 byte boundaries.
The following diagram illustrates the format of DHCP messages sent
between clients and servers:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type | transaction-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. options .
. (variable) .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type Identifies the DHCP message type; the
available message types are listed in
section 5.3.
transaction-id The transaction ID for this message exchange.
options Options carried in this message; options are
described in section 22.
7. Relay Agent/Server Message Formats
Relay agents exchange messages with servers to relay messages between
clients and servers that are not connected to the same link.
All values in the message header and in options are in network byte
order.
Options are stored serially in the options field, with no padding
between the options. Options are byte-aligned but are not aligned in
any other way such as on 2 or 4 byte boundaries.
There are two relay agent messages, which share the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type | hop-count | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| link-address |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| peer-address |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
. .
. options (variable number and length) .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following sections describe the use of the Relay Agent message
header.
7.1. Relay-forward Message
The following table defines the use of message fields in a Relay-
forward message.
msg-type RELAY-FORW
hop-count Number of relay agents that have relayed this
message.
link-address A global or site-local address that will be used by
the server to identify the link on which the client
is located.
peer-address The address of the client or relay agent from which
the message to be relayed was received.
options MUST include a "Relay Message option" (see
section 22.10); MAY include other options added by
the relay agent.
7.2. Relay-reply Message
The following table defines the use of message fields in a Relay-reply message. msg-type RELAY-REPL hop-count Copied from the Relay-forward message link-address Copied from the Relay-forward message peer-address Copied from the Relay-forward message options MUST include a "Relay Message option"; see section 22.10; MAY include other options8. Representation and Use of Domain Names
So that domain names may be encoded uniformly, a domain name or a list of domain names is encoded using the technique described in section 3.1 of RFC 1035 [10]. A domain name, or list of domain names, in DHCP MUST NOT be stored in compressed form, as described in section 4.1.4 of RFC 1035.9. DHCP Unique Identifier (DUID)
Each DHCP client and server has a DUID. DHCP servers use DUIDs to identify clients for the selection of configuration parameters and in the association of IAs with clients. DHCP clients use DUIDs to identify a server in messages where a server needs to be identified. See sections 22.2 and 22.3 for the representation of a DUID in a DHCP message. Clients and servers MUST treat DUIDs as opaque values and MUST only compare DUIDs for equality. Clients and servers MUST NOT in any other way interpret DUIDs. Clients and servers MUST NOT restrict DUIDs to the types defined in this document, as additional DUID types may be defined in the future. The DUID is carried in an option because it may be variable length and because it is not required in all DHCP messages. The DUID is designed to be unique across all DHCP clients and servers, and stable for any specific client or server - that is, the DUID used by a client or server SHOULD NOT change over time if at all possible; for example, a device's DUID should not change as a result of a change in the device's network hardware.
The motivation for having more than one type of DUID is that the DUID must be globally unique, and must also be easy to generate. The sort of globally-unique identifier that is easy to generate for any given device can differ quite widely. Also, some devices may not contain any persistent storage. Retaining a generated DUID in such a device is not possible, so the DUID scheme must accommodate such devices.9.1. DUID Contents
A DUID consists of a two-octet type code represented in network byte order, followed by a variable number of octets that make up the actual identifier. A DUID can be no more than 128 octets long (not including the type code). The following types are currently defined: 1 Link-layer address plus time 2 Vendor-assigned unique ID based on Enterprise Number 3 Link-layer address Formats for the variable field of the DUID for each of the above types are shown below.9.2. DUID Based on Link-layer Address Plus Time [DUID-LLT]
This type of DUID consists of a two octet type field containing the value 1, a two octet hardware type code, four octets containing a time value, followed by link-layer address of any one network interface that is connected to the DHCP device at the time that the DUID is generated. The time value is the time that the DUID is generated represented in seconds since midnight (UTC), January 1, 2000, modulo 2^32. The hardware type MUST be a valid hardware type assigned by the IANA as described in RFC 826 [14]. Both the time and the hardware type are stored in network byte order. The link-layer address is stored in canonical form, as described in RFC 2464 [2]. The following diagram illustrates the format of a DUID-LLT: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | hardware type (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . link-layer address (variable length) . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The choice of network interface can be completely arbitrary, as long as that interface provides a globally unique link-layer address for the link type, and the same DUID-LLT 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-LLT. Clients and servers using this type of DUID MUST store the DUID-LLT in stable storage, and MUST continue to use this DUID-LLT even if the network interface used to generate the DUID-LLT is removed. Clients and servers that do not have any stable storage MUST NOT use this type of DUID. Clients and servers that use this DUID SHOULD attempt to configure the time prior to generating the DUID, if that is possible, and MUST use some sort of time source (for example, a real-time clock) in generating the DUID, even if that time source could not be configured prior to generating the DUID. The use of a time source makes it unlikely that two identical DUID-LLTs will be generated if the network interface is removed from the client and another client then uses the same network interface to generate a DUID-LLT. A collision between two DUID-LLTs is very unlikely even if the clocks have not been configured prior to generating the DUID. This method of DUID generation is recommended for all general purpose computing devices such as desktop computers and laptop computers, and also for devices such as printers, routers, and so on, that contain some form of writable non-volatile storage. Despite our best efforts, it is possible that this algorithm for generating a DUID could result in a client identifier collision. A DHCP client that generates a DUID-LLT using this mechanism MUST provide an administrative interface that replaces the existing DUID with a newly-generated DUID-LLT.
9.3. DUID Assigned by Vendor Based on Enterprise Number [DUID-EN]
This form of DUID is assigned by the vendor to the device. It consists of the vendor's registered Private Enterprise Number as maintained by IANA [6] followed by a unique identifier assigned by the vendor. The following diagram summarizes the structure of a DUID-EN: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | enterprise-number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | enterprise-number (contd) | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | . identifier . . (variable length) . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The source of the identifier is left up to the vendor defining it, but each identifier part of each DUID-EN MUST be unique to the device that is using it, and MUST be assigned to the device at the time it is manufactured and stored in some form of non-volatile storage. The generated DUID SHOULD be recorded in non-erasable storage. The enterprise-number is the vendor's registered Private Enterprise Number as maintained by IANA [6]. The enterprise-number is stored as an unsigned 32 bit number. An example DUID of this type might look like this: +---+---+---+---+---+---+---+---+ | 0 | 2 | 0 | 0 | 0 | 9| 12|192| +---+---+---+---+---+---+---+---+ |132|221| 3 | 0 | 9 | 18| +---+---+---+---+---+---+ This example includes the two-octet type of 2, the Enterprise Number (9), followed by eight octets of identifier data (0x0CC084D303000912).9.4. DUID Based on Link-layer Address [DUID-LL]
This type of DUID consists of two octets containing the DUID type 3, a two octet network hardware type code, followed by the link-layer address of any one network interface that is permanently connected to the client or server device. For example, a host that has a network interface implemented in a chip that is unlikely to be removed and
used elsewhere could use a DUID-LL. The hardware type MUST be a valid hardware type assigned by the IANA, as described in RFC 826 [14]. The hardware type is stored in network byte order. The link-layer address is stored in canonical form, as described in RFC 2464 [2]. The following diagram illustrates the format of a DUID-LL: 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) . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The choice of network interface can be completely arbitrary, as long as that interface provides a unique link-layer address and is permanently attached to the device on which the DUID-LL is being generated. The same DUID-LL SHOULD be used in configuring all network interfaces connected to the device, regardless of which interface's link-layer address was used to generate the DUID. DUID-LL is recommended for devices that have a permanently-connected network interface with a link-layer address, and do not have nonvolatile, writable stable storage. DUID-LL MUST NOT be used by DHCP clients or servers that cannot tell whether or not a network interface is permanently attached to the device on which the DHCP client is running.10. Identity Association
An "identity-association" (IA) is a construct through which a server and a client can identify, group, and manage a set of related IPv6 addresses. Each IA consists of an IAID and associated configuration information. A client must associate at least one distinct IA with each of its network interfaces for which it is to request the assignment of IPv6 addresses from a DHCP server. The client uses the IAs assigned to an interface to obtain configuration information from a server for that interface. Each IA must be associated with exactly one interface. The IAID uniquely identifies the IA and must be chosen to be unique among the IAIDs on the client. The IAID is chosen by the client. For any given use of an IA by the client, the IAID for that IA MUST be consistent across restarts of the DHCP client. The client may maintain consistency either by storing the IAID in non-volatile
storage or by using an algorithm that will consistently produce the same IAID as long as the configuration of the client has not changed. There may be no way for a client to maintain consistency of the IAIDs if it does not have non-volatile storage and the client's hardware configuration changes. The configuration information in an IA consists of one or more IPv6 addresses along with the times T1 and T2 for the IA. See section 22.4 for the representation of an IA in a DHCP message. Each address in an IA has a preferred lifetime and a valid lifetime, as defined in RFC 2462 [17]. The lifetimes are transmitted from the DHCP server to the client in the IA option. The lifetimes apply to the use of IPv6 addresses, as described in section 5.5.4 of RFC 2462.11. Selecting Addresses for Assignment to an IA
A server selects addresses to be assigned to an IA according to the address assignment policies determined by the server administrator and the specific information the server determines about the client from some combination of the following sources: - The link to which the client is attached. The server determines the link as follows: * If the server receives the message directly from the client and the source address in the IP datagram in which the message was received is a link-local address, then the client is on the same link to which the interface over which the message was received is attached. * If the server receives the message from a forwarding relay agent, then the client is on the same link as the one to which the interface, identified by the link-address field in the message from the relay agent, is attached. * If the server receives the message directly from the client and the source address in the IP datagram in which the message was received is not a link-local address, then the client is on the link identified by the source address in the IP datagram (note that this situation can occur only if the server has enabled the use of unicast message delivery by the client and the client has sent a message for which unicast delivery is allowed). - The DUID supplied by the client. - Other information in options supplied by the client.
- Other information in options supplied by the relay agent. Any address assigned by a server that is based on an EUI-64 identifier MUST include an interface identifier with the "u" (universal/local) and "g" (individual/group) bits of the interface identifier set appropriately, as indicated in section 2.5.1 of RFC 2373 [5]. A server MUST NOT assign an address that is otherwise reserved for some other purpose. For example, a server MUST NOT assign reserved anycast addresses, as defined in RFC 2526, from any subnet.12. Management of Temporary Addresses
A client may request the assignment of temporary addresses (see RFC 3041 [12] for the definition of temporary addresses). DHCPv6 handling of address assignment is no different for temporary addresses. DHCPv6 says nothing about details of temporary addresses like lifetimes, how clients use temporary addresses, rules for generating successive temporary addresses, etc. Clients ask for temporary addresses and servers assign them. Temporary addresses are carried in the Identity Association for Temporary Addresses (IA_TA) option (see section 22.5). Each IA_TA option contains at most one temporary address for each of the prefixes on the link to which the client is attached. The IAID number space for the IA_TA option IAID number space is separate from the IA_NA option IAID number space. The server MAY update the DNS for a temporary address, as described in section 4 of RFC 3041.13. Transmission of Messages by a Client
Unless otherwise specified in this document, or in a document that describes how IPv6 is carried over a specific type of link (for link types that do not support multicast), a client sends DHCP messages to the All_DHCP_Relay_Agents_and_Servers. A client uses multicast to reach all servers or an individual server. An individual server is indicated by specifying that server's DUID in a Server Identifier option (see section 22.3) in the client's message (all servers will receive this message but only the indicated server will respond). All servers are indicated by not supplying this option.
A client may send some messages directly to a server using unicast, as described in section 22.12.14. Reliability of Client Initiated Message Exchanges
DHCP clients are responsible for reliable delivery of messages in the client-initiated message exchanges described in sections 17 and 18. If a DHCP client fails to receive an expected response from a server, the client must retransmit its message. This section describes the retransmission strategy to be used by clients in client-initiated message exchanges. Note that the procedure described in this section is slightly modified when used with the Solicit message. The modified procedure is described in section 17.1.2. The client begins the message exchange by transmitting a message to the server. The message exchange terminates when either the client successfully receives the appropriate response or responses from a server or servers, or when the message exchange is considered to have failed according to the retransmission mechanism described below. The client retransmission behavior is controlled and described by the following variables: RT Retransmission timeout IRT Initial retransmission time MRC Maximum retransmission count MRT Maximum retransmission time MRD Maximum retransmission duration RAND Randomization factor With each message transmission or retransmission, the client sets RT according to the rules given below. If RT expires before the message exchange terminates, the client recomputes RT and retransmits the message. Each of the computations of a new RT include a randomization factor (RAND), which is a random number chosen with a uniform distribution between -0.1 and +0.1. The randomization factor is included to minimize synchronization of messages transmitted by DHCP clients.
The algorithm for choosing a random number does not need to be
cryptographically sound. The algorithm SHOULD produce a different
sequence of random numbers from each invocation of the DHCP client.
RT for the first message transmission is based on IRT:
RT = IRT + RAND*IRT
RT for each subsequent message transmission is based on the previous
value of RT:
RT = 2*RTprev + RAND*RTprev
MRT specifies an upper bound on the value of RT (disregarding the
randomization added by the use of RAND). If MRT has a value of 0,
there is no upper limit on the value of RT. Otherwise:
if (RT > MRT)
RT = MRT + RAND*MRT
MRC specifies an upper bound on the number of times a client may
retransmit a message. Unless MRC is zero, the message exchange fails
once the client has transmitted the message MRC times.
MRD specifies an upper bound on the length of time a client may
retransmit a message. Unless MRD is zero, the message exchange fails
once MRD seconds have elapsed since the client first transmitted the
message.
If both MRC and MRD are non-zero, the message exchange fails whenever
either of the conditions specified in the previous two paragraphs are
met.
If both MRC and MRD are zero, the client continues to transmit the
message until it receives a response.
15. Message Validation
Clients and servers SHOULD discard any messages that contain options
that are not allowed to appear in the received message. For example,
an IA option is not allowed to appear in an Information-request
message. Clients and servers MAY choose to extract information from
such a message if the information is of use to the recipient.
A server MUST discard any Solicit, Confirm, Rebind or
Information-request messages it receives with a unicast destination
address.
Message validation based on DHCP authentication is discussed in section 21.4.2. If a server receives a message that contains options it should not contain (such as an Information-request message with an IA option), is missing options that it should contain, or is otherwise not valid, it MAY send a Reply (or Advertise as appropriate) with a Server Identifier option, a Client Identifier option if one was included in the message and a Status Code option with status UnSpecFail.15.1. Use of Transaction IDs
The "transaction-id" field holds a value used by clients and servers to synchronize server responses to client messages. A client SHOULD generate a random number that cannot easily be guessed or predicted to use as the transaction ID for each new message it sends. Note that if a client generates easily predictable transaction identifiers, it may become more vulnerable to certain kinds of attacks from off-path intruders. A client MUST leave the transaction ID unchanged in retransmissions of a message.15.2. Solicit Message
Clients MUST discard any received Solicit messages. Servers MUST discard any Solicit messages that do not include a Client Identifier option or that do include a Server Identifier option.15.3. Advertise Message
Clients MUST discard any received Advertise messages that meet any of the following conditions: - the message does not include a Server Identifier option. - the message does not include a Client Identifier option. - the contents of the Client Identifier option does not match the client's DUID. - the "transaction-id" field value does not match the value the client used in its Solicit message. Servers and relay agents MUST discard any received Advertise messages.
15.4. Request Message
Clients MUST discard any received Request messages. Servers MUST discard any received Request message that meet any of the following conditions: - the message does not include a Server Identifier option. - the contents of the Server Identifier option do not match the server's DUID. - the message does not include a Client Identifier option.15.5. Confirm Message
Clients MUST discard any received Confirm messages. Servers MUST discard any received Confirm messages that do not include a Client Identifier option or that do include a Server Identifier option.15.6. Renew Message
Clients MUST discard any received Renew messages. Servers MUST discard any received Renew message that meets any of the following conditions: - the message does not include a Server Identifier option. - the contents of the Server Identifier option does not match the server's identifier. - the message does not include a Client Identifier option.15.7. Rebind Message
Clients MUST discard any received Rebind messages. Servers MUST discard any received Rebind messages that do not include a Client Identifier option or that do include a Server Identifier option.
15.8. Decline Messages
Clients MUST discard any received Decline messages. Servers MUST discard any received Decline message that meets any of the following conditions: - the message does not include a Server Identifier option. - the contents of the Server Identifier option does not match the server's identifier. - the message does not include a Client Identifier option.15.9. Release Message
Clients MUST discard any received Release messages. Servers MUST discard any received Release message that meets any of the following conditions: - the message does not include a Server Identifier option. - the contents of the Server Identifier option does not match the server's identifier. - the message does not include a Client Identifier option.15.10. Reply Message
Clients MUST discard any received Reply message that meets any of the following conditions: - the message does not include a Server Identifier option. - the "transaction-id" field in the message does not match the value used in the original message. If the client included a Client Identifier option in the original message, the Reply message MUST include a Client Identifier option and the contents of the Client Identifier option MUST match the DUID of the client; OR, if the client did not include a Client Identifier option in the original message, the Reply message MUST NOT include a Client Identifier option. Servers and relay agents MUST discard any received Reply messages.
15.11. Reconfigure Message
Servers and relay agents MUST discard any received Reconfigure messages. Clients MUST discard any Reconfigure messages that meets any of the following conditions: - the message was not unicast to the client. - the message does not include a Server Identifier option. - the message does not include a Client Identifier option that contains the client's DUID. - the message does not contain a Reconfigure Message option and the msg-type must be a valid value. - the message includes any IA options and the msg-type in the Reconfigure Message option is INFORMATION-REQUEST. - the message does not include DHCP authentication: * the message does not contain an authentication option. * the message does not pass the authentication validation performed by the client.15.12. Information-request Message
Clients MUST discard any received Information-request messages. Servers MUST discard any received Information-request message that meets any of the following conditions: - The message includes a Server Identifier option and the DUID in the option does not match the server's DUID. - The message includes an IA option.15.13. Relay-forward Message
Clients MUST discard any received Relay-forward messages.15.14. Relay-reply Message
Clients and servers MUST discard any received Relay-reply messages.
16. Client Source Address and Interface Selection
When a client sends a DHCP message to the All_DHCP_Relay_Agents_and_Servers address, it SHOULD send the message through the interface for which configuration information is being requested. However, the client MAY send the message through another interface attached to the same link, if and only if the client is certain the two interfaces are attached to the same link. The client MUST use a link-local address assigned to the interface for which it is requesting configuration information as the source address in the header of the IP datagram. When a client sends a DHCP message directly to a server using unicast (after receiving the Server Unicast option from that server), the source address in the header of the IP datagram MUST be an address assigned to the interface for which the client is interested in obtaining configuration and which is suitable for use by the server in responding to the client.17. DHCP Server Solicitation
This section describes how a client locates servers that will assign addresses to IAs belonging to the client. The client is responsible for creating IAs and requesting that a server assign IPv6 addresses to the IA. The client first creates an IA and assigns it an IAID. The client then transmits a Solicit message containing an IA option describing the IA. Servers that can assign addresses to the IA respond to the client with an Advertise message. The client then initiates a configuration exchange as described in section 18. If the client will accept a Reply message with committed address assignments and other resources in response to the Solicit message, the client includes a Rapid Commit option (see section 22.14) in the Solicit message.17.1. Client Behavior
A client uses the Solicit message to discover DHCP servers configured to assign addresses or return other configuration parameters on the link to which the client is attached.17.1.1. Creation of Solicit Messages
The client sets the "msg-type" field to SOLICIT. The client generates a transaction ID and inserts this value in the "transaction-id" field.
The client MUST include a Client Identifier option to identify itself to the server. The client includes IA options for any IAs to which it wants the server to assign addresses. The client MAY include addresses in the IAs as a hint to the server about addresses for which the client has a preference. The client MUST NOT include any other options in the Solicit message, except as specifically allowed in the definition of individual options. The client uses IA_NA options to request the assignment of non- temporary addresses and uses IA_TA options to request the assignment of temporary addresses. Either IA_NA or IA_TA options, or a combination of both, can be included in DHCP messages. The client SHOULD include an Option Request option (see section 22.7) to indicate the options the client is interested in receiving. The client MAY additionally include instances of those options that are identified in the Option Request option, with data values as hints to the server about parameter values the client would like to have returned. The client includes a Reconfigure Accept option (see section 22.20) if the client is willing to accept Reconfigure messages from the server.17.1.2. Transmission of Solicit Messages
The first Solicit message from the client on the interface MUST be delayed by a random amount of time between 0 and SOL_MAX_DELAY. In the case of a Solicit message transmitted when DHCP is initiated by IPv6 Neighbor Discovery, the delay gives the amount of time to wait after IPv6 Neighbor Discovery causes the client to invoke the stateful address autoconfiguration protocol (see section 5.5.3 of RFC 2462). This random delay desynchronizes clients which start at the same time (for example, after a power outage). The client transmits the message according to section 14, using the following parameters: IRT SOL_TIMEOUT MRT SOL_MAX_RT MRC 0 MRD 0
If the client has included a Rapid Commit option in its Solicit message, the client terminates the waiting process as soon as a Reply message with a Rapid Commit option is received. If the client is waiting for an Advertise message, the mechanism in section 14 is modified as follows for use in the transmission of Solicit messages. The message exchange is not terminated by the receipt of an Advertise before the first RT has elapsed. Rather, the client collects Advertise messages until the first RT has elapsed. Also, the first RT MUST be selected to be strictly greater than IRT by choosing RAND to be strictly greater than 0. A client MUST collect Advertise messages for the first RT seconds, unless it receives an Advertise message with a preference value of 255. The preference value is carried in the Preference option (section 22.8). Any Advertise that does not include a Preference option is considered to have a preference value of 0. If the client receives an Advertise message that includes a Preference option with a preference value of 255, the client immediately begins a client- initiated message exchange (as described in section 18) by sending a Request message to the server from which the Advertise message was received. If the client receives an Advertise message that does not include a Preference option with a preference value of 255, the client continues to wait until the first RT elapses. If the first RT elapses and the client has received an Advertise message, the client SHOULD continue with a client-initiated message exchange by sending a Request message. If the client does not receive any Advertise messages before the first RT has elapsed, it begins the retransmission mechanism described in section 14. The client terminates the retransmission process as soon as it receives any Advertise message, and the client acts on the received Advertise message without waiting for any additional Advertise messages. A DHCP client SHOULD choose MRC and MRD to be 0. If the DHCP client is configured with either MRC or MRD set to a value other than 0, it MUST stop trying to configure the interface if the message exchange fails. After the DHCP client stops trying to configure the interface, it SHOULD restart the reconfiguration process after some external event, such as user input, system restart, or when the client is attached to a new link.
17.1.3. Receipt of Advertise Messages
The client MUST ignore any Advertise message that includes a Status Code option containing the value NoAddrsAvail, with the exception that the client MAY display the associated status message to the user. Upon receipt of one or more valid Advertise messages, the client selects one or more Advertise messages based upon the following criteria. - Those Advertise messages with the highest server preference value are preferred over all other Advertise messages. - Within a group of Advertise messages with the same server preference value, a client MAY select those servers whose Advertise messages advertise information of interest to the client. For example, the client may choose a server that returned an advertisement with configuration options of interest to the client. - The client MAY choose a less-preferred server if that server has a better set of advertised parameters, such as the available addresses advertised in IAs. Once a client has selected Advertise message(s), the client will typically store information about each server, such as server preference value, addresses advertised, when the advertisement was received, and so on. If the client needs to select an alternate server in the case that a chosen server does not respond, the client chooses the next server according to the criteria given above.17.1.4. Receipt of Reply Message
If the client includes a Rapid Commit option in the Solicit message, it will expect a Reply message that includes a Rapid Commit option in response. The client discards any Reply messages it receives that do not include a Rapid Commit option. If the client receives a valid Reply message that includes a Rapid Commit option, it processes the message as described in section 18.1.8. If it does not receive such a Reply message and does receive a valid Advertise message, the client processes the Advertise message as described in section 17.1.3.
If the client subsequently receives a valid Reply message that
includes a Rapid Commit option, it either:
processes the Reply message as described in section 18.1.8, and
discards any Reply messages received in response to the Request
message, or
processes any Reply messages received in response to the Request
message and discards the Reply message that includes the Rapid
Commit option.
17.2. Server Behavior
A server sends an Advertise message in response to valid Solicit
messages it receives to announce the availability of the server to
the client.
17.2.1. Receipt of Solicit Messages
The server determines the information about the client and its
location as described in section 11 and checks its administrative
policy about responding to the client. If the server is not
permitted to respond to the client, the server discards the Solicit
message. For example, if the administrative policy for the server is
that it may only respond to a client that is willing to accept a
Reconfigure message, if the client indicates with a Reconfigure
Accept option in the Solicit message that it will not accept a
Reconfigure message, the servers discard the Solicit message.
If the client has included a Rapid Commit option in the Solicit
message and the server has been configured to respond with committed
address assignments and other resources, the server responds to the
Solicit with a Reply message as described in section 17.2.3.
Otherwise, the server ignores the Rapid Commit option and processes
the remainder of the message as if no Rapid Commit option were
present.
17.2.2. Creation and Transmission of Advertise Messages
The server sets the "msg-type" field to ADVERTISE and copies the
contents of the transaction-id field from the Solicit message
received from the client to the Advertise message. The server
includes its server identifier in a Server Identifier option and
copies the Client Identifier from the Solicit message into the
Advertise message.
The server MAY add a Preference option to carry the preference value for the Advertise message. The server implementation SHOULD allow the setting of a server preference value by the administrator. The server preference value MUST default to zero unless otherwise configured by the server administrator. The server includes a Reconfigure Accept option if the server wants to require that the client accept Reconfigure messages. The server includes options the server will return to the client in a subsequent Reply message. The information in these options may be used by the client in the selection of a server if the client receives more than one Advertise message. If the client has included an Option Request option in the Solicit message, the server includes options in the Advertise message containing configuration parameters for all of the options identified in the Option Request option that the server has been configured to return to the client. The server MAY return additional options to the client if it has been configured to do so. The server must be aware of the recommendations on packet sizes and the use of fragmentation in section 5 of RFC 2460. If the Solicit message from the client included one or more IA options, the server MUST include IA options in the Advertise message containing any addresses that would be assigned to IAs contained in the Solicit message from the client. If the client has included addresses in the IAs in the Solicit message, the server uses those addresses as hints about the addresses the client would like to receive. If the server will not assign any addresses to any IAs in a subsequent Request from the client, the server MUST send an Advertise message to the client that includes only a Status Code option with code NoAddrsAvail and a status message for the user, a Server Identifier option with the server's DUID, and a Client Identifier option with the client's DUID. If the Solicit message was received directly by the server, the server unicasts the Advertise message directly to the client using the address in the source address field from the IP datagram in which the Solicit message was received. The Advertise message MUST be unicast on the link from which the Solicit message was received. If the Solicit message was received in a Relay-forward message, the server constructs a Relay-reply message with the Advertise message in the payload of a "relay-message" option. If the Relay-forward messages included an Interface-id option, the server copies that option to the Relay-reply message. The server unicasts the Relay-reply message directly to the relay agent using the address in
the source address field from the IP datagram in which the Relay- forward message was received.17.2.3. Creation and Transmission of Reply Messages
The server MUST commit the assignment of any addresses or other configuration information message before sending a Reply message to a client in response to a Solicit message. DISCUSSION: When using the Solicit-Reply message exchange, the server commits the assignment of any addresses before sending the Reply message. The client can assume it has been assigned the addresses in the Reply message and does not need to send a Request message for those addresses. Typically, servers that are configured to use the Solicit-Reply message exchange will be deployed so that only one server will respond to a Solicit message. If more than one server responds, the client will only use the addresses from one of the servers, while the addresses from the other servers will be committed to the client but not used by the client. The server includes a Rapid Commit option in the Reply message to indicate that the Reply is in response to a Solicit message. The server includes a Reconfigure Accept option if the server wants to require that the client accept Reconfigure messages. The server produces the Reply message as though it had received a Request message, as described in section 18.2.1. The server transmits the Reply message as described in section 18.2.8.
(page 38 continued on part 3)