Network Working Group R. Droms, Ed. Request for Comments: 3315 Cisco Category: Standards Track J. Bound Hewlett Packard B. Volz Ericsson T. Lemon Nominum C. Perkins Nokia Research Center M. Carney Sun Microsystems July 2003 Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract The Dynamic Host Configuration Protocol for IPv6 (DHCP) enables DHCP servers to pass configuration parameters such as IPv6 network addresses to IPv6 nodes. It offers the capability of automatic allocation of reusable network addresses and additional configuration flexibility. This protocol is a stateful counterpart to "IPv6 Stateless Address Autoconfiguration" (RFC 2462), and can be used separately or concurrently with the latter to obtain configuration parameters. Droms, et al. Standards Track [Page 1] RFC 3315 DHCP for IPv6 July 2003 Table of Contents 1. Introduction and Overview . . . . . . . . . . . . . . . . . . 5 1.1. Protocols and Addressing . . . . . . . . . . . . . . . 6 1.2. Client-server Exchanges Involving Two Messages . . . . 6 1.3. Client-server Exchanges Involving Four Messages. . . . 7 2. Requirements. . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Background. . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. IPv6 Terminology . . . . . . . . . . . . . . . . . . . 9 4.2. DHCP Terminology . . . . . . . . . . . . . . . . . . . 10 5. DHCP Constants. . . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Multicast Addresses. . . . . . . . . . . . . . . . . . 13 5.2. UDP Ports. . . . . . . . . . . . . . . . . . . . . . . 13 5.3. DHCP Message Types . . . . . . . . . . . . . . . . . . 13 5.4. Status Codes . . . . . . . . . . . . . . . . . . . . . 15 5.5. Transmission and Retransmission Parameters . . . . . . 16 5.6 Representation of time values and "Infinity" as a time value. . . . . . . . . . . . . . . . . . . . . . . . . 16 6. Client/Server Message Formats . . . . . . . . . . . . . . . . 16 7. Relay Agent/Server Message Formats. . . . . . . . . . . . . . 17 7.1. Relay-forward Message. . . . . . . . . . . . . . . . . 18 7.2. Relay-reply Message. . . . . . . . . . . . . . . . . . 19 8. Representation and Use of Domain Names. . . . . . . . . . . . 19 9. DHCP Unique Identifier (DUID) . . . . . . . . . . . . . . . . 19 9.1. DUID Contents. . . . . . . . . . . . . . . . . . . . . 20 9.2. DUID Based on Link-layer Address Plus Time [DUID-LLT]. 20 9.3. DUID Assigned by Vendor Based on Enterprise Number [DUID-EN]. . . . . . . . . . . . . . . . . . . . . . . 22 9.4. DUID Based on Link-layer Address [DUID-LL] . . . . . . 22 10. Identity Association. . . . . . . . . . . . . . . . . . . . . 23 11. Selecting Addresses for Assignment to an IA . . . . . . . . . 24 12. Management of Temporary Addresses . . . . . . . . . . . . . . 25 13. Transmission of Messages by a Client. . . . . . . . . . . . . 25 14. Reliability of Client Initiated Message Exchanges . . . . . . 26 15. Message Validation. . . . . . . . . . . . . . . . . . . . . . 27 15.1. Use of Transaction IDs . . . . . . . . . . . . . . . . 28 15.2. Solicit Message. . . . . . . . . . . . . . . . . . . . 28 15.3. Advertise Message. . . . . . . . . . . . . . . . . . . 28 15.4. Request Message. . . . . . . . . . . . . . . . . . . . 29 15.5. Confirm Message. . . . . . . . . . . . . . . . . . . . 29 15.6. Renew Message. . . . . . . . . . . . . . . . . . . . . 29 15.7. Rebind Message . . . . . . . . . . . . . . . . . . . . 29 15.8. Decline Messages . . . . . . . . . . . . . . . . . . . 30 15.9. Release Message. . . . . . . . . . . . . . . . . . . . 30 15.10. Reply Message. . . . . . . . . . . . . . . . . . . . . 30 15.11. Reconfigure Message. . . . . . . . . . . . . . . . . . 31 15.12. Information-request Message. . . . . . . . . . . . . . 31 Droms, et al. Standards Track [Page 2] RFC 3315 DHCP for IPv6 July 2003 15.13. Relay-forward Message. . . . . . . . . . . . . . . . . 31 15.14. Relay-reply Message. . . . . . . . . . . . . . . . . . 31 16. Client Source Address and Interface Selection . . . . . . . . 32 17. DHCP Server Solicitation. . . . . . . . . . . . . . . . . . . 32 17.1. Client Behavior. . . . . . . . . . . . . . . . . . . . 32 17.1.1. Creation of Solicit Messages . . . . . . . . . 32 17.1.2. Transmission of Solicit Messages . . . . . . . 33 17.1.3. Receipt of Advertise Messages. . . . . . . . . 35 17.1.4. Receipt of Reply Message . . . . . . . . . . . 35 17.2. Server Behavior. . . . . . . . . . . . . . . . . . . . 36 17.2.1. Receipt of Solicit Messages . . . . . . . . . 36 17.2.2. Creation and Transmission of Advertise Messages 36 17.2.3. Creation and Transmission of Reply Messages. . 38 18. DHCP Client-Initiated Configuration Exchange. . . . . . . . . 38 18.1. Client Behavior. . . . . . . . . . . . . . . . . . . . 39 18.1.1. Creation and Transmission of Request Messages. 39 18.1.2. Creation and Transmission of Confirm Messages. 40 18.1.3. Creation and Transmission of Renew Messages. . 41 18.1.4. Creation and Transmission of Rebind Messages . 43 18.1.5. Creation and Transmission of Information- request Messages . . .. . . . . . . . . . . . 44 18.1.6. Creation and Transmission of Release Messages. 44 18.1.7. Creation and Transmission of Decline Messages. 46 18.1.8. Receipt of Reply Messages. . . . . . . . . . . 46 18.2. Server Behavior. . . . . . . . . . . . . . . . . . . . 48 18.2.1. Receipt of Request Messages. . . . . . . . . . 49 18.2.2. Receipt of Confirm Messages. . . . . . . . . . 50 18.2.3. Receipt of Renew Messages. . . . . . . . . . . 51 18.2.4. Receipt of Rebind Messages . . . . . . . . . . 51 18.2.5. Receipt of Information-request Messages. . . . 52 18.2.6. Receipt of Release Messages. . . . . . . . . . 53 18.2.7. Receipt of Decline Messages. . . . . . . . . . 53 18.2.8. Transmission of Reply Messages . . . . . . . . 54 19. DHCP Server-Initiated Configuration Exchange. . . . . . . . . 54 19.1. Server Behavior. . . . . . . . . . . . . . . . . . . . 55 19.1.1. Creation and Transmission of Reconfigure Messages . . . . . . . . . . . . . . . . . . . 55 19.1.2. Time Out and Retransmission of Reconfigure Messages . . . . . . . . . . . . . . . . . . . 56 19.2. Receipt of Renew Messages. . . . . . . . . . . . . . . 56 19.3. Receipt of Information-request Messages. . . . . . . . 56 19.4. Client Behavior. . . . . . . . . . . . . . . . . . . . 57 19.4.1. Receipt of Reconfigure Messages. . . . . . . . 57 19.4.2. Creation and Transmission of Renew Messages. . 58 19.4.3. Creation and Transmission of Information- request Messages . . . . . . . . . . . . . . . 58 19.4.4. Time Out and Retransmission of Renew or Information-request Messages . . . . . . . . . 58 Droms, et al. Standards Track [Page 3] RFC 3315 DHCP for IPv6 July 2003 19.4.5. Receipt of Reply Messages. . . . . . . . . . . 58 20. Relay Agent Behavior. . . . . . . . . . . . . . . . . . . . . 58 20.1. Relaying a Client Message or a Relay-forward Message . 59 20.1.1. Relaying a Message from a Client . . . . . . . 59 20.1.2. Relaying a Message from a Relay Agent. . . . . 59 20.2. Relaying a Relay-reply Message . . . . . . . . . . . . 60 20.3. Construction of Relay-reply Messages . . . . . . . . . 60 21. Authentication of DHCP Messages . . . . . . . . . . . . . . . 61 21.1. Security of Messages Sent Between Servers and Relay Agents . . . . . . . . . . . . . . . . . . . . . . . 61 21.2. Summary of DHCP Authentication . . . . . . . . . . . . 63 21.3. Replay Detection . . . . . . . . . . . . . . . . . . . 63 21.4. Delayed Authentication Protocol. . . . . . . . . . . . 63 21.4.1. Use of the Authentication Option in the Delayed Authentication Protocol. . . . . . . . . . . . 64 21.4.2. Message Validation . . . . . . . . . . . . . . 65 21.4.3. Key Utilization . . . . . . . . . . . . . . . 65 21.4.4. Client Considerations for Delayed Authentication Protocol . . . . . . . . . . . . . . . . . . . 66 21.4.5. Server Considerations for Delayed Authentication Protocol . . . . . . . . . . . . . . . . . . . 67 21.5. Reconfigure Key Authentication Protocol. . . . . . . . 68 21.5.1. Use of the Authentication Option in the Reconfigure Key Authentication Protocol. . . . 69 21.5.2. Server considerations for Reconfigure Key protocol . . . . . . . . . . . . . . . . . . . 69 21.5.3. Client considerations for Reconfigure Key protocol . . . . . . . . . . . . . . . . . . . 70 22. DHCP Options. . . . . . . . . . . . . . . . . . . . . . . . . 70 22.1. Format of DHCP Options . . . . . . . . . . . . . . . . 71 22.2. Client Identifier Option . . . . . . . . . . . . . . . 71 22.3. Server Identifier Option . . . . . . . . . . . . . . . 72 22.4. Identity Association for Non-temporary Addresses Option 72 22.5. Identity Association for Temporary Addresses Option. . 75 22.6. IA Address Option. . . . . . . . . . . . . . . . . . . 76 22.7. Option Request Option. . . . . . . . . . . . . . . . . 78 22.8. Preference Option. . . . . . . . . . . . . . . . . . . 79 22.9. Elapsed Time Option. . . . . . . . . . . . . . . . . . 79 22.10. Relay Message Option . . . . . . . . . . . . . . . . . 80 22.11. Authentication Option. . . . . . . . . . . . . . . . . 81 22.12. Server Unicast Option. . . . . . . . . . . . . . . . . 82 22.13. Status Code Option . . . . . . . . . . . . . . . . . . 82 22.14. Rapid Commit Option. . . . . . . . . . . . . . . . . . 83 22.15. User Class Option. . . . . . . . . . . . . . . . . . . 84 22.16. Vendor Class Option. . . . . . . . . . . . . . . . . . 85 22.17. Vendor-specific Information Option . . . . . . . . . . 86 22.18. Interface-Id Option. . . . . . . . . . . . . . . . . . 87 22.19. Reconfigure Message Option . . . . . . . . . . . . . . 88 Droms, et al. Standards Track [Page 4] RFC 3315 DHCP for IPv6 July 2003 22.20. Reconfigure Accept Option. . . . . . . . . . . . . . . 89 23. Security Considerations . . . . . . . . . . . . . . . . . . . 89 24. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 91 24.1. Multicast Addresses. . . . . . . . . . . . . . . . . . 92 24.2. DHCP Message Types . . . . . . . . . . . . . . . . . . 93 24.3. DHCP Options . . . . . . . . . . . . . . . . . . . . . 94 24.4. Status Codes . . . . . . . . . . . . . . . . . . . . . 95 24.5. DUID . . . . . . . . . . . . . . . . . . . . . . . . . 95 25. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 95 26. References. . . . . . . . . . . . . . . . . . . . . . . . . . 96 26.1. Normative References . . . . . . . . . . . . . . . . . 96 26.2. Informative References . . . . . . . . . . . . . . . . 97 A. Appearance of Options in Message Types . . . . . . . . . . . . 98 B. Appearance of Options in the Options Field of DHCP Options . . 99 Chair's Address . . . . . . . . . . . . . . . . . . . . . . . . . 99 Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 100 Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 101 1. Introduction and Overview This document describes DHCP for IPv6 (DHCP), a client/server protocol that provides managed configuration of devices. DHCP can provide a device with addresses assigned by a DHCP server and other configuration information, which are carried in options. DHCP can be extended through the definition of new options to carry configuration information not specified in this document. DHCP is the "stateful address autoconfiguration protocol" and the "stateful autoconfiguration protocol" referred to in "IPv6 Stateless Address Autoconfiguration" [17]. The operational models and relevant configuration information for DHCPv4 [18][19] and DHCPv6 are sufficiently different that integration between the two services is not included in this document. If there is sufficient interest and demand, integration can be specified in a document that extends DHCPv6 to carry IPv4 addresses and configuration information. The remainder of this introduction summarizes DHCP, explaining the message exchange mechanisms and example message flows. The message flows in sections 1.2 and 1.3 are intended as illustrations of DHCP operation rather than an exhaustive list of all possible client-server interactions. Sections 17, 18, and 19 explain client and server operation in detail. Droms, et al. Standards Track [Page 5] RFC 3315 DHCP for IPv6 July 2003 1.1. Protocols and Addressing Clients and servers exchange DHCP messages using UDP [15]. The client uses a link-local address or addresses determined through other mechanisms for transmitting and receiving DHCP messages. DHCP servers receive messages from clients using a reserved, link-scoped multicast address. A DHCP client transmits most messages to this reserved multicast address, so that the client need not be configured with the address or addresses of DHCP servers. To allow a DHCP client to send a message to a DHCP server that is not attached to the same link, a DHCP relay agent on the client's link will relay messages between the client and server. The operation of the relay agent is transparent to the client and the discussion of message exchanges in the remainder of this section will omit the description of message relaying by relay agents. Once the client has determined the address of a server, it may under some circumstances send messages directly to the server using unicast. 1.2. Client-server Exchanges Involving Two Messages When a DHCP client does not need to have a DHCP server assign it IP addresses, the client can obtain configuration information such as a list of available DNS servers [20] or NTP servers [21] through a single message and reply exchanged with a DHCP server. To obtain configuration information the client first sends an Information-Request message to the All_DHCP_Relay_Agents_and_Servers multicast address. Servers respond with a Reply message containing the configuration information for the client. This message exchange assumes that the client requires only configuration information and does not require the assignment of any IPv6 addresses. When a server has IPv6 addresses and other configuration information committed to a client, the client and server may be able to complete the exchange using only two messages, instead of four messages as described in the next section. In this case, the client sends a Solicit message to the All_DHCP_Relay_Agents_and_Servers requesting the assignment of addresses and other configuration information. This message includes an indication that the client is willing to accept an immediate Reply message from the server. The server that is willing to commit the assignment of addresses to the client Droms, et al. Standards Track [Page 6] RFC 3315 DHCP for IPv6 July 2003 immediately responds with a Reply message. The configuration information and the addresses in the Reply message are then immediately available for use by the client. Each address assigned to the client has associated preferred and valid lifetimes specified by the server. To request an extension of the lifetimes assigned to an address, the client sends a Renew message to the server. The server sends a Reply message to the client with the new lifetimes, allowing the client to continue to use the address without interruption. 1.3. Client-server Exchanges Involving Four Messages To request the assignment of one or more IPv6 addresses, a client first locates a DHCP server and then requests the assignment of addresses and other configuration information from the server. The client sends a Solicit message to the All_DHCP_Relay_Agents_and_Servers address to find available DHCP servers. Any server that can meet the client's requirements responds with an Advertise message. The client then chooses one of the servers and sends a Request message to the server asking for confirmed assignment of addresses and other configuration information. The server responds with a Reply message that contains the confirmed addresses and configuration. As described in the previous section, the client sends a Renew message to the server to extend the lifetimes associated with its addresses, allowing the client to continue to use those addresses without interruption. 2. Requirements The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [1]. This document also makes use of internal conceptual variables to describe protocol behavior and external variables that an implementation must allow system administrators to change. The specific variable names, how their values change, and how their settings influence protocol behavior are provided to demonstrate protocol behavior. An implementation is not required to have them in the exact form described here, so long as its external behavior is consistent with that described in this document. Droms, et al. Standards Track [Page 7] RFC 3315 DHCP for IPv6 July 2003 3. Background The IPv6 Specification provides the base architecture and design of IPv6. Related work in IPv6 that would best serve an implementor to study includes the IPv6 Specification [3], the IPv6 Addressing Architecture [5], IPv6 Stateless Address Autoconfiguration [17], IPv6 Neighbor Discovery Processing [13], and Dynamic Updates to DNS [22]. These specifications enable DHCP to build upon the IPv6 work to provide both robust stateful autoconfiguration and autoregistration of DNS Host Names. The IPv6 Addressing Architecture specification [5] defines the address scope that can be used in an IPv6 implementation, and the various configuration architecture guidelines for network designers of the IPv6 address space. Two advantages of IPv6 are that support for multicast is required and nodes can create link-local addresses during initialization. The availability of these features means that a client can use its link-local address and a well-known multicast address to discover and communicate with DHCP servers or relay agents on its link. IPv6 Stateless Address Autoconfiguration [17] specifies procedures by which a node may autoconfigure addresses based on router advertisements [13], and the use of a valid lifetime to support renumbering of addresses on the Internet. In addition, the protocol interaction by which a node begins stateless or stateful autoconfiguration is specified. DHCP is one vehicle to perform stateful autoconfiguration. Compatibility with stateless address autoconfiguration is a design requirement of DHCP. IPv6 Neighbor Discovery [13] is the node discovery protocol in IPv6 which replaces and enhances functions of ARP [14]. To understand IPv6 and stateless address autoconfiguration, it is strongly recommended that implementors understand IPv6 Neighbor Discovery. Dynamic Updates to DNS [22] is a specification that supports the dynamic update of DNS records for both IPv4 and IPv6. DHCP can use the dynamic updates to DNS to integrate addresses and name space to not only support autoconfiguration, but also autoregistration in IPv6. 4. Terminology This sections defines terminology specific to IPv6 and DHCP used in this document. Droms, et al. Standards Track [Page 8] RFC 3315 DHCP for IPv6 July 2003 4.1. IPv6 Terminology IPv6 terminology relevant to this specification from the IPv6 Protocol [3], IPv6 Addressing Architecture [5], and IPv6 Stateless Address Autoconfiguration [17] is included below. address An IP layer identifier for an interface or a set of interfaces. host Any node that is not a router. IP Internet Protocol Version 6 (IPv6). The terms IPv4 and IPv6 are used only in contexts where it is necessary to avoid ambiguity. interface A node's attachment to a link. link A communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IP. Examples are Ethernet (simple or bridged); Token Ring; PPP links, X.25, Frame Relay, or ATM networks; and Internet (or higher) layer "tunnels", such as tunnels over IPv4 or IPv6 itself. link-layer identifier A link-layer identifier for an interface. Examples include IEEE 802 addresses for Ethernet or Token Ring network interfaces, and E.164 addresses for ISDN links. link-local address An IPv6 address having a link-only scope, indicated by having the prefix (FE80::/10), that can be used to reach neighboring nodes attached to the same link. Every interface has a link-local address. multicast address An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to all interfaces identified by that address. neighbor A node attached to the same link. Droms, et al. Standards Track [Page 9] RFC 3315 DHCP for IPv6 July 2003 node A device that implements IP. packet An IP header plus payload. prefix The initial bits of an address, or a set of IP addresses that share the same initial bits. prefix length The number of bits in a prefix. router A node that forwards IP packets not explicitly addressed to itself. unicast address An identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address. 4.2. DHCP Terminology Terminology specific to DHCP can be found below. appropriate to the link An address is "appropriate to the link" when the address is consistent with the DHCP server's knowledge of the network topology, prefix assignment and address assignment policies. binding A binding (or, client binding) is a group of server data records containing the information the server has about the addresses in an IA or configuration information explicitly assigned to the client. Configuration information that has been returned to a client through a policy - for example, the information returned to all clients on the same link - does not require a binding. A binding containing information about an IA is indexed by the tuple (where IA-type is the type of address in the IA; for example, temporary). A binding containing configuration information for a client is indexed by . Droms, et al. Standards Track [Page 10] RFC 3315 DHCP for IPv6 July 2003 configuration parameter An element of the configuration information set on the server and delivered to the client using DHCP. Such parameters may be used to carry information to be used by a node to configure its network subsystem and enable communication on a link or internetwork, for example. DHCP Dynamic Host Configuration Protocol for IPv6. The terms DHCPv4 and DHCPv6 are used only in contexts where it is necessary to avoid ambiguity. DHCP client (or client) A node that initiates requests on a link to obtain configuration parameters from one or more DHCP servers. DHCP domain A set of links managed by DHCP and operated by a single administrative entity. DHCP realm A name used to identify the DHCP administrative domain from which a DHCP authentication key was selected. DHCP relay agent (or relay agent) A node that acts as an intermediary to deliver DHCP messages between clients and servers, and is on the same link as the client. DHCP server (or server) A node that responds to requests from clients, and may or may not be on the same link as the client(s). DUID A DHCP Unique IDentifier for a DHCP participant; each DHCP client and server has exactly one DUID. See section 9 for details of the ways in which a DUID may be constructed. Identity association (IA) A collection of addresses assigned to a client. Each IA has an associated IAID. A client may have more than one IA assigned to it; for example, one for each of its interfaces. Droms, et al. Standards Track [Page 11] RFC 3315 DHCP for IPv6 July 2003 Each IA holds one type of address; for example, an identity association for temporary addresses (IA_TA) holds temporary addresses (see "identity association for temporary addresses"). Throughout this document, "IA" is used to refer to an identity association without identifying the type of addresses in the IA. Identity association identifier (IAID) An identifier for an IA, chosen by the client. Each IA has an IAID, which is chosen to be unique among all IAIDs for IAs belonging to that client. Identity association for non-temporary addresses (IA_NA) An IA that carries assigned addresses that are not temporary addresses (see "identity association for temporary addresses") Identity association for temporary addresses (IA_TA) An IA that carries temporary addresses (see RFC 3041 [12]). message A unit of data carried as the payload of a UDP datagram, exchanged among DHCP servers, relay agents and clients. Reconfigure key A key supplied to a client by a server used to provide security for Reconfigure messages. relaying A DHCP relay agent relays DHCP messages between DHCP participants. transaction ID An opaque value used to match responses with replies initiated either by a client or server. 5. DHCP Constants This section describes various program and networking constants used by DHCP. Droms, et al. Standards Track [Page 12] RFC 3315 DHCP for IPv6 July 2003 5.1. Multicast Addresses DHCP makes use of the following multicast addresses: All_DHCP_Relay_Agents_and_Servers (FF02::1:2) A link-scoped multicast address used by a client to communicate with neighboring (i.e., on-link) relay agents and servers. All servers and relay agents are members of this multicast group. All_DHCP_Servers (FF05::1:3) A site-scoped multicast address used by a relay agent to communicate with servers, either because the relay agent wants to send messages to all servers or because it does not know the unicast addresses of the servers. Note that in order for a relay agent to use this address, it must have an address of sufficient scope to be reachable by the servers. All servers within the site are members of this multicast group. 5.2. UDP Ports Clients listen for DHCP messages on UDP port 546. Servers and relay agents listen for DHCP messages on UDP port 547. 5.3. DHCP Message Types DHCP defines the following message types. More detail on these message types can be found in sections 6 and 7. Message types not listed here are reserved for future use. The numeric encoding for each message type is shown in parentheses. SOLICIT (1) A client sends a Solicit message to locate servers. ADVERTISE (2) A server sends an Advertise message to indicate that it is available for DHCP service, in response to a Solicit message received from a client. REQUEST (3) A client sends a Request message to request configuration parameters, including IP addresses, from a specific server. CONFIRM (4) A client sends a Confirm message to any available server to determine whether the addresses it was assigned are still appropriate to the link to which the client is connected. Droms, et al. Standards Track [Page 13] RFC 3315 DHCP for IPv6 July 2003 RENEW (5) A client sends a Renew message to the server that originally provided the client's addresses and configuration parameters to extend the lifetimes on the addresses assigned to the client and to update other configuration parameters. REBIND (6) A client sends a Rebind message to any available server to extend the lifetimes on the addresses assigned to the client and to update other configuration parameters; this message is sent after a client receives no response to a Renew message. REPLY (7) A server sends a Reply message containing assigned addresses and configuration parameters in response to a Solicit, Request, Renew, Rebind message received from a client. A server sends a Reply message containing configuration parameters in response to an Information-request message. A server sends a Reply message in response to a Confirm message confirming or denying that the addresses assigned to the client are appropriate to the link to which the client is connected. A server sends a Reply message to acknowledge receipt of a Release or Decline message. RELEASE (8) A client sends a Release message to the server that assigned addresses to the client to indicate that the client will no longer use one or more of the assigned addresses. DECLINE (9) A client sends a Decline message to a server to indicate that the client has determined that one or more addresses assigned by the server are already in use on the link to which the client is connected. RECONFIGURE (10) A server sends a Reconfigure message to a client to inform the client that the server has new or updated configuration parameters, and that the client is to initiate a Renew/Reply or Information-request/Reply transaction with the server in order to receive the updated information. Droms, et al. Standards Track [Page 14] RFC 3315 DHCP for IPv6 July 2003 INFORMATION-REQUEST (11) A client sends an Information-request message to a server to request configuration parameters without the assignment of any IP addresses to the client. RELAY-FORW (12) A relay agent sends a Relay-forward message to relay messages to servers, either directly or through another relay agent. The received message, either a client message or a Relay-forward message from another relay agent, is encapsulated in an option in the Relay-forward message. RELAY-REPL (13) A server sends a Relay-reply message to a relay agent containing a message that the relay agent delivers to a client. The Relay-reply message may be relayed by other relay agents for delivery to the destination relay agent. The server encapsulates the client message as an option in the Relay-reply message, which the relay agent extracts and relays to the client. 5.4. Status Codes DHCPv6 uses status codes to communicate the success or failure of operations requested in messages from clients and servers, and to provide additional information about the specific cause of the failure of a message. The specific status codes are defined in section 24.4. Droms, et al. Standards Track [Page 15] RFC 3315 DHCP for IPv6 July 2003 5.5. Transmission and Retransmission Parameters This section presents a table of values used to describe the message transmission behavior of clients and servers. Parameter Default Description ------------------------------------- SOL_MAX_DELAY 1 sec Max delay of first Solicit SOL_TIMEOUT 1 sec Initial Solicit timeout SOL_MAX_RT 120 secs Max Solicit timeout value REQ_TIMEOUT 1 sec Initial Request timeout REQ_MAX_RT 30 secs Max Request timeout value REQ_MAX_RC 10 Max Request retry attempts CNF_MAX_DELAY 1 sec Max delay of first Confirm CNF_TIMEOUT 1 sec Initial Confirm timeout CNF_MAX_RT 4 secs Max Confirm timeout CNF_MAX_RD 10 secs Max Confirm duration REN_TIMEOUT 10 secs Initial Renew timeout REN_MAX_RT 600 secs Max Renew timeout value REB_TIMEOUT 10 secs Initial Rebind timeout REB_MAX_RT 600 secs Max Rebind timeout value INF_MAX_DELAY 1 sec Max delay of first Information-request INF_TIMEOUT 1 sec Initial Information-request timeout INF_MAX_RT 120 secs Max Information-request timeout value REL_TIMEOUT 1 sec Initial Release timeout REL_MAX_RC 5 MAX Release attempts DEC_TIMEOUT 1 sec Initial Decline timeout DEC_MAX_RC 5 Max Decline attempts REC_TIMEOUT 2 secs Initial Reconfigure timeout REC_MAX_RC 8 Max Reconfigure attempts HOP_COUNT_LIMIT 32 Max hop count in a Relay-forward message 5.6 Representation of time values and "Infinity" as a time value All time values for lifetimes, T1 and T2 are unsigned integers. The value 0xffffffff is taken to mean "infinity" when used as a lifetime (as in RFC2461 [17]) or a value for T1 or T2. 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. Droms, et al. Standards Track [Page 16] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 17] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 18] RFC 3315 DHCP for IPv6 July 2003 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 options 8. 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. Droms, et al. Standards Track [Page 19] RFC 3315 DHCP for IPv6 July 2003 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) . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Droms, et al. Standards Track [Page 20] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 21] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 22] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 23] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 24] RFC 3315 DHCP for IPv6 July 2003 - 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. Droms, et al. Standards Track [Page 25] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 26] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 27] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 28] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 29] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 30] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 31] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 32] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 33] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 34] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 35] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 36] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 37] RFC 3315 DHCP for IPv6 July 2003 the source address field from the IP datagram in which the Relay- forward message was received. 17.2.3. Creation and Transmission of Reply Messages The server MUST commit the assignment of any addresses or other configuration information message before sending a Reply message to a client in response to a Solicit message. DISCUSSION: When using the Solicit-Reply message exchange, the server commits the assignment of any addresses before sending the Reply message. The client can assume it has been assigned the addresses in the Reply message and does not need to send a Request message for those addresses. Typically, servers that are configured to use the Solicit-Reply message exchange will be deployed so that only one server will respond to a Solicit message. If more than one server responds, the client will only use the addresses from one of the servers, while the addresses from the other servers will be committed to the client but not used by the client. The server includes a Rapid Commit option in the Reply message to indicate that the Reply is in response to a Solicit message. The server includes a Reconfigure Accept option if the server wants to require that the client accept Reconfigure messages. The server produces the Reply message as though it had received a Request message, as described in section 18.2.1. The server transmits the Reply message as described in section 18.2.8. 18. DHCP Client-Initiated Configuration Exchange A client initiates a message exchange with a server or servers to acquire or update configuration information of interest. The client may initiate the configuration exchange as part of the operating system configuration process, when requested to do so by the application layer, when required by Stateless Address Autoconfiguration or as required to extend the lifetime of an address (Renew and Rebind messages). Droms, et al. Standards Track [Page 38] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 39] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 40] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 41] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 42] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 43] RFC 3315 DHCP for IPv6 July 2003 - 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. Droms, et al. Standards Track [Page 44] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 45] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 46] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 47] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 48] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 49] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 50] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 51] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 52] RFC 3315 DHCP for IPv6 July 2003 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. Droms, et al. Standards Track [Page 53] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 54] RFC 3315 DHCP for IPv6 July 2003 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 Droms, et al. Standards Track [Page 55] RFC 3315 DHCP for I