Dynamic Host Configuration Protocol for IPv6 (DHCPv6) bisInternet Systems Consortium, Inc.950 Charter StreetRedwood CityCA94063USAtomasz.mrugalski@gmail.comInternet Systems Consortium, Inc.950 Charter St.Redwood CityCA94063USAmsiodelski@gmail.comCisco Systems, Inc.1414 Massachusetts AveBoxborough, MA 01719USAvolz@cisco.comCisco Systems, Inc.De Kleetlaan, 7DiegemB-1831Belgiumayourtch@cisco.comSandelman Software Works470 Dawson AvenueOttawaONK1Z 5V7CAmcr+ietf@sandelman.cahttp://www.sandelman.ca/Huawei Technologies Co., LtdQ14, Huawei Campus, No.156 Beiqing RoadHai-Dian District, Beijing, 100095P.R. Chinajiangsheng@huawei.comNominum, Inc.800 Bridge St.Redwood CityCA94043USATed.Lemon@nominum.com
Internet
Dynamic Host Configuration (DHC)DHCPv6IPv6DHCP
This document describes the Dynamic Host Configuration Protocol
for IPv6 (DHCPv6): an extensible mechanism for configuring hosts
with network configuration parameters, IP addresses, and
prefixes. Parameters can be provided statelessly, or in
combination with stateful assignment of one or more IPv6
addresses and/or IPv6 prefixes. DHCPv6 can operate either in
place of or in addition to stateless address autoconfiguration
(SLAAC).
This document updates the text from RFC 3315, the original DHCPv6
specification, and incorporates the stateless DHCPv6 extensions (RFC 3736)
and prefix delegation (RFC 3633), clarifying the interactions between
these modes of operation (RFC 7550) and providing a mechanism for
throttling DHCPv6 clients when DHCPv6 service is not available (RFC 7083).
As such, this document obsoletes RFC3315, RFC3633, RFC3736, RFC7083,
RFC7550.This document describes DHCP for IPv6 (DHCPv6), a client/server
protocol that provides managed configuration of devices. Relay agent
functionality is also defined for enabling communication between
clients and servers that are not on the same link.DHCPv6 can provide a device with addresses assigned by a DHCPv6 server
and other configuration information, which are carried in options. DHCPv6
can be extended through the definition of new options to carry
configuration information not specified in this document.DHCPv6 is the "stateful address autoconfiguration protocol" and the
"stateful autoconfiguration protocol" referred to in "IPv6 Stateless
Address Autoconfiguration" .This document also provides a mechanism for automated delegation
of IPv6 prefixes using DHCPv6. Through this mechanism, a delegating
router can delegate prefixes to requesting routers.DHCPv6 can also operate in mode, where only configuration options (but
no addresses or prefixes) are provided. That implies that the server
doesn't have to track any state, and thus the mode is called stateless
DHCPv6. Mechanisms necessary to support stateless DHCPv6 are much smaller
than to support stateful DHCPv6.The remainder of this introduction summarizes relation to the previous
DHCPv6 standards , clarifies the stance
with regards to DHCPv4 . More detailed description
of the message exchange mechanisms and example message flows in and are intended as
illustrations of DHCP operation rather than an exhaustive list of all
possible client-server interactions. provides an
overview of common operational models. , , and explain client and
server operation in detail.The initial specification of DHCPv6 was defined in and a number of follow up extensions published over
the years. Several notable extensions were published: prefix delegation , stateless , update to
SOL_MAX_RT and INF_MAX_RT option values and
harmonization between addresses and prefixes support . Understanding a protocol which definition is spread
between large number of documents may be cumbersome. Furthermore,
a significant operational experience has been gained over the years and
certain small elements of the protocol have been reworked. This document
provides a unified, corrected and cleaned up definition of the
DHCPv6 that also covers all erratas filled against older RFCs.
As such, it obsoletes a number of aforementioned RFCs. There is a small
number of mechanisms that were obsoleted. They are listed in
.The operational models and relevant configuration information for
DHCPv4 and DHCPv6
are sufficiently different that integration between the two
services is not included in this document.
suggested that future work might be to extend DHCPv6 to carry IPv4
address and configuration information. However, the current
consensus of the IETF is that DHCPv4 should be used rather than
DHCPv6 when conveying IPv4 configuration information to
nodes. describes a transport mechanism to
carry DHCPv4 messages using the DHCPv6 protocol for the dynamic
provisioning of IPv4 address and configuration information across
IPv6-only networks.Clients and servers exchange DHCP messages using UDP . The client uses a link-local address or addresses
determined through other mechanisms for transmitting and receiving
DHCP messages.A DHCP client sends most messages using a reserved, link-scoped
multicast destination 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.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 or NTP
servers 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 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.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.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in .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.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 , the IPv6
Addressing Architecture , IPv6 Stateless Address
Autoconfiguration , IPv6 Neighbor Discovery
Processing , and Dynamic Updates to DNS . 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 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
specifies procedures by which a node may autoconfigure addresses based
on router advertisements , 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 is the node
discovery protocol in IPv6 which replaces and enhances functions of ARP
. To understand IPv6 and stateless address
autoconfiguration, it is strongly recommended that implementors
understand IPv6 Neighbor Discovery.Dynamic Updates to DNS 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.This section defines terminology specific to IPv6 and DHCP used in
this document.IPv6 terminology relevant to this specification from the IPv6
Protocol , IPv6 Addressing Architecture , and IPv6 Stateless Address Autoconfiguration is included below.An IP layer identifier for an interface or a set of
interfaces.Any node that is not a router. Internet Protocol Version 6 (IPv6). The terms IPv4 and IPv6 are
used only in contexts where it is necessary to avoid ambiguity. A node's attachment to a 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. 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. 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. 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. A node attached to the same link. A device that implements IP. An IP header plus payload. The initial bits of an address, or a set of IP addresses
that share the same initial bits. The number of bits in a prefix. A node that forwards IP packets not explicitly addressed to
itself. An identifier for a single interface. A packet sent
to a unicast address is delivered to the interface identified by that
address.Terminology specific to DHCP can be found below. 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. 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 <DUID,
IA-type, IAID> (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 <DUID>. 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.The router that acts as a
DHCP server, and is responding to the prefix request.Dynamic Host Configuration Protocol for IPv6. The terms DHCPv4
and DHCPv6 are used only in contexts where it is necessary to avoid
ambiguity. A node that initiates requests on a link to
obtain configuration parameters from one or more DHCP servers. Depending on the
purpose of the client, it may feature the requesting router functionality, if it
supports prefix delegation. A set of links managed by DHCP and operated by a single
administrative entity. A name used to identify the DHCP administrative domain
from which a DHCP authentication key was selected. A node that acts as an
intermediary to deliver DHCP messages between clients and servers. In certain
configurations there may be more than one relay agent between clients and servers,
so a relay agent may send DHCP messages to another relay agent. A node that responds to requests from
clients, and may or may not be on the same link as the client(s). Depending
on its capabilities, it may also feature the functionality of delegating router,
if it supports prefix delegation. A DHCP Unique IDentifier for a DHCP participant; each DHCP
client and server has exactly one DUID. See for details of
the ways in which a DUID may be constructed. Identity Association: A collection of
leases 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.
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") and identity association
for prefix delegation (IA_PD) holds delegated prefixes. Throughout this
document, "IA" is used to refer to an identity association without
identifying the type of a lease in the IA. At the time of writing
this document, there are 3 IA types defined: IA_NA, IA_TA and IA_PD. New IA types
may be defined in the future.Identity Association IDentifier: An identifier for an IA,
chosen by the client. Each IA has an IAID, which is chosen to be unique
among IAIDs for IAs of a specific type, belonging to that client.Identity association for Non-temporary Addresses: An IA that
carries assigned addresses that are not temporary addresses (see
"identity association for temporary addresses")Identity Association for Temporary Addresses: An IA that
carries temporary addresses (see ). Identity Association for Prefix Delegation:
A collection of prefixes assigned to the requesting
router. Each IA_PD has an associated IAID. A requesting
router may have more than one IA_PD assigned to it; for
example, one for each of its interfaces. It is an address assigned by the DHCP server to the client
or a delegated prefix assigned by the delegating router to the requesting
router. Leases are carried in the IA Address or IA Prefix options within the
IA_NA/IA_TA and IA_PD options respectively. A unit of data carried as the payload of a UDP datagram,
exchanged among DHCP servers, relay agents and clients. A key supplied to a client by a server used to
provide security for Reconfigure messages.The router that acts as a
DHCP client and is requesting prefix(es) to be assigned.An option that is allowed to appear only once.
Most options are singletons. A DHCP relay agent relays DHCP messages between DHCP
participants.An option conveyed in a DHCP message directly,
i.e. not encapsulated in any other option, as described in Section 9 of
. An opaque value used to match responses with replies
initiated either by a client or server.
This section describes some of the current most common
DHCP operational models. The described models are not
mutually exclusive and are sometimes used together. For
example, a device may start in stateful mode to obtain
an address, and at a later time when an application
is started, request additional parameters using stateless
mode.
This document assumes that the DHCP servers and the client,
communicating with the servers via specific interface, belong
to a single provisioning domain.
Stateless DHCP is used when DHCP
is not used for obtaining a lease, but a node
(DHCP client) desires one or more DHCP "other
configuration" parameters, such as a list of DNS recursive
name servers or DNS domain search lists .
Stateless may be used when a node initially boots or at
any time the software on the node requires some missing
or expired configuration information that is available via
DHCP.
This is the simplest and most basic operation for DHCP
and requires a client (and a server) to support only two
messages - Information-request and Reply. Note that DHCP
servers and relay agents typically also need to support
the Relay-Forw and Relay-Reply messages to accommodate
operation when clients and servers are not on the same link.
This model of operation was the original motivation for
DHCP and is the "stateful address autoconfiguration
protocol" for IPv6 . It is
appropriate for situations where stateless address
autoconfiguration is not desired, because of network
policy, additional requirements (such as updating the
DNS with forward or reverse resource records), or client
specific requirements (i.e., some prefixes are only
available to some clients) which are not possible using
stateless address autoconfiguration.
The model of operation for non-temporary address
assignment is as follows. The server is provided with
IPv6 prefixes from which it may allocate addresses to
clients, as well as any related network topology
information as to which prefixes are present on which
links. A client requests a non-temporary address to be
assigned by the server. The server allocates an address
or addresses appropriate for the link on which the
client is connected. The server returns the allocated
address or addresses to the client.
Each address has an associated preferred and valid lifetime,
which constitutes an agreement about the length of time
over which the client is allowed to use the address. A
client can request an extension of the lifetimes on an
address and is required to terminate the use of an address
if the valid lifetime of the address expires.
Typically clients request other configuration parameters,
such as the domain server addresses and search lists, when
requesting addresses.
The prefix delegation mechanism, originally described in
, is another stateful mode of
operation and intended for simple delegation of prefixes
from a delegating router (DHCP server) to requesting
routers (DHCP clients). It is appropriate for
situations in which the delegating router does not have
knowledge about the topology of the networks to which the
requesting router is attached, and the delegating router
does not require other information aside from the
identity of the requesting router to choose a prefix for
delegation. For example, these options would be used by a
service provider to assign a prefix to a Customer Edge
Router device acting as a router between the
subscriber's internal network and the service provider's
core network.
The design of this prefix delegation mechanism meets the
requirements for prefix delegation in .
The model of operation for prefix delegation is as follows.
A delegating router is provided IPv6 prefixes to be
delegated to requesting routers. A requesting
router requests prefix(es) from the delegating router, as
described in . The
delegating router chooses prefix(es) for delegation, and
responds with prefix(es) to the requesting router. The
requesting router is then responsible for the delegated
prefix(es). For example, the requesting router might assign
a subnet from a delegated prefix to one of its interfaces,
and begin sending router advertisements for the prefix on
that link.
Each prefix has an associated valid and preferred lifetime,
which constitutes an agreement about the length of time over
which the requesting router is allowed to use the prefix.
A requesting router can request an extension of the
lifetimes on a delegated prefix and is required to terminate
the use of a delegated prefix if the valid lifetime of the
prefix expires.
The mechanism through which the delegating router
selects prefix(es) for delegation is not specified in this
document. Examples of ways in which the server might select
prefix(es) for a client include: static assignment based on
subscription to an ISP; dynamic assignment from a pool of
available prefixes; selection based on an external authority
such as a RADIUS server using the Framed-IPv6-Prefix option as
described in .
This prefix delegation mechanism would be appropriate for
use by an ISP to delegate a prefix to a subscriber, where
the delegated prefix would possibly be subnetted and
assigned to the links within the subscriber's network.
illustrates a network
architecture in which prefix delegation could be used.
In this example, the delegating router is configured
with a set of prefixes to be used for assignment to
customers at the time of each customer's first connection
to the ISP service. The prefix delegation process begins
when the requesting router requests configuration
information through DHCP. The DHCP messages from the
requesting router are received by the delegating router
in the aggregation device. When the delegating router
receives the request, it selects an available prefix or
prefixes for delegation to the requesting router. The
delegating router then returns the prefix or prefixes to
the requesting router.
The requesting router subnets the delegated prefix and
assigns the longer prefixes to links in the subscriber's
network. In a typical scenario based on the network
shown in , the
requesting router subnets a single delegated /48 prefix
into /64 prefixes and assigns one /64 prefix to each of
the links in the subscriber network.
The prefix delegation options can be used in conjunction
with other DHCP options carrying other configuration
information to the requesting router. The requesting
router may, in turn, provide DHCP service to hosts
attached to the internal network. For example, the
requesting router may obtain the addresses of DNS and NTP
servers from the ISP delegating router, and then pass
that configuration information on to the subscriber hosts
through a DHCP server in the requesting router.
If the requesting router assigns a delegated prefix to a
link to which the router is attached, and begins to send
router advertisements for the prefix on the link, the
requesting router MUST set the valid lifetime in those
advertisements to be no later than the valid lifetime
specified in the IA_PD Prefix option. A requesting router MAY
use the preferred lifetime specified in the IA_PD Prefix
option.
The DHCP requirements and network architecture for
Customer Edge Routers are described in . This model of operation combines
address assignment (see )
and prefix delegation (see ).
In general, this model assumes that a
single set of transactions between the client and server
will assign or extend the client's non-temporary addresses
and delegated prefixes.
Temporary addresses were originally introduced to avoid
privacy concerns with stateless address autoconfiguration,
which based 64-bits of the address on the EUI-64 (see
and ).
They were added to DHCP to provide complementary support
when stateful address assignment is used.
Temporary address assignment works mostly like non-temporary
address assignment (see ), however
these addresses are generally intended to be used for a short
period of time and not to have their lifetimes extended,
though they can be if required.
This section describes various program and networking constants used
by DHCP.DHCP makes use of the following multicast addresses: 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. 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.Clients listen for DHCP messages on UDP port 546. Servers and relay
agents listen for DHCP messages on UDP port 547.DHCP defines the following message types. More detail on these
message types can be found in and . Message types not
listed here are reserved for future use. The numeric encoding for each
message type is shown in parentheses. A client sends a Solicit message to locate servers. 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. A client sends a Request message to request
configuration parameters, including IP addresses, from a specific
server. 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. 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. 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. 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. 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. 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. 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. A client
sends an Information-request message to a server to request
configuration parameters without the assignment of any IP addresses to
the client. 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. 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.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
.If the Status Code option does not appear in a message in which
the option could appear, the status of the message is assumed to be Success.This section presents a table of values used to describe the
message transmission behavior of clients and servers.ParameterDefaultDescriptionSOL_MAX_DELAY1 secMax delay of first SolicitSOL_TIMEOUT1 secInitial Solicit timeoutSOL_MAX_RT3600 secsMax Solicit timeout valueREQ_TIMEOUT1 secInitial Request timeoutREQ_MAX_RT30 secsMax Request timeout valueREQ_MAX_RC10Max Request retry attemptsCNF_MAX_DELAY1 secMax delay of first ConfirmCNF_TIMEOUT1 secInitial Confirm timeoutCNF_MAX_RT4 secsMax Confirm timeoutCNF_MAX_RD10 secsMax Confirm durationREN_TIMEOUT10 secsInitial Renew timeoutREN_MAX_RT600 secsMax Renew timeout valueREB_TIMEOUT10 secsInitial Rebind timeoutREB_MAX_RT600 secsMax Rebind timeout valueINF_MAX_DELAY1 secMax delay of first Information-requestINF_TIMEOUT1 secInitial Information-request timeoutINF_MAX_RT3600 secsMax Information-request timeout valueREL_TIMEOUT1 secInitial Release timeoutREL_MAX_RC4MAX Release retry attemptsDEC_TIMEOUT1 secInitial Decline timeoutDEC_MAX_RC4Max Decline retry attemptsREC_TIMEOUT2 secsInitial Reconfigure timeoutREC_MAX_RC8Max Reconfigure attemptsHOP_COUNT_LIMIT32Max hop count in a Relay-forward messageAll 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 ) or a value for T1 or T2.All DHCP messages sent between clients and servers share an identical
fixed format header and a variable format area for options.All values in the message header and in options are in network byte
order.Options are stored serially in the options field, with no padding
between the options. Options are byte-aligned but are not aligned in any
other way such as on 2 or 4 byte boundaries.The following diagram illustrates the format of DHCP messages sent
between clients and servers:Identifies the DHCP message type; the
available message types are listed in .
The transaction ID for this message exchange.Options carried in this message; options are
described in .
Relay agents exchange messages with servers to relay messages between
clients and servers that are not connected to the same link.All values in the message header and in options are in network byte
order.Options are stored serially in the options field, with no padding
between the options. Options are byte-aligned but are not aligned in any
other way such as on 2 or 4 byte boundaries.There are two relay agent messages, which share the following
format:The following sections describe the use of the Relay Agent message
header.The following table defines the use of message fields in a
Relay-forward message.RELAY-FORWNumber of relay agents that have relayed this
message.An address that will be used by
the server to identify the link on which the client is located.
This is typically global, site-scoped or ULA ,
but see discussion in .The address of the client or relay agent from which
the message to be relayed was received.MUST include a "Relay Message option" (see );
MAY include other options added by the relay agent.The following table defines the use of message fields in a
Relay-reply message.RELAY-REPLCopied from the Relay-forward messageCopied from the Relay-forward messageCopied from the Relay-forward messageMUST include a "Relay Message option"; see ;
MAY include other optionsSo 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 . 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 .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
and for
the representation of a DUID in a DHCP message.Clients and servers MUST treat DUIDs as opaque values and MUST only
compare DUIDs for equality. Clients and servers MUST NOT in any other
way interpret DUIDs. Clients and servers MUST NOT restrict DUIDs to the
types defined in this document, as additional DUID types may be defined
in the future.The DUID is carried in an option because it may be variable length
and because it is not required in all DHCP messages. The DUID is
designed to be unique across all DHCP clients and servers, and stable
for any specific client or server - that is, the DUID used by a client
or server SHOULD NOT change over time if at all possible; for example, a
device's DUID should not change as a result of a change in the device's
network hardware.The motivation for having more than one type of DUID is that the DUID
must be globally unique, and must also be easy to generate. The sort of
globally-unique identifier that is easy to generate for any given device
can differ quite widely. Also, some devices may not contain any
persistent storage. Retaining a generated DUID in such a device is not
possible, so the DUID scheme must accommodate such devices.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. The length of the DUID (not including the type code)
is at least 1 octet and at most 128 octets. The following types are
currently defined:TypeDescription1Link-layer address plus time2Vendor-assigned unique ID based on Enterprise Number3Link-layer address4Universally Unique IDentifier (UUID) - see Formats for the variable field of the DUID for the first 3 of the
above types are shown below. The fourth type, DUID-UUID , can
be used in situations where there is a UUID stored in a device's firmware settings.
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 . 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 .The following diagram illustrates the format of a DUID-LLT: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.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 followed by a unique
identifier assigned by the vendor. The following diagram summarizes
the structure of a DUID-EN: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 no later than
at the first usage and stored in some form of non-volatile storage.
This typically means being assigned during manufacture process in case
of physical devices or when the image is created or booted for the
first time in case of virtual machines. 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 . The enterprise-number
is stored as an unsigned 32 bit number.An example DUID of this type might look like this:This example includes the two-octet type of 2, the Enterprise
Number (9), followed by eight octets of identifier data
(0x0CC084D303000912).This type of DUID consists of two octets containing the DUID type
3, a two octet network hardware type code, followed by the link-layer
address of any one network interface that is permanently connected to
the client or server device. For example, a host that has a network
interface implemented in a chip that is unlikely to be removed and
used elsewhere could use a DUID-LL. The hardware type MUST be a valid
hardware type assigned by the IANA, as described in . The hardware type is stored in network byte order.
The link-layer address is stored in canonical form, as described in
. The following diagram illustrates
the format of a DUID-LL: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.An "identity-association" (IA) is a construct through which a server
and a client can identify, group, and manage a set of related IPv6
addresses or delegated prefixes. Each IA consists of an IAID and
associated configuration information.The IAID uniquely identifies the IA and must be chosen to be unique
among the IAIDs for that IA type on the client. The IAID is chosen
by the client. For any given use of an IA by the client, the IAID
for that IA MUST be consistent across restarts of the DHCP client.
The client may maintain consistency either by storing the IAID in
non-volatile storage or by using an algorithm that will consistently
produce the same IAID as long as the configuration of the client has
not changed. There may be no way for a client to maintain consistency
of the IAIDs if it does not have non-volatile storage and the
client's hardware configuration changes. If the client uses only one
IAID, it can use a well-known value, e.g., zero.A client must associate at least one distinct IA with each of its
network interfaces for which it is to request the assignment of IPv6
addresses from a DHCP server. The client uses the IAs assigned to an
interface to obtain configuration information from a server for that
interface. Each IA must be associated with exactly one interface.
The configuration information in an IA consists of one or more IPv6
addresses along with the times T1 and T2 for the IA. See 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 . 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 .
An IA_PD is different from an IA for address assignment, in that it
does not need to be associated with exactly one interface. One IA_PD
can be associated with the requesting router, with a set of interfaces
or with exactly one interface. A requesting router must create at
least one distinct IA_PD. It may associate a distinct IA_PD with each
of its downstream network interfaces and use that IA_PD to obtain a
prefix for that interface from the delegating router.The configuration information in an IA_PD consists of one or more
IPv6 prefixes along with the times T1 and T2 for the IA_PD. See
for the representation of an IA_PD in a DHCP message.
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. According to , the server MUST
ignore any link-address field whose value is zero. The link address field referes
to the link-address field of the Relay-Forward message, and the link-address fields
in any Relay-Forward messages that may be nested within the Relay-Forward message. If the server receives the message directly from the client and the
source address in the IP datagram in which the message was received is
not a link-local address, then the client is on the link identified by
the source address in the IP datagram (note that this situation can
occur only if the server has enabled the use of unicast message delivery
by the client and the client has sent a message for which unicast
delivery is allowed). The DUID supplied by the client. Other information in options supplied by the client, e.g. IA
Address options that include the client's requests for specific addresses. Other information in options supplied by the relay agent.Any address assigned by a server that is based on an EUI-64
identifier MUST include an interface identifier with the "u"
(universal/local) and "g" (individual/group) bits of the interface
identifier set appropriately, as indicated in section 2.5.1 of
.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 , from any subnet.A client may request the assignment of temporary addresses (see
for the definition of temporary
addresses). DHCPv6 handling of address assignment is no different for
temporary addresses.Clients ask for temporary addresses and servers assign them.
Temporary addresses are carried in the Identity Association for
Temporary Addresses (IA_TA) option (see ). Each IA_TA option
contains at most one temporary address for each of the prefixes on the
link to which the client is attached.The lifetime of the assigned temporary address is set in the IA Address Option
(see ) with in the IA_TA option. It is
RECOMMENDED to set short lifetimes, typically shorter than
TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME (see Section 5, .The IAID number space for the IA_TA option IAID number space is
separate from the IA_NA option IAID number space.A DHCPv6 server implementation MAY generate temporary addresses
referring to the algorithm defined in Section 3.2.1, ,
with additional condition that the new address is not duplicated with any assigned addresses. The server MAY update the DNS for a temporary address, as described
in section 4 of .On the clients, by default, temporary addresses are preferred in source address selection,
according to Rule 7, . However, this policy is overridable.One of the most important properties of temporary address is
unlinkability of different actions over time. So, it is NOT RECOMMENDED
for a client to renew expired temporary addresses, though DHCPv6
provides such possibility (see ).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 ) in the client's message (all
servers will receive this message but only the indicated server will
respond). All servers are indicated by not supplying this option.A client may send some messages directly to a server using unicast,
as described in .In order to avoid prolonged message bursts that may be caused by
possible logic loops, a DHCPv6 client MUST limit the rate of DHCPv6
messages it transmits. One example is that a client obtains an address,
but does not like the response; it reverts back to Solicit procedure,
discovers the same (sole) server, requests an address and gets the
same address as before (the server still has the lease that was
requested just previously). This loops can repeat infinitely if there is not
a quit/stop mechanism. Therefore, a client must not initiate
transmissions too frequently.A recommended method for implementing the rate limiting function
is a token bucket, limiting the average rate of transmission to a
certain number in a certain time. This method of bounding burstiness
also guarantees that the long-term transmission rate will not exceed.Transmission Rate Limit
The Transmission Rate Limit parameter (TRT) SHOULD be configurable.
A possible default could be 20 packets in 20 seconds.For a device that has multiple interfaces, the limit MUST be
enforced on a per interface basis.Rate limiting of forwarded DHCPv6 messages and server-side
messages are out of scope of this specification.In certain cases, T1 and/or T2 timers may be set to zero. Currently
there are three such cases: 1. a client received IA_NA option with
zeroed values; 2. a client received IA_PD option with zeroed values;
3. a client received IA_TA option (which does not contain T1 or T2
fields). Additional cases may appear in the future. This is an
indication that the transmission times are left at client's
discretion. They are not completely discretionary, though.When T1 and/or T2 timers are set to zero, client MUST choose
transmission time to avoid packet storms. In particular, it MUST NOT
transmit immediately. If the client received multiple IA containers, it
SHOULD pick renew and/or rebind transmission time so all IA containers
are received in one exchange, if possible. Client MUST choose the
transmission times to not violate rate limiting restrictions, defined in
.DHCP clients are responsible for reliable delivery of messages in the
client-initiated message exchanges described in
and . 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 .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:
Retransmission timeout Initial retransmission time Maximum retransmission count Maximum retransmission time Maximum retransmission duration Randomization factorWith each message transmission or retransmission, the client sets RT
according to the rules given below. If RT expires before the message
exchange terminates, the client recomputes RT and retransmits the
message.Each of the computations of a new RT include a randomization factor
(RAND), which is a random number chosen with a uniform distribution
between -0.1 and +0.1. The randomization factor is included to minimize
synchronization of messages transmitted by DHCP clients.The algorithm for choosing a random number does not need to be
cryptographically sound. The algorithm SHOULD produce a different
sequence of random numbers from each invocation of the DHCP client.RT for the first message transmission is based on IRT:RT for each subsequent message transmission is based on the previous
value of RT: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:MRC specifies an upper bound on the number of times a client may
retransmit a message. Unless MRC is zero, the message exchange fails
once the client has transmitted the message MRC times.MRD specifies an upper bound on the length of time a client may
retransmit a message. Unless MRD is zero, the message exchange fails
once MRD seconds have elapsed since the client first transmitted the
message.If both MRC and MRD are non-zero, the message exchange fails whenever
either of the conditions specified in the previous two paragraphs are
met.If both MRC and MRD are zero, the client continues to transmit the
message until it receives a response.A client is not expected to listen for a response during the entire
period between transmission of Solicit or Information-request
messages.Clients and servers might get messages that contain options
not allowed to appear in the received message. For example, an
IA option is not allowed to appear in an Information-request message.
Clients and servers MAY choose either to extract information from such a
message if the information is of use to the recipient, or to ignore
such message completely and just drop it.If a server receives a message that contains options it should not
contain (such as an Information-request message with an IA option), is
missing options that it should contain, or is otherwise not valid, it
MAY send a Reply (or Advertise as appropriate) with a Server Identifier
option, a Client Identifier option if one was included in the message
and a Status Code option with status UnSpecFail.Clients, relay agents and servers MUST NOT discard messages that contain
unknown options (or instances of vendor options with unknown
enterprise-numbers). These should be ignored as if they weren't
present.A server MUST discard any Solicit, Confirm, Rebind or
Information-request messages it receives with a unicast destination
address.A client or server MUST silently discard any received DHCPv6
messages with an unknown message type.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.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.Clients MUST discard any received Advertise message that meets any
of the following conditions:
the message does not include a Server Identifier option. the message does not include a Client Identifier option. the contents of the Client Identifier option does not match the
client's DUID. the "transaction-id" field value does not match the value the
client used in its Solicit message.Servers and relay agents MUST discard any received Advertise
messages.Clients MUST discard any received Request messages.Servers MUST discard any received Request message that meets any of
the following conditions:
the message does not include a Server Identifier option. the contents of the Server Identifier option do not match the
server's DUID. the message does not include a Client Identifier option.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.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.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.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.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.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.Servers and relay agents MUST discard any received Reconfigure
messages.Clients MUST discard any Reconfigure message that meets any of the
following conditions:
the message was not unicast to the client. the message does not include a Server Identifier option. the message does not include a Client Identifier option that
contains the client's DUID. the message does not contain a Reconfigure Message option. the Reconfigure Message option msg-type is not a valid value. the message includes any IA options and the msg-type in the
Reconfigure Message option is INFORMATION-REQUEST. the message does not include DHCP authentication:
the message does not contain an authentication option. the message does not pass the authentication validation performed
by the client.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.Clients MUST discard any received Relay-forward messages.Clients and servers MUST discard any received Relay-reply
messages.Client's behavior is different depending on the purpose of the
configuration.When a client sends a DHCP message to the
All_DHCP_Relay_Agents_and_Servers address, it SHOULD send the message
through the interface for which configuration information is being
requested. However, the client MAY send the message through another
interface if the interface is a logical interface without direct link
attachment or the client is certain that two interfaces are attached
to the same link.
When a client sends a DHCP message directly to a server using unicast
(after receiving the Server Unicast option from that server), the source
address in the header of the IPv6 datagram MUST be an address assigned to
the interface for which the client is interested in obtaining
configuration and which is suitable for use by the server in responding
to the client.Delegated prefixes are not associated with a particular
interface in the same way as addresses are for address
assignment, and mentioned above.When a client (acting as requesting router) sends a DHCP
message for the purpose of prefix delegation, it SHOULD be sent
on the interface associated with the upstream router (ISP
network). The upstream interface is typically determined by
configuration. This rule applies even in the case where a
separate IA_PD is used for each downstream interface.When a requesting router sends a DHCP message directly to a
delegating router using unicast (after receiving the Server
Unicast option from that delegating router), the source address
SHOULD be an address from the upstream interface and which is
suitable for use by the delegating router in responding to the
requesting router.This section describes how a client locates servers that will assign
addresses and delegated prefixes to IAs belonging to the client.The client is responsible for creating IAs and requesting that a
server assign IPv6 addresses and/or delegated prefixes to the IAs. The client
first creates the IAs and assigns IAIDs to them. The client then transmits
a Solicit message containing the IA options describing the IAs. The client
MUST NOT be using any of the addresses or delegated prefixes for which
it tries to obtain the bindings by sending the Solicit message. In particular,
if the client had some valid bindings and has chosen to start the
server solicitation process to obtain the bindings from a different
server, the client MUST stop using the addresses and delegated prefixes
for the bindings it had obtained from the previous server, and which
it is now trying to obtain from a new server.Servers
that can assign addresses or delegated prefixes to the IAs respond to
the client with an Advertise message. The client then initiates a
configuration exchange as described in .If the client will accept a Reply message with committed leases
assignments and other resources in response to the Solicit message, the
client includes a Rapid Commit option (see ) in the Solicit
message.A client uses the Solicit message to discover DHCP servers
configured to assign leases or return other configuration
parameters on the link to which the client is attached.The client sets the "msg-type" field to SOLICIT. The client
generates a transaction ID and inserts this value in the
"transaction-id" field.The client MUST include a Client Identifier option to identify
itself to the server. The client includes IA options for any IAs to
which it wants the server to assign leases.The client MAY include addresses in the IA_NA and IA_TA options
as hints to the server about the addresses for which the client has
a preference.The client MAY include values in the IA Prefix option encapsulated
within IA_PD option as hints for the delegated prefix and/or prefix
length for which the client has a preference.The client MUST NOT include
any other options in the Solicit message, except as specifically
allowed in the definition of individual options.The client uses IA_NA options to request the assignment of
non-temporary addresses, IA_TA options to request the
assignment of temporary addresses and IA_PD options to request
prefix delegation. Either IA_NA, IA_TA or IA_PD options, or
a combination of all, can be included in DHCP messages. In addition,
multiple instances of any IA option type can be included.The client MUST include an Option Request option (see
) to request the SOL_MAX_RT option
(see ) and any
other options the client is interested in receiving.
The client MAY additionally include instances of those options that
are identified in the Option Request option, with data values as
hints to the server about parameter values the client would like to
have returned.The client includes a Reconfigure Accept option (see
) if the client is willing to accept
Reconfigure messages from the server.The first Solicit message from the client on the interface MUST
be delayed by a random amount of time between 0 and SOL_MAX_DELAY.
In the case of a Solicit message transmitted when DHCP is initiated
by IPv6 Neighbor Discovery, the delay gives the amount of time to
wait after IPv6 Neighbor Discovery causes the client to invoke the
stateful address autoconfiguration protocol (see section 5.5.3 of
). This random delay desynchronizes clients which start at
the same time (for example, after a power outage).The client transmits the message according to , using
the following parameters:
SOL_TIMEOUT SOL_MAX_RT 0 0If 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 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 (). 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 ) 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 . 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.The client MUST process SOL_MAX_RT and INF_MAX_RT options in an
Advertise message, even if the message contains a Status Code
option indicating a failure, and the Advertise message will be
discarded by the client.The client MUST ignore any Advertise message that contains no
addresses (IAADDR options encapsulated in IA_NA or IA_TA options)
and no delegated prefixes (IAPREFIX options encapsulated in IA_PD
options) with the exception that the client:
MUST process an included SOL_MAX_RT option and MUST process an included INF_MAX_RT option.A client can display any associated status message(s) to the user
or activity log.The client ignoring this Advertise message MUST NOT restart the
Solicit retransmission timer.Upon receipt of one or more valid Advertise messages, the client
selects one or more Advertise messages based upon the following
criteria.
Those Advertise messages with the highest server preference
value are preferred over all other Advertise messages. Within a group of Advertise messages with the same server
preference value, a client MAY select those servers whose Advertise
messages advertise information of interest to the client. The client MAY choose a less-preferred server if that
server has a better set of advertised parameters, such as the available
set of IAs, as well as the set of other configuration options
advertised.Once a client has selected Advertise message(s), the client will
typically store information about each server, such as server
preference value, addresses advertised, when the advertisement was
received, and so on.In practice, this means that the client will maintain
independent per-IA state machines per each selected server.If the client needs to select an alternate server in the case
that a chosen server does not respond, the client chooses the next
server according to the criteria given above.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 . 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
.If the client subsequently receives a valid Reply
message that includes a Rapid Commit option, it either: processes the Reply message as described in
, 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.A server sends an Advertise message in response to valid Solicit
messages it receives to announce the availability of the server to the
client.The server determines the information about the client and its
location as described in and checks its administrative
policy about responding to the client. If the server is not
permitted to respond to the client, the server discards the Solicit
message. For example, if the administrative policy for the server is
that it may only respond to a client that is willing to accept a
Reconfigure message, if the client does not include a Reconfigure
Accept option (see ) in the
Solicit 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
lease assignments and other resources, the server responds to the
Solicit with a Reply message as described in .
Otherwise, the server ignores the Rapid Commit option and
processes the remainder of the message as if no Rapid Commit
option were present.The server sets the "msg-type" field to ADVERTISE and copies the
contents of the transaction-id field from the Solicit message
received from the client to the Advertise message. The server
includes its server identifier in a Server Identifier option and
copies the Client Identifier from the Solicit message into the
Advertise message.The server MAY add a Preference option to carry
the preference value for the Advertise message. The server
implementation SHOULD allow the setting of a server preference value
by the administrator. The server preference value MUST default to
zero unless otherwise configured by the server administrator.The server includes a Reconfigure Accept option if the server
wants to require that the client accept Reconfigure messages.The server includes options the server will return to the client
in a subsequent Reply message. The information in these options may
be used by the client in the selection of a server if the client
receives more than one Advertise message. If the client has included
an Option Request option in the Solicit message, the server includes
options in the Advertise message containing configuration parameters
for all of the options identified in the Option Request option that
the server has been configured to return to the client. The server
MAY return additional options to the client if it has been
configured to do so. The server must be aware of the recommendations
on packet sizes and the use of fragmentation in section 5 of .If the Solicit message from the client included one or more IA
options, the server MUST include IA options in the Advertise message
containing any addresses and/or delegated prefixes that would be assigned
to IAs contained in the Solicit message from the client. If the client
has included addresses in the IA in the Solicit message, the server
MAY use those addresses as hints about the addresses that the client
would like to receive. If the client has included IA Prefix option
in the IA_PD, the server MAY use the prefix contained in the
IPv6 prefix field and/or the prefix length contained in the
"prefix-length" field as a hints about the prefixes the client
would like to receive. If the server is not going to assign an address
or delegated prefix received as a hint in the Solicit message, the
server MUST NOT include this address or delegated prefix in the
Advertise messageIf the server will not assign any addresses to an IA (IA_NA or
IA_IA) in subsequent Request from the client, the server MUST include
the IA in the Advertise message with no addresses in the IA and a
Status Code option encapsulated in the IA containing status code
NoAddrsAvail.If the server will not assign any prefixes to an IA_PD in
subsequent Request from the client, the server MUST include the
IA_PD in the Advertise message with no prefixes in the IA and a
Status Code option encapsulated in the IA_PD containing status code
NoPrefixAvail.If the Solicit message was received directly by the server, the
server unicasts the Advertise message directly to the client using
the address in the source address field from the IP datagram in
which the Solicit message was received. The Advertise message MUST
be unicast on the link from which the Solicit message was
received.If the Solicit message was received in a Relay-forward message,
the server constructs a Relay-reply message with the Advertise
message in the payload of a "relay-message" option. If the
Relay-forward messages included an Interface-id option, the server
copies that option to the Relay-reply message. The server unicasts
the Relay-reply message directly to the relay agent using the
address in the source address field from the IP datagram in which
the Relay-forward message was received.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 leases before sending the Reply message.
The client can assume it has been assigned the leases in the
Reply message and does not need to send a Request message for those
addresses.Typically, servers that are configured to use the Solicit-Reply
message exchange will be deployed so that only one server will
respond to a Solicit message. If more than one server responds, the
client will only use the leases from one of the servers, while
the leases from the other servers will be committed to the client
but not used by the client.The server includes a 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 . The server
transmits the Reply message as described in .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 address(es) and/or delegated prefix(es), using
Renew and Rebind messages.According to a terminology for the prefix delegation, a client requesting a
delegation of a prefix is referred to as a requesting router and a server
delegating the prefix is referred to as a delegating router. The requesting
router and the delegating router use the IA_PD Prefix option to exchange
information about prefix(es) in much the same way as IA Address options
are used for assigned addresses. Typically, a single DHCP session is
used to exchange information about addresses and prefixes, i.e.
IA_NA and IA_PD options are carried in the same message.A client uses Request, Renew, Rebind, Release and Decline messages
during the normal life cycle of addresses and delegated prefixes. When a
client detects it may have moved to a new link, it uses Confirm if it
only has addresses and Rebind if it has delegated prefixes (and
addresses). It uses Information-request messages when it needs
configuration information but no addresses and no prefixes.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 () 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.The client uses a Request message to populate IAs with leases
and obtain other configuration information. The client includes one
or more IA options in the Request message. The server then returns
leases and other information about the IAs to the client in IA
options in a Reply message.The client generates a transaction ID and inserts this value in
the "transaction-id" field.The client places the identifier of the destination server in a
Server Identifier option.The client MUST include a Client Identifier option to identify
itself to the server. The client adds any other appropriate options,
including one or more IA options (if the client is requesting that
the server assign it some network addresses or delegated prefixes).
The client MUST include an Option Request option (see
) to indicate the options the client is interested in receiving.
The client MAY include options with data values as hints to the
server about parameter values the client would like to have
returned.The client includes a Reconfigure Accept option (see
) indicating whether or not the client is willing to accept
Reconfigure messages from the server.The client transmits the message according to , using
the following parameters:
REQ_TIMEOUT REQ_MAX_RT REQ_MAX_RC 0If 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
. Terminate the configuration process and report failure.Whenever a client may have moved to a new link, the
prefixes/addresses assigned to the interfaces on that link may no longer
be appropriate for the link to which the client is attached.
Examples of times when a client may have moved to a new link
include:
The client reboots (and has no stable storage or persisted DHCP state). The client is physically connected to a wired connection. The client returns from sleep mode. The client using a wireless technology changes access
points.In any situation when a client may have moved to a new link and
the client does not have any delegated prefixes obtained from the
DHCP server from which it has obtained the addresses, the
client SHOULD initiate a Confirm/Reply message exchange. The client
includes any IAs assigned to the interface that may have moved to a
new link, along with the addresses associated with those IAs, in its
Confirm message. Any responding servers will indicate whether those
addresses are appropriate for the link to which the client is
attached with the status in the Reply message it returns to the
client.If the client has any valid delegated prefixes obtained from the
DHCP server from which it has obtained the addresses, the client
initiates Rebind/Reply exchange as described in
instead of sending the
Confirm message.The client sets the "msg-type" field to CONFIRM. The client
generates a transaction ID and inserts this value in the
"transaction-id" field.The client MUST include a Client Identifier option to identify
itself to the server. The client 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 , using the
following parameters:
CNF_TIMEOUT CNF_MAX_RT 0 CNF_MAX_RDIf the client receives no responses before the message
transmission process terminates, as described in , 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.To extend the valid and preferred lifetimes for the leases
assigned to the IAs, the client sends a Renew message to the server
from which the leases were obtained, which includes IA options
for the IAs whose lease lifetimes are to be extended. The client
includes IA Address options within IA_NA and IA_TA options for the
addresses assigned to the IAs. The client includes IA Prefix options
within IA_PD options for the delegated prefixes assigned to the IAs.
The server determines new lifetimes for the leases according to the
administrative configuration of the server. The server may also add
leases to the IAs. The server can remove leases from the IAs
by returning IA Address options (for IA_NA and IA_TA) and IA Prefix
options (for IA_PD) with preferred and valid lifetimes set to 0.The server controls the time at which the client contacts the server
to extend the lifetimes on assigned leases through the T1 and T2
parameters assigned to an IA. However, as the client Renews/Rebinds all
IAs from the server at the same time, the client MUST select a T1 and
T2 time from all IA options, which will guarantee that the client will
send Renew/Rebind messages not later than at the T1/T2 times associated
with any of the client's bindings.At time T1, the client initiates a Renew/Reply message
exchange to extend the lifetimes on any leases in the IA.If T1 or T2 had been set to 0 by the server (for an IA_NA or IA_PD) or
there are no T1 or T2 times (for an IA_TA) in a previous Reply, the client
may send a Renew or Rebind message, respectively, at the client's discretion.
The client MUST follow the rules defined in .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.For IAs to which leases have been assigned, the client includes
a corresponding IA option containing an IA Address option for each
address assigned to the IA and IA Prefix option for each prefix
assigned to the IA. The client MUST NOT include addresses and
prefixes in any IA option that the client did not obtain from the
server or that are no longer valid (that have a valid lifetime of 0).The client MAY include an IA option for each binding it desires but
has been unable to obtain. In this case, if the client includes the
IA_PD option to request prefix delegation, the client MAY include
the IA Prefix option encapsulated within the IA_PD option, with
the IPv6 prefix field set to 0 and the "prefix-length" field set
to the desired length of the prefix to be delegated. The server MAY
use this value as a hint for the prefix length. The client SHOULD NOT
include IA Prefix option with the IPv6 prefix field set to 0 unless
it is supplying a hint for the prefix length.The client MUST include an Option Request option (see
) 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 , using
the following parameters:
REN_TIMEOUT REN_MAX_RT 0 Remaining time until T2The message exchange is terminated when time T2 is reached (see
), at which time the client
begins a Rebind message exchange. At time T2 (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 constructs the Rebind message as described in
with the following differences:
The client sets the "msg-type" field to REBIND. The client does not include the Server Identifier
option in the Rebind message.The client transmits the message according to ,
using the following parameters:
REB_TIMEOUT REB_MAX_RT 0 Remaining time until valid lifetimes of all
addresses in all IAs have expiredIf all leases for an IA have expired, the client may choose to
include this IA in subsequent Rebind messages to indicate that the
client is interested in assignment of the leases to this IA.The message exchange is terminated when the valid lifetimes of all
leases across all IAs have expired, at which time the client uses
the Solicit message to locate a new DHCP server and sends a Request
for the expired IAs to the new server.The client uses an Information-request message to obtain
configuration information without having addresses and/or
delegated prefixes assigned to it.The client sets the "msg-type" field to INFORMATION-REQUEST. The
client generates a transaction ID and inserts this value in the
"transaction-id" field.The client SHOULD include a Client Identifier option to identify
itself to the server. If the client does not include a Client
Identifier option, the server will not be able to return any
client-specific options to the client, or the server may choose not
to respond to the message at all. The client MUST include a Client
Identifier option if the Information-request message will be
authenticated.The client MUST include an Option Request option (see
) to request the INF_MAX_RT option
(see ) and any
other options the client is interested in receiving.
The client MAY include options with data values as hints to the
server about parameter values the client would like to have
returned.The 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
, using the following parameters:
INF_TIMEOUT INF_MAX_RT 0 0To release one or more leases, a client sends a Release
message to the server.The client sets the "msg-type" field to RELEASE. The client
generates a transaction ID and places this value in the
"transaction-id" field.The client places the identifier of the server that allocated the
lease(s) in a Server Identifier option.The client MUST include a Client Identifier option to identify
itself to the server. The client includes options containing the IAs
for the leases it is releasing in the "options" field. The
leases to be released MUST be included in the IAs. Any leases
for the IAs the client wishes to continue to use MUST NOT be added
to the IAs.The client MUST NOT use any of the addresses it is releasing as
the source address in the Release message or in any subsequently
transmitted message.Because Release messages may be lost, the client should
retransmit the Release if no Reply is received. However, there are
scenarios where the client may not wish to wait for the normal
retransmission timeout before giving up (e.g., on power down).
Implementations SHOULD retransmit one or more times, but MAY choose
to terminate the retransmission procedure early.The client transmits the message according to , using
the following parameters:
REL_TIMEOUT 0 REL_MAX_RC 0The client MUST stop using all of the leases being released
before the client begins the Release message exchange process.
For an address, this means the address MUST have been removed from
the interface. For a delegated prefix, this means the prefix
MUST have been advertised with a Preferred Lifetime and a Valid
Lifetime of zero in a Router Advertisement message as described in
Section 5.5.3, (e) of - also see L-13 in
Section 4.3 of .If leases are released but the Reply from a DHCP server is lost, the
client will retransmit the Release message, and the server may
respond with a Reply indicating a status of NoBinding. Therefore,
the client does not treat a Reply message with a status of NoBinding
in a Release message exchange as if it indicates an error.Note that if the client fails to release the lease, each
lease assigned to the IA will be reclaimed by the server when the
valid lifetime of that lease expires.If a client detects that one or more addresses assigned to it by
a server are already in use by another node, the client sends a
Decline message to the server to inform it that the address is
suspect.The Decline message is not used in prefix delegation and thus
the client MUST NOT include IA_PD options in the Decline message.The client sets the "msg-type" field to DECLINE. The client
generates a transaction ID and places this value in the
"transaction-id" field.The client places the identifier of the server that allocated the
address(es) in a Server Identifier option.The client MUST include a Client Identifier option to identify
itself to the server. The client 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 , using
the following parameters:
DEC_TIMEOUT 0 DEC_MAX_RC 0If addresses are declined but the Reply from a DHCP server is
lost, the client will retransmit the Decline message, and the server
may respond with a Reply indicating a status of NoBinding.
Therefore, the client does not treat a Reply message with a status
of NoBinding in a Decline message exchange as if it indicates an
error.The client SHOULD NOT send a Release message for other bindings
it may have received just because it sent a Decline message. The
client SHOULD retain the non-conflicting bindings. The client SHOULD
treat the failure to acquire a binding as a result of the conflict,
to be equivalent to not having received the binding, insofar as it
behaves when sending Renew and Rebind messages.Upon the receipt of a valid Reply message in response to a Solicit
(with a Rapid Commit option), Request, Confirm, Renew, Rebind, or
Information-request message, the client extracts the top-level Status
Code option if present.If the client receives a Reply message with a status code of
UnspecFail, the server is indicating that it was unable to process
the message due to an unspecified failure condition. If the client
retransmits the original message to the same server to retry the
desired operation, the client MUST limit the rate at which it
retransmits the message and limit the duration of the time during
which it retransmits the message (see
).If the client receives a Reply message with a status code of
UseMulticast, the client records the receipt of the message and
sends subsequent messages to the server through the interface on
which the message was received using multicast. The client resends
the original message using multicast.Otherwise (no status code or another status code), the client
processes the Reply as described below based on the original
message for which the Reply was received.The client MAY choose to report any status code or message from
the Status Code option in the Reply message.If the client receives a NotOnLink status from the server in
response to a Solicit (with a Rapid Commit option) or a Request, the
client can either re-issue the message without specifying any
addresses or restart the DHCP server discovery process (see
).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 leases in the IA option to the IA as
recorded by the client. Update lifetimes for any leases in the IA option
that the client already has recorded in the IA. Discard any leases from the IA, as recorded by the
client, that have a valid lifetime of 0 in the IA Address or
IA Prefix option. Leave unchanged any information about leases the
client has recorded in the IA but that were not included in the IA
from the server.If the client can operate with the addresses and/or prefixes
obtained from the server:
The client uses the addresses, delegated prefixes,
and other information from any IAs that do not contain a Status Code
option with the NoAddrsAvail or NoPrefixAvail status code. The
client MAY include the IAs for which it received the NoAddrsAvail or
NoPrefixAvail status code, with no addresses or prefixes, in
subsequent Renew and Rebind messages sent to the server, to retry
obtaining the addresses or prefixes for these IAs.The client SHOULD perform duplicate address detection
on each of the received addresses in any IAs,
on which it has not performed duplicate address detection during
processing of any of the previous Reply messages from the server.
The client performs the duplicate address detection before using
the received addresses for the traffic. If any of the addresses
are found to be in use on the link, the client sends a Decline
message to the server for those addresses as described in
.For each IA_PD the delegating router assigns a subnet from each
of the delegated prefixes to each of the links to which the associated
interfaces are attached.Management of the specific configuration information is detailed in
the definition of each option in .If the Reply message contains any IAs, but the client finds no usable
addresses and/or delegated prefixes in any of these IAs, the client
may either try another server (perhaps restarting the DHCP server
discovery process) or use the Information-request message to obtain
other configuration information only.When the client receives a Reply message in response to a Renew or
Rebind message, the client:
Sends a Request message if any of the IAs in the
Reply message contains the NoBinding status code. The client places
IA options in this message for only those IAs for which the server
returned the NoBinding status code in the Reply message. The client
continues to use other bindings for which the server did not
return an error.Sends a Renew/Rebind if any of the IAs are not in the
Reply message, but in this case the client MUST limit the rate at which
it sends these messages, to avoid the Renew/Rebind storm.Otherwise accepts the information in the IA.Whenever a client restarts the DHCP server discovery process
or selects an alternate server, as described in
, the client SHOULD stop using
all the addresses and delegated prefixes for which it has the bindings
and try to obtain all required leases from the new server. This
facilitates the client using a single state machine for all bindings.
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.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 , and
client-initiated configuration, as described in
. If the client receives any Reply
messages that indicate a success status (explicit or implicit),
the client can use the addresses in the IA and ignore any messages
that indicate a NotOnLink status.In some circumstances the requesting router may need
verification that the delegating router still has a valid
binding for the requesting router. Examples of times when a
requesting router may ask for such verification include:The requesting router reboots.The requesting router's upstream link flaps.The requesting router is physically disconnected from a
wired connection.If such verification is needed the requesting router MUST
initiate a Rebind/Reply message exchange as described in
, with the exception
that the retransmission parameters should be set as for the
Confirm message, described in .
The requesting router includes any IA_PDs, along with
prefixes associated with those IA_PDs in its Rebind
message.For this discussion, the Server is assumed to have been configured
in an implementation specific manner with configuration of interest to
clients.In most instances, the server will send a Reply in response to a
client message. This Reply message MUST always contain the Server
Identifier option containing the server's DUID and the Client
Identifier option from the client message if one was present.In most Reply messages, the server includes options containing
configuration information for the client. The server must be aware of
the recommendations on packet sizes and the use of fragmentation in
section 5 of . 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.When the server receives a Request message via unicast from a
client to which the server has not sent a unicast option (or is
not currently configured to send a unicast option to the client),
the server
discards the Request message and responds with a Reply message
containing a Status Code option with the value UseMulticast, a
Server Identifier option containing the server's DUID, the Client
Identifier option from the client message, and no other options.When the server receives a valid Request message, the server
creates the bindings for that client according to the server's
policy and configuration information and records the IAs and other
information requested by the client.The server constructs a Reply message by setting the "msg-type"
field to REPLY, and copying the transaction ID from the Request
message into the transaction-id field.The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Request
message in the Reply message.The server examines all IAs in the message from the client.For each IA_NA and IA_TA the server checks if the prefixes on
included IP addresses are appropriate for the link to which the
client is connected. If any of the prefixes on the included IP
addresses is not appropriate for the link to which the client is
connected, the server MUST return the IA to the client with a Status Code
option with the value NotOnLink. If the server does not send the
NotOnLink status code but it cannot assign any IP addresses to an IA,
the server MUST return the IA in the Reply message with no addresses
in the IA and a Status Code option containing status code NoAddrsAvail.For any IA_PD to which the server cannot assign any delegated
prefixes, the server MUST return the IA_PD option in the Reply
message with the Status Code option containing status code NoPrefixAvail.The server MAY assign different addresses and/or delegated prefixes
to an IA than included in the IA within the Request message sent by the
client.For all IAs to which the server
can assign addresses or delegated prefixes, the server includes the IAs
with addresses (for IA_NA and IA_TA), prefixes (for IA_PD) and other
configuration parameters, and records the IA as a new client
binding. The server MUST NOT include any addresses or delegated prefixes
in the IA which the server does not assign to the client.The 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
.If the server finds that the client has included an IA in the
Request message for which the server already has a binding that
associates the IA with the client, the server sends a new Reply
message with existing bindings, possibly with updated lifetimes.
The server may update the bindings according to its local policies,
but the server SHOULD generate the response again and not simply
retransmit previously sent information, even if the transaction-id
matches previous transmission. The server MUST NOT cache its responses.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.When the server receives a Renew message via unicast from a client to
which the server has not sent a unicast option (or is not currently
configured to send a unicast option to the client), the server discards
the Renew message and responds with a Reply message containing a
Status Code option with the value UseMulticast, a Server Identifier
option containing the server's DUID, the Client Identifier option
from the client message, and no other options.For each IA in the Renew message from a client, the server locates
the client's binding and verifies that the information in the IA
from the client matches the information stored for that client.If the server finds the client entry for the IA, the
server sends back the IA to the client with new lifetimes and,
if applicable, T1/T2 times. If the server is unable to extend the
lifetimes of an address or delegated prefix in the IA, the server MAY
choose not to include
the IA Address or IA Prefix option for this address or delegated prefix.The server may choose to change the list of addresses or delegated prefixes and the
lifetimes in IAs that are returned to the client.If the server finds that any of the addresses in the IA are not
appropriate for the link to which the client is attached, the server
returns the address to the client with lifetimes of 0.If the server finds that any of the delegated prefixes in the IA are not
appropriate for the link to which the client is attached, the server
returns the delegated prefix to the client with lifetimes of 0.For each IA for which the server cannot find a client entry, the server
has the following choices depending on the server's policy and
configuration information:
If the server is configured to create new bindings
as a result of processing Renew messages, the server SHOULD create
a binding and return the IA with assigned addresses or delegated prefixes with lifetimes
and, if applicable, T1/T2 times and other information requested by
the client. If the client included the IA Prefix option within the
IA_PD option with the non-zero value in the "prefix-length" field,
the server MAY use this value as a hint for the length of the prefixes
to be assigned (see
for
further details on prefix length hints).
If the server is configured to create new bindings
as a result of processing Renew messages, but the server will not
assign any leases to an IA, the server returns the IA option
containing a Status Code option with the NoAddrsAvail or NoPrefixAvail status code
and a status message for a user.If the server does not support creation of new
bindings for the client sending a Renew message, or if this behavior
is disabled according to the server's policy or configuration
information, the server returns the IA option containing a Status
Code option with the NoBinding status code and a status message for
a user.The server constructs a Reply message by setting the "msg-type" field
to REPLY and copying the transaction ID from the Renew message into
the "transaction-id" field.The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Renew message
in the Reply message.The server includes other options containing configuration
information to be returned to the client as described in
.The T1/T2 times set in each applicable IA option for a Reply
MUST be the same values across all IAs. The server MUST determine
the T1/T2 times across all of the applicable client's bindings in
the Reply. This facilitates the client being able to renew all of
the bindings at the same time.When the server receives a Rebind message that contains an IA
option from a client, it locates the client's binding and verifies
that the information in the IA from the client matches the
information stored for that client.If the server finds the client entry for the IA and the
server determines that the addresses or delegated prefixes in the
IA are appropriate for the link to which the client's interface is
attached according to the server's explicit configuration information,
the server SHOULD send back the IA to the client with new lifetimes and,
if applicable, T1/T2 times. If the server is unable to extend the
lifetimes of an address in the IA, the server MAY choose not to
include the IA Address option for this address. If the server is unable
to extend the lifetimes of a delegated prefix in the IA, the server MAY
choose not to include the IA Prefix option for this prefix.If the server finds that the client entry for the IA and any of the
addresses or delegated prefixes are no longer appropriate for the link
to which the client's interface is attached according to the server's
explicit configuration information, the server returns the address or
delegated prefix to the client with lifetimes of 0.If the server cannot find a client entry for the IA, the server checks
if the IA contains addresses (for IA_NA and IA_TA) or delegated prefixes
(for IA_PD). The server checks if the addresses and delegated prefixes
are appropriate for the link to which the client's interface is attached
according to the server's explicit configuration information. For any
address which is not appropriate for the link to which the client's
interface is attached, the server MAY include the IA Address option with
the lifetimes of 0. For any delegated prefix which is not appropriate for
the link to which the client's interface is attached, the server MAY
include the IA Prefix option with the lifetimes of 0. The Reply with
lifetimes of 0 constitutes an explicit notification to the client that
the specific addresses and delegated prefixes are no longer valid and
MUST NOT be used by the client. In this situation, if the server does
not send a Reply message, it silently discards the Rebind message.Otherwise, for each IA for which the server cannot find a client entry,
the server has the following choices depending on the server's policy and
configuration information:
If the server is configured to create new bindings
as a result of processing Rebind messages (also see the note about
the Rapid Commit option below), the server SHOULD create a binding and
return the IA with allocated leases with lifetimes and, if applicable,
T1/T2 times and other information requested by the client. The
server MUST NOT return any addresses or delegated prefixes in
the IA which the server does not assign to the client.If the server is configured to create new bindings
as a result of processing Rebind messages, but the server will not
assign any leases to an IA, the server returns the IA option
containing a Status Code option with the NoAddrsAvail or NoPrefixAvail status code
and a status message for a user.If the server does not support creation of new
bindings for the client sending a Rebind message, or if this
behavior is disabled according to the server's policy or
configuration information, the server returns the IA option
containing a Status Code option with the NoBinding status code
and a status message for a user.When the server creates new bindings for the IA, it is possible that
other servers also create bindings as a result of receiving the
same Rebind message. This is the same issue as in the Discussion
under "Rapid Commit Option"; see .
Therefore, the server SHOULD only create new bindings during processing
of a Rebind message if the server is configured to respond with a Reply
message to a Solicit message containing the Rapid Commit option.The server constructs a Reply message by setting the "msg-type"
field to REPLY and copying the transaction ID from the Rebind
message into the "transaction-id" field.The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Rebind
message in the Reply message.The server includes other options containing configuration
information to be returned to the client as described in
.The T1/T2 times set in each applicable IA option for a Reply
MUST be the same values across all IAs. The server MUST determine
the T1/T2 times across all of the applicable client's bindings in
the Reply. This facilitates the client being able to renew all of
the bindings at the same time.When the server receives an Information-request message, the
client is requesting configuration information that does not include
the assignment of any leases. The server determines all
configuration parameters appropriate to the client, based on the
server configuration policies known to the server.The server constructs a Reply message by setting the "msg-type"
field to REPLY, and copying the transaction ID from the
Information-request message into the transaction-id field.The server MUST include a Server Identifier option containing the
server's DUID in the Reply message. If the client included a Client
Identification option in the Information-request message, the server
copies that option to the Reply message.The server includes options
containing configuration information to be returned to the client as
described in .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.When the server receives a Release message via unicast from a
client to which the server has not sent a unicast option (or is not currently
configured to send a unicast option to the client), the server
discards the Release message and responds with a Reply message
containing a Status Code option with value UseMulticast, a Server
Identifier option containing the server's DUID, the Client
Identifier option from the client message, and no other options.Upon the receipt of a valid Release message, the server examines
the IAs and the leases in the IAs for validity. If the IAs in the
message are in a binding for the client, and the leases in the
IAs have been assigned by the server to those IAs, the server
deletes the leases from the IAs and makes the leases available
for assignment to other clients. The server ignores leases not
assigned to the IA, although it may choose to log an error.After all the leases have been processed, the server generates
a Reply message and includes a Status Code option with value
Success, a Server Identifier option with the server's DUID, and a
Client Identifier option with the client's DUID. For each IA in the
Release message for which the server has no binding information, the
server adds an IA option using the IAID from the Release message,
and includes a Status Code option with the value NoBinding in the IA
option. No other options are included in the IA option.A server may choose to retain a record of assigned leases and
IAs after the lifetimes on the leases have expired to allow the
server to reassign the previously assigned leases to a
client.When the server receives a Decline message via unicast from a
client to which the server has not sent a unicast option (or is not currently
configured to send a unicast option to the client), the server
discards the Decline message and responds with a Reply message
containing a Status Code option with the value UseMulticast, a
Server Identifier option containing the server's DUID, the Client
Identifier option from the client message, and no other options.Upon the receipt of a valid Decline message, the server examines the
IAs and the addresses in the IAs for validity. If the IAs in the
message are in a binding for the client, and the addresses in the
IAs have been assigned by the server to those IAs, the server
deletes the addresses from the IAs. The server ignores addresses not
assigned to the IA (though it may choose to log an error if it finds
such an address).The client has found any addresses in the Decline messages to be
already in use on its link. Therefore, the server SHOULD mark the
addresses declined by the client so that those addresses are not
assigned to other clients, and MAY choose to make a notification
that addresses were declined. Local policy on the server determines
when the addresses identified in a Decline message may be made
available for assignment.After all the addresses have been processed, the server generates
a Reply message 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 Decline message and
includes a Status Code option with the value NoBinding in the IA
option. No other options are included in the IA option.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 ). 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.A server initiates a configuration exchange to cause DHCP clients to
obtain new addresses and other configuration information. For example,
an administrator may use a server-initiated configuration exchange when
links in the DHCP domain are to be renumbered. Other examples include
changes in the location of directory servers, addition of new services
such as printing, and availability of new software.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.The server sets the "msg-type" field to RECONFIGURE. The server
sets the transaction-id field to 0. The server includes a Server
Identifier option containing its DUID and a Client Identifier option
containing the client's DUID in the Reconfigure message.The server MAY include an Option Request option to inform the
client of what information has been changed or new information that
has been added. In particular, the server specifies the IA option in
the Option Request option if the server wants the client to obtain
new address information. If the server identifies the IA option in
the Option Request option, the server MUST include an IA option
to identify each IA that is to be reconfigured on the client.
The IA options included by the server MUST NOT contain any
options.Because of the risk of denial of service attacks against DHCP
clients, the use of a security mechanism is mandated in Reconfigure
messages. The server MUST use DHCP authentication in the Reconfigure
message.The server MUST include a Reconfigure Message option (defined in
) to select whether the client responds with a Renew
message, a Rebind message, or an Information-request message.The server MUST NOT include any other options in the Reconfigure
except as specifically allowed in the definition of individual
options.A server sends each Reconfigure message to a single DHCP client,
using an IPv6 unicast address of sufficient scope belonging to the
DHCP client. If the server does not have an address to which it can
send the Reconfigure message directly to the client, the server uses
a Relay-reply message (as described in ) to send the
Reconfigure message to a relay agent that will relay the message to
the client. The server may obtain the address of the client (and the
appropriate relay agent, if required) through the information the
server has about clients that have been in contact with the server,
or through some external agent.To reconfigure more than one client, the server unicasts a
separate message to each client. The server may initiate the
reconfiguration of multiple clients concurrently; for example, a
server may send a Reconfigure message to additional clients while
previous reconfiguration message exchanges are still in
progress.The Reconfigure message causes the client to initiate a
Renew/Reply, a Rebind/Reply, or Information-request/Reply message exchange with the
server. The server interprets the receipt of a Renew, a Rebind, or
Information-request message (whichever was specified in the original
Reconfigure message) from the client as satisfying the Reconfigure
message request.If the server does not receive a Renew, Rebind, or Information-request
message from the client in REC_TIMEOUT milliseconds, the server
retransmits the Reconfigure message, doubles the REC_TIMEOUT value
and waits again. The server continues this process until REC_MAX_RC
unsuccessful attempts have been made, at which point the server
SHOULD abort the reconfigure process for that client.Default and initial values for REC_TIMEOUT and REC_MAX_RC are
documented in .In response to a Renew message, the server generates
and sends a Reply message to the client as
described in and
, including options for
configuration parameters.In response to a Rebind message, the server generates and sends a
Reply message to the client as described in
and , including options
for configuration parameters.The server MAY include options containing the IAs and new values
for other configuration parameters in the Reply message, even if those
IAs and parameters were not requested in the Renew or Rebind message from the
client.The server generates and sends a Reply message to the client as
described in and
, including options for
configuration parameters.The server MAY include options containing
new values for other configuration parameters in the Reply message,
even if those parameters were not requested in the Information-request
message from the client.A client receives Reconfigure messages sent to the UDP port 546 on
interfaces for which it has acquired configuration information through
DHCP. These messages may be sent at any time. Since the results of a
reconfiguration event may affect application layer programs, the
client SHOULD log these events, and MAY notify these programs of the
change through an implementation-specific interface.Upon receipt of a valid Reconfigure message, the client responds
with either a Renew message, a Rebind message, or an Information-request message as
indicated by the Reconfigure Message option (as defined in
). The client ignores the transaction-id field in the received
Reconfigure message. While the transaction is in progress, the
client discards any Reconfigure messages it receives.DISCUSSION:The Reconfigure message acts as a trigger that signals the client
to complete a successful message exchange. Once the client has
received a Reconfigure, the client proceeds with the message
exchange (retransmitting the Renew or Information-request message if
necessary); the client ignores any additional Reconfigure messages
until the exchange is complete. Subsequent Reconfigure messages
cause the client to initiate a new exchange.How does this mechanism work in the face of duplicated or
retransmitted Reconfigure messages? Duplicate messages will be
ignored because the client will begin the exchange after the receipt
of the first Reconfigure. Retransmitted messages will either trigger
the exchange (if the first Reconfigure was not received by the
client) or will be ignored. The server can discontinue
retransmission of Reconfigure messages to the client once the server
receives the Renew or Information-request message from the
client.It might be possible for a duplicate or retransmitted Reconfigure
to be sufficiently delayed (and delivered out of order) to arrive at
the client after the exchange (initiated by the original
Reconfigure) has been completed. In this case, the client would
initiate a redundant exchange. The likelihood of delayed and out of
order delivery is small enough to be ignored. The consequence of the
redundant exchange is inefficiency rather than incorrect
operation.When responding to a Reconfigure, the client creates and sends
the Renew message in exactly the same manner as outlined in
, with the exception that the client copies the Option Request
option and any IA options from the Reconfigure message into the
Renew message. The client MUST include a Server Identifier
option in the Renew message, identifying the server with which
the client most recently communicated.When responding to a Reconfigure, the client creates and sends the
Rebind message in exactly the same manner as outlined in
, with the exception that the client copies the
Option Request option and any IA options from the Reconfigure message
into the Rebind message.If a client is currently sending Rebind messages, as described in
, the client ignores any received
Reconfigure messages.When responding to a Reconfigure, the client creates and sends
the Information-request message in exactly the same manner as
outlined in , with the exception that the client
includes a Server Identifier option with the identifier from the
Reconfigure message to which the client is responding.The client uses the same variables and retransmission algorithm
as it does with Renew, Rebind, or Information-request messages generated as
part of a client-initiated configuration exchange. See
, , and
for details. If the client does not receive a
response from the server by the end of the retransmission process,
the client ignores and discards the Reconfigure message.Upon the receipt of a valid Reply message, the client processes
the options and sets (or resets) configuration parameters
appropriately. The client records and updates the lifetimes for any
addresses specified in IAs in the Reply message.This section describes prefix delegation in Reconfigure
message exchanges.The delegating router initiates a configuration message
exchange with a requesting router, as described in ,
by sending a Reconfigure message (acting as a DHCP server)
to the requesting router, as described in .
The delegating router specifies the IA_PD option in the
Option Request option to cause the requesting router to
include an IA_PD option to obtain new information about
delegated prefix(es).The requesting router responds to a Reconfigure message,
acting as a DHCP client, received from a delegating router
as described in
The requesting router MUST include the IA_PD Prefix option(s) (in
an IA_PD option) for prefix(es) that have been delegated to
the requesting router by the delegating router from which the
Reconfigure message was received.The relay agent MAY be configured to use a list of destination
addresses, which MAY include unicast addresses, the All_DHCP_Servers
multicast address, or other addresses selected by the network
administrator. If the relay agent has not been explicitly configured, it
MUST use the All_DHCP_Servers multicast address as the default.If the
relay agent relays messages to the All_DHCP_Servers multicast address or
other multicast addresses, it sets the Hop Limit field to 32.If the relay agent receives a message other than Relay-forward
and Relay-reply and the relay agent does not recognize its
message type, it MUST forward them as described in .A relay agent relays both messages from clients and Relay-forward
messages from other relay agents. When a relay agent receives a valid
message (for a definition of a valid message, see Section 4.1 of ) to be relayed, it constructs a new Relay-forward message. The
relay agent copies the source address from the header of the IP
datagram in which the message was received to the peer-address field
of the Relay-forward message. The relay agent copies the received DHCP
message (excluding any IP or UDP headers) into a Relay Message option
in the new message. The relay agent adds to the Relay-forward message
any other options it is configured to include. defines a Lightweight DHCPv6 Relay
Agent (LDRA) that allows Relay Agent Information to be inserted by an
access node that performs a link- layer bridging (i.e., non-routing)
function.If the relay agent received the message to be relayed from a
client, the relay agent places a global, ULA
or site-scoped address with
a prefix assigned to the link on which the client should be assigned
an address in the link-address field. If not addresses of other scopes
are available the relay agent may fill in the link-address field with
a link-local address from the interface the original message was received
on. That is not recommended as it requires additional information
to be provided in the server configuration. See Section 3.2 of
for detailed discussion.This address will be used by
the server to determine the link from which the client should be
assigned an address and other configuration information. The
hop-count in the Relay-forward message is set to 0.If the relay agent cannot use the address in the link-address
field to identify the interface through which the response to the
client will be relayed, the relay agent MUST include an Interface-id
option (see ) in the Relay-forward message. The server
will include the Interface-id option in its Relay-reply message. The
relay agent fills in the link-address field as described in the
previous paragraph regardless of whether the relay agent includes an
Interface-id option in the Relay-forward message.If the message received by the relay agent is a Relay-forward
message and the hop-count in the message is greater than or equal to
HOP_COUNT_LIMIT, the relay agent discards the received message.The relay agent copies the source address from the IP datagram in
which the message was received from the relay agent into the peer-address
field in the Relay-forward message and sets the hop-count field to
the value of the hop-count field in the received message incremented
by 1.If the source address from the IP datagram header of the received
message is a global or site-scoped address (and the device on which
the relay agent is running belongs to only one site), the relay
agent sets the link-address field to 0; otherwise the relay agent
sets the link-address field to a global or site-scoped address
assigned to the interface on which the message was received, or
includes an Interface-ID option to identify the interface on which
the message was received.A relay agent forwards messages containing Prefix Delegation
options in the same way as described earlier in this section.If a delegating router communicates with a requesting router
through a relay agent, the delegating router may need a protocol
or other out-of-band communication to configure routing information
for delegated prefixes on any router through which the requesting
router may forward traffic.The relay agent processes any options included in the Relay-reply
message in addition to the Relay Message option, and then discards
those options.The relay agent extracts the message from the Relay Message option
and relays it to the address contained in the peer-address field of
the Relay-reply message. Relay agents MUST NOT modify the message.If the Relay-reply message includes an Interface-id option, the
relay agent relays the message from the server to the client on the
link identified by the Interface-id option. Otherwise, if the
link-address field is not set to zero, the relay agent relays the
message on the link identified by the link-address field.If the relay agent receives a Relay-reply message, it
MUST process the message as defined above, regardless of the
type of message encapsulated in the Relay Message option.A server uses a Relay-reply message to return a response to a
client if the original message from the client was relayed to the
server in a Relay-forward message or to send a Reconfigure message to
a client if the server does not have an address it can use to send the
message directly to the client.A response to the client MUST be relayed through the same relay
agents as the original client message. The server causes this to
happen by creating a Relay-reply message that includes a Relay Message
option containing the message for the next relay agent in the return
path to the client. The contained Relay-reply message contains another
Relay Message option to be sent to the next relay agent, and so on.
The server must record the contents of the peer-address fields in the
received message so it can construct the appropriate Relay-reply
message carrying the response from the server.For example, if client C sent a message that was relayed by relay
agent A to relay agent B and then to the server, the server would send
the following Relay-Reply message to relay agent B:When sending a Reconfigure message to a client through a relay
agent, the server creates a Relay-reply message that includes a Relay
Message option containing the Reconfigure message for the next relay
agent in the return path to the client. The server sets the
peer-address field in the Relay-reply message header to the address of
the client, and sets the link-address field as required by the relay
agent to relay the Reconfigure message to the client. The server
obtains the addresses of the client and the relay agent through prior
interaction with the client or through some external mechanism.Some network administrators may wish to provide authentication of the
source and contents of DHCP messages. For example, clients may be
subject to denial of service attacks through the use of bogus DHCP
servers, or may simply be misconfigured due to unintentionally
instantiated DHCP servers. Network administrators may wish to constrain
the allocation of addresses to authorized hosts to avoid denial of
service attacks in "hostile" environments where the network medium is
not physically secured, such as wireless networks or college residence
halls.The DHCP authentication mechanism is based on the design of
authentication for DHCPv4 .Relay agents and servers that exchange messages securely use the
IPsec mechanisms for IPv6 . If a client
message is relayed through multiple relay agents, each of the relay
agents must have established independent, pairwise trust
relationships. That is, if messages from client C will be relayed by
relay agent A to relay agent B and then to the server, relay agents A
and B must be configured to use IPsec for the messages they exchange,
and relay agent B and the server must be configured to use IPsec for
the messages they exchange.Relay agents and servers that support secure relay agent to server
or relay agent to relay agent communication use IPsec under the
following conditions:
Relay agents are manually configured with the addresses
of the relay agent or server to which DHCP messages are to be
forwarded. Each relay agent and server that will be using IPsec for
securing DHCP messages must also be configured with a list of the
relay agents to which messages will be returned. The selectors for the
relay agents and servers will be the pairs of addresses defining relay
agents and servers that exchange DHCP messages on DHCPv6 UDP port 547. Relay agents and servers use transport mode and ESP. The
information in DHCP messages is not generally considered confidential,
so encryption need not be used (i.e., NULL encryption can be
used). Because the relay agents and servers are used within
an organization, public key schemes are not necessary. Because the
relay agents and servers must be manually configured, manually
configured key management may suffice, but does not provide defense
against replayed messages. Accordingly, IKE with preshared secrets
SHOULD be supported. IKE with public keys MAY be supported. DHCP messages between relay agents and servers
should only be accepted from DHCP peers as identified in the local
configuration. Shared keys, indexed to the source IP address of the
received DHCP message, are adequate in this application. Appropriate IPsec implementations are likely to be
available for servers and for relay agents in more featureful devices
used in enterprise and core ISP networks. IPsec is less likely to be
available for relay agents in low end devices primarily used in the
home or small office markets.Authentication of DHCP messages is accomplished through the use of
the Authentication option (see ). The authentication
information carried in the Authentication option can be used to
reliably identify the source of a DHCP message and to confirm that the
contents of the DHCP message have not been tampered with.The Authentication option provides a framework for multiple
authentication protocols. Two such protocols are defined here. Other
protocols defined in the future will be specified in separate
documents.Any DHCP message MUST NOT include more than one Authentication
option.The protocol field in the Authentication option identifies the
specific protocol used to generate the authentication information
carried in the option. The algorithm field identifies a specific
algorithm within the authentication protocol; for example, the
algorithm field specifies the hash algorithm used to generate the
message authentication code (MAC) in the authentication option. The
replay detection method (RDM) field specifies the type of replay
detection used in the replay detection field.The Replay Detection Method (RDM) field determines the type of
replay detection used in the Replay Detection field.If the RDM field contains 0x00, the replay detection field MUST be
set to the value of a strictly monotonically increasing counter. Using a
counter value, such as the current time of day (for example, an
NTP-format timestamp ), can reduce the danger
of replay attacks. This method MUST be supported by all protocols.The Reconfigure key authentication protocol provides protection
against misconfiguration of a client caused by a Reconfigure message
sent by a malicious DHCP server. In this protocol, a DHCP server sends
a Reconfigure Key to the client in the initial exchange of DHCP
messages. The client records the Reconfigure Key for use in
authenticating subsequent Reconfigure messages from that server. The
server then includes an HMAC computed from the Reconfigure Key in
subsequent Reconfigure messages.Both the Reconfigure Key sent from the server to the client and the
HMAC in subsequent Reconfigure messages are carried as the
Authentication information in an Authentication option. The format of
the Authentication information is defined in the following
section.The Reconfigure Key protocol is used (initiated by the server) only
if the client and server are not using any other authentication
protocol and the client and server have negotiated to use Reconfigure
messages.The following fields are set in an Authentication option for the
Reconfigure Key Authentication Protocol:
3 1 0The format of the Authentication information for the Reconfigure
Key Authentication Protocol is: Type of data in Value field carried in this option:
Reconfigure Key value (used in Reply message). HMAC-MD5 digest of the message (used in Reconfigure
message). Data as defined by the Type field.The server selects a Reconfigure Key for a client during the
Request/Reply, Solicit/Reply or Information-request/Reply message
exchange. The server records the Reconfigure Key and transmits that
key to the client in an Authentication option in the Reply
message.The Reconfigure Key is 128 bits long, and MUST be a
cryptographically strong random or pseudo-random number that cannot
easily be predicted.To provide authentication for a Reconfigure message, the server
selects a replay detection value according to the RDM selected by
the server, and computes an HMAC-MD5 of the Reconfigure message
using the Reconfigure Key for the client. The server computes the
HMAC-MD5 over the entire DHCP Reconfigure message, including the
Authentication option; the HMAC-MD5 field in the Authentication
option is set to zero for the HMAC-MD5 computation. The server
includes the HMAC-MD5 in the authentication information field in an
Authentication option included in the Reconfigure message sent to
the client.The client will receive a Reconfigure Key from the server in the
initial Reply message from the server. The client records the
Reconfigure Key for use in authenticating subsequent Reconfigure
messages.To authenticate a Reconfigure message, the client computes an
HMAC-MD5 over the DHCP Reconfigure message, using the Reconfigure
Key received from the server. If this computed HMAC-MD5 matches the
value in the Authentication option, the client accepts the
Reconfigure message.Options are used to carry additional information and parameters in
DHCP messages. Every option shares a common base format, as described in
. All values in options are represented in network byte
order.This document describes the DHCP options defined as part of the base
DHCP specification. Other options may be defined in the future in
separate documents. See for guidelines regarding
new options definition.Unless otherwise noted, each option may appear only in the options
area of a DHCP message and may appear only once. If an option does
appear multiple times, each instance is considered separate and the data
areas of the options MUST NOT be concatenated or otherwise combined.Options that are allowed to appear only once are called singleton options.
The only non-singleton options defined in this document are
IA_NA (see ),
IA_TA (see ), and
IA_PD (see ) options.
Also, IAAddress (see ) and
IAPrefix (see ) may appear in their
respective IA options more than once.The format of DHCP options is: An unsigned integer identifying the specific option
type carried in this option. An unsigned integer giving the length of the option-data
field in this option in octets. The data for the option; the format of this data
depends on the definition of the option.DHCPv6 options are scoped by using encapsulation. Some options
apply generally to the client, some are specific to an IA, and some
are specific to the addresses within an IA. These latter two cases are
discussed in and .The Client Identifier option is used to carry a DUID (see
) identifying a client between a client
and a server. The format of the Client Identifier option is: OPTION_CLIENTID (1). Length of DUID in octets. The DUID for the client.The Server Identifier option is used to carry a DUID (see
) identifying a server between a client
and a server. The format of the Server Identifier option is: OPTION_SERVERID (2). Length of DUID in octets. The DUID for the server.The Identity Association for Non-temporary Addresses option (IA_NA
option) is used to carry an IA_NA, the parameters associated with the
IA_NA, and the non-temporary addresses associated with the IA_NA.Addresses appearing in an IA_NA option are not temporary addresses
(see ).The format of the IA_NA option is: OPTION_IA_NA (3). 12 + length of IA_NA-options field. The unique identifier for this IA_NA; the IAID must be unique
among the identifiers for all of this client's IA_NAs. The number
space for IA_NA IAIDs is separate from the number space for IA_TA
IAIDs. The time at which the client contacts the server from which the
addresses in the IA_NA were obtained to extend the lifetimes of the
addresses assigned to the IA_NA; T1 is a time duration relative to the
current time expressed in units of seconds. The time at which the client contacts any available server to
extend the lifetimes of the addresses assigned to the IA_NA; T2 is a
time duration relative to the current time expressed in units of
seconds. Options associated with this IA_NA.The IA_NA-options field encapsulates those options that are
specific to this IA_NA. For example, all of the IA Address Options
carrying the addresses associated with this IA_NA are in the
IA_NA-options field.Each IA_NA carries one "set" of non-temporary addresses; that is, at
most one address from each prefix assigned to the link to which the
client is attached.An IA_NA option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_NA options.The status of any operations involving this IA_NA is indicated in a
Status Code option in the IA_NA-options field.Note that an IA_NA has no explicit "lifetime" or "lease length" of
its own. When the valid lifetimes of all of the addresses in an IA_NA
have expired, the IA_NA can be considered as having expired. T1 and T2
are included to give servers explicit control over when a client
recontacts the server about a specific IA_NA.In a message sent by a client to a server, the T1 and T2
fields SHOULD be set to 0. The server MUST ignore any values in
these fields in messages received from a client.In a message sent by a server to a client, the client MUST use the
values in the T1 and T2 fields for the T1 and T2 parameters, unless
those values in those fields are 0. The values in the T1 and T2 fields
are the number of seconds until T1 and T2.The server selects the T1 and T2 times to allow the client to
extend the lifetimes of any addresses in the IA_NA before the
lifetimes expire, even if the server is unavailable for some short
period of time. Recommended values for T1 and T2 are .5 and .8 times
the shortest preferred lifetime of the addresses in the IA that the
server is willing to extend, respectively. If the "shortest" preferred
lifetime is 0xffffffff ("infinity"), the recommended T1 and T2 values
are also 0xffffffff. If the time at which the addresses in an IA_NA
are to be renewed is to be left to the discretion of the client, the
server sets T1 and T2 to 0. The client MUST follow the rules defined
in .If a server receives an IA_NA with T1 greater than T2, and both T1
and T2 are greater than 0, the server ignores the invalid values of T1
and T2 and processes the IA_NA as though the client had set T1 and T2
to 0.If a client receives an IA_NA with T1 greater than T2, and both T1
and T2 are greater than 0, the client discards the IA_NA option and
processes the remainder of the message as though the server had not
included the invalid IA_NA option.Care should be taken in setting T1 or T2 to 0xffffffff
("infinity"). A client will never attempt to extend the lifetimes of
any addresses in an IA with T1 set to 0xffffffff. A client will never
attempt to use a Rebind message to locate a different server to extend
the lifetimes of any addresses in an IA with T2 set to 0xffffffff.This option MAY appear in a Confirm message if the lifetimes on the
non-temporary addresses in the associated IA have not expired.The Identity Association for the Temporary Addresses (IA_TA) option
is used to carry an IA_TA, the parameters associated with the IA_TA
and the addresses associated with the IA_TA. All of the addresses in
this option are used by the client as temporary addresses, as defined
in . The format of the IA_TA option
is: OPTION_IA_TA (4). 4 + length of IA_TA-options field. The unique identifier for this IA_TA; the IAID must be unique
among the identifiers for all of this client's IA_TAs. The number
space for IA_TA IAIDs is separate from the number space for IA_NA
IAIDs. Options associated with this IA_TA.The IA_TA-Options field encapsulates those options that are
specific to this IA_TA. For example, all of the IA Address Options
carrying the addresses associated with this IA_TA are in the
IA_TA-options field.Each IA_TA carries one "set" of temporary addresses.An IA_TA option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_TA options.The status of any operations involving this IA_TA is indicated in a
Status Code option in the IA_TA-options field.Note that an IA has no explicit "lifetime" or "lease length" of its
own. When the valid lifetimes of all of the addresses in an IA_TA have
expired, the IA can be considered as having expired.An IA_TA option does not include values for T1 and T2. A client MAY
request that the lifetimes on temporary addresses be extended by
including the addresses in a IA_TA option sent in a Renew or Rebind
message to a server. For example, a client would request an extension
on the lifetime of a temporary address to allow an application to
continue to use an established TCP connection.The client obtains new temporary addresses by sending an IA_TA
option with a new IAID to a server. Requesting new temporary addresses
from the server is the equivalent of generating new temporary
addresses as described in . The server will generate new
temporary addresses and return them to the client. The client should
request new temporary addresses before the lifetimes on the previously
assigned addresses expire.A server MUST return the same set of temporary address for the same
IA_TA (as identified by the IAID) as long as those addresses are still
valid. After the lifetimes of the addresses in an IA_TA have expired,
the IAID may be reused to identify a new IA_TA with new temporary
addresses.This option MAY appear in a Confirm message if the lifetimes on the
temporary addresses in the associated IA have not expired.The IA Address option is used to specify IPv6 addresses associated
with an IA_NA or an IA_TA. The IA Address option must be encapsulated
in the Options field of an IA_NA or IA_TA option. The Options fields
of the IA_NA or IA_TA option encapsulates those options that are
specific to this address.The format of the IA Address option is: OPTION_IAADDR (5). 24 + length of IAaddr-options field. An IPv6 address. The preferred lifetime for the IPv6 address in
the option, expressed in units of seconds. The valid lifetime for the IPv6 address in the
option, expressed in units of seconds. Options associated with this address.In a message sent by a client to a server, the preferred
and valid lifetime fields SHOULD be set to 0. The server MUST ignore
any received values.The client SHOULD NOT send the IA Address option with unspecified
address (::).In a message sent by a server to a
client, the client MUST use the values in the preferred and valid
lifetime fields for the preferred and valid lifetimes. The values in
the preferred and valid lifetimes are the number of seconds remaining
in each lifetime.A client discards any addresses for which the
preferred lifetime is greater than the valid lifetime. A server
ignores the lifetimes set by the client if the preferred lifetime is
greater than the valid lifetime and ignores the values for T1 and T2
set by the client if those values are greater than the preferred
lifetime.Care should be taken in setting the valid lifetime of an address to
0xffffffff ("infinity"), which amounts to a permanent assignment of an
address to a client.More than one IA Address Option can appear in an IA_NA option
or an IA_TA option.The status of any operations involving this IA Address is indicated
in a Status Code option in the IAaddr-options field, as specified
in .The Option Request option is used to identify a list of options in
a message between a client and a server. The format of the Option
Request option is: OPTION_ORO (6). 2 * number of requested options. The option code for an option requested by
the client.A client MAY include an Option Request option in a Solicit,
Request, Renew, Rebind, Confirm or Information-request message to
inform the server about options the client wants the server to send to
the client. A server MAY include an Option Request option in a
Reconfigure message to indicate which options the client should request
from the server. If there is a need to request encapsulated options,
top-level Option Request option MUST be used for that purpose. There is
no need request IAADDR or IAPREFIX.The Preference option is sent by a server to a client to affect the
selection of a server by the client.The format of the Preference option is: OPTION_PREFERENCE (7). 1. The preference value for the server in this message.A server MAY include a Preference option in an Advertise message to
control the selection of a server by the client. See
for the use of the Preference option by the client and the
interpretation of Preference option data value. OPTION_ELAPSED_TIME (8). 2. The amount of time since the client began its current
DHCP transaction. This time is expressed in hundredths of a second
(10^-2 seconds).A client MUST include an Elapsed Time option in messages to
indicate how long the client has been trying to complete a DHCP
message exchange. The elapsed time is measured from the time at which
the client sent the first message in the message exchange, and the
elapsed-time field is set to 0 in the first message in the message
exchange. Servers and Relay Agents use the data value in this option
as input to policy controlling how a server responds to a client
message. For example, the elapsed time option allows a secondary DHCP
server to respond to a request when a primary server has not answered
in a reasonable time. The elapsed time value is an unsigned, 16 bit
integer. The client uses the value 0xffff to represent any elapsed
time values greater than the largest time value that can be
represented in the Elapsed Time option.The Relay Message option carries a DHCP message in a Relay-forward
or Relay-reply message.The format of the Relay Message option is: OPTION_RELAY_MSG (9) Length of DHCP-relay-message In a Relay-forward message, the received
message, relayed verbatim to the next relay agent or server; in a
Relay-reply message, the message to be copied and relayed to the relay
agent or client whose address is in the peer-address field of the
Relay-reply messageThe Authentication option carries authentication information to
authenticate the identity and contents of DHCP messages. The use of
the Authentication option is described in . The format of
the Authentication option is: OPTION_AUTH (11). 11 + length of authentication information field. The authentication protocol used in this authentication
option. The algorithm used in the authentication protocol. The replay detection method used in this authentication
option. The replay detection information for the RDM.The authentication information, as
specified by the protocol and algorithm used in this authentication
option.The server sends this option to a client to indicate to the client
that it is allowed to unicast messages to the server. The format of
the Server Unicast option is: OPTION_UNICAST (12). 16. The IP address to which the client should send
messages delivered using unicast.The server specifies the IPv6 address to which the client is to
send unicast messages in the server-address field. When a client
receives this option, where permissible and appropriate, the client
sends messages directly to the server using the IPv6 address specified
in the server-address field of the option.When the server sends a Unicast option to the client, some messages
from the client will not be relayed by Relay Agents, and will not
include Relay Agent options from the Relay Agents. Therefore, a server
should only send a Unicast option to a client when Relay Agents are
not sending Relay Agent options. A DHCP server rejects any messages
sent inappropriately using unicast to ensure that messages are relayed
by Relay Agents when Relay Agent options are in use.Details about when the client may send messages to the server using
unicast are in .This option returns a status indication related to the DHCP message
or option in which it appears. The format of the Status Code option
is: OPTION_STATUS_CODE (13). 2 + length of status-message. The numeric code for the status encoded in
this option. A UTF-8 encoded text string suitable for display to
an end user, which MUST NOT be null-terminated.A Status Code option may appear in the options field of a DHCP
message and/or in the options field of another option. If the Status
Code option does not appear in a message in which the option could
appear, the status of the message is assumed to be Success.The status-codes values previously defined by
and are:NameCodeDescriptionSuccess0Success.UnspecFail1Failure, reason unspecified; this
status code is sent by either a client or a server to indicate
a failure not explicitly specified in this document.NoAddrsAvail2Server has no addresses available
to assign to the IA(s).NoBinding3Client record (binding) unavailable.NotOnLink4The prefix for the address is not
appropriate for the link to which the client is attached.UseMulticast5Sent by a server to a client to
force the client to send messages to the server using the
All_DHCP_Relay_Agents_and_Servers address.NoPrefixAvail6Delegating router has no prefixes
available to assign to the IAPD(s).The Rapid Commit option is used to signal the use of the two
message exchange for address assignment. The format of the Rapid
Commit option is: OPTION_RAPID_COMMIT (14). 0.A client MAY include this option in a Solicit message if the client
is prepared to perform the Solicit-Reply message exchange described in
.A server MUST include this option in a Reply message sent in
response to a Solicit message when completing the Solicit-Reply
message exchange.DISCUSSION:Each server that responds with a Reply to a Solicit that includes a
Rapid Commit option will commit the assigned addresses in the Reply
message to the client, and will not receive any confirmation that the
client has received the Reply message. Therefore, if more than one
server responds to a Solicit that includes a Rapid Commit option, some
servers will commit addresses that are not actually used by the
client.The problem of unused addresses can be minimized, for example, by
designing the DHCP service so that only one server responds to the
Solicit or by using relatively short lifetimes for assigned
addresses, or the DHCP client initiatively releases unused addresses
using the Release message.
The User Class option is used by a client to identify the type or
category of user or applications it represents.The format of the User Class option is: OPTION_USER_CLASS (15). Length of user class data field. The user classes carried by the client.The information contained in the data area of this option is
contained in one or more opaque fields that represent the user class
or classes of which the client is a member. A server selects
configuration information for the client based on the classes
identified in this option. For example, the User Class option can be
used to configure all clients of people in the accounting department
with a different printer than clients of people in the marketing
department. The user class information carried in this option MUST be
configurable on the client.The data area of the user class option MUST contain one or more
instances of user class data. Each instance of the user class data is
formatted as follows:The user-class-len is two octets long and specifies the length of
the opaque user class data in network byte order.A server interprets the classes identified in this option according
to its configuration to select the appropriate configuration
information for the client. A server may use only those user classes
that it is configured to interpret in selecting configuration
information for a client and ignore any other user classes. In
response to a message containing a User Class option, a server
includes a User Class option containing those classes that were
successfully interpreted by the server, so that the client can be
informed of the classes interpreted by the server.This option is used by a client to identify the vendor that
manufactured the hardware on which the client is running. The
information contained in the data area of this option is contained in
one or more opaque fields that identify details of the hardware
configuration. The format of the Vendor Class option is: OPTION_VENDOR_CLASS (16). 4 + length of vendor class data field. The vendor's registered Enterprise Number as
registered with IANA . The hardware configuration of the host on which
the client is running.The vendor-class-data is composed of a series of separate items,
each of which describes some characteristic of the client's hardware
configuration. Examples of vendor-class-data instances might include
the version of the operating system the client is running or the
amount of memory installed on the client.Each instance of the vendor-class-data is formatted as follows:The vendor-class-len is two octets long and specifies the length of
the opaque vendor class data in network byte order.Servers and clients MUST NOT include more than one instance
of OPTION_VENDOR_CLASS with the same Enterprise Number. Each instance
of OPTION_VENDOR_CLASS can carry multiple sub-options.This option is used by clients and servers to exchange
vendor-specific information.The format of the Vendor-specific Information option is: OPTION_VENDOR_OPTS (17). 4 + length of option-data field. The vendor's registered Enterprise Number as
registered with IANA . An opaque object, interpreted by
vendor-specific code on the clients and servers.The definition of the information carried in this option is vendor
specific. The vendor is indicated in the enterprise-number field. Use
of vendor-specific information allows enhanced operation, utilizing
additional features in a vendor's DHCP implementation. A DHCP client
that does not receive requested vendor-specific information will still
configure the host device's IPv6 stack to be functional.The encapsulated vendor-specific options field MUST be encoded as a
sequence of code/length/value fields of identical format to the DHCP
options field. The option codes are defined by the vendor identified
in the enterprise-number field and are not managed by IANA. Each of
the encapsulated options is formatted as follows: The code for the encapsulated option. An unsigned integer giving the length of the option-data
field in this encapsulated option in octets. The data area for the encapsulated option.Multiple instances of the Vendor-specific Information option may
appear in a DHCP message. Each instance of the option is interpreted
according to the option codes defined by the vendor identified by the
Enterprise Number in that option. Servers and clients MUST NOT send
more than one instance of Vendor-specific Information option with the
same Enterprise Number. Each instance of Vendor-specific Information option
MAY contain multiple encapsulated options.A client that is interested in receiving a Vendor-specific Information Option:
MUST specify the Vendor-specific Information Option in an
Option Request Option. MAY specify an associated Vendor Class Option. MAY specify the Vendor-specific Information Option with any
data.Severs only return the Vendor-specific Information Options if specified in
Option Request Options from clients and: MAY use the Enterprise Numbers in the associated Vendor Class
Options to restrict the set of Enterprise Numbers in the Vendor-specific
Information Options returned. MAY return all configured Vendor-specific Information Options. MAY use other information in the packet or in its configuration
to determine which set of Enterprise Numbers in the Vendor-specific Information
Options to return.The relay agent MAY send the Interface-id option to identify the
interface on which the client message was received. If a relay agent
receives a Relay-reply message with an Interface-id option, the relay
agent relays the message to the client through the interface
identified by the option.The format of the Interface ID option is: OPTION_INTERFACE_ID (18). Length of interface-id field. An opaque value of arbitrary length generated by the
relay agent to identify one of the relay agent's interfaces.The server MUST copy the Interface-Id option from the Relay-forward
message into the Relay-reply message the server sends to the relay
agent in response to the Relay-forward message. This option MUST NOT
appear in any message except a Relay-forward or Relay-reply
message.Servers MAY use the Interface-ID for parameter assignment policies.
The Interface-ID SHOULD be considered an opaque value, with policies
based on exact match only; that is, the Interface-ID SHOULD NOT be
internally parsed by the server. The Interface-ID value for an
interface SHOULD be stable and remain unchanged, for example, after
the relay agent is restarted; if the Interface-ID changes, a server
will not be able to use it reliably in parameter assignment
policies.A server includes a Reconfigure Message option in a Reconfigure
message to indicate to the client whether the client responds with a
Renew message, a Rebind message, or an Information-request message.
The format of this option is: OPTION_RECONF_MSG (19). 1. 5 for Renew message, 6 for Rebind,
11 for Information-request message.The Reconfigure Message option can only appear in a Reconfigure
message.A client uses the Reconfigure Accept option to announce to the
server whether the client is willing to accept Reconfigure messages,
and a server uses this option to tell the client whether or not to
accept Reconfigure messages. The default behavior, in the absence of
this option, means unwillingness to accept Reconfigure messages, or
instruction not to accept Reconfigure messages, for the client and
server messages, respectively. The following figure gives the format
of the Reconfigure Accept option: OPTION_RECONF_ACCEPT (20). 0.The IA_PD option is used to carry a prefix delegation
identity association, the parameters associated with the IA_PD
and the prefixes associated with it.OPTION_IA_PD (25).12 + length of IA_PD-options
field.The unique identifier for this IA_PD;
the IAID must be unique among the identifiers for all of
this requesting router's IA_PDs.The time at which the requesting router
should contact the delegating router from which the
prefixes in the IA_PD were obtained to extend the
lifetimes of the prefixes delegated to the IA_PD; T1 is a
time duration relative to the current time expressed in
units of seconds.The time at which the requesting router
should contact any available delegating router to extend
the lifetimes of the prefixes assigned to the IA_PD; T2 is
a time duration relative to the current time expressed in
units of seconds.Options associated with this
IA_PD.The IA_PD-options field encapsulates those options that are
specific to this IA_PD. For example, all of the IA_PD Prefix
Options carrying the prefixes associated with this IA_PD are in
the IA_PD-options field.An IA_PD option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_PD options.The status of any operations involving this IA_PD is indicated
in a Status Code option in the IA_PD-options field.Note that an IA_PD has no explicit "lifetime" or "lease
length" of its own. When the valid lifetimes of all of the
prefixes in a IA_PD have expired, the IA_PD can be considered as
having expired. T1 and T2 are included to give delegating
routers explicit control over when a requesting router should
contact the delegating router about a specific IA_PD.In a message sent by a requesting router to a delegating
router, the T1 and T2 fields SHOULD be set to 0. The delegating
router MUST ignore any values in these fields in messages received
from a requesting router.In a message sent by a delegating router to a requesting router,
the delegating router MUST use the values in the T1 and T2 fields
for the T1 and T2 parameters, unless those values in those fields
are 0. The values in the T1 and T2 fields are the number of seconds
until T1 and T2.The delegating router selects the T1 and T2 times to allow
the requesting router to extend the lifetimes of any prefixes in
the IA_PD before the lifetimes expire, even if the delegating
router is unavailable for some short period of time.
Recommended values for T1 and T2 are .5 and .8 times the
shortest preferred lifetime of the prefixes in the IA_PD that
the delegating router is willing to extend, respectively. If the
time at which the prefixes in an IA_PD are to be renewed is to
be left to the discretion of the requesting router, the
delegating router sets T1 and T2 to 0. The requesting router MUST
follow the rules defined in .If a delegating router receives an IA_PD with T1 greater than
T2, and both T1 and T2 are greater than 0, the delegating router
ignores the invalid values of T1 and T2 and processes the IA_PD
as though the requesting router had set T1 and T2 to 0.If a requesting router receives an IA_PD with T1 greater than
T2, and both T1 and T2 are greater than 0, the requesting router discards
the IA_PD option and processes the remainder of the message as
though the requesting router had not included the IA_PD
option.The IA_PD Prefix option is used to specify IPv6 address
prefixes associated with an IA_PD. The IA_PD Prefix option must
be encapsulated in the IA_PD-options field of an IA_PD
option.OPTION_IAPREFIX (26).25 + length of
IAprefix-options field.The recommended
preferred lifetime for the IPv6 prefix in the option,
expressed in units of seconds. A value of 0xFFFFFFFF
represents infinity.The valid lifetime for the
IPv6 prefix in the option, expressed in units of
seconds. A value of 0xFFFFFFFF represents infinity.Length for this prefix in
bits.An IPv6 prefix.Options associated with
this prefix.In a message sent by a requesting router to a delegating
router, the preferred and valid lifetime fields SHOULD be set to 0.
The server MUST ignore any received values in these lifetime fields.A requesting router may set the IPv6 prefix field to zero and a
given value in the prefix-length field to indicate a preference
for the size of the prefix to be delegated.In a message sent by a delegating router the preferred and
valid lifetimes should be set to the values of
AdvPreferredLifetime and AdvValidLifetime as specified in
section 6.2.1, "Router Configuration Variables" of , unless administratively configured.A requesting router discards any prefixes for which the
preferred lifetime is greater than the valid lifetime. A
delegating router ignores the lifetimes set by the requesting
router if the preferred lifetime is greater than the valid
lifetime and ignores the values for T1 and T2 set by the
requesting router if those values are greater than the preferred
lifetime.The values in the preferred and valid lifetimes are the
number of seconds remaining for each lifetime.An IA_PD Prefix option may appear only in an IA_PD
option. More than one IA_PD Prefix Option can appear in a single
IA_PD option.The status of any operations involving this IA_PD Prefix
option is indicated in a Status Code option in the
IAprefix-options field.A DHCP server sends the SOL_MAX_RT option to a client to override
the default value of SOL_MAX_RT. The value of SOL_MAX_RT in the
option replaces the default value defined in .
One use for the SOL_MAX_RT option is to set a longer value for
SOL_MAX_RT, which reduces the Solicit traffic from a client that has
not received a response to its Solicit messages. The format of the SOL_MAX_RT option is: OPTION_SOL_MAX_RT (82). 4. Overriding value for SOL_MAX_RT
in seconds; MUST be in range: 60 <= "value" <= 86400 (1 day).A DHCP client MUST include the SOL_MAX_RT option code in any Option
Request option (see ) it sends.The DHCP server MAY include the SOL_MAX_RT option in any response
it sends to a client that has included the SOL_MAX_RT option code in
an Option Request option. The SOL_MAX_RT option is sent in the main
body of the message to client, not as an encapsulated option in,
e.g., an IA_NA, IA_TA, or IA_PD option.A DHCP client MUST ignore any SOL_MAX_RT option values
that are less than 60 or more than 86400.If a DHCP client receives a message containing a SOL_MAX_RT option
that has a valid value for SOL_MAX_RT, the client MUST set its
internal SOL_MAX_RT parameter to the value contained in the
SOL_MAX_RT option. This value of SOL_MAX_RT is then used by the
retransmission mechanism defined in
and .Updated SOL_MAX_RT value applies only to the network interface on
which the client received SOL_MAX_RT option.A DHCP server sends the INF_MAX_RT option to a client to override
the default value of INF_MAX_RT. The value of INF_MAX_RT in the
option replaces the default value defined in .
One use for the INF_MAX_RT option is to set a longer value for INF_MAX_RT,
which reduces the Information-request traffic from a client that has not
received a response to its Information-request messages.The format of the INF_MAX_RT option is: OPTION_INF_MAX_RT (83). 4. Overriding value for INF_MAX_RT
in seconds; MUST be in range: 60 <= "value" <= 86400 (1 day).A DHCP client MUST include the INF_MAX_RT option code in any Option
Request option (see ) it sends.The DHCP server MAY include the INF_MAX_RT option in any response
it sends to a client that has included the INF_MAX_RT option code in
an Option Request option. The INF_MAX_RT option is sent in the main
body of the message to client, not as an encapsulated option in,
e.g., an IA_NA, IA_TA, or IA_PD option.A DHCP client MUST ignore any INF_MAX_RT option values
that are less than 60 or more than 86400.If a DHCP client receives a message containing an INF_MAX_RT option
that has a valid value for INF_MAX_RT, the client MUST set its
internal INF_MAX_RT parameter to the value contained in the
INF_MAX_RT option. This value of INF_MAX_RT is then used by the
retransmission mechanism defined in
and .Updated INF_MAX_RT value applies only to the network interface on
which the client received INF_MAX_RT option.This section discusses security considerations that are not
related to privacy. For dedicated privacy discussion, see
.The threat to DHCP is inherently an insider threat (assuming a
properly configured network where DHCPv6 ports are blocked on the
perimeter gateways of the enterprise). Regardless of the gateway
configuration, however, the potential attacks by insiders and outsiders
are the same.Use of manually configured preshared keys for IPsec between relay
agents and servers does not defend against replayed DHCP messages.
Replayed messages can represent a DOS attack through exhaustion of
processing resources, but not through mis-configuration or exhaustion of
other resources such as assignable addresses.One attack specific to a DHCP client is the establishment of a
malicious server with the intent of providing incorrect configuration
information to the client. The motivation for doing so may be to mount a
"man in the middle" attack that causes the client to communicate with a
malicious server instead of a valid server for some service such as DNS
or NTP. The malicious server may also mount a denial of service attack
through misconfiguration of the client that causes all network
communication from the client to fail.A malicious DHCP server might cause a client to set its SOL_MAX_RT
and INF_MAX_RT parameters to an unreasonably high value with the
SOL_MAX_RT and INF_MAX_RT options, which may cause an undue delay in a
client completing its DHCP protocol transaction in the case no other
valid response is received. Assuming the client also receives a response
from a valid DHCP server, large values for SOL_MAX_RT and INF_MAX_RT
will not have any effect.There is another threat to DHCP clients from mistakenly or
accidentally configured DHCP servers that answer DHCP client requests
with unintentionally incorrect configuration parameters.A DHCP client may also be subject to attack through the receipt of a
Reconfigure message from a malicious server that causes the client to
obtain incorrect configuration information from that server. Note that
although a client sends its response (Renew or Information-request
message) through a relay agent and, therefore, that response will only
be received by servers to which DHCP messages are relayed, a malicious
server could send a Reconfigure message to a client, followed (after an
appropriate delay) by a Reply message that would be accepted by the
client. Thus, a malicious server that is not on the network path between
the client and the server may still be able to mount a Reconfigure
attack on a client. The use of transaction IDs that are
cryptographically sound and cannot easily be predicted will also reduce
the probability that such an attack will be successful.The threat specific to a DHCP server is an invalid client
masquerading as a valid client. The motivation for this may be for theft
of service, or to circumvent auditing for any number of nefarious
purposes.The threat common to both the client and the server is the resource
"denial of service" (DoS) attack. These attacks typically involve the
exhaustion of available addresses, or the exhaustion of CPU or network
bandwidth, and are present anytime there is a shared resource.In the case where relay agents add additional options to Relay
Forward messages, the messages exchanged between relay agents and
servers may be used to mount a "man in the middle" or denial of service
attack.This threat model does not consider the privacy of the contents of
DHCP messages to be important. DHCP is not used to exchange
authentication or configuration information that must be kept secret
from other networks nodes.DHCP authentication provides for
authentication of the identity of DHCP clients and servers, and for the
integrity of messages delivered between DHCP clients and servers. DHCP
authentication does not provide any privacy for the contents of DHCP
messages.Because of the opportunity for attack through the Reconfigure
message, a DHCP client MUST discard any Reconfigure message that does
not include authentication or that does not pass the validation process
for the authentication protocol.The Reconfigure Key protocol described in provides
protection against the use of a Reconfigure message by a malicious DHCP
server to mount a denial of service or man-in-the-middle attack on a
client. This protocol can be compromised by an attacker that can
intercept the initial message in which the DHCP server sends the key to
the client.Communication between a server and a relay agent, and communication
between relay agents, can be secured through the use of IPsec, as
described in . The use of manual configuration and
installation of static keys are acceptable in this instance because
relay agents and the server will belong to the same administrative
domain and the relay agents will require other specific configuration
(for example, configuration of the DHCP server address) as well as the
IPsec configuration.A rogue delegating router can issue bogus prefixes to a requesting
router. This may cause denial of service due to unreachability.A malicious requesting router may be able to mount a denial of
service attack by repeated requests for delegated prefixes that exhaust
the delegating router's available prefixes.To guard against attacks through prefix delegation, requesting
routers and delegating routers SHOULD use DHCP authentication as
described in .
For point to point links, where one trusts that there is no man in the
middle, or one trusts layer two authentication, DHCP authentication or
IPsec may not be necessary. Because a requesting router and delegating
routers must each have at least one assigned IPv6 address, the routers
may be able to use IPsec for authentication of DHCPv6 messages. The
details of using IPsec for DHCPv6 are under development.Networks configured with delegated prefixes should be configured to
preclude intentional or inadvertent inappropriate advertisement of these
prefixes.The following sections focuses on the server considerations. For
extended discussion about privacy considerations for the client, see . It particular, Section 3 of said
document discuss various identifiers that could be misused to track the
client. Section 4 discusses existing mechanisms that may have an impact on
client's privacy. Finally, Section 5 discusses potential attack
vectors. For recommendations how to address or mitigate those issues, see
.This specification does not define any allocation strategies.
Implementors are expected to develop their own algorithm for the server to
choose a resource out of the available pool. Several possible allocation
strategies are mentioned in Section 4.3 of . Please keep in mind that this list
is not exhaustive and there are certainly possible other strategies. Here
are some observations for the implementor to consider.Assigning addresses using some kind of sequential
algorithm (prefix::1, prefix::2, prefix::3,...) is fast, but greatly
facilitate scanning of the network. Also, it makes any attacks that
require guessing the next address much easier to conduct.Deriving the IID (Interface Identifier) par of the
addresses from the link layer address of the client exposes information
about the client hardware and enables tracking the client across
multiple subnets. Also, since the address will likely be used to access
remote services, this tracking can be conducted remotely.This document does not define any new DHCPv6 name spaces or definitions.IANA is requested to update the
http://www.iana.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xhtml
page to add a reference to this document for definitions previously created by
, , and .
This specification of the DHCPv6 is mostly a corrected and
cleaned up version of the original spec
along with numerous additions from later RFCs. However, there is
a small number of mechanisms that didn't get much traction,
were not widely deployed, underspecified or had other
operational issues. Those mechanisms are now considered
decprecated. Legacy implementations MAY support it, but
implementations conformant to this document MUST NOT rely on them.
The following mechanism are now obsolete:Delayed Authentication. This mechanism was underspecified and
had significant operational burden. As a result, after 10 years
its adoption was extremely limited at best.Lifetime hints sent by a client. Client used to be allowed to
send lifetime values as hints. This mechanism was not widely
implemented and there were known misimplementations that sent
remaining lifetimes rather than total lifetimes. That in turn
was sometimes misunderstood by the servers as a request for ever
decreasing lease lifetimes, which caused issues when values
started approaching zero.The following people are authors of the original RFC 3315:
Ralph Droms, Jim Bound, Bernie Volz, Ted Lemon, Charles Perkins,
and Mike Carney. The following people are authors of the original
RFC 3633: Ole Troan and Ralph Droms. This document is merely a
refinement of their work and would not be possible without their
original work.A number of additional people have contributed to identifying
issues with RFC 3315 and RFC 3633 and proposed resolutions to
these issues as reflected in this document (in no particular
order): Ole Troan, Robert Marks, Leaf Yeh, Tim Winters, Michelle
Cotton, Pablo Armando, John Brzozowski, Suresh Krishnan,
Hideshi Enokihara, Alexandru Petrescu, Yukiyo Akisada, Tatuya
Jinmei, Fred Templin and Christian Huitema. With special thanks to
Ralph Droms for answering many questions related to the original RFC
3315 work.The following acknowledgements are from the original RFC 3315 and RFC 3633:Thanks to the DHC Working Group and the members of the IETF for their
time and input into the specification. In particular, thanks also for
the consistent input, ideas, and review by (in alphabetical order)
Bernard Aboba, Bill Arbaugh, Thirumalesh Bhat, Steve Bellovin, A. K.
Vijayabhaskar, Brian Carpenter, Matt Crawford, Steve Deering, Francis
Dupont, Dave Forster, Brian Haberman, Richard Hussong, Tatuya Jinmei,
Kim Kinnear, Fredrik Lindholm, Tony Lindstrom, Josh Littlefield, Gerald
Maguire, Jack McCann, Shin Miyakawa, Thomas Narten, Erik Nordmark, Jarno
Rajahalme, Yakov Rekhter, Pekka Savola, Mark Stapp, Matt Thomas,
Sue Thomson, Tatuya Jinmei, Bernie Volz, Trevor Warwick, Phil Wells and
Toshi Yamasaki.Thanks to Steve Deering and Bob Hinden, who have consistently taken
the time to discuss the more complex parts of the IPv6
specifications.And, thanks to Steve Deering for pointing out at IETF 51 in London
that the DHCPv6 specification has the highest revision number of any
Internet Draft.Private Enterprise Numbers registry
http://www.iana.org/assignments/enterprise-numbersIANAIncorporated RFC3315 errata (ids: 294, 1373, 2928, 1815, 3577,
2509, 295).Partially incorporated RFC3315 errata id 2472 (place other IA options
if NoAddrsAvail is sent in Advertise).Clarified section 21.4.1 of RFC3315 by defining length of "key ID" field
and specifying that 'DHCP realm' is Domain Name encoded as per section 8 of RFC3315.
Ticket #43.Added DUID-UUID and reference to RFC6355. Ticket #54.Specified a minimum length for the DUID in section "9.1. DUID Contents".
Ticket #39.Removed the use of term "sub-options" from section "19.1.1. Creation and
Transmission of Reconfigure Messages". Ticket #40.Added text to section 22.6 "IA Address Option" about the usage of
unspecified address to express the client hints for Preferred and Valid
lifetimes. Ticket #45.Updated text in 21.4.2 of RFC3315 ("Message Validation") as suggested in
section 3.1 of draft-ietf-dhc-dhcpv6-clarify-auth-01. Ticket #87.Merged RFC7083, "Modification to Default Values of SOL_MAX_RT and
INF_MAX_RT", into this document. Ticket #51.Incorporated RFC3315 errata (id 2471), into section 17.1.3. Ticket #25.Added text that relay agents MUST NOT modify the relayed message to section
20.1.2. Ticket #57.Modified the text in section 21.4.4.5, Receiving Reply Messages, to remove
special treatment of a Reply validation failure (client ignores message).
Ticket #89.Appendix C updated: Authentication option is no longer allowed in Relay-forward and
Relay-reply messages, ORO is no longer allowed in Confirm, Release and Decline
messages; Preference option is no longer allowed in Reply messages (only in
Advertise). Ticket #10.Removed "silently" from several instances of "silently ignores" or "silently"
discards. It is up to software vendor if and how to log such events (debug log
message, event log, message pop-up etc.). Ticket #50.Clarified that: there should be no more that one instance of Vendor Class
option with a given Enterprise Number; that one instance of Vendor Class can
contain multiple encapsulated options; the same applies to Vendor Specific
Information option. Ticket #22.Clarified relay agent definition. Ticket #12.Changed REL_MAX_RC and DEC_MAX_RC defaults from 5 to 4 and added retry to
parameter description. Ticket #84.Clarify handling process for Vendor-specific Information Option
and Vendor Class Option. Ticket #20.Replace "monotonic" with "strictly monotonic" in Section 21.3.
Ticket #11.Incorporate everything of RFC 6644, except for Security
Considerations Section, which has already covered in a more
abstracted way. Ticket #55 & #56.Clarify the server behavior process when a client violates
Delayed Authentication Protocol, in Section 21.4. Ticket #90.Updated titles of sections 19.4.2. and 19.4.4. to include Rebind
messages.Applied many of the review comments from a review done by Fred
Templin in August 2006. Ticket #14.Reworded the first paragraph of Section 15 to relax the "SHOULD"
requirement to drop the messages which contain the options not expected
in the current message. Ticket #17.Changed WG to DHC, added keywordsLoosened requirements for DUID-EN, so that DUID type can be used
for virtual machines. Ticket #16.Clarified that IA may contain other resources than just address.
Ticket #93.Clarified that most options are singletons (i.e. can appear only
once). Ticket #83.Merged sections 1 (Ticket #96), 2 (Ticket #97), 3 (Ticket #98),
4 (Ticket #99), 6 (Ticket #101), 8 (Ticket #103), 9 (Ticket #104),
10 (Ticket #105), 11 (Ticket #106), 13 (Ticket #108), 14 (Ticket #109),
15 (Ticket #110), 16 (Ticket #111), 17 (Ticket #112) and 19
(Ticket #113) from RFC3633 (Prefix Delegation).Clarified that encapsulated options must be requested using
top-level ORO (ticket #38).Clarified that configuration for interface X should be requested
over interface X (ticket #48).CONFIRM is now an optional message (MUST send Confirm eased to
SHOULD) (ticket #120).Added reference to RFC7227: DHCPv6 Option Guidelines (ticket #121).
Added new section 5 providing an overview of DHCPv6 operational
modes and removed two prefix delegation sections from section 1.
See tickets #53, #100, and #102.Addressed ticket #115 - don't use DHCPv6 for DHCPv4 configuration.Revised IANA Considerations based on ticket #117.Updated IAID description in the terminology with the clarification
that the IAID is unique among IAs of a specific type, rather than
globally unique among all IAs (ticket #94).Merged Section 12 from RFC3633 (ticket #107)Clarified behavior for unknown messages (RFC7283), ticket #58.Addressed tickets #123 and #126, and clarified that the client
SHOULD abandon its bindings when restarts the server solicitation.Clarified link-address field usage, ticket #73.Clarified that retransmitting at client's discretion (t1,t2=0) does
not mean immediate transmission (ticket #71).Merged section 4.4. of RFC7550 (stateful-issues). This includes
new Renew and Rebind processing by the server, i.e. a client may
request allocation of new addresses/prefixes during Renew and
Rebind. This addresses tickets #62 and #63.Merged section 4.1 and 4.2 of RFC7550 (stateful-issues). This
addresses tickets #59 and #60.Added normative reference to RFC7550 and to obsoleted list.
Also added terse text of Abstract and section 1.0 to indicate what
this document is about.Merged section 4.6 of RFC7550 (stateful-issues). This addresses
ticket #85.Clarified that the document assumes single provisioning domain.
This resolves ticket #66.Removed the Delayed Auth Protocol. This resolves ticket #86.Added text the text suggesting to not send all-zero address/prefix
unless the client wishes to hint the lifetimes and/or prefix lengths.
This resolves ticket #82.Cleaned up Confirm text regarding rebooting client an stable storage.Updated server processing section to clarify that server unicast
check can be based on current configuration of unicast setting so server
is not required to remember what it sent. This resolves ticket #130.Removed references to lifetime hints from the client; only IAPREFIX
prefix-length hints are now allowed. Servers will ignore any lifetime
hints; clients should set lifetime to 0. This resolves ticket #148. Also,
removed T1/T2 hints from clients to servers - while not explicitly
discussed, seems to make sense? And, fixed some of the message processing
text to include PD, not just addresses (though I think Marcin will
eventually replace much of this text? Plus a few other minor edits.Clarified that the server may return different addresses in the Reply
than requested by the client in the Request message. Also clarified that
the server must not include addresses that it will not assign. This
resolves ticket #69.Introduction updated, including relation to previous RFCs
(ticket #136).Added section with deprecated mechanisms (ticket #149).Combined text for prefix delegation and address assignment in sections
18 and 19. Also, introduced a term "lease" replacing "allocable resources".
This resolves ticket #146.Reorganized text about Reply processing on the client side (ticket #140).
Revised the "Creation and Transmission of Release Messages" section to
clarify what a client must do before initiating a Release
(ticket #151).Privacy considerations added (ticket #145).Abstract updated (#133).Removed references to the text that server was allowed to cache its
replies (#80).Incorporated RFC3633 errata (ids: 248, 1880, 2468, 2469, 2470, 3736)...The following table indicates with a "*" the options are allowed in
each DHCP message type:NOTE:Only included in Information-request messages that are sent in
response to a Reconfigure (see ).The following table indicates with a "*" where options can appear
in the options field of other options:Note: "Relay Forw" / "Relay Reply" options appear in the options
field of the message but may only appear in these messages.