Dynamic Host Configuration Protocol for IPv6
(DHCPv6) bisInternet Systems Consortium,
Inc.950 Charter StreetRedwood CityCA94063USAtomasz.mrugalski@gmail.comInternet Systems Consortium,
Inc.950 Charter StreetRedwood 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.comUniversity of New Hampshire, Interoperability Lab (UNH-IOL)Durham, NHUSAtwinters@iol.unh.edu
Internet
Dynamic Host Configuration (DHC)DHCPv6IPv6DHCPThis document describes the Dynamic Host Configuration Protocol for
IPv6 (DHCPv6): an extensible mechanism for configuring nodes 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 RFC3315, the original DHCPv6
specification, and incorporates prefix delegation (RFC3633),
stateless DHCPv6 (RFC3736), an option to specify an upper
bound for how long a client should wait before refreshing information
(RFC4242), a mechanism for throttling DHCPv6 clients when DHCPv6
service is not available (RFC7083), incorporates relay agent handling
of unknown messages (RFC7283), and clarifies the interactions
between modes of operation (RFC7550).
As such, this document obsoletes RFC3315, RFC3633, RFC3736, RFC4242,
RFC7083, RFC7283, and RFC7550.This document describes DHCP for IPv6 (DHCPv6), a client/server
protocol that provides managed configuration of devices. The basic
operation of DHCPv6 provides configuration for clients connected to
the same link as the server. 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 also provides a mechanism for automated delegation of
IPv6 prefixes using DHCPv6, originally specified in .
Through this mechanism, a delegating router
can delegate prefixes to requesting routers. Use of this mechanism is
specified as part of and by .DHCP can also be used just to provide other configuration options
(i.e., no addresses or prefixes). That implies that the
server does not have to track any state, and thus this mode is called
stateless DHCPv6. Mechanisms necessary to support stateless DHCPv6 are
much smaller than to support stateful DHCPv6 (
was written to document just those portions of DHCPv6 needed to support
DHCPv6 stateless operation).The remainder of this introduction summarizes the relationship to the
previous DHCPv6 standards in and
clarifies the stance with regards to DHCPv4 in .
describes the message exchange mechanisms
to illustrate DHCP operation rather than provide an exhaustive
list of all possible interactions and
provides an overview of common operational models.
explains client and server operation
in detail.The initial specification of DHCPv6 was defined in and a number of follow up documents were
published over the years:
IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6
, Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6s , Information Refresh
Time Option for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
, Modification to
Default Values of SOL_MAX_RT and INF_MAX_RT ,
Handling Unknown DHCPv6 Messages , and
Issues and Recommendations with Multiple Stateful DHCPv6 Options
. This document
provides a unified, corrected, and cleaned up definition of DHCPv6
that also covers all errata filed against older RFCs (see
list in ). As such, it
obsoletes a number of the aforementioned RFCs. And, there are a small
number of mechanisms that were obsoleted, listed in . Also see .The operational models and relevant configuration information for
DHCPv4 ( and )
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.
For IPv6-only networks, describes a
transport mechanism to carry DHCPv4 messages using the DHCPv6 protocol
for the dynamic provisioning of IPv4 address and configuration
information.Merging DHCPv4 and DHCPv6 configuration is out of scope of
this document. discusses some issues
and possible strategies for running DHCPv4 and DHCPv6 services
together. While this document is a bit dated, it provides a
good overview of the issues at hand.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 BCP 14
when, and only when, they appear in all capitals, as shown here.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 implementer to study
includes the IPv6 Specification , the IPv6
Addressing Architecture , IPv6 Stateless
Address Autoconfiguration , and IPv6 Neighbor
Discovery Processing . These specifications
enable DHCP to build upon the IPv6 work to provide robust stateful
autoconfiguration.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. 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 implementers
understand IPv6 Neighbor Discovery.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);
PPP and PPPoE links; and Internet (or higher) layer "tunnels",
such as tunnels over IPv4 or IPv6 itself.A link-layer identifier for an
interface. For example, IEEE 802 addresses for Ethernet or
Token Ring network interfaces.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 IPv6 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 or delegated prefixes in an IA or configuration information
explicitly assigned to the client. Configuration information that
has been returned to a client through a policy, such as 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 lease in the IA; for example, temporary). A binding
containing configuration information for a client is indexed by
<DUID>. See below for definitions of DUID, IA, and IAID.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.An option that encapsulates other
options (for example, the IA_NA option, see ,
may contain IA Address options, see ).The router that acts as a DHCP
server, and responds to requests for delegated prefixes. This document
primarily uses the term "DHCP server" or "server" when
discussing the "delegating router" functionality of prefix
delegation (see ).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. The node may act as a requesting router (see
below) if it supports prefix delegation.A set of links managed by DHCP and
operated by a single administrative entity.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.A DHCPv6 option that is usually
only contained in another option. For example, the IA Address option
is contained in IA_NA or IA_TA options (see ).
See Section 9 of
for a more complete definition.Identity Association: A collection of leases
assigned to a client. Each IA has an associated IAID (see below). 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 lease; for example, an
identity association for temporary addresses (IA_TA) holds 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 three IA types defined: IA_NA, IA_TA and IA_PD. New IA types may
be defined in the future.At the time of writing this document,
one or more IA_NA, IA_TA, and/or IA_PD options. 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 "IA_TA"). See
for details on the IA_NA option.Identity Association for Temporary Addresses:
An IA that carries temporary addresses (see ). See
for details on the IA_TA option.Identity Association for Prefix Delegation: An
IA that carries delegated prefixes. See
for details on the IA_PD option.A contract by which the server grants the use
of an address or delegated prefix to the client for a specified
period of time.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 (see
).A DHCP relay agent relays DHCP messages
between DHCP participants.The router that acts as a DHCP
client and is requesting prefix(es) to be assigned. This document
primarily uses the term "DHCP client" or "client" when
discussing the "requesting router" functionality of prefix
delegation (see ).Another attempt to send the same
DHCP message by a client or server, as a result of not
receiving a valid response to the previously sent messages.
The retransmitted message is typically modified prior to sending,
as required by the DHCP specifications. In particular, the
client updates the value of the Elapsed Time option in the
retransmitted message.The Reconfiguration Key Authentication
Protocol, see .An option that is allowed to appear
only once as a top-level option or at any encapsulation level.
Most options are singletons.The time interval after which the client is
expected to contact the server that did the assignment to extend
(renew) the lifetimes of the addresses assigned (via IA_NA option(s)) and/or
prefixes delegated (via IA_PD option(s)) to the client. T1 is expressed as
an absolute value in messages (in seconds), is conveyed within IA
containers (currently the IA_NA and IA_PD options), and is interpreted as a time
interval since the packet's reception. The value stored in the T1 field in
IA options is referred to as the T1 value. The actual time when the
timer expires is referred to as the T1 time.The time interval after which the client is
expected to contact any available server to extend (rebind) the
lifetimes of the addresses assigned (via IA_NA option(s)) and/or prefixes
delegated (via IA_PD option(s)) to the client. T2 is expressed as an absolute value
in messages (in seconds), is conveyed within IA containers
(currently the IA_NA and IA_PD options), and is interpreted as a time interval
since the packet's reception. The value stored in the T2 field in IA options
is referred to as the T2 value. The actual time when the timer expires
is referred to as the T2 time.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.Clients and servers exchange DHCP messages using UDP BCP 145 . 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 or delegated prefixes, the client can obtain other
configuration information such as a
list of available DNS servers or NTP
servers through a single message and
reply exchange with a DHCP server. To obtain other 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 other configuration information
for the client.A client may also request the server to expedite address assignment
and/or prefix delegation by using a two message
exchange instead of the normal four message exchange as discussed
in the next section. Expedited assignment can be requested by the
client, and servers may or may not honor the request (see
and
for more details and why servers may not honor this request).
Clients may request this expedited service in environments where it
is likely that there is only one server available on a link and no
expectation that a second server would become available, or when
completing the configuration process as quickly as possible is a
priority.To request the expedited two message exchange, the client
sends a Solicit message to the All_DHCP_Relay_Agents_and_Servers multicast address
requesting the assignment of addresses and/or delegated prefixes and other configuration
information. This message includes an indication (the Rapid Commit option,
see ) 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 and/or
delegated prefixes to the client immediately responds with a Reply
message. The configuration information and the addresses and/or
delegated prefixes in the Reply message are then
immediately available for use by the client.Each address or delegated prefix 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 or delegated prefix, 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 or
delegated prefix without interruption. If the server is unable to extend
the lifetime of an address or delegated prefix, it indicates this by
returning the address or delegated prefix with lifetimes of 0. At the same
time, the server may assign other addresses or delegated prefixes.There are additional two message exchanges between the client and
server described later in this document.To request the assignment of one or more addresses and/or delegated prefixes, a client
first locates a DHCP server and then requests the assignment of
addresses and/or delegated prefixes and other configuration information from the server. The
client sends a Solicit message to the
All_DHCP_Relay_Agents_and_Servers multicast 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/or delegated prefixes and other configuration information. The
server responds with a Reply message that contains the confirmed
addresses, delegated prefixes, and configuration.As described in the previous section, the client can request an
extension of the lifetimes assigned to addresses or delegated prefixes
(this is a two message exchange).A server that has previously communicated with a client and negotiated
for the client to listen for Reconfigure messages, may send the client
a Reconfigure message to initiate the client to update its configuration
by sending an Information-request, Renew, or Rebind message. The client then
performs the two message exchange as described earlier. This can be used to
expedite configuration changes to a client, such as the need to renumber
a network (see ).
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 a specific interface, belong to a
single provisioning domain.DHCP may be extended to support additional stateful services that
may interact with one or more of the models described below. Such
interaction should be considered and documented as part of any future
protocol extension.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 DHCP 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-forward 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.
It is appropriate for situations where
stateless address autoconfiguration alone is insufficient or
impractical, e.g., because of network policy, additional requirements
such as dynamic updates to the DNS, or client-specific requirements.The model of operation for non-temporary address assignment is as
follows. The server is provided with 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 DNS name server addresses and domain search lists, when requesting
addresses.Clients can also request more than one address or set of
addresses (see and ).The prefix delegation mechanism, originally described in , is another stateful mode of operation and
was originally 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 .While assumed that the DHCP client
is a router (hence the use of "requesting router") and that the DHCP
server was a router (hence the use of "delegating router"), DHCP
prefix delegation itself does not require that the client forward
IP packets not addressed to itself, and thus does not require
that the client (or server) be a router as defined in
. Also, in many cases (such as tethering
or hosting virtual machines), hosts are already forwarding IP
packets and thus operating as routers as defined in
. Therefore, this document mostly replaces
"requesting router" with client and "delegating router" with
server.The model of operation for prefix delegation is as follows. A
server is provisioned with prefixes to be delegated to
clients. A client requests prefix(es) from the
server, as described in . The server chooses
prefix(es) for delegation, and responds with prefix(es) to the
client. The client is then responsible for the
delegated prefix(es). For example, the client 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
client is allowed to use the prefix. A client
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.This prefix delegation mechanism is 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. and describe in detail such use. illustrates a network
architecture in which prefix delegation could be used.In this example, the server (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 client (requesting router) requests configuration
information through DHCP. The DHCP messages from the client
are received by the server in the aggregation device. When
the server receives the request, it selects an available
prefix or prefixes for delegation to the client. The
server then returns the prefix or prefixes to the
client.The client 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 client 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
client. The client may, in turn, provide DHCP
service to nodes attached to the internal network. For example, the
client may obtain the addresses of DNS and NTP servers from
the ISP server, and then pass that configuration
information on to the subscriber hosts through a DHCP server in the
client (requesting router).If the client uses a delegated prefix to configure addresses on
interfaces on itself or other nodes behind it, the preferred and
valid lifetimes of those addresses MUST be no larger than the
remaining preferred and valid lifetimes, respectively, for the
delegated prefix at any time. In particular, if the delegated
prefix or a prefix derived from it is advertised for stateless
address autoconfiguration , the advertised
preferred and valid lifetimes MUST NOT exceed the corresponding
remaining lifetimes of the delegated prefix.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 . 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.The protocol allows a client to receive multiple
addresses. During typical operation, a client sends one instance
of an IA_NA option and the server assigns at most one address from
each prefix assigned to the link the client is attached to. In
particular, the server can be configured to serve addresses out
of multiple prefixes for a given link. This is useful in cases
such as when a network renumbering event is in progress. In a typical
deployment the server will grant one address per each IA_NA
option (see ).A client can explicitly request multiple addresses by sending
multiple IA_NA options (and/or IA_TA options,
see ). A client can send multiple
IA_NA (and/or IA_TA) options in its initial transmissions.
Alternatively, it can send an extra Request message
with additional new IA_NA (and/or IA_TA) options (or include them
in a Renew message).The same principle also applies to Prefix Delegation. In
principle the protocol allows a client to request new prefixes
to be delegated by sending additional IA_PD options
(see ). However, a
typical operator usually prefers to delegate a single, larger
prefix. In most deployments it recommended for the client to
request a larger prefix in its initial transmissions rather than
request additional prefixes later on.The exact behavior of the server (whether to grant additional
addresses and prefixes or not) is up to the server policy and is
outside of scope of this document.For more information on how the server distinguishes between IA
option instances, see .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 on the interfaces which are within the site.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
. Additional message types have been
defined and may be defined in the future - see
https://www.iana.org/assignments/dhcpv6-parameters.
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 addresses and/or
delegated prefixes, 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 leases and
configuration parameters to extend the lifetimes on the leases
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 leases 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 leases and configuration parameters in response to a
Solicit, Request, Renew, or 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 leases to the client to indicate that the
client will no longer use one or more of the assigned
leases.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 leases 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.DHCP makes extensive use of options in messages and some of these
are defined later in . Additional options
are defined in other documents or may be defined in the future.DHCP 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 (see ) 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. Some of
the values are adjusted by a randomization factor and backoffs
(see ) and transmissions may also
be influenced by rate limiting (see ).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_LIMIT8Max hop count in a Relay-forward messageIRT_DEFAULT86400 secs (24 hours)Default information refresh timeIRT_MINIMUM600 secsMin information refresh timeMAX_WAIT_TIME60 secsMaximum required time to wait for a responseAll time values for lifetimes, T1, and T2 are unsigned 32-bit integers
and are expressed in seconds. The value 0xffffffff is taken to mean
"infinity" when used as a lifetime (as in ) or a
value for T1 or T2.Setting the valid lifetime of an address or a
delegated prefix to 0xffffffff ("infinity") amounts to a permanent
address or delegation of the prefix to a client and should only be used in cases
were permanent assignments are desired.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.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 . A one octet long field.The transaction ID for this message
exchange. A three octets long field.Options carried in this message; options
are described in . A variable length
field (4 octets less than the size of the message).Relay agents exchange messages with other relay agents and 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-FORW (12). A one octet long field.Number of relay agents that
have already relayed this message. A one octet long field.An address that may be used by the
server to identify the link on which the client is located. This
is typically a globally unique address (including unique
local address, ), but see discussion in . A 16 octets long fieldThe address of the client or relay
agent from which the message to be relayed was received. A 16
octets long field.MUST include a Relay Message option
(see ); MAY include other
options, such as the Interface-Id option (see
), added by the relay agent. A
variable length field
(34 octets less than the size of the message).See for an explanation
how link-address is used.The following table defines the use of message fields in a
Relay-reply message.RELAY-REPL (13). A one octet long field.Copied from the Relay-forward
message. A one octet long field.Copied from the Relay-forward
message. A 16 octets long field.Copied from the Relay-forward
message. A 16 octets long field.MUST include a Relay Message option
(see ); MAY include other
options, such as the Interface-Id option (see
). A variable length field (34
octets less than the size of the message).So that domain names may be encoded uniformly, a domain name or a
list of domain names is encoded using the technique described in section
3.1 of . 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 SHOULD 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. It should be noted that an attempt to parse a DUID to obtain
a client's link-layer address is unreliable as there is no guarantee that
the client is still using the same link-layer address as when it generated
its DUID. And, such an attempt will be more and more unreliable as more
clients adopt privacy measures, such as those defined in . It is recommended to rely on the mechanism defined in
.The DUID is carried in an option because it may be variable in 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 stability of the DUID includes changes to virtual
interfaces, such as logical PPP (over Ethernet) interfaces that may
come and go in Customer Premise Equipment routers. The client
may change its DUID as specified in .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 octets 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 three 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 octets type field containing the
value 1, a two octets 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
IANA, see . Both the time and
the hardware type are stored in network byte order. For Ethernet
hardware types, 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.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 four octet 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 octets 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 octets 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 node 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 IANA, see .
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 SHOULD 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.This type of DUID consists of 16 octets containing a 128-bit
UUID. details when to use this type, and
how to pick an appropriate source of the UUID.
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 (i.e., IA_NA with IAID 0 is
unique from IA_TA with IAID 0). 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.If the client wishes to obtain a distinctly new address or prefix and
deprecate the existing one, the client sends a Release message
to the server for the IAs using the original IAID. Then the client creates
a new IAID, to be used in future messages to obtain leases for the new IA.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 such IA must be associated with exactly one interface.The configuration information in an IA_NA option consists of one or
more IPv6 addresses along with the T1 and T2 values for the IA. See for the representation of an IA_NA in a
DHCP message.The configuration information in an IA_TA option consists of one or more IPv6
addresses. See for the representation of an
IA_TA 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 Address
option (see ).
The lifetimes apply to the use of 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 client, with a set of interfaces
or with exactly one interface. A client configured to request
delegated prefixes 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 server.The configuration information in an IA_PD option consists of one or more
prefixes along with the T1 and T2 values for the IA_PD. See for the representation of an IA_PD in a
DHCP message.Each delegated prefix 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 Prefix option
(see ).
The lifetimes apply to the use of delegated prefixes, as described in
section 5.5.4 of .A server selects addresses to be assigned to an IA_NA 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 in this case may come from any of the
Relay-forward messages encapsulated in the received
Relay-forward, and in general the most encapsulated
(closest Relay-forward to the client) has the most
useful value.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 (see )
that include the client's requests for specific addresses.Other information in options supplied by the relay
agent.By default, DHCP server implementations SHOULD NOT generate
predictable addresses (see Section 4.7 of ). Server
implementers are encouraged to review ,
, and as to
possible considerations for how to generate addresses.A server MUST NOT assign an address that is otherwise reserved for
some other purpose. For example, a server MUST NOT assign addresses
that use a reserved IPv6 Interface Identifier (,
, ).See for a more detailed
discussion on how servers determine a client's location on the network.A client may request the assignment of temporary addresses (see
for the definition of temporary
addresses). DHCP 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 typically contains
at least 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 ) encapsulated 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, ).A DHCP server implementation MAY generate temporary addresses
referring to the algorithm defined in Section 3.2.1, , with the additional condition that any new
address is not the same as any assigned address.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 a temporary address is to
make it difficult to link the address to different actions over time.
So, it is NOT RECOMMENDED for a client to renew temporary
addresses, though DHCP provides for such a possibility (see ).The mechanism through which the server 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 .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 multicast address.DHCP servers SHOULD NOT care if the layer-2 address used was
multicast or not, as long as the layer-3 address was correct.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 DHCP client MUST limit the rate of DHCP
messages it transmits or retransmits. One example is that a client obtains an
address or delegated prefix, but does not like the response; so it reverts back to Solicit
procedure, discovers the same (sole) server, requests an address or delegated prefix and
gets the same address or delegated prefix as before (as the server has this previously
requested lease assigned to this client). This loop 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 interval. This method of bounding burstiness also
guarantees that the long-term transmission rate will not be exceeded.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 DHCP messages and server-side messages
are out of scope of this specification.In certain cases, T1 and/or T2 values may be set to zero. Currently
there are three such cases:
a client received an IA_NA option (see ) with a zero value a client received an IA_PD option (see ) with a zero value a client received an IA_TA option (see ) (which does not
contain T1 and T2 fields and are not generally renewed)
This is an indication that the renew and rebind times are left
at the client's discretion. However, they are not completely
discretionary.When T1 and/or T2 values are set to zero, the client MUST choose a
time to avoid packet storms. In particular, it MUST NOT transmit
immediately. If the client received multiple IA options, it SHOULD
pick renew and/or rebind transmission times so all IA options are
handled in one exchange, if possible. The client MUST choose renew
and rebind 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 . If a DHCP client fails to
receive an expected response from a server, the client must retransmit
its message according to the retransmission strategy described
in this section.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 MUST update an "elapsed-time" value within an Elapsed
Time option (see ) in the
retransmitted message. In some cases, the client may also need to modify
values in IA Address (see ) or
IA Prefix options (see ) if a valid lifetime for
any of the client's leases expires before retransmission. Thus, whenever
this document refers to a "retransmission" of a client's message, it
refers to both modifying the original message and sending this new message
instance to the server.The client retransmission behavior is controlled and described by the
following variables: Retransmission timeoutInitial retransmission timeMaximum retransmission countMaximum retransmission timeMaximum retransmission durationRandomization factorSpecific values for each of these parameters relevant to the
various messages are given in the sub-sections of
using values defined in
in .
The algorithm for RAND is common across all message transmissions.With each message transmission or retransmission, the client sets RT
according to the rules given below. If RT expires before the message
exchange terminates, the client recomputes RT and retransmits the
message.Each of the computations of a new RT include a randomization factor
(RAND), which is a random number chosen with a uniform distribution
between -0.1 and +0.1. The randomization factor is included to minimize
synchronization of messages transmitted by DHCP clients.The algorithm for choosing a random number does not need to be
cryptographically sound. The algorithm SHOULD produce a different
sequence of random numbers from each invocation of the DHCP client.RT for the first message transmission is based on IRT:RT 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
RT period and may turn off listening capabilities after waiting at least
the shorter of RT and MAX_WAIT_TIME due to power consumption saving or
other reasons. Of course, a client MUST listen for a Reconfigure if it
has negotiated for its use with the server.This section describes which options are valid in which kinds of
message types. Should a client or server receive messages which
contain known options which are invalid for the message, this section
explains how to process it.
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 discard it.If a server receives a message that it considers invalid, it
MAY send a Reply (or Advertise as appropriate) with a Server Identifier
option (see ), a Client Identifier option
(see ) if one was included in the message
and a Status Code option (see ) 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 were not
present. This is critical to provide for later extension of the DHCP
protocol.A server MUST discard any Solicit, Confirm, Rebind or
Information-request messages it receives with a layer-3 unicast
destination address.A client or server MUST discard any received DHCP 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 (see ).the message does not include a Client Identifier
option (see ).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 (see ).the contents of the Server Identifier option do
not match the server's DUID.the message does not include a Client Identifier
option (see ).Clients MUST discard any received Confirm messages.Servers MUST discard any received Confirm messages that do not
include a Client Identifier option (see )
or that do include a Server Identifier option (see
).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 (see ).the contents of the Server Identifier option does
not match the server's identifier.the message does not include a Client Identifier
option (see ).Clients MUST discard any received Rebind messages.Servers MUST discard any received Rebind messages that do not
include a Client Identifier option (see )
or that do include a Server Identifier option (see
).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 (see ).the contents of the Server Identifier option does
not match the server's identifier.the message does not include a Client Identifier
option (see ).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 (see ).the contents of the Server Identifier option does
not match the server's identifier.the message does not include a Client Identifier
option (see ).Clients MUST discard any received Reply message that meets any of
the following conditions: the message does not include a Server Identifier
option (see ).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 (see )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 (see ).the message does not include a Client Identifier
option (see ) that contains the
client's DUID.the message does not include a Reconfigure Message
option (see ).the Reconfigure Message option msg-type is not a
valid value.the message does not include authentication (such
as RKAP, see ) or fails
authentication validation.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
(see ) 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 regarding interface selection is different
depending on the purpose of the configuration.When a client sends a DHCP message to the
All_DHCP_Relay_Agents_and_Servers multicast address, it SHOULD send the message
through the interface for which configuration information
(including the addresses) is being
requested. However, the client MAY send the message through another
interface if the interface which configuration is being requested for
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, see
, 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, as mentioned
in above.When a client sends a DHCP message
for the purpose of prefix delegation, it SHOULD be sent on the
interface associated with the upstream router (typically, connected
to an ISP network); see . 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 client sends a DHCP message directly to a
server using unicast (after receiving the Server Unicast
option, see , from that server), the
source address SHOULD be an
address from the upstream interface and which is suitable for use by
the server in responding to the client.A client initiates a message exchange with a server or servers to
acquire or update configuration information of interest. A client has
many reasons to initiate the configuration exchange. Some of the
more common ones are:
as part of the operating system configuration/bootstrap process, when requested to do so by the application layer (through an
operating system specific API), when Router Advertisement indicates DHCPv6 is available for
address configuration (see Section 4.2 of ), as required to extend the lifetime of address(es) and/or
delegated prefix(es), using Renew and Rebind messages, or when requested to do so by a server - upon the receipt of a
Reconfigure message.The client is responsible for creating IAs and requesting that a
server assign 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 discovery process to obtain the
same 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 (see for more details
on what stop using means), and which it is now trying to obtain from a
new server.A DHCP client that does not need to have a DHCP server assign it IP
addresses or delegated prefixes, can obtain configuration information
such as a list of available DNS servers or
NTP servers through a single message and
reply exchange with a DHCP server. To obtain configuration
information the client first sends an Information-request message
(see ) to the All_DHCP_Relay_Agents_and_Servers
multicast address. Servers respond with a Reply message containing
the configuration information for the client (see ).To request the assignment of one or more addresses or delegated
prefixes, a client first locates a DHCP server and then requests the
assignment of addresses/prefixes and other configuration information from the
server. The client does this by sending the Solicit message
(see ) to the
All_DHCP_Relay_Agents_and_Servers multicast address and collecting
Advertise messages from the servers which respond to the client's
message and selects a server from which it wants to obtain configuration
information. This process is referred to as server discovery. When
the client has selected the server it sends a Request
message to this server as described in .A client willing to perform the Solicit/Reply message
exchange described in includes a Rapid
Commit option (see ) in its
Solicit message.Servers that can assign addresses or delegated prefixes to the IAs
respond to the client with an Advertise message or Reply message if the
client included a Rapid Commit option and the server is configured to
accept it.If the server responds with an Advertise message, the client initiates
a configuration exchange as described in
.A server may initiate a message exchange with a client by sending a
Reconfigure message to cause the client to send a Renew, Rebind or
Information-request message to refresh its configuration information as
soon as the Reconfigure message is received by the client. shows a timeline diagram of the
messages exchanged between a client and two servers for the
typical lifecycle of one or more leases. This is a combination
of the 4-message exchange (to select a server and assign the
lease(s) to the client) followed by two 2-message exchanges (to
extend the lifetime on the lease(s) and eventually release the
lease(s)).This document assumes that a client SHOULD use a single transaction
for all of the IA options required on an interface as this simplifies
the client implementation and reduces the potential number of
transactions required (for the background on this design
choice, refer to Section 4 of ). To facilitate
a client's use of a single transaction for all IA options, servers
MUST return the same T1/T2 values for all IA options in a Reply
(see , ,
and ), so that the client will
generate a single transaction when renewing or rebinding its leases.
However, because some servers may not yet conform to this requirement,
a client MUST be prepared to select appropriate T1/T2 times as
described in .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.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.When a client requests multiple IA option types or multiple
instances of the same IA types in a Solicit, Request, Renew, or
Rebind, it is possible
that the available server(s) may only be configured to offer a subset
of them. When possible, the client SHOULD use the best configuration
available and continue to request the additional IAs in subsequent
messages. This allows the client to maintain
a single session and state machine. In practice, especially in the
case of handling IA_NA and IA_PD requests ,
this situation should be rare or a result of a temporary operational
error.
Thus, it is more likely for the client to get all configuration if
it continues, in each subsequent configuration exchange, to request
all the configuration information it is programmed to try to obtain,
including any stateful configuration options for which no results
were returned in previous message exchanges.Upon receipt of a Reconfigure message from the server, a client
responds with a Renew, Rebind or an Information-request message as
indicated by the Reconfigure Message option (see ).
The client SHOULD be suspicious of the Reconfigure message (they
may be faked), and it MUST NOT abandon any resources it might have
already obtained.
The client SHOULD treat the Reconfigure message as if the T1 timer
had expired. The client will expect the server to send IAs
and/or other configuration information to the client in a Reply
message.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 (see ) from the
server, the client SHOULD unicast any Request, Renew, Release and
Decline messages to the server.Use of unicast may avoid delays due to the relaying of messages
by relay agents, as well as avoid overhead on servers due to
the delivery of client messages to multiple
servers. However, 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 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 (see
) 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 MUST include an Elapsed Time option (see
) to indicate how long the client has
been trying to complete the current DHCP message exchange.The client uses IA_NA options (see )
to request the assignment of non-temporary addresses, IA_TA options
(see ) to request the assignment of
temporary addresses, and IA_PD options (see
) 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 MAY include addresses in IA Address options (see
) encapsulated
within 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 IA Prefix options (see
)
encapsulated within IA_PD options as hints for the delegated prefix
and/or prefix length for which the client has a preference. See
for more on prefix length hints.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 client MUST NOT include any other options in the Solicit
message, except as specifically allowed in the definition of
individual options.The first Solicit message from the client on the interface SHOULD
be delayed by a random amount of time between 0 and SOL_MAX_DELAY.
This random delay helps desynchronize clients which start a DHCP
session at the same time, such as after recovery from a power
failure or after a router outage after seeing that DHCP is
available in Router Advertisement messages (see Section 4.2 of
).The client transmits the message according to , using the following parameters: SOL_TIMEOUTSOL_MAX_RT00A client that wishes to use the Rapid Commit 2-message exchange
includes a Rapid Commit option (see )
in its Solicit message.
The client may receive a number of different replies from
different servers. The client will make note of any valid Advertise
messages that it receives. The client will discard any Reply
messages that do not contain the Rapid Commit option.
Upon receipt of a valid Reply with the Rapid Commit option,
the client processes the message as described in
At the end of the first RT period, if no suitable Reply
messages are received, but the client has valid Advertise
messages, then the client processes the Advertise 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, orprocesses any Reply messages received in
response to the Request message and discards the Reply message
that includes the Rapid Commit option.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 valid 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 valid Advertise messages for the first RT
seconds, unless it receives a valid Advertise message with a preference
value of 255. The preference value is carried in the Preference
option (see ). Any valid Advertise that
does not include a Preference option is considered to have a
preference value of 0. If the client receives a valid 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 a valid 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 a valid Advertise message,
the client SHOULD continue with a client-initiated message exchange
by sending a Request message.If the client does not receive any valid 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
valid 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 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 MUST include the identifier of the destination server in a
Server Identifier option (see ).The client MUST include a Client Identifier option (see
) to identify
itself to the server. The client adds any other appropriate options,
including one or more IA options.The client MUST include an Elapsed Time option (see
) to indicate how long the client has
been trying to complete the current DHCP message exchange.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 client transmits the message according to , using the following parameters: REQ_TIMEOUTREQ_MAX_RTREQ_MAX_RC0If 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.The client uses a Confirm message when it has only addresses
(no delegated prefixes) assigned by a DHCP server to determine if it
is still connected to the same link when the client detects a
change in network information as described in .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 (see
) to identify itself to the server.The client MUST include an Elapsed Time option (see
) to indicate how long the client has
been trying to complete the current DHCP message exchange.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 (see
) and the preferred-lifetime and
valid-lifetime fields in the IA Address options (see
) 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_TIMEOUTCNF_MAX_RT0CNF_MAX_RDIf the client receives no responses before the message
transmission process terminates, as described in , the client SHOULD continue to use any
leases, using the last known lifetimes for those leases,
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 and obtain new addresses or delegated prefixes
for 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 (see ) within IA_NA
(see ) and IA_TA
(see ) options for the addresses
assigned to the IAs. The client includes IA Prefix options
(see ) within IA_PD options
(see ) for the delegated prefixes
assigned to the IAs.The server controls the time at which the client should contact the
server to extend the lifetimes on assigned leases through the T1 and
T2 values assigned to an IA. However, as the client SHOULD
renew/rebind all IAs from the server at the same time, the client
MUST select T1 and T2 times from all IA options that will
guarantee the client initiates transmissions of Renew/Rebind
messages not later than at the T1/T2 times associated with any of
the client's bindings (earliest T1/T2).At time T1, the client initiates a Renew/Reply message exchange
to extend the lifetimes on any leases in the IA.A client MUST also initiate a Renew/Reply message exchange before
time T1 if the client's link-local address used in previous
interactions with the server is no longer valid and it is willing
to receive Reconfigure messages.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 MUST include a Server Identifier option
(see ) in the Renew
message, identifying the server with which the client most recently
communicated.The client MUST include a Client Identifier option
(see ) to identify
itself to the server. The client adds any appropriate options,
including one or more IA options.The client MUST include an Elapsed Time option (see
) to indicate how long the client has
been trying to complete the current DHCP message exchange.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 includes 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
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_TIMEOUTREN_MAX_RT0Remaining time until earliest T2The message exchange is terminated when earliest time T2 is
reached. If the client is responding to a Reconfigure, the client
ignores and discards the Reconfigure message. In this case, the
client continues to operate as if Reconfigure message was not
received, i.e., it uses T1/T2 times associated with the client's
leases to determine when it should send Renew or Rebind to the
server. The client begins a Rebind message exchange
(see ) when the
earliest time T2 is reached.At time T2 (which will only be reached if the server to which the
Renew message was sent starting at time T1 has not responded), the
client initiates a Rebind/Reply message exchange with any available
server.A Rebind is also used to verify delegated prefix bindings but
with different retransmission parameters as described in
.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 (see ) in
the Rebind message.The client transmits the message according to , using the following parameters: REB_TIMEOUTREB_MAX_RT0Remaining time until valid lifetimes of all
leases 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. If the terminated
Rebind exchange was initiated as a result of receiving a Reconfigure
message, the client ignores and discards the Reconfigure
message.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
(see ) to identify
itself to the server (see section 4.3.1 of for
reasons why a client may not want to include this option). 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 an Elapsed Time option (see
) to indicate how long the client has
been trying to complete the current DHCP message exchange.The client MUST include an Option Request option (see ) to request the INF_MAX_RT option (see
), the Information
Refresh Time 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.When responding to a Reconfigure, the client includes a Server
Identifier option (see ) with the
identifier from the Reconfigure message
to which the client is responding.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_TIMEOUTINF_MAX_RT00To 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
(see ).The client MUST include a Client Identifier option
(see ) to identify
itself to the server.The client MUST include an Elapsed Time option (see
) to indicate how long the client has
been trying to complete the current DHCP message exchange.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 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 (e) of Section
5.5.3 of - also see L-13 in
Section 4.3 of .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_TIMEOUT0REL_MAX_RC0If 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
(see ) 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
(see ).The client MUST include a Client Identifier option
(see ) to identify
itself to the server.The client MUST include an Elapsed Time option (see
) to indicate how long the client has
been trying to complete the current DHCP message exchange.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_TIMEOUT0DEC_MAX_RC0If 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 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 SHOULD be preferred over all other Advertise
messages. 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.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.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.The client MUST process any SOL_MAX_RT option
(see )
and INF_MAX_RT option (see ) present in an
Advertise message, even if the message contains a Status Code option
(see ) indicating a failure, and the
Advertise message will be discarded by
the client. A client SHOULD only update its SOL_MAX_RT and INF_MAX_RT
values if all received Advertise messages that contained the
corresponding option specified the same value, otherwise it should
use the default value (see ).The client MUST ignore any Advertise message that contains no
addresses (IA Address options, see
encapsulated in IA_NA, see , or IA_TA,
see , options)
and no delegated prefixes (IA Prefix options,
see , encapsulated in IA_PD
options, see ) with the exception
that the client: MUST process an included SOL_MAX_RT option
andMUST 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 an Advertise message MUST NOT restart the
Solicit retransmission timer.Upon the receipt of a valid Reply message in response to a
Solicit with a Rapid Commit option (see ),
Request, Confirm, Renew, Rebind, or Information-request message,
the client extracts the top-level Status Code option
(see ) if present.The client MUST process any SOL_MAX_RT option
(see ) and INF_MAX_RT option
(see ) present in a
Reply message, even if the message contains a Status Code option
indicating a failure.If the client receives a Reply message with a status code of
UnspecFail, the server is indicating that it was unable to process
the client's 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.When a client received a configuration option in an earlier
Reply, then sends a Renew, Rebind or Information-request and
the requested option is not present in the Reply, the client
SHOULD stop using the previously received configuration
information. In other words, the client should behave as if
it never received this configuration option and return to the
relevant default state. If there is no viable way to stop using
the received configuration information, the values
received/configured from the option MAY persist if there are
no other sources for that data and they have no external impact.
For example, a client that previously received a Client FQDN
option (see ) and used it to set up
its hostname is allowed to continue
using it if there is no reasonable way for a node to unset its
hostname and it has no external impact. As a counter example,
a client that previously received an NTP server address from
the DHCP server and does not receive it any more, MUST stop
using the configured NTP server address. The client
SHOULD be open to other sources of the same configuration
information. This behavior does not apply to any IA options,
as their processing is described in detail in the next section.When a client receives a requested option that has an updated
value from what was previously received, the client SHOULD make
use of that updated value as soon as possible for its configuration
information.If the client receives a NotOnLink status from the server in
response to a Solicit (with a Rapid Commit option,
see ) 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: Calculate T1 and T2 times (based on T1 and T2
values sent in the packet and the packet reception time), if
appropriate for the IA type.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 MUST perform duplicate address
detection as per Section 5.4, which
does list some exceptions, 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 any 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 assigned address, which does not have any
associated reachability information, in order to avoid the
problems described in , the client
MUST NOT assume that any addresses are reachable on-link as a
result of receiving an IA_NA or IA_TA. Addresses obtained from
IA_NA or IA_TA MUST NOT be used to form an implicit prefix
with a length other than 128.For each delegated prefix, the client assigns a
subnet to each of the links to which the associated interfaces are
attached.
When a client subnets a delegated prefix, it must assign
additional bits to the prefix to generate unique, longer prefixes.
For example, if the client in were delegated
2001:db8:0::/48, it might generate 2001:db8:0:1::/64 and
2001:db8:0:2::/64 for assignment to the two links in the subscriber
network. If the client were delegated 2001:db8:0::/48
and 2001:db8:5::/48, it might assign 2001:db8:0:1::/64 and
2001:db8:5:1::/64 to one of the links, and 2001:db8:0:2::/64 and
2001:db8:5:2::/64 for assignment to the other link.
If the client uses a delegated prefix to configure addresses on
interfaces on itself or other nodes behind it, the preferred and
valid lifetimes of those addresses MUST be no larger than the
remaining preferred and valid lifetimes, respectively, for the
delegated prefix at any time. In particular, if the delegated
prefix or a prefix derived from it is advertised for stateless
address autoconfiguration [RFC4862], the advertised valid and
preferred lifetimes MUST NOT exceed the corresponding remaining
lifetimes of the delegated prefix.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 to the server that
responded if any of the IAs in the Reply message contains the
NoBinding status code. The client places IA options in this
message for all IAs. 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 as this likely indicates the server
that responded does not support that IA type, sending immediately
is unlikely to produce a different result. Therefore, the client
MUST rate limit its transmissions (see )
and MAY just wait for the normal retransmission time (as if the
Reply message had not been received). The client continues to use
other bindings for which the server did return information.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 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
(see ) 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.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. When the client
only receives one or more Replies with the NotOnLink status in
response to a Confirm message, the client performs DHCP server
discovery as described in .Refer to for details on how the
Information Refresh Time option (whether or not present in the
Reply) should be handled by 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 (see ). 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.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, Rebind, or
Information-request message if necessary); the client MUST ignore
any additional Reconfigure messages until the exchange is
complete.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 MAY
discontinue retransmission of Reconfigure messages to the client
once the server receives the Renew, Rebind 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.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 stable storage and
persisted DHCP state).The client is reconnected to a link on which
it has obtained leases.The client returns from sleep mode.The client changes access points (such as if
using a wireless technology).When the client detects that it may have moved to a new link and it
has obtained addresses and no delegated prefixes from a server, 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, the client 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
(see ). The client includes IA_NAs, IA_TAs, and IA_PDs,
along with the associated leases, in its Rebind message.If the client has only obtained network information using Information-request/Reply
message exchanges, the client MUST initiate a Information-request/Reply message exchange as
described in .If not associated with one of the above mentioned conditions, a client SHOULD
initiate a Renew/Reply exchange (as if the T1 time expired) as described in
or an Information-request/Reply exchange as
described in if the client detects a
significant change regarding the prefixes
available on the link (when new are added or existing are deprecated) as this
may indicate a configuration change. However, a client MUST rate limit such
attempts to avoid flooding a server with requests when there are link issues
(for example, only doing one of these at most every 30 seconds).For this discussion, the Server is assumed to have been configured
in an implementation specific manner with configuration of interest to
clients.A server sends an Advertise message in response to each valid Solicit
message it receives to announce the availability of the server to the
client.In most cases, the server will send a Reply in response to a
Request, Confirm, Renew, Rebind, Decline, Release, and Information-request
messages sent by a client. The server will also send a Reply in
response to a Solicit with a Rapid Commit option
(see ), when the server is
configured to respond with committed lease assignments.These Advertise and Reply messages MUST always contain the
Server Identifier option (see ) containing
the server's DUID and the Client Identifier option
(see ) from the
client message if one was present.In most response 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 (see ) in its
message, the server includes options in
the response 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.Any message sent from a client may arrive at the server
encapsulated in one or more Relay-forward messages. The server MUST
use the received message to construct the proper Relay-reply
message to allow the response to the received message to be
relayed through the same relay agents (in reverse order) as
the original client message; see
for more details.
The server may also need to record this information with each
client in case it is needed to send
a Reconfigure message at a later time unless the server has been
configured with addresses that can be used to send Reconfigure
messages directly to the client (see
). Note that servers
that support leasequery also need
to record this information.The server MAY initiate a configuration exchange, by sending
Reconfigure messages, to cause DHCP clients to obtain new addresses,
prefixes 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 or when other
configuration options are updated, perhaps because servers
are moved, added, or removed.When a client receives a Reconfigure message from the server, the
client initiates sending a Renew, Rebind or Information-request message
as indicated by msg-type in the Reconfigure Message option (see
). The server sends IAs and/or
other configuration information to the client in a Reply message. 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 client's message.See for handling Solicit message
received via unicast. Unicast transmission of Solicit
is not allowed, regardless of whether the Server Unicast option
(see ) is configured or not.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 server
discards the Solicit message.If the server is permitted to respond to the client, the client
has not included a Rapid Commit option
(see ) in the Solicit message or
the server has not been configured to respond with committed assignment
of leases and other resources, the server sends an Advertise
message to the client as described in .
If the client has included a Rapid Commit option in the Solicit
message and the server has been configured to respond with committed
assignments of leases and other resources, the server responds to the
Solicit with a Reply message. 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 .
The server MUST commit the assignment
of any addresses and delegated prefixes or other configuration
information before sending a Reply message to a client. In this case
the server includes a Rapid Commit option in the Reply message to
indicate that the Reply is 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 leases.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.See for handling Request message
received via unicast.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
(see ) containing the
server's DUID and the Client Identifier option
(see ) from the Request
message in the Reply message.The server examines all IAs in the message from the client.For each IA_NA option (see ) and
IA_TA option (see ) in the Request
message the server
checks if the prefixes of included addresses are appropriate for
the link to which the client is connected. If any of the prefixes of
the included 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 (see )
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 option in the Reply
message with no addresses in the IA and a Status Code option
containing status code NoAddrsAvail in the IA.For any IA_PD option (see ) in the
Request message, to which the server cannot
assign any delegated prefixes, the server MUST return the IA_PD
option in the Reply message with no prefixes in the IA_PD and with a
Status Code option containing status code NoPrefixAvail in the IA_PD.The server MAY assign different addresses and/or delegated prefixes
to an IA than those included within the IA of the client's Request
message.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 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.The server SHOULD include a Reconfigure Accept option
(see ) if the server
policy enables reconfigure mechanism and the client supports it.
Currently sending this option in a Reply is technically redundant, as
the use of the reconfiguration mechanism requires authentication
and currently the only defined one is the Reconfigure Key Authentication
Protocol (see ) and
the presence of the reconfigure key signals support for Reconfigure
acceptance. However, there may be better security mechanisms defined
in the future that would cause RKAP to not be used anymore.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 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 a previous transmission. The server MUST NOT cache its
responses.DISCUSSION:The reason why cached replies are bad is because
lifetimes need to be updated (either decrease the timers by
the amount of time elapsed since the original transmission
or keep the lifetime values and update the lease
information in the server's database). Also, if the message uses any
security protection (such as RDM described in ), its value must be updated. Additionally,
any digests must be updated. Given all of the above, caching
replies is far more complex than simply sending the
same buffer as before and it is easy to miss some of those
steps.See for handling Confirm
message received via unicast. Unicast transmission of Confirm
is not allowed, regardless of whether the Server Unicast
option (see ) is configured or not.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 (see ).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
(see ) containing the
server's DUID and the Client Identifier option
(see ) from the Confirm
message in the Reply message. The server includes a Status Code
option (see ) indicating the
status of the Confirm message.See for handling Renew message
received via unicast.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 option (see ) for
that address or IA Prefix option (see )
for that delegated prefix. If the server chooses to include the IA Address or
IA Prefix option for such an address or delegated prefix, the server
SHOULD set T1 and T2 values to the valid lifetime for the IA option unless
the server also includes other addresses or delegated prefixes which
the server is able to extend for the IA. Setting T1 and T2
to values equal to valid lifetime informs the client that the leases
associated with said IA will not be extended, so there is no
point in trying. Also, it avoids generating unnecessary
traffic as the remaining lifetime approaches 0.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 (see )
with zero value in the "IPv6 prefix" field and
non-zero value in the "prefix-length" field, the server MAY use
the "prefix-length" 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
(see ) 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
(see ) containing the
server's DUID and the Client Identifier option
(see ) 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 server MAY include options containing the IAs and values for
other configuration parameters, even if those parameters were not
requested in the Renew message.The T1/T2 values set in each applicable IA option for a Reply MUST
be the same across all IAs. The server MUST determine the
T1/T2 values 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.See for handling Rebind message
received via unicast. Unicast transmission of Rebind
is not allowed, regardless of whether the Server Unicast option
(see ) is configured or not.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 values. 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 (see )
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
(see ) 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. If the server chooses to not include any IAs
containing IA Address or IA Prefix options with lifetimes of 0 and
the server does not include any other IAs with leases and/or status
codes, the server does not send a Reply message. In this situation
the server 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
(see ) below), the server SHOULD
create a binding and return the IA with allocated leases with
lifetimes and, if applicable, T1/T2 values 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
(see ) 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 - see the Discussion in
. 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
(see ) containing the
server's DUID and the Client Identifier option
(see ) 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 server MAY include options containing the IAs and values for
other configuration parameters, even if those IAs and parameters
were not requested in the Rebind message.The T1 values set in each applicable IA option for a Reply MUST be
the same values across all IAs. The T2 values set in each applicable
IA option for a Reply MUST be the same values across all IAs. The
server MUST determine the T1 values across all of the applicable
client's bindings in the Reply. The server MUST determine the T2
values 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.See for handling
Information-request message received via unicast.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
(see ) containing the
server's DUID in the Reply message. If the client included a Client
Identifier option (see ) 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 . The server MAY include additional
options that were not requested by the client in the
Information-request message.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.See for handling Release message
received via unicast.The server constructs a Reply message by setting the "msg-type"
field to REPLY, and copying the transaction ID from the Release
message into the transaction-id field.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
(see ) with value Success,
a Server Identifier option (see )
with the server's DUID, and a Client Identifier option (see
) 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.See for handling Decline message
received via unicast.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 by setting the "msg-type" field to REPLY, and
copying the transaction ID from the Decline message into the
transaction-id field. The client includes a Status Code option
(see ) with
the value Success, a Server Identifier option
(see ) with the server's
DUID, and a Client Identifier option
(see ) 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.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
(see ) and
copies the Client Identifier option
(see ) from the Solicit message into
the Advertise message.The server MAY add a Preference option
(see ) 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
(see ) if the server
wants to indicate it supports Reconfigure mechanism. This
information may be used by the client during the server
selection process.The server includes the 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. The server MUST include
options in the Advertise message containing configuration parameters
for all of the options identified in the Option Request option
(see ) in the Solicit message
that the server has been configured to return to the client. If the
Option Request option includes a container option the server MUST include all
the options that are eligible to be encapsulated in the container. The Option
Request option MAY be used to signal support for a feature even when that option is
encapsulated as in the case of the Prefix Exclude option .
In this case, special processing is required by the server.
The server MAY return additional options to the client if it has been
configured to do so.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 Address options
(see ) 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 options (see ),
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 message.If the server will not assign any addresses to an IA_NA or IA_TA
in subsequent Request from the client, the server MUST
include the IA option in the Advertise message with no addresses in that IA
and a Status Code option (see )
encapsulated in the IA option 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 option (see )
in the Advertise message with no prefixes in the IA_PD
option and a Status Code option encapsulated in the IA_PD containing
status code NoPrefixAvail.Transmission of the Advertise message is described in the next
section.If the original message was received directly by the server, the
server unicasts the Advertise or 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 Advertise or 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 (see ),
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. See for more details on the
construction of Relay-reply messages.The server sets the "msg-type" field to RECONFIGURE. The server
sets the transaction-id field to 0. The server includes a Server
Identifier option (see )
containing its DUID and a Client Identifier option
(see )
containing the client's DUID in the Reconfigure message.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 (see ).The server MUST include a Reconfigure Message option (see
) 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 (see
), 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.When transmitting the Reconfigure message, the server sets
the retransmission time (RT) to REC_TIMEOUT. If the server does
not receive a Renew, Rebind, or Information-request message from
the client before the RT elapses, the server retransmits the
Reconfigure message, doubles the RT 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 .Unless otherwise stated in sections dedicated to specific
messages reception (see dedicated sections in
), the server is not supposed to accept
unicast traffic when it is not explicitly configured to do
so. For some messages (Solicit, Rebind, and Confirm) unicast
transmission is not allowed, even if Server Unicast option
(see ) is
configured. For Request, Renew, Informaton-request, Release,
and Decline messages, it is allowed only if Server Unicast
option is configured.When the server receives a message via unicast from a client
to which the server has not sent a Server Unicast option (or is not
currently configured to send a Server Unicast option to the client),
the server discards that message and responds with an Advertise
(when responding to Solicit) or Reply (when responding to any
other messages) message containing a Status Code option
(see ) with
value UseMulticast, a Server Identifier option
(see ) containing the
server's DUID, the Client Identifier option
(see ) from the client
message (if any), and no other options.The relay agent SHOULD be configured to use a list of destination
addresses, which include unicast addresses. The list of destination addresses
MAY include 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
8.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 Relay-forward message, a recognized message type for which it is
not the intended target, or an unrecognized message type
(), 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 into 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
(see )
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 address (including unique
local address, ) with a prefix
assigned to the link on which the client should be assigned
leases into the link-address field. If such an address is not
available, the relay agent may set the link-address field
to a link-local address from the interface the original message
was received on. That is not recommended as it may require additional
information to be provided in the server configuration. See Section
3.2 of for a detailed
discussion.This address will be used by the server to determine the link
from which the client should be assigned leases 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 sets the
link-address field as described in the earlier paragraphs 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 address (including unique local address,
), the relay
agent sets the link-address field to 0; otherwise the relay agent
sets the link-address field to a global address (including
unique local address)
assigned to the interface on which the message was received, or
includes an Interface-Id option (see )
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 server communicates with a client through a relay agent
about delegated prefixes, the server may need a protocol or
other out-of-band communication to configure routing information for
delegated prefixes on any router through which the client
may forward traffic.The relay agent processes any options included in the Relay-reply
message in addition to the Relay Message option
(see ).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
(see ), 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 (see )
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.Each time a packet is relayed by a relay agent towards a server, a
new encapsulation level is added around the packet. Each relay is
allowed to insert additional options on the encapsulation level it
added, but MUST NOT change anything in the packet being encapsulated. If
there are multiple relays between a client and a server, multiple
encapsulations are used. Although it makes packet processing slightly
more complex, it has a big advantage of having clear indication which
relay inserted which option. The response packet is expected to travel
through the same relays, but in reverse order. Each time a response
packet is relayed back towards a client, one encapsulation level is
removed.In certain cases relays can add one or more options. These options
can be added for several reasons. First, relays can provide additional
information about the client. That source of information is usually more
trusted by a server administrator as it comes from the network
infrastructure rather then the client and cannot be easily
spoofed. These options can be used by the server to determine its
allocation policy.Second, a relay may need some information to send a response back to
the client. Relay agents are expected to be stateless (not retain any
state after a packet has been processed). A relay agent may include
the Interface-Id option (see ), which
will be echoed back in the response. It can include other options and
ask the server to echo one or more of the options back in the response.
These options can then be used by the relay agent to send the response
back to the client or for other needs. The client will never see these
options. See for details.Third, sometimes a relay is the best device to provide values for
certain options. A relay can insert an option into the packet being
forwarded to the server and ask the server to pass that option back to the
client. The client will receive that option. It should be noted that the
server is the ultimate authority here and depending on its configuration, it
may send the option back to the client or not. See
for details.Servers may need to retain the relay information after the packet
processing is completed for various reasons. One is a bulk leasequery
mechanism that may ask for all addresses and/or prefixes that were
assigned via a specific relay. A second is for the reconfigure mechanism. The
server may chose to not send the Reconfigure message directly to the
client, but rather send it via relays. This particular behavior is
considered an implementation detail and is out of scope for this
document.Within this document, two security mechanisms are introduced for the
authentication of DHCP messages: authentication (and encryption) of messages
sent between servers and relay agents using IPsec, and protection against
misconfiguration of a client caused by a Reconfigure message sent by a
malicious DHCP server.The delayed authentication protocol, defined in
, has been obsoleted by this document
(see ).Relay agents and servers that exchange messages can use
IPsec as detailed in .
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. One such protocol, the Reconfigure key
authentication protocol, is defined in . 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 of the Authentication
option (see ) 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 64-bit unsigned
integer (modulo 2^64). Using this technique can reduce the danger
of replay attacks. This method MUST be supported by all Authentication
option protocols. One choice might be to use the 64-bit NTP Timestamp
format ).A client that receives a message with the RDM field set to 0x00 MUST
compare its replay detection field with the previous value sent by
that same server (based on the Server Identifier option, see
). If this is the first time a client
processes an Authentication option sent by a server, the client
MUST record the replay detection value, but otherwise skip the
replay detection check.Servers that support the reconfigure mechanism MUST ensure the
replay detection value is retained between restarts. Failing to do so
may cause clients to refuse Reconfigure messages sent by the server,
effectively rendering the reconfigure mechanism useless.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
(see . 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 have negotiated to use Reconfigure
messages.The following fields are set in an Authentication option
(see for the
Reconfigure Key Authentication Protocol: 310The format of the authentication information for the Reconfigure
Key Authentication Protocol is:Type of data in the Value field carried in
this option:
Reconfigure Key value (used in Reply
message).HMAC-MD5 digest of the message (used in
Reconfigure message).
A one octet long field.Data as defined by the Type field. A
16 octets long 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
(see ) 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 an
Authentication option (see ) 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 Reconfigure message, with zeroes substituted
for the HMAC-MD5 field, 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. See for additional
information about a registry maintained by IANA.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 ), Vendor Class (see ),
Vendor-specific Information (see ),
and IA_PD (see ) options. Also, IA Address (see ) and IA Prefix (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. A two octets long
field.An unsigned integer giving the length
of the option-data field in this option in octets. A two octets
long field.The data for the option; the format
of this data depends on the definition of the option. A variable
length field (the length, in octets, is specified by option-len).DHCP 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 other IA option types (i.e., IA_TA and IA_PD).
A four octets long field containing an unsigned integer.The time interval after which the client should contact 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. A four octets long field containing an unsigned integer.The time interval after which the client should contact 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. A four octets long field containing
an unsigned integer.Options associated with this
IA_NA. A variable length field (12 octets less than the
value in the option-len field).The IA_NA-options field encapsulates those options that are
specific to this IA_NA. For example, all of the IA Address options
(see )
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;
it is up to the server policy to determine how many addresses are
assigned, but typically at most one address is assigned from each
prefix assigned to the link to which the client is attached to.An IA_NA option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_NA options (though
each must have a unique IAID).The status of any operations involving this IA_NA is indicated in a
Status Code option (see )
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 times, 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 and are calculated since
reception of the message.As per , the value 0xffffffff is taken to
mean "infinity" and should be used carefully.The server selects the T1 and T2 values 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 values to 0. The client MUST follow the rules defined
in .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.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 other IA option types (i.e., IA_NA and IA_PD).
A four octets long field containing an unsigned integer.Options associated with this
IA_TA. A variable length field (4 octets less than the
value in the option-len field).The IA_TA-Options field encapsulates those options that are
specific to this IA_TA. For example, all of the IA Address options
(see )
carrying the addresses associated with this IA_TA are in the
IA_TA-options field.Each IA_TA carries one "set" of temporary addresses. It is
up to the server policy to determine how many addresses are
assigned.An IA_TA option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_TA options (though
each must have a unique IAID).The status of any operations involving this IA_TA is indicated in a
Status Code option (see )
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 valid lifetime 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 valid lifetime of a temporary address to allow an application to
continue to use an established TCP connection. Extending only the
valid, but not the preferred lifetime means the address will end up
in deprecated state eventually. Existing connections could continue, but
no new ones would be created using that address.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.The IA Address option is used to specify an address associated
with an IA_NA or an IA_TA. The IA Address option must be encapsulated
in the Options field of an IA_NA (see )
or IA_TA (see ) option. The IAaddr-options fields
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. A client MUST NOT
form an implicit prefix with a length other than 128 for this
address. And, a client MUST NOT assume any length of prefix that
matches this address is on-link (see ).
A 16 octets long field.The preferred lifetime for the
address in the option, expressed in units of seconds. A four
octets long field containing an unsigned integer.The valid lifetime for the
address in the option, expressed in units of seconds. A four
octets long field containing an unsigned integer.Options associated with this
address. A variable length field (24 octets less than the
value in the option-len field).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 an 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.The client MUST discard any addresses for which the preferred lifetime is
greater than the valid lifetime.As per , the valid lifetime of an address
0xffffffff is taken to mean "infinity" and should be used carefully.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. Each option-code is a two
octets long field containing an unsigned integer.A client MUST include an Option Request option in a Solicit,
Request, Renew, Rebind, or Information-request message to
inform the server about options the client wants the server to send to
the client. For certain message types, some option codes MUST be
included in the Option Request option, see
for details.The Option Request option MUST NOT include the
following options:
Client Identifier (see ),
Server Identifier (see ),
IA_NA (see ),
IA_TA (see ),
IA_PD (see ),
IA Address (see ),
IA Prefix (see ),
Option Request,
Elapsed Time (see ),
Preference (see ),
Relay Message (see ),
Authentication (see ),
Server Unicast (see ),
Status Code (see ),
Rapid Commit (see ),
User Class (see ),
Vendor Class (see ),
Interface-Id (see ),
Reconfigure Message (see ), and
Reconfigure Accept (see ).
Other top-level options MUST appear in the Option Request
option or they will not be sent by the server. Only top-level options MAY
appear in the Option Request option. Options encapsulated in a container
option SHOULD NOT appear in an Option Request option; see
for an example of container options. However, options MAY be defined which
specify exceptions to this restriction on including
encapsulated options in an Option Request option. For example, the Option Request
option MAY be used to signal support for a feature even when that option is
encapsulated, as in the case of the Prefix Exclude option .
See .
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 one-octet unsigned integer.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 two octets long field
containing an unsigned integer.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 message. The
length, in octets, is specified by option-len.The Authentication option carries authentication information to
authenticate the identity and contents of DHCP messages. The use of
the Authentication option is described in . The delayed authentication protocol,
defined in , has been obsoleted by this
document, due to lack of usage. The format of the Authentication
option is:OPTION_AUTH (11).11 + length of authentication
information field.The authentication protocol used in this
authentication option. A one-octet unsigned integer.The algorithm used in the
authentication protocol. A one-octet unsigned integer.The replay detection method used in this
Authentication option. A one-octet unsigned integer.The replay detection information
for the RDM. A 64-bit (8 octets) long fieldThe authentication
information, as specified by the protocol and algorithm used in
this Authentication option. A variable length field (11 octets
less than the value in option-len).IANA maintains a registry for the protocol, algorithm, and RDM
values at https://www.iana.org/assignments/auth-namespaces.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 128-bit address to
which the client should send messages delivered using
unicast.The server specifies the 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 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 two octets long field containing
an unsigned integer.A UTF-8 encoded text string
suitable for display to an end user, which MUST NOT be
null-terminated. A variable length field (2 octets
less than the value in option-len).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-code 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 multicast
address.NoPrefixAvail6Server has no prefixes available to assign to the
IA_PD(s).See for additional information about the
registry maintained by IANA with the complete list of status codes.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 leases
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 leases
that are not actually used by the client, which could result in
bad information in the DNS server if the DHCP server updates DNS
or in response to leasequery requests
.The problem of unused leases can be minimized by designing
the DHCP service so that only one server responds to the
Solicit or by using relatively short lifetimes for newly assigned
leases.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 length, in octets, is specified by option-len.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 . A four octets long field containing
an unsigned integer.The hardware configuration of
the node on which the client is running. A variable length field
(4 octets less than the value in option-len).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 vendor-class-data instances.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 . A four octets long field containing
an unsigned integer.Vendor options, interpreted by
vendor-specific code on the clients and servers. A variable
length field (4 octets less than the value in option-len).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 node device's IPv6 stack to be functional.The vendor-option-data field MUST be encoded as a
sequence of code/length/value fields of identical format to the DHCP
options field. The sub-option codes are defined by the vendor identified
in the enterprise-number field, and are not managed by IANA. Each of
the sub-options is formatted as follows:The code for the sub-option. A
two octets long field.An unsigned integer giving the length
of the sub-option-data field in this sub-option in octets. A two
octets long field.The data area for the sub-option.
The length, in octets, is specified by sub-option-len.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 sub-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
(see ).MAY specify the Vendor-specific Information option
with appropriate data.Servers 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 length, in octets, is specified by option-len.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. A one-octet unsigned integer.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. The format of this option is: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
client's IA_PDs. The number space for IA_PD IAIDs is separate from
the number space for other IA option types (i.e., IA_NA and IA_TA).
A four octets long field containing an unsigned integer.The time interval after which the client should
contact the server 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 message reception
time expressed in units of seconds. A four octets long field containing
an unsigned integer.The time interval after which the client should
contact any available server to extend the lifetimes of
the prefixes assigned to the IA_PD; T2 is a time duration relative
to the message reception time expressed in units of seconds.
A four octets long field containing an unsigned integer.Options associated with this
IA_PD. A variable length field (12 octets less than the
value in the option-len field).The IA_PD-options field encapsulates those options that are
specific to this IA_PD. For example, all of the IA Prefix options
(see )
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 (though
each must have a unique IAID).The status of any operations involving this IA_PD is indicated in a
Status Code option (see )
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
fields are included to give the server explicit control over when a
client should contact the server about a
specific IA_PD.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 timers, 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 prefixes in the IA_PD
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
prefixes in the IA_PD that the server 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 client, the
server sets T1 and T2 to 0. The client MUST
follow the rules defined in .If a client receives an IA_PD with T1 greater than T2,
and both T1 and T2 are greater than 0, the client discards
the IA_PD option and processes the remainder of the message as though
the server had not included the IA_PD option.The IA Prefix option is used to specify a prefix
associated with an IA_PD. The IA Prefix option must be encapsulated
in the IA_PD-options field of an IA_PD option
(see ).OPTION_IAPREFIX (26).25 + length of IAprefix-options
field.The preferred
lifetime for the prefix in the option, expressed in units of
seconds. A value of 0xFFFFFFFF represents "infinity"
(see . A four
octets long field containing an unsigned integer.The valid lifetime for the
prefix in the option, expressed in units of seconds. A value of
0xFFFFFFFF represents "infinity". A four octets long field
containing an unsigned integer.Length for this prefix in bits.
A one-octet unsigned integer.An IPv6 prefix. A 16 octets long
field.Options associated with this
prefix. A variable length field (25 octets less than the
value in the option-len field).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 in these lifetime fields.The client SHOULD NOT send an IA Prefix option with 0 in the
prefix-length field (and an unspecified value (::) in the
IPv6-prefix field). A client MAY send a non-zero value in the
prefix-length field and the unspecified value (::) in the
IPv6-prefix field to indicate a preference for the size of the
prefix to be delegated. See for
further details on prefix length hints.The client MUST discard any prefixes for which the preferred
lifetime is greater than the valid lifetime.The values in the preferred and valid lifetimes are the number of
seconds remaining for each lifetime. See
for more details
on how these values are used for delegated prefixes.As per , the preferred and valid lifetime values of
0xffffffff is taken to mean "infinity" and should be used carefully.An IA Prefix option may appear only in an IA_PD option. More
than one IA Prefix option can appear in a single IA_PD option.The status of any operations involving this IA Prefix option is
indicated in a Status Code option (see )
in the IAprefix-options field.This option is requested by clients and returned by servers to
specify an upper bound for how long a client should wait before
refreshing information retrieved
from a DHCP server. It is only used in Reply messages in response to
Information-request messages. In other messages there will usually
be other information that indicates when the client should contact the
server, e.g., T1/T2 times and lifetimes. This option is useful
when the configuration parameters change or during renumbering event as
clients running in the stateless mode will be able to update their
configuration.The format of the Information Refresh Time option is:OPTION_INFORMATION_REFRESH_TIME (32).4.Time duration relative to
the current time, expressed in units of seconds. A four octets
long field containing an unsigned integer.A DHCP client MUST request this option in the Option Request option
(see ) when sending Information-request
messages. A client MUST NOT request this option in the Option Request
option in any other messages.A server sending a Reply to an Information-request message SHOULD
include this option if it is requested in the Option Request option
of the Information-request. The option value MUST NOT be smaller
than IRT_MINIMUM. This
option MUST only appear in the top-level option area of Reply messages.If the Reply to an Information-request message does not contain this
option, the client MUST behave as if the option with value IRT_DEFAULT
was provided.A client MUST use the refresh time IRT_MINIMUM if it receives the
option with a value less than IRT_MINIMUM.As per , the value 0xffffffff is taken to
mean "infinity" and implies that the client should not refresh its
configuration data without some other trigger (such as detecting
movement to a new link).If a client contacts the server to obtain new data or refresh some
existing data before the refresh time expires, then it SHOULD also
refresh all data covered by this option.When the client detects that the refresh time has expired, it SHOULD
try to update its configuration data by sending an Information-
Request as specified in , except that the
client MUST delay sending the first Information-request by a random
amount of time between 0 and INF_MAX_DELAY.A client MAY have a maximum value for the refresh time, where that
value is used whenever the client receives this option with a value
higher than the maximum. This also means that the maximum value is
used when the received value is "infinity". A maximum value might
make the client less vulnerable to attacks based on forged DHCP
messages. Without a maximum value, a client may be made to use wrong
information for a possibly infinite period of time. There may
however be reasons for having a very long refresh time, so it may be
useful for this maximum value to be configurable.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 four octets long field containing an unsigned integer.A DHCP client MUST include the SOL_MAX_RT option code in any Option
Request option (see ) it sends in a
Solicit message.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 as a top-level
option in the message to the client.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 .The purpose of this mechanism is to give network
administrator a way to avoid large DHCP traffic if all DHCP
servers become unavailable. Therefore this value is expected
to be retained for as long as practically possible.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 four octets long field containing an unsigned integer.A DHCP client MUST include the INF_MAX_RT option code in any Option
Request option (see ) it sends in
an Information-request message.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 a top-level
option in the message to the client.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 DHCP 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.DHCP lacks end-to-end encryption between clients and servers, thus
hijacking, tampering, and eavesdropping attacks are all possible
as a result. Some network environments (discussed below) can be
secured through various means to minimize these attacks.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 (see ) and INF_MAX_RT
(see ) 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.A malicious server can also send a Server Unicast option (see
) to a client in an Advertise
message, thus potentially causing the client to bypass relays and
communicate only with the malicious server for subsequent Request
and Renew messages.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, Rebind, 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.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 "in plain text" to the client.Many of these rogue server attacks can be mitigated by making use of
the mechanism described in and .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 assigned address or delegatable prefixes, or the
exhaustion of CPU or network bandwidth, and are present anytime there is
a shared resource. Some forms of these exhaustion attacks can be
partially mitigated by appropriate server policy, e.g., limiting the
maximum number of leases any one client can get.The messages exchanged between relay agents and servers may be used to
mount a "man in the middle" or denial of service attack. Communication
between a server and a relay agent, and communication between relay
agents, can be authenticated and encrypted through the use of IPsec, as
described in .However, the use of manually configured pre-shared 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 address and delegatable
prefixes.Various network environments also offer levels of security if deployed
as described below.
In enterprise and factory networks, use of
authentication can prevent unknown or untrusted clients from connecting to
the network. However, this does not necessarily assure that the connected
client will be a good DHCP or network actor.For wired networks where clients typically are connected to a switch
port, snooping DHCP multicast (or unicast traffic) becomes difficult as
the switches limit the traffic delivered to a port. The client's DHCP
multicast packets (with destination address fe02::1:2) are only forwarded
to the DHCP server's (or relay's) switch port - not all ports. And the
server's (or relay's) unicast replies are only delivered to the target
client's port - not all ports.In public networks (such as a WiFi network in a coffee shop or airport),
it is possible for others within radio range to snoop DHCP and other
traffic. But in these environments, there is very little if anything that
can be learned from the DHCP traffic itself (either from client to server,
or server to client) if the privacy considerations (see ) are followed. For devices that do not follow the
privacy considerations, there is also little that can be learned that
would not be available from subsequent communications anyway (such as the
device's mac-address). Or, that cannot be inferred by the bad actor
initiating a DHCP request itself (since all clients will typically receive
similar configuration details). As mentioned above, one threat is that the
RKAP key for a client can be learned (if the initial Solicit / Advertise /
Request / Reply exchange is monitored) and trigger a premature
reconfiguration - but this is relatively easy to prevent by disallowing
direct client-to-client communication on these networks or using and .This section focuses on the server considerations. For extended
discussion about privacy considerations for the client, see . In particular, Section 3
of that document discusses 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.
Implementers 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 other possible
strategies. Readers are also encouraged to read , in particular Section 4.1.2 that discusses
the problems with certain allocation strategies.This document does not define any new DHCP name spaces or
definitions.The publication of this document does not change the
assignment rules for new values for message types, option codes,
DUID types or status codes.The list of assigned values used in DHCPv6 is available at
https://www.iana.org/assignments/dhcpv6-parametersIANA is requested to update the
https://www.iana.org/assignments/dhcpv6-parameters
page to add a reference to this document for definitions previously
created by , ,
and .IANA is requested to add two columns to the DHCPv6 Option table at
https://www.iana.org/assignments/dhcpv6-parameters
to indicate which options are allowed to appear in a client's Option
Request option
(see ) and which options are singleton
options (only allowed to appear once as a top-level or encapsulated
option - see Section 16 of ).
provides the data for the options
assigned by IANA at the time of writing.OptionOption Name (OPTION prefix removed)Client ORO (1)Singleton Option1CLIENTIDNoYes2SERVERIDNoYes3IA_NANoNo4IA_TANoNo5IAADDRNoNo6ORONoYes7PREFERENCENoYes8ELAPSED_TIMENoYes9RELAY_MSGNoYes11AUTHNoYes12UNICASTNoYes13STATUS_CODENoYes14RAPID_COMMITNoYes15USER_CLASSNoYes16VENDOR_CLASSNoNo (2)17VENDOR_OPTSOptionalNo (2)18INTERFACE_IDNoYes19RECONF_MSGNoYes20RECONF_ACCEPTNoYes21SIP_SERVER_DYesYes22SIP_SERVER_AYesYes23DNS_SERVERSYesYes24DOMAIN_LISTYesYes25IA_PDNoNo26IAPREFIXNoNo27NIS_SERVERSYesYes28NISP_SERVERSYesYes29NIS_DOMAIN_NAMEYesYes30NISP_DOMAIN_NAMEYesYes31SNTP_SERVERSYesYes32INFORMATION_REFRESH_TIMERequired for Information-requestYes33BCMCS_SERVER_DYesYes34BCMCS_SERVER_AYesYes36GEOCONF_CIVICYesYes37REMOTE_IDNoYes38SUBSCRIBER_IDNoYes39CLIENT_FQDNYesYes40PANA_AGENTYesYes41NEW_POSIX_TIMEZONEYesYes42NEW_TZDB_TIMEZONEYesYes43ERONoYes44LQ_QUERYNoYes45CLIENT_DATANoYes46CLT_TIMENoYes47LQ_RELAY_DATANoYes48LQ_CLIENT_LINKNoYes49MIP6_HNIDFYesYes50MIP6_VDINFYesYes51V6_LOSTYesYes52CAPWAP_AC_V6YesYes53RELAY_IDNoYes54IPv6_Address-MoSYesYes55IPv6_FQDN-MoSYesYes56NTP_SERVERYesYes57V6_ACCESS_DOMAINYesYes58SIP_UA_CS_LISTYesYes59OPT_BOOTFILE_URLYesYes60OPT_BOOTFILE_PARAMYesYes61CLIENT_ARCH_TYPENoYes62NIIYesYes63GEOLOCATIONYesYes64AFTR_NAMEYesYes65ERP_LOCAL_DOMAIN_NAMEYesYes66RSOONoYes67PD_EXCLUDEYesYes68VSSNoYes69MIP6_IDINFYesYes70MIP6_UDINFYesYes71MIP6_HNPYesYes72MIP6_HAAYesYes73MIP6_HAFYesYes74RDNSS_SELECTIONYesNo75KRB_PRINCIPAL_NAMEYesYes76KRB_REALM_NAMEYesYes77KRB_DEFAULT_REALM_NAMEYesYes78KRB_KDCYesYes79CLIENT_LINKLAYER_ADDRNoYes80LINK_ADDRESSNoYes81RADIUSNoYes82SOL_MAX_RTRequired for SolicitYes83INF_MAX_RTRequired for Information-requestYes84ADDRSELYesYes85ADDRSEL_TABLEYesYes86V6_PCP_SERVERYesNo87DHCPV4_MSGNoYes88DHCP4_O_DHCP6_SERVERYesYes89S46_RULENoNo (3)90S46_BRNoNo91S46_DMRNoYes92S46_V4V6BINDNoYes93S46_PORTPARAMSNoYes94S46_CONT_MAPEYesNo95S46_CONT_MAPTYesYes96S46_CONT_LWYesYes974RDYesYes984RD_MAP_RULEYesYes994RD_NON_MAP_RULEYesYes100LQ_BASE_TIMENoYes101LQ_START_TIMENoYes102LQ_END_TIMENoYes103DHCP Captive-PortalYesYes104MPL_PARAMETERSYesYes105ANI_ATTNoYes106ANI_NETWORK_NAMENoYes107ANI_AP_NAMENoYes108ANI_AP_BSSIDNoYes109ANI_OPERATOR_IDNoYes110ANI_OPERATOR_REALMNoYes111S46_PRIORITYYesYes112MUD_URL_V6 (TEMPORARY)NoYes113V6_PREFIX64YesNo114F_BINDING_STATUSNoYes115F_CONNECT_FLAGSNoYes116F_DNS_REMOVAL_INFONoYes117F_DNS_HOST_NAMENoYes118F_DNS_ZONE_NAMENoYes119F_DNS_FLAGSNoYes120F_EXPIRATION_TIMENoYes121F_MAX_UNACKED_BNDUPDNoYes122F_MCLTNoYes123F_PARTNER_LIFETIMENoYes124F_PARTNER_LIFETIME_SENTNoYes125F_PARTNER_DOWN_TIMENoYes126F_PARTNER_RAW_CLT_TIMENoYes127F_PROTOCOL_VERSIONNoYes128F_KEEPALIVE_TIMENoYes129F_RECONFIGURE_DATANoYes130F_RELATIONSHIP_NAMENoYes131F_SERVER_FLAGSNoYes132F_SERVER_STATENoYes133F_START_TIME_OF_STATENoYes134F_STATE_EXPIRATION_TIMENoYes135RELAY_PORTNoYes143IPv6_ADDRESS-ANDSFYesYesNotes for :
For the "Client ORO" column: a "Yes" for an option
means that the client includes this option code in the Option Request
option (see ) if it desires that
configuration information; a "No" means that
the option MUST NOT be included (and servers SHOULD silently ignore
that option code if it appears in a client's Option Request option). For each enterprise-number, there MUST only be
a single instance. See for details.IANA is requested to correct the range of possible Status Codes
in the Status Codes table at https://www.iana.org/assignments/dhcpv6-parameters
by replacing 23-255 (as Unassigned) with 23-65535 (the codes are
16-bit unsigned integers).IANA is requested to update the All_DHCP_Relay_Agents_and_Servers
(ff02::1:2) and All_DHCP_Servers (ff05::1:3)
table entries in the IPv6 multicast address space registry at
https://www.iana.org/assignments/ipv6-multicast-addresses
to reference this document instead of .IANA is requested
to add an "Obsolete" annotation into the "DHCPv6 Delayed
Authentication" entry in the "Authentication Suboption (value 8) -
Protocol identifier values" registry at
https://www.iana.org/assignments/bootp-dhcp-parameters, and
to add an "Obsolete" annotation into the "Delayed Authentication"
entity in the "Protocol Name Space Values" registry at
https://www.iana.org/assignments/auth-namespaces. IANA is also
requested to update these pages to reference this document
instead of .
IANA is requested to add a reference to this document for the
RDM value of 0 to the "RDM Name Space Values" registry at
https://www.iana.org/assignments/auth-namespaces.
IANA is requested to update the "Service Name and Transport
Protocol Port Number Registry" at
https://www.iana.org/assignments/service-names-port-numbers as
follows:
Add a reference to this document.Add a reference to this document.Add a reference to .Add a reference to .This specification is mostly a corrected and cleaned up version
of the original specification, , along with
numerous additions from later RFCs. However, there are a small number of
mechanisms that were not widely deployed, were
underspecified or had other operational issues. Those mechanisms are now
considered deprecated. Legacy implementations MAY support them, but
implementations conformant to this document MUST NOT rely on them.The following mechanisms 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. Clients used to be allowed to send
lifetime values as hints. This mechanism was not widely implemented and
there were known misimplementations that sent the remaining lifetimes rather
than total desired lifetimes. That in turn was sometimes misunderstood by
servers as a request for ever decreasing lease lifetimes, which caused
issues when values started approaching zero. Clients now SHOULD set
lifetimes to 0 in IA Address and IA Prefix options, and servers MUST
ignore any requested lifetime value.T1/T2 hints sent by a client. These had similar issues to the
lifetime hints. Clients now SHOULD set the T1/T2 values to 0 in IA_NA and
IA_PD options, and servers MUST ignore any client supplied T1/T2
values.This document is merely a refinement of earlier work by the
authors of RFC3315 (Ralph Droms, Jim Bound, Bernie Volz,
Ted Lemon, Charles Perkins, and Mike Carney), RFC3633 (Ole Troan
and Ralph Droms), RFC3736 (Ralph Droms), RFC4242 (Stig Venaas,
Tim Chown, and Bernie Volz), RFC7083 (Ralph Droms), and RFC7550
(Ole Troan, Bernie Volz, and Marcin Siodelski) and would not be
possible without their original work.A number of additional people have contributed to identifying issues
with RFC3315 and RFC3633 and proposed resolutions to these issues as
reflected in this document (in no particular order): Ole Troan, Robert
Marks, Leaf Yeh, Michelle Cotton, Pablo Armando, John Brzozowski,
Suresh Krishnan, Hideshi Enokihara, Alexandru Petrescu,
Yukiyo Akisada, Tatuya Jinmei, Fred Templin and Christian Huitema.We also thank the following, not otherwise acknowledged and in no
particular order, for their review comments: Jeremy Reed, Francis Dupont,
Tatuya Jinmei, Lorenzo Colitti, Tianxiang Li, Ian Farrer, Yogendra Pal,
Kim Kinnear, Shawn Routhier, Tim Chown, Michayla Newcombe, Alissa Cooper,
Allison Mankin, Adam Roach, Kyle Rose, Elwyn Davies, Eric Rescorla,
Ben Campbell, Warren Kumari, and Kathleen Moriarty.And, special thanks to Ralph Droms for answering many questions related
to the original RFC3315 and RFC3633 work and for shepherding this document
through the IETF process.Private Enterprise Numbers registry
https://www.iana.org/assignments/enterprise-numbers
IANA
Reserved IPv6 Interface Identifiers
https://www.iana.org/assignments/ipv6-interface-ids
IANA
Hardware Types
https://www.iana.org/assignments/arp-parameters
IANATR-187 - IPv6 for PPP Broadband AccessBroadband Forum802.1X-2010 - IEEE Standard for Local and metropolitan area networks--Port-Based Network Access ControlIEEEThis appendix provides a summary of the significant changes made
to this updated DHCPv6 specification.The Introduction was reorganized and
updated. In particular, the client/server message exchanges were moved into
a new (and expanded) section on their own (see
). And, new sections were added to
discuss the relation to previous DHCPv6 documents and also to DHCPv4.The Requirements and Background
had very minor edits.The Terminology had minor edits.The DHCP Terminology was expanded to
incorporate definitions from RFC3633, add T1/T2 definitions,
add a few new definitions useful in a document that combined address and prefix
delegation assignments, and improve some existing definitions.The Client-Server Exchanges was
added from material previously in the Introduction Section 1 of RFC3315 and
was expanded.The Operational Models is new and provides
information on the kinds of DHCP clients and how they operate.The DHCP Constants was primarily updated
to add constants from RFC4242 and RFC7083. Note that the HOP_COUNT_LIMIT
was reduced from 32 to 8.The Client/Server Message Formats , Relay
Agent/Server Message Formats , and Representation and
Use of Domain Names had only very minor changes.The DHCP Unique Identifier (DUID) now discourages,
rather than disallows, a server to parse the DUID, now includes some information
on the DUID-UUID (RFC6355), and has other minor edits.The Identity Association was expanded to better
explain the concept and also included prefix delegation.The Assignment to an IA incorporates material from
two sections (11 and 12) of RFC3315 and also includes a section on
prefix delegation.The Transmission of Messages by a Client was
expanded to include rate limiting by clients and how clients should handle T1
or T2 values of 0.The Reliability of Client Initiated Message Exchanges
was expanded to clarify that the Elapsed Time option must be updated in
retransmitted messages and that a client is not required to listen for DHCP
traffic for the entire retransmission period.The Message Validation had minor edits.The Client Source Address and Interface Selection
was expanded to include prefix delegation.The DHCP Configuration Exchanges
consolidates what used to be in the RFC3315 DHCP Server Solicitation Section
17, DHCP Client-Initiated Configuration Exchange Section 18, and
DHCP Server-Initiated Configuration Exchange Section 19. This material was
reorganized and enhanced, and incorporates prefix delegation from RFC3633
and other changes from RFC4242, RFC7083, and RFC7550. A few changes of note:
The Option Request option is no longer optional for some
messages (Solicit and Information-request) as RFC7083 requires
clients to request SOL_MAX_RT or INF_MAX_RT options.The Reconfigure message should no longer contain
IA_NA/IA_PD, ORO, or other options to indicate to the
client what was reconfigured. The client should request
everything it needs in the response to the Reconfigure.The lifetime and T1/T2 hints should not be sent by a client
(it should send 0 values in these fields) and any non-zero
values should be ignored by the server.Clarified that a server may return different addresses
in the Reply than requested by a client in the Request message.
Also clarified that a server must not include addresses that it
will not assign.
Also, a Refreshing Configuration Information
was added indicating use cases for when a client should try to refresh network
information.The Relay Agent Behavior incorporates
and had minor edits. A new section,
Interaction between Relay Agents and Servers
, was added.The Authentication of DHCP Messages had
significant changes: IPsec materials were mostly removed and replaced
with a reference to , and the
Delay Authentication Protocol was removed (see ).
Note that the Reconfigure Key Authentication Protocol is retained.The DHCP Options was expanded to incorporate
the prefix delegation options from RFC3633, the Information Refresh Time
option from RFC4242, and the SOL_MAX_RT and INF_MAX_RT options from RFC7083.
In addition, some additional edits were made to clarify option handling,
such as which options should not be in an Option Request option.The Security Considerations were updated to
expand the discussion of security threats and incorporate material from the
incorporated documents, primarily RFC3633.The new Privacy Considerations was added to
consider privacy issues.The IANA Considerations was rewritten to reflect
the changes requested for this document as other documents have already
made the message, option, DUID, and status code assignments and
this document does not add any new assignments.The new Obsoleted Mechanisms documents what
this specification obsoletes.The Appearance of Options in Message Types
and Appearance of Options in the Options Field of DHCP
were updated to reflect the incorporated
options from RFC3633, RFC4242, and RFC7083.Where appropriate, informational references have been added to
provide further background and guidance throughout the document (as
can be noted by the vast increase in references).Changes were made to incorporate the following errata for :
Erratum IDs 294, 295, 1373, 1815, 2471, 2472, 2509, 2928, 3577;
: Erratum IDs 248, 1880, 2468, 2469, 2470, 3736; and
: Erratum ID 3796.General changes to other IPv6 specifications, such as removing
the use of site-local unicast addresses and adding unique local addresses,
were made to the document. Note that in a few places, older obsoleted RFCs
(such as RFC2462 related to M and O bit handling) are still referenced as
the material cited was not added in the replacement RFC.It should be noted that this document does not refer to all DHCPv6
functionality and specifications. Readers of this specification should
visit https://www.iana.org/assignments/dhcpv6-parameters
and https://datatracker.ietf.org/wg/dhc/ to learn of the RFCs that define
DHCPv6 messages, options, status-codes, and more.The following tables indicates with a "*" the options are allowed in
each DHCP message type.These tables are informational and should they conflict with text
earlier in this document, that text should be considered authoritative.NOTE: Server ID option (see ) is only included in
Information-request messages that are sent in response to a Reconfigure
(see ).The following table indicates with a "*" where options defined in
this document can appear as top-level options or encapsulated in other options
defined in this document. Other RFC's may define additional situations
where options defined in this document are encapsulated in other options.This table is informational and should it conflict with text earlier in
this document, that text should be considered authoritative.Notes: Options asterisked in the "Top-Level" column appear in the
options field of client messages (see ). Options
asterisked in the "RELAY-FORW" / "RELAY-REPLY" column appear in the
options field of the Relay-forward and Relay-reply messages
(see ).