Internet Engineering Task Force J. Bound
INTERNET DRAFT Compaq Computer Corp.
DHC Working Group M. Carney
Obsoletes: draft-ietf-dhc-dhcpv6-15.txt Sun Microsystems, Inc
C. Perkins
Nokia Research Center
R. Droms(ed.)
Cisco Systems
22 November 2000
Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
draft-ietf-dhc-dhcpv6-16.txt
Status of This Memo
This document is a submission by the Dynamic Host Configuration
Working Group of the Internet Engineering Task Force (IETF). Comments
should be submitted to the dhcp-v6@bucknell.edu mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at:
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at:
http://www.ietf.org/shadow.html.
Abstract
The Dynamic Host Configuration Protocol for IPv6 (DHCP) enables
DHCP servers to pass configuration parameters such as IPv6 network
addresses to IPv6 nodes. It offers the capability of automatic
allocation of reusable network addresses and additional configuration
flexibility. This protocol is a stateful counterpart to ``IPv6
Stateless Address Autoconfiguration'' [14], and can be used
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separately or concurrently with the latter to obtain configuration
parameters.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Terminology 2
2.1. IPv6 Terminology . . . . . . . . . . . . . . . . . . . . 2
2.2. DHCP Terminology . . . . . . . . . . . . . . . . . . . . 3
3. DHCP Constants 4
3.1. Multicast Addresses . . . . . . . . . . . . . . . . . . . 5
3.2. UDP ports . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. DHCP message types . . . . . . . . . . . . . . . . . . . 5
3.4. Error Values . . . . . . . . . . . . . . . . . . . . . . 7
3.4.1. Generic Error Values . . . . . . . . . . . . . . 7
3.4.2. Server-specific Error Values . . . . . . . . . . 7
3.5. Configuration Variables . . . . . . . . . . . . . . . . . 8
4. Requirements 8
5. Background 9
6. Design Goals 10
7. Non-Goals 11
8. Overview 11
8.1. How does a node know to use DHCP? . . . . . . . . . . . . 11
8.2. How does a client find out about DHCP agents? . . . . . . 11
8.3. What if the client and server(s) are on different links? 11
8.4. How does a client request configuration parameters from
servers? . . . . . . . . . . . . . . . . . . . . . . . 12
8.5. How do clients and servers identify and manage addresses? 13
8.6. Can a client release its assigned addresses before the lease
expires? . . . . . . . . . . . . . . . . . . . . . . . 13
8.7. What if the client determines one or more of its assigned
addresses are already being used by another client? . 13
8.8. How are clients notified of server configuration changes? 13
9. Message Formats and Identity Associations 14
9.1. DHCP Solicit Message Format . . . . . . . . . . . . . . . 14
9.2. DHCP Advertise Message Format . . . . . . . . . . . . . . 15
9.3. DHCP Request Message Format . . . . . . . . . . . . . . . 16
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9.4. DHCP Reply Message Format . . . . . . . . . . . . . . . . 17
9.5. DHCP Release Message Format . . . . . . . . . . . . . . . 18
9.6. DHCP Reconfigure Message Format . . . . . . . . . . . . . 18
9.7. DHCP Reconfigure-reply Message Format . . . . . . . . . . 18
9.8. DHCP Reconfigure-init Message Format . . . . . . . . . . 19
9.9. Relay-forward message . . . . . . . . . . . . . . . . . . 20
9.10. Server-forward message . . . . . . . . . . . . . . . . . 20
9.11. Identity association . . . . . . . . . . . . . . . . . . 21
10. DHCP Server Solicitation 21
10.1. Solicit Message Validation . . . . . . . . . . . . . . . 21
10.2. Advertise Message Validation . . . . . . . . . . . . . . 21
10.3. Client Behavior . . . . . . . . . . . . . . . . . . . . . 22
10.3.1. Creation and sending of the Solicit message . . . 22
10.3.2. Time out and retransmission of Solicit Messages . 22
10.3.3. Receipt of Advertise messages . . . . . . . . . . 23
10.4. Relay Behavior . . . . . . . . . . . . . . . . . . . . . 23
10.4.1. Relaying of Solicit messages . . . . . . . . . . 23
10.4.2. Relaying of Advertise messages . . . . . . . . . 24
10.5. Server Behavior . . . . . . . . . . . . . . . . . . . . . 24
10.5.1. Receipt of Solicit messages . . . . . . . . . . . 24
10.5.2. Creation and sending of Advertise messages . . . 24
11. DHCP Client-Initiated Configuration Exchange 25
11.1. Request Message Validation . . . . . . . . . . . . . . . 25
11.2. Reply Message Validation . . . . . . . . . . . . . . . . 26
11.3. Release Message Validation . . . . . . . . . . . . . . . 26
11.4. Client Behavior . . . . . . . . . . . . . . . . . . . . . 26
11.4.1. Creation and sending of Request messages . . . . 27
11.4.2. Time out and retransmission of Request Messages . 27
11.4.3. Receipt of Reply message in response to a Request 28
11.4.4. Creation and sending of Release messages . . . . 28
11.4.5. Time out and retransmission of Release Messages . 29
11.4.6. Receipt of Reply message in response to a Release 29
11.4.7. When a client should send a Request message . . . 29
11.4.8. Initialization . . . . . . . . . . . . . . . . . 29
11.4.9. Confirming the validity of IPv6 addresses . . . . 29
11.4.10. Extending the lifetimes on IPv6 addresses . . . . 30
11.5. Relay Behavior . . . . . . . . . . . . . . . . . . . . . 31
11.5.1. Relaying of Request or Release messages . . . . . 31
11.6. Server Behavior . . . . . . . . . . . . . . . . . . . . . 31
11.6.1. Receipt of Request messages . . . . . . . . . . . 31
11.6.2. Receipt of Release messages . . . . . . . . . . . 31
11.6.3. Creation and sending of Reply messages . . . . . 32
12. DHCP Server-Initiated Configuration Exchange 33
12.1. Reconfigure Message Validation . . . . . . . . . . . . . 33
12.2. Reconfigure-reply Message Validation . . . . . . . . . . 33
12.3. Reconfigure-init Message Validation . . . . . . . . . . . 33
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12.4. Server Behavior . . . . . . . . . . . . . . . . . . . . . 33
12.4.1. Creation and sending of Reconfigure messages . . 34
12.4.2. Time out and retransmission of Reconfigure
messages . . . . . . . . . . . . . . . . . 34
12.4.3. Receipt of Reconfigure-reply messages . . . . . . 34
12.4.4. Creation and sending of Reconfigure-init messages 34
12.4.5. Time out and retransmission of Reconfigure-init
messages . . . . . . . . . . . . . . . . . 35
12.4.6. Receipt of Request messages . . . . . . . . . . . 35
12.5. Client Behavior . . . . . . . . . . . . . . . . . . . . . 35
12.5.1. Receipt of Reconfigure-init messages . . . . . . 35
12.5.2. Creation and sending of Request messages . . . . 36
12.5.3. Time out and retransmission of Request messages . 36
12.5.4. Receipt of Reply messages . . . . . . . . . . . . 36
13. Using DHCP for network renumbering 36
14. DHCP Client Implementor Notes 37
14.1. Primary Interface . . . . . . . . . . . . . . . . . . . . 37
14.2. Advertise Message and Configuration Parameter Caching . . 37
14.3. Time out and retransmission variables . . . . . . . . . . 37
14.4. Server Preference . . . . . . . . . . . . . . . . . . . . 38
15. DHCP Server Implementor Notes 38
15.1. Client Bindings . . . . . . . . . . . . . . . . . . . . . 38
15.2. Reconfigure-init Considerations . . . . . . . . . . . . . 38
15.3. Server Preference . . . . . . . . . . . . . . . . . . . . 39
15.4. Request Message Transaction-ID Cache . . . . . . . . . . 39
16. DHCP Relay Implementor Notes 39
17. Open Issues for Working Group Discussion 39
17.1. Authentication . . . . . . . . . . . . . . . . . . . . . 39
17.2. DHCP-DNS interaction . . . . . . . . . . . . . . . . . . 39
17.3. Release vs. Decline . . . . . . . . . . . . . . . . . . 40
17.4. Request messages . . . . . . . . . . . . . . . . . . . . 40
17.5. Use of term ``agent'' . . . . . . . . . . . . . . . . . . 40
17.6. Use of terms ``subnet'' and ``network'' . . . . . . . . . 40
18. Security 40
19. Year 2000 considerations 41
20. IANA Considerations 41
21. Acknowledgments 41
22. DHCP options 42
22.1. Format of DHCP options . . . . . . . . . . . . . . . . . 42
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22.2. Identity association option . . . . . . . . . . . . . . . 43
22.3. Option request option . . . . . . . . . . . . . . . . . . 44
22.4. Client message option . . . . . . . . . . . . . . . . . . 45
22.5. Server message option . . . . . . . . . . . . . . . . . . 45
22.6. Retransmission parameter option . . . . . . . . . . . . . 46
22.7. Authentication option . . . . . . . . . . . . . . . . . . 46
23. Changes in this draft 46
23.1. Order of sections . . . . . . . . . . . . . . . . . . . . 47
23.2. Reconfigure message . . . . . . . . . . . . . . . . . . . 47
23.3. Releasable resources . . . . . . . . . . . . . . . . . . 47
23.4. DHCP message header . . . . . . . . . . . . . . . . . . . 47
23.5. Design goals . . . . . . . . . . . . . . . . . . . . . . 47
23.6. Overview . . . . . . . . . . . . . . . . . . . . . . . . 47
23.7. Message formats, 9 . . . . . . . . . . . . . . . . . . . 47
23.8. Solicit and Advertise messages, (section 10) . . . . . . 48
23.9. Prefix advertisement . . . . . . . . . . . . . . . . . . 48
23.10. Identity Associations . . . . . . . . . . . . . . . . . . 48
23.11. Extensions renamed options; defined in this document . . 48
23.12. Transaction-ID ranges . . . . . . . . . . . . . . . . . . 48
23.13. Release messages and relays . . . . . . . . . . . . . . . 48
23.14. Discovering relay agents . . . . . . . . . . . . . . . . 48
A. Comparison between DHCPv4 and DHCPv6 49
B. Full Copyright Statement 51
Chair's Address 54
Author's Address 54
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1. Introduction
This document describes DHCP for IPv6 (DHCP), a UDP [13] client
/ server protocol designed to reduce the cost of management of
IPv6 nodes in environments where network managers require more
control over the allocation of IPv6 addresses and configuration
of network stack parameters than that offered by ``IPv6 Stateless
Autoconfiguration'' [14]. DHCP is a stateful counterpart to
stateless autoconfiguration. Note that both stateful and stateless
autoconfiguration can be used concurrently in the same environment,
leveraging the strengths of both mechanisms in order to reduce the
cost of ownership and management of network nodes.
DHCP reduces the cost of ownership by centralizing the management
of network resources such as IP addresses, routing information, OS
installation information, directory service information, and other
such information on a few DHCP servers, rather than distributing such
information in local configuration files among each network node.
DHCP is designed to be easily extended to carry new configuration
parameters through the addition of new DHCP ``options'' defined to
carry this information. (What were called ``extensions'' in the -15
draft are now called ``options''; see section 23.11.)
Those readers familiar with DHCP for IPv4 [6] will find DHCP for IPv6
provides a superset of features, and benefits from the additional
features of IPv6 and freedom from BOOTP [4]-backward compatibility
constraints. For more information about the differences between DHCP
for IPv6 and DHCP for IPv4, see Appendix A.
This document is organized as follows. Section 2 defines terminology
used throughout this document. Section 3 defines constant values
used by DHCP. Section 4 briefly discusses requirement levels.
Section 5 points the reader to helpful background specifications
covering related IPv6 protocols. Section 6 discusses the design
goals that influenced DHCP. Section 7 identifies some of the
non-goals of this specification. Section 8 gives a high level
overview of DHCP, its message types, and identifies DHCP functional
entities (client, relay, server). Section 9 describes in detail
the format of each DHCP message type. Section 10 discusses DHCP
server solicitation. Section 11 discusses DHCP client-initiated
configuration information exchange. Section 12 discusses DHCP
server-initiated configuration information exchange. Section 14
presents helpful notes for DHCP client implementors. Section 15
presents helpful notes for DHCP server implementors. Section 16
presents helpful notes for DHCP relay implementors. Section 18
discusses security considerations for DHCP.
Section 23 describes the changes between this version of the DHCPv6
specification and draft-ietf-dhc-dhcpv6-15.txt.
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2. Terminology
2.1. IPv6 Terminology
IPv6 terminology relevant to this specification from the IPv6
Protocol [5], IPv6 Addressing Architecture [7], and IPv6 Stateless
Address Autoconfiguration [14] is included below.
address An IP layer identifier for an interface or a set of
interfaces.
unicast address
An identifier for a single interface. A packet sent
to a unicast address is delivered to the interface
identified by that address.
multicast address
An identifier for a set of interfaces (typically
belonging to different nodes). A packet sent to a
multicast address is delivered to all interfaces
identified by that address.
host Any node that is not a router.
IP Internet Protocol Version 6 (IPv6). The terms IPv4 and
IPv6 are used only in contexts where it is necessary to
avoid ambiguity.
interface
A node's attachment to a link.
link A communication facility or medium over which nodes
can communicate at the link layer, i.e., the layer
immediately below IP. Examples are Ethernet (simple or
bridged); Token Ring; PPP links, X.25, Frame Relay, or
ATM networks; and Internet (or higher) layer "tunnels",
such as tunnels over IPv4 or IPv6 itself.
link-layer identifier
a link-layer identifier for an interface. Examples
include IEEE 802 addresses for Ethernet or Token Ring
network interfaces, and E.164 addresses for ISDN links.
link-local address
An IP address having link-only scope, indicated by
having the prefix (FE80::0000/64), that can be used
to reach neighboring nodes attached to the same link.
Every interface has a link-local address.
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message A unit of data carried in a packet, exchanged between
DHCP agents and clients.
neighbor A node attached to the same link.
node A device that implements IP.
packet An IP header plus payload.
prefix A bit string that consists of some number of initial
bits of an address.
router A node that forwards IP packets not explicitly
addressed to itself.
2.2. DHCP Terminology
Terminology specific to DHCP can be found below.
abort status
A status value returned to the application that has
invoked a DHCP client operation, indicating anything
other than success.
agent address
The address of a neighboring DHCP Agent on the same
link as the DHCP client.
binding A binding (or, client binding) is a group of server
data records indexed by <prefix, UUID> containing the
server's information about the addresses and other
information assigned to the IA.
DHCP Dynamic Host Configuration Protocol for IPv6. The
terms DHCPv4 and DHCPv6 are used only in contexts where
it is necessary to avoid ambiguity.
configuration parameter
An element of the configuration information set on the
server and delivered to the client using DHCP. Such
parameters may be used to carry information to be used
by a node to configure its network subsystem and enable
communication on a link or internetwork, for example.
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DHCP client (or client)
A node that initiates requests on a link to obtain
configuration parameters from one or more DHCP servers.
DHCP domain
A chunk of network topology managed by DHCP and
operated by a single administrative entity.
DHCP server (or server)
A server is a node that responds to requests from
clients, and may or may not be on the same link as the
client(s).
DHCP relay (or relay)
A node that acts as an intermediary to deliver DHCP
messages between clients and servers, and is on the
same link as a client.
DHCP agent (or agent)
Either a DHCP server on the same link as a client, or a
DHCP relay.
Identity association (IA)
A collection of addresses assigned to a client. Each
IA has an associated UUID. A server identifies an IA by
the tuple (prefix, UUID), where ``prefix'' is a prefix
assigned to the link to which the client is attached,
An IA may have 0 or more addresses associated with it.
Releasable resource
(Removed; see section 23.3.)
transaction-ID
An unsigned integer to match responses with replies
initiated either by a client or server.
UUID
A universally unique identifier for a client.
DISCUSSION:
Rules for choosing a UUID are TBD.
3. DHCP Constants
This section describes various program and networking constants used
by DHCP.
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3.1. Multicast Addresses
DHCP makes use of the following multicast addresses:
All DHCP Agents address: FF02::1:2
This link-local multicast address is used by clients to
communicate with the on-link agent(s) when they do not
know those agents' link-local address(es). All agents
(servers and relays) are members of this multicast
group.
All DHCP Servers address: FF05::1:3
This site-local multicast address is used by clients or
relays to communicate with server(s), either because
they want to send messages to all servers or because
they do not know the server(s) unicast address(es).
Note that in order for a client to use this address,
it must have an address of sufficient scope to be
reachable by the server(s). All servers within the
site are members of this multicast group.
3.2. UDP ports
DHCP uses the following destination UDP [13] port numbers. While
source ports MAY be arbitrary, client implementations SHOULD permit
their specification through a local configuration parameter to
facilitate the use of DHCP through firewalls.
546 Client port. Used by agents to send messages to
clients. Also used by servers to send messages to
relays.
547 Agent port. Used by clients to send messages to
agents. Also used by relays to send messages to
servers.
3.3. DHCP message types
DHCP defines the following message types. More detail on these
message types can be found in Section 9. Message types 0 and 9--255
are reserved and MUST be silently ignored.
01 DHCP Solicit
The DHCP Solicit (or Solicit) message is used by clients
to locate servers. This message is multicast using the
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All-DHCP-Agents address. Relay(s) forward Solicits as
necessary to off-link servers.
Section 9.1 contains more details about the Solicit message.
02 DHCP Advertise
The DHCP Advertise (or Advertise) message is used by servers
responding to Solicits. This message is unicast to the
client's link-local address (if the server and client are
on the same link) or unicast to the relay through which the
Solicit was sent for final delivery to the client.
Section 9.2 contains more details about the Advertise message.
03 DHCP Request
The DHCP Request (or Request) message is used by clients to
request configuration parameters from servers. This message is
multicast using the All-DHCP-Agents address. Relay(s) forward
Requests as necessary to off-link servers.
Section 9.3 contains more details about the Request message.
04 DHCP Reply
The DHCP Reply (or Reply) message is used by servers responding
to Request and Release messages. In the case of responding to
a Request message, the Reply contains configuration parameters
destined for the client. This message is unicast to the client
if the client has an address of sufficient scope that is
reachable by the server. Otherwise, it is unicast to the relay
through which the Request or Release message was sent for final
delivery to the client.
Section 9.4 contains more details about the Reply message.
05 DHCP Release
The DHCP Release (or Release) message is used by clients to
return one or more IP addresses to servers. The server will
acknowledge the receipt of the Release message by sending the
client a Reply message.
Section 9.5 contains more details about the Release message.
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06 DHCP Reconfigure
07 DHCP Reconfigure-reply
Removed; see section 23.2.
08 DHCP Reconfigure-init
The DHCP Reconfigure-init (or Reconfigure-init) message is set
by server(s) to inform client(s) that the server(s) has new or
updated configuration parameters, and that the client(s) are
to initiate a Request/Reply transaction with the server(s) in
order to receive the updated information.
Section 9.8 contains more details about the Reconfigure-init
message.
3.4. Error Values
This section describes error values exchanged between DHCP
implementations.
3.4.1. Generic Error Values
The following symbolic names are used between client and server
implementations to convey error conditions. The following table
contains the actual numeric values for each name. Note that the
numeric values do not start at 1, nor are they consecutive. The
errors are organized in logical groups.
_______________________________________________________________
|Error_Name___|Error_ID|_Description_________________________|_
|Success______|00______|_Success_____________________________|_
|UnspecFail___|16______|_Failure,_reason_unspecified_________|_
|AuthFailed___|17______|_Authentication_failed_or_nonexistent|_
|PoorlyFormed_|18______|_Poorly_formed_message_______________|_
|Unavail______|19______|_Addresses_unavailable_______________|_
3.4.2. Server-specific Error Values
The following symbolic names are used by server implementations to
convey error conditions to clients. The following table contains the
actual numeric values for each name.
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_______________________________________________________________
|Error_Name____|Error_ID|_Description________________________|_
|NoBinding_____|20______|_Client_record_(binding)_unavailable|_
|InvalidSource_|21______|_Invalid_Client_IP_address__________|_
|NoServer______|23______|_Relay_cannot_find_Server_Address___|_
|ICMPError_____|64______|_Server_unreachable_(ICMP_error)____|_
3.5. Configuration Variables
This section presents a table of client and server configuration
variables and the default or initial values for these variables. The
client-specific variables MAY be configured on the server and MAY be
delivered to the client through the ``DHCP Retransmission Parameter
Option'' in a Reply message. This option is TBD.
______________________________________________________________
|Parameter__________|Default|_Description___________________|_
|MIN_SOL_DELAY______|1______|_MIN_(secs)_to_delay_1st_mesg__|_
|MAX_SOL_DELAY______|5______|_MAX_(secs)_to_delay_1st_mesg__|_
|ADV_MSG_TIMEOUT____|500____|_SOL_Retrans_timer_(msecs)_____|_
|ADV_MSG_MAX________|30_____|_MAX_timer_value_(secs)________|_
|SOL_MAX_ATTEMPTS___|-1_____|_MAX_attempts_(-1_=_infinite)__|_
|REP_MSG_TIMEOUT____|250____|_REQ_Retrans_timer_(msecs)_____|_
|REQ_MSG_ATTEMPTS___|10_____|_MAX_Request_attempts__________|_
|REL_MSG_ATTEMPTS___|5______|_MAX_Release_attempts__________|_
|RECREP_MSG_TIMEOUT_|2000___|_Retrans_timer_(msecs)_________|_
|REC_MSG_ATTEMPTS___|10_____|_Reconfigure_attempts__________|_
|REC_REP_MIN________|5______|_Minimum_pause_interval_(secs)_|_
|REC_REP_MAX________|7200___|_Maximum_pause_interval_(secs)_|_
|REC_THRESHOLD______|100____|_%_of_required_clients_________|_
|SRVR_PREF_WAIT_____|2______|_Advertise_Collect_timer_(secs)|_
4. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [2].
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.
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5. Background
Related work in IPv6 that would best serve an implementor to study
is the IPv6 Specification [5], the IPv6 Addressing Architecture [7],
IPv6 Stateless Address Autoconfiguration [14], IPv6 Neighbor
Discovery Processing [11], and Dynamic Updates to DNS [16]. These
specifications enable DHCP to build upon the IPv6 work to provide
both robust stateful autoconfiguration and autoregistration of DNS
Host Names.
The IPv6 Specification provides the base architecture and design of
IPv6. A key point for DHCP implementors to understand is that IPv6
requires that every link in the Internet have an MTU of 1280 octets
or greater (in IPv4 the requirement is 68 octets). This means that
a UDP packet of 536 octets will always pass through an internetwork
(less 40 octets for the IPv6 header), as long as there are no IP
options prior to the UDP header in the packet. But, IPv6 does not
support fragmentation at routers, so that fragmentation takes place
end-to-end between hosts. If a DHCP implementation needs to send a
packet greater than 1500 octets it can either fragment the UDP packet
into fragments of 1500 octets or less, or use Path MTU Discovery [9]
to determine the size of the packet that will traverse a network
path.
DHCP clients use Path MTU discovery when they have an address of
sufficient scope to reach the DHCP server. If a DHCP client does not
have such an address, that client MUST fragment its packets if the
resultant message size is greater than the minimum 1280 octets.
Path MTU Discovery for IPv6 is supported for both UDP and TCP and
can cause end-to-end fragmentation when the PMTU changes for a
destination.
The IPv6 Addressing Architecture specification [7] 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. This means that a client can immediately use
its link-local address and a well-known multicast address to begin
communications to discover neighbors on the link. For instance, a
client can send a Solicit message and locate a server or relay.
IPv6 Stateless Address Autoconfiguration [14] (Addrconf) specifies
procedures by which a node may autoconfigure addresses based on
router advertisements [11], and the use of a valid lifetime to
support renumbering of addresses on the Internet. In addition the
protocol interaction by which a node begins stateless or stateful
autoconfiguration is specified. DHCP is one vehicle to perform
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stateful autoconfiguration. Compatibility with addrconf is a design
requirement of DHCP (see Section 6).
IPv6 Neighbor Discovery [11] is the node discovery protocol in IPv6
which replaces and enhances functions of ARP [12]. To understand
IPv6 and Addrconf it is strongly recommended that implementors
understand IPv6 Neighbor Discovery.
Dynamic Updates to DNS [16] is a specification that supports the
dynamic update of DNS records for both IPv4 and IPv6. DHCP can use
the dynamic updates to DNS to integrate addresses and name space
to not only support autoconfiguration, but also autoregistration
in IPv6. The security model to be used with DHCPv6 should conform
as closely as possible to the authentication model outlined in
RFC2402 [8].
6. Design Goals
- DHCP is a mechanism rather than a policy. Network administrators
set their administrative policies through the configuration
parameters they place upon the DHCP servers in the DHCP domain
they're managing. DHCP is simply used to deliver parameters
according to that policy to each of the DHCP clients within the
domain.
- DHCP is compatible with IPv6 stateless autoconf [14].
- DHCP does not require manual configuration of network parameters
on DHCP clients, except in cases where such configuration is
needed for security reasons. A node configuring itself using
DHCP should require no user intervention.
- DHCP does not require a server on each link. To allow for scale
and economy, DHCP must work across DHCP relays.
- DHCP coexists with statically configured, non-participating nodes
and with existing network protocol implementations.
- DHCP clients can operate on a link without IPv6 routers present.
- DHCP will provide the ability to renumber network(s) when
required by network administrators [3].
- A DHCP client can make multiple, different requests for
configuration parameters when necessary from one or more DHCP
servers at any time.
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- DHCP will contain the appropriate time out and retransmission
mechanisms to efficiently operate in environments with high
latency and low bandwidth characteristics.
7. Non-Goals
This specification explicitly does not cover the following:
- Specification of a DHCP server to server protocol.
- How a DHCP server stores its DHCP data.
- How to manage a DHCP domain or DHCP server.
- How a DHCP relay is configured or what sort of information it may
log.
8. Overview
This section provides a general overview of the interaction
between the functional entities of DHCP. The overview is organized
as a series of questions and answers. Details of DHCP such
as message formats and retransmissions are left to sections 9,
10, 11, 12, 14, 15, and 16.
8.1. How does a node know to use DHCP?
An unconfigured node determines that it is to use DHCP for
configuration of an interface by detecting the presence (or absence)
of routers on the link. If router(s) are present, the node examines
router advertisements to determine if DHCP should be used to
configure the interface. If there are no routers present, then
the node MUST use DHCP to configure the interface. Detail on
this process can be found in neighbor discovery [11] and stateless
autoconfiguration [14].
8.2. How does a client find out about DHCP agents?
(Section removed, see 23.6
8.3. What if the client and server(s) are on different links?
Use of DHCP in such environments requires one or more DHCP relays
be set up on the client's link, because a client may only have a
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link-local address. Relays receive the Solicit and Request messages
from the client and forward them to some set of servers within the
DHCP domain. The client message is forwarded verbatim as the payload
in a message from the relay to the server. A relay will include
one of its own addresses (of sufficient scope) from the interface
on the same link as the client, as well as the prefix length of
that address, in its message to the server. Servers receiving
the forwarded traffic use this information to aid in selecting
configuration parameters appropriate to the client's link. The
servers also use the relay's address as the destination to forward
client-destined messages for final delivery by the relay.
Relays forward client messages to servers using some combination of
the FF05::1:3(All Servers) site-local multicast address, some other
(perhaps a combination) of site-local multicast addresses set up
within the DHCP domain to include the servers in that domain, or a
list of unicast addresses for servers. The network administrator
makes relay configuration decisions based upon the topological
requirements (scope) of the DHCP domain they are managing. Note
that if the DHCP domain spans more than the site-local scope, then
the relays MUST be configured with global addresses for the client's
link so as to be reachable by servers outside the relays' site-local
environment.
8.4. How does a client request configuration parameters from servers?
To request configuration parameters, the client forms a Request
message, and sends it to the server either directly (client has an
address of sufficient scope) or indirectly (through the on-link
relay). The client MAY include a Option Request Option 22.3 (ORO)
along with other options to request specific information from the
server. Note that the client MAY form multiple Request messages
and send each of them to different servers to request potentially
different information (perhaps based upon what was advertised) in
order to satisfy its needs. As a client's needs may change over time
(perhaps based upon an application's requirements), the client may
form additional Request messages to request additional information as
it is needed.
The server(s) respond with Reply messages containing the requested
configuration parameters, which can include status information
regarding the information requested by the client. The Reply MAY
also include additional information, such as a reconfiguration event
multicast group for the client to join to monitor reconfiguration
events, as described in section 8.8.
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8.5. How do clients and servers identify and manage addresses?
Servers and clients manage addresses in groups called ``identity
associations.'' Each identity associations is identified using
a unique identifier. An identity association may contain one or
more IPv6 addresses. DHCP servers assign addresses to identity
associations. DHCP clients use the addresses in an identity
association to configure interfaces. There is always at least one
identity association per interface that a client wishes to configure.
Each address in an IA has its own preferred and valid lifetime. Over
time, the server may change the characteristics of the addresses in
an IA; for example, by changing the preferred or valid lifetime for
an address in the IA. The server may also add or delete addresses
from an IA; for example, deleting old addresses and adding new
addresses to renumber a client. A client can request the current
list of addresses assigned to an IA from a server through an exchange
of protocol messages.
8.6. Can a client release its assigned addresses before the lease
expires?
A client forms a Release message, including options identifying
the IA to be released. The client sends the Release to the server
which assigned the addresses to the client initially. If that
server cannot be reached after a certain number of attempts (see
section 3.5), the client can abandon the Release attempt. In this
case, the address(es) in the IA will be reclaimed by the server(s)
when the lifetimes on the addresses expire.
8.7. What if the client determines one or more of its assigned addresses
are already being used by another client?
If the client determines through a mechanism like Duplicate Address
Detection [14] that the address it was assigned by the server is
already in use by another client, the client will form a Release
message, including the option carrying the in-use address. The
option's status field MUST be set to the value reflecting the ``in
use'' status of the address.
8.8. How are clients notified of server configuration changes?
There are two possibilities. Either the clients discover the new
information when they revisit the server(s) to request additional
configuration information / extend the lifetime on an address. or
through a server-initiated event known as a reconfigure event.
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The reconfiguration feature of DHCP offers network administrators
the opportunity to update configuration information on DHCP clients
whenever necessary. To signal the need for client reconfiguration,
the server will unicast a Reconfigure-init message to each
client individually. The server may use multicast to signal the
reconfiguration to multiple clients simultaneously. (Note that
there is no mechanism defined in the protocol to guarantee that
every client actually performs a reconfiguration in response to a
multicast reconfigure-init message.) A Reconfigure-init is a trigger
which will cause the client(s) to initiate a standard Request/Reply
exchange with the server in order to acquire the new or updated
addresses.
9. Message Formats and Identity Associations
All reserved fields in a message MUST be transmitted as zeroes and
ignored by the receiver of the message.
DISCUSSION:
Each DHCP message has an identical fixed format header; some
messages also allow a variable format area for options. Not
all fields in the header are used in every message. In this
section, every field is included in every message format
diagram and fields that are not used in a message are marked
as ``unused''. As an alternative, the unused fields could
be labeled ``unused'' in the format diagram.
9.1. DHCP Solicit Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type = 1 | preference | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| client-link-local-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| server-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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preference
(unused) MUST be 0
transaction-ID
An unsigned integer generated by the client used to
identify this Solicit message.
client-link-local-address
The link-local address of the interface for which the
client is using DHCP.
server-address (unused) MUST be 0
9.2. DHCP Advertise Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type = 2 | preference | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| client-link-local-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| server-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| options (variable number and length) .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
preference An unsigned integer indicating a server's willingness
to provide service to the client.
transaction-ID An unsigned integer used to identify this Advertise
message. Copied from the client's Solicit message.
client-link-local-address
The IP link-local address of the client interface
from which the client issued the Solicit message.
server-address
The IP address of the server. If the DHCP domain
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crosses site boundaries, then this address MUST be
globally-scoped.
options Options are described elsewhere in this document
See Sections 14.4 and 15.3 for information about how clients and
servers handle the preference field.
9.3. DHCP Request Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type = 3 | preference | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| client-link-local-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| server-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| options (variable number and length) .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
preference
(unused) MUST be 0
transaction-ID
An unsigned integer generated by the client used to
identify this Request message.
client-link-local-address
The link-local address of the client interface from
which the client will issue the Request message.
server-address
The IP address of the server to which the the client's
Request message is directed, copied from an Advertise
message.
options
Options are described elsewhere in this document.
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9.4. DHCP Reply Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type = 4 | preference | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| client-link-local-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| server-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| options (variable number and length) .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
preference An unsigned integer indicating a server's willingness
to provide service to the client.
transaction-ID
An unsigned integer used to identify this Reply
message. Copied from the client's Request message.
client-link-local-address
The link-local address of the interface for which the
client is using DHCP.
server-address
The IP address of the server. If the DHCP domain
crosses site boundaries, then this address MUST be
globally-scoped.
options
Options are described elsewhere in this document.
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9.5. DHCP Release Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type = 5 | preference | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| client-link-local-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| server-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| options (variable number and length) .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
preference (unused) MUST be 0
transaction-ID
An unsigned integer generated by the client used to
identify this Release message.
P (unused) MUST be 0
client-link-local-address
The client's link-local address for the interface from
which the client issued the Release message.
server-address
The IP address of the server that assigned the
addresses.
options See section 22.
9.6. DHCP Reconfigure Message Format
The Reconfigure message has been deleted (see section 23.2).
9.7. DHCP Reconfigure-reply Message Format
The Reconfigure-reply message has been deleted (see section 23.2).
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9.8. DHCP Reconfigure-init Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type = 8 | preference | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| client-link-local-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| server-address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| options (variable number and length) .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
preference (unused) MUST be 0
transaction-ID
An unsigned integer generated by the server to identify
this Reconfigure-init message
client-link-local-address
(unused) MUST be 0
server-address
The IP address of the DHCP server issuing the
Reconfigure-init message. MUST be of sufficient scope
to be reachable by all clients.
options SHOULD only include an ``Options request option''
(ORO) and/or authentication options. No configuration
information SHOULD be included. See section 22 more
information about options.
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9.9. Relay-forward message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type TBD | prefix length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| relay-address |
| |
| |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| options (variable number and length) .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type TBD
prefix-length
The length of the prefix in the address in the
``relay-address'' field.
relay-address
An address assigned to the interface through which the
message from the client was received.
options MUST include a ``Client message option''; see
section 22.4.
9.10. Server-forward message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type TBD | prefix length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| relay-address |
| |
| |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| options (variable number and length) .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type TBD
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prefix-length
The length of the prefix in the address in the
``relay-address'' field.
relay-address
An address identifying the interface through which the
message from the server should be forwarded; copied
from the ``client-forward'' message.
options MUST include a ``Server message option''; see
section 22.5.
9.11. Identity association
An ``identity-association'' (IA) is a construct through which a
server and a client can identify, group and manage IPv6 addresses.
Each IA consists of a UUID and a list of associated IPv6 addresses
(the list may be empty). A client associates an IA with one of
its interfaces and uses the IA to obtain IPv6 addresses for that
interface from a server.
10. DHCP Server Solicitation
This section describes how a client locates servers. The behavior of
client, server, and relay implementations is discussed, along with
the messages they use.
(Prefix advertisements have been deleted; see 23.9.)
10.1. Solicit Message Validation
Clients MUST silently discard any received Solicit messages.
Agents MUST silently discard any received Solicit messages if
the ``client-link-local-address'' field does not contain a valid
link-local address.
10.2. Advertise Message Validation
Servers MUST discard any received Advertise messages.
Clients MUST discard any Advertise messages that meet any of the
following criteria:
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o The ``Transaction-ID'' field value does not match the value the
client used in its Solicit message.
o The ``client-link-local-address'' field value does not match the
link-local address of the interface upon which the client sent
the Solicit message.
10.3. Client Behavior
Clients use the Solicit message to discover DHCP servers configured
to serve addresses on the link to which the client is attached.
(Prefix advertisement by servers has been deleted; see section 23.9.)
10.3.1. Creation and sending of the Solicit message
The client sets the ``msg-type'' field to 1, and places the
link-local address of the interface it wishes to configure in the
``client-link-local-address'' field. The client sets all other
fields to zero.
The client sends the Solicit message to the FF02::1:2 (All DHCP
Agents) multicast address, destination port 547. The source port
selection can be arbitrary, although it SHOULD be possible using a
client configuration facility to set a specific source port value.
10.3.2. Time out and retransmission of Solicit Messages
The client's first Solicit message on the interface MUST be delayed
by a random amount of time between the interval of MIN_SOL_DELAY and
MAX_SOL_DELAY. This random delay desynchronizes clients which start
at the same time (e.g., after a power outage).
The client waits ADV_MSG_TIMEOUT, collecting Advertise messages.
If no Advertise messages are received, the client retransmits
the Solicit, and doubles the ADV_MSG_TIMEOUT value. This process
continues until either one or more Advertise messages are received or
ADV_MSG_TIMEOUT reaches the ADV_MSG_MAX value. Thereafter, Solicits
are retransmitted every ADV_MSG_MAX until SOL_MAX_ATTEMPTS have been
made, at which time the client stops trying to DHCP configure the
interface. An event external to DHCP is required to restart the DHCP
configuration process.
Default and initial values for MIN_SOL_DELAY, MAX_SOL_DELAY,
ADV_MSG_TIMEOUT, AND ADV_MSG_MAX are documented in section 3.5.
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10.3.3. Receipt of Advertise messages
Upon receipt of one or more validated Advertise messages, the client
selects one or more Advertise messages based upon the following
criteria.
- Those Advertise messages with the highest server preference
value (see section 14.4) are preferred over all other Advertise
messages.
- Within a group of Advertise messages with the same server
preference value, a client MAY select those servers whose
Advertise messages advertise information of interest to
the client. For example, one server may be advertising the
availability of IP addresses which have an address scope 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. Depending on the requirements of the client's
invoking user, the client MAY initiate a configuration exchange with
the server(s) immediately, or MAY defer this exchange until later.
10.4. Relay Behavior
For this discussion, the Relay may be configured to use a list of
server destination addresses, which may include unicast addresses,
the FF05::1:3 (All DHCP Servers) multicast address, or other
multicast addresses selected by the network administrator. If
the Relay has not been explicitly configured, it will use the
FF05::1:3 (All DHCP Servers) multicast address as the default.
10.4.1. Relaying of Solicit messages
When a Relay receives a valid Solicit message, it constructs a
Relay-forward message. The client Solicit message is carried as the
payload of a ``client-message'' option. The relay places an address
from the interface on which the Solicit message was received in the
``relay-address'' field and the prefix length for that address in
the ``prefix-length'' field. The Relay then sends the Relay-forward
message to the list of server destination addresses that it has been
configured with.
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10.4.2. Relaying of Advertise messages
When the relay receives a Relay-reply message, it extracts the server
message from the ``server-message'' option and forwards the server
message to the address in the client-link-local-address field in
the server message. The relay forwards the server message through
the interface identified in the ``relay-address'' field in the
Relay-reply message.
10.5. Server Behavior
For this discussion, the Server is assumed to have been configured in
an implementation specific manner. This configuration is assumed to
contain all network topology information for the DHCP domain, as well
as any necessary authentication information.
10.5.1. Receipt of Solicit messages
If the server receives a Solicit message, the client must be on the
same link as the server. If the server receives a Relay-forward
message containing a Solicit message, the client must be on the
link to which the prefix identified by the ``relay-address'' and
``prefix-length'' fields in the Relay-forward message is assigned.
The server records the ``relay-address'' field from the Relay-forward
message and extracts the solicit message from the ``client-message''
option.
If administrative policy permits the server to respond to a client on
that link, the server will generate and send an Advertise message to
the client.
10.5.2. Creation and sending of Advertise messages
The server sets the ``msg-type'' field to 2 and copies the values
of the following fields from the client's Solicit to the Advertise
message:
o transaction-ID
o client-link-local-address
The server places one of its IP addresses (determined through
administrator setting) in the ``server-address'' field of the
Advertise message. The server sets the ``preference'' field
according to its configuration information. See section 15.3 for a
description of server preference.
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If the Solicit message was received in a Relay-forward message, the
server constructs a Relay-reply message with the Advertise message
in the payload of a ``server-message'' option. The server unicasts
the Relay-reply message to the address in the ``relay-address'' field
from the Relay-forward message.
If the Solicit message was received directly by the server, the
server unicasts the Advertise message directly to the client using
the ``client-link-local-address'' field value as the destination
address. The Advertise message MUST be unicast through the interface
on which the Solicit message was received.
DISCUSSION:
(From Ted Lemon) There is a danger in using Solicit versus
DHCPDISCOVER: in the Solicit paradigm, the client has to
choose the DHCP server before it knows if the DHCP server
will give it an IP address, or which addresses the server is
willing to assign to the client. It may be that there are
two or more DHCP servers owned by the same administrative
domain, and both are theoretically willing to give the
client addresses, but only one actually has any addresses to
give.
11. DHCP Client-Initiated Configuration Exchange
A client uses the Request-Reply message exchange to acquire
configuration information of interest. The client may initiate the
configuration exchange as part of the operating system configuration
process or when requested to do so by the application layer.
A client uses the Release-Reply message exchange to indicate to the
DHCP server that the client will no longer be using the addresses in
the released IA.
11.1. Request Message Validation
Clients MUST silently discard any received Request messages.
Agents MUST discard any Request messages in which the
``client-link-local-address'' field does not contain a valid
link-local address.
Servers MUST discard any received Request message which meets any of
the following criteria:
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o The ``server-address'' field value does not match any of the
server's addresses.
o The ``options'' field contains an authentication option, and the
server cannot successfully authenticate the client.
11.2. Reply Message Validation
Servers MUST silently discard any received Reply messages.
Clients MUST discard any Reply message that meets any of the
following criteria:
o The ``transaction-ID'' field value does not match the value the
client used in its Request or Release message.
o The ``client-link-local-address'' field value does not match the
link-local address of the interface upon which the client sent in
its Request or Release message.
o The Reply message contains an authentication option, and the
client's attempt to authenticate the message fails.
Relays MUST discard any Relay-reply message in which the
``client-link-local-address'' in the encapsulated Reply message does
not contain a valid link-local address.
11.3. Release Message Validation
Clients MUST silently discard any received Release messages.
Agents MUST discard any Release message in which the
``client-link-local-address'' field does not contain a valid
link-local address.
Servers MUST discard any received Release message in which the
``options'' field contains an authentication option, and the server
cannot successfully authenticate the client.
11.4. Client Behavior
A client will generate one or more Request messages to acquire
configuration information. A client may initiate such an exchange
automatically in order to acquire the necessary network parameters
to communicate with nodes off-link. The client uses the server
address information from previous Advertise message(s) for use in
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constructing Request message(s). Note that a client may request
configuration information from one or more servers at any time.
A client uses the Release message in the management of IAs when:
o The client has determined through DAD or some other method that
one or more of the addresses assigned by the server in the IA is
already in use by a different client.
o The client has been instructed to release the IA prior to the IA
expiration time since it is no longer needed.
11.4.1. Creation and sending of Request messages
The client sets the ``msg-type'' field to 3, and places the
link-local address of the interface it wishes to acquire
configuration information for in the ``client-link-local-address''
field.
The client generates a transaction ID inserts this value in the
``transaction-ID'' field.
The client places the address of the destination server in the
``server-address'' field.
The client adds any appropriate options, including one or more IA
options (if the client is requesting that the server assign it some
network addresses). If the client does include any IA options,
it MUST include the list of addresses the client currently has
associated with that IA. If the client is requesting configuration of
a new IA, the list of addresses MUST be empty.
11.4.2. Time out and retransmission of Request Messages
The server will respond to the Request message with a Reply
message. If no Reply message is received within REP_MSG_TIMEOUT
milliseconds, the client retransmits the Request with the same
transaction-ID, and doubles the REP_MSG_TIMEOUT value, and waits
again. The client continues this process until a Reply is received
or REQUEST_MSG_ATTEMPTS unsuccessful attempts have been made, at
which time the client MUST abort the configuration attempt. The
client SHOULD report the abort status to the application layer.
Default and initial values for REP_MSG_TIMEOUT and REQ_MSG_ATTEMPTS
are documented in section 3.5.
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11.4.3. Receipt of Reply message in response to a Request
Upon the receipt of a valid Reply message, the client extracts the
configuration information contained in the Reply. If the ``status''
field contains a non-zero value, the client reports the error status
to the application layer.
The client records the T1 and T2 times for each IA in the Reply
message. The client records any addresses included with IAs in
the Reply message. The client updates the preferred and valid
lifetimes for the addresses in the IA from the lifetime information
in the IA option. The client leaves any addresses that the client
has associated with the IA that are not included in the IA option
unchanged.
Management of the specific configuration information is detailed in
the definition of each option, in section 22.
11.4.4. Creation and sending of Release messages
The client sets the ``msg-type'' field to 5, and places the
link-local address of the interface associated with the configuration
information it wishes to release in the ``client-link-local-address''
field.
The client generates a transaction ID and places this value in the
``transaction-ID'' field.
The client includes options containing the IAs it is releasing in the
``options'' field. The appropriate ``status'' field in the options
MUST be set to indicate the reason for the release.
The client places the IP address of the server that allocated the
address(es) in the ``server-address'' field.
If the client is configured to use authentication, the client
generates the appropriate authentication option, and adds this option
to the ``options'' field. Note that the authentication option MUST
be the last option in the ``options'' field. See section 22.7 for
more details about the authentication option.
(The client always forwards Release messages to the server through a
relay; see section 11.5.)
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11.4.5. Time out and retransmission of Release Messages
If no Reply message is received within REP_MSG_TIMEOUT milliseconds,
the client retransmits the Release, doubles the REP_MSG_TIMEOUT
value, and waits again. The client continues this process until a
Reply is received or REL_MSG_ATTEMPTS unsuccessful attempts have been
made, at which time the client SHOULD abort the release attempt.
The client SHOULD return the abort status to the application, if an
application initiated the release.
Default and initial values for REP_MSG_TIMEOUT and REL_MSG_ATTEMPTS
are documented in section 3.5.
Note that if the client fails to release the IA, the addresses
assigned to the IA will be reclaimed by the server when the lease
associated with it expires.
11.4.6. Receipt of Reply message in response to a Release
Upon receipt of a valid Reply message, the client can consider the
Release event successful, and SHOULD return the successful status to
the application layer, if an application initiated the release.
11.4.7. When a client should send a Request message
The description of the Request/Reply message exchange in this section
makes no assumptions about the timing or state of the client when
it initiates a Request/Reply message exchange. Sections 11.4.8
through 11.4.10 describe when a client MAY initiate a Request/Reply
message exchange. The procedures for timeout and retransmission of
Request messages are described in section 11.4.2.
11.4.8. Initialization
If a client has no valid IPv6 addresses of sufficient scope to
communicate with a DHCP server, it may a Request message to obtain
new addresses. The client includes one or more IAs in the Request
message, to which the server assigns new addresses. The server then
returns to IA(s) to the client in a Reply message.
11.4.9. Confirming the validity of IPv6 addresses
Whenever a client may have moved to a new link, its IPv6 addresses
may no longer be valid. Examples of times when a client may have
moved to a new link include:
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o The client reboots
o The client is physically disconnected from a wired connection
o The client returns from sleep mode
o The client using a wireless technology changes cells
In any situation when a client may have moved to a new link, the
client MUST initiate a Request/Reply message exchange. The client
includes any IAs, along with the addresses associated with those IAs,
in its Request message. The server returns the IAs with updated list
of addresses and associated lifetimes.
11.4.10. Extending the lifetimes on IPv6 addresses
IPv6 addresses assigned to a client through an IA use the same
preferred and valid lifetimes as IPv6 addresses obtained through
stateless autoconfiguration. The server assigns preferred and valid
lifetimes to the IPv6 addresses it assigns to an IA. To extend those
lifetimes, the client sends a Request to the server containing an
``IA option'' for the IA and its associated addresses. The server
determines new lifetimes for the addresses in the IA according to
the server's administrative configuration. The server may also add
new addresses to the IA. The server remove addresses from the IA by
setting the preferred and valid lifetimes of those addresses to zero.
The server controls the time at which the client contacts the server
to extend the lifetimes on assigned addresses through the T1 and
T2 parameters assigned to an IA. If the server does not assign an
explicit value to T1 or T2 for an IA, T1 defaults to 0.5 times the
shortest preferred lifetime of any address assigned to the IA and
T2 defaults to 0.875 times the shortest preferred lifetime of any
address assigned to the IA.
At time T1 for an IA, the client initiates a Request/Reply message
exchange to extend the lifetimes on any addresses in the IA. The
client includes an IA option with all addresses currently assigned
to the IA in its Request message. The client unicasts this Request
message to the server that originally assigned the addresses to the
IA.
At time T2 for an IA (which will only be reached if the server to
which the Request message was sent at time T1 has not responded),
the client initiates a Request/Reply message exchange. The client
includes an IA option with all addresses currently assigned to the
IA in its Request message. The client multicasts this message to
the FF02::1:2 (All DHCP Agents) multicast address.
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11.5. Relay Behavior
11.5.1. Relaying of Request or Release messages
When a Relay receives a valid Request or Release message, it
constructs a Relay-forward message. The client message is carried
as the payload of a ``client-message'' option. The relay places an
address from the interface on which the client message was received
in the ``relay-address'' field and the prefix length for that
address in the ``prefix-length'' field. The Relay then forwards the
Relay-forward message to the list of server destination addresses
that it has been configured with.
11.6. Server Behavior
For this discussion, the Server is assumed to have been configured in
an implementation specific manner with configuration of interest to
clients.
11.6.1. Receipt of Request messages
Upon the receipt of a valid Request message from a client the server
can respond to, (implementation-specific administrative policy
satisfied) the server scans the options field.
The server then constructs a Reply message and sends it to the
client.
DISCUSSION:
This section needs text about managing IAs and determining
options to be returned to client.
11.6.2. Receipt of Release messages
Upon the receipt of a valid Release message, the server examines the
IAs and the addresses in the IAs for validity. If the IAs in the
message are in a binding for the client and the addresses in the IAs
have been assigned by the server to those IA, the server deletes
the addresses from the IAs and makes the addresses available for
assignment to other clients.
The server then generates a Reply message. If all of the IAs were
valid and the addresses successfully released,, the server sets the
``status'' field to ``Success''. If any of the IAs were invalid or
if any of the addresses were not successfully released, the server
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releases none of the addresses in the message and sets the ``status''
field to ``NoBinding''(section 3.4).
DISCUSSION:
What is the behavior of the server relative to a ``partially
released'' IA; i.e., an IA for which some but not all
addresses are released?
Can a client send an empty IA to release all addresses in
the IA?
If the IA becomes empty - all addresses are released - can
the server discard any record of the IA?
11.6.3. Creation and sending of Reply messages
DISCUSSION:
XXX - This section needs to be fixed (see section 11.6.1).
The server sets the ``msg-type'' field to 4 and copies the values
of the following fields from the client's Request or Release to the
Reply message:
o transaction-ID
o client's link-local address
o server-address
The server sets the ``status'' field appropriately (see the table
in section 3.4) based upon the results of processing the client's
request.
If the Request or Release message from the client was originally
received by the server, the server unicasts the Reply message to the
link-local address in the ``client-link-local-address'' field.
If the message was originally received in a Forward-request or
Forward-release message from a relay, the server places the Reply
message in the options field of a Response-reply message and unicasts
the message to the relay's address from the original message.
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12. DHCP Server-Initiated Configuration Exchange
A server initiates a configuration exchange on behalf of the
administrator of the DHCP domain. An administrator may initiate such
an exchange when new links are added to the domain or existing links
are to be renumbered. Other examples include changes in the location
of directory servers, addition of new services such as printing, and
availability of new software (system or application).
DISCUSSION:
Changed ``networks'' to ``links'' here (ed.). Why would
adding new links cause a server-initiated configuration
exchange?
12.1. Reconfigure Message Validation
Reconfigure messages have been deleted; see section 23.2.
12.2. Reconfigure-reply Message Validation
Reconfigure-reply messages have been deleted; see section 23.2.
12.3. Reconfigure-init Message Validation
Agents MUST silently discard any received Reconfigure-init messages.
Clients MUST discard any Reconfigure-init messages that do
not contain an authentication option or that fail the client's
authentication check.
12.4. Server Behavior
For this discussion, the server is assumed to have a
implementation-specific interface by which an administrator
may initiate a reconfiguration event with some set of clients.
A server sends a Reconfigure-init message to trigger a client to
initiate immediately a Request/Reply message exchange with the
server. A server can send Reconfigure-init messages only to those
clients who have an address of sufficient scope to be reachable by
the server. Thus, those clients who have not requested an IP address
and are off-link cannot be reconfigured by the server.
DISCUSSION:
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It would be possible to forward Reconfigure-init messages
through relays if the server records the client's link-local
address and the relay's address from the client's Request
message.
12.4.1. Creation and sending of Reconfigure messages
Reconfigure messages have been deleted; see section 23.2.
12.4.2. Time out and retransmission of Reconfigure messages
12.4.3. Receipt of Reconfigure-reply messages
12.4.4. Creation and sending of Reconfigure-init messages
The server sets the ``msg-type'' field to 8. The server generates
a transaction-ID and inserts it in the ``transaction-ID'' field.
The server places its address (of appropriate scope) in the
``server-address'' field.
The server MAY include an ORO option to inform the client of what
information has been changed or new information that has been added.
The server MUST include an authentication option with the appropriate
settings and add that option as the last option in the ``options''
field of the Reconfigure-init message.
Typically, the server will not provide more than an ORO and / or
Authentication option, since it will provide the new configuration
information as part of the Request/Reply transaction triggered by the
Reconfigure-init message.
The server may either unicast the Reconfigure-init message to one
client or multicast the message to one or more Reconfigure Multicast
Addresses previously sent as options to the clients. The server
may unicast Reconfigure-init messages to more than one client
concurrently; for example, to reliably reconfigure all clients, the
server will unicast a Reconfigure-init message to each client.
If the server unicasts to one or more clients, it waits for a Request
message from those clients confirming that it has received the
Reconfigure-init and are thus initiating a Request/Reply transaction
with the server. The server can determine that a Request message is
in response to a Reconfigure-init because the transaction-ID in the
Request will be the same value as was used in the Reconfigure-init
message.
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If the server multicasts the Reconfigure-init message, it must use
some TBD authentication mechanism that can authenticate the server to
multiple clients. There is no reliability mechanism for multicast
Reconfigure-init messages. A server might use multicast in the
case where it does not have a list of its clients; for example, a
server that distributes configuration information to clients using
stateless autoconfiguration might not keep a list of clients it has
communicated with.
12.4.5. Time out and retransmission of Reconfigure-init messages
It the server does not receive a Request message from the client
in RECREP_MSG_TIMEOUT milliseconds, the server retransmits
the Reconfigure-init message, doubles the RECREP_MSG_TIMEOUT
value and waits again. The server continues this process until
REC_MSG_ATTEMPTS unsuccessful attempts have been made, at which point
the server SHOULD abort the reconfigure process.
Default and initial values for RECREP_MSG_TIMEOUT and
REC_MSG_ATTEMPTS are documented in section 3.5.
12.4.6. Receipt of Request messages
The server generates and sends Reply message(s) to the client as
described in section 11.6.3, including in the ``option'' field new
values for configuration parameters.
12.5. Client Behavior
A client MUST always monitor UDP port 546 for Reconfigure-init
messages on interfaces upon which it has acquired DHCP parameters.
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.
12.5.1. Receipt of Reconfigure-init messages
Upon receipt of a valid Reconfigure-init message, the client
initiates a Request/Reply transaction with the server.
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12.5.2. Creation and sending of Request messages
When responding to a Reconfigure-init, the client creates and
sends the Request message in exactly the same manner as outlined in
section 11.4.1 with the following differences:
transaction-ID
The client copies the transaction-ID from the
Reconfigure-init message into the Request message.
IAs
The client includes IA options containing the addresses
the client currently has assigned to those IAs for the
interface through which the Reconfigure-init message was
received.
Pause before sending Request
The client pauses before sending the Request for
a random value within the range REC_REP_MIN and
REC_REP_MAX seconds. This delay helps reduce the
load on the server generated by processing large
numbers of triggered Request messages from a multicast
Reconfigure-init message.
12.5.3. Time out and retransmission of Request messages
The client uses the same variables and retransmission algorithm as it
does with Request messages generated as part of a client-initiated
configuration exchange. See section 11.4.2 for details.
12.5.4. Receipt of Reply messages
Upon the receipt of a valid Reply message, the client extracts the
contents of the ``option'' field, and sets (or resets) configuration
parameters appropriately. The client records and updates the
lifetimes for any addresses specified in IAs in the Reply message.
If the configuration parameters changed were requested by the
application layer, the client notifies the application layer of the
changes using an implementation-specific interface.
13. Using DHCP for network renumbering
This section has been deleted (to be moved to ``Notes about DHCP''
doc?).
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14. DHCP Client Implementor Notes
This section provides helpful information for the client implementor
regarding their implementations. The text described here is not part
of the protocol, but rather a discussion of implementation features
we feel the implementor should consider during implementation.
14.1. Primary Interface
Since configuration parameters acquired through DHCP can be
interface-specific or more general, the client implementor SHOULD
provide a mechanism by which the client implementation can be
configured to specify which interface is the primary interface. The
client SHOULD always query the DHCP data associated with the primary
interface for non-interface specific configuration parameters. An
implementation MAY implement a list of interfaces which would be
scanned in order to satisfy the general request. In either case, the
first interface scanned is considered the primary interface.
By allowing the specification of a primary interface, the client
implementor identifies which interface is authoritative for
non-interface specific parameters, which prevents configuration
information ambiguity within the client implementation.
14.2. Advertise Message and Configuration Parameter Caching
If the hardware the client is running on permits it, the implementor
SHOULD provide a cache for Advertise messages and a cache of
configuration parameters received through DHCP. Providing these
caches prevents unnecessary DHCP traffic and the subsequent load
this generates on the servers. The implementor SHOULD provide a
configuration knob for setting the amount of time the cache(s) are
valid.
14.3. Time out and retransmission variables
Note that the client time out and retransmission variables outlined
in section 3.5 can be configured on the server and sent to the client
through the use of the ``DHCP Retransmission Parameter Option'',
which is documented in section 22.6. A client implementation SHOULD
be able to reset these variables using the values from this option.
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14.4. Server Preference
A client MUST wait for SRVR_PREF_WAIT seconds after sending a DHCP
Solicit message to collect Advertise messages and compare their
preferences (see section 15.3), unless it receives an Advertise
message with a preference of 255. If the client receives an
Advertise message with a preference of 255, then the client MAY act
immediately on that Advertise without waiting for any more additional
Advertise messages.
15. DHCP Server Implementor Notes
This section provides helpful information for the server implementor.
15.1. Client Bindings
A server implementation MUST use the IA's UUID and the prefix
specification from which the client sent its Request message(s) as an
index for finding configuration parameters assigned to the client.
While it isn't critical to keep track of the other parameters
assigned to a client, the server MUST keep track of the addresses it
has assigned to an IA.
The server should periodically scan its bindings for addresses whose
leases have expired. When the server finds expired addresses, it
MUST delete the assignment of those addresses, thereby making these
addresses available to other clients.
The client bindings MUST be stored in non-volatile storage.
The server implementation should provide policy knobs to control
whether or not the lifetimes on assigned addresses are renewable, and
by how long.
15.2. Reconfigure-init Considerations
A server implementation MUST provide an interface to the
administrator for initiating reconfigure-init events.
A server implementation may provide a mechanism for allowing the
specification of how many clients comprise a reconfigure multicast
group. This enables the administrator to control the hit a server
takes when a reconfigure-init event occurs.
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15.3. Server Preference
The server implementation SHOULD allow the setting of a server
preference value by the administrator. The server preference
variable is an unsigned single octet value (0--255), with the lowest
preference being 0 and the highest 255. Clients will choose higher
preference servers over those with lower preference values. If you
don't choose to implement this feature in your server, you MUST set
the server preference field to 0 in the Advertise messages generated
by your server.
15.4. Request Message Transaction-ID Cache
In order to improve performance, a server implementation MAY include
an in memory transaction-ID cache. This cache is indexed by client
binding and transaction-ID, and enables the server to quickly
determine whether a Request is a retransmission or a new Request
without the cost of a database lookup. If an implementor chooses to
implement this cache, then they SHOULD provide a configuration knob
to tune the lifetime of the cache entries.
16. DHCP Relay Implementor Notes
A relay implementation SHOULD allow the specification of a list of
destination addresses for forwarded messages. This list MAY contain
any mixture of unicast addresses and multicast addresses.
If a relay receives an ICMP message in response to a DHCP message it
has forwarded, it SHOULD log this event.
17. Open Issues for Working Group Discussion
This section contains some items for discussion by the working group.
17.1. Authentication
Authentication is not discussed in this document.
17.2. DHCP-DNS interaction
Interaction among DHCP servers, clients and DNS servers is not
discussed in this document.
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17.3. Release vs. Decline
Should there be a separate Decline message through which the client
informs the server that it has discovered an address that is in use
by some other host?
17.4. Request messages
In DHCPv4, there has been much confusion about overloading
DHCPREQUEST with the actions of initial address allocation
(INIT), address confirmation (INIT-REBOOT), and extending leases
(RENEW/REBIND).
The model for DHCPv6 messages described in section 11 also uses one
type of message, Request, in each of the scenarios in sections 11.4.8
through 11.4.10. The DHCPv6 specification in this document does not
differentiate the actions taken by a server based on different times
at which a client might initiate a Request/Reply exchange with a
server. That is, the description of server actions in section 11.6.1
does not differentiate among Requests received from clients based on
the client behavior described in sections 11.4.8 through 11.4.10.
It may be necessary to define different server behaviors for each of
the client scenarios. For example, in the address-reconfirmation
scenario (section 11.4.9), servers cannot safely assign new addresses
to a client. The reconfirmation Request is broadcast to multiple
servers, which cannot coordinate the assignment of any addresses.
Therefore, in this scenario, servers can only acknowledge or deny the
validity of addresses but cannot allocate any new addresses.
17.5. Use of term ``agent''
The term ``agent'', taken to mean ``relay agent or server'', may be
confusing. ``relay agent or server'' might be clearer.
17.6. Use of terms ``subnet'' and ``network''
The term ``subnet'' has been eliminated from the document. The term
``network'' is no longer used to describe a link, collection of links
or collection of IPv6 addresses.
18. Security
This document references an ``authentication option'' which is TBD.
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DISCUSSION:
Based on the discussion of security issues at the
8/31/00 design team teleconference and subsequent
DHC WG mailing list discussion, DHCPv6 will use
the security model from DHCPv4, as described in
draft-ietf-dhc-authentication-15.txt.
19. Year 2000 considerations
Since all times are relative to the current time of the transaction,
there is no problem within the DHCPv6 protocol related to any
hardcoded dates or two-digit representation of the current year.
20. IANA Considerations
This document defines message types 1--8 to be received by UDP at
port numbers 546 and 547. Additional message types may be defined in
the future.
Section 3.1 lists several multicast addresses used by DHCP.
This document also defines several status codes that are to
be returned with the Reply and Reconfigure-reply messages (see
sections 9.4 and 9.7). The non-zero values for these status codes
which are currently specified are shown in the table in section 3.4.
There is a DHCPv6 option described in section 22.6, which allows
clients and servers to exchange values for some of the timing
and retransmission parameters defined in section 3.5. Adding new
parameters in the future would require extending the values by which
the parameters are indicated in the DHCP option. Since there needs
to be a list kept, the default values for each parameter should also
be stored as part of the list.
All of these protocol elements may be specified to assume new values
at some point in the future. New values should be approved by the
process of IETF Consensus [10].
21. Acknowledgments
Thanks to the DHC Working Group for their time and input into the
specification. Ralph Droms and Thomas Narten have had a major
role in shaping the continued improvement of the protocol by their
careful reviews. Many thanks to Matt Crawford, Erik Nordmark, Gerald
Maguire, and Mike Carney for their studied review as part of the
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Last Call process. Thanks also for the consistent input, ideas, and
review by (in alphabetical order) Brian Carpenter, Jack McCann, Yakov
Rekhter, Matt Thomas, Sue Thomson, and Phil Wells.
Thanks to Steve Deering and Bob Hinden, who have consistently
taken the time to discuss the more complex parts of the IPv6
specifications.
22. DHCP options
Options are used to carry additional information and parameters
in DHCP messages. Every option shares a common base format, as
described in section 22.1.
this document describes the DHCP options defined as part of the base
DHCP specification. Other options may be defined in the future in a
separate document.
22.1. Format of DHCP options
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-data |
| (option-len octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code
An unsigned integer identifying the specific option type
carried in this option.
option-len
An unsigned integer giving the length of the data in
this option in bytes.
option-data
The data for the option; the format of this data depends
on the definition of the option.
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22.2. Identity association option
The identity association option is used to carry an identity
association, the parameters associated with the IA and the addresses
assigned to the IA.
The format of the IA option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD | variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IA UUID |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| num-addrs | IPv6 address |
+-+-+-+-+-+-+-+-+ (16 octets) |
| |
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | pref. len | preferred lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| pref. lifetime (cont.) | valid lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| valid lifetime (cont.) | IPv6 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code
TBD
option-len
Variable; equal to 17 + num-addrs*25
IA UUID
The unique identifier for this IA; chosen by the client
T1 The time at which the client contacts the server from
which the addresses in the IA were obtained to extend
the lifetimes of the addresses assigned to the IA.
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Internet Draft DHCP for IPv6 22 November 2000
T2 The time at which the client contacts any available
server to extend the lifetimes of the addresses assigned
to the IA.
num-addrs
An unsigned integer giving the number of addresses
carried in this IA option (MAY be zero).
IPv6 address
An IPv6 address assigned to this IA.
preferred lifetime
The preferred lifetime for the associated IPv6 address.
valid lifetime
The valid lifetime for the associated IPv6 address.
The ``IPv6 address'', ``preferred lifetime'' and ``valid lifetime''
fields are repeated for each address in the IA option (as determined
by the ``num-addrs'' field).
DISCUSSION:
The details of the format and the selection of an IA's UUID
are TBD.
DISCUSSION:
An IA has no explicit ``lifetime'' or ``lease length'' of
its own. When the lifetimes of all of the addresses in an
IA have expired, the IA 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.
22.3. Option request option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| requested-option-code-1 | requested-option-code-2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Internet Draft DHCP for IPv6 22 November 2000
option-code TBD.
option-len
Variable; equal to twice the number of option codes
carried in this option.
option-data
A list of the option codes for the options requested in
this option.
22.4. Client message option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DHCP client message |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code TBD
option-len
Variable; equal to the length of the forwarded DHCP
client message.
option-data
The message received from the client; forwarded verbatim
to the server.
22.5. Server message option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DHCP server message |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code TBD
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Internet Draft DHCP for IPv6 22 November 2000
option-len
Variable; equal to the length of the forwarded DHCP
server message.
option-data
The message received from the server; forwarded verbatim
to the client.
22.6. Retransmission parameter option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-data |
| (option-len octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code
An unsigned integer identifying the specific option type
carried in this option.
option-len
An unsigned integer giving the length of the data in
this option in bytes.
option-data
The data for the option; the format of this data depends
on the definition of the option.
22.7. Authentication option
The authentication option is TBD.
23. Changes in this draft
This section describes the changes between this version of the DHCPv6
specification and draft-ietf-dhc-dhcpv6-15.txt.
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Internet Draft DHCP for IPv6 22 November 2000
23.1. Order of sections
New sections have been added at the end of this document to minimize
changes in section numbering. Those sections will be rearranged in a
future revision.
23.2. Reconfigure message
DHCP Reconfigure and Reconfigure-reply messages and the associated
mechanisms have been removed from this draft of the specification.
23.3. Releasable resources
``Releasable resources'' have been removed from this draft.
23.4. DHCP message header
A common fixed DHCP message header has been defined. Not all fields
are used in all messages.
23.5. Design goals
The second sentence in the 8th design goal bullet has been removed.
23.6. Overview
Section 8.2 (DHCP agents) has been removed. DHCP clients no longer
need to know about specific DHCP agents.
Section 8.3 has been modified to reflect the new encapsulating
mechanism through which relays forward client messages to servers.
Section 8.6 and 8.7 have been modified to describe ``identity
associations''.
Section 8.8 has been modified to reflect the deletion of
``reconfigure'' and ``reconfigure-reply'' messages.
23.7. Message formats, 9
Message formats have been changed. All messages share a common fixed
message header followed by options. The various control bits (``P'',
``C'') have been removed from the message header.
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23.8. Solicit and Advertise messages, (section 10)
The description of the message exchanges have been changed to
reflect:
- New relay behavior - encapsulated client messages
- Use of IAs
23.9. Prefix advertisement
Servers no longer advertise prefixes.
23.10. Identity Associations
Section 9.11 describes IAs in detail. A definition of ``IA'' has
been added to section 2. The description of messages exchanges
have been extended to include IAs. The IA option is defined in
section 22.2
23.11. Extensions renamed options; defined in this document
``extensions'' are now called ``options''; the options referenced in
this document are defined in section 22.
23.12. Transaction-ID ranges
Solicit, Advertise, Request, Reply, Release and Reconfigure-init
messages all use an unsigned 16-bit integer ``Transaction-ID''.
Transaction-IDs generated by clients are considered to be chosen from
a different namespace than those chosen by servers. There is no
need to restrict clients and servers to select Transaction-IDs from
specific ranges to avoid conflicts.
23.13. Release messages and relays
Release/Reply messages are forwarded through relays. This mechanism
eliminates the need for an 'R' bit.
23.14. Discovering relay agents
Clients no longer learn the identity of relay agents. When the
client only has a link-local address (e.g., the client has no
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Internet Draft DHCP for IPv6 22 November 2000
assigned addresses), it now multicasts Request message, which is then
forwarded by a relay agent on the same link.
A. Comparison between DHCPv4 and DHCPv6
This appendix is provided for readers who will find it useful to see
a model and architecture comparison between DHCPv4 [6, 1] and DHCPv6.
There are three key reasons for the differences:
o IPv6 inherently supports a new model and architecture for
communications and autoconfiguration of addresses.
o DHCPv6 benefits from the new IPv6 features.
o New features were added to support the expected evolution and
the existence of more complicated Internet network service
requirements.
IPv6 Architecture/Model Changes:
o The link-local address permits a node to have an address
immediately when the node boots, which means all clients have a
source IP address at all times to locate an on-link server or
relay.
o The need for BOOTP compatibility and the broadcast flag have been
removed.
o Multicast and address scoping in IPv6 permit the design of
discovery packets that would inherently define their range by the
multicast address for the function required.
o Stateful autoconfiguration has to coexist and integrate with
stateless autoconfiguration supporting Duplicate Address
Detection and the two IPv6 lifetimes, to facilitate the dynamic
renumbering of addresses and the management of those addresses.
o Multiple addresses per interface are inherently supported in
IPv6.
o Some DHCPv4 options are unnecessary now because the configuration
parameters are either obtained through IPv6 Neighbor Discovery or
the Service Location protocol [15].
DHCPv6 Architecture/Model Changes:
o The message type is the first byte in the packet.
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o IPv6 Address allocations are now handled in a message option as
opposed to the message header.
o Client/Server bindings are now mandatory and take advantage of
the client's link-local address to always permit communications
either directly from an on-link server, or from a off-link server
through an on-link relay.
o Servers are discovered by a client Solicit, followed by a server
Advertise message
o The client will know if the server is on-link or off-link.
o The on-link relay may locate off-link server addresses from
system configuration or by the use of a site-wide multicast
packet.
o ACKs and NAKs are not used.
o The server assumes the client receives its responses unless it
receives a retransmission of the same client request. This
permits recovery in the case where the network has faulted.
o Clients can issue multiple, unrelated Request messages to the
same or different servers.
o The function of DHCPINFORM is inherent in the new packet design;
a client can request configuration parameters other than IPv6
addresses in the optional option headers.
o Clients MUST listen to their UDP port for the new Reconfigure
message from servers.
o New options have been defined.
With the changes just enumerated, we can support new user features,
including
o Configuration of Dynamic Updates to DNS
o Address deprecation, for dynamic renumbering.
o Relays can be preconfigured with server addresses, or use of
multicast.
o Authentication
o Clients can ask for multiple IP addresses.
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Internet Draft DHCP for IPv6 22 November 2000
o Addresses can be reclaimed using the Reconfigure-init message.
o Integration between stateless and stateful address
autoconfiguration.
o Enabling relays to locate off-link servers.
B. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However,
this document itself may not be modified in any way, such as by
removing the copyright notice or references to the Internet Society
or other Internet organizations, except as needed for the purpose
of developing Internet standards in which case the procedures
for copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
References
[1] S. Alexander and R. Droms. DHCP Options and BOOTP Vendor
Extensions. Request for Comments (Draft Standard) 2132,
Internet Engineering Task Force, March 1997.
[2] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. Request for Comments (Best Current Practice) 2119,
Internet Engineering Task Force, March 1997.
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Internet Draft DHCP for IPv6 22 November 2000
[3] S. Bradner and A. Mankin. The Recommendation for the IP Next
Generation Protocol. Request for Comments (Proposed Standard)
1752, Internet Engineering Task Force, January 1995.
[4] W. J. Croft and J. Gilmore. Bootstrap Protocol. Request for
Comments 951, Internet Engineering Task Force, September 1985.
[5] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. Request for Comments (Draft Standard) 2460,
Internet Engineering Task Force, December 1998.
[6] R. Droms. Dynamic Host Configuration Protocol. Request for
Comments (Draft Standard) 2131, Internet Engineering Task Force,
March 1997.
[7] R. Hinden and S. Deering. IP Version 6 Addressing Architecture.
Request for Comments (Proposed Standard) 2373, Internet
Engineering Task Force, July 1998.
[8] S. Kent and R. Atkinson. IP Authentication Header. Request for
Comments (Proposed Standard) 2402, Internet Engineering Task
Force, November 1998.
[9] J. McCann, S. Deering, and J. Mogul. Path MTU Discovery for
IP version 6. Request for Comments (Proposed Standard) 1981,
Internet Engineering Task Force, August 1996.
[10] T. Narten and H. Alvestrand. Guidelines for Writing an IANA
Considerations Section in RFCs. Request for Comments (Best
Current Practice) 2434, Internet Engineering Task Force, October
1998.
[11] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for
IP Version 6 (IPv6). Request for Comments (Draft Standard)
2461, Internet Engineering Task Force, December 1998.
[12] D. C. Plummer. Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet address
for transmission on Ethernet hardware. Request for Comments
(Standard) 826, Internet Engineering Task Force, November 1982.
[13] J. Postel. User Datagram Protocol. Request for Comments
(Standard) 768, Internet Engineering Task Force, August 1980.
[14] S. Thomson and T. Narten. IPv6 Stateless Address
Autoconfiguration. Request for Comments (Draft Standard) 2462,
Internet Engineering Task Force, December 1998.
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Internet Draft DHCP for IPv6 22 November 2000
[15] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. Service
Location Protocol. Request for Comments (Proposed Standard)
2165, Internet Engineering Task Force, June 1997.
[16] P. Vixie, Ed., S. Thomson, Y. Rekhter, and J. Bound. Dynamic
Updates in the Domain Name System (DNS UPDATE). Request for
Comments (Proposed Standard) 2136, Internet Engineering Task
Force, April 1997.
Bound, Carney, Perkins, Droms (ed.) Expires 1 May 2001 [Page 53]
Internet Draft DHCP for IPv6 22 November 2000
Chair's Address
The working group can be contacted via the current chair:
Ralph Droms
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-4733
E-mail: rdroms@cisco.com
Author's Address
Questions about this memo can be directed to:
Jim Bound
Compaq Computer Corporation
Mail Stop: ZK03-3/U14
110 Spitbrook Road
Nashua, NH 03062
USA
Phone: +1-603-884-0400
Email: bound@zk3.dec.com
Mike Carney
Sun Microsystems, Inc
Mail Stop: UMPK17-202
901 San Antonio Road
Palo Alto, CA 94303-4900
USA
Phone: +1-650-786-4171
Email: mwc@eng.sun.com
Charles E. Perkins
Communications Systems Lab
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
Phone: +1-650 625-2986
EMail: charliep@iprg.nokia.com
Fax: +1 650 625-2502
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