Internet Engineering Task Force J. Bound
INTERNET DRAFT Digital Equipment Corp.
DHC Working Group C. Perkins
Obsoletes: draft-ietf-dhc-dhcpv6-08.txt Sun Microsystems
27 February 1997
Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
draft-ietf-dhc-dhcpv6-09.txt
Status of This Memo
This document is a submission to the DHC Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the dhcp-v6@bucknell.edu mailing list.
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this document is unlimited.
Abstract
The Dynamic Host Configuration Protocol (DHCPv6) provides a framework
for passing configuration information, via extensions, to IPv6 nodes.
It offers the capability of automatic allocation of reusable network
addresses and additional configuration flexibility. This protocol
should be considered a stateful counterpart to the IPv6 Stateless
Address Autoconfiguration protocol specification.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Terminology and Definitions 2
2.1. IPv6 Terminology . . . . . . . . . . . . . . . . . . . . 2
2.2. DHCPv6 Terminology . . . . . . . . . . . . . . . . . . . 3
2.3. Specification Language . . . . . . . . . . . . . . . . . 4
3. Protocol Design Model 4
3.1. Design Goals . . . . . . . . . . . . . . . . . . . . . . 4
3.2. DHCP Messages . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Request/Response Processing Model . . . . . . . . . . . . 7
4. DHCP Message Formats and Field Definitions 8
4.1. DHCP Solicit Message Format . . . . . . . . . . . . . . . 8
4.2. DHCP Advertise Message Format . . . . . . . . . . . . . . 9
4.3. DHCP Request Message Format . . . . . . . . . . . . . . . 10
4.4. DHCP Reply Message Format . . . . . . . . . . . . . . . . 12
4.5. DHCP Release Message Format . . . . . . . . . . . . . . . 13
4.6. DHCP Reconfigure Message Format . . . . . . . . . . . . . 14
5. DHCP Client Considerations 15
5.1. Sending DHCP Solicit Messages . . . . . . . . . . . . . . 16
5.2. Receiving DHCP Advertise Messages . . . . . . . . . . . . 16
5.3. Sending DHCP Request Messages . . . . . . . . . . . . . . 17
5.4. Receiving DHCP Reply Messages . . . . . . . . . . . . . . 18
5.5. Sending DHCP Release Messages . . . . . . . . . . . . . . 19
5.6. Receiving DHCP Reconfigure Messages . . . . . . . . . . . 19
6. DHCP Server Considerations 20
6.1. Receiving DHCP Solicit Messages . . . . . . . . . . . . . 21
6.2. Sending DHCP Advertise Messages . . . . . . . . . . . . . 21
6.3. DHCP Request and Reply Messages . . . . . . . . . . . . . 21
6.4. Receiving DHCP Release Messages . . . . . . . . . . . . . 23
6.5. Sending DHCP Reconfigure Messages . . . . . . . . . . . . 24
7. DHCP Relay Considerations 24
7.1. DHCP Solicit and DHCP Advertise Message Processing . . . 24
7.2. DHCP Request Message Processing . . . . . . . . . . . . . 25
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7.3. DHCP Reply Message Processing . . . . . . . . . . . . . . 25
8. Retransmission and Configuration Variables 26
9. Security Considerations 28
10. Acknowledgements 29
A. Related Work in IPv6 29
B. Change History 30
B.1. Changes from November 95 to February 96 Drafts . . . . . 30
B.2. Changes from February 96 to June 96 Drafts . . . . . . . 31
B.3. Changes from June 96 to August 96 Drafts . . . . . . . . 31
B.4. Changes from August 96 to November 96 Drafts . . . . . . 32
B.5. Changes from November 96 to February 97 Drafts . . . . . 33
C. Comparison between DHCPv4 and DHCPv6 34
Chair's Address 38
Author's Address 38
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1. Introduction
The Dynamic Host Configuration Protocol (DHCPv6, or in this
document usually DHCP) provides configuration parameters to Internet
nodes. DHCP consists of a protocol for delivering node-specific
configuration parameters from a DHCP server to a client, and a
mechanism for allocation of network addresses and other related
parameters to IPv6 [3] nodes.
DHCP is built on a client-server model, where designated DHCP
servers allocate network addresses and automatically deliver
configuration parameters to dynamically configurable clients.
Throughout the remainder of this document, the term "server"
refers to a node providing initialization parameters by way of the
DHCP protocol, and the term "client" refers to a node requesting
initialization parameters from a DHCP server. DHCP servers maintain
state for their clients, in contrast to IPv6 Stateless Address
Autoconfiguration [11], where IPv6 nodes should get the same results
if they repeat the autoconfiguration procedure multiple times.
DHCPv6 uses Request and Reply messages to support a client/server
processing model whereby both client and server are assured that
requested configuration parameters have been received and accepted
by the client. DHCP supports optional configuration parameters and
processing for nodes through extensions described in its companion
document ``Extensions for the Dynamic Host Configuration Protocol for
IPv6'' [7].
The IPv6 Addressing Architecture [4] and IPv6 Stateless Address
Autoconfiguration specifications provide new features not available
in IP version 4 (IPv4) [10], which are used to simplify and
generalize the operation of DHCP clients.
Section 2 provides definitions for terminology used throughout
this document. Section 3 provides an overview of the protocol
design model that guided the design choices in the specification;
section 3.2 briefly describes the protocol messages and their
semantics. Section 4 provides the message formats and field
definitions used for each message. Sections 5, 6, and 7 specify
how clients, servers, and relays interact. Appendix A summarizes
related work in IPv6 that will provide helpful context; it is not
part of this specification, but included for informational purposes.
Appendix B itemizes changes between different versions of this
protocol specification. Appendix C discusses the differences between
DHCPv4 and DHCPv6.
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2. Terminology and Definitions
Relevant terminology from the IPv6 Protocol [3], IPv6 Addressing
Architecture [4], and IPv6 Stateless Address Autoconfiguration [11]
will be provided, and then the DHCPv6 terminology.
2.1. IPv6 Terminology
IP Internet Protocol Version 6 (IPv6). The terms IPv4 and
IPv6 are used only in contexts where necessary to avoid
ambiguity.
node A device that implements IP.
router A node that forwards IP datagrams not explicitly
addressed to itself.
host Any node that is not a router.
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 that can be used
to reach neighboring nodes attached to the same link.
All interfaces have a link-local address.
neighbor Nodes attached to the same link.
interface
A node's attachment to the link.
address An IP layer identifier for an interface or a set of
interfaces.
message A unit of data carried in a datagram, exchanged between
DHCP agents and clients.
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datagram An IP header plus payload.
unicast address
An identifier for a single interface. A datagram 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 datagram sent to
a multicast address is delivered to all interfaces
identified by that address.
2.2. DHCPv6 Terminology
configuration parameter
Any parameter that can be used by a node to configure
its network environment and enable communication on a
link or internetwork.
DHCP client A node that initiates requests on a link to obtain
configuration parameters.
DHCP server A server is a node that responds to requests from
clients to provide: addresses, prefix lengths, or
other configuration parameters.
DHCP relay A node that acts as an intermediary to deliver DHCP
messages between clients and servers.
DHCP Agent
Either a DHCP server or a DHCP relay.
agent address
The address of a neighboring DHCP relay or DHCP server
on the same link as the DHCP client.
transaction-ID
The transaction-ID is a monotonically increasing
integer identifier specified by the client or server,
and used to match a response to a pending message.
binding A binding (or, client binding) in DHCP contains the
data which a DHCP server maintains for each of its
clients (see Section 6).
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2.3. Specification Language
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that
the definition is an absolute requirement of the
specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means
that there may exist valid reasons in particular
circumstances to ignore this item, but the full
implications must be understood and carefully
weighed before choosing a different course.
Unexpected results may result otherwise.
MAY This word, or the adjective "optional", means that
this item is one of an allowed set of alternatives.
An implementation which does not include this option
MUST be prepared to interoperate with another
implementation which does include the option.
silently discard
The implementation discards the datagram without
further processing, and without indicating an error
to the sender. The implementation SHOULD provide
the capability of logging the error, including the
contents of the discarded datagram, and SHOULD
record the event in a statistics counter.
3. Protocol Design Model
This section is provided for implementors to understand the DHCPv6
protocol design model from an architectural perspective. The goals,
conceptual models and implementation examples presented in this
section do not specify requirements of the DHCPv6 protocol.
3.1. Design Goals
The following list gives general design goals for this DHCP
specification.
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- DHCP should be a mechanism rather than a policy. DHCP MUST allow
local system administrators control over configuration parameters
where desired; e.g., local system administrators should be able
to enforce local policies concerning allocation and access to
local resources where desired.
- DHCP MUST NOT introduce any requirement for manual configuration
of DHCP clients, except possibly for manually configured
keys. Each node should be able to discover appropriate
local configuration parameters without user intervention, and
incorporate those parameters into its own configuration.
- DHCP MUST NOT require a server on each link. To allow for scale
and economy, DHCP MUST work across DHCP relays.
- A DHCP client MUST be prepared to receive multiple (possibly
different) responses to solicitations for DHCP servers. Some
installations may include multiple, overlapping DHCP servers to
enhance reliability and/or to increase performance.
- DHCP MUST coexist with statically configured, non-participating
nodes and with existing network protocol implementations.
- DHCPv6 MUST be compatible with IPv6 Stateless Address
Autoconfiguration [11].
- DHCP MUST support the requirements of automated renumbering of IP
addresses [1].
- DHCP servers should be able to support Dynamic Updates to
DNS [12].
- DHCP servers MUST be able to handle multiple IPv6 addresses for
each client.
- A DHCP server to server protocol is NOT part of this
specification.
- It is NOT a design goal of DHCP to specify how a server
configuration parameter database is maintained or determined.
Methods for configuring DHCP servers are outside the scope of
this document.
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3.2. DHCP Messages
Each DHCP message contains a type, which defines their function
within the protocol. Processing details for these DHCP messages are
specified in Sections 5, 6, and 7. The message types are as follows:
01 DHCP Solicit
The DHCP Solicit message is a DHCP message sent to one or more
DHCP Agents.
02 DHCP Advertise
The DHCP Advertise is an IP unicast message from a DHCP Agent
in response to a client DHCP Solicit message.
03 DHCP Request
The DHCP Request is an IP unicast message from a client to a
server to request configuration parameters on a network.
04 DHCP Reply
The DHCP Reply is an IP unicast message sent by a server to
respond to a client's DHCP Request. Extensions [7] to the DHCP
Reply describe the resources that the DHCP Server has committed
and allocated to the client, and may contain other information
for use by the client.
05 DHCP Release
The DHCP Release message is used by a DHCP client to inform
the server that the client is releasing a particular address,
or set of addresses or resources, so that the server may
subsequently mark the addresses as invalid, or release
resources in the server's binding for the client.
06 DHCP Reconfigure
The DHCP Reconfigure message is used by a DHCP server to inform
its client that the server has new configuration information of
importance to the client. The client is expected to initiate a
new Request/Reply transaction.
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3.3. Request/Response Processing Model
The request/response processing for DHCPv6 is transaction based
and uses a best-effort set of messages to guarantee a completed
transaction.
Transactions are usually started by a client with a DHCP Request,
which may be issued after the client knows the server's address.
The response (DHCP Reply) is from the server (possibly via a DHCP
Relay). At this point in the flow all data has been transmitted
and, hopefully, received. To provide a method of recovery if either
the client or server do not receive the messages to complete the
transaction, the client is required to retransmit any DHCP Request
message until it elicits the corresponding DHCP Reply or Replies,
or until it can be reasonably certain that the desired DHCP Server
is unavailable. The timeout and retransmission guidelines and
configuration variables are discussed in Section 8.
All DHCP Agents (Servers and Relays) MUST join the link-local
All-DHCP-Agent multicast group at the well-known multicast address
FF02:0:0:0:0:0:1:2. All DHCP Servers MUST, in addition, join
the site-local All-DHCP-Servers multicast group at the well-known
multicast address FF05:0:0:0:0:0:1:3. All DHCP Relays MUST, on the
other hand, join in addition the site-local All-DHCP-Relays multicast
group at the well-known multicast address FF05:0:0:0:0:0:1:4.
DHCP uses the UDP [9] protocol to communicate between clients
and servers. UDP is not reliable, but DHCP has to provide some
reliability between clients and servers. If a response is not
received after transmission of a DHCP message, the message MUST be
retransmitted according to the rules specified in Section 8. The
All-DHCP-Relays address will be used eventually when DHCP Servers
wish to automatically configure all site DHCP Relays.
A client MUST transmit all messages over UDP using port 547 as the
destination port. A client MUST receive all messages from UDP port
546.
A DHCP Agent MUST transmit all messages to clients over UDP using
port 546 as the destination port. A DHCP Agent MUST receive all
messages over UDP using port 547. The source port for DHCP messages
is arbitrary.
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4. DHCP Message Formats and Field Definitions
All fields in DHCP messages MUST be initialized to binary zeroes by
both the client and server unless otherwise noted. DHCP message
types not defined here (msg-types 0 and 7-255) are reserved.
4.1. DHCP Solicit Message Format
A DHCP client (or DHCP relay on behalf of a client) transmits a DHCP
Solicit message to obtain one or more DHCP server addresses.
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 |C|A| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| relay address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 1
C If set, the client requests that all servers receiving
the message deallocate the resources associated with
the client.
A If set, the relay's address is present
reserved 0
client's link-local address
The IP link-local address of the client interface from
which the client issued the DHCP Request message
relay address
If present, the IP address of the interface on which
the relay received the client's DHCP Solicit message
If a DHCP client does not know any DHCP Agent address, or wants
to locate a new server to receive configuration parameters, the
client SHOULD use, as the destination IP address, the well-known All
DHCP Agents multicast address FF02:0:0:0:0:0:1:2. Any DHCP Relay
receiving the solicitation MUST forward it to the All-DHCP-Servers
multicast address, to instruct DHCP Servers to send their
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advertisements to the prospective client. In that case, the relay
MUST copy the client's link-local address into the message, copy the
address of its interface from which the client's solicitation was
received into the agent's address field, and set the 'A' bit.
4.2. DHCP Advertise Message Format
A DHCP agent sends a DHCP Advertise message to inform a prospective
client about the IP address of a DHCP Agent to which a DHCP Request
message may be sent. When the client and server are on different
links, the server sends the advertisement back through the DHCP Relay
whence the solicitation came.
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 |S| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 2
S If set, the agent address is also a server address.
reserved 0
agent address
The IP address of a DHCP Agent interface on the same
link as the client.
client's link-local address
The IP link-local address of the client interface
from which the client issued the DHCP Request message
server address
The IP address of the DHCP server
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extensions See [7].
Suppose that a DHCP server on the same link as a client issues the
DHCP Advertise in response to a DHCP Solicit message sent to the
All-DHCP-Agent multicast address. Then the agent address will be the
IP address of one of the server's interfaces, the 'S' bit will be
set, the agent address will be an address of the server, and there
will be no server address sent in the DHCP Advertise message. It is
an error for server-count to be zero if the 'S' bit is not set.
The DHCP Server MUST copy the link-local address into the
advertisement which is sent in response to a DHCP Solicit. The
source IP address of the IP header of any DHCP Advertise message MUST
have sufficient scope to be reachable by the DHCP Client. Moreover,
the source address of any DHCP Advertise message sent by a DHCP relay
MUST NOT be a link-local address. In situations where there are no
routers sending Router Advertisements, then a DHCP Server MUST be
configured on the same link as prospective clients.
4.3. DHCP Request Message Format
In order to request parameters from a DHCP server, a client sends a
DHCP Request message, and MAY append the appropriate extensions [7].
If the client does not know any DHCP server address, it MUST first
obtain a server address by multicasting a DHCP Solicit message (see
Section 4.1). If the client does not have a valid IP address of
sufficient scope for the DHCP server to communicate with the client,
the client MUST use the unicast IP address of a local DHCP relay
(which then becomes the agent address in the message header) as the
Destination IP address. In this case, the client cannot send the
message directly to the DHCP server because the server could not
return any response to the client. Otherwise, the client MAY omit
the server address in the DHCP Request message; in this case, the
client MUST send the DHCP Request message directly to the server,
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using the server address as the IP destination address in the IP
header.
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 |S|C| reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 3
S If set, the server address is present
C If set, the client requests the server to clear all
other existing resources and bindings (not requested
in extensions) currently associated with the client,
deallocating as needed.
reserved 0
transaction-ID
A monotonically increasing number used to identify this
Request, and copied into the Reply.
server address
If present, the IP address of the DHCP server which
should receive the client's DHCP Request message.
agent address
The IP address of a relay or server interface, copied
from a DHCP Advertisement message.
client's link-local address
The IP link-local address of the client interface from
which the client issued the DHCP Request message
extensions See [7].
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4.4. DHCP Reply Message Format
The server sends one DHCP Reply message in response to every DHCP
Request or DHCP Release received. If the request comes with the 'S'
bit set, the client could not directly send the Request to the server
and had to use a neighboring relay agent. In that case, the server
sends back the DHCP Reply with the 'L' bit set, and the DHCP Reply is
addressed to the agent address found in the DHCP Request message. If
the 'L' bit is set, then the client's link-local address will also be
present.
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 |L| error code | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 4
L If set, the link-local address is present
error code
One of the following values:
0 Success
16 Failure, reason unspecified
17 Authentication failed or nonexistent
18 Poorly formed Request or Release
19 Resources unavailable
20 Client record unavailable
21 Invalid client IP address in Release
23 Relay cannot find Server Address
24 Cannot understand selected Character Set
64 Server unreachable (ICMP error)
transaction-ID
A monotonically increasing number used to identify this
Reply, and copied from the client's Request.
client's link-local address
If present, the IP address of the client interface
which issued the corresponding DHCP Request message.
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extensions
See [7].
If the 'L' bit is set, and thus the link-local address is present in
the Reply message, the Reply is sent by the server to the relay's
address which was specified as the agent address in the DHCP Request
message, and the relay uses the link-local address to deliver
the Reply message to the client. If the length in the UDP header
preceding the DHCP message does not match that which is expected in
the DHCP Request, error code 23 MUST be sent.
4.5. DHCP Release Message Format
The DHCP Release message is sent without the assistance of any DHCP
relay. When a client sends a Release message, it is assumed to
have a valid IP address with sufficient scope to allow access to
the target server. Only the parameters which are specified in the
extensions are released. The DHCP server acknowledges the Release
message by sending a DHCP Reply (Section 4.4, 6.3).
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 |D| reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 5
D If the 'D' ("Direct") flag is set, the client instructs
the server to send the DHCP Reply directly back to the
client, instead of using the given agent address and
link-local address to relay the Reply message.
reserved 0
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transaction-ID
A monotonically increasing number used to identify this
Release, and copied into the Reply.
agent address
The IP address of the agent interface to which the
client issued the DHCP Request message
client's link-local address
The IP link-local address of the client interface from
which the the client issued the DHCP Request message
client address
The IP address of the client interface from which the
the client issued the DHCP Request message. The client
address field is present whenever the 'D' bit is set,
even if it is equal to the link-local address.
extensions See [7]
Suppose that the client has an IP address that will still be valid
after the server performs the operations requested in the extensions
to the DHCP Release message. In that case, and only then, the client
SHOULD then specify the 'D' flag. When the 'D' flag is set, the
server MUST send the DHCP Reply back to the client's address as shown
in the client address field of the Release message. Otherwise, when
the 'D' bit is not set, the server MUST use the agent address and
link-local address in its DHCP Reply message to forward the Reply
message back to the releasing client.
4.6. DHCP Reconfigure Message Format
The DHCP Reconfigure message is sent without the assistance of any
DHCP relay. When a server sends a Reconfigure message, the client
to which it is sent is assumed to have a valid IP address with
sufficient scope to be accessible by the server. Only the parameters
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which are specified in the extensions to the Reconfigure message need
be requested again by the client.
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 | reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 6
reserved 0
transaction-ID
A monotonically increasing number used to identify
this Reconfigure message, and copied into the
client's Request.
server address
The IP address of the DHCP server issuing the DHCP
Reconfigure message.
extensions See [7]
5. DHCP Client Considerations
A DHCP client MUST silently discard any DHCP Solicit, DHCP Request,
or DHCP Release message it receives.
A DHCP client MAY retain its configured parameters and resources
across client system reboots and DHCP client program restarts.
However, in these circumstances a DHCP client MUST also formulate a
DHCP Request message to verify that its configured parameters and
resources are still valid. This Request message MUST have the 'C'
bit set, to clean up stale client binding information at the server
which may no longer be in use by the client; stale information is
that which the client does not include in extensions to such request
messages.
If the server does not respond to the DHCP Request message, the
client may still use any addresses which have not yet expired. In
this case, however, the client MUST begin to search for another
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server by multicasting a new DHCP Solicit message, again with the 'C'
bit set, containing its IP address in the appropriate extension.
This also handles the case wherein a client restarts on a new
network, so that its IP address is no longer valid. When the client
multicasts a new DHCP Discover message, servers will respond with
the information needed for the client to release its old address, if
need be, and request an address reachable on the new network. In
this situation, when the client receives a new IP address and the old
IP address is no longer reachable, the client MUST release its old
IP address by issuing a DHCP Release message with the appropriate
extension.
5.1. Sending DHCP Solicit Messages
If a node wishes to become a new DHCP client, it MUST first
locate a DHCP Server. The client does this by multicasting a DHCP
Solicit message to the All-DHCP-Agents address multicast address
FF02:0:0:0:0:0:1:2, setting the Hop Limit == 1. If there are
no DHCP servers on the same link as the node, then a DHCP Relay
MUST be present for further handling of the solicitation. The
prospective client SHOULD wait for ADV_WAIT seconds to get all the
DHCP Advertisement messages which may be sent in response to the
solicitation.
If a DHCP client reboots and does not have a valid IP address,
it MUST set the 'C' bit in the DHCP Solicit message it sends
when restarting. By setting the 'C' bit in the solicitation, a
DHCP client requests that all the DHCP Servers that receive the
solicitation should clean up their stale client records that match
its link-local address.
If a client sends a DHCP Solicit message after it reboots, the
solicitation SHOULD be delayed after reception of the first Router
Advertisement [6] message, by at least some random amount of time
between MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY seconds. This delay
is intended to help stagger requests to DHCP Servers (and avoid
link-layer collisions) after a power outage causes many nodes to
reboot all at once. Each subsequent DHCP Solicit message that is
issued before receiving an advertisement MUST be delayed by twice the
amount by which the previous DHCP Solicit message was delayed.
5.2. Receiving DHCP Advertise Messages
When a DHCP client receives a DHCP Advertise message, it may
formulate a DHCP Request message to receive configuration information
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and resources from the DHCP servers listed in the advertisement.
If the Advertise message has no server address field and does
not have the 'S' bit set, the client MUST silently discard the
message. If the server's address is shown as a Multicast address,
the advertisement MUST be silently discarded.
If the 'S' bit is set, the DHCP Advertise message was transmitted
by a DHCP server on the same link as the client. In this case, the
client MUST use the agent address as the destination address for any
future DHCP message transactions sent to that server.
Advertisements may have extensions; this might allow the DHCP client
to select the configuration that best meets its needs from among
several prospective servers.
5.3. Sending DHCP Request Messages
A DHCP client obtains configuration information from a DHCP server by
sending a DHCP Request message. The client MUST know the server's
address before sending the Request message, and client MUST have
acquired a (possibly identical) DHCP agent address. If the client
and server are on the same link, the agent address used by the client
MUST be the same as the DHCP server's address. A DHCP Request
message MUST NOT be sent to any multicast address, since otherwise
multiple DHCP agents would possibly allocate resources to the client
in response to the same Request, and the client would have no way to
know which servers had made the allocations, if any datagrams were
lost due to collisions, etc.
If the client has no valid IP address of sufficient scope, and the
DHCP server is off-link, then the client MUST include the server
address in the appropriate field of the DHCP Request message and set
the 'S' bit. In this case, the IP destination address of the Request
message will be a DHCP relay address.
Otherwise, if the client already has a valid IP address of sufficient
scope and knows the IP address of a candidate DHCP server, it
SHOULD send the Request message directly to the DHCP server without
requiring the services of the local DHCP relay.
If a client wishes to instruct a DHCP server to deallocate all
unknown previous resources, configuration information, and bindings
associated with its agent address and link-local address, it sets the
'C' bit in the DHCP Request. A client MAY send in such a Request
even when it is no longer attached to the link on which the relay
address is attached.
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In any case, after choosing a transaction-ID which is numerically
greater than its previous transaction-ID, and filling in the
appropriate fields of the DHCP Request message, the client MAY append
various DHCP Extensions to the message. These extensions denote
specific requests by the client; for example, a client may request
a particular IP address, or request that the server send an update
containing the client's new IP address to a Domain Name Server. When
all desired extensions have been applied, the DHCP client unicasts
the DHCP Request to the appropriate DHCP Agent.
For each pending DHCP Request message, a client MUST maintain the
following information:
- The transaction-ID of the request message,
- The server address,
- The agent address (which can be the same as the server address),
- The time at which the next retransmission will be attempted, and
- All extensions appended to the request message.
If a client does not receive a DHCP Reply message (Section 5.4) with
the same transaction-ID as a pending DHCP Request message within
REPLY_MSG_INITIAL_TIMEOUT seconds, or if the received DHCP Reply
message contains a DHCP Authentication extension which fails to
provide the correct authentication information, the client MUST
retransmit the Request with the same transaction-ID and continue to
retransmit according to the rules in Section 8.
If the client transmits a DHCP Request in response to a DHCP
Reconfigure message (see Section5.6), the client can continue to
operate with its existing configuration information and resources
until it receives the corresponding DHCP Reply from the server. The
same retransmission rules apply as for any other DHCP Request message
from the client.
5.4. Receiving DHCP Reply Messages
When a client receives a DHCP Reply message, it MUST check whether
the transaction-ID in the Reply message matches the transaction-ID
of a pending DHCP Request message. If no match is found, the Reply
message MUST be silently discarded.
If the Reply message is acceptable, the client processes each
Extension [7], extracting the relevant configuration information
and parameters for its network operation. The client can determine
when all extensions in the Reply have been processed by using the
Length field of the Reply. Some extensions in the Reply may have
error codes, when the server was unable to honor the request, which
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will indicate to the client the reason for failure. If the server is
simply unable to honor the request in an extension included by the
client, that extension may simply be omitted from the Reply.
Some configuration information extracted from the extensions to the
DHCP Reply message MUST remain associated with the DHCP server that
sent the message. The particular extensions that require this extra
measure of association with the server are indicated in the DHCP
Extensions document [7]. These "resource-server" associations are
used when sending DHCP Release messages.
5.5. Sending DHCP Release Messages
If a DHCP client determines that some of its network configuration
parameters are no longer needed, it SHOULD enable the DHCP server to
release allocated resources which are no longer in use by sending a
DHCP Release message to the server. The client consults its list
of resource-server associations in order to determine which server
should receive the desired Release message. If a client wishes to
ask the server to release all information and resources relevant to
the client, the client specifies no extensions; this is preferable
to sending a DHCP Request message with the 'C' bit set and no
extensions.
Suppose a client wishes to release resources which were granted to
it on another link. In that case, the client MUST instruct the
server to send the DHCP Reply directly back to the client, instead
of performing the default processing of sending the DHCP Reply back
through the agent-address included in the DHCP Release. This is done
by setting the 'D' bit in the DHCP Release message. Note that it is
an error (Error Code 21) to include within the DHCP Release message
both the 'D' bit and an IP address extension which has the IP address
used as the client IP address field of the DHCP Release message
header.
5.6. Receiving DHCP Reconfigure Messages
Each DHCP client MUST listen at UDP port 546 to receive possible
DHCP Reconfigure messages, except in cases where the client knows
that no Reconfigure message will ever be issued. In some cases,
the IP address at which the client listens will be a multicast
address sent to the client by the DHCP server in an extension to
an earlier DHCP Reply message. If the client does not listen for
DHCP Reconfigure messages, it is possible that the client will
not receive notification that its DHCP server has deallocated the
client's IP address and/or other resources allocated to the client.
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See discussion in 6.5. The client MAY receive an update to the
prefix for their addresses and then MUST use that prefix for their
addresses.
If a DHCP client receives a DHCP Reconfigure message, it is a request
for the client to initiate a new DHCP Request/Reply transaction with
the server which sent the Reconfigure message. The server sending
the Reconfigure message MAY be different than the server which sent a
DHCP Reply message containing the original configuration information.
For each Extension which is present in the Reconfigure message, the
client appends a matching Extension to its Request message, which
it formulates to send to the server specified in the server address
field of the message. The client also copies a transaction-ID from
the Reconfigure message into the Request message. From then on,
processing is the same as specified above in Section 5.3.
Resources held by the client which are not identified by Extensions
in the server's Reconfigure message are not affected.
Note that a server may ask its client to join a multicast group
for the purpose of receiving DHCP Reconfigure messages. When a
Reconfigure message is delivered to the client by way of the selected
multicast address, the client MUST delay its further response for
a random amount of time uniformly distributed within the interval
between RECONF_MSG_MIN_RESP and RECONF_MSG_MAX_RESP seconds. This
will minimize the likelihood that the server will be bombarded with
DHCP Request messages all at the same time.
6. DHCP Server Considerations
A server MUST ignore any DHCP Advertise, DHCP Reply, or DHCP
Reconfigure message it receives.
A server maintains a collection of client records, called
``bindings''. Each binding is uniquely identifiable by the ordered
pair <link-local address, agent address>, since the link-local
address is guaranteed to be unique [11] on the link identified
by the agent address. An implementation MUST support bindings
consisting of at least a client's link-local address, agent address,
preferred lifetime and valid lifetime [11] for each client address,
and the transaction-ID. A client binding may be used to store any
other information, resources, and configuration data which will be
associated with the client. A DHCP server MUST retain its clients'
bindings across server reboots, and, whenever possible, a DHCP client
should be assigned the same configuration parameters despite server
system reboots and DHCP server program restarts. A DHCP server MUST
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support fixed or permanent allocation of configuration parameters to
specific clients.
Servers on the same link as the client MUST use the source address
in the IP header from the client as the destination address in DHCP
response messages sent by the server to the client.
6.1. Receiving DHCP Solicit Messages
If the DHCP Solicit message was received at the All-DHCP-Servers
multicast address, the DHCP Server MUST check to make sure that the
source address is not a link-local address. In that case, if the
source address is a link-local address, the server MUST silently
discard the packet. If the UDP length disagrees with the length
determined by the format of the DHCP Solicit message, the server
MUST drop the packet and SHOULD log the error. Note that if the
client sends a DHCP Solicit message from a link-local address, the
multicast destination will be the All-DHCP-Agents address, not the
All-DHCP-Servers address.
6.2. Sending DHCP Advertise Messages
Upon receiving and verifying the correctness of a DHCP Solicit
message, a server constructs a DHCP Advertise message and transmits
it on the same link as the solicitation was received from. The
destination address of the advertisement MUST be the source address
of the solicitation. The DHCP server MUST use an IP address of the
interface on which it received the Solicit message as the source
address field of the IP header of the message.
The DHCP server MAY append extensions to the Advertisement, in order
to offer the soliciting node the best possible information about
the services and resources which the server may be able to make
available.
6.3. DHCP Request and Reply Messages
The DHCP server MUST check to ensure that the client's link-local
address field of the Request message contains an address which could
be a valid link-local address. If not, the message MUST be silently
discarded. Otherwise, it checks for the presence of the 'S' bit. If
the 'S' bit is set, the server MUST check that the server address
matches the destination IP address at which the Request message was
received by the server. If the server address does not match, the
Request message MUST be silently discarded.
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If the received agent address and link-local address do not
correspond to any binding known to the server, then the server MAY
create a new binding for the previously unknown client; otherwise, it
SHOULD return a DHCP Reply with an error code of 20.
While processing the Request, the server MUST first determine whether
or not the Request is a retransmission of an earlier DHCP Request
from the same client. This is done by comparing the transaction-ID
to all those transaction-IDs received from the same client during the
previous XID_TIMEOUT seconds. If the transaction-ID is the same as
one received during that time, the server MUST take the same action
(e.g., retransmit the same DHCP Reply to the client) as it did after
processing the previous DHCP Request with the same transaction-ID.
Otherwise, if the transaction-ID has not been recently used, the
server identifies and allocates all the relevant information,
resources, and configuration data that is associated with the client.
Then it sends that information to its DHCP client by constructing a
DHCP Reply message and including the client's information in DHCP
Extensions to the Reply message. The DHCP Reply message uses the
same transaction-ID as found in the received DHCP Request message.
Note that the reply message MAY contain information not specifically
requested by the client.
If the DHCP Request message has the 'S' bit set in the message
header, then the Request was sent to the server by a DHCP Relay. In
this case, the DHCP server MUST send the corresponding DHCP Reply
message to the agent address found in the Request (see section 7.2).
The DHCP Request may contain extensions, which are interpreted
(by default) as advisory information from the client about its
configuration preferences. For instance, if the IP Address Extension
is present, the DHCP server SHOULD attempt to allocate or extend the
lifetime of the address indicated by the extension. Some extensions
may be marked by the client as required.
The DHCP server may accept some extensions for successful processing
and allocation, while still rejecting others, or the server may
reject various extensions for different reasons. The server sets
the Error Code appropriately for those extensions which return error
status to the client. The DHCP server sends a single Reply message
in response to each DHCP Request, with the same transaction-ID as the
Request.
Whenever it is able to, the server includes an extension in the
Reply message for every extension sent by the client in the Request
message. If the client requests some extensions that cannot be
supplied by the server, the server can simply fail to provide them,
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not including them in the Reply. Other extensions can be rejected by
including them in the Reply with an appropriate error code indicating
failure.
When a client DHCP Request is received that has the 'C' bit set,
the server should check to find out whether the extensions listed
in the Request message match those which it has associated with the
client's binding. Any resources which are not indicated by the
client are presumed to be unknown by the client, and thus possible
candidates for reallocation to satisfy requests from other clients.
The DHCP Server MUST deallocate all resources associated with the
client upon reception of a DHCP Request with the 'C' bit set, except
for those which the server is willing to reallocate response to the
client's request. It may be more efficient to avoid deallocating any
resources until after the list of extensions to the request have been
inspected.
6.4. Receiving DHCP Release Messages
If the server receives a DHCP Release Message, it MUST verify that
the link-local address field of the message contains an address
which could be a valid link-local address (i.e., one with the prefix
FE80::0000/64). If not, the message MUST be silently discarded.
In response to a DHCP Release Message with a valid client's
link-local address and agent address, the DHCP server formulates a
DHCP Reply message that will be sent back to the releasing client by
way of the client's link-local address. A DHCP Reply message sent
in response to a DHCP Release message MUST be sent to the client's
link-local address via the agent address in the Release message
and set the 'L' bit in the Reply, unless the 'D' bit is set in the
Release message.
If the received agent address and link-local address do not
correspond to any binding known to the server, then the server SHOULD
return a DHCP Reply with an error code of 20.
Otherwise, if the agent address and link-local address indicate a
binding known to the server, then the server continues processing the
Release message. If there are any extensions, the server releases
the particular configuration items specified in the extensions.
Otherwise, if there are no extensions, the server releases all
configuration information in the client's binding.
After performing the operations indicated in the DHCP Release message
and its extensions, the DHCP server formulates a DHCP Reply message,
copying the transaction-ID, from the DHCP Release message. For
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each Extension in the DHCP Release message successfully processed
by the server, a matching Extension is appended to the DHCP Reply
message. For extensions in the DHCP Release message which cannot be
successfully processed by the server, a DHCP Reply message containing
extensions with the appropriate error codes MUST be returned by the
server.
6.5. Sending DHCP Reconfigure Messages
If a DHCP server needs to change the configuration associated to any
of its clients, it constructs a DHCP Reconfigure message and sends it
to each such client [7]. The Reconfigure MAY be sent to a multicast
address chosen by the server and sent to each of its clients in an
extension to a previous DHCP Reply message.
7. DHCP Relay Considerations
The DHCP protocol is constructed so that a relay does not have
to maintain any state in order to facilitate DHCP client/server
interactions.
All relays MUST use the IP address of the interface from which the
DHCP request was received as the source address for the IP header of
that DHCP message.
The main purpose of the DHCP Relay is to enable clients and servers
to carry out DHCP protocol transactions. DHCP Solicit messages are
issued by the relay when initiated by prospective DHCP clients.
By default, the relay discovers local DHCP Servers by use of
multicasting DHCP solicitations to the All-DHCP-Servers multicast
address, but relays SHOULD allow this behavior to be configurable.
The relay SHOULD NOT send such a multicast solicitation on the
interface from which it received the solicitation from the client.
7.1. DHCP Solicit and DHCP Advertise Message Processing
Upon receiving a DHCP Solicit message from a prospective client, a
relay, by default, forwards the message to all DHCP Servers at a site
according to the following procedure:
- copying the prospective client's solicitation message fields into
the appropriate fields of the outgoing solicitation,
- setting the 'A' bit,
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- copying the address of its interface from which the solicitation
was received from the client into the DHCP Relay address field,
and
- finally, sending the resulting message to the All-DHCP-Servers
multicast address, FF05:0:0:0:0:0:1:3, over all interfaces except
that from which the client's solicitation was received.
When the relay receives a DHCP advertisement with the 'A' bit set, it
relays the advertisement to the client at the indicated link-local
address by way of the interface indicated in the agent's address
field.
7.2. DHCP Request Message Processing
When a relay receives a DHCP Request message, it MUST check that the
message is received from a link-local address, that the link-local
address matches the link-local address field in the Request message
header, and that the agent address field of the message matches an
IP address associated to the interface from which the DHCP Request
message was received. If any of these checks fail, the Relay MUST
silently discard the Request message.
The relay MUST also check whether the 'S' bit is set in the message
header. If not, the datagram is discarded, and and the relay SHOULD
return a DHCP Reply message to the source address of the Request
message with error code 18.
If the received request message is acceptable, the relay then
transmits the DHCP Request message to the address of the DHCP
server found in the Server IP Address field of the received DHCP
Request message. All of the fields of DHCP Request message header
transmitted by the relay are copied over unchanged from the DHCP
Request received from the client. Only the fields in the IP header
will differ from the datagram received from the client, not the
payload. If the Relay receives an ICMP error, the Relay SHOULD
return a DHCP Reply message to the client address (which can be found
in the payload of the ICMP message [2]), with error code 64.
7.3. DHCP Reply Message Processing
When the relay receives a DHCP Reply, it MUST check whether the
message has the 'L' bit set. It MUST check whether the link-local
address field contains an IP address that has prefix FE80::0000/64.
If all the checks are satisfied, the relay MUST send a DHCP Reply
message to the link-local address listed in the received Reply
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message. Only the fields in the IP header will differ from the
datagram received from the server, not the payload.
8. Retransmission and Configuration Variables
When a DHCP client does not receive a DHCP Reply in response to a
pending DHCP Request, the client MUST retransmit the identical DHCP
Request, with the same transaction-ID, to the same server again
until it can be reasonably sure that the DHCP server is unavailable
and an alternative can be chosen. The DHCP Server assumes that the
client has received the configuration information included with the
extensions to the DHCP Reply message, and it is up to the client
to continue to try for a reasonable amount of time to complete the
transaction in order to make that assumption hold true. All the
actions specified for DHCP Request in this section hold also for DHCP
Release messages sent by the DHCP Client.
Similarly, when a client sends a DHCP Request message in response to
a Reconfigure message from the server, the client assumes that the
DHCP server has received the Request. The server MUST retransmit the
identical DHCP Reconfigure to the client for a reasonable amount of
time, to try to elicit the Request message from the client, in order
to make the best effort for that assumption to hold true. If no
corresponding DHCP Request is ever received by the server, the server
MAY erase or deallocate information as needed from the client's
binding.
These retransmissions occur using the following configuration
variables for a DHCP implementation that MUST be configurable by a
client or server:
ADV_WAIT
The amount of time a client waits to hear DHCP Advertisements
after issuing a DHCP Solicit to the All-DHCP Agents multicast
address.
Default: 5 seconds
REPLY_MSG_INITIAL_TIMEOUT
The time in seconds that a DHCP client waits to receive a
server's DHCP Reply before retransmitting a DHCP Request.
Default: 2 seconds.
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REPLY_MSG_MIN_RETRANS
The minimum number of DHCP Request transmissions that a DHCP
client should retransmit, before aborting the request, possibly
retrying the Request with another Server, and logging a DHCP
System Error.
Default: 10 retransmissions.
REPLY_MSG_RETRANS_INTERVAL
The time between successive retransmissions of DHCP Request
messages.
Default: 2 seconds.
RECONF_MSG_INITIAL_TIMEOUT
The time in seconds that a DHCP server waits to receive
a client's DHCP Request before retransmitting its DHCP
Reconfigure.
Default: 2 seconds.
RECONF_MSG_MIN_RETRANS
The minimum number of DHCP Reconfigure messages that a DHCP
server should retransmit, before assuming the the client is
unavailable and that the server can proceed with the needed
reconfiguration of that client's resources, and logging a DHCP
System Error.
Default: 10 retransmissions.
RECONF_MSG_RETRANS_INTERVAL
The least time between successive retransmissions of DHCP
Reconfigure messages.
Default: 2 seconds.
RECONF_MSG_MIN_RESP
The minimum amount of time before a client can respond to a
DHCP Reconfigure message sent to a multicast address.
Default: 2 second.
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RECONF_MSG_MAX_RESP
The maximum amount of time before a client MUST respond to a
DHCP Reconfigure message sent to a multicast address.
Default: 10 seconds.
MIN_SOLICIT_DELAY
The maximum amount of time a prospective client is required
to wait, after determining from a Router Discovery message
that the client should perform stateful address configuration,
before sending a DHCP Solicit to a DHCP Server.
Default: 1 second
MAX_SOLICIT_DELAY
The maximum amount of time a prospective client is required
to wait, after determining from a Router Discovery message
that the client should perform stateful address configuration,
before sending a DHCP Solicit to a DHCP Server.
Default: 5 seconds
XID_TIMEOUT
The amount of time a DHCP server has to keep track of
client transaction-IDs in order to make sure that client
retransmissions using the same transaction-ID are idempotent.
Default: 600 seconds
Note that, if a client receives a DHCP message which fails
authentication, it should continue to wait for another message which
might be correctly authenticated just as if the failed message had
never arrived; however, receiving such failed messages SHOULD be
logged.
9. Security Considerations
DHCP clients and servers often have to authenticate the messages they
exchange. For instance, a DHCP server may wish to be certain that a
DHCP Request originated from the client identified by the <link-local
address, agent address> fields included within the Request message
header. Conversely, it is often essential for a DHCP client to
be certain that the configuration parameters and addresses it has
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received were sent to it by an authoritative DHCP server. Similarly,
a DHCP server should only accept a DHCP Release message which seems
to be from one of its clients, if it has some assurance that the
client actually did transmit the Release message. At the time of
this writing, there is no generally accepted mechanism useful with
DHCPv4 that can be extended for use with DHCPv6.
The IPv6 Authentication Header can provide security for DHCPv6
messages when both endpoints have a suitable IP address. However,
a client often has only a link-local address, and such an address
is not sufficient for a DHCP server which is off-link. In those
circumstances the DHCP relay is involved, so that the DHCP message
MUST have the relay's address in the IP destination address field,
even though the client aims to deliver the message to the DHCP
server. The DHCP Client-Server Authentication Extension [7] is
intended to be used in these circumstances.
10. Acknowledgements
Thanks to the DHC Working Group for their time and input into the
specification. A special thanks for the consistent input, ideas,
and review by (in alphabetical order) Brian Carpenter, Ralph Droms,
Thomas Narten, 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. Thanks to Stuart Cheshire for his excellent minutes.
A. Related Work in IPv6
The related work in IPv6 that would best serve an implementor
to study is the IPv6 Specification [3], the IPv6 Addressing
Architecture [4], IPv6 Stateless Address Autoconfiguration [11], IPv6
Neighbor Discovery Processing [6], and Dynamic Updates to DNS [12].
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 576 octets
or greater (in IPv4 the requirement is 68 octets). This means that
a UDP datagram of 536 octets will always pass through an internet
(less 40 octets for the IPv6 header), as long as there are no IP
options prior to the UDP header in the datagram. But, IPv6 does
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not support fragmentation at routers, so that fragmentation takes
place end-to-end between hosts. If a DHCP implementation needs
to send a datagram greater than 536 octets it can either fragment
the UDP datagram in UDP or use Path MTU Discovery [5] to determine
the size of the datagram that will traverse a network path. It is
implementation dependent how this is accomplished in DHCP.
The IPv6 Addressing Architecture specification [4] 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 multicast
addressing is required, and nodes can create link-local addresses
during initialization of the nodes environment. This means that a
client immediately can configure an IP address at initialization
for an interface, before communicating in any manner on the link.
The client can then use a well-known multicast address to begin
communications to discover neighbors on the link, or to send a DHCP
Solicit and locate a DHCP server or relay.
IPv6 Stateless Address Autoconfiguration [11] (addrconf) specifies
procedures by which a node may autoconfigure addresses based on
router advertisements [6], and the use of a validation lifetime to
support renumbering of addresses on the Internet. In addition the
protocol interaction by which a node begins stateless or stateful
autoconfiguration is specified. DHCP is one vehicle to perform
stateful autoconfiguration. Compatibility with addrconf is a design
requirement of DHCP (see Section 3.1).
IPv6 Neighbor Discovery [6] is the node discovery protocol in IPv6
(replaces and enhances functions of ARP [8]). To truly understand
IPv6 and addrconf it is strongly recommended that implementors
understand IPv6 Neighbor Discovery.
Dynamic Updates to DNS [12] 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 now integrate addresses and name space
to not only support autoconfiguration, but also autoregistration in
IPv6.
B. Change History
B.1. Changes from November 95 to February 96 Drafts
Substituted use of client's link-local address for previous uses of
client's interface token.
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Reorganized DHCP messages into Solicit/Advertise, Request/Reply,
Release, and Reconfigure.
Made message-specific formats instead of using the same DHCP header
for each message.
Eliminated retransmission message types.
Server commits after receiving DHCP Request, and optimistically
depends on client retransmissions as negative acknowledgement.
Eliminated total-addrs.
Eliminated all definitions and most fields related to allocating IPv6
addresses (moved to the Extensions specification).
Renamed "gateway address" to be "agent address".
Added "Considerations" sections.
B.2. Changes from February 96 to June 96 Drafts
Added language referring to DHCP Client-Server Authentication
extension.
Moved the 'L' bit in the DHCP Reply Message format to save 32 bits.
Added language for multicast Reconfigure message handling.
Added initial capability for the DHCP Relay to multicast and obtain
DHCP Server addresses.
Added capability for Servers to add Extensions to their
Advertisements.
Added 'C' bit to DHCP Solicit for deallocating resources after client
crash.
Added DHCP Advertisement lifetimes for use by DHCP Relay agents that
need to periodically update their list of DHCP servers.
B.3. Changes from June 96 to August 96 Drafts
Since the working group indicated that DHCP solicitation traffic
was not considered to be a significant factor affecting network
load, it was decided to modify the handling of solicitations so
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that DHCP relays, by default, multicast DHCP Solicit message to all
DHCP servers at a site. This entailed a number of changes to the
protocol, namely:
- Adding fields to the DHCP Solicit and DHCP Advertise messages to
contain the DHCP client's link-local addresses.
- Adding the 'L' bit to the DHCP Solicit and DHCP Advertise
messages to indicate whether the link-local address is present
- Adding a 'P' bit to the DHCP Solicit message so that the client
can allow the Relay to use its non-default behavior, which is to
return cached DHCP Server addresses to the client in response to
the client's DHCP Solicit message.
- Specified a new multicast address (the All-DHCP-Servers address)
for use by DHCP Relays when "relaying" client solicitations.
Added a random backoff after reboot so that clients' solicitations
don't immediately swamp DHCP Servers after power outages.
Added new multicast addresses for All DHCP Servers and All DHCP
Relays.
B.4. Changes from August 96 to November 96 Drafts
Clarified language regarding treatment by the DHCP server of DHCP
Requests with the 'C' bit set.
Specified that the UDP source port for DHCP messages is arbitrary.
Added description for Appendix C.
Changed must to MUST where appropriate.
Changed definitions for client, server, and relay to be definitions
for DHCP client, DHCP server, and DHCP relay.
Changed definitions of DHCP multicast addresses to conform to recent
IANA allocations.
Corrected references to "leases", to more accurately refer to IPv6
address lifetimes.
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B.5. Changes from November 96 to February 97 Drafts
Clients can continue to use valid addresses, after restarts or any
request triggered by a DHCP Reconfigure message, at least until it
receives the DHCP Request from the server.
All extensions sent in response to a single DHCP Request now must be
part of the same DHCP Reply message. If some requested resources and
configuration parameters are not available or cannot be allocated,
each particular extension will either have the appropriate error code
indicating the particular problem, or simply will not be included
in the Reply. The extensions are to be modified to have fields for
error codes whenever the server might have to indicate to the client
a reason why the information requested in its extension was unable to
be supplied.
If a client receives a DHCP Reconfigure message which does not list
some the client's configuration information, it can continue to
assume that configuration information is valid.
If a client reboots, it MUST set the 'C' bit and transmit a DHCP
Request. If it doesn't have a valid server address, it MUST set the
'C' bit in its DHCP Solicit message.
Relays are no longer allowed to cache server addresses. The
DHC working group decided to ice this plan until there was some
determination that it might be useful. This caused the elimination
of the 'P' bit, and quite a bit of discussion about the 'P' bit and
DHCP server address caching was eliminated. The 'server count' field
of the Advertisement and the lifetime field were eliminated, since
relays never keep track of server addresses and clients have to
solicit again whenever they lose their DHCP server.
The working group decided to make programming as simple as possible,
and therefore to include IP addresses in the appropriate DHCP
message headers whenever those addresses would otherwise have to be
discovered by manipulating the IP header itself. This caused many
changes to the message header formats. The 'L' bit in the DHCP
Solicit and DHCP Advertise messages is no longer necessary, because
the link-local address of the client is always present in the header.
Previously, there was language which required the client to match
pending Requests with Reply messages with the same destination
agent addresses. Those agent addresses were to be determined by
inspecting the IP headers of the DHCP Reply messages. We deleted
the requirement, in preference to loading possibly two more agent
addresses in every DHCP Advertise message and DHCP Reply message.
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The DHCP Reconfigure message now has a transaction ID which the
client copies into the corresponding DHCP Request, and then which
subsequently the server copies again into the corresponding DHCP
Reply message.
Clients now use the DHCP server address found in the appropriate
field of the DHCP Reconfigure message header instead of inspecting
the IP header of the Reconfigure message.
C. 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 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 in its design was able to take advantage of the inherent
benefits of IPv6.
o New features were added to support the evolution and the
existence of mature Internet users in the industry.
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 a server or relay agent
on the local link.
o The need for bootp compatibility and broadcast flags are removed,
which permitted a great deal of freedom in designing the new
packet formats for the client and server interaction.
o Multicast and the scoping methods in IPv6 permitted 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.
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o Most DHCPv4 options are unnecessary now because the configuration
parameters are either obtained through IPv6 Neighbor Discovery or
the Service Location protocol.
DHCPv6 Architecture/Model Changes:
o The message type is the first byte in the packet.
o IPv6 Address allocations are now handled in a message extension
as opposed to the main 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 remote server
through an on-link relay-agent.
o Servers are now discovered by a client solicit and server or
relay-agent advertisement model.
o The client will know if the server is on-link or off-link.
o The client after a solicit will be returned the addresses of
available servers either from an on-link server or from an
on-link relay-agent as agents providing the advertisements.
o The on-link relay-agent will obtain the location of remote server
addresses from system configuration or by the use of a site wide
DHCPv6 Multicast packet.
o The protocol is optimized and removes the use of ACKs and NAKs
once the client and server set-up is complete.
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 DHCPINFORM is inherent in the new packet design; a client can
request configuration parameters other than IPv6 addresses in the
optional extension headers.
o Clients MUST listen to their UDP port for the new Reconfigure
message type from servers, unless they join the appropriate
multicast group as specified by the DHCP server.
o Dynamic Updates to DNS are supported in the IPv6 Address
extension.
o New extensions have been defined.
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New Internet User Features:
o Configuration of Dynamic Updates to DNS to support multiple
implementation policy requirements.
o Configuration of what policy is enforced when addresses are
deprecated for dynamic renumbering can be implemented.
o Configuration of how relay-agents locate remote servers for a
link can be implemented.
o An Authentication extension has been added.
o Configuration of additional addresses for server applications can
be requested by a client in an implementation.
o Reclaiming addresses allocated with very long lifetimes can be
implemented using the Reconfigure message type.
o Configuration of tightly coupled integration between stateless
and stateful address autoconfiguration can be implemented.
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References
[1] S. Bradner and A. Mankin. The Recommendation for the IP Next
Generation Protocol. RFC 1752, January 1995.
[2] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6). RFC 1885,
December 1995.
[3] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883, December 1995.
[4] R. Hinden and S. Deering. IP Version 6 Addressing Architecture.
RFC 1884, December 1995.
[5] J. McCann, S. Deering, and J. Mogul. Path MTU Discovery for IP
version 6. RFC 1981, August 1996.
[6] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for
IP version 6 (IPv6). RFC 1970, August 1996.
[7] C. Perkins. Extensions to DHCPv6, February 1997.
draft-ietf-dhc-dhcpv6ext-05.txt, work in progress.
[8] David C. Plummer. An Ethernet Address Resolution Protocol:
Or Converting Network Protocol Addresses to 48.bit Ethernet
Addresses for Transmission on Ethernet Hardware. RFC 826,
November 1982.
[9] J. B. Postel. User Datagram Protocol. RFC 768, August 1980.
[10] J. B. Postel, Editor. Internet Protocol. RFC 791, September
1981.
[11] S. Thomson and T. Narten. IPv6 stateless address
autoconfiguration. RFC 1971, August 1996.
[12] P. Vixie, S. Thomson, Y. Rekhter, and J. Bound.
Dynamic Updates in the Domain Name System (DNS).
draft-ietf-dnsind-dynDNS-11.txt, November 1996. (work
in progress).
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Chair's Address
The working group can be contacted via the current chair:
Ralph Droms
Computer Science Department
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
E-mail: droms@bucknell.edu
Author's Address
Questions about this memo can be directed to:
Jim Bound Charles Perkins
Digital Equipment Corporation Netcentricity Group
110 Spitbrook Road, ZKO3-3/U14 Sun Microsystems, Inc.
Nashua, NH 03062 2550 Garcia Avenue.
Mountain View, CA 94043
Phone: +1-603-881-0400 +1-415-336-7153
Fax: +1-415-336-0673
E-mail: bound@zk3.dec.com charles.perkins@corp.sun.com
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