Internet Engineering Task Force J. Bound
INTERNET DRAFT Compaq Computer Corp.
DHC Working Group C. Perkins
Obsoletes: draft-ietf-dhc-dhcpv6-12.txt Sun Microsystems
28 June 1998
Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
draft-ietf-dhc-dhcpv6-13.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.
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Abstract
The Dynamic Host Configuration Protocol (DHCPv6) enables DHCP
servers to pass configuration information, via extensions, 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 the IPv6 Stateless Address
Autoconfiguration protocol, and can be used separately or together
with the latter to obtain configuration information.
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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
2.4. Error Values . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Design Model 4
3.1. Design Goals . . . . . . . . . . . . . . . . . . . . . . 4
3.2. DHCP Messages . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Request/Response Processing Model . . . . . . . . . . . . 6
4. DHCP Message Formats and Field Definitions 7
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. Verifying Resource Allocations After Restarts . . . . . . 15
5.2. Sending DHCP Solicit Messages . . . . . . . . . . . . . . 16
5.3. Receiving DHCP Advertise Messages . . . . . . . . . . . . 16
5.4. Sending DHCP Request Messages . . . . . . . . . . . . . . 17
5.5. Receiving DHCP Reply Messages . . . . . . . . . . . . . . 19
5.6. Sending DHCP Release Messages . . . . . . . . . . . . . . 19
5.7. Receiving DHCP Reconfigure Messages . . . . . . . . . . . 20
5.8. Interaction with Stateless Address Autoconfiguration . . 21
6. DHCP Server Considerations 21
6.1. Receiving DHCP Solicit Messages . . . . . . . . . . . . . 22
6.2. Sending DHCP Advertise Messages . . . . . . . . . . . . . 22
6.3. DHCP Request and Reply Message Processing . . . . . . . . 23
6.3.1. Processing for Extensions to DHCP Request and Reply
Messages . . . . . . . . . . . . . . . . . 24
6.3.2. Client Requests to Deallocate Unknown Resources . 24
6.4. Receiving DHCP Release Messages . . . . . . . . . . . . . 25
6.5. Sending DHCP Reconfigure Messages . . . . . . . . . . . . 26
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6.6. Client-Resource timeouts . . . . . . . . . . . . . . . . 26
7. DHCP Relay Considerations 26
7.1. DHCP Solicit and DHCP Advertise Message Processing . . . 27
7.2. DHCP Request Message Processing . . . . . . . . . . . . . 27
7.3. DHCP Reply Message Processing . . . . . . . . . . . . . . 28
8. Retransmission and Configuration Variables 28
9. Security Considerations 31
10. Year 2000 considerations 32
11. IANA Considerations 32
12. Acknowledgements 32
A. Changes for this revision 33
B. Related Protocol Specifications 33
C. Comparison between DHCPv4 and DHCPv6 34
D. Full Copyright Statement 37
Chair's Address 40
Author's Address 40
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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
extensions for allocation of network addresses and other related
parameters to IPv6 [6] nodes.
DHCP uses a client-server model, where designated DHCP servers
automatically allocate network addresses and 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.
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'' [13].
The IPv6 Addressing Architecture [8] and IPv6 Stateless Address
Autoconfiguration [17] specifications provide new features not
available in IP version 4 (IPv4) [16], which are used to simplify
and generalize the operation of DHCP clients. This document is
intended to complement those specifications for clients attached to
the kinds of Internet media for which those specifications apply. In
particular, the specification in this document does not necessarily
apply to nodes which do not enjoy a bidirectional link to the
Internet.
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 respectively interact. The timeout
and retransmission guidelines as well as configuration variables for
DHCP protocol operations are discussed in Section 8. Appendix B
summarizes related work in IPv6 that will provide helpful context;
it is not part of this specification, but included for informational
purposes. Appendix C discusses the differences between DHCPv4 and
DHCPv6.
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2. Terminology and Definitions
Relevant terminology from the IPv6 Protocol [6], IPv6 Addressing
Architecture [8], and IPv6 Stateless Address Autoconfiguration [17]
is given, followed by DHCPv6 terminology.
2.1. IPv6 Terminology
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 routing 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. DHCPv6 Terminology
Agent Address
The address of a neighboring DHCP Agent on the same
link as the DHCP client.
binding A binding (or, client binding) containing the data
which a DHCP server maintains for each of its clients
(see Section 6).
resource-server association
An association between a resource and a DHCP server,
maintained by the client which received that resource
from that DHCP server.
configuration parameter
Any parameter that can be used by a node to configure
its network subsystem and enable communication on a
link or internetwork.
DHCP agent (or agent)
Either a DHCP server or a DHCP relay.
DHCP client (or client)
A node that initiates requests on a link to obtain
configuration parameters.
DHCP relay (or relay)
A node that acts as an intermediary to deliver DHCP
messages between clients and servers.
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DHCP server (or server)
A server is a node that responds to requests from
clients
transaction-ID
A monotonically increasing unsigned integer used to
match a response to a pending message.
2.3. Specification Language
In this document, the words MUST, MUST NOT, SHOULD, SHOULD NOT, and
MAY are used to signify the requirements of the specification, in
accordance with RFC 2119 [2].
2.4. Error Values
This specification document uses symbolic names for the errors known
to DHCP clients and servers, as used for instance in the status field
of the DHCP Reply message (see section 4.4). The symbolic names have
the actual values listed below:
Error Name ErrorID
================================
PoorlyFormed 18
Unavail 19
NoBinding 20
InvalidSource 21
NoServer 23
ICMPError 64
3. Protocol Design Model
This section is provided for implementors to understand the DHCPv6
protocol design model from an architectural perspective. Goals and
conceptual models are presented in this section.
3.1. Design Goals
The following list gives general design goals for this DHCP
specification.
- 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
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to enforce local policies concerning allocation and access to
local resources where desired.
- DHCP MUST NOT require manual configuration of DHCP clients,
except as dictated by security requirements. Each node should be
able to obtain appropriate local configuration parameters without
user intervention.
- DHCP MUST NOT require a server on each link. To allow for scale
and economy, DHCP MUST work across DHCP relays.
- In some installations, clients on certain subnets can be served
by more than one DHCP server, improving reliability and/or
performance. Therefore, a DHCP client MUST be prepared to
receive multiple (possibly different) responses to a DHCP Solicit
message.
- DHCP MUST coexist with statically configured, non-participating
nodes and with existing network protocol implementations.
- DHCPv6 MUST be compatible with IPv6 Stateless Address
Autoconfiguration [17].
- A DHCPv6 Client implementation MAY be started in the absence of
any IPv6 routers on the client's link.
- DHCP architecture MUST support automated renumbering of IP
addresses [3].
- DHCP servers SHOULD be able to support Dynamic Updates to
DNS [20].
- DHCP servers MUST be able to support multiple IPv6 addresses for
each client.
On the other hand, a DHCP server to server protocol is NOT part of
this specification. Furthermore, 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.
3.2. DHCP Messages
Each DHCP message contains a type, which defines its function within
the protocol. Formats for the messages are found in section 4.
Processing details for these DHCP messages are specified in
Sections 5, 6, and 7. The message types are as follows:
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01 DHCP Solicit
The DHCP Solicit message is an IP multicast message sent by a
client to one or more agents, or forwarded by a relay to one or
more servers.
02 DHCP Advertise
The DHCP Advertise is an IP unicast message sent by a DHCP
Agent in response to a client's DHCP Solicit message.
03 DHCP Request
The DHCP Request is an IP unicast message sent by 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 in
response to a client's DHCP Request, or by the relay that
relayed that client's DHCP Request. Extensions [13] to the
DHCP Reply describe the resources that the server has committed
and allocated to this client, and may contain other information
for use by this client.
05 DHCP Release
The DHCP Release is an IP unicast message sent by a client to
inform the server that the client is releasing resources.
06 DHCP Reconfigure
The DHCP Reconfigure is an IP unicast or multicast message sent
by a server to inform one or more clients that the server has
new configuration information of importance to the client.
Each client is expected to initiate a new Request/Reply
transaction.
DHCP message types not defined here (msg-types 0 and 7-255) are
reserved and SHOULD be silently ignored.
3.3. Request/Response Processing Model
The request/response processing for DHCPv6 is transaction based and
uses a set of best-effort messages to complete the transaction.
To find a server, a client sends a DHCP Solicit from the interface
which it wishes to configure. The client then awaits a DHCP
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Advertise message containing an IP address of a DHCP server.
Transactions are started by a client with a DHCP Request, which may
be issued after the client knows the server's address. The server
then unicasts a DHCP Reply, possibly via a relay. At this point in
the flow all data has been transmitted and is presumed to have been
received. To provide a method of recovery if either the client or
server does not receive its messages, the client retransmits each
DHCP Request message until it elicits the corresponding DHCP Reply,
or until it can be reasonably certain that the desired DHCP server
is unavailable, or it determines that it does not want a response
(i.e., it MAY abort the transaction). The timeout and retransmission
guidelines and configuration variables are discussed in Section 8.
DHCP uses UDP [15] to communicate between clients and servers. UDP
is not reliable, but the DHCP retransmission scheme just described
provides reliability between clients and servers. The following
well-known multicast addresses are used by DHCP agents and clients:
FF02:0:0:0:0:0:1:2
All DHCP Agents (servers and relays) MUST join the
link-local All-DHCP-Agents multicast group at the address
FF02:0:0:0:0:0:1:2.
FF05:0:0:0:0:0:1:3
All DHCP servers MUST join the site-local
All-DHCP-Servers multicast group at the address
FF05:0:0:0:0:0:1:3.
FF05:0:0:0:0:0:1:4
All DHCP relays MUST join the site-local All-DHCP-Relays
multicast group at the address FF05:0:0:0:0:0:1:4.
A DHCP server or agent MUST transmit all messages to DHCP clients on
UDP port 546. A DHCP client MUST transmit all messages to a DHCP
agent over UDP using port 547. A DHCP server MUST transmit all
messages to DHCP relays over UDP on port 546. The source port for
DHCP messages is arbitrary.
For the proper operation of the DHCP protocol to operate within a
network where one or more firewalls [4] are used, DHCP transactions
sent to the IP addresses of DHCP servers at UDP destination ports 546
and 547 will need to be permitted.
4. DHCP Message Formats and Field Definitions
All reserved fields in a message MUST be transmitted as zeroes and
ignored by the receiver of the message.
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4.1. DHCP Solicit Message Format
A client transmits a DHCP Solicit message over the interface to be
configured, to obtain one or more 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| reserved | prefix-size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| relay-address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 1
C If set, the client requests that all servers receiving
the message deallocate the resources associated with
the client.
prefix-size A nonzero prefix-size is the number of leftmost bits
of the agent's IPv6 address which make up the routing
prefix.
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 nonzero, the IP address of the interface on which
the relay received the client's DHCP Solicit message
A client SHOULD send a DHCP Solicit message to the All-DHCP-Agents
multicast group (see section 3.3), setting the relay-address to
zero. Any relay receiving the solicitation MUST forward it to the
All-DHCP-Servers multicast group. In that case, the relay MUST copy
a non-link-local address of its interface from which the client's
solicitation was received into the relay-address field. Servers
receiving the solicitation then send their advertisements to the
prospective client.
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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 server 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 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 | preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent-address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server-address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 2
S If set, the server-address is present.
reserved 0
preference An octet (unsigned) indicating a server's willingness
to provide service to the client (see Section 5.3).
client's link-local address
The IP link-local address of the client interface
from which the client issued the DHCP Request message
agent-address
The IP address of a DHCP Agent interface on the same
link as the client.
server-address
If present, the IP address of the DHCP server
extensions See [13].
Suppose that a server on the same link as a client issues the
DHCP Advertise in response to a DHCP Solicit message sent to the
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All-DHCP-Agents multicast address. Then the agent-address will be an
IP address of one of the server's interfaces on the same link as the
client, and the `S' bit will be set to zero, indicating the absence
of the server-address in the DHCP Advertise message. See section 5.3
for information about how clients handle the preference field.
The server MUST copy the client's link-local address into the
advertisement which is sent in response to a DHCP Solicit. Both
server-address (if present) and agent-address of the DHCP Advertise
message MUST have sufficient scope to be reachable by the client.
Moreover, the agent-address of any DHCP Advertise message sent by a
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. The DHCPv6
protocol design does not apply to situations where the client is
unable to route messages to a server not on the same link.
4.3. DHCP Request Message Format
In order to request configuration parameters from a server, a client
sends a DHCP Request message, and MAY append extensions [13]. If
the client does not know any server address, it MUST first obtain
one by multicasting a DHCP Solicit message (see Section 4.1).
Typically, when a client reboots, it does not have a valid IP address
of sufficient scope for the server to communicate with the client.
In such cases, the client MUST NOT send the message directly to
the server because the server could not return any response to the
client; the client MUST send the message to the local relay and
insert the relay-address as the agent-address in the message header.
Otherwise, the client MAY omit the server-address in the DHCP Request
message; in this case, the client MUST clear the S-bit and send the
message directly to the server, using the server's address as the IP
destination address in the IP header.
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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|S|R| rsvd | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent-address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server-address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 3
C If set, the client requests the server to remove all
resources associated with the client binding, except
those resources provided as extensions.
S If set, the server-address is present
R If set, the client has rebooted and requests that all
of its previous transaction-IDs be expunged and made
available for re-use.
rsvd 0
transaction-ID
A monotonically increasing unsigned integer used to
identify this Request, and copied into the Reply.
client's link-local address
The IP link-local address of the client interface from
which the client issued the DHCP Request message
agent-address
The IP address of an agent's interface, copied from a
DHCP Advertisement message.
server-address
If present, the IP address of the server which should
receive the client's DHCP Request message.
extensions See [13].
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When the client sets the `C' bit and adds extensions, the server
is expected to deallocate all other resources not listed in the
extension. The resources explicitly requested in extensions to the
Request message SHOULD be reallocated by the server to the client,
assuming the client is still authorized to receive them.
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| status | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 4
L If set, the client's link-local address is present
status One of the following decimal 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
64 Server unreachable (ICMP error)
transaction-ID
A monotonically increasing unsigned integer used to
identify this Reply, and copied from the client's
Request.
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client's link-local address
If present, the IP address of the client interface
which issued the corresponding DHCP Request message.
extensions
See [13].
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.
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. If parameters are specified in the extensions, only
those parameters are released. The DHCP server acknowledges the
Release message by sending a DHCP Reply (Sections 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent-address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client-address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 5
D If the `D' 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 unsigned integer used to
identify this Release, and copied into the Reply.
client's link-local address
The IP link-local address of the client interface from
which the the client issued the DHCP Release message
agent-address
The IP address of the agent interface to which the
client issued a previous DHCP Request message
client-address
The IP address of the client interface from which
the the client issued the DHCP Release 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 [13]
It is an error (status code ``InvalidSource'' (see Section 2.4)) 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-address
field of the DHCP Release message header.
4.6. DHCP Reconfigure Message Format
DHCP Reconfigure messages can only be sent to clients which have
established an IP address which routes to the link at which they are
reachable, hence the DHCP Reconfigure message is sent without the
assistance of any DHCP relay. When a server sends a Reconfigure
message, the receivers are assumed to have a valid IP address with
sufficient scope to be accessible by the server. Only the parameters
which are specified in the extensions to the Reconfigure message need
be requested again by the client. A Reconfigure message can either
be unicast or multicast by the server.
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 |N| reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server-address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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msg-type 6
N The `N' flag indicates that the client should not
expect a DHCP Reply in response to the DHCP Request
it sends as a result of the DHCP Reconfigure message.
reserved 0
transaction-ID
A monotonically increasing unsigned integer 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 [13]
5. DHCP Client Considerations
A node which is not a DHCP agent MUST silently discard any DHCP
Solicit, DHCP Request, or DHCP Release message it receives.
5.1. Verifying Resource Allocations After Restarts
A client MAY retain its configured parameters and resources across
client system reboots and program restarts. However, in these
circumstances the 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 after
REQUEST_MSG_MIN_RETRANS (see section 8), the client may still
use any resources whose lifetimes have not yet expired. In such
cases, however, the client MUST begin to search for another server
by multicasting a DHCP Solicit message with the `C' bit set (see
section 5.2). The client SHOULD log a DHCP System Error.
This also handles the case wherein a client restarts on a new
network, where its IP address is no longer valid. In this situation,
when the client receives a new IP address and the old IP address
is no longer needed, the client MUST release its old IP address by
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issuing a DHCP Release message with the appropriate extension if it
can communicate with its previous server.
5.2. Sending DHCP Solicit Messages
A client MUST have the address of a server to send a Request message.
The client SHOULD locate a DHCP server by multicasting a DHCP Solicit
message to the All-DHCP-Agents link-local multicast address (see
Section 3.3), 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 if solicitations sent from a client's link-local address are
to be handled.
When sending a DHCP Solicit message, a client MUST set the Relay
Address field to 16 octets of zeros, and zero the prefix-size field.
If a 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 client requests that all
the DHCP servers that receive the solicitation should clean up their
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 [11] message, by at least some random amount of time
between MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY (see section 8).
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, plus
a small random delay between MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY
seconds.
5.3. Receiving DHCP Advertise Messages
After a client has received a DHCP Advertise message, it has the
address of a server for subsequent DHCP Request messages. If the `S'
bit is zero, the DHCP Advertise message was transmitted by a server
on the same link as the client, and the client uses the agent-address
as the server's address; otherwise, the server's IP address is
located in the server-address field. If the server-address is a
multicast address, the advertisement MUST be silently discarded.
A server MAY append extensions to its advertisements; this might
allow the client to select the configuration that best meets its
needs from among several prospective servers.
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Unless a DHCP Advertisement is received with a preference equal
to 255 (see Section 4.2), the client MUST wait CLIENT_ADV_WAIT
seconds after issuing the DHCP Solicit message in order to receive
the Advertisement with the highest preference. After waiting for
that period of time, a client MUST select the highest preference
server as the target of its DHCP request. If a client receives an
advertisement with a preference of 255, it does not have to wait for
any more advertisements.
If a client sends a DHCP Request to a highly preferred server but
fails to receive a DHCP reply from that server after following the
retransmission algorithm in section 8, the client SHOULD then try to
send a DHCP Request to a less preferred server.
A client is free to cache the result of any DHCP Advertisement it
hears. This is purely a potential performance enhancement, because
the results might change over time. A client may not get a DHCP
Reply if it uses a cached server-address, and in that case SHOULD
multicast another DHCP Solicit message.
5.4. Sending DHCP Request Messages
A client obtains configuration information from a server by sending
a DHCP Request message. The client MUST know the server's address
before sending the Request message, and the 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 packets were lost due to
collisions, etc.
If the DHCP server is off-link, and the client has no valid IP
address of sufficient scope, then the client MUST include the
server-address and set the `S' bit in the DHCP Request message. In
this case, the IP destination address in the IP header will be a DHCP
relay address.
Otherwise, if the client has a valid IP address of sufficient scope
and knows the IP address of a candidate server, it MUST send the
Request message directly to the server without requiring the services
of the local DHCP relay.
If a client wishes to instruct a server to deallocate all unknown
previous resources, configuration information, and bindings
associated with its agent-address and link-local address, it
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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. The `C' bit allows better
reclamation of available resources when a client lost records for
its resource-server associations and might not otherwise be able to
release the associated resources.
When a client reboots and loses its previous state, the server
should no longer keep track of the transaction IDs associated with
that previous state. In order to inform the server that the client
no longer wishes any association to be maintained between used
transaction-IDs and previous transactions, the client SHOULD set the
`R' bit in its DHCP Request.
In any case, after choosing a transaction-ID which is numerically
greater than its last recorded 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 client sends the DHCP
Request to the appropriate 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 5.5) with
the same transaction-ID as a pending DHCP Request message within
REPLY_MSG_TIMEOUT (see section 8) 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 (after following
those rules) the client has not received a Reply message, it SHOULD
start over again by multicasting a new DHCP Solicit message to find a
different server.
If the client receives an ICMP error message in response to such a
DHCP Request, it likewise SHOULD start over again by multicasting a
new DHCP Solicit message, to find a different server.
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If the client transmits a DHCP Request in response to a DHCP
Reconfigure message (see Section 5.7) with the `N' bit not set,
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. When the `N' bit is
set, a DHCP Request sent in response to a DHCP Reconfigure message
will not elicit a DHCP Reply message from the server.
5.5. 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 [13], 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 UDP
Length field of the Reply. Some extensions in the Reply may have
status codes, which indicate to the client the reason for failure
when the server was unable to honor the request. If the server is
unable to honor the request in an extension included by the client,
that extension may simply be omitted from the Reply. The server MAY
also provide the client with configuration parameters the client did
not specifically request.
Some configuration information extracted from the extensions to the
DHCP Reply message MUST remain associated with the 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 [13]. These "resource-server" associations are
used when sending DHCP Release messages.
5.6. Sending DHCP Release Messages
If a client wishes to ask the server to release all information and
resources relevant to the client, the client SHOULD send a DHCP
Release message without any extensions; this is preferable to sending
a DHCP Request message with the `C' bit set and no extensions. If a
client wishes to retain some of its network configuration parameters,
but determines that others are no longer needed, it SHOULD enable
the server to release allocated resources which are no longer in
use by sending a DHCP Release message to the server, and including
extensions to identify the unneeded items. The client consults its
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list of resource-server associations in order to determine which
server should receive the desired Release message.
Suppose a client wishes to release resources which were granted to
it at another IP address. Further, 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,
and which has sufficient scope to be reachable from the server. In
that case, and only then, the client MUST set the `D' bit in the DHCP
Release message (see section 4.5). This instructs the server to
send the DHCP Reply directly back to the client at the latter valid
IP address, instead of performing the default processing of sending
the DHCP Reply back through the agent-address included in the DHCP
Release.
5.7. Receiving DHCP Reconfigure Messages
Each client implementation MUST support listening at UDP port 546
to receive possible DHCP Reconfigure messages; in cases where the
client knows that no Reconfigure message will ever be issued, the
client MAY be configured to avoid executing this supported feature.
In some cases, the IP address at which the client listens will be a
multicast address sent to the client by the 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 server has deallocated the client's
IP address and/or other resources allocated to the client. See
discussion in 6.5. The client MAY receive a prefix update for one
or more of their addresses and then MUST use that prefix for those
addresses.
If a 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.
Reconfigure messages can be retransmitted by the server with the
same transaction-ID. When a client receives such a retransmitted
Reconfigure message within XID_TIMEOUT of the last received
Reconfigure message with the same transaction-ID, the client MUST
reformulate exactly the same DHCP Request message and retransmit the
request message to the server again. In this way, the server can
make use of the retransmission algorithm to ensure that all affected
clients have received the Reconfigure message.
For each Extension which is present in the Reconfigure message, the
client MUST append a matching Extension to its Request message, which
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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. If the `N' bit is
not set, processing from then on is the same as specified above in
Section 5.4.
Resources held by the client which are not identified by Extensions
in the server's Reconfigure message are not affected.
If a client has recently sent a DHCP Request to the server from which
it subsequently received the DHCP Reconfigure message, the client
SHOULD silently discard the Reconfigure message until the server
sends the DHCP Reply message with the same transaction-ID as the
client's DHCP Request message.
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_MMSG_MIN_RESP and RECONF_MMSG_MAX_RESP seconds (see
section 8). This will minimize the likelihood that the server will
be flooded with DHCP Request messages.
5.8. Interaction with Stateless Address Autoconfiguration
Please refer to the Stateless Address Autoconfiguration
Protocol specification [17] and its follow-on, Stateless Address
Autoconfiguration version 2 [18] for details regarding the actions
taken by clients upon receiving Router Advertisements with changing
values for the `M' and `O' bits.
6. DHCP Server Considerations
A node which is not a client or relay 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 prefix>, since the link-local
address is guaranteed to be unique [17] on the link identified
by the agent-address and prefix. An implementation MUST support
bindings consisting of at least a client's link-local address,
agent-address prefix, preferred lifetime and valid lifetime [17] for
each client address. A server MAY, at the discretion of the network
administrator, be configured so that client bindings are identified
by the client's MAC address, without need to use the additional
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information supplied by the agent-address. A client binding may be
used to store any other information, resources, and configuration
data which will be associated with the client. A server MUST retain
its clients' bindings across server reboots, and, whenever possible,
a client should be assigned the same configuration parameters
despite server system reboots and DHCP server program restarts. A
server MUST support fixed or permanent allocation of configuration
parameters to specific clients.
In addition to the client binding a server must maintain an
XID_TIMEOUT binding cache to determine if a previous transaction-ID
is being retransmitted by a client. An implementation of an
XID_TIMEOUT binding cache MUST support at least a tuple consisting of
the client's link-local address, agent-address prefix, IPv6 address,
and XID_TIMEOUT value when the cache entry can be deleted (see
Section 8).
6.1. Receiving DHCP Solicit Messages
If the DHCP Solicit message was received at the All-DHCP-Servers
multicast address, the server MUST check to make sure that the
relay-address is present, and is not a link-local address. If the
relay-address is not present, or if it is a link-local address,
the server MUST silently discard the packet. 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.
When the `C' bit is set in the solicitation, the server deallocates
all resources that match its link-local address. The server MUST
take the relay-address and use the prefix-size to locate the client
binding.
As an optimization, a server processing a Solicit message from relays
MAY check the prefix of the IP source address in the IP header to
determine whether the server has received the Solicit from multiple
relays on the same link. The prefix-size field in the solicitation
enables the server to ascertain when two relay addresses belong to
the same link.
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. When the
solicitation is received at the All-DHCP-Servers multicast address,
the server SHOULD delay the transmission of its advertisement
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for a random amount of time between SERVER_MIN_ADV_DELAY and
SERVER_MAX_ADV_DELAY (see section 8).
If the relay-address is nonzero, the server MUST copy it into the
agent-address field of the advertisement message, and send the
advertisement to the relay-address. Otherwise, the server MUST
send the advertisement to the client's link-local address. An IP
address of the interface on which the server received the Solicit
message MUST appear in the server-address field of the corresponding
advertisement.
The server MAY append extensions to the Advertisement, in order to
offer the soliciting node the best possible information about the
available services and resources.
6.3. DHCP Request and Reply Message Processing
DHCP server MUST check to ensure that the client's link-local address
field of the Request message contains a link-local address. If not,
the message MUST be silently discarded. If the `S' bit is set, the
server MUST check that the server-address matches one of its own IP
addresses. If the server-address does not match, the Request message
MUST be silently discarded.
If the received agent-address and link-local address do not
correspond to any binding known to the server, then the server
SHOULD create a new binding for the previously unknown client,
and send a DHCP Reply with any resources allocated to the new
binding. Otherwise, if the server cannot create a new binding,
it SHOULD return a DHCP Reply with a status of ``NoBinding'' (see
Section 2.4). If the client is updating its resources but the
database is temporarily unavailable, the server SHOULD return a DHCP
Reply with a status of ``Unavail'' (see Section 2.4).
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 server has no record of a message from the client
with the same transaction-ID, 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 client by constructing a DHCP Reply message and including the
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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, the server MUST send the corresponding DHCP Reply message to
the agent-address found in the Request (see section 7.2). Otherwise,
the server SHOULD send the corresponding DHCP Reply message to the
IP source address in the IP header received from the client Request
message.
6.3.1. Processing for Extensions to DHCP Request and Reply Messages
The DHCP Request may contain extensions [13], 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 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 server may accept some extensions for successful processing and
allocation, while still rejecting others, or it may reject various
extensions for different reasons, and set the status appropriately
for those extensions which return status to the client. The server
sends a single DHCP 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,
not including them in the Reply. Other extensions can be rejected
by including them in the Reply with an appropriate status indicating
failure. The server can include extensions in the reply that were
not requested by the client.
6.3.2. Client Requests to Deallocate Unknown Resources
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 server MUST deallocate all resources associated with the client
upon reception of a DHCP Request with the `C' bit set, except for
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those which the server is willing to reallocate in 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 has 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 (see Section 2.1). 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 server formulates a DHCP
Reply message that will be sent back to the releasing client. When
the `D' flag is set, the server MUST send the DHCP Reply back to
the client using the client-address field of the Release message.
Otherwise, if the `D' bit is not set, the server MUST send its DHCP
Reply message (with the `L' bit set) to the agent-address in the
Release message, so that the relay agent can subsequently forward the
Reply back to the releasing client at the client's link-local address
indicated in the Reply 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, indicating the error by setting the status code
to ``NoBinding'' (see Section 2.4).
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.
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 server formulates a DHCP Reply message,
copying the transaction-ID from the DHCP Release message. For
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 status MUST be returned by the
server. If the Release message contains no extensions, the server
does not include any extensions in the corresponding DHCP Reply
message to the client.
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6.5. Sending DHCP Reconfigure Messages
If a server needs to change the configuration associated with any of
its clients, it constructs a DHCP Reconfigure message and sends it to
each such client. The Reconfigure MAY be sent to a multicast address
chosen by the server and previously sent to each such client in an
extension to a previous DHCP Reply message.
It may happen that a client does not send a DHCP Request message
after the DHCP Reconfigure message has been issued and retransmitted
RECONF_MSG_MIN_RETRANS times, according to the algorithm specified
in Section 8. This can happen when the client is not listening for
the Reconfigure message, possibly because the client is a mobile
node disconnected from the network, or because the client node
has sustained a power outage or operating system crash. In such
cases, the server SHOULD reserve any resources issued to the client
until the client responds at some future time, until the resource
allocation times out (see section 6.6), or until administrative
intervention causes the resources to be manually returned to use.
The server SHOULD also log a DHCP System Error.
If the server gets another DHCP Request from a client, with a
transaction-ID which does not match that of the recently transmitted
reconfigure message, the server SHOULD send a DHCP Reply to
the client, and wait for RECONF_MSG_RETRANS_INTERVAL, before
retransmitting the DHCP Reconfigure again.
6.6. Client-Resource timeouts
Some resources (for instance, a client's IP address) may only be
allocated to a client for a particular length of time (for instance,
the valid lifetime of an IP address). If the client does not renew
the resource allocation for such a resource, the server MAY make the
resource available for allocation to another client. However, under
administrative control, the server MAY reserve any resources issued
to the client until the client responds at some future time.
7. DHCP Relay Considerations
The DHCP protocol is constructed so that a relay does not have
to maintain any state in order to mediate DHCP client/server
interactions.
The DHCP relay enables clients and servers to carry out DHCP protocol
transactions. DHCP Solicit messages are issued by the relay when
initiated by prospective clients. By default, the relay locates
servers by use of multicasting solicitations to the All-DHCP-Servers
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multicast group, but relays SHOULD allow this behavior to be
configurable. The relay MUST be able to determine which of its
interfaces received the client's solicitation message.
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 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,
- copying a non-link-local address of its interface from which the
solicitation was received from the client into the relay-address
field, and
- setting the prefix-size field appropriately,
- by default, setting the hop-count field in the IP header of
the solicitation to the value DEFAULT_SOLICIT_HOPCOUNT (see
section 8).
- setting the IP source address to be a site-local or global-scope
address belonging to the relay's interface on which the client's
original solicitation was received,
- finally, sending the resulting message to one or more servers.
By default, the relay sends solicitations to the All-DHCP-Servers
multicast address, FF05:0:0:0:0:0:1:3. However, the relay MAY be
configured with an alternate server address, or the FQDN of a server.
Methods for automatically updating such alternately configured server
addresses are not specified in this document.
When the relay receives a DHCP advertisement, it relays the
advertisement to the client at the client's 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 SHOULD check that
the IP source address in the IP header is 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 with the interface from
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which the DHCP Request message was received. If any of these checks
fail, the relay MUST silently discard the Request message.
The relay MUST check whether the `S' bit is set in the message
header. If not, the packet is discarded, and the relay SHOULD
return a DHCP Reply message to the address contained in the client's
link-local address field of the Request message, with status
``PoorlyFormed'' (see Section 2.4).
If the received request message is acceptable, the relay then
transmits the DHCP Request message to the address of the server found
in the server-address field of the received DHCP Request message.
All of the fields of DHCP Request message 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
packet received from the client. All relays MUST send DHCP Request
messages using the source IP address from the interface where the
DHCP request was received. 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 [5]), with status
``ICMPError'' (see Section 2.4), along with the DHCP Relay ICMP Error
extension [13].
7.3. DHCP Reply Message Processing
When the relay receives a DHCP Reply, it MUST check that the message
has the `L' bit set. It MUST check that the client's link-local
address field contains a link-local address. If either check fails,
the packet MUST be silently discarded. If both checks are satisfied,
the relay MUST send a DHCP Reply message to the link-local address
listed in the received Reply message. Only the fields in the IP
header will differ from the packet received from the server, not the
payload.
8. Retransmission and Configuration Variables
When a 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 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. All the
actions specified for DHCP Request in this section hold also for DHCP
Release messages sent by the client.
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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 a reasonable number
of times to try to elicit the Request message from the client.
If no corresponding DHCP Request is received by the server after
REQUEST_MSG_MIN_RETRANS retransmissions, the server MAY erase or
deallocate information as needed from the client's binding, but see
section 6.5.
Retransmissions occur using the following configuration variables
for a DHCP implementation. Note that the retransmission algorithm
does not use exponential backoff techniques. These configuration
variables MUST be configurable by a client or server:
CLIENT_ADV_WAIT
The minimum amount of time a client waits to receive DHCP
Advertisements after transmitting a DHCP Solicit to the
All-DHCP Agents multicast address (see section 5.3).
Default: 2 seconds
DEFAULT_SOLICIT_HOPCOUNT
The default hop-count used in the IP header by DHCP relays when
sending DHCP Solicit messages on behalf of a client.
Default: 4
SERVER_MIN_ADV_DELAY
The minimum amount of time a server waits to transmit a
DHCP Advertisement after receiving a DHCP Solicit at the
All-DHCP-Servers or All-DHCP-Agents multicast address.
Default: 100 milliseconds
SERVER_MAX_ADV_DELAY
The maximum amount of time a server waits to transmit a DHCP
Advertisement after receiving a DHCP Solicit at the All-DHCP
Agents multicast address.
Default: 1 second
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REPLY_MSG_TIMEOUT
The time in seconds that a client waits to receive a server's
DHCP Reply before retransmitting a DHCP Request.
Default: 2 seconds.
REQUEST_MSG_MIN_RETRANS
The minimum number of DHCP Request transmissions that a client
should retransmit, before aborting the request.
Default: 10 retransmissions.
RECONF_MSG_MIN_RETRANS
The minimum number of DHCP Reconfigure messages that a
server should retransmit, before assuming the the client is
unavailable.
Default: 10 retransmissions.
RECONF_MSG_RETRANS_INTERVAL
The least time in seconds that a server waits for a
client's DHCP Request before each retransmission of the DHCP
Reconfigure.
Default: 12 seconds.
RECONF_MMSG_MIN_RESP
The minimum amount of time before a client can respond to a
DHCP Reconfigure message sent to a multicast address.
Default: 2 seconds.
RECONF_MMSG_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 minimum amount of time a prospective client is required to
wait, after determining from a Router Advertisement message
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that the client should perform stateful address configuration,
before sending a DHCP Solicit to a server.
Default: 1 second
MAX_SOLICIT_DELAY
The maximum amount of time a prospective client is required to
wait, after determining from a Router Advertisement message
that the client should perform stateful address configuration,
before sending a DHCP Solicit to a 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
9. Security Considerations
Clients and servers often have to authenticate the messages they
exchange. For instance, a 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 quite often essential for a client to
be certain that the configuration parameters and addresses it has
received were sent to it by an authoritative server. Similarly, a
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. Again, a client might
wish to only accept DHCP Reconfigure messages that are certain to
have originated from a server with authority to issue them.
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 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 server.
The DHCP Client-Server Authentication Extension [13] is intended to
be used in these circumstances.
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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.
10. 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.
11. IANA Considerations
This document defines message types 1-7 to be received by UDP at port
numbers 546 and 547. Additional message types may be defined in the
future.
Section 3.3 specifies the use of several multicast groups, with
multicast addresses FF02:0:0:0:0:0:1:2, FF05:0:0:0:0:0:1:3, and
FF05:0:0:0:0:0:1:4.
This document also defines several status codes that are to be
returned with the DHCP Reply message (see section 4.4). The nonzero
values for these status codes which are currently specified are shown
in section 2.4.
There is a DHCPv6 extension [13] which allows clients and servers to
exchange values for some of the timing and retransmission parameters
defines in section 8. Adding new parameters in the future would
require extending the values by which the parameters are indicated in
the DHCP extension. 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 [12].
12. Acknowledgements
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
Last Call process. Thanks also for the consistent input, ideas, and
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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.
A. Changes for this revision
- Changed the preference field to be 8 bits and always present
- Eliminated the `P' bit from the DHCP Advertise message
- Noted that relays SHOULD be able to determine which interface
receives a message
Should this be here?
B. Related Protocol Specifications
Related work in IPv6 that would best serve an implementor to study
is the IPv6 Specification [6], the IPv6 Addressing Architecture [8],
IPv6 Stateless Address Autoconfiguration [17], IPv6 Neighbor
Discovery Processing [11], and Dynamic Updates to DNS [20]. 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 1500 octets
or greater (in IPv4 the requirement is 68 octets). This means that
a UDP packet 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 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 [10]
to determine the size of the packet that will traverse a network
path. It is implementation dependent how this is accomplished in
DHCP. 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 [8] defines the
address scope that can be used in an IPv6 implementation, and the
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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 DHCP Solicit and locate a server or relay.
IPv6 Stateless Address Autoconfiguration [17] (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
stateful autoconfiguration. Compatibility with Addrconf is a design
requirement of DHCP (see Section 3.1).
IPv6 Neighbor Discovery [11] is the node discovery protocol in IPv6
which replaces and enhances functions of ARP [14]. To understand
IPv6 and Addrconf it is strongly recommended that implementors
understand IPv6 Neighbor Discovery.
Dynamic Updates to DNS [20] 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 [9].
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 [7, 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
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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 is 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 Many DHCPv4 options are unnecessary now because the configuration
parameters are either obtained through IPv6 Neighbor Discovery or
the Service Location protocol [19].
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 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 remote server
through an on-link relay-agent.
o Servers are discovered by a client solicit, followed by a server
or relay-agent advertisement.
o The client will know if the server is on-link or off-link.
o The on-link relay-agent locates remote 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.
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o Clients can issue multiple, unrelated DHCP 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 extension headers.
o Clients MUST listen to their UDP port for the new Reconfigure
message from servers.
o New extensions 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.
o Addresses allocated with too-long lifetimes can be reclaimed
using the Reconfigure message.
o Integration between stateless and stateful address
autoconfiguration.
o Enabling relay-agents to locate remote servers for a link.
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D. Full Copyright Statement
Copyright (C) The Internet Society (1998). 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. RFC 2132, March 1997.
[2] S. Bradner. Key Words for Use in RFCs to Indicate Requirement
Levels. RFC 2119, March 1997.
[3] S. Bradner and A. Mankin. The Recommendation for the IP Next
Generation Protocol. RFC 1752, January 1995.
[4] William R. Cheswick and Steven Bellovin. Firewalls and Internet
Security. Addison-Wesley, Reading, Massachusetts, 1994. (ISBN:
0-201-63357-4).
[5] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6). RFC 1885,
December 1995.
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[6] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883, December 1995.
[7] R. Droms. Dynamic Host Configuration Protocol. RFC 2131, March
1997.
[8] R. Hinden and S. Deering. IP Version 6 Addressing Architecture.
RFC 1884, December 1995.
[9] Stephen Kent and Randall Atkinson. IP Authentication Header.
draft-ietf-ipsec-auth-header-06.txt, May 1998. (work in
progress).
[10] J. McCann, S. Deering, and J. Mogul. Path MTU Discovery for IP
version 6. RFC 1981, August 1996.
[11] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for
IP version 6 (IPv6). RFC 1970, August 1996.
[12] Thomas Narten and Harald Tveit Alvestrand. Guidelines
for Writing an IANA Considerations Section in RFCs.
draft-iesg-iana-considerations-04.txt, May 1998. (work in
progress).
[13] C. Perkins. Extensions for the Dynamic Host Configuration
Protocol for IPv6. draft-ietf-dhc-dhcpv6ext-11.txt, June 1998.
(work in progress).
[14] 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.
[15] J. B. Postel. User Datagram Protocol. RFC 768, August 1980.
[16] J. B. Postel, Editor. Internet Protocol. RFC 791, September
1981.
[17] S. Thomson and T. Narten. IPv6 Stateless Address
Autoconfiguration. RFC 1971, August 1996.
[18] S. Thomson and T. Narten. IPv6 Address Autoconfiguration.
draft-ietf-ipngwg-addrconf-v2-00.txt, November 1997. (work in
progress).
[19] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. Service
Location Protocol. RFC 2165, July 1997.
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[20] P. Vixie, S. Thomson, Y. Rekhter, and J. Bound. Dynamic Updates
in the Domain Name System (DNS). RFC 2136, April 1997.
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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
Compaq Computer Corporation Technology Development
110 Spitbrook Road, ZKO3-3/U14 Sun Microsystems, Inc.
Nashua, NH 03062 901 San Antonio Rd.
Palo Alto, CA 94303
Phone: +1-603-884-0400 +1-650-786-6464
Fax: +1-650-786-6445
E-mail: bound@zk3.dec.com charles.perkins@sun.com
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