Dynamic Host Configuration Working Group Rich Woundy
INTERNET DRAFT Comcast Cable
Kim Kinnear
Cisco Systems
March 2003
Expires September 2003
DHCP Lease Query
<draft-ietf-dhc-leasequery-05.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
A DHCP server contains considerable authoritative information
concerning the IP addresses it has leased to DHCP clients. Other
processes and devices, many that already send and receive DHCP format
packets, sometimes need to access this information. The leasequery
protocol is designed to give these processes and devices a
lightweight way to access information that may be critical to their
operation.
Woundy & Kinnear Expires September 2003 [Page 1]
Internet Draft DHCP Lease Query March 2003
1. Introduction
A DHCP server contains considerable authoritative information con-
cerning the IP addresses it has leased to DHCP clients. Other
processes and devices, many that already send and receive DHCP format
packets, sometimes need to access this information. The leasequery
protocol is designed to give these processes and devices a light-
weight way to access information that may be critical to their opera-
tion.
For example, access concentrators that act as DHCP relay agents some-
times derive information important to their operation by extracting
data out of the DHCP packets they forward, a process known as "glean-
ing". Unfortunately, the typical access concentrator loses its
gleaned information when the access concentrator is rebooted or is
replaced. This memo proposes that when gleaned DHCP information is
not available, the access concentrator/relay agent can obtain the
location information directly from the DHCP server(s) using the new
lightweight DHCPLEASEQUERY message.
To continue this example in more depth, in many broadband access net-
works, the access concentrator needs to associate an IP address lease
to the correct endpoint location, which includes knowledge of the
host hardware address, the port or virtual circuit that leads to the
host, and/or the hardware address of the intervening subscriber
modem. This is particularly important when one or more IP subnets
are shared among many ports, circuits, and modems. Representative
cable and DSL environments are depicted in Figures 1 and 2 below.
+--------+ +---------------+
| DHCP | | DOCSIS CMTS |
| Server |-...-| or DVB INA |-------------------
+--------+ | (Relay Agent) | | |
+---------------+ +------+ +------+
|Modem1| |Modem2|
+------+ +------+
| | |
+-----+ +-----+ +-----+
|Host1| |Host2| |Host3|
+-----+ +-----+ +-----+
Figure 1: Cable Environment for DHCPLEASEQUERY
Woundy & Kinnear Expires September 2003 [Page 2]
Internet Draft DHCP Lease Query March 2003
+--------+ +---------------+
| DHCP | | DSL Access | +-------+
| Server |-...-| Concentrator |-...-| DSLAM |
+--------+ | (Relay Agent) | +-------+
+---------------+ | |
+------+ +------+
|Modem1| |Modem2|
+------+ +------+
| | |
+-----+ +-----+ +-----+
|Host1| |Host2| |Host3|
+-----+ +-----+ +-----+
Figure 2: DSL Environment for DHCPLEASEQUERY
Knowledge of this location information can benefit the access concen-
trator in several ways:
1. The access concentrator can forward traffic to the access net-
work using the correct access network port, down the correct
virtual circuit, through the correct modem, to the correct
hardware address.
2. The access concentrator can perform IP source address verifica-
tion of datagrams received from the access network. The verif-
ication may be based on the datagram source hardware address,
the incoming access network port, the incoming virtual circuit,
and/or the transmitting modem.
3. The access concentrator can encrypt datagrams which can only be
decrypted by the correct modem, using mechanisms such as [BPI]
or [BPI+].
The access concentrator in this example obtains the location informa-
tion primarily from "gleaning" information from DHCP server responses
sent through the relay agent. When location information is not
available from "gleaning", e.g. due to reboot, the access concentra-
tor can query the DHCP server(s) for location information using the
DHCPLEASEQUERY message defined in this document.
The DHCPLEASEQUERY message is a new DHCP message type transmitted
from a DHCP relay agent to a DHCP server. A DHCPLEASEQUERY-aware
relay agent sends the DHCPLEASEQUERY message when it needs to know
the location of an IP endpoint. The DHCPLEASEQUERY-aware DHCP server
Woundy & Kinnear Expires September 2003 [Page 3]
Internet Draft DHCP Lease Query March 2003
replies with a DHCPLEASEKNOWN, DHCPLEASEACTIVE or DHCPLEASEUNKNOWN
message. The DHCPLEASEACTIVE response to a DHCPLEASEQUERY message
allows the relay agent to determine the IP endpoint location, and the
remaining duration of the IP address lease. The DHCPLEASEKNOWN is
similar to a DHCPLEASEACTIVE message but indicates that there is no
currently active lease on the resultant IP address. The DHCPLEASEUN-
KNOWN message indicates that the DHCP server has no knowledge of the
information specified in the query (e.g., IP address, MAC address, or
client-id option).
The DHCPLEASEQUERY message does not presuppose a particular use for
the information it returns -- it is simply designed to return infor-
mation for which the DHCP server is an authoritative source to a
client which requests that information. It is designed to make it
straightforward for processes and devices which already interpret
DHCP packets to access information from the DHCP server.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
This document uses the following terms:
o "access concentrator"
An access concentrator is a router or switch at the broadband
access provider's edge of a public broadband access network.
This document assumes that the access concentrator includes the
DHCP relay agent functionality.
o "DHCP client"
A DHCP client is an Internet host using DHCP to obtain confi-
guration parameters such as a network address.
o "DHCP relay agent"
A DHCP relay agent is a third-party agent that transfers BOOTP
and DHCP messages between clients and servers residing on dif-
ferent subnets, per [RFC 951] and [RFC 1542].
o "DHCP server"
A DHCP server is an Internet host that returns configuration
parameters to DHCP clients.
Woundy & Kinnear Expires September 2003 [Page 4]
Internet Draft DHCP Lease Query March 2003
o "downstream"
Downstream is the direction from the access concentrator towards
the broadband subscriber.
o "gleaning"
Gleaning is the extraction of location information from DHCP
messages, as the messages are forwarded by the DHCP relay agent
function.
o "location information"
Location information is information needed by the access concen-
trator to forward traffic to a broadband-accessible host. This
information includes knowledge of the host hardware address, the
port or virtual circuit that leads to the host, and/or the
hardware address of the intervening subscriber modem.
o "MAC address"
In the context of a DHCP packet, a MAC address consists of the
fields: hardware type "htype", hardware length "hlen", and
client hardware address "chaddr".
o "reservation"
At times it is convenient for an administrator to assign a fixed
IP address to a particular DHCP client. The DHCP server must be
configured with this DHCP client to IP address mapping, typi-
cally using the MAC address as the way to identify the client. A
reservation defines a mapping between a client and an IP address
but doesn't establish or record a lease binding for the IP
address. The DHCP client to IP address mapping, configured in
the DHCP server, is called a reservation for the purposes of
this document.
o "primary DHCP server"
The primary DHCP server in a DHCP Failover environment is con-
figured to provide primary service to a set of DHCP clients for
a particular set of subnet address pools.
o "secondary DHCP server"
The secondary DHCP server in a DHCP Failover environment is con-
figured to act as backup to a primary server for a particular
set of subnet address pools.
Woundy & Kinnear Expires September 2003 [Page 5]
Internet Draft DHCP Lease Query March 2003
o "stable storage"
Every DHCP server is assumed to have some form of what is called
"stable storage". Stable storage is used to hold information
concerning IP address bindings (among other things) so that this
information is not lost in the event of a server failure which
requires restart of the server.
o "upstream"
Upstream is the direction from the broadband subscriber towards
the access concentrator.
3. Background
The focus of this document is to enable processes and devices which
wish to access information from the DHCP server in a lightweight and
convenient manner. It is especially appropriate for processes and
devices which already interpret DHCP packets.
One important motivating example is that the DHCPLEASEQUERY message
allows access concentrators to send DHCPLEASEQUERY messages to DHCP
servers, to obtain location information of broadband access network
devices.
This document assumes that many access concentrators have an embedded
DHCP relay agent functionality. Typical access concentrators include
DOCSIS Cable Modem Termination Systems (CMTSs) [DOCSIS], DVB Interac-
tive Network Adapters (INAs) [EUROMODEM], and DSL Access Concentra-
tors.
The DHCPLEASEQUERY message is an optional extension to the DHCP pro-
tocol [RFC 2131].
In a DHCP Failover environment [FAILOVER], the DHCPLEASEQUERY message
can be sent to the primary or secondary DHCP server. In order for the
secondary DHCP server to answer DHCPLEASEQUERY messages, the primary
DHCP server must send "interesting options" (such as the relay-
agent-information option [RFC 3046]) in Failover BNDUPD messages to
the secondary DHCP server, as recommended by section 7.1.1 of [FAIL-
OVER].
The DHCPLEASEQUERY message is a query message only, and does not
affect the state of the IP address or the binding information associ-
ated with it.
Woundy & Kinnear Expires September 2003 [Page 6]
Internet Draft DHCP Lease Query March 2003
4. Design Goals
The goal of this document is to provide a lightweight mechanism for
processes or devices to access information contained in the DHCP
server. It is designed to allow processes and devices which already
process and interpret DHCP messages to access this information in a
rapid and lightweight manner.
Some of this information might be acquired in a different way, and
the following sections discuss some of these alternative approaches.
4.1. Broadcast ARP is Undesirable
The access concentrator can transmit a broadcast ARP Request [RFC
826], and observe the origin and contents of the ARP Reply, to recon-
struct the location information.
The ARP mechanism is undesirable for three reasons:
1. the burden on the access concentrator to transmit over multiple
access ports and virtual circuits (assuming that IP subnets
span multiple ports or virtual circuits),
2. the burden on the numerous subscriber hosts to receive and pro-
cess the broadcast, and
3. the ease by which a malicious host can misrepresent itself as
the IP endpoint.
4.2. SNMP and LDAP Client Functionality is Lacking
Access concentrator implementations typically do not have SNMP
management client interfaces nor LDAP client interfaces (although
they typically do include SNMP management agents). This is a primary
reason why this document does not leverage the proposed DHCP Server
MIB [DHCPMIB].
4.3. DHCP Relay Agent Functionality is Common
Access concentrators commonly act as DHCP relay agents. Furthermore,
many access concentrators already glean location information from
DHCP server responses, as part of the relay agent function.
The gleaning mechanism as a technique to determine the IP addresses
valid for a particular downstream link is preferred over other
Woundy & Kinnear Expires September 2003 [Page 7]
Internet Draft DHCP Lease Query March 2003
mechanisms (ARP, SNMP, LDAP) because of the lack of additional net-
work traffic, but sometimes gleaning information can be incomplete.
The access concentrator usually cannot glean information from any
DHCP unicast (i.e. non-relayed) messages due to performance reasons.
Furthermore, the DHCP-gleaned location information often does not
persist across access concentrator reboots (due to lack of stable
storage), and almost never persists across concentrator replacements.
4.4. DHCP Servers as a Reliable Source of Location Information
DHCP servers are the most reliable source of location information for
access concentrators, particularly when the location information is
dynamic and not reproducible by algorithmic means (e.g. when a sin-
gle IP subnet extends behind many broadband modems). DHCP servers
participate in all IP lease transactions (and therefore in all loca-
tion information updates) with DHCP clients, whereas access concen-
trators sometimes miss some important lease transactions.
In a DHCP Failover environment [FAILOVER], the access concentrator
can query either the primary or secondary DHCP server, so that no one
DHCP server is a single point of failure.
4.5. Minimal Additional Configuration is Required
Access concentrators can usually query the same set of DHCP servers
used for forwarding by the relay agent, thus minimizing configuration
requirements.
5. Protocol Overview
In the following discussion of the DHCPLEASEQUERY message, the client
of the message is assumed to be an access concentrator. Note that
access concentrators are not the only allowed (or required) consumers
of the information provided by the DHCPLEASEQUERY message, but they
do give reader a concrete feel for how the message might be used.
The access concentrator initiates all DHCPLEASEQUERY message conver-
sations. This document assumes that the access concentrator gleans
location information in its DHCP relay agent function. However, the
location information is usually unavailable after the reboot or
replacement of the access concentrator.
Suppose the access concentrator is a router, and further suppose that
the router receives an IP datagram to forward downstream to the pub-
lic broadband access network. If the location information for the
Woundy & Kinnear Expires September 2003 [Page 8]
Internet Draft DHCP Lease Query March 2003
downstream next hop is missing, the access concentrator sends one or
more DHCPLEASEQUERY message(s), each containing the IP address of the
downstream next hop in the "ciaddr" field.
An alternative approach is to send in a DHCPLEASEQUERY message with
the "ciaddr" field empty and the MAC address (i.e., "htype", "hlen",
and "chaddr" fields) with a valid MAC address or a Client-identifier
option (option 61) appearing in the options area. In this case, the
DHCP server SHOULD return an IP address in the "ciaddr" if it has any
record of the client described by the Client-identifier or MAC
address. In the absence of specific configuration information to the
contrary (see Section 6.4) it MUST be the IP address most recently
used by the client described by the MAC address or Client-identifier
option (or the client described by both, if both appear).
The DHCP servers that implement this protocol always send a response
to the DHCPLEASEQUERY message: either a DHCPLEASEKNOWN, DHCPLEASEAC-
TIVE or DHCPLEASEUNKNOWN (or in some cases, DHCPUNIMPLEMENTED). The
reasons why a DHCPLEASEKNOWN, DHCPLEASEACTIVE or DHCPLEASEUNKNOWN
message might be generated are explained in the specific query
regimes, below.
Servers which do not implement the DHCPLEASEQUERY message fall into
two classes. Those that simply do not know about the DHCPLEASEQUERY
message will simply not respond to it, so clients which send the
DHCPLEASEQUERY message MUST be prepared to deal with this behavior.
Servers which are aware of the DHCPLEASEQUERY message but do not
implement it SHOULD respond with a DHCPUNIMPLEMENTED message but MAY
simply not respond.
The DHCPLEASEQUERY message can support three query regimes:
o Query by IP address:
For this query, the requester supplies only an IP address in the
DHCPLEASEQUERY message. The DHCP server will return any infor-
mation that it has on the most recent client to have been
assigned that IP address.
The DHCP server replies with a DHCPLEASEKNOWN or DHCPLEASEACTIVE
message if the IP address in the DHCPLEASEQUERY message
corresponds to an IP address about which the server has defini-
tive information (ie., it is authorized to lease this IP
address). The server replies with a DHCPLEASEUNKNOWN message if
the server does not have definitive information concerning the
address in the DHCPLEASEQUERY message.
A server which implements the DHCPLEASEQUERY message MUST
Woundy & Kinnear Expires September 2003 [Page 9]
Internet Draft DHCP Lease Query March 2003
implement this capability.
o Query by MAC address:
For this query, the requester supplies only a MAC address in the
DHCPLEASEQUERY message. The DHCP server will return any infor-
mation that it has on the IP address most recently accessed by a
client with that MAC address. In addition, it may supply addi-
tion IP addresses which have been associated with that MAC
address in different subnets. Information about these bindings
can then be found using the Query by IP Address, described
above.
The DHCP server replies with a DHCPLEASEKNOWN or DHCPLEASEACTIVE
message if the MAC address in the DHCPLEASEQUERY message
corresponds to an IP address about which the server has defini-
tive information (ie., it is authorized to lease this IP
address). The server replies with a DHCPLEASEUNKNOWN message if
the server does not have definitive information concerning the
MAC address in the DHCPLEASEQUERY message.
A server which implements the DHCPLEASEQUERY message SHOULD
implement this capability. If it does not, it SHOULD respond
with a DHCPUNIMPLEMENTED message when it receives a query by MAC
address.
o Query by Client-identifier option:
For this query, the requester supplies only a client-id option
in the DHCPLEASEQUERY message. The DHCP server will return any
information that it has on the IP address most recently accessed
by a client with that client-id. In addition, it may supply
addition IP addresses which have been associated with client-id
in different subnets. Information about these bindings can then
be found using the Query by IP Address, described above.
The DHCP server replies with a DHCPLEASEKNOWN or DHCPLEASEACTIVE
message if the client-id in the DHCPLEASEQUERY message
corresponds to an IP address about which the server has defini-
tive information (ie., it is authorized to lease this IP
address). The server replies with a DHCPLEASEUNKNOWN message if
the server does not have definitive information concerning the
client-id in the DHCPLEASEQUERY message.
A server which implements the DHCPLEASEQUERY message SHOULD
implement this capability. If it does not, it SHOULD respond
with a DHCPUNIMPLEMENTED message when it receives a query by
Client-identifier option address.
Woundy & Kinnear Expires September 2003 [Page 10]
Internet Draft DHCP Lease Query March 2003
Generally, the query by IP address is likely to be the most efficient
and widely implemented form of leasequery, and it SHOULD be used if
at all possible. Use of the other two query formats SHOULD be minim-
ized, as they can potentially place a large load on some servers.
The DHCPLEASEKNOWN or DHCPLEASEACTIVE message reply MUST always con-
tain the IP address in the ciaddr field and SHOULD contains the phy-
sical address of the IP address lease owner in the "htype", "hlen",
and "chaddr" fields. The Parameter Request List (option 55) can be
used to request specific options to be returned about the IP address
in the ciaddr. The reply often contains the time until expiration of
the lease, and the original contents of the Relay Agent Information
option [RFC 3046]. The access concentrator uses the "chaddr" and
Relay Agent Information option to construct location information,
which can be cached on the access concentrator until lease expira-
tion.
Any DHCP server which supports the DHCPLEASEQUERY message SHOULD save
the information from the most recent Relay Agent Information option
(option 82) [RFC 3046] associated with every IP address which it
serves. It is assumed that most clients which generate the DHCPLEASE-
QUERY message will ask for the Relay Agent Information option (option
82) in the Parameter Request List (option 55), and so supporting the
DHCPLEASEQUERY message without having the Relay Agent Information
option around to return to the client is likely to be less than help-
ful.
A server which implements DHCPLEASEQUERY SHOULD also save the infor-
mation on the most recent Vendor class identifier, option 60, associ-
ated with each IP address, since this option is also a likely candi-
date to be requested by clients sending the DHCPLEASEQUERY message.
6. Protocol Details
6.1. Definitions required for DHCPLEASEQUERY processing
The operation of the DHCPLEASEQUERY message requires the definition
of the following new and extended values for the DHCP packet beyond
those defined by [RFC 2131] and [RFC 2132]. See also Section 8, IANA
considerations.
1. The message type option (option 53) from [RFC 2132] requires
five new values: The DHCPLEASEQUERY message itself and its
three possible responses DHCPLEASEKNOWN, DHCPLEASEACTIVE,
DHCPLEASEUNKNOWN, and DHCPUNIMPLEMENTED. The values of these
message types are shown below in a reproduction of the table
Woundy & Kinnear Expires September 2003 [Page 11]
Internet Draft DHCP Lease Query March 2003
from [RFC 2132]:
Value Message Type
----- ------------
1 DHCPDISCOVER
2 DHCPOFFER
3 DHCPREQUEST
4 DHCPDECLINE
5 DHCPACK
6 DHCPNAK
7 DHCPRELEASE
8 DHCPINFORM
TBD DHCPLEASEQUERY
TBD DHCPLEASEKNOWN
TBD DHCPLEASEUNKNOWN
TBD DHCPLEASEACTIVE
TBD DHCPUNIMPLEMENTED
2. There is a new bit defined in the "flags" field of the DHCP
packet (see Section 1, Figure 1 and Table 1 of [RFC 2131]). It
is called the R: RESERVATION flag. The revised Figure 2 from
[RFC 2131] is show here:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B|R| MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
B: BROADCAST flag
R: RESERVATION FLAG
MBZ: MUST BE ZERO (reserved for future use)
Revised Figure 2 from RFC2131:
Format of the 'flags' field
3. There is a new option, the client-last-transaction-time:
client-last-transaction-time
Woundy & Kinnear Expires September 2003 [Page 12]
Internet Draft DHCP Lease Query March 2003
This option allows the receiver to determine the time of the
most recent access of the client. It is particularly useful
when DHCPLEASEKNOWN messages from two different DHCP servers
need to be compared, although it can be useful in other situa-
tions. The value is a duration in seconds from the current
time into the past when this IP address was most recently the
subject of communication between the client and the DHCP
server.
This MUST NOT be an absolute time. This MUST NOT be an abso-
lute number of seconds since Jan 1, 1970. Instead, this MUST
be an integer number of seconds in the past from the time the
DHCPLEASEKNOWN message is sent that the client last dealt with
this server about this IP address. In the same way that the IP
Address Lease Time option (option 51) encodes a lease time
which is a number of seconds into the future from the time the
message was sent, this option encodes a value which is a number
of seconds into the past from when the message was sent.
The code for the this option is TBD. The length of the this
option is 4 octets.
Code Len Seconds in the past
+-----+-----+-----+-----+-----+-----+
| TBD | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+
4. There in a second new option, the associated-ip option:
associated-ip
This option is used to return all of the IP addresses associ-
ated with the DHCP client specified in a particular DHCPLEASE-
QUERY message.
The code for this option is TBD. The minimum length for this
option is 4 octets, and the length MUST always be a multiple of
4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
| TBD | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
Woundy & Kinnear Expires September 2003 [Page 13]
Internet Draft DHCP Lease Query March 2003
6.2. Sending the DHCPLEASEQUERY Message
The DHCPLEASEQUERY message is typically sent by an access concentra-
tor. The DHCPLEASEQUERY message uses the DHCP message format as
described in [RFC 2131], and uses message number TBD in the DHCP Mes-
sage Type option (option 53). The DHCPLEASEQUERY message has the
following pertinent message contents:
o The giaddr MUST be set to the IP address of the requester (i.e.
the access concentrator). The giaddr is independent of the
"ciaddr" field to be searched -- it is simply the return address
of for the DHCPLEASEKNOWN or DHCPLEASEUNKNOWN message from the
DHCP server.
o The Parameter Request List option (option 55) SHOULD be set to
the options of interest to the requester. The interesting
options are likely to include the IP Address Lease Time option
(option 51), the Relay Agent Information option (option 82) and
possibly the Vendor class identifier option (option 60). In the
absence of a Parameter Request List option, the server will
return the same options it would return for a DHCPREQUEST mes-
sage which didn't contain a DHCPLEASEQUERY message, which
includes those mandated by [RFC 2131, Section 4.3.1] as well as
any options which the server was configured to always return to
a client.
o The Reservation bit in the "flags" field of the DHCP packet (see
[RFC 2131] and Section 6.1 of this document) is not used when
sending a DHCPLEASEQUERY message.
Additional details concerning different query types are:
o Query by IP address:
The values of htype, hlen, and chaddr MUST be set to 0.
The "ciaddr" field MUST be set to the IP address of the lease to
be queried.
The Client-identifier option (option 61) MUST NOT appear in the
packet.
o Query by MAC address:
The values of htype, hlen, and chaddr MUST be set to the value
of the MAC address to search for.
The "ciaddr" field MUST be set to zero.
Woundy & Kinnear Expires September 2003 [Page 14]
Internet Draft DHCP Lease Query March 2003
The Client-identifier option (option 61) MUST NOT appear in the
packet.
o Query by Client-identifier option:
There MUST be a Client-identifier option (option 61) in the
DHCPLEASEQUERY message.
The "ciaddr" field MUST be set to zero.
The values of htype, hlen, and chaddr MUST be set to 0.
The DHCPLEASEQUERY message SHOULD be sent to a DHCP server which is
known to possess authoritative information concerning the IP address.
The DHCPLEASEQUERY message MAY be sent to more than one DHCP server,
and in the absence of information concerning which DHCP server might
possess authoritative information concerning the IP address, it
SHOULD be sent to all DHCP servers configured for the associated
relay agent (if any are known).
6.3. Receiving the DHCPLEASEQUERY Message
A DHCPLEASEQUERY message MUST have a non-zero giaddr. The DHCPLEASE-
QUERY message MUST have exactly one of: a non-zero ciaddr, a non-
zero "htype"/"hlen"/"chaddr", or a Client-identifier.
The DHCP server which receives a DHCPLEASEQUERY message MUST base its
response on the particular data item used in the query.
The giaddr is used only for the destination address of any generated
response and, while required, is not otherwise used in generating the
response to the DHCPLEASEQUERY message. It MUST NOT be used to res-
trict the processing of the query in any way, and MUST NOT be used
locate a subnet to which the ciaddr (if any) must belong.
6.4. Responding to the DHCPLEASEQUERY Message
There are four possible responses to a DHCPLEASEQUERY message:
o DHCPLEASEKNOWN
The server MUST respond with a DHCPLEASEKNOWN message if this
server has information about the IP address or client in ques-
tion, but there is no active lease for the IP address or client
specified in the query. If the query was by IP address, then
the DHCPLEASEKNOWN message indicates that this server manages
this IP address. In the case where a client was specified either
by Client-identifier or MAC address, then the DHCPLEASEKNOWN
Woundy & Kinnear Expires September 2003 [Page 15]
Internet Draft DHCP Lease Query March 2003
message indicates that the client is known to the DHCP server,
and was the most recent client associated with a particular IP
address.
For any type of query, if the client specified in the DHCPLEASE-
KNOWN message has a reservation for the IP address specifed in
the ciaddr, then the server MUST set the R (reservation) bit in
the DHCPLEASEKNOWN message.
o DHCPLEASEUNKNOWN
The DHCPLEASEKNOWN message indicates that the server knows noth-
ing about the IP address or client specified in the DHCPLEASE-
QUERY message.
The server MUST response with a DHCPLEASEKNOWN message when this
server has no information about the IP address or client speci-
fied in the DHCPLEASEQUERY message.
When responding with a DHCPLEASEUNKNOWN, the DHCP server SHOULD
NOT include other DHCP options in the response. The R (reserva-
tion) bit MUST NOT be set in the "flags" field of the DHCP
packet.
o DHCPLEASEACTIVE
The DHCPLEASEACTIVE message indicates that the server not only
knows about the IP address and client specified in the
DHCPLEASEACTIVE message but also that there is an active lease
by that client for that IP address.
In some cases, the DHCP server MAY be configured to return a
DHCPLEASEACTIVE message when there is no active lease but when
there is a reservation by the specified client for the IP
address in the "ciaddr" field of the DHCPLEASEACTIVE message. A
server would be so configured when it was desired that the
access concentrator would allow access to IP addresses which are
not DHCP clients. In this case the DHCP server SHOULD NOT place
an IP Address Lease Time (option 51) in the DHCPLEASEACTIVE mes-
sage, allowing the access concentrator to determine that this is
a DHCPLEASEACTIVE message for an IP address without a currently
active lease.
The server MUST respond with a DHCPLEASEACTIVE message when the
IP address returned in the "ciaddr" field is currently leased.
If the client returned in the DHCPLEASEACTIVE message has a
reservation for that IP address recorded in the DHCP server,
then the R (reservation) bit MUST be set in the "flags" field of
Woundy & Kinnear Expires September 2003 [Page 16]
Internet Draft DHCP Lease Query March 2003
the DHCP packet.
o DHCPUNIMPLEMENTED
The DHCPUNIMPLEMENTED response to the DHCPLEASEQUERY message
indicates that the particular form of DHCPLEASEQUERY used is not
implemented in this DHCP server. It may mean that the DHCPLEASE-
QUERY message as a whole is not implemented by this DHCP server
although it is usually used to indicate that a query by Client-
identifier or MAC address is not implemented by a DHCP server
that otherwise supports a DHCPLEASEQUERY by IP address.
The DHCPUNIMPLEMENTED message can apply to any unimplemented
messages, and MAY be used to respond to messages other than
DHCPLEASEQUERY.
6.4.1. Determining the IP address to which to respond
Since the response to a DHCPLEASEQUERY request can only contain full
information about one IP address -- the one that appears in the
"ciaddr" field -- determination of which IP address to which to
respond is a key issue. Of course, the values of additional IP
addresses for which a client has a lease must also be returned in the
associated-ip option (Section 6.1, #4). This is the only information
returned not directly associated with the IP address in the "ciaddr"
field.
In the event that an IP address appears in the "ciaddr" field of a
DHCPLEASEQUERY message, if that IP address is one managed by the DHCP
server, then that IP address MUST be set in the "ciaddr" field of a
DHCPLEASEKNOWN message.
If the IP address is not managed by the DHCP server, then a
DHCPLEASEUNKNOWN message must be returned.
If the "ciaddr" field of the DHCPLEASEQUERY is zero, then the
DHCPLEASEQUERY message is a query by Client-identifier or MAC
address. In this case, the client's identity is any client which has
proffered an identical Client-identifier option (if the Client-
identifier option appears in the DHCPLEASEQUERY message), or an
identical MAC address (if the MAC address fields in the DHCPLEASE-
QUERY message are non-zero). This client matching approach will, for
the purposes of this section, be described as "Client-identifier or
MAC address".
The Reservations bit (the R bit) has no meaning in the DHCPLEASEQUERY
message and is used only to indicate the existence of a reservation
in a DHCPLEASEKNOWN or DHCPLEASEACTIVE message.
Woundy & Kinnear Expires September 2003 [Page 17]
Internet Draft DHCP Lease Query March 2003
If the "ciaddr" field is zero in a DHCPLEASEQUERY message, then the
IP address placed in the "ciaddr" field of the DHCPLEASEKNOWN or
DHCPLEASEACTIVE message MUST be that of an IP address for which the
client that most recently used the IP address matches the Client-
identifier or MAC address specified in the DHCPLEASEQUERY message.
If there is only a single IP address which fulfills this criteria,
then it MUST be placed in the "ciaddr" field of the DHCPLEASEKNOWN or
DHCPLEASEACTIVE message.
In the case where more than one IP address has been accessed by the
client specified by the MAC address or Client-identifier option, then
the DHCP server MUST return the IP address returned to the client in
the most recent transaction with the client unless the DHCP server
has been configured by the server administrator to use some other
preference mechanism.
If, after all of the above processing, no value is set in the
"ciaddr" field of the DHCPLEASEKNOWN or DHCPLEASEACTIVE message, then
a DHCPLEASEUNKNOWN message MUST be returned instead.
6.4.2. Building a DHCPLEASEKNOWN or DHCPLEASEACTIVE message once the
"ciaddr" field is set
Once the "ciaddr" field of the DHCPLEASEKNOWN or DHCPLEASEACTIVE mes-
sage is set, the rest of the processing largely involves returning
information about the IP address specified in the "ciaddr" field.
If the IP address in the "ciaddr" field of the DHCPLEASEKNOWN or
DHCPLEASEACTIVE message is currently leased by the client specified
in the Client-identifier or MAC address returned in the DHCPLEASE-
KNOWN or DHCPLEASEACTIVE message, then the message MUST be a
DHCPLEASEACTIVE message, otherwise it MUST be a DHCPLEASEKNOWN mes-
sage.
It MAY be possible to configure a DHCP server to return a
DHCPLEASEACTIVE message even though the IP address specified in the
"ciaddr" field is not currently leased if there is a reservation for
that IP address by the client specified in the Client-identifier or
MAC address fields of the DHCPLEASEACTIVE message. In this case,
there MUST NOT be an IP Address Lease Time option (option 51) in the
packet.
The R (reservation) bit must be set in the "flags" field if the IP
address in the "ciaddr" field is reserved for the client returned in
the MAC address or Client-identifier option.
The IP address in the "ciaddr" field of the DHCPLEASEKNOWN or
Woundy & Kinnear Expires September 2003 [Page 18]
Internet Draft DHCP Lease Query March 2003
DHCPLEASEACTIVE message MUST be one for which this server is respon-
sible (or a DHCPLEASEUNKNOWN message would be have already been
returned early in the processing described in the previous section).
The MAC address of the DHCPLEASEKNOWN or DHCPLEASEACTIVE message MUST
be set to the values which identify the client associated with the IP
address in the "ciaddr" field of the DHCPLEASEKNOWN message. This
may be derived from a real DHCP client or from reservation informa-
tion configured into the DHCP server.
If the Client-identifier option (option 61) is specified in the
Parameter Request List option (option 55), then the Client-identifier
(if any) of the client associated with the IP address in the "ciaddr"
field SHOULD be returned in the DHCPLEASEKNOWN or DHCPLEASEACTIVE
message. This may be derived from a real DHCP client, or from reser-
vation information configured into the DHCP server.
In the case where more than one IP address has been involved in a
DHCP message exchange with the client specified by the MAC address
and/or Client-identifier option, then the list of all of the IP
addresses SHOULD be returned in the associated-ip option (option
TBD), if that option was requested as part of the Parameter Request
List option.
If the IP Address Lease Time option (option 51) is specified in the
Parameter Request List and if there is a currently valid lease for
the IP address specified in the ciaddr, then the DHCP server MUST
return this option in the DHCPLEASEKNOWN with its value equal to the
time remaining until lease expiration. If there is no valid lease
for the IP address, then the server MUST NOT return the IP Address
Lease Time option (option 51).
If there is no currently valid lease on the IP address in the
"ciaddr" field, and if the "ciaddr" in the DHCPLEASEKNOWN message is
currently reserved for the client specified by the client-id or MAC
address, then the DHCP server MAY synthesize an IP Address Lease Time
option for the DHCPLEASEKNOWN message if configured to do so. Typi-
cally the value of this option would itself be a configuration param-
eter of the DHCP server.
A request for the Renewal (T1) Time Value option or the Rebinding
(T2) Time Value option in the Parameter Request List of the
DHCPLEASEQUERY message MUST be handled like the IP Address Lease Time
option is handled. If there is a valid lease, then the DHCP server
SHOULD return these options (when requested) with the remaining time
until renewal or rebinding, respectively. If there is not currently
a valid lease for this IP address, the DHCP server MUST NOT return
these options.
Woundy & Kinnear Expires September 2003 [Page 19]
Internet Draft DHCP Lease Query March 2003
If the Relay Agent Information (option 82) is specified in the Param-
eter Request List and if the DHCP server has saved the information
contained in the most recent Relay Agent Information option, the DHCP
server MUST include that information in a Relay Agent Information
option in the DHCPLEASEKNOWN.
The DHCPLEASEKNOWN or DHCPLEASEACTIVE message SHOULD include the
values of all other options not specifically discussed above that
were requested in the Parameter Request List of the DHCPLEASEQUERY
message. The DHCP server uses information from its lease binding
database to supply the DHCPLEASEKNOWN or DHCPLEASEACTIVE option
values. The values of the options that were returned to the DHCP
client would generally be preferred, but in the absence of those,
options that were sent in DHCP client requests would be acceptable.
In order to accommodate DHCPLEASEQUERY messages sent to a DHCP Fail-
over secondary server [FAILOVER] when the primary server is down, the
primary server MUST communicate the Relay Agent Information option
(option 82) values to the secondary server via the DHCP Failover
BNDUPD messages.
6.4.3. Sending a DHCPLEASEKNOWN, DHCPLEASEACTIVE, or DHCPLEASEUNKNOWN
message
The server expects a giaddr in the DHCPLEASEQUERY message, and uni-
casts the DHCPLEASEKNOWN, DHCPLEASEACTIVE or DHCPLEASEUNKNOWN message
to the giaddr. If the giaddr field is zero, then the DHCP server MUST
NOT reply to the DHCPLEASEQUERY message.
6.5. Receiving a DHCPLEASEKNOWN, DHCPLEASEACTIVE, or DHCPLEASEUNKNOWN
Message
When a DHCPLEASEACTIVE message is received in response to the
DHCPLEASEQUERY message it means that there is a currently active
lease for this IP address in this DHCP server. The access concentra-
tor SHOULD use the information in the htype, hlen, and chaddr fields
of the DHCPLEASEACTIVE as well as any Relay Agent Information option
information included in the packet to refresh its location informa-
tion for this IP address.
When a DHCPLEASEKNOWN message is received in response to the
DHCPLEASEQUERY message that means that there is no currently active
lease for the IP address present in the DHCP server. In this case,
the access concentrator SHOULD cache this information in order to
prevent unacceptable loads on the access concentrator and the DHCP
server in the face of a malicious or seriously compromised device
downstream of the access concentrator. This cacheing could be as
simple as simply setting a bit saying that a response was received
Woundy & Kinnear Expires September 2003 [Page 20]
Internet Draft DHCP Lease Query March 2003
from a server which knew about this IP address but that there was no
current lease. This would of course need to be cleared when the
access concentrator next "gleaned" that a lease for this IP address
came into existance.
If the R (reservation) bit is set in the "flags" field of the
DHCPLEASEKNOWN message, it means that a reservation exists in the
DHCP server for the IP address and associated client. The access
concentrator MAY be configured to allow the client access even though
no currently outstanding lease is in place for this
In either case, when a DHCPLEASEKNOWN or DHCPLEASEACTIVE message is
received in response to a DHCPLEASEQUERY message, it means that the
DHCP server which responded is a DHCP server which manages the IP
address present in the ciaddr, and the Relay Agent SHOULD cache this
information for later use.
When a DHCPLEASEUNKNOWN message is received by an access concentrator
which has sent out a DHCPLEASEQUERY message, it means that the DHCP
server contacted supports the DHCPLEASEQUERY message but that the
DHCP server does not have definitive information concerning the IP
address contained in the "ciaddr" field of the DHCPLEASEQUERY mes-
sage. If there is no IP address in the "ciaddr" field of the
DHCPLEASEQUERY message, then a DHCPLEASEUNKNOWN message means that
the DHCP server does not have definitive information concerning the
any DHCP client specified in the "hlen", "htype", and "chaddr" fields
or the Client-identifier option of the DHCPLEASEQUERY message.
The access concentrator SHOULD cache this information, and only
infrequently direct a DHCPLEASEQUERY message to a DHCP server that
responded to a DHCPLEASEQUERY message for a particular "ciaddr" field
with a DHCPLEASEUNKNOWN.
When a DHCPUNIMPLEMENTED message is received by an access concentra-
tor, it means that the particular aspect of DHCPLEASEQUERY processing
requested is not implemented in the responding server. It may or may
not be the case that other aspects of DHCPLEASEQUERY processing are
not implemented in that server.
6.6. Receiving no response to the DHCPLEASEQUERY Message
When an access concentrator receives no response to a DHCPLEASEQUERY
message, there are several possible reasons:
o The DHCPLEASEQUERY or a corresponding DHCPLEASEKNOWN,
DHCPLEASEACTIVE or DHCPLEASEUNKNOWN were lost during transmis-
sion or the DHCPLEASEQUERY arrived at the DHCP server but it was
dropped because the server was too busy.
Woundy & Kinnear Expires September 2003 [Page 21]
Internet Draft DHCP Lease Query March 2003
o The DHCP server doesn't support DHCPLEASEQUERY.
In the first of the cases above, a retransmission of the DHCPLEASE-
QUERY would be appropriate, but in the second of the two cases, a
retransmission would not be appropriate. There is no way to tell
these two cases apart (other than, perhaps, because of a DHCP
server's response to other DHCPLEASEQUERY messages indicating that it
does or does not support the DHCPLEASEQUERY message).
An access concentrator which utilizes the DHCPLEASEQUERY message
SHOULD attempt to resend DHCPLEASEQUERY messages to servers which do
not respond to them using a backoff algorithm for the retry time that
approximates an exponential backoff. The access concentrator SHOULD
adjust the backoff approach such that DHCPLEASEQUERY messages do not
arrive at a server which is not otherwise known to support the
DHCPLEASEQUERY message at a rate of more than approximately one
packet every 10 seconds, and yet (if the access concentrator needs to
send DHCPLEASEQUERY messages) not less than one DHCPLEASEQUERY per
minute.
In practice this approach would probably best be handled by a per-
server timer that backs off exponentially to once a minute, and a
per-message backoff timer that also backs off to once a minute. The
per-server timer would start off expired, and in the expired state
only one DHCPLEASEQUERY message would be queued for the associated
server. This DHCPLEASEQUERY message would be sent with the backoff
quickly moving to once a minute until a DHCPLEASEACTIVE, DHCPLEASE-
KNOWN, or DHCPLEASEUNKNOWN message reply was received. Whenever one
of these messages is received, the per-server timer is reset, and
whenever the per-server timer has not expired, more than one indivi-
dual DHCPLEASEQUERY messages can be outstanding to the DHCP server at
one time. It is recommended that this number be limited to a rela-
tively small number, for example, 100 or 200, to avoid swamping the
DHCP server. Each of these messages should have its own per-message
retry timer. This would retransmit each message and backoff as dis-
cussed above. In the event the per-server timer goes off, then all
outstanding messages SHOULD be dropped except for a single DHCPLEASE-
QUERY message which is used to poll the server until such time as
another DHCPLEASEACTIVE, DHCPLEASEKNOWN, or DHCPLEASEUNKNOWN message
is received.
6.7. Using the DHCPLEASEQUERY message in a failover environment
When using the DHCPLEASEQUERY message in an environment where multi-
ple DHCP server may contain authoritative information about the same
IP address (such as when failover [FAILOVER] is operating), there
could be some difficulty in deciding which results are the most use-
ful if two servers respond with DHCPLEASEKNOWN messages to the same
Woundy & Kinnear Expires September 2003 [Page 22]
Internet Draft DHCP Lease Query March 2003
query.
In this case, the client-last-transaction-time can be used to decide
which server has more recent information concerning the IP address
returned in the "ciaddr" field.
7. Security Considerations
Access concentrators that use DHCP gleaning, refreshed with
DHCPLEASEQUERY messages, will maintain accurate location information.
Location information accuracy ensures that the access concentrator
can forward data traffic to the intended location in the broadband
access network, can perform IP source address verification of
datagrams from the access network, and can encrypt traffic which can
only be decrypted by the intended access modem (e.g. [BPI] and
[BPI+]). As a result, the access concentrator does not need to
depend on ARP broadcasts across the access network, which is suscep-
tible to malicious hosts which masquerade as the intended IP end-
points. Thus, the DHCPLEASEQUERY message allows an access concentra-
tor to provide considerably enhanced security.
DHCP servers SHOULD prevent exposure of location information (partic-
ularly the mapping of hardware address to IP address lease, which can
be an invasion of broadband subscriber privacy) by leveraging DHCP
authentication [RFC 3118]. With respect to authentication, the
access concentrator acts as the "client". The use of "Authentication
Protocol 0" (using simple unencoded authentication token(s) between
the access concentrator and the DHCP server) is straightforward.
Alternatively, use of IPsec would also be a way to ensure security
between the relay agent and the DHCP server.
Access concentrators SHOULD minimize potential denial of service
attacks on the DHCP servers by minimizing the generation of
DHCPLEASEQUERY messages. In particular, the access concentrator
should employ negative cacheing (i.e. cache both DHCPLEASEKNOWN and
DHCPLEASEUNKNOWN responses to DHCPLEASEQUERY messages) and ciaddr
restriction (i.e. don't send a DHCPLEASEQUERY message with a ciaddr
outside of the range of the attached broadband access networks).
Together, these mechanisms limit the access concentrator to transmit-
ting one DHCPLEASEQUERY message (excluding message retries) per legi-
timate broadband access network IP address after a reboot event.
In some environments it may be appropriate to configure a DHCP server
with the IP addresses of the relay agents for which it may respond to
DHCPLEASEQUERY messages, thereby allowing it to respond only to to
requests from only a handful of relay agents. This does not provide
any true security, but may be useful to thwart unsophisticated
attacks of various sorts.
Woundy & Kinnear Expires September 2003 [Page 23]
Internet Draft DHCP Lease Query March 2003
8. IANA Considerations
IANA has assigned seven values and one bit position for this docu-
ment. See Section 6.1 for details. There are five new messages
types, which are the value of the message type option (option 53)
from [RFC 2132]. The value for DHCPLEASEQUERY is TBD, the value for
DHCPLEASEKNOWN is TBD, the value for DHCPLEASEACTIVE is TBD, the
value for DHCPLEASEUNKNOWN is TBD and the value for DHCPUNIMPLEMENTED
is TBD. There is a new bit defined for the "flags" field of the DHCP
packet (see Section 1, Figure 1 and Table 1 of [RFC 2131]). The flag
is called "R: RESERVATION flag", and its value is TBD. Finally,
there are two new DHCP option defined; the client-last-transaction-
time option -- option code TBD, and the associated-ip option --
option code TBD.
9. Acknowledgments
Jim Forster, Joe Ng, Guenter Roeck, and Mark Stapp contributed
greatly to the initial creation of the DHCPLEASEQUERY message.
Patrick Guelat suggested several improvements to support static IP
addressing.
10. References
[RFC 826] Plummer, D., "Ethernet Address Resolution Protocol: Or con-
verting network protocol addresses to 48.bit Ethernet address for
transmission on Ethernet hardware", RFC 826, November 1982.
[RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC
951, September 1985.
[RFC 1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
Woundy & Kinnear Expires September 2003 [Page 24]
Internet Draft DHCP Lease Query March 2003
[RFC 3118] Droms, R., Arbaugh, W., "Authentication for DHCP Mes-
sages", RFC 3118, June 2001.
[BPI] CableLabs, "Baseline Privacy Interface Specification", SP-BPI-
I02-990319, March 1999, available at http://www.cablemodem.com/.
[BPI+] CableLabs, "Baseline Privacy Plus Interface Specification",
SP-BPI+-I04-000407, April 2000, available at
http://www.cablemodem.com/.
[DHCPMIB] Hibbs, R., Waters, G., "Dynamic Host Configuration Protocol
(DHCP) Server MIB", draft-ietf-dhc-server-mib-06.txt, February
2002.
[DOCSIS] CableLabs, "Data-Over-Cable Service Interface Specifica-
tions: Cable Modem Radio Frequency Interface Specification SP-
RFI-I05-991105", November 1999.
[EUROMODEM] ECCA, "Technical Specification of a European Cable Modem
for digital bi-directional communications via cable networks",
Version 1.0, May 1999.
[FAILOVER] Droms, R., Kinnear, K., Stapp, M., Volz, B., Gonczi, S.,
Rabil, G., Dooley, M., Kapur, A., "DHCP Failover Protocol",
draft-ietf-dhc-failover-10.txt, January 2002.
11. Author's information
Rich Woundy
Comcast Cable
27 Industrial Ave.
Chelmsford, MA 01824
Phone: (978) 244-4010
EMail: richard_woundy@cable.comcast.com
Kim Kinnear
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 497-8000
Woundy & Kinnear Expires September 2003 [Page 25]
Internet Draft DHCP Lease Query March 2003
EMail: kkinnear@cisco.com
12. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any intel-
lectual property or other rights that might be claimed to pertain to
the implementation or use of the technology described in this document
or the extent to which any license under such rights might or might not
be available; neither does it represent that it has made any effort to
identify any such rights. Information on the IETF's procedures with
respect to rights in standards-track and standards-related documentation
can be found in BCP-11. Copies of claims of rights made available for
publication and any assurances of licenses to be made available, or the
result of an attempt made to obtain a general license or permission for
the use of such proprietary rights by implementors or users of this
specification can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
which may cover technology that may be required to practice this stan-
dard. Please address the information to the IETF Executive Director.
13. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to oth-
ers, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and dis-
tributed, 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 Stan-
dards 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
Woundy & Kinnear Expires September 2003 [Page 26]
Internet Draft DHCP Lease Query March 2003
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 FIT-
NESS FOR A PARTICULAR PURPOSE.
Woundy & Kinnear Expires September 2003 [Page 27]