Dynamic Host Configuration Working Group Rich Woundy
INTERNET DRAFT Kim Kinnear
Cisco Systems
March 2002
Expires September 2002
DHCP Lease Query
<draft-ietf-dhc-leasequery-03.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
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The list of current Internet-Drafts can be accessed at
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
Access concentrators that act as DHCP relay agents need to determine
the endpoint locations of IP addresses across public broadband access
networks such as cable, DSL, and wireless networks. Because ARP
broadcasts are undesirable in public networks, many access
concentrator implementations "glean" location information from DHCP
messages forwarded by its relay agent function. 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 obtains the location information directly
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from the DHCP server(s) using a new, lightweight DHCPLEASEQUERY
message.
1. Introduction
In many broadband access networks, 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
+--------+ +---------------+
| DHCP | | DSL Access | +-------+
| Server |-...-| Concentrator |-...-| DSLAM |
+--------+ | (Relay Agent) | +-------+
+---------------+ | |
+------+ +------+
|Modem1| |Modem2|
+------+ +------+
| | |
+-----+ +-----+ +-----+
|Host1| |Host2| |Host3|
+-----+ +-----+ +-----+
Figure 2: DSL Environment for DHCPLEASEQUERY
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Knowledge of this location information benefits the access concentra-
tor 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 premise of this document is that the access concentrator obtains
this location information 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 concentrator can query the DHCP server(s) for location
information using the DHCPLEASEQUERY message. The DHCPLEASEQUERY
mechanism is the focus of this document.
The DHCPLEASEQUERY message is a new DHCP message type transmitted
from a DHCP relay agent to a DHCP server. The DHCPLEASEQUERY-aware
relay agent sends the DHCPLEASEQUERY message when it needs to know
the location of an IP endpoint. The DHCPLEASEQUERY-aware DHCP server
replies with a DHCPKNOWN, DHCPACTIVE or DHCPUNKNOWN message. The
DHCPACTIVE response to a DHCPLEASEQUERY message allows the relay
agent to determine the IP endpoint location, and the remaining dura-
tion of the IP address lease.
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.
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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.
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
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configured with this DHCP client to IP address mapping, typi-
cally using the MAC address as the way to identify the client.
The DHCP client to IP address mapping, configured in the DHCP
server, is called a reservation for the purposes of this docu-
ment.
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.
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 access concentrators to send
DHCPLEASEQUERY messages to DHCP servers, to obtain location informa-
tion 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]. Unlike previous DHCP message types, the DHCP relay
agent originates and sends the DHCPLEASEQUERY message to the DHCP
server, and processes the reply from the DHCP server (a DHCPKNOWN or
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DHCPUNKNOWN).
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.
4. Design Goals
The core requirement of this document is to provide a lightweight
mechanism for access concentrator implementations to obtain location
information for broadband access network devices. The specifics of
the broadband environment that drove the approach of this document
follow.
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
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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
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
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
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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
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 DHCPKNOWN, DHCPACTIVE or
DHCPUNKNOWN (or in some cases, DHCPUNIMPLEMENTED). The reasons why a
DHCPKNOWN, DHCPACTIVE or DHCPUNKNOWN message might be generated are
explained in the specific query regimes, below. Servers which do not
support the DHCPLEASEQUERY message SHOULD (and are expected to) drop
the DHCPLEASEQUERY message silently, although they MAY respond with a
DHCPUNIMPLEMENTED message. The DHCPLEASEQUERY message can support
three query regimes:
o Query by IP address:
For this query, the "ciaddr" field MUST contain an IP address.
It MUST NOT contain a MAC address or Client-identifier option
(option 61). The DHCP server will return any information that
it has on the most recent client to have been allocated that IP
address. Any server which supports the DHCPLEASEQUERY message
MUST support query by IP address.
The DHCP server replies to the DHCPLEASEQUERY message with a
DHCPKNOWN or DHCPACTIVE message if the "ciaddr" corresponds to
an IP address about which the server has definitive information
(i.e., it is authorized to lease this IP address). The server
replies with a DHCPUNKNOWN message if the server does not have
definitive location information concerning the lease implied by
the "ciaddr".
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o Query by MAC address:
For this query, the "ciaddr" field MUST be zero and there MUST
be a MAC address is specified in the "htype", "hlen", and
"chaddr" fields. There MUST NOT be a Client-identifier option
(option 61) in the packet. The DHCP server looks up all IP
addresses for which clients with this MAC address are the most
recent user. In contrast to the query by IP address, there may
be multiple IP addresses which show the client specified by the
MAC address as having been the most recent user. The DHCP server
places the IP address most recently accessed by a DHCP client
with this MAC address (unless specifically configured otherwise,
see Section 6.4) in the "ciaddr" field, and returns other infor-
mation associated with that IP address. If requested, the DHCP
server SHOULD return information on all of the IP addresses it
found to be associated with the DHCP client with the MAC address
in a single Requested IP address option (option 50) [RFC 2132]
with multiple IP addresses in it. A server which implements the
DHCPLEASEQUERY message SHOULD implement this capability. If it
does not, it MUST respond with a DHCPUNIMPLEMENTED message when
it receives a query by MAC address.
The DHCP server replies to the DHCPLEASEQUERY message with a
DHCPKNOWN or DHCPACTIVE message if the MAC address corresponds
to a DHCP client which was the most recent user of an IP address
controlled by this DHCP server. The server replies with a
DHCPUNKNOWN message if the MAC address does not correspond to
such an IP address.
o Query by Client-identifier option:
For this query, the "ciaddr" field MUST be zero, there MUST be a
Client-identifier option (option 61) in the packet and there
MUST NOT be a MAC address in the packet (i.e., the hlen, htype,
and chaddr MUST all be zero). The DHCP server looks up all IP
addresses for which a client with this Client-identifier is the
most recent user. In contrast to the query by IP address, there
may be multiple IP addresses which show the client specified by
this Client-identifier as having been the most recent user. The
DHCP server places the IP address most recently accessed by a
DHCP client with this Client-identifier (unless specifically
configured otherwise, see Section 6.4) in the "ciaddr" field,
and returns other information associated with that IP address.
If requested, the DHCP server SHOULD return information on all
of the IP addresses it found to be associated with the DHCP
client with the Client-identifier in a single Requested IP
address option (option 50) containing multiple IP addresses. A
server which implements the DHCPLEASEQUERY message SHOULD
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implement this capability. If it does not, it MUST respond with
a DHCPUNIMPLEMENTED message when it received a query by Client-
identifier option address.
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 DHCPKNOWN or DHCPACTIVE message reply MUST always contain the IP
address in the ciaddr field and SHOULD contains the physical 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 expiration.
Any DHCP server which supports the DHCPLEASEQUERY message SHOULD save
the information from the most recent Relay Agent Information option
[RFC 3046] associated with every IP address which it serves. A
server which implements DHCPLEASEQUERY SHOULD also save the informa-
tion on the most recent vendor-class-identifier, option 60, associ-
ated with each IP address.
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 DHCPKNOWN, DHCPACTIVE, DHCPUNKNOWN,
and DHCPUNIMPLEMENTED. The values of these message types are
shown below in a reproduction of the table from [RFC 2132]:
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Value Message Type
----- ------------
1 DHCPDISCOVER
2 DHCPOFFER
3 DHCPREQUEST
4 DHCPDECLINE
5 DHCPACK
6 DHCPNAK
7 DHCPRELEASE
8 DHCPINFORM
TBD DHCPLEASEQUERY
TBD DHCPKNOWN
TBD DHCPUNKNOWN
TBD DHCPACTIVE
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| tbd 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 one new option defined which can be used to return
important information in a DHCPKNOWN response to a DHCPLEASE-
QUERY message -- the client-last-transaction-time.
client-last-transaction-time
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This option allows the receiver to determine the time of the
most recent access of the client. It is particularly useful
when DHCPKNOWN messages from two different DHCP servers need to
be compared, although it can be useful in other situations.
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
DHCPKNOWN 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. The Requested IP Address option is extended to allow for multi-
ple IP addresses in the option.
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 50, and its minimum length is 4 and
its maximum length MUST be a multiple of 4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
| 50 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
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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 DHCPKNOWN or DHCPUNKNOWN message from the DHCP
server.
o The Parameter Request List 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) and the
Relay Agent Information option (option 82).
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.
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
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DHCPLEASEQUERY message.
The "ciaddr" field MUST be set to zero.
The values of htype, hlen, and chaddr MUST be set to 0.
The access concentrator SHOULD ensure that the "ciaddr" field men-
tioned in the DHCPLEASEQUERY message (if a query by IP address) is a
local subnet of the interface specified for the client.
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.
6.4. Responding to the DHCPLEASEQUERY Message
There are four possible responses to a DHCPLEASEQUERY message:
o DHCPKNOWN
The DHCPKNOWN message indicates that the server knows about the
IP address or client specified in the DHCPLEASEQUERY message,
but there is no currently active lease for the IP address
returned in the "ciaddr" field of the DHCPKNOWN message. The R
(reservation) bit MAY be set in the case where there is a reser-
vation for this IP address by the client returned in the
DHCPKNOWN message, allowing the access concentrator to consider
a reservation equivalent to a currently active lease on the IP
address.
The server MUST respond with a DHCPKNOWN message if this server
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has information about the IP address or client in question, but
that there is no active lease for the IP address or client
specified in the query. If the query was by IP address, then
the DHCPKNOWN message indicates that this server manages this IP
address. If there is a reservation for this IP address, then
the DHCP server MUST set the R (reservation) bit in the "flags"
field of the DHCP packet, and the DHCP server MUST return what-
ever client information is known in the DHCPKNOWN message.
In the case where a client was specified either by Client-
identifier or MAC address, then the DHCPKNOWN message indicates
that the client is known to the DHCP server, and was the most
recent client associated with a particular IP address. In the
case where the client specified has a reservation for the IP
address returned in the ciaddr, the R (reservation) bit is set
in the "flags" field of the DHCP packet.
o DHCPUNKNOWN
The DHCPKNOWN message indicates that the server knows nothing
about the IP address or client specified in the DHCPLEASEQUERY
message.
The server MUST response with a DHCPKNOWN message when this
server has no information about the IP address or client speci-
fied in the DHCPLEASEQUERY message.
When responding with a DHCPUNKNOWN, the DHCP server SHOULD NOT
include other DHCP options in the response. The R (reservation)
bit MUST NOT be set in the "flags" field of the DHCP packet.
o DHCPACTIVE
The DHCPACTIVE message indicates that the server not only knows
about the IP address and client specified in the DHCPACTIVE mes-
sage 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
DHCPACTIVE 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 DHCPACTIVE 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 DHCPACTIVE message, allowing the
access concentrator to determine that this is a DHCPACTIVE mes-
sage for an IP address without a currently active lease.
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The server MUST respond with a DHCPACTIVE message when the IP
address returned in the "ciaddr" field is currently leased. If
the client returned in the DHCPACTIVE 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 the DHCP
packet.
o DHCPUNIMPLEMENTED
The DHCPUNIMPLEMENTED message indicates that the particular form
of DHCPLEASEQUERY used is not implemented in this DHCP server.
It may mean that the DHCPLEASEQUERY 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.
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 may also be returned in mul-
tiple Requested IP address options (option 50). This is the only
information returned not directly associated with the IP address in
the "ciaddr" field.)
6.4.1. Determining the IP address to which to respond
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
DHCPKNOWN message.
If the IP address is not managed by the DHCP server, then a DHCPUN-
KNOWN 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
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in a DHCPKNOWN or DHCPACTIVE message.
If the "ciaddr" field is zero in a DHCPLEASEQUERY message, then the
IP address placed in the "ciaddr" field of the DHCPKNOWN or DHCPAC-
TIVE 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 DHCPKNOWN or
DHCPACTIVE message.
In the case where more than one IP 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 prefer-
ence mechanism.
If, after all of the above processing, no value is set in the
"ciaddr" field of the DHCPKNOWN or DHCPACTIVE message, then a DHCPUN-
KNOWN message MUST be returned instead.
6.4.2. Building a DHCPKNOWN or DHCPACTIVE message once the "ciaddr"
field is set
Once the "ciaddr" field of the DHCPKNOWN or DHCPACTIVE message is
set, the rest of the processing largely involves returning informa-
tion about the IP address specified in the "ciaddr" field.
If the IP address in the "ciaddr" field of the DHCPKNOWN or DHCPAC-
TIVE message is currently leased by the client specified in the
Client-identifier or MAC address returned in the DHCPKNOWN or DHCPAC-
TIVE message, then the message MUST be a DHCPACTIVE message, other-
wise it MUST be a DHCPKNOWN message.
It MAY be possible to configure a DHCP server to return a DHCPACTIVE
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 DHCPACTIVE 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 DHCPKNOWN or DHCPACTIVE
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message MUST be one for which this server is responsible (or a
DHCPUNKNOWN message would be have already been returned early in the
processing described in the previous section).
The MAC address of the DHCPKNOWN or DHCPACTIVE message MUST be set
from the client associated with the IP address in the "ciaddr" field
of the DHCPKNOWN message. This may be derived from a real DHCP
client or from reservation information 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 DHCPKNOWN or DHCPACTIVE message. This
may be derived from a real DHPC client, or from reservation informa-
tion configured into the DHCP server.
In the case where more than one IP has been accessed by the client
specified by the MAC address and/or Client-identifier option, then
the list of all of the IP addresses SHOULD be returned as multiple
Requested IP address options (option 50), 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 DHCPKNOWN 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). This allows the requester (i.e. the access con-
centrator) to determine if there is currently a valid lease for the
IP address as well as the time until the lease expiration.
If there is no currently valid lease on the IP address in the
"ciaddr" field, and if the R bit is set in the DHCPLEASEQUERY and in
the DHCPKNOWN messages (i.e., if the sender of the DHCPLEASEQUERY
message requested reservation information, and the "ciaddr" in the
DHCPKNOWN message was derived from reservation information), then the
DHCP server MAY synthesize an IP Address Lease Time option for the
DHCPKNOWN message if configured to do so. Typically the value of
this option would itself be a configuration parameter 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
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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.
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 DHCPKNOWN.
The DHCPKNOWN or DHCPACTIVE 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 the lease binding database to supply the
DHCPKNOWN or DHCPACTIVE 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 DHCPKNOWN, DHCPACTIVE, or DHCPUNKNOWN message
The server expects a giaddr in the DHCPLEASEQUERY message, and uni-
casts the DHCPKNOWN, DHCPACTIVE or DHCPUNKNOWN 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 DHCPKNOWN, DHCPACTIVE, or DHCPUNKNOWN Message
When a DHCPACTIVE message is received in response to the DHCPLEASE-
QUERY message it means that there is a currently active lease for
this IP address in this DHCP server. The access concentrator SHOULD
use the information in the htype, hlen, and chaddr fields of the
DHCPACTIVE as well as any Relay Agent Information option information
included in the packet to refresh its location information for this
IP address.
When a DHCPKNOWN message is received in response to the DHCPLEASE-
QUERY 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 unac-
ceptable loads on the access concentrator and the DHCP server in the
face of a malicious or seriously compromised device downstream of the
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access concentrator.
If the R (reservation) bit is set in the "flags" field of the
DHCPKNOWN message, it means that a reservation exists in the DHCP
server for the IP address and associated client. The access concen-
trator MAY be configured to allow the client access even though no
currently outstanding lease is in place for this
In either case, when a DHCPKNOWN or DHCPACTIVE 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 DHCPUNKNOWN 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 not have definitive information concerning the IP address
contained in the "ciaddr" field of the DHCPLEASEQUERY message. If
there is no IP address in the "ciaddr" field of the DHCPLEASEQUERY
message, then a DHCPUNKNOWN message means that the DHCP server does
not have definitive information concerning the any DHCP client speci-
fied 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 DHCPUNKNOWN.
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 DHCPKNOWN, DHCPACTIVE or
DHCPUNKNOWN were lost during transmission or the DHCPLEASEQUERY
arrived at the DHCP server but it was dropped because the server
was too busy.
o The DHCP server doesn't support DHCPLEASEQUERY.
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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
supports 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 DHCPACTIVE, DHCPKNOWN, or
DHCPUNKNOWN message reply was received. Whenever one of these mes-
sages is received, the per-server timer is reset, and whenever the
per-server timer has not expired, more than one individual DHCPLEASE-
QUERY messages can be outstanding to the DHCP server at one time. It
is recommended that this number be limited to a relatively 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 discussed above. In
the event the per-server timer goes off, then all outstanding mes-
sages SHOULD be dropped except for a single DHCPLEASEQUERY message
which is used to poll the server until such time as another DHCPAC-
TIVE, DHCPKNOWN, or DHCPUNKNOWN 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 DHCPKNOWN messages to the same query.
In this case, the client-last-transaction-time can be used to decide
which server has more recent information concerning the IP address
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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 DHCPKNOWN and
DHCPUNKNOWN responses to DHCPLEASEQUERY messages) and ciaddr restric-
tion (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 transmitting one
DHCPLEASEQUERY message (excluding message retries) per legitimate
broadband access network IP address after a reboot event.
8. IANA Considerations
IANA has assigned seven values for this document. 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 DHCPKNOWN is TBD, the value
for DHCPACTIVE is TBD, the value for DHCPUNKNOWN 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
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value is TBD. Finally, there is one new DHCP option defined, which
is the client-last-transaction-time option, and its option code is
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.
[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/.
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[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
Kim Kinnear
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 497-8000
EMail: rwoundy@cisco.com
kkinnear@cisco.com
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Woundy & Kinnear Expires September 2002 [Page 24]
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Woundy & Kinnear Expires September 2002 [Page 25]
