Dynamic Host Configuration Working Group Rich Woundy
INTERNET DRAFT Kim Kinnear
Cisco Systems
March 2001
Expires September 2001
DHCP Lease Query
<draft-ietf-dhc-leasequery-01.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
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Copyright Notice
Copyright (C) The Internet Society (2001). 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 or DHCPUNKNOWN message. The DHCPKNOWN
response to a DHCPLEASEQUERY message allows the relay agent to deter-
mine the IP endpoint location, and the remaining duration 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 "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
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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
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) in Failover BNDUPD messages to the secon-
dary DHCP server, as recommended by section 7.1.1 of [FAILOVER].
The DHCPLEASEQUERY message is a query message only, and does not
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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 reconstruct 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 process
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 manage-
ment client interfaces nor LDAP client interfaces (although they typi-
cally do include SNMP management agents). This is a primary reason why
this document does not leverage the proposed DHCP Server MIB [DHCPMIB]
nor leverage the proposed DHCP LDAP schema [DHCPSCHEMA].
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
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valid for a particular downstream link is preferred over other mechan-
isms (ARP, SNMP, LDAP) because of the lack of additional network
traffic, but sometimes gleaning information can be incomplete. The
access concentrator usually cannot glean information from any DHCP uni-
cast (i.e. non-relayed) messages due to performance reasons. Further-
more, 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 Are Most 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 single
IP subnet extends behind many broadband modems). DHCP servers partici-
pate in all IP lease transactions (and therefore in all location infor-
mation updates) with DHCP clients, whereas access concentrators some-
times 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
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
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the "ciaddr" field empty and the MAC address (i.e., "htype", "hlen",
and "chaddr" fields) with a valid MAC address and/or a client-id
option (option 61) appearing in the options area. In this case, the
DHCP server SHOULD return an IP address in the "ciaddr". It MUST be
the IP address most recently used by the client described by the MAC
address or client-id option (or both, if both appear).
The DHCP servers that implement this protocol always sends a response
to the DHCPLEASEQUERY message: either a DHCPKNOWN or DHCPUNKNOWN. The
DHCP server replies to the DHCPLEASEQUERY message with a DHCPKNOWN
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". Note that non-DHCPLEASEQUERY-
literate DHCP servers SHOULD (and are expected to) drop the
DHCPLEASEQUERY message silently. The DHCPLEASEQUERY message can sup-
port three different query regimes:
o Query by IP address:
For this query, the client passes in an IP address and the DHCP
server the IP address and returns any information that it has on
the most recent client to utilized that IP address. Any server
which supports the DHCPLEASEQUERY message MUST support query by
IP address. If an IP address appears in the "ciaddr" field,
then the query MUST be by IP address regardless of the contents
of the MAC address or client-id option (if any).
o Query by MAC address:
For this query, the MAC address is specified in the "htype",
"hlen", and "chaddr" fields and no IP address is given in the
"ciaddr" field. The DHCP server looks up all IP addresses for
which clients with this MAC address are the most recent acces-
sor. It returns information associated with the IP address most
recently accessed by a DHCP client with this MAC 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 the associated-ip option (option TBD).
A server which implements the DHCPLEASEQUERY message SHOULD
implement this capability.
o Query by client-id option:
This query is similar to the query by MAC address, except that a
client-id option is present in the DHCPLEASEQUERY packet. In
this case, information on the IP address most recently accessed
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by a client with the included client-id will be returned in the
DHCPACK. If no MAC address is given in the DHCPLEASEQUERY
request, then all IP addresses which have been accessed by any
client with the included client-id SHOULD be returned in the
associated-ip option (option TBD). If a MAC address is present
in the DHCP packet, then the client-id and the MAC address both
must match the client information for an IP address for informa-
tion about that IP address to be returned either in the "ciaddr"
or the associated-ip option.
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 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
dhcp-parameter-request option 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 values for the DHCP packet beyond those defined
by [RFC 2131].
1. The message type option (option 53) from [RFC 2132] requires
three new values: The DHCPLEASEQUERY message itself and its
two responses DHCPKNOWN and DHCPUNKNOWN. 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
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 are three new options defined which can be used to return
important information in a DHCPKNOWN response to a DHCPLEASE-
QUERY message: associated-ip, client-last-transaction-time, and
client-requested-host-name. See Section 6.8 for details.
DISCUSSION:
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The associated-ip option is necessary to support returning
multiple IP addresses in a single DHCPKNOWN message.
The client-last-transaction-time is necessary in order to
allow an entity that receives multiple DHCPKNOWN messages
from different DHCP servers to compare the results and
extract the most recently used IP address from among the
multiple replies.
The client-requested-host-name is distinguished from the
host-name option in that the client-requested-host-name
option is used to return the name that the client requested
by either the host-name (option 12) or client-FQDN option
(option 81). It is different from the actual host-name
given to the client, which would be returned in the host-
name option. This may be a distinction which is not
interesting in general, and we might want to drop the
requirement for allocating an option for this purpose.
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 requestor (i.e.
the access concentrator). The giaddr is independent of the
ciaddr 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 requestor. The interesting options are likely
to include the IP Address Lease Time option (option 51) and the
Relay Agent Information 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 used to specify
if the response should include information encoded into reserva-
tions.
Additional details concerning different query types are:
o Query by IP address:
The values of htype, hlen, and chaddr MUST be set to 0.
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The ciaddr MUST be set to the IP address of the lease to be
queried.
The client-id 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 MUST be set to zero.
The client-id option (option 61) MUST NOT appear in the packet.
o Query by client-id option:
There MUST be a client-id option (option 61) in the DHCPLEASE-
QUERY message.
The ciaddr MUST be set to zero.
The values of htype, hlen, and chaddr MAY be set to the value of
the MAC address to search for. In this case, the search MUST
match both the values in the client-id option and the MAC
address specified in the "htype", "hlen", or "chaddr".
The access concentrator SHOULD ensure that the ciaddr mentioned in
the DHCPLEASEQUERY message (if a query by IP address) is a local sub-
net 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 at least one of: a non-zero ciaddr, a non-
zero "htype"/"hlen"/"chaddr", or a client-id. It MAY have more than
one.
The DHCP server which receives a DHCPLEASEQUERY message MUST base its
response (if any) on the IP address represented by the ciaddr in the
DHCPLEASEQUERY message if one is given.
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If an IP address is not given, then the receiving DHCP server MUST
base its response on the client-id and any MAC address contained in
the "htype", "hlen", and "chaddr" fields of the DHCP packet.
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
The DHCP server MUST respond to a DHCPLEASEQUERY message with a
DHCPKNOWN message if the ciaddr corresponds to an IP address which is
managed by the DHCP server or if there is an IP address which has
most recently been acccess by any DHCP client described by any
client-id option and/or MAC address information in the "htype",
"hlen", and "chaddr" fields of the DHCPLEASEQUERY request.
In the event that an IP address appears in the "ciaddr" field, then
the information returned should be about that IP address regardless
of the values of the MAC address and/or client-id option.
If the Reservation bit is not set in the "flags" field of the DHCP
packet (see [RFC 2131]), then the DHCP server SHOULD NOT respond to a
DHCPLEASEQUERY message with a DHCPKNOWN if the "ciaddr" corresponds
to an IP address about which the DHCP server has definitive informa-
tion but which has no DHCP client information associated with it. As
well, if the "ciaddr" does not contain an IP address and there is a
MAC address or client-id in the DHCPLEASEQUERY request, if the Reser-
vation bit is not set then the DHCP server SHOULD NOT respond with a
DHCPKNOWN unless the client specified in the DHCPLEASEQUERY has
accessed an IP address.
Conversely, if the Reservation bit is set in the "flags" field of the
DHCP packet, then the DHCP server SHOULD respond with information
contained in the reservation associated with either the IP address
specified in the "ciaddr" or the client specified in the MAC adddress
and/or client-id if there is no actual usage information concerning
the association of the IP address or specified client.
If the DHCP server uses reservation information to fill in the infor-
mation of a DHCPKNOWN message (other than using it to include an IP
address in an associated-ip option), the the DHCP server MUST set the
Reservation bit in the "flags" field of the DHCPKNOWN message.
Thus, a DHCP server SHOULD, but doesn't have to implement reservation
support if it implements support for the DHCPLEASEQUERY message, but
if it does, it MUST set the Reservation bit in the "flags" field
whenever the primary information it returns in the DHCPKNOWN message
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is based on a reservation.
The DHCP server MUST respond to the DHCPLEASEQUERY with a DHCPUNKNOWN
if the DHCP server supports the DHCPLEASEQUERY message but does not
have definitive information concerning the IP address in the ciaddr
(if any) or if it does not have definitive information concerning the
DHCP client specified in the "htype", "hlen", and "chaddr" fields or
the client-id option. When responding with a DHCPUNKNOWN, the DHCP
server SHOULD NOT include other DHCP options in the response.
A DHCP server which does not support the DHCPLEASEQUERY message MUST
NOT respond to the DHCPLEASEQUERY message.
When responding to a DHCPLEASEQUERY message with a DHCPKNOWN:
o In the case where more than one IP has been accessed by the
client specified by the MAC address and/or client-id option,
then the IP address most recently accessed by that client SHOULD
be used as the IP address to place into the "ciaddr".
In this case, all of the IP addresses which are recorded as hav-
ing been most recently been accessed by this client should be
returned in the associated-ip option (option TBD) if that option
is included in the dhcp-parameter-request-list option in the
request. They should appear in order of increasing age of
access in that option.
o If the IP Address Lease Time (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 reques-
tor (i.e. the access concentrator) to determine if there is
currently a valid lease for the IP address as well as the time
until the lease expiration.
A request for the Renewal (T1) Time Value option or the Rebind-
ing (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.
o If the DHCP server has information about the most recent device
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associated with the IP address specified in the ciaddr, then the
DHCP server MUST encode the physical address of that device in
the htype, hlen, and chaddr fields. Otherwise, the values of
htype, hlen, and chaddr MUST be set to 0 in the DHCPKNOWN. If
the IP Address Lease Time (option 51) is returned in the
DHCPKNOWN (indicating a currently valid lease by some device for
this IP address), the DHCP server MUST encode the physical
address of the device which owns the lease in the htype, hlen,
and chaddr fields.
o If the Relay Agent Information (option 82) is specified in the
Parameter 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.
In environments with non-DHCP-enabled devices, when the DHCP
server knows the network access information (perhaps through
server configuration), the DHCP server MAY generate its own
Relay Agent Information option value in the DHCPKNOWN; in such
cases, the DHCP server MUST generate an option value that the
access concentrator can process.
o The DHCPKNOWN 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 option values.
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
(82) values to the secondary server via the DHCP Failover BNDUPD mes-
sages.
The server expects a giaddr in the DHCPLEASEQUERY message, and uni-
casts the DHCPKNOWN or DHCPUNKNOWN to the giaddr. If the giaddr
field is zero, then the DHCP server does not reply to the DHCPLEASE-
QUERY message.
6.5. Receiving a DHCPKNOWN or DHCPUNKNOWN response to the DHCPLEASE-
QUERY Message
When a DHCPKNOWN message is received in response to the DHCPLEASE-
QUERY message and the DHCPKNOWN has an IP Address Lease Time option
value that is non-zero, it means that there is a currently active
lease for this IP address in this DHCP server. The access
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concentrator SHOULD use the information in the htype, hlen, and
chaddr fields of the DHCPKNOWN 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 and the DHCPKNOWN has no IP Address Lease Time option
(though one was requested in the Parameter Request List), 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 seri-
ously compromised device downstream of the access concentrator.
In either case, when a DHCPKNOWN 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 of the DHCPLEASEQUERY message. If there is
no IP address in the ciaddr of the DHCPLEASEQUERY message, then a
DHCPUNKNOWN message means that the DHCP server does not have defini-
tive information concering the any DHCP client specified in the
"hlen", "htype", and "chaddr" fields or the client-id 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 with a
DHCPUNKNOWN.
6.6. Receiving the 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 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.
In the first of the cases above, a retransmission of the
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DHCPLEASEQUERY 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 not 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.
6.7. Utilizing the DHCPLEASEQUERY message in a failover environment
When utilizing 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 useful 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
returned in the "ciaddr" field.
6.8. New options defined for responding to DHCPLEASEQUERY messages.
Three new options are defined for responding to DHCPLEASEQUERY mes-
sages:
1. client-last-transaction-time
2. associated-ip
3. client-requested-host-name
6.8.1. client-last-transaction-time
This option SHOULD record the time of the most recent access of the
client. It is particularly useful when DHCPLEASEQUERY responses from
two different DHCP servers need to be compared, although it can be
useful in other situations. The value is a duration in seconds in
the past from when this IP address was most recently accessed by the
client specified.
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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 |
+-----+-----+-----+-----+-----+-----+
6.8.2. associated-ip
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 | ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
6.8.3. client-requested-host-name
This option SHOULD contain the value of the host name requested by
the client in the host-name option (option 12) or the FQDN option
(option 81).
This option specifies the name of the client. The name may or may
not be qualified with the local domain name.
The code for this option is TBD, and its minimum length is 1.
Code Len Host Name
+-----+-----+-----+-----+-----+-----+-----+-----+--
| TBD | n | h1 | h2 | h3 | h4 | h5 | h6 | ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
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
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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 [DHCPAUTH]. 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. The
use of "Authentication Protocol 1" (using "delayed authentication")
is under investigation, since it requires two message round trips.
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 caching (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. 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.
9. 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.
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[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.
[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/.
[DHCPAUTH] Droms, R., Arbaugh, W., "Authentication for DHCP Mes-
sages", draft-ietf-dhc-authentication-14.txt, July 2000.
[DHCPMIB] Hibbs, R., Waters, G., "Dynamic Host Configuration Protocol
(DHCP) Server MIB", draft-ietf-dhc-server-mib-05.txt, November
2000.
[DHCPSCHEMA] Bennett, A., Volz, B., "DHCP Schema for LDAP", draft-
ietf-dhc-schema-02.txt, March 2000.
[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-08.txt, November 2000.
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10. Author's information
Rich Woundy
Kim Kinnear
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8000
EMail: rwoundy@cisco.com
kkinnear@cisco.com
11. Full Copyright Statement
Copyright (C) The Internet Society (2000). 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
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.
Open Issues
These issues need to be resolved by the working group:
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1. Should the DHCPLEASEQUERY message be extended to find lease
information by physical address or by DHCP Client ID? This
might be useful for non-router access concentrators.
[?] This capability has been added to the current draft since
we found it quite useful, and thought that others might as
well.
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