WDIFF

draft-ietf-dhc-leasequery-00.txt --> draft-ietf-dhc-leasequery-01.txt

Dynamic Host Configuration Working Group Rich Woundy
INTERNET DRAFT Kim Kinnear
Cisco Systems

November 2000

March 2001
Expires May September 2001

DHCP Lease Query
<draft-ietf-dhc-leasequery-00.txt>
<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
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.

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 DHCPACK DHCPKNOWN or DHCPNAK DHCPUNKNOWN message. The DHCPACK DHCPKNOWN
response to a DHCPLEASEQUERY message allows the relay agent to determine 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

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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 DHCPACK DHCPKNOWN or
DHCPNAK).
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 lease. or the binding information associ-
ated with it.

4. Design Goals

The core requirement of this document is to provide a lightweight

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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 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 pro-
cess 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
management manage-
ment client interfaces nor LDAP client interfaces (although they typically 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
mechanisms mechan-
isms (ARP, SNMP, LDAP) because of the lack of additional net-
work network
traffic, but sometimes gleaning information can be incomplete. The
access concentrator usually cannot glean information from any DHCP unicast uni-
cast (i.e. non-relayed) messages due to performance reasons.
Furthermore, Further-
more, the DHCP-gleaned location information often does not

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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 sin-
gle single
IP subnet extends behind many broadband modems). DHCP servers
participate partici-
pate in all IP lease transactions (and therefore in all loca-
tion information location infor-
mation updates) with DHCP clients, whereas access concen-
trators sometimes 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 DHCPACK DHCPKNOWN or DHCPNAK. DHCPUNKNOWN. The
DHCP server replies to the DHCPLEASEQUERY message with a DHCPACK 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 DHCPNAK DHCPUNKNOWN message if
the server does not have definitive location information concerning
the lease

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Internet Draft DHCP Lease Query November 2000 implied by the "ciaddr". Note that non-DHCPLEASEQUERY-literate non-DHCPLEASEQUERY-
literate DHCP servers SHOULD (and are expected to to) drop the
DHCPLEASEQUERY message silently. The DHCPACK DHCPLEASEQUERY message reply contains can sup-
port three different query regimes:

o Query by IP address:

For this query, the physical client passes in an IP address of and the DHCP
server the IP address lease owner 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 "htype", "hlen", and "chaddr" fields. The
reply often contains "ciaddr" field,
then the time until expiration query MUST be by IP address regardless of the lease, and the
original contents
of the Relay Agent Information MAC address or client-id option [RELAYAGEN-
TINFO]. The access concentrator uses (if any).

o Query by MAC address:

For this query, the MAC address is specified in the "htype",
"hlen", and "chaddr" fields and Relay Agent
Information option to construct location information, which can be
cached on no IP address is given in the access concentrator until lease expiration.

Any
"ciaddr" field. The DHCP server looks up all IP addresses for
which supports the DHCPLEASEQUERY message SHOULD save
the information from clients with this MAC address are the most recent Relay Agent Information option
[RELAYAGENTINFO] acces-
sor. It returns information associated with every the IP address which it serves.

6. Protocol Details

6.1. Sending the DHCPLEASEQUERY Message

The DHCPLEASEQUERY message is typically sent most
recently accessed by an access concentra-
tor. The DHCPLEASEQUERY message uses a DHCP client with this MAC address. If
requested, the DHCP message format as
described in [RFC 2131], and uses message number TBD in server SHOULD return information on all of
the IP addresses it found to be associated with the DHCP Mes-
sage Type client
with the MAC address in the associated-ip option (option 53). The DHCPLEASEQUERY message has TBD).
A server which implements the
following pertinent DHCPLEASEQUERY message contents: SHOULD
implement this capability.

o The values of htype, hlen, and chaddr MUST be set to 0. Query by client-id option:

This
DHCP message query is used for querying on IP similar to the query by MAC address, not except that a
client-id option is present in the DHCPLEASEQUERY packet. In
this case, information on hardware the IP address or most recently accessed

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by a client ID.

o The ciaddr MUST with the included client-id will be set to 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 of is present
in the lease 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
queried.

o The giaddr MUST be set to returned either in the "ciaddr"
or the associated-ip option.

Generally, the query by IP address of is likely to be the requestor (i.e. most efficient
and widely implemented form of leasequery, and it SHOULD be used if
at all possible. Use of the access concentrator). other two query formats SHOULD be minim-
ized, as they can potentially place a large load on some servers.

The giaddr is independent of DHCPKNOWN message reply MUST always contain the IP address in the
ciaddr to be searched -- it is simply field and SHOULD contains the return physical address of for the DHCPACK or DHCPNAK message from IP
address lease owner in the DHCP server.

o "htype", "hlen", and "chaddr" fields. The Parameter Request List SHOULD
dhcp-parameter-request option can be set used to the request specific options of
interest
to be returned about the requestor. IP address in the ciaddr. The interesting options are likely
to include reply often
contains the IP Address Lease Time option (option 51) time until expiration of the lease, and the original
contents of the Relay Agent Information option (82). [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 ensure that save
the ciaddr mentioned in 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 is a local subnet requires the definition
of the interface speci-
fied following new values for the client. 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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The March 2001

Value Message Type
----- ------------
1 DHCPDISCOVER
2 DHCPOFFER
3 DHCPREQUEST
4 DHCPDECLINE
5 DHCPACK
6 DHCPNAK
7 DHCPRELEASE
8 DHCPINFORM
TBD DHCPLEASEQUERY message SHOULD be sent to a DHCP server which
TBD DHCPKNOWN
TBD DHCPUNKNOWN

2. There is
known to possess authoritative information concerning a new bit defined in the flags field of the IP address.
The DHCPLEASEQUERY message MAY be sent to more than one DHCP server,
packet (see Section 1, Figure 1 and in Table 1 of [RFC 2131]). It
is called the absence 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 information concerning which DHCP server might
possess authoritative information concerning the IP address, it
SHOULD 'flags' field

3. There are three new options defined which can be sent used to all DHCP servers configured for the associated
relay agent (if any are known).

6.2. Receiving the DHCPLEASEQUERY Message

A DHCPLEASEQUERY message MUST have return
important information in a non-zero ciaddr DHCPKNOWN response to a DHCPLEASE-
QUERY message: associated-ip, client-last-transaction-time, and MUST have
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
non-zero giaddr. single DHCPKNOWN message.

The DHCP server which client-last-transaction-time is necessary in order to
allow an entity that receives a DHCPLEASEQUERY
message MUST base its response (if any) on multiple DHCPKNOWN messages
from different DHCP servers to compare the results and
extract the most recently used IP address represented
by from among the ciaddr
multiple replies.

The client-requested-host-name is distinguished from the
host-name option in that the DHCPLEASEQUERY message. The giaddr client-requested-host-name
option is used only
for to return the destination address of any generated response and, while
required, name that the client requested
by either the host-name (option 12) or client-FQDN option
(option 81). It is not otherwise used in generating different from the response actual host-name
given to the
DHCPLEASEQUERY message.

6.3. Responding 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 DHCP server MUST respond to a DHCPLEASEQUERY message with a
DHCPACK message if the ciaddr corresponds to an IP address which is
managed typically sent by an access concentra-
tor. The DHCPLEASEQUERY message uses the DHCP server.

The message format as
described in [RFC 2131], and uses message number TBD in the DHCP server SHOULD respond to a Mes-
sage Type option (option 53). The DHCPLEASEQUERY message with a
DHCPACK if has the ciaddr corresponds
following pertinent message contents:

o The giaddr MUST be set to an the IP address about which of the
DHCP server has definitive information, even if requestor (i.e.
the access concentrator). The giaddr is independent of the
ciaddr does not
correspond to an IP address which might be dynamically allocated by
the DHCP server searched -- for example, a statically allocated IP address
which it is known to be reserved simply the return address of for a particular device by
the DHCPKNOWN or DHCPUNKNOWN message from the DHCP server.

o The DHCP server MUST respond Parameter Request List SHOULD be set to the DHCPLEASEQUERY with a DHCPNAK if
the DHCP server supports the DHCPLEASEQUERY message but does not have
definitive information concerning the IP address in the ciaddr. When
responding with a DHCPNAK, 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 of
interest to the DHCPLEASEQUERY message.

When responding requestor. The interesting options are likely
to a DHCPLEASEQUERY message with a DHCPACK:

o If include 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
Relay Agent Information option (82).

o The Reservation bit in the ciaddr, then "flags" field of the DHCP server
MUST return packet (see
[RFC 2131] and Section 6.1 of this option in the DHCPACK with its value equal document) is used to specify
if the time remaining until lease expiration. If there is no valid 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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lease for March 2001

The ciaddr MUST be set to the IP address, then address of the server lease to be
queried.

The client-id option (option 61) MUST NOT return appear in the IP
Address Lease Time option (option 51). This allows the reques-
tor (i.e. the access concentrator) packet.

o Query by MAC address:

The values of htype, hlen, and chaddr MUST be set 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 value
of the
DHCPLEASEQUERY message MAC address to search for.

The ciaddr MUST be handled like the IP Address Lease
Time set to zero.

The client-id 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 (option 61) MUST NOT return these options.

o If the DHCP server has information about the most recent device
associated with the IP address specified appear in the ciaddr, then the
DHCP server packet.

o Query by client-id option:

There MUST encode the physical address of that device be a client-id option (option 61) in the htype, hlen, and chaddr fields. Otherwise, the DHCPLEASE-
QUERY message.

The ciaddr MUST be set to zero.

The values of htype, hlen, and chaddr MUST MAY be set to 0 in the DHCPACK. If the
IP Address Lease Time (option 51) is returned in value of
the DHCPACK
(indicating a currently valid lease by some device for MAC address to search for. In this IP
address), case, the DHCP server search MUST encode the physical address of
the device which owns
match both the lease values in the htype, hlen, client-id option and chaddr
fields.

o If the Relay Agent Information (option 82) is MAC
address specified in the
Parameter Request List and if "htype", "hlen", or "chaddr".

The access concentrator SHOULD ensure that the DHCP server has saved the
information contained ciaddr mentioned in
the most recent Relay Agent Information
option, the DHCP server MUST include that information in DHCPLEASEQUERY message (if a Relay
Agent Information option in query by IP address) is a local sub-
net of the DHCPACK.

In environments with non-DHCP-enabled devices, when interface specified for the client.

The DHCPLEASEQUERY message SHOULD be sent to a DHCP server knows the network access which is
known to possess authoritative information (perhaps through
server configuration), concerning the DHCP server IP address.
The DHCPLEASEQUERY message MAY generate its own
Relay Agent Information option value in the DHCPACK; be sent to more than one DHCP server,
and in such
cases, the absence of information concerning which DHCP server MUST generate an option value that might
possess authoritative information concerning the
access concentrator can process.

o The DHCPACK message IP address, it
SHOULD include the values of be sent to all other
options not specifically discussed above that were requested in DHCP servers configured for the Parameter Request List of associated
relay agent (if any are known).

6.3. Receiving the DHCPLEASEQUERY message. 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 uses information from which receives a DHCPLEASEQUERY message MUST base its
response (if any) on the lease binding database to
supply IP address represented by the DHCPACK option values.

In order to accommodate DHCPLEASEQUERY messages sent to a DHCP Fail-
over secondary server [FAILOVER] when ciaddr in the primary server
DHCPLEASEQUERY message if one is down, the given.

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primary server MUST communicate the Relay Agent Information option
(82) values to the secondary server via March 2001

If an IP address is not given, then the receiving DHCP Failover BNDUPD mes-
sages.

The server expects a giaddr in MUST
base its response on the DHCPLEASEQUERY message, client-id and uni-
casts the DHCPACK or DHCPNAK to any MAC address contained in
the giaddr. If "htype", "hlen", and "chaddr" fields of the DHCP packet.

The giaddr field is
zero, then used only for the DHCP server does destination address of any generated
response and, while required, is not reply otherwise used in generating the
response to the DHCPLEASEQUERY mes-
sage. message.

6.4. Receiving a DHCPACK or DHCPNAK response Responding to the DHCPLEASEQUERY Mes-
sage

When a DHCPACK message is received in response Message

The DHCP server MUST respond to the a DHCPLEASEQUERY message and with a
DHCPKNOWN message if the DHCPACK has ciaddr corresponds to an IP Address Lease Time option value
that address which is non-zero, it means that
managed by the DHCP server or if there is a currently active lease for
this an IP address in this which has
most recently been acccess by any DHCP server. The access concentrator SHOULD
use the client described by any
client-id option and/or MAC address information in the htype, hlen, "htype",
"hlen", and chaddr "chaddr" fields of the
DHCPACK as well as any Relay Agent Information option information
included in DHCPLEASEQUERY request.

In the packet to refresh its location information for this event that an IP address.

When a DHCPACK message is received address appears in response to the DHCPLEASEQUERY
message and "ciaddr" field, then
the DHCPACK has no information returned should be about that IP Address Lease Time option (though
one was requested in address regardless
of the Parameter Request List), that means that
there is no currently active lease for values of the IP MAC address present and/or client-id option.

If the Reservation bit is not set in the
DHCP server. In this case, the access concentrator SHOULD cache this
information in order to prevent unacceptable loads on "flags" field of the access con-
centrator and DHCP
packet (see [RFC 2131]), then the DHCP server in the face of a malicious or seriously
compromised device downstream of the access concentrator.

In either case, when a DHCPACK message is received in response SHOULD NOT respond to a
DHCPLEASEQUERY message, it means that message with a DHCPKNOWN if the DHCP server "ciaddr" corresponds
to an IP address about which responded
is a the DHCP server has definitive informa-
tion but which manages has no DHCP client information associated with it. As
well, if the "ciaddr" does not contain an IP address present in the ciaddr, and the Relay Agent SHOULD cache this information for later use.

When a DHCPNAK message there is received by an access concentrator which
has sent out a
MAC address or client-id in the DHCPLEASEQUERY message, it means that request, if the Reser-
vation bit is not set then the DHCP server
contacted supports SHOULD NOT respond with a
DHCPKNOWN unless the client specified in the DHCPLEASEQUERY message but that has
accessed an IP address.

Conversely, if the Reservation bit is set in the "flags" field of the
DHCP packet, then the DHCP server not have definitive SHOULD respond with information concerning
contained in the reservation associated with either the IP address con-
tained
specified in the ciaddr "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 DHCPLEASEQUERY message. It doesn't
manage this IP address.

The access concentrator SHOULD cache this information, and only
infrequently direct address or specified client.

If the DHCP server uses reservation information to fill in the infor-
mation of a DHCPLEASEQUERY DHCPKNOWN message (other than using it to a include an IP
address in an associated-ip option), the the DHCP server that
responded to MUST set the
Reservation bit in the "flags" field of the DHCPKNOWN message.

Thus, a DHCPLEASEQUERY message DHCP server SHOULD, but doesn't have to implement reservation
support if it implements support for a particular ciaddr with a
DHCPNAK. 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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6.5. Receiving no response to the DHCPLEASEQUERY Message

When an access concentrator receives no response to March 2001

is based on a DHCPLEASEQUERY
message, there are several possible reasons:

o reservation.

The DHCPLEASEQUERY or a corresponding DHCPACK or DHCPNAK were
lost during transmission or DHCP server MUST respond to the DHCPLEASEQUERY arrived at with a DHCPUNKNOWN
if the DHCP server but it was dropped because supports the server was too busy.

o The DHCP server doesn't support DHCPLEASEQUERY.

In DHCPLEASEQUERY message but does not
have definitive information concerning the first of IP address in the cases above, a retransmission of ciaddr
(if any) or if it does not have definitive information concerning the DHCPLEASE-
QUERY would be appropriate, but
DHCP client specified in the second of "htype", "hlen", and "chaddr" fields or
the two cases, a
retransmission would not be appropriate. There is no way to tell
these two cases apart (other than, perhaps, because of client-id option. When responding with a DHCPUNKNOWN, the DHCP
server's response to
server SHOULD NOT include other DHCPLEASEQUERY messages indicating that it
supports DHCP options in the DHCPLEASEQUERY message).

An access concentrator response.

A DHCP server which utilizes does not support the DHCPLEASEQUERY message
SHOULD attempt MUST
NOT respond to resend the DHCPLEASEQUERY messages to servers which do
not respond message.

When responding 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 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
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 not otherwise known particularly useful when DHCPLEASEQUERY responses from
two different DHCP servers need to support 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
DHCPLEASEQUERY message at a rate name of not more than approximately one
packet every 10 seconds, and yet (if the access concentrator needs to
send DHCPLEASEQUERY messages) client. The name may or may
not less than one DHCPLEASEQUERY per
minute. 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

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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 DHCPACK DHCPKNOWN and DHCPNAK
DHCPUNKNOWN responses to DHCPLEASEQUERY messages) and ciaddr restriction 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.

Ralph Droms, Mark Stapp and Andy Sudduth contributed to making the
draft more complete and helped add clarity.

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.

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[RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
Extensions", Internet 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.

[RELAYAGENTINFO] Patrick, M., "DHCP Relay Agent Information Option",
draft-ietf-dhc-agent-options-12.txt, October 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

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EMail: rwoundy@cisco.com
kkinnear@cisco.com

11. Full Copyright Statement

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. May the DHCPLEASEQUERY message be sent by parties other than
relay agents?

[Resolved] Sure, you can't stop them in any case.

2. 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.

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[Resolved] There has been no working group interest in this
aspect of the DHCPLEASEQUERY message, so it

[?] This capability has been specifi-
cally excluded.

3. How can the DHCPLEASEQUERY message exchange be modified added to lev-
erage the better DHCP authentication protocol types?

[Unresolved] current draft since
we found it quite useful, and thought that others might as
well.

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