Internet Engineering Task Force R. Johnson
Internet Draft Jay Kumarasamy
Expiration: April 2003 Kim Kinnear
File: draft-johnson-dhc-subnet-alloc-00.txt Mark Stapp
Cisco Systems, Inc.
Subnet Allocation using DHCP
<draft-johnson-dhc-subnet-alloc-00.txt>
October 24, 2002
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a new DHCP option which is passed between the
DHCP Client to the DHCP Server to request dynamic allocation of a
subnet, give specifications of subnet(s) allocated, and report usage
statistics.
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1.0 Introduction
There is a need for a DHCP Client to be able to allocate a subnet
from a DHCP Server. Alternate methods of allocation, such as AAA are
not appropriate for various reasons and the most straight-forward way
to accomplish this allocation is via DHCP. A DHCP option to support
this may be utilized by a simple Dialin Aggregation box, or even to
implement a Hierarchical chain of DHCP Servers, each one in turn
leasing one or more subnets to the next DHCP Server down the chain.
This new DHCP option [2], the Subnet Allocation option is specified
in such a way as to use one DHCP Option number, while using suboption
numbers for each function. The Subnet-Request suboption tells what
types of subnets are needed and how many. The "Subnet Information"
suboption gives the actual subnet number(s) and allows for extra
flags to convey additional information about each subnet. The
"Subnet Name" suboption allows a method of passing additional
information about the requested subnet(s), such as department name,
user name, customer number, etc. The DHCP Server has the option of
not supplying all subnets requested or even returning smaller subnets
than was requested. The "Subnet Usage Statistics" suboption is used
to report usage information from the DHCP Client back to the DHCP
Server.
1.1 Conventions
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 [3].
This document also uses the following terms:
"DHCP Client"
DHCP Client or "Client" is an Internet host using DHCP to
obtain configuration parameters such as a network address.
"DHCP Server"
A DHCP Server or "Server" is an Internet host that returns
configuration parameters to DHCP Clients.
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2.0 Option Formats:
2.1 Subnet Allocation Option Format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Len | Flags | Suboptions ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code = Subnet Allocation option code (1 octet) TBD (220 currently)
Len = length of the entire option including all sub-options and
excluding the "Code" and "Len" fields above (1 octet)
Flags = Various flags: (None currently defined)
One or more sub-options may be specified as described below.
2.2 Subnet Request suboption format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Len | Flags |i|h| Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = length of the suboption excluding the subcode and Len
fields
(1 octet)
Flags = Flags field. (all unused bits MBZ)
"h" = "1" = Client will be allocating addresses from this
subnet.
"0" = Client will be relaying DHCP requests to the
Server from this subnet.
"i" = "1" = Client is seeking information about previously
allocated subnets.
"0" = Client is seeking a new subnet allocation.
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Prefix = size of the subnet needed [4] (number of bits in subnet
prefix)
(zero (0) means no suggested size is given)
(1 octet)
The DHCP Server SHOULD NOT include the Subnet Request suboption in
any replies to the DHCP Client. Enough information will be present
in the Subnet Information suboption, such that the Subnet Request
suboption qis not needed in replies to the Client.
The DHCP Server SHOULD allocate a subnet with prefix size less than
or equal to the size P specified in the request. In other words, a
subnet at least the size requested and possibly bigger.
A size P of zero (0) MAY be specified by the DHCP Client. In this
case, no suggested size is given and the Server is free to return
subnet(s) of whatever size is deemed appropriate by the Server.
Multiple Subnet Request suboptions in a DHCP packet indicate that
multiple sizes of subnets are being requested.
Each Subnet Request suboption MUST result in no more than one (1)
Subnet Information suboption in the DHCP OFFER message from the
Server, and may result in zero (0) Subnet Information suboptions.
Setting the "h" flag to "1" indicates the Client will be allocating
addresses from the allocated subnet(s) itself. This can be thought
of as a "Hierarchial DHCP" design in that control of allocation for
the subnet(s) will be passed to the Client.
Setting the "h" flag to "0" indicates the Client wants the DHCP
Server to retain control over allocation of addresses from the
subnet(s). In this case, the Server should simply mark the subnet(s)
as "used" by this Client and not return the subnet(s) in any Subnet
Information suboptions to any other client. (Any address allocation
requests on the subnet will be relayed back to the DHCP Server.)
Setting the "i" flag to "1" indicates the Client is NOT seeking
allocation of any subnets, but is simply seeking information from the
Server as to what subnet(s) have been allocated (or reserved) for
this Client. If the "i" flag is set to "1", then the "P" field
SHOULD be set to "0" and MUST be ignored by the Server.
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2.3 Subnet Information suboption format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Len | Flags |c|s| SP1, SP2, ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = length of the sub-option excluding the Subcode and Len
fields (1 octet)
Flags = Various flags which apply to ALL Subnet Prefix Information
fields specified in this suboption
"c" = Client flag (explained below)
"s" = Server flag (explained below)
SP1,SP2 = Subnet Prefix information as specified below (variable
sized)
The "Client flag" ("c") is set to "1" if this Subnet Information
suboption is in response to a Client request for information from the
Server as to what subnet(s) have been allocated. This flag is only
used in response to a Subnet Request suboption with the "i" flag set
and should be zero (0) otherwise.
The "Server flag" ("s") is set to "1" if the Server has additional
subnet information for the Client.
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2.4 Subnet Prefix Information suboption Format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix | Flags |h|d| Stat-len | Optional statistics...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Addr = IPv4 address (4 octets)
Prefix = specifying number of prefix bits in the subnet (1 octet)
Flags = flags field (Undefined bits must be zero) (1 octet)
"d" = "1" = Deprecate this subnet
"h" = "1" = Client will be allocating addresses from this
subnet.
"0" = Client will be relaying DHCP requests to the
Server from this subnet.
Stat-len = length of the optional statistics information field
Subnet Prefix suboption format:
The "d" flag may only be returned by the Server to the Client. It's
presensce means that the Client should prepare to give up the subnet.
For example, if the Client is assigning addresses from this subnet to
other clients, it should cease doing so immediately and should not
renew any leases when client's ask for renewal. As soon as all
addresses in the subnet are unallocated, the Client should send a
DHCP RELEASE message to the Server, including a Subnet Prefix
Information suboption for the subnet in order to release the Subnet.
The format of this message is described below.
The "h" flag tells the Client how the Server intends the Client to
use the allocated subnet. It is interpreted in the same manner as
that in the Subnet Request suboption. In response to a Subnet
Request, the Server should normally specify the "h" flag in the same
mannor was it was in the Subnet Request suboption from the Client.
The Server MAY, however, change the "h" flag from that specified in
the Subnet Request suboption if it has been configured to override
the Client's request.
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If any usage statistics information is to be included, then the
"Stat-len" field specifies the number of bytes of statistics
information which is included. See below for more information. If
no statistics information is included, then this byte MUST be zero.
2.4.1 Subnet Usage Statistics
The Subnet Information suboption may also include usage statistics
information. If this information is included, then the "Stat-len"
(Statistics length) field MUST be set to the number of bytes of
statistics information which is being included. The statistics
information MUST be in the following form and order:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| High water |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Currently in use |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unusable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
"High water" refers the to "high water mark" of allocated addresses
within the subnet.
"Currently in use" refers to the number of addresses currently
allocated in the subnet.
"Unusable" refers to the number of addresses which are currently
unusable for any reason (such as a client returning a DHCP DECLINE,
or finding the address already in use).
Additional statistics may be added to this option in the future. If
so, they MUST be appended to the end of the option. All statistics
fields MUST remain in the same order. Use the all ones value
(0xFFFF) in order to skip reporting a number for a particular field.
Fewer fields may be included than what is specified in any current
RFC, but all fields which are included MUST remain in order specifed
here.
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2.5 Subnet Name suboption format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | Len | Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = length of the sub-option excluding the Subcode and Len fields
(1 octet)
The Subnet Name suboption may be used in order to pass a subnet name
to the server for use during allocation. This name may be used for
any purpose but is intended to tell the server something extra about
the needed subnet; for example, "sales department", "customer 1002",
"address pool FOO", or some such. The "name" should NOT be NULL
terminated since the "len" field already specifies the length of the
name.
3.0 Requesting allocation of a subnet
3.1 Client DISCOVER message
The DHCP Client creates a DHCP DISCOVER message including the
Subnet Allocation option, and its set of suboptions, to request
allocation of a subnet. The DHCP Client should include the Subnet
Request suboption, specifying the prefix size of the subnet
requested. The "h" bit should be set to "1" if the Client intends
to control allocation of addresses within the subnet itself, or
"0" if the Server should retain control of addresses within the
subnet. More than one Subnet Allocation option may appear in a
DHCP DISCOVER message, however the client SHOULD limit the number
of requests, noting that the DHCP replies will need to include the
Subnet Information suboption, which takes up more space.
If more than one subnet size is being requested, multiple Subnet
Request suboptions MAY be included or multiple DHCP DISCOVER
messages MAY be sent instead. The prefix size field of each
Subnet Request suboption MUST be either zero (0), or in the range
of 1 to 30 inclusive.
The DHCP "IP address lease time" option (code 51) MAY be included
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in the DHCP DISCOVER message to specify the lease time the Client
is requesting. If not present, no suggested lease time is given.
The DHCP "Client ID" option (code 61) MAY be included in the DHCP
DISCOVER message as it may be used by the Server in performing the
subnet allocation.
3.2 Server OFFER message
Upon receiving a DHCP DISCOVER containing the Subnet Allocation
option, the DHCP Server should respond with a DHCP OFFER message
including the Subnet Information suboption in order to specify the
subnet(s) which it is willing to allocate to the Client in order
to fill the request(s).
The Server need not reserve the subnets which are being OFFERed,
but SHOULD strive to not OFFER the same subnets to another DHCP
Client until a reasonable time period (implementation independent)
has passed.
The Server MUST NOT include the Subnet Request suboption in the
OFFER. The same information is already present in the Subnet
Information suboption(s) which SHOULD be included in the OFFER.
The Server SHOULD also include the IP address lease time option
(option 51) in the DHCP OFFER message. This gives the lease time
for all subnets given in all Subnet Request suboptions contained
in the DHCP OFFER message. The Server MAY also include the
Renewal and/or Rebinding options in order to further control the
"T1" and "T2" lease timers of the client. There MUST be only one
IP address lease time, rebind, and/or renew option in the DHCP
OFFER message. If different lease times are required for some of
the allocated subnets, then the server should only return Subnet
Information suboption(s) for those subnets with the same lease
time. If the Client requires more subnets, another DHCP DISCOVER
message will need to be issued to collect the other needed
subnets. Also see the "s" flag described below.
The Server MAY set the "Server flag" ("s") to "1" to indicate that
it would like to allocate one or more additional subnet(s) to the
Client. This indicates that the Client should send another
DISCOVER message specifying a zero prefix size field, P, in order
to request the additional subnet allocation(s) information. This
may be necessary if the subnets are to be allocated with different
lease times, for example.
The "Client flag" ("c") MUST be set to zero (0) to indicate this
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is a response to a client request for a new subnet allocation and
not a request for information about already allocated subnets.
The Server SHOULD set the DHCP yiaddr value to all zeros (0.0.0.0)
and the Client MUST ignore fields having to do with address
assignment if the packet contains a Subnet Allocation option. In
other words, a DHCP packet exchange can not provide subnet
allocation and address assignment simultaneously.
3.3 Client DHCP REQUEST message
When sending a DHCP REQUEST, the Client MUST NOT modify any fields
of all Subnet Information suboptions received from the Server.
However, the Client MAY choose to not include some Subnet
Information suboptions when issuing the DHCP REQUEST. Subnet
Request suboptions MUST NOT be included in the DHCP REQUEST
message, only the Subnet Information suboption(s) should be
included.
3.4 Server DHCP ACK message
The DHCP Server, upon receipt of the Client's REQUEST message, MAY
refuse allocation of any subnets (for example, if they have been
allocated elsewhere in the meantime), however since the Server
should have set aside the subnets offered for a short period of
time, and since the Client should have requested the subnets
within a short period of time after receiving the offer(s) from
the server(s), this last minute rejection should be rare. The
DHCP Server MAY NOT change the subnet address(es) or prefix
size(s), however it MAY remove some Subnet Information suboptions
from the list.
The Server SHOULD include the IP address lease time option
specifying the lease period for all subnet(s) in the ACK. All
subnets allocated in one DHCP message will have the same lease
time and only one IP address lease time option must appear in the
DHCP message.
If the Server has internal information which states that the
Client should be allocated more subnets than were requested, the
Server MAY set the "s" bit in the Subnet Information suboption to
indicate that the Client needs to request more subnets (as
described above).
The allocatable unit is the tuple (subnet address, prefix size).
Multiple subnets may be allocated in one DHCP ACK, however since
there can be only one Lease-time option, each subnet allocated is
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assigned the same lease time. Each subnet lease MAY be RENEWed or
RELEASEd individually.
4.0 Client renewal of subnet lease
4.1 Client RENEW REQUEST message
The Client MUST renew all subnets allocated with a lease time in
much the same manner as renewing an allocated IP address. Renewal
timers need not be set in exactly the same manner, however. If
Renewal and/or Rebinding options were included in the ACK of the
subnet allocation, then these "T1" and "T2" timers should be used
just as they would be in the case of address allocation timers.
The REQUEST message MUST include a Subnet Information suboption
for which the Client is seeking renewal of the lease. This Subnet
Information suboption may optionally include subnet usage
statistics, as described above w.r.t. the Subnet Information
suboption format.
The subnet IP address field (Address) and subnet prefix field
(Prefix) MUST agree with the values as they were originally
allocated to the Client by the Server. In any of the statistics
fields (High, Current, Ususable), a value of all ones (0xffff)
SHOULD be used if the Client has no information to report for a
statistic.
4.2 Server RENEW response
The Server MAY respond to a subnet RENEW request with either an
ACK or NAK response. If a NAK response is given the Client MUST
immediately stop using the subnet(s) specified and, if possible,
notify all Clients with addresses allocated from this subnet that
their addresses are no longer valid. The Client MAY, of course,
send a DHCP DISCOVER message containing the Subnet Allocation
option and the Subnet Request suboption in order to acquire
another subnet for use. In general, the Server should ask the
Client to "free" subnets by using the "Deprecate" bit of the
Subnet Information suboption in an ACK message (see below).
If an ACK response is given, the "Deprecate" ("d") bit of the
flags field in the Subnet Information suboption may also be set.
This indicates the DHCP Client should "prepare to stop using this
subnet". If the Client is allocating IP addresses for other
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clients out of this subnet (probably via DHCP), the Client SHOULD
immediately stop allocating such addresses. Once all allocated
addresses in the subnet have been released, the Client SHOULD send
a DHCP RELEASE message, including the Subnet Information suboption
(with optional usage statistics) in order to release the subnet(s)
back to the Server.
4.3 Client RELEASE message
The DHCP Client should send a DHCP RELEASE message in order to
release allocated subnet(s) back to the Server when it is finished
using them. This message MUST NOT include the Subnet Request
suboption, but MUST include one or more Subnet Information
suboptions, and optionally including usage statistics.
The Client MUST release the same subnet(s) of the same prefix size
(Prefix), as was originally allocated. The Client MAY release a
subset of the subnets which were allocated originally. In other
words, the allocatable unit is the tuple (subnet address, prefix
size). Multiple subnets may be allocated in one DHCP ACK, however
each subnet MAY be released individually.
4.4 Server RECONFIGURE message
The DHCP Server may issue a DHCP RECONFIGURE message containing the
Subnet Allocation option and the Subnet Information suboption. This
message effectively immediately times out the Client's lease(s) for
the allocated subnet(s). Upon receiving this message, the DHCP
Client MUST issue a DHCP REQUEST message to the DHCP Server in order
to renew the lease on the subnet mentioned. No other subnets
allocated to the Client are effected. As is the case with all DHCP
Renewal messages, the Client may include subnet usage information in
the Subnet Information suboption in order to report subnet usage
statistics, or set the "Stat-len" field to zero (0) if no statistics
are to be reported.
If the Server responds to this REQUEST with a DHCP NAK message, then
the Client MUST immediately stop using the subnet(s) and, if
possible, notify all Clients with addresses allocated from this/these
subnet(s) that their addresses are no longer valid. The Client MAY,
of course, send a DHCP DISCOVER message containing the Subnet
Allocation option and the Subnet Request suboption in order to
acquire another subnet for use.
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5.0 Client requesting subnet allocation information:
The DHCP Client may request from the DHCP Server a list of what
subnets are currently allocated to Client. This may be used to
recover from a restart if the Client does not have local storage in
order to retain the information itself.
5.1 Client DISCOVER message
The DHCP Client DISCOVER message, in order to discover already
allocated subnet information, should contain a Subnet Request
suboption, with the "Prefix" field set to zero (0) and with the
"i" flag set to "1" to indicate that the Client is seeking already
allocated subnet information from the Server. No Subnet
Information suboptions should be included in this message.
This DISCOVER message MAY be unicast to a particular DHCP Server,
or broadcast in the normal fashion.
5.2 Server OFFER response
Any DHCP Server which has allocated subnets to the Client should
respond to the DISCOVER message with a DHCP OFFER message The
OFFER message should contain one or more Subnet Information
suboption(s) telling the subnet address(es) and prefix(es) of the
subnet(s) allocated to the Client.
The Server SHOULD, internally, retain an ordered list of subnets
which are allocated to each Client. The subnet(s) information
returned in the OFFER message are the first subnet(s) from this
list. If the end of the list has been reached, then the "s" bit
should be set to "0". If there are more subnets in the list, the
"s" bit should be set to "1". to indicate to the Client that more
information is available. If this is the initial OFFER to the
client, the "c" flag should be set to "1".
5.3 Client additional DISCOVER messages
The Client, upon receiving any Server OFFER messages containing
Subnet Information suboption information with the "c" ("Client")
bit set, should gather the subnet address and prefix information
from the message.
If the "s" bit is set in the Subnet Information suboption, then
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the client MUST construct a new DHCP DISCOVER message containing
the Subnet Allocation option and the Subnet Information suboption,
and send this message back to the same DHCP Server originating the
OFFER message. The "c" and "s" bits MUST retain the same settings
they had in the Server's OFFER message and the subnet address
("A") and prefix size ("P") fields MUST be unaltered as well.
If the "s" bit in the Subnet Information suboption from the Server
was "0", then it indicates the Server has no more information
about subnets allocated to the Client. In this case, the Client
MUST NOT send a REQUEST response to the Server.
5.4 Server additional OFFER messages
The Server, upon receiving a DISCOVER message from a Client
containing a Subnet Information suboption with the "c" and the "s"
bits set, MUST use the subnet address ("A") and prefix size ("P")
fields in order to locate the position in the internal table of
allocated subnets for this Client, and then return an OFFER
message containing a Subnet Information suboption giving
information about the next set of subnets allocated to this
Client. If this finishes the list in the table for this Client,
then the "s" bit MUST be set to "0" to indicate there is no more
information.
6.0 DHCP Server Subnet Allocation method
The actual method of allocating subnets on the DHCP Server, as well
as the configuration of what networks may be subnetted and how, is
left up to the implementation.
7.0 Examples
Only the Subnet Allocation option and accompanying suboptions are
displayed in these examples. All other fields in the DHCP messages
are described in [1]. For the purposes of these examples, "SAC"
stands for the actual code number allocated for the "Subnet
Allocation option Code".
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7.1 Example 1:
DHCP Client requesting a subnet with prefix size 24 from which the
Client will allocate addresses to other clients. The Server responds
with allocation of exactly the size requested:
Client sends DHCP DISCOVER including the Subnet Allocation option
with the Subnet-Request suboption:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 5 | 0 | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 |0|0| 24 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Server responds with DHCP OFFER including Subnet Allocation option
with a Subnet Information suboption:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | 0 |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
Client sends DHCP REQUEST including Subnet Allocation option with
Subnet Information suboption:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | 0 |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
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Server responds with DHCP ACK including Subnet Allocation option
with Subnet-Info suboption:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | 0 |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
Later Client sends DHCP RELEASE including Subnet Allocation option
with Subnet Information suboption:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | 0 |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
7.2 Example 2:
DHCP Client requesting a subnet with prefix size 24 and a subnet with
prefix size 30:
Client sends DHCP DISCOVER including the Subnet Allocation option
with the Subnet-Request suboption:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 9 | 0 | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 |0|0| 24 | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 |0|0| 30 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Server responds with DHCP OFFER including Subnet Allocation option
with Subnet Information suboption:
Offer includes 1 subnet of size 24 and 1 subnet of size 28.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 18 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 15 | |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 10 | 0 | 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 28 | |0|0| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Client sends DHCP REQUEST including Subnet Allocation option with
Subnet Information suboption:
Client decides that the subnet of size 28 is not sufficient so
doesn't include it into the request message.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
Server responds with DHCP ACK including Subnet Allocation option
with Subnet Information suboption:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 0 |
+-+-+-+-+-+-+-+-+
Later Client sends DHCP REQUEST message in order to renew the
lease on the one subnet, including subnet usage information. It
reports that a maximum of 10 addresses were allocated from the
subnet since the last report, 7 addresses are currently allocated,
and 2 addresses were found to be unusable.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 17 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 | 0 | 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Server responds with ACK, however signals Client that the subnet
should be deprecated.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 | 24 | |0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
Client crashes at this point and upon recovery sends a DISCOVER
asking for information about all subnets which were allocated to
it.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 5 | 0 | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | |1|0| 0 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Server responds with an OFFER, giving the subnet information of
the one subnet which is allocated to the Client. Also the Server
specifies that the one allocated subnet should be immediately
deprecated. Note that the "s" ("Server") bit is zero (0) thus
indicating that there is no more information available for this
Client.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | |1|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 | 24 | |0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
Client responds with RELEASE message after having deprecated the
subnet:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAC | 11 | 0 | SIS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | |0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0 | 24 | |0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
8.0 Security Considerations
Potential exposures to attack are discussed in section 7 of the DHCP
protocol specification [1]. The Subnet Allocation option can be used
to hoard all allocatable subnets on a network.
It is suggested that DHCP Authentication be used with this option.
Message authentication in DHCP for intradomain use where the out-of-
band exchange of a shared secret is feasible is defined in RFC 3118
[5]. Potential exposures to attack are discussed in section 7 of the
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Internet Draft Subnet Allocation using DHCP October 2002
DHCP protocol specification in RFC 2131 [1].
9.0 IANA Considerations
IANA has assigned a value of TBD for the DHCP option code described
in this document. No assignment of values for the suboption codes
need be made at this time. New values may only be defined by IETF
Consensus, as described in [6]. Basically, this means that they are
defined by RFCs approved by the IESG.
10.0 Intellectual Property Rights
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
Bucknell University, March 1997.
[2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
University, March 1997.
[3] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, Harvard University, March 1997.
[4] Pummill, T., Manning, B., "Variable Length Subnet Table For
IPv4", RFC1878, Alantec, USC/Information Sciences Institute,
December, 1995
[5] Droms, R, "Authentication for DHCP Messages",
RFC 3118, June 2001
Author Information:
Richard Johnson
Cisco Systems, Inc.
San Jose, Ca.
email: raj@cisco.com
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Internet Draft Subnet Allocation using DHCP October 2002
Jay Kumarasamy
Cisco Systems, Inc.
San Jose, Ca.
email: jayk@cisco.com
Kim Kinnear
Cisco Systems, Inc.
San Jose, Ca.
email: kkinnear@cisco.com
Mark Stapp
Cisco Systems, Inc.
San Jose, Ca.
email: mjs@cisco.com
Johnson, et. al. [Page 21]