One document matched: draft-ietf-dhc-subnet-alloc-03.txt
Differences from draft-ietf-dhc-subnet-alloc-02.txt
Network Working Group R. Johnson
Internet-Draft J. Jumarasamy
Expires: December 31, 2005 K. Kinnear
M. Stapp
Cisco Systems, Inc.
June 29, 2005
Subnet Allocation Option
draft-ietf-dhc-subnet-alloc-03.txt
Status of this Memo
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This Internet-Draft will expire on December 31, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
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. The option number currently in use is 220. This memo
documents the current usage of the option in agreement with RFC-
3942[7] , which declares that any pre-existing usages of option
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numbers in the range 128 - 223 should be documented and the working
group will try to officially assign those numbers to those options.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Subnet Allocation Option format . . . . . . . . . . . . . . . 5
3.1 Request Suboption format . . . . . . . . . . . . . . . . . 5
3.2 Subnet Information Suboption format . . . . . . . . . . . 7
3.3 Subnet Prefix Information Suboption format . . . . . . . . 7
3.3.1 Subnet Usage Statistics . . . . . . . . . . . . . . . 8
3.4 Subnet Name Suboption format . . . . . . . . . . . . . . . 9
3.5 Subnet Suggested Lease Time Suboption format . . . . . . . 10
4. Requesting allocation of a subnet . . . . . . . . . . . . . . 11
4.1 Client DISCOVER message . . . . . . . . . . . . . . . . . 11
4.2 Server OFFER message . . . . . . . . . . . . . . . . . . . 11
4.3 Client DHCP REQUEST message . . . . . . . . . . . . . . . 12
4.4 Server DHCP ACK message . . . . . . . . . . . . . . . . . 12
5. Client renewal of subnet lease . . . . . . . . . . . . . . . . 14
5.1 Client RENEW REQUEST message . . . . . . . . . . . . . . . 14
5.2 Server RENEW response . . . . . . . . . . . . . . . . . . 14
5.3 Client RELEASE message . . . . . . . . . . . . . . . . . . 15
5.4 Server RECONFIGURE message . . . . . . . . . . . . . . . . 15
6. Client requesting subnet allocation information . . . . . . . 16
6.1 Client DISCOVER message . . . . . . . . . . . . . . . . . 16
6.2 Server OFFER response . . . . . . . . . . . . . . . . . . 16
6.3 Client additional DISCOVER messages . . . . . . . . . . . 16
6.4 Server additional OFFER messages . . . . . . . . . . . . . 17
7. DHCP Server Subnet Allocation method . . . . . . . . . . . . . 18
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1 Example 1: . . . . . . . . . . . . . . . . . . . . . . . . 19
8.2 Example 2: . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Differences with DHCPv6 Prefix Delegation . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . 25
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 26
12. Intellectual Property Rights . . . . . . . . . . . . . . . . 27
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . 29
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1. 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 [3], 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.
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2. 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 [1].
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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3. 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) 220
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.
3.1 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 [6] (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 is 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.
The Client MAY also include the Subnet Information suboption
(described below) in order to request a particular subnet. In this
case, the Client SHOULD only include one (1) Subnet Request
suboption, since it would otherwise be unclear which Subnet
Information suboption refered to which Subnet Request suboption.
Multiple subnet requests can be made by sending multiple DHCP
DISCOVER packets.
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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3.2 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.
3.3 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 stats...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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)
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'd' Flag
'1' = Deprecate this subnet
'h' Flag
'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
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.
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.
3.3.1 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:
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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. I.e., the largest number of addresses which were
ever allocated at one time from 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.
3.4 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.
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3.5 Subnet Suggested Lease Time 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | Len (4) | t1 | t2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| t3 | t4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = length of the sub-option excluding the Subcode and Len fields
(always 4 for this suboption) (1 octet)
The Subnet Suggested Lease Time suboption MAY be included by the
Server in order to suggest the lease time to be used by the Client
when allocating addresses from the subnet allocated. The four (4)
octet value of the lease time is in the same format as that of the
"IP Address Lease Time" option (option 51), as described in [3].
If included, this suboption may appear only once. (Multiple such
suboptions would make things ambiguous as to which applied to which
subnet.) If different suggested lease times are needed, the Server
SHOULD, instead, reply with only one offered subnet and should set
the "Server flag" in the Subnet Information suboption to indicate to
the Client that it should send another subnet request to gather the
others.
If this suboption is not included, the Client is free to use whatever
lease time it wants for addresses allocated from this subnet.
In all cases, the Client should, at each address allocation time, use
a lease time which is the lesser of the remaining lease time of the
subnet itself and the Server Suggested Lease Time suboption.
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4. Requesting allocation of a subnet
4.1 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 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.
4.2 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
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(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 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.
4.3 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.
4.4 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
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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
assigned the same lease time. Each subnet lease MAY be RENEWed or
RELEASEd individually.
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5. Client renewal of subnet lease
5.1 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.
5.2 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 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.
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5.3 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.
5.4 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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6. 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.
6.1 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.
6.2 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".
6.3 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 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
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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.
6.4 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.
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7. 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.
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8. Examples
Only the Subnet Allocation option and accompanying suboptions are
displayed in these examples. All other fields in the DHCP messages
are described in [2]. For the purposes of these examples, "SAC"
stands for the actual code number allocated for the "Subnet
Allocation option Code".
8.1 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 |
+-+-+-+-+-+-+-+-+
8.2 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 |
+-+-+-+-+-+-+-+-+
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9. Differences with DHCPv6 Prefix Delegation
The following differences may be noticed between Subnet Allocation as
described in this document and DHCPv6 Prefix Delegation as described
in [5]:
o This option does not use anything like an "IA_PD" as is used in
DHCPv6.
o If the Server can not allocate a subnet, it remains silent,
instead of returning a special saying nothing is available.
o DHCPv6 Prefix Delegation has no mechanism for returning subnet/
prefix usage statistics.
o DHCPv6 has no equivalent to the "subnet deprecation" flag as
described here.
o DHCPv6 Prefix Delegation makes no mention of what Client actions
should result from receiving a NAK during a RENEW of a delegation.
o DHCPv6 has no equivalent of the subnet allocation "Network name"
suboption, which may be used by the Server for various purposes,
such as to specify a pool to use when allocating a subnet.
o DHCPv6 Prefix Delegation corresponds to "Hierarchical Subnet
Allocation" (setting the "h" flag in the Prefix Information
suboption). There is no V6 equivalent of clearing the "h" flag,
in which the Server retains authority over allocation of addresses
from the subnet.
o DHCPv6 Prefix Delegation has nothing to correspond to the
Suggested Lease Time suboption.
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10. Security Considerations
Potential exposures to attack are discussed in section 7 of the DHCP
protocol specification [2]. 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
[4]. Potential exposures to attack are discussed in section 7 of the
DHCP protocol specification in RFC 2131 [2].
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11. IANA Considerations
This option is in current usage with the number 150. As per RFC-
3942, this already existing number assignment should simply be made
"official" by IANA, unless there is a conflict with some other usage.
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 [5]. Basically, this means that they are defined by RFCs approved
by the IESG.
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12. 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.
13. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Droms, R. and S. Alexander, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[4] Droms, R., "Authentication for DHCP Messages", RFC 3118,
June 2001.
[5] Troan, O. and R. Droms, "IPv6 Prefix Options for DHCPv6",
RFC 3633, February 2003.
[6] Pummill, T. and B. Manning, "Variable Length Subnet Table For
IPv4", RFC 1878, December 1995.
[7] Volz, B., "Reclassifying Dynamic Host Configuration Protocol
version 4 (DHCPv4) Options", RFC 3942, November 2004.
Authors' Addresses
Richard A. Johnson
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: raj@cisco.com
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Jay Kumarasamy
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: jayk@cisco.com
Kim Kinnear
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: kkinnear@cisco.com
Mark Stapp
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
Email: mjs@cisco.com
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Intellectual Property Statement
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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