





Internet Engineering Task Force (IETF)                        R. Johnson
Request for Comments: 6656                                    K. Kinnear
Category: Informational                                         M. Stapp
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                               July 2012


   Description of Cisco Systems' Subnet Allocation Option for DHCPv4

Abstract

   This memo documents a DHCPv4 option that currently exists and was
   previously privately defined for the operation and usage of the Cisco
   Systems' Subnet Allocation Option for DHCPv4.  The option is passed
   between the DHCPv4 Client and the DHCPv4 Server to request dynamic
   allocation of a subnet, give specifications of the subnet(s)
   allocated, and report usage statistics.  This memo documents the
   current usage of the option in agreement with RFC 3942, which
   declares that any preexisting usages of option numbers in the range
   128-223 should be documented and that the working group will try to
   officially assign those numbers to those options.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6656.














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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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RFC 6656         Cisco Systems' DHCP Subnet Alloc Option       July 2012


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Subnet Allocation Option Format  . . . . . . . . . . . . . . .  5
     3.1.  Subnet-Request Suboption Format  . . . . . . . . . . . . .  5
     3.2.  Subnet-Information Suboption Format  . . . . . . . . . . .  7
       3.2.1.  Subnet Prefix Information Block Format . . . . . . . .  8
     3.3.  Subnet-Name Suboption Format . . . . . . . . . . . . . . .  9
     3.4.  Suggested-Lease-Time Suboption Format  . . . . . . . . . . 10
   4.  Requesting Allocation of a Subnet  . . . . . . . . . . . . . . 10
     4.1.  Client DHCPDISCOVER Message  . . . . . . . . . . . . . . . 11
     4.2.  Server DHCPOFFER Message . . . . . . . . . . . . . . . . . 11
     4.3.  Client DHCPREQUEST Message . . . . . . . . . . . . . . . . 12
     4.4.  Server DHCPACK Message . . . . . . . . . . . . . . . . . . 13
   5.  Client Renewal of Subnet Lease . . . . . . . . . . . . . . . . 13
     5.1.  Client RENEW DHCPREQUEST Message . . . . . . . . . . . . . 13
     5.2.  Server RENEW DHCPREQUEST Response  . . . . . . . . . . . . 14
     5.3.  Client DHCPRELEASE Message . . . . . . . . . . . . . . . . 14
     5.4.  Server DHCPFORCERENEW Message  . . . . . . . . . . . . . . 15
   6.  Client Requesting Previously Allocated Subnet Information  . . 15
     6.1.  Initial Client DHCPDISCOVER Message  . . . . . . . . . . . 15
     6.2.  Initial Server DHCPOFFER Response  . . . . . . . . . . . . 16
     6.3.  Additional Client DHCPDISCOVER Messages  . . . . . . . . . 16
     6.4.  Additional Server DHCPOFFER Responses  . . . . . . . . . . 16
   7.  DHCP Server Subnet Allocation Method . . . . . . . . . . . . . 17
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  Differences from DHCPv6 Prefix Delegation  . . . . . . . . . . 21
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     12.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 23















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RFC 6656         Cisco Systems' DHCP Subnet Alloc Option       July 2012


1.  Introduction

   This memo documents a DHCP option [RFC2132], the Subnet Allocation
   option, that was developed by Cisco and allows a DHCP Client to
   allocate a subnet given information from a DHCP Server.  This
   protocol makes use of DHCP option number 220, one of the option
   numbers reclassified by [RFC3942].  That RFC specifies in Section 4,
   procedure 2, "Vendors that currently use one or more of the
   reclassified options have 6 months following this RFC's publication
   date to notify the DHC WG and IANA that they are using particular
   options numbers and agree to document that usage in an RFC".  This
   memo serves as that documentation.  The DHC WG has had no input into
   any of the details of the protocol operation and makes no statement
   about the correctness or any other aspect of the protocol.  The WG
   also has seen no interest in further implementation or deployment of
   this protocol other than privately, and therefore has decided to
   publish this document solely for informational purposes.

   The Subnet Allocation option provides a straightforward way to
   allocate a subnet from DHCP, useful for a simple Dial-in Aggregation
   box, or 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 option is specified in such a way as to use one DHCP
   option number, while using suboption numbers for each function.

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 [RFC2119].

   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 suboptions
           (1 octet)
   Flags = Various flags: (None currently defined)

   One or more suboptions may be specified as described below.

3.1.  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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |      Len      |    Flags  :i:h|    Prefix     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Len   = Length of the suboption (always 2 for this suboption)
           (1 octet)
   Flags = Flags field. (all unused bits must be zero)

       'h' = Hierarchical flag
             1 : Client will be allocating addresses from this subnet.
             0 : Client will be relaying DHCP requests to the Server
                 from this subnet.
       'i' = Information flag
             1 : Client is seeking information about previously
                 allocated subnets.
             0 : Client is seeking a new subnet allocation.

   Prefix = Network prefix length requested
            (0 means no suggested length 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 length [RFC4632]
   less than or equal to the "Prefix" field length specified in the
   request.  In other words, a subnet containing a number of addresses
   at least the size indicated by the prefix length requested and



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   possibly more.  The DHCP Server MAY allocate a subnet with a prefix
   length greater than that specified in the request (or a subnet with a
   number of addresses less than requested), but this is not encouraged.

   A "Prefix" field size of 0 MAY be specified by the DHCP Client.  In
   this case, no suggested prefix length is given.

   Multiple Subnet-Request suboptions in a DHCP packet indicate that
   multiple subnets are being requested.  Note that multiple occurrences
   of this suboption MUST NOT be concatenated in the sense of [RFC3396].

   Each Subnet-Request suboption MUST result in no more than one Subnet-
   Information suboption in the DHCPOFFER message from the Server, and
   may result in no Subnet-Information suboptions.

   The Client MAY also include the Subnet-Information suboption in order
   to request a particular subnet.  In this case, the Client SHOULD only
   include one Subnet-Request suboption, since it would otherwise be
   unclear which Subnet-Information suboption referred to which Subnet-
   Request suboption.  Multiple subnet requests can be made by sending
   multiple DHCPDISCOVER 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 "Hierarchical 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).
   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 "Prefix" field
   SHOULD be set to 0 and MUST be ignored by the Server.  In this case,
   the conversation between the Client and the Server will only progress
   to the DHCPOFFER packet from the Server, giving the information, as
   described in Section 6 below.

   Any undefined flags (those other than 'i' and 'h', mentioned above)
   should be ignored by the DHCP 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 suboption (min. length of 8) (1 octet)
   Flags = Various flags that apply to ALL Subnet Prefix
           Information fields specified in this suboption.
           Unused flags must be zero.

       'c' : Client flag (explained below)
             1 : This information is in response to a Client request
                 (or has been echoed back by the Client, when asking
                 for the next previously allocated subnet from the
                 Server).
             0 : Otherwise.
       's' : Server flag (explained below)
             1 : Server has additional subnet information for this
                 Client.
             0 : Otherwise.

   SP1, SP2, ...  Subnet Prefix Information blocks as specified below
                  (variable size)

   Setting the 'c' flag to 1 indicates 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 used in response
   to a Subnet-Request suboption with the 'i' flag set and should be 0
   in other Server responses.  Note, the flag is echoed back from the
   Client to the Server when requesting the "next previously allocated
   subnet".  Another way to think of the 'c' bit would be that it
   indicates that the subnet information contained in this suboption
   does not represent a new allocation by the Server or a new request
   for allocation by the Client, but instead represents previously
   allocated subnet information.

   Setting the 's' flag to 1 indicates the Server has additional subnet
   information for the Client.

   Any undefined flags (those other than 'c' and 's', mentioned above)
   should be ignored by the DHCP Server.






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   The Subnet-Information suboption is used by the DHCP Server in order
   to return information as to what subnets are offered (in the case of
   a DHCPOFFER packet) or allocated (in the case of a DHCPACK packet).
   As is indicated above, multiple subnets may be returned in one
   Subnet-Information suboption.

   The Subnet-Information suboption is also used by the DHCP Client in
   order to request allocation of specific subnets in a DHCPREQUEST
   packet.  In this case, the "Network", "Prefix", and "Flags" fields
   contained in the associated Subnet Prefix Information blocks MUST NOT
   be changed from the information that was received in the DHCPOFFER
   packet from the Server.  The Client MAY, however, use multiple
   Subnet-Information suboptions in order to request subnets that were
   originally specified by the Server inside one Subnet-Information
   suboption.

3.2.1.  Subnet Prefix Information Block 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Network                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Prefix     |     Flags :h:d|   Stat-len    |  Optional stats...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Network = IPv4 network number (4 octets)
   Prefix  = Prefix length (1 octet)
   Flags   = Flags field (Undefined bits must be zero) (1 octet)

       'd' = Deprecate flag (explained below)
             1 : Deprecation of this subnet is requested.
             0 : No deprecation is needed.

       'h' = Hierarchical flag (explained below)
             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
              (zero if no statistics are included) (1 octet)

   The 'd' flag may only be returned by the Server to the Client (inside
   a DHCPACK packet, in response to a DHCP RENEW).  Its presence 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



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   any leases when Clients ask for renewal.  As soon as all addresses in
   the subnet are unallocated, the Client should send a DHCPRELEASE
   message to the Server, including a Subnet Prefix Information block
   for the subnet in order to release the subnet.  The format of this
   message is described farther below.

   The 'h' flag tells the Client how the Server intends for 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
   manner as 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.

   Any undefined flags (those other than 'd' and 'h', mentioned above)
   should be ignored by the DHCP Server.

   If any usage statistics information is to be included, then the
   "Stat-len" field specifies the number of bytes of statistics
   information that is included.  See below for more information.  If no
   statistics information is included, then this byte MUST be zero.  The
   "Stat-len" field SHOULD always be zero when this suboption is sent by
   the DHCP Server.  The usage statistics information is intended for
   use only to report usage statistics from the Client to the Server.

3.2.1.1.  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 that 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           High water          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Currently in use      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Unusable            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   "High water" refers the to "high-water mark" of allocated addresses
   within the subnet.  That is, the largest number of addresses that
   were ever allocated at one time from the subnet.




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   "Currently in use" refers to the number of addresses currently
   allocated in the subnet.

   "Unusable" refers to the number of addresses that are currently
   unusable for any reason (such as a Client returning a DHCPDECLINE, or
   finding the address already in use).

   Additional statistics may be added to this option in the future.  If
   so, they MUST be appended immediately after the already defined
   statistics fields.  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 above; for example, if "Stat-len" is "4", then the
   "Unusable" field has not been included.  All fields that are included
   MUST remain in order specified here.

3.3.  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 suboption (min. length of 1) (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.  The "Name" in this suboption MUST be given using UTF-8
   [RFC3629].

3.4.  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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       4       |     Len (4)   |       t1      |       t2      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       t3      |       t4      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Len  = length of the suboption (always 4 for this suboption) (1
      octet)




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   The 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 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 [RFC2132].

   If included, this suboption MUST appear only once.  (Inclusion of
   multiple such suboptions would result in ambiguity 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.

   The Client SHOULD use a lease time, when allocating addresses from
   the subnet, that is the lesser of the remaining lease time of the
   subnet itself and the Suggested-Lease-Time suboption.

4.  Requesting Allocation of a Subnet

4.1.  Client DHCPDISCOVER Message

   The DHCP Client creates a DHCPDISCOVER 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 length 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 DHCPDISCOVER 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 than the Subnet-Request
   suboption.

   If more than one subnet is being requested, multiple Subnet-Request
   suboptions MAY be included or multiple DHCPDISCOVER messages MAY be
   sent instead.  The prefix length field of each Subnet-Request
   suboption MUST be either 0, or in the range of 1 to 30 inclusive.

   The DHCP "IP address lease time" option (code 51) MAY be included in
   the DHCPDISCOVER message to specify the lease time the Client is
   requesting for the subnet.  If not present, no suggested lease time
   is given.

   The DHCP "Client ID" option (code 61) MAY be included in the
   DHCPDISCOVER message as it may be used by the Server in performing
   the subnet allocation.



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4.2.  Server DHCPOFFER Message

   Upon receiving a DHCPDISCOVER containing the Subnet Allocation
   option, the DHCP Server SHOULD respond with a DHCPOFFER message
   including the Subnet-Information suboption in order to specify the
   subnet(s) that it is willing to allocate to the Client in order to
   fulfill the request(s).

   The Server need not reserve the subnets that are being offered, but
   SHOULD not offer the same subnets to another DHCP Client until a
   reasonable time period (implementation dependent) has passed.  (This
   is similar to normal DHCP address allocation.)

   The Server MUST NOT include the Subnet-Request suboption in the
   DHCPOFFER.  The same information is already present in the Subnet
   Information suboption(s) that SHOULD be included in the DHCPOFFER.

   The Server SHOULD also include the IP address lease time option
   (option 51) in the DHCPOFFER message.  This gives the lease time for
   all subnets given in all Subnet-Request suboptions contained in the
   DHCPOFFER 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 exactly one IP address lease
   time (and optionally one Rebinding and/or one Renewal option) in the
   DHCPOFFER message.

   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
   DHCPDISCOVER message specifying a prefix length field, P, of zero 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 0 to indicate this is a Server
   response to a Client request for a new subnet allocation and not a
   response to a request for information about already allocated
   subnets.

   If the packet contains a Subnet Allocation option, 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.  In other words,
   a DHCP packet exchange cannot provide subnet allocation and address
   assignment simultaneously.







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4.3.  Client DHCPREQUEST Message

   When sending a DHCPREQUEST, the Client MUST NOT modify any fields of
   any Subnet-Information suboptions received from the Server.  However,
   the Client MAY choose not to include some Subnet-Information
   suboptions when issuing the DHCPREQUEST.  Subnet-Request suboptions
   MUST NOT be included in the DHCPREQUEST message; only the Subnet-
   Information suboption(s) should be included.

4.4.  Server DHCPACK Message

   The DHCP Server, upon receipt of the Client's DHCPREQUEST 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 MUST NOT
   change the network number(s) or prefix length(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 DHCPACK.  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 that states that the Client
   should be allocated more subnets than were requested, the Server MAY
   set the 's' bit in the last Subnet-Information suboption to indicate
   that the Client needs to request more subnets (as described above).

   The allocable unit is the tuple (network number, prefix length).
   Multiple subnets may be allocated in one DHCPACK; however, since
   there can be only one Lease-time option, each subnet allocated is
   assigned the same lease time.  Each subnet lease tuple (network
   number, prefix length) MAY be renewed or released individually.

5.  Client Renewal of Subnet Lease

5.1.  Client RENEW DHCPREQUEST 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 DHCPACK 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.




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   The DHCPREQUEST 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 in Section 3.2 ("Subnet-Information Suboption
   Format").

   The subnet network number field ("Network") and subnet prefix length
   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 water, Currently in use, Unusable), a value of all ones
   (0xffff) SHOULD be used if the Client has no information to report
   for a statistic.

5.2.  Server RENEW DHCPREQUEST Response

   The Server MAY respond to a subnet RENEW with either a DHCPACK or
   DHCPNAK response.  If a DHCPNAK 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 DHCPDISCOVER 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 deprecate
   subnets by using the 'd' bit of the Subnet-Information suboption in a
   DHCPACK message (see below).

   If a DHCPACK 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
   from this subnet (e.g., 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 DHCPRELEASE
   message, including the Subnet-Information suboption (with optional
   usage statistics) in order to release the subnet(s) back to the
   Server.

5.3.  Client DHCPRELEASE Message

   The DHCP Client SHOULD send a DHCPRELEASE 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 may
   optionally include usage statistics.

   The Client MUST release the same subnet(s) of the same prefix length
   ("Prefix"), as were originally allocated.  The Client MAY release a
   subset of the subnets that were allocated originally.  In other



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   words, the allocable unit is the tuple (network number, prefix
   length).  Multiple subnets may be allocated in one DHCPACK; however,
   each subnet MAY be released individually.

5.4.  Server DHCPFORCERENEW Message

   The DHCP Server MAY issue a DHCPFORCERENEW [RFC3203] message
   containing the Subnet Allocation option and the Subnet-Information
   suboption.  Upon receiving this message, the DHCP Client MUST issue a
   DHCPREQUEST message to the DHCP Server in order to renew the lease on
   the subnet mentioned.  No other subnets allocated to the Client are
   affected.  As is the case with all DHCP RENEW 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 0 if no statistics are to be reported.

   If the Server responds to this DHCPREQUEST with a DHCPNAK 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 DHCPDISCOVER message containing the Subnet
   Allocation option and the Subnet-Request suboption in order to
   acquire another subnet for use.

6.  Client Requesting Previously Allocated Subnet Information

   A DHCP Client MAY request from the DHCP Server a list of what subnets
   are currently allocated to the 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.  (For an example of this, see
   Section 8.2 below.)

6.1.  Initial Client DHCPDISCOVER Message

   The DHCP Client DHCPDISCOVER message, when used to discover already
   allocated subnet information, SHOULD contain a Subnet-Request
   suboption with the "Prefix" field set to 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.  Note, no Subnet-Information suboption
   is included in this message, since the Client would not know of any
   subnet to request at that point.

   This DHCPDISCOVER message MAY be unicast to a particular DHCP Server,
   or broadcast in the normal fashion.






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6.2.  Initial Server DHCPOFFER Response

   Any DHCP Server that has allocated subnets to the Client SHOULD
   respond to the DHCPDISCOVER message with a DHCPOFFER message.  The
   DHCPOFFER message should contain one or more Subnet-Information
   suboption(s) telling the prefix of the subnet(s) allocated to the
   Client.

   The Server SHOULD, internally, retain an ordered list of subnets that
   are allocated to each Client.  In response to an initial DHCPDISCOVER
   message requesting allocated subnet information (i.e., one with the
   'i' flag set to 1, but not carrying a Subnet-Information suboption),
   the Server returns in the DHCPOFFER message the subnet information
   for the first subnet(s) from this list.  If the end of the list has
   been reached, then the 's' bit of the last Subnet-Information
   suboption included in the message MUST be set to 0.  If there are
   more subnets in the list, the 's' bit MUST be set to 1 to indicate to
   the Client that more information is available.  Since this
   information is in response to a Client request for previously
   allocated subnet information, the 'c' bit MUST be set to 1.

6.3.  Additional Client DHCPDISCOVER Messages

   The Client, upon receiving any Server DHCPOFFER messages containing
   Subnet Information suboption information with the 'c' ("Client") bit
   set, SHOULD gather the network number ("Network") and prefix length
   ("Prefix") information from the message.

   If the 's' bit is set in the last of the Subnet-Information
   suboptions included in the message, then the Client SHOULD construct
   a new DHCPDISCOVER message containing the Subnet Allocation option
   and the last Subnet-Information suboption from the Server's message.
   This message SHOULD then be sent back to the same DHCP Server
   originating the DHCPOFFER message.  The 'c' and 's' bits MUST retain
   the same settings they had from the Server's DHCPOFFER message and
   the network number ("Network") and prefix length ("Prefix") fields
   MUST be unaltered as well.

   If the 's' bit in all of the Subnet-Information suboptions from the
   Server was 0, then it indicates the Server has no more information
   about subnets allocated to the Client.

6.4.  Additional Server DHCPOFFER Responses

   The Server, upon receiving from a Client an additional DHCPDISCOVER
   message for allocated subnet information retrieval, with the 'i' flag
   set to 1 and containing one or more Subnet Information suboptions
   with the 'c' and the 's' bits set, MUST use the network number



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   ("Network") and prefix length ("Prefix") fields contained in the last
   such Subnet Information suboption.  This is in order to locate the
   position in the internal table of allocated subnets for this Client.
   Then, the Server MUST return an DHCPOFFER 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.  Any Subnet Information suboptions
   encountered without both the 'c' and 's' bits set should be ignored
   by the Server.

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.

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 [RFC2131].

8.1.  Example 1

   A DHCP Client requesting a subnet with prefix length 24 from which
   the Client will allocate addresses to other Clients.  The Server
   responds with an allocation of exactly the size requested:

   The Client sends a DHCPDISCOVER message including a Subnet Allocation
   option with the Subnet-Request suboption:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |       5       |       0       |       1       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |     0     |0|0|       24      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Server responds with a DHCPOFFER message including a Subnet
   Allocation option with a Subnet-Information suboption, offering the
   subnet 10.0.1.0/24.










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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      11       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |     0     |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |       0       |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

   The Client sends a DHCPREQUEST including a Subnet Allocation option
   with a Subnet-Information suboption:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      11       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |     0     |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |       0       |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

   The Server responds with a DHCPACK message including a Subnet
   Allocation option with a Subnet-Information suboption:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      11       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |     0     |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |       0       |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

   Later, the Client sends a DHCPRELEASE message including a Subnet
   Allocation option with a Subnet-Information suboption:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      11       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |     0     |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |       0       |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+



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8.2.  Example 2

   A DHCP Client requesting two subnets, each with prefix length 24:

   The Client sends a DHCPDISCOVER message including a Subnet Allocation
   option with a Subnet-Request suboption:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |       9       |       0       |       1       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |     0     |0|0|       24      |       1       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |     0     |0|0|       24      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Server responds with a DHCPOFFER message including a Subnet
   Allocation option with a Subnet-Information suboption:

   The DHCPOFFER specifies one subnet of size 24 and one subnet of size
   28.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      18       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       15      |           |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |      0        |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |     10        |       0       |       3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |     28        |           |0|0|       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Client sends a DHCPREQUEST message including a Subnet Allocation
   option with a Subnet-Information suboption:

   The Client decides that the subnet of size 28 is not sufficient so it
   doesn't include that subnet in the DHCPREQUEST message.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      11       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |           |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |      0        |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+



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   The Server responds with a DHCPACK message including a Subnet
   Allocation option with a Subnet-Information suboption:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      11       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |           |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |      0        |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

   Later, the Client sends a DHCPREQUEST message in order to renew the
   lease on the one subnet and includes 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.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      17       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      14       |           |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |      0        |      24       |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       6       |       0       |      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       7       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Server responds with a DHCPACK message; however, it signals to
   the Client that the subnet should be deprecated.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |      11       |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |           |0|0|      10       |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |      0        |      24       |           |0|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

   The Client reloads at this point and, upon completion of the reload,
   sends a DHCPDISCOVER asking for information about all subnets that
   were allocated to it.




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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |       5       |       0       |       1       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |           |1|0|       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Server responds with a DHCPOFFER, giving the subnet information
   about the one subnet that 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 0, thus indicating
   that there is no more information available for this Client.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |       11      |       0       |       2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |           |1|0|       10      |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |      0        |       24      |           |0|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

   The Client responds with a DHCPRELEASE message after having
   deprecated the subnet:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      220      |       11      |       0       |     SIS       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |           |0|0|       10      |       0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |      0        |       24      |           |0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |
   +-+-+-+-+-+-+-+-+

9.  Differences from DHCPv6 Prefix Delegation

   The following differences may be noticed between Subnet Allocation as
   described in this document and DHCPv6 Prefix Delegation as described
   in [RFC3633]:

   o  This option does not use anything like an "IA_PD" as is used in
      DHCPv6.

   o  If the Server cannot allocate a subnet, it remains silent, instead
      of returning a special response saying nothing is available.





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   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 DHCPNAK 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
      block).  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.

10.  Security Considerations

   Potential exposures to attack are discussed in Section 7 of the DHCP
   protocol specification [RFC2131].  The Subnet Allocation option can
   be used to hoard all allocable subnets on a network.

   Implementations should consider using the DHCP Authentication option
   [RFC3118] in order to provide a higher level of security if it is
   deemed necessary in their environment.

11.  IANA Considerations

   IANA has assigned DHCP option number 220 for this option, in
   accordance with [RFC3942].

   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 [RFC5226].  Basically, this means that they are defined by RFCs
   approved by the IESG.









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12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3942]  Volz, B., "Reclassifying Dynamic Host Configuration
              Protocol version 4 (DHCPv4) Options", RFC 3942,
              November 2004.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

12.2.  Informative References

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC3203]  T'Joens, Y., Hublet, C., and P. De Schrijver, "DHCP
              reconfigure extension", RFC 3203, December 2001.

   [RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
              Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
              November 2002.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, August 2006.








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Appendix A.  Acknowledgments

   The authors gratefully acknowledge the contributions of Jay
   Kumarasamy.

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


   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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