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rfc:rfc6656

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.

Johnson, et al. Informational [Page 1] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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
 to this document.  Code Components extracted from this document must
 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.

Johnson, et al. Informational [Page 2] 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

Johnson, et al. Informational [Page 3] 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.

Johnson, et al. Informational [Page 4] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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

Johnson, et al. Informational [Page 5] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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.

Johnson, et al. Informational [Page 6] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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.

Johnson, et al. Informational [Page 7] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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

Johnson, et al. Informational [Page 8] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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.

Johnson, et al. Informational [Page 9] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 "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)

Johnson, et al. Informational [Page 10] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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.

Johnson, et al. Informational [Page 11] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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.

Johnson, et al. Informational [Page 12] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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.

Johnson, et al. Informational [Page 13] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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

Johnson, et al. Informational [Page 14] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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.

Johnson, et al. Informational [Page 15] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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

Johnson, et al. Informational [Page 16] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 ("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.

Johnson, et al. Informational [Page 17] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      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       |
 +-+-+-+-+-+-+-+-+

Johnson, et al. Informational [Page 18] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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       |
 +-+-+-+-+-+-+-+-+

Johnson, et al. Informational [Page 19] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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.

Johnson, et al. Informational [Page 20] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      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.

Johnson, et al. Informational [Page 21] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

 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.

Johnson, et al. Informational [Page 22] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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.

Johnson, et al. Informational [Page 23] RFC 6656 Cisco Systems' DHCP Subnet Alloc Option July 2012

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

Johnson, et al. Informational [Page 24]

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