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Versions: 12

Internet Engineering Task Force                                 J. Bound
INTERNET DRAFT                                     Compaq Computer Corp.
DHC Working Group                                              M. Carney
Obsoletes:  draft-ietf-dhc-dhcpv6exts-11.txt       Sun Microsystems, Inc
                                                              C. Perkins
                                                   Nokia Research Center
                                                              5 May 2000
     Extensions for the Dynamic Host Configuration Protocol for IPv6
                   draft-ietf-dhc-dhcpv6exts-12.txt
Status of This Memo


   This document is a submission by the Dynamic Host Configuration
   Working Group of the Internet Engineering Task Force (IETF). Comments
   should be submitted to the dhcp-v6@bucknell.edu mailing list.


   Distribution of this memo is unlimited.


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at
   any time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."


   The list of current Internet-Drafts can be accessed at:
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at:
        http://www.ietf.org/shadow.html.

Abstract


   The Dynamic Host Configuration Protocol for IPv6 [4] (DHCP) provides
   a framework for passing configuration information to hosts on
   a TCP/IP network.  Configuration parameters and other control
   information are carried in typed data items that are stored in the
   ``extensions'' field of the DHCP message.  The data items themselves
   are also called ``extensions.''  This document specifies the initial
   set of DHCP extensions, which will be periodically updated as new
   extensions are defined until this document reaches proposed standard.

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   After that time, individual extensions will be defined in separate
   documents, to be reviewed by the DHCP WG (if it still exists) and the
   IESG.


                                Contents
Status of This Memo                                                    i


Abstract                                                               i


 1. Introduction                                                       1


 2. DHCP Extension Field Format                                        2


 3. DHCP Relay Considerations                                          4


 4. Releasable Resource Extensions                                     4
     4.1. IP Address Extension  . . . . . . . . . . . . . . . . . .    4
           4.1.1. IP Address Lifetimes  . . . . . . . . . . . . . .    8
           4.1.2. Client Considerations . . . . . . . . . . . . . .    9
           4.1.3. Server Considerations . . . . . . . . . . . . . .   11


 5. General Extensions                                                14
     5.1. IEEE 1003.1 POSIX Timezone Extension  . . . . . . . . . .   14
           5.1.1. IEEE 1003.1 POSIX Timezone specifier  . . . . . .   15
           5.1.2. An Example: . . . . . . . . . . . . . . . . . . .   16
     5.2. Domain Name Server Extension  . . . . . . . . . . . . . .   17
     5.3. Domain Name Suffix Extension  . . . . . . . . . . . . . .   17
     5.4. Service Location Protocol Directory Agent Extension . . .   17
     5.5. Service Location Protocol Service Scope Extension . . . .   19
     5.6. Network Time Protocol Servers Extension . . . . . . . . .   22
     5.7. Network Information Service (NIS) Domain Name Extension .   22
     5.8. Network Information Service (NIS) Servers Extension . . .   23
     5.9. Network Information Service V2 (NIS+) Domain Extension  .   23
    5.10. Network Information Service V2 (NIS+) Servers Extension .   23
    5.11. TCP-specific Extensions . . . . . . . . . . . . . . . . .   24
          5.11.1. TCP Keepalive Interval Extension  . . . . . . . .   24


 6. DHCP-specific Extensions                                          24
     6.1. Maximum DHCP Message Size Extension . . . . . . . . . . .   25
     6.2. DHCP Retransmission and Configuration Parameter Extension   25
     6.3. Extension Request Extension (ERE) . . . . . . . . . . . .   26
           6.3.1. Client Considerations . . . . . . . . . . . . . .   26
           6.3.2. Server Considerations . . . . . . . . . . . . . .   26
     6.4. Subnet Prefix Extension . . . . . . . . . . . . . . . . .   26
           6.4.1. Client Considerations . . . . . . . . . . . . . .   27
           6.4.2. Server Considerations . . . . . . . . . . . . . .   27
     6.5. Platform Specific Information Extension . . . . . . . . .   27
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     6.6. Platform Class Identifier Extension . . . . . . . . . . .   28
           6.6.1. Client Considerations . . . . . . . . . . . . . .   29
           6.6.2. Server Considerations . . . . . . . . . . . . . .   29
     6.7. User Class Identifier Extension . . . . . . . . . . . . .   29
           6.7.1. Client Considerations . . . . . . . . . . . . . .   30
           6.7.2. Server Considerations . . . . . . . . . . . . . .   30
     6.8. Reconfigure Multicast Address Extension . . . . . . . . .   30
     6.9. Renumber DHCP Server Address Extension  . . . . . . . . .   31
    6.10. Client-Server Authentication Extension  . . . . . . . . .   31
    6.11. Client Key Selection Extension  . . . . . . . . . . . . .   32


 7. Security Considerations                                           33
     7.1. Replay Protection . . . . . . . . . . . . . . . . . . . .   33
     7.2. Default Authentication Algorithm  . . . . . . . . . . . .   33


 8. IANA Considerations                                               34


 9. Acknowledgements                                                  35


10. Full Copyright Statement                                          35


Chair's Address                                                       39


Authors' Addresses                                                    39
1. Introduction


   This document specifies extensions for use with the Dynamic Host
   Configuration Protocol for IP version 6 (DHCP). The DHCP message
   formats are described in the DHCP protocol document [4].  In this
   document, several words are used to signify the requirements of the
   specification, in accordance with RFC 2119 [5].  These words (MUST,
   SHOULD, MAY, MUST NOT, etc) are often capitalized.


   This document defines the overall format of information in the
   ``extensions'' field of DHCP messages.  The extensions defined
   within this document specify a generalized way to distribute
   information useful to a wide class of devices, operating systems
   and configurations.  Sites with a DHCP server that is shared among
   heterogeneous clients may choose to define other, site-specific
   formats for the use of the ``extensions'' field.


   Section 2 of this memo describes the formats of DHCP extensions.
   Information on registering new extensions is contained in section 8.
   The other sections organize the format descriptions of various
   extensions according to their general type, as follows:


     o Releasable Resource Extensions (section 4)
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     o General Extensions (section 5)


        *  TCP-specific Extensions


     o DHCP-specific Extensions (section 6)


   Future applications will make extensive use of an ever-increasing
   number and variety of network services.  It is expected that client
   requirements for locating these network services will be satisfied
   by the Service Location Protocol [20], and not the DHCP. The DHCP
   is expected to be used for the kinds of configuration that enable
   clients to become fully functional as self-contained network
   entities.
2. DHCP Extension Field Format


   Extensions may be fixed length or variable length.  All extensions
   begin with a ``Type'' field, which is a an two octet unsigned
   network-order integer that uniquely identifies the extension.  Every
   extension has a two octet unsigned network-order integer ``Length''
   field following the ``Type'' field.  The ``Length'' field contains
   the number of octets of extension data that follow the ``Length''
   field.  Thus, the ``Length'' field value does not include the number
   of octets needed to carry the ``Type'' and ``Length'' fields.  For
   some extensions, the ``Length'' field is always the same number, but
   it MUST still be specified.  There is no requirement for alignment of
   data fields within existing DHCP extensions.  Any extensions defined
   subsequent to this document MUST contain a two-octet ``Length'' field
   even if the value it would contain would always be fixed or zero.


   Unrecognized extensions MUST be silently ignored by skipping over the
   number of octets specified in the ``Length'' field, and processing
   continued for subsequent extensions.  Unless and until specified
   otherwise by use of the ``Maximum DHCP message size'' extension
   (section 6.1), DHCP implementations MUST assume that that the maximum
   DHCP message size including extensions is limited to 1280 octets.


   All multi-octet quantities are in network-order.


   Extension Type 0 (zero) is reserved.


   There can be 65535 different extensions, which are divided up into
   the following ranges:


      Releasable Resource Range (1--8192)


         Extensions carrying data which identifies a resource which is
         leased by the server to a client for a finite period of time
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         known as a ``lease''.  The client agrees to stop using the
         resource when the lease expires, and the server guarantees that
         it will not allocate the resource to another client until the
         lease expires or the client signals that it is done using the
         resource.


         A server MUST NOT return a releasable resource to a client
         unless the client explicitly requests an instance of that
         resource from the server.  This requirement ensures that only
         clients capable of managing a releasable resource receive them.


         A client MUST remember which server allocated the client a
         releasable resource, in order to contact that server to extend
         the lease on the resource or release the resource back to the
         server when it is finished with it.


         As of this writing, the only example of a type of releasable
         resource is an IP address, carried in the ``IP Address
         Extension'' (section 4.1).  See the ``DHCP for IPv6'' companion
         document ([4]) for more details.


      General Range (8193-49152)


         Extensions in this range are informational in nature, and may
         point to resources which may be shared by any number of nodes.
         General extension proposals are reviewed by the DHC WG and IESG
         for general usefulness to the IETF community at large.


         Servers MAY return any general range extension to clients if
         administrative policy requires it; however, a server SHOULD
         only return general extensions if the client requests them
         using the ``Extension Request Extension'' (ERE) (section 6.3).


         Examples of general extensions include Domain Name Service
         parameters, Network Time Protocol (NTP, [14]) parameters,
         those carrying information pertaining to the DHCP, such
         as the ``Maximum DHCP Message Size'' (section 6.1), ``DHCP
         Retransmission and Configuration Parameter'' (section 6.2, and
         ``Platform Specific Information'' (section 6.5) Extensions.


      Site-specific Range (49153--65535)


         Extensions in this range are reserved for use by Site managers
         (administrators of the DHCP domain).  Their type and content is
         entirely up to the administrator.  DHCP implementations SHOULD
         permit the definition of site-specific extensions, including
         such information as data type and format.  Note that both
         client and server implementations MAY need to be configured in
         order to properly exchange site extensions.
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         A server SHOULD only return site-specific extensions to the
         client if it explicitly requests them using the ``Extension
         Request Extension'' (ERE) (section 6.3).


   All of the extensions described in this document MUST also have
   their default values specified, if any.  Whenever an extension is
   received as part of a DHCP message, any reserved fields of the
   message MUST be ignored, and processing continued as if the reserved
   fields were zero.  Typically, the value of the ``Type'' field is
   shown directly in the format illustration, and for some fixed-length
   extensions the value of the ``Length'' field is also shown in the
   format illustration for the extension.
3. DHCP Relay Considerations


   The DHCP Relay MUST NOT change any information in any DHCP Extension
   fields.  All Extension information flows between DHCP Server and DHCP
   Client without modification by any Relay.
4. Releasable Resource Extensions


   Releasable resource extensions contain data identifying a specific
   resource leased by the server to the client for a specific period of
   time known as a ``lease''.  Because the allocation of such extensions
   requires extension-specific management of the lease by both the
   client and the server, these extensions MUST only be returned to the
   client if they have been explicitly requested by the client.


   How the resource and its lease is managed is resource-specific
   (extension-specific).


   A client MUST remember in non-volatile storage which server allocated
   which releasable resource, in order to appropriately manage the lease
   associated with that resource.


   As of this writing, the only example of a releasable resource is
   an IP address, which is carried in the ``IP Address Extension'',
   discussed below.
4.1. IP Address Extension


   The IP address extension is used by clients and servers to refer to
   a particular IP address and related information such as the status
   of the host name associated with the IP address.  All information
   relevant to a particular IP address allocation has been collected
   together within the ``IP address'' extension.
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   The ``lease'' feature of the ``IP address'' extension is implemented
   by the ``Preferred lifetime'' and ``Valid lifetime'' fields.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 1            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    status     |C|I|L|Q|A|P|   reserved    |scope| prefix-len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         (if present)                          |
   |                    IP address (16 octets)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          (if present) preferred lifetime (4 octets)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            (if present) valid lifetime (4 octets)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         (if present) DNS name (variable length)  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Type     1


      Length   (unsigned integer, variable) The length of the Extension
               in octets.


      status   The receiver's result of its attempt to honor the
               sender's request.


      C        If the ``C'' bit is set, the field containing the IP
               address is present in the extension.


      I        If the ``I'' bit is set, the client is informing the
               server that the IP address listed *was not* received from
               DHCP (e.g.  received from Stateless Autoconfiguration,
               or manually configured).  The ``I'' bit MUST NOT be set
               if the ``C'' bit is not set (IP address field MUST be
               present).  The ``L'' bit MUST NOT be set if the ``I'' bit
               is set.


      L        If the ``L'' bit is set, the preferred and valid
               lifetimes are present in the extension.


      Q        If the ``Q'' bit is set, the fields included by the
               client are required, and must be made available by the
               server or else the extension must be rejected.


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      A        If the ``A'' bit is set, the client requests that
               the server updates DNS with a new AAAA/A6 record, as
               specified by the client's FQDN.


      P        If the ``P'' bit is set, the client requests that the
               server updates DNS with a new PTR record, as specified by
               the client's FQDN.


      reserved MUST be zero.


      scope
               This 3-bit field is used by the client to request an IP
               address of a certain scope.  The 3 bits form a number
               (0--7) which can have the following settings:


                 0 Don't Care


                 1 Globally-scoped address


                 2 Site local-scoped address


                 3 reserved


                 4 reserved


                 5 reserved


                 6 reserved


                 7 reserved


      prefix-len
               If the IP address field is present (the ``C'' bit is
               set), a non-zero prefix-len is the number of left-most
               bits of the IP address which make up the subnet prefix.
               Otherwise, if the ``C'' bit is not set, prefix-len MUST
               be zero.  The prefix-len field is 7-bits in length.


      IP address
               The IP address to be conveyed to the receiver from the
               sender.  (16 octets long).


      preferred lifetime
               The preferred lifetime of the IP address in seconds.


      valid lifetime
               The valid lifetime of the IP address in seconds.
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      DNS name
               The DNS name (a string of NVT-ASCII octets) associated
               with the IP address.


   The following values for the status field are defined within this
   document:


        0   request granted, no errors

        1   IP address is already in use by a different node

        2   Extension settings (bit combinations) illegal

        3   IP address scope requested is not available

        4   IP address requested by client is not available

       18   Security parameters failed for this client

       20   Resource AAAA/A6 Record Parameter Problem

       21   Resource PTR Record Parameter Problem

       23   DNS name string error

       24   dynDNS Not Implemented

       25   Authoritative DNS Server could not be found

       33   The name server was unable to interpret the request
          due to a format error.

       34   dynDNS unavailable at this time (SERVFAIL)

       35   Some name that ought to exist, does not exist
          (NXDOMAIN)

       36   The name server does not support the specified Opcode
          (NOTIMP)

       37   The name server refuses to perform the specified
          operation for policy or security reasons (REFUSED)

       38   Some name that ought not to exist, does exist
          (YXDOMAIN)

       39   Some RRset that ought not to exist, does exist
          (YXRRSET)

       40   Some RRset that ought to exist, does not exist
          (NXRRSET)

       41   The server is not authoritative for the zone named in
          the Zone Section (NOTAUTH)

       42   A name used in the Prerequisite or Update Section
          is not within the zone denoted by the Zone Section
          (NOTZONE)

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   Status values 33 through 42 are described more fully within Dynamic
   Updates to DNS (RFC 2136 [21]).  Up-to-date values for the values
   of the status field are specified in the most recent ``Assigned
   Numbers'' document [17].


   The DNS name can be a host name, which does not contain the ``.''
   ASCII character as a separator between DNS hierarchy components.
   Any name containing the ``.''  is treated as a Fully Qualified
   Domain Name (FQDN). The length of the DNS name may be determined by
   subtracting, from the Length, the length of those fixed length fields
   which are present.


   If the ``Q'' bit is set, the values or actions requested by the C, I,
   L, A, P bits and the scope field are required, and MUST be provided,
   or the extension MUST be rejected with the appropriate ``status''
   field value, indicating the reason why the server was unable to
   fulfill the required extension attributes.  The ``Q'' bit is NEVER
   used by the server in ``IP address'' extensions it generates.


   A DHCP client can include an IP address in its IP Address extension
   and set the ``I'' bit and the ``A'' bit and/or ``P'' bit to ask the
   DHCP Server to use the information contained in the extension to
   update the DNS on the client's behalf.  This would be done for IP
   addresses obtained by a method other than the DHCP, such as Stateless
   Address Autoconfiguration, RFC 2462 [19].


   If the client wishes to have its FQDN associated with one of several
   existing IP addresses which it has received from the DHCP Server, the
   client MUST supply that IP address in the IP address extension along
   with the FQDN.


   By default, the client SHOULD update the AAAA/A6 (See [7] for
   information about the A6 record type) record, and the server SHOULD
   update the PTR record.  The IP Address extension permit clients and
   servers to use a different behavior than the default through the use
   of the 'Q' and 'A' bits and associated fields.
4.1.1. IP Address Lifetimes


   The lifetime value contained in the ``preferred'' and ``valid''
   fields is a value relative to when the sender sent the message
   containing the ``IP address'' extension.  The receiver adds these
   times to the current clock time in order to determine the absolute
   times for the ``preferred'' and ``valid'' lifetimes.



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4.1.2. Client Considerations


      Sent in a DHCP Request Message


         In a DHCP Request message (for each IP address extension), a
         client MUST initialize the ``status'' field value to zero.


         A client may include multiple IP Address extensions in a single
         DHCP Request, in order to request as many IP addresses of
         varying scopes or subnet prefixes as it requires.


         In a DHCP Request (for each address extension), a client MAY:


           o Request that any IP address of a specific scope be
             returned.  The client does this by setting the scope
             value to the desired value.  The ``C'' bit and prefix-len
             fields MUST NOT be set, as the client is not requesting a
             particular IP address.


           o Request the lease of some IP address on a specific network
             (subnet prefix) or a specific IP address (interface ID
             specified).  The client does this by setting the ``C'' bit
             and including the desired information in the ``IP address''
             and ``prefix-len'' fields in the extension.  Note that
             the client MUST set the prefix-len field to the number of
             left-most bits representing the subnet prefix, even if it
             is requesting a specific IP address.


           o Ask that the IP address returned have the ``preferred''
             and ``valid'' lifetimes suggested by the client.  The
             client does this by setting the ``L'' bit and including the
             desired lifetime values in the ``preferred'' and ``valid''
             lifetime fields.  The client MUST use the lifetimes
             returned by the DHCP server.


           o Request that the DHCP server perform a DNS AAAA/A6 record
             update (``A'' bit is set) and/or DNS PTR record update
             (``P'' bit is set) for either an IP address the server will
             assign (``I'' bit not set) or the IP address the client
             has provided (``I'' bit set) which it acquired through a
             means other than DHCP, for the host name or Fully Qualified
             Domain Name (FQDN) the client has provided.


           o Specify that the attributes of the request carried by the
             ``IP address'' extension are required by the client by
             setting the ``Q'' bit.

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      Received in a DHCP Reply Message in response to a DHCP Request


         When the client receives an IP address extension within a DHCP
         Reply message, it first validates that the bits / fields set in
         the extension are valid.  If they aren't, the client generates
         a DHCP Release message including the ill-formed IP address
         extension, and sets the ``status'' field to 2, and sends it to
         the server.  If the extension is valid, the client inspects
         the ``status'' field value to see whether the client's request
         has been granted.  If the status is nonzero, the client should
         log the error, and display the error condition for action by
         the user and/or the network administrator.  Non-zero status
         almost always indicates that the client will be need to modify
         its request before it could be satisfied by the replying DHCP
         server, or alternatively that the replying DHCP server will
         need to be given updated configuration information for the
         client.


         Upon reception of a new IP address, the client MUST perform
         Duplicate Address Detection (DAD) as specified in RFC
         2462 [19].  If the IP address has already been allocated to the
         client and the client is merely requesting a renewal of the
         lifetime of the IP address, the client MUST NOT perform DAD, as
         it is using this IP address.  If the client finds that the new
         IP address is in use by another node, the client forms a DHCP
         Release message including the IP address extension containing
         the in-use IP address, and sets the ``status'' field value
         to 1, and sends the Release to the DHCP server.


         If the client receives an IP address with zero valid lifetime
         and:


          -  The DHCP Reply message has been authenticated, the client
             MUST immediately discontinue using that IP address.


          -  The DHCP Reply message has no authentication, the client
             sets the valid lifetime for the address to 2 hours.


         When the preferred lifetime of an IP address leased from the
         DHCP server is 80% exhausted, the client SHOULD begin sending
         DHCP requests to the server requesting a renewal of the lease
         on that IP address.  If the client is unsuccessful at its
         attempts and the valid lifetime expires, the client MUST
         immediately stop using that IP address.


      Sent in a DHCP Release Message


         A client sends IP address extension(s) in a DHCP Release
         message when:
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           o It is releasing an IP address back to the server because it
             is finished using it.


           o It has discovered through DAD that the IP address assigned
             by the DHCP server is already being used by a different
             node.


           o The IP address extension received from the DHCP server has
             an illegal combination of bit/fields settings.


           o The client wishes to delete the DNS records associated with
             the IP address/hostname it will present to the server.
4.1.3. Server Considerations


   This section contains information specifying the handling of the ``IP
   Address'' extension by DHCP servers.


   Note that a server implementation MUST scan its client bindings from
   time to time to locate bindings whose lifetimes have expired.  Those
   bindings SHOULD be deleted, and any DNS operations performed which
   are recorded in those bindings MUST be reversed.


      The DHCP Advertise Message and the ``IP address'' Extension


         The ``IP address'' extension is not used in the DHCP Advertise
         message.


      Received in a DHCP Request Message


         When a server processes an ``IP address'' extension within a
         DHCP Request, the server first validates the combination of
         bits / fields contained within the extension.  If these bits /
         fields are set incorrectly, the server generates a DHCP Reply
         message, which includes the incorrect IP address extension
         from the client's request, with the ``status'' field set to 2,
         thereby notifying the client of the error.  If the IP address
         extension is correct, the server processes the extension as
         follows:


           o If no IP address field is present, then the client is
             requesting that the server allocate an IP address of the
             scope identified by the ``scope'' field value.  If the IP
             address field is present and the ``I'' bit is not set, then
             the client is requesting the assignment of:


              *  A specific IP address (interface ID present)


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              *  Any IP address in the specified network (interface ID
                 is zero)


             The prefix-len field specifies the length of the subnet
             prefix in either case.


             The server consults its allocation tables and attempts
             to select an IP address meeting the client's request
             which is appropriate for the link to which the client is
             attached.  The link can be determined by the contents of
             the relay-agent address and prefix-len fields of the DHCP
             Request message.  If these fields are set, then the client
             is off-link, otherwise the client is attached to one of the
             same links as the server.


           o If the client is requesting that the server update the DNS
             on its behalf (either for the IP address the server will
             assign or the one it provided which was acquired through
             some other means (not the DHCP)), the server makes the
             appropriate DNS dynamic update requests and records the
             status of the update within the ``status'' field of the IP
             address extension it will include in the DHCP Reply message
             sent to the client.  If the ``Q'' bit is set, then the
             server will ensure that the DNS operation has completed
             successfully before responding to the client.  If the ``Q''
             bit is not set, then the server SHOULD register the update
             request with the DNS, and MAY immediately return its DHCP
             Reply without waiting for the result of the DNS operation.


             If the client has requested that the server perform
             DNS updates as part of the IP address allocation and
             configuration, the server MUST maintain this fact as part
             of the client's binding.  Then, if the client eventually
             releases the IP address by sending a DHCP Release message
             or the lifetimes associated with the IP address expire
             because the client has not renewed them, the server MUST
             perform the reverse service by updating DNS again to remove
             the changes it has made on the client's behalf.


           o If the client has set the ``Q'' bit, then all fields
             within the IP address extension which represent attributes
             of interest to the client are requirements, and must be
             met, otherwise a DHCP Reply message is generated with the
             ``status'' field set identifying the portion of the request
             the server could not fulfill.  Note that if more than one
             attribute of the request could not be provided, the server
             can only identify one of the problems in the ``status''
             field.


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      Sent in a DHCP Reply Message in response to a DHCP Request


         If the server is assigning an IP address (or extending the
         lifetimes of an existing IP address binding the client holds),
         the server MUST include an IP address extension for the IP
         address with the following settings:


          -  the preferred lifetime


          -  the valid lifetime


         If the DHCP Reply is a response to a DHCP Release, the
         lifetimes MUST both be zero.


         If the server has performed DNS operations on behalf of the
         client, it sets the ``A'' and ``P'' bits if the AAAA/A6 record
         and PTR record respectively have been updated by the DNS.


         The ``status'' field of the extension MUST be set by the server
         indicating the result of the server's attempt to honor the
         client's IP address-related request.


         If the server receives a DHCP Request from one of its clients
         whose address it wishes to invalidate, it can cause the client
         to discontinue use of the old address by including valid and
         preferred lifetimes with a value of zero.


         To perform renumbering, the server will include two IP address
         extensions, one to reduce the preferred and valid lifetimes
         for the old address, and another to give the client its new
         address.


      Received in a DHCP Release Message


         When a server processes an ``IP address'' extension within a
         DHCP Release, the server first validates the combination of
         bits / fields contained within the extension.  If these bits /
         fields are set incorrectly, the server generates a DHCP Reply
         message, which includes the incorrect IP address extension
         from the client's request, with the ``status'' field set to 2,
         thereby notifying the client of the error.  If the IP address
         extension is correct, the server continues to processes the
         extension.


         The client generates a DHCP Release for the following reasons:


           o Client is finished with the IP address.  In this case, the
             client has determined it no longer needs the IP address,
             and is returning it to the server for use by other clients.
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             The server removes the IP address from the client's
             binding, returning it to the general pool of IP addresses.
             If the server has performed DNS operations on behalf of the
             client regarding this IP address, the server contacts the
             DNS service and deletes the changes it has made regarding
             the FQDN/IP address.  The server generates a DHCP Reply
             including the client's IP address extension, with the
             ``status'' field set to indicate the results of the release
             operation.


           o Client has discovered through DAD that the IP address is
             already in use by another node.  The server MUST mark
             the errant IP address as unavailable for assignment, and
             SHOULD generate a log message indicating the problem to
             the administrator.  The server then generates a DHCP Reply
             message containing the client's IP address extension, with
             the ``status'' field set to 0 to indicate that it has
             received the client's release.


           o The client has requested that the DHCP server serve as a
             DNS update proxy for a name associated with an IP address
             that it acquired outside of the DHCP. The server will undo
             the DNS operations it performed on behalf of this client,
             deletes its knowledge of those operations, and generates
             a DHCP Reply message including the client's IP address
             extension with the ``status'' field set to indicate the
             result of the release operation.


      The ``IP Address'' Extension and the DHCP Reconfigure-init
         Messages


         A server MUST NOT include an ``IP address'' extension in DHCP
         Reconfigure or DHCP Reconfigure-init messages.  IP addresses
         may be changed during the DHCP Request/Reply exchange set in
         motion by DHCP Reconfigure-init message(s).
5. General Extensions


   General extensions (in the range 8193-49152) are important for many
   DHCP clients, and are not specific to any upper-level protocol.
5.1. IEEE 1003.1 POSIX Timezone Extension


   This extension allows delivery of timezone information in the form of
   a IEEE 1003.1 POSIX Timezone specifier, as detailed in section 5.1.1.
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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Type = 8193           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    IEEE 1003.1 POSIX Timezone string (variable length) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   If a DHCP client finds that the POSIX Timezone extension value is
   misformatted, it SHOULD notify the the user of the problem and MUST
   discard the entire extension value.
5.1.1. IEEE 1003.1 POSIX Timezone specifier


   The format of the IEEE 1003.1 POSIX timezone string is specified as

      StdOffset[Dst[Offset],[Start[/Time],End[/Time]]]


   where '[' and ']' enclose optional fields, '|' indicates choice
   of exactly one of the alternatives, ',' and '/' represent literal
   characters present in the string, and:


      Std      three or more octets for the standard timezone (Std).
               Any characters (or case) except a leading colon, digits,
               comma, minus or plus sign are allowed.


      Offset   Indicates the value one must add to local time to
               arrive at UTC, of the form:  [+|-]hh[:mm[:ss]].  Offset
               following Std is required.  Digits are always interpreted
               as decimal number.  If preceded by a '-', the timezone is
               east of the Prime Meridian, otherwise it is west ('+' is
               optional) The permissible values for hh[:mm[:ss]] are as
               follows:


                  hh       0 <= hh <= 23


                  mm       0 <= mm <= 60


                  ss       0 <= ss <= 60


               Offset has no default value.


      Dst      three or more octets for the daylight savings timezone.
               If Dst is missing, then daylight savings time does not
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               apply in this locale.  If no Offset follows Dst, then
               Dst is assumed to be one hour ahead of standard time.
               Any characters (or case) except a leading colon, digits,
               comma, minus or plus sign are allowed.


      Start    Indicates the day of the year, in one of the formats
               indicated below, when to change to daylight savings time.
               The ``Time'' field (which follows immediately after a
               ``/'' character, if present) indicates when the change is
               made, in local time.


      End      Indicate the day of the year, in one of the formats
               indicated below, when to change back from daylight
               savings time.  The ``Time'' field (which follows
               immediately after a ``/'' character, if present)
               indicates when the change is made, in local time.


      Time     Time has the same format as Offset, except that no
               leading ``-'' or ``+'' is permitted.  The default is
               02:00:00.


   The day of the year can be given in one of the following formats:


      Jn       The julian day n, (1 <= n <= 365).  Leap days are not
               counted.


      n        Zero-based julian day, (0 <= n <= 365).  Leap days are
               counted so it is possible to refer to Feb 29.


      Mm.n.d   The ``d''th day, (0 <= d <= 6) of week ``n'' of month
               ``m'' of the year (1 <= n <= 5, 1 <= m <= 12, where week
               5 means last ``d'' day in month ``m'' which may occur in
               either the fourth or the fifth week.  Week ``1'' is the
               first week in which the ``d'' day occurs.
5.1.2. An Example:


   For Eastern USA time zone, 1986, the Posix timezone string is as
   follows:

      EST5EDT4,116/02:00:00,298/02:00:00


   Here, ``5'' is the Offset for Std, and ``4'' is the Offset for Dst.
   Start is the 116th day at 2am, and End is 298th day at 2am.
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5.2. Domain Name Server Extension

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Type = 8194          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Domain Name System server addresses              |
   |                       (16 octets each)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The Domain Name Server extension specifies a list of Domain Name
   System (STD13 [16]) name servers available to the client.  Servers
   SHOULD be listed in order of preference.


   The minimum Length for this extension is 16 octets, and MUST always
   be a multiple of 16.
5.3. Domain Name Suffix Extension


   This extension specifies the default domain name suffix that client
   should use when resolving hostnames via the Domain Name System.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8195         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Domain Name Suffix (variable length)  ...        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The minimum length for this extension is 1.  The domain name is a
   NVT-ASCII string, ``Length'' octets in size.  If the Domain Name
   Suffix extension is not specified, and the IP Address extension
   received by a client contains a FQDN, then the client MAY take the
   part of the FQDN after the first ``.''  octet as the Domain Name
   Suffix.
5.4. Service Location Protocol Directory Agent Extension


   Entities using the Service Location Protocol (SLP) [20] need to find
   out the address of one or more Directory Agents in order to transact
   messages, and possibly the correct scope to be used in conjunction
   with the service attributes which are exchanged using the Service
   Location Protocol.

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   The Directory Agent extension requests or specifies a Directory Agent
   (DA), along with zero or more scopes supported by that DA. Note
   that this extension MAY be included multiple times in the same DHCP
   Request or DHCP Reply.  If so, then the extensions SHOULD be included
   in order of decreasing preference.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8196         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |D|F|M|T|        reserved       |           DA length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Directory Agent (variable length) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        (if present) Typed-Scope-List (variable length) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Length   (unsigned integer, variable) The length of the Extension
               in octets.


      D        If the ``D'' bit is set, the Directory Agent field and
               the DA Length fields are present.


      F        If the ``F'' bit is set, the Directory Agent is indicated
               by including its variable length host name or Fully
               Qualified Domain Name (FQDN) instead of its IP address.


      M        If the ``M'' bit is set, the Directory Agent address
               MUST be present, and multicast methods for discovering
               Directory Agents MUST NOT be used.


      T        If the ``T'' bit is set, the Typed-Scope-List is present.


      rsv      reserved; ignored upon reception; MUST be sent as zero


      DA Length
               The length (in octets) of the Directory Agent field.


      Directory Agent
               The FQDN, host name, or IP address of the Directory
               Agent.


      Typed-Scope-List
               The string denoting the typed-scope-list formatted
               as explained in the description of the service scope
               extension (section 5.5).


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   In order to simplify administration of the configuration of DAs for
   clients using SLP, the DA can be indicated by presenting its host
   name or FQDN instead of its IP address.  This allows renumbering to
   proceed more smoothly as outlined in RFC1900 [6].  When the FQDN or
   host name is used, the server sets the ``F'' bit.  The host name can
   be distinguished from the FQDN by the presence of a ``.''  character.
   In any case, the DA length field is set to be the length of the
   Directory Agent field.  When the ``F'' bit is not set, the DA Length
   MUST be 16.


   Note that more than one Directory Agent extension may be present in
   a DHCP message.  Each such extension may have the same or different
   typed-scope-list.  The client may request any Directory Agent with
   a particular scope, by including the Directory Agent extension in a
   DHCP Request message with no Directory Agent address included (the
   ``D'' bit set to zero), and a nonempty typed-scope-list.  The length
   of the Typed-Scope-List is only indicated implicitly by the overall
   length of the extension.


   The format of the Typed-Scope-List field is described in the service
   scope extension (section 5.5).


   The ``M'' bit MUST NOT be set when the extension is used as part of a
   DHCP Request message.


   Extension type 8196 MUST include one or more scopes if a DA address
   is returned.  Using extension type 8196, it is not possible for
   different service types on the same node to be configured with
   different directory agents.  In other words, all service agents of
   the same service type on the same node will be configured with the
   same directory agent.
5.5. Service Location Protocol Service Scope Extension


   This extension indicates a scope that should be used by a Service
   Agent (SA) as described in RFC 2165 [20], when responding to Service
   Request messages as specified by the Service Location Protocol (SLP).


   This extension MAY be included multiple times in the same DHCP
   Request or DHCP Reply.


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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8197         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Typed-Scope-List ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Length   (unsigned integer, variable) The length of the Extension
               in octets.


      Typed-Scope-List
               In Service Location Protocol (SLP) [20], multiple
               service types can be hosted on the same network node.
               It is possible that different service types on the same
               computer would be administered from different scopes.
               Thus, extension types 8196 and 8197 have additional
               syntax to allow this more detailed style of service
               configuration.


               In particular, the list of scopes contained in the
               extensions is syntactically separated into lists
               pertaining to each service type.


               Grammatically, a typed-scope-list extension in a DHCP
               Reply is structured as follows:


                    typed-scope-list = one or more
                     maybe-typed-scope-items,
               separated by commas
                    maybe-typed-scope-item =
                     typed-scope-item,
               or scope-list
                    typed-scope-item = '(' service-type
                     '=' scope-list ')'
                    scope-list = one or more
                     scope-items, comma-separated
               A typed-scope-list extension in a DHCP Request is
               structured as follows:


                    typed-scope-list = one or more
                     maybe-typed-scope-items,
               separated by commas
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                    maybe-typed-scope-item = typed-scope-item,
                     or maybe-empty-scope-list
                    typed-scope-item = '(' service-type '='
                     maybe-empty-scope-list ')'
                    maybe-empty-scope-list = zero or
                     more scope-items, comma-separated
               A service type has the format defined in RFC 2609 ([9]),
               and a scope-item has the format defined in RFC 2608
               ([10]) for ``strval''.  Basically, a scope-item is
               a character string that has alphanumeric characters
               not including control characters or `(',`)',`,',
               \',`!',`<',`=',`>', or `"' Service schemes are special
               cases of schemes as defined for general URLs in RFC 1738
               ([3]).


               The typed-scope-list MAY contain both untyped-scope-lists
               and typed-scope-lists.  Each scope-item in each
               untyped-scope-list applies to every service type on the
               node.  The string containing the typed-scope-list is NOT
               null-terminated.  The typed-scope-list string must be
               UTF-8 character encoded.


               As an example, the scope-list ``A,B,C'' denotes scopes A,
               B and C for all service types on the client.  In a DHCP
               Request, this scope string would indicate that the client
               wishes a directory agent which supports ANY of these
               three scopes.  In a DHCP Reply, the scope indicates that
               the directory agent supports ALL of the three scopes.


               Suppose instead that service types ``netman'' and
               ``proxystuff'' are residing on a DHCP client.  Then, the
               typed-scope-list in a DHCP Reply could be:


               (netman=mgmt),(proxystuff=math-dept,labs)
               Assuming the DHCP client with two service types
               ``netman'' and ``proxystuff'' did not make any scope
               restriction, a corresponding typed-scope-list in a DHCP
               Request could be:


               (netman=),(proxystuff=)
               asking for scopes for those service types.
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   The Typed-Scope-List is described in section 5.5.  The DHCP
   client (i.e., user agent or service agent) which receives this
   extension will use the indicated scope for in all SLP requests and
   registrations.


   DHCP clients MAY use extension 8197 to request scopes for one or
   more particular service types.  Note that more than one Service
   Scope extension may be present in a DHCP message.  The length of the
   typed-scope-list is only indicated implicitly by the overall length
   of the extension.
5.6. Network Time Protocol Servers Extension


   This extension specifies a list of IP addresses indicating NTP [13]
   servers available to the client.  Servers SHOULD be listed in order
   of preference.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Type = 8198          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     NTP server addresses                      |
   |                       (16 octets each)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The minimum Length for this extension is 16, and the Length MUST be a
   multiple of 16.
5.7. Network Information Service (NIS) Domain Name Extension


   This extension specifies the name of the client's NIS domain.  The
   domain is formatted as a character string consisting of characters
   from the NVT-ASCII character set.  The minimum length for this
   extension is 1.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Type = 8199          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              NIS Domain Name (variable length)  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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5.8. Network Information Service (NIS) Servers Extension


   This extension specifies a list of IP addresses indicating NIS
   servers available to the client.  Servers SHOULD be listed in order
   of preference.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Type = 8200          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     NIS server addresses                      |
   |                       (16 octets each)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The minimum Length for this extension is 16, and the length MUST be a
   multiple of 16.
5.9. Network Information Service V2 (NIS+) Domain Extension


   This extension specifies the name of the client's NIS+ domain.  The
   domain is formatted as a character string consisting of characters
   from the NVT-ASCII character set.  The minimum Length for this
   extension is 1.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Type = 8201          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              NIS+ Client Domain Name (variable length)  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.10. Network Information Service V2 (NIS+) Servers Extension


   This extension specifies a list of IP addresses indicating NIS+
   servers available to the client.  Servers SHOULD be listed in order
   of preference.

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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Type = 8202          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     NIS+ server addresses                     |
   |                       (16 octets each)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The minimum Length for this extension is 16, and the length MUST be a
   multiple of 16.
5.11. TCP-specific Extensions


   This section lists the extensions that affect the operation of the
   TCP layer on a per-interface basis.
5.11.1. TCP Keepalive Interval Extension


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Type = 8203          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Keepalive Time Interval                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   This extension specifies the interval (in seconds) that the
   client TCP should wait before sending a keepalive message on a TCP
   connection.  The time is specified as a 32-bit unsigned integer.
   A value of zero indicates that the client should not generate
   keepalive messages on connections unless specifically requested by an
   application.


   The length for this extension is 4.
6. DHCP-specific Extensions


   This section details the extensions that are used by the DHCP.
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6.1. Maximum DHCP Message Size Extension


   This extension specifies the maximum size in octets of any DHCP
   message that the sender of the extension is willing to accept.
   The size is specified as an unsigned 32-bit integer.  A client
   may use the maximum DHCP message size extension in DHCP Request
   messages, but MUST NOT use the extension in other DHCP messages.


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Type = 8204         |           Length = 4          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Max DHCP Message Length                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The Length for this extension is 4.  The minimum permissible value is
   1280, as specified in RFC 2460 [8].
6.2. DHCP Retransmission and Configuration Parameter Extension


   This extension allows configuration of values for DHCP
   retransmission and configuration variables, as specified for
   use when sending or receiving DHCP messages.  These variables
   are discussed in detail in the section on ``Configuration
   Variables'' in the ``DHCP for IPv6'' companion document [4].


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Type = 8205         |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Configuration Variable Identifier (2 octets)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 New Variable Value  (8 octets)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The length for this extension is 10 octets.  The ``Configuration
   Variable Identifier'' field carries an unsigned 16-bit network-order
   integer representing the configuration variable.  The ``New Variable
   Value'' field carries a 64-bit network-order integer representing the
   value of the configuration variable.  If a client uses this extension
   in a DHCP Request message, then the ``New Variable Value'' field
   MUST be 0 (zero).  If a client does not receive a setting for the
   ``Configuration Variable Identifier'' it has requested, it MUST use


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   the default values defined in the ``Configuration Variables'' section
   of the ``DHCP for IPv6'' document [4].
6.3. Extension Request Extension (ERE)


   The ``Extension Request Extension'' (ERE) MAY be used by DHCP
   implementations to indicate which DHCP extensions they are interested
   in (client), or what DHCP information (e.g.  extensions) are
   available (server).

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8206         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | A | B | C | D | E | ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The extension contains a list of extension ``Type'' values indicating
   the extension of interest.  Since an extension ``Type'' field is
   an unsigned two-octet network-order integer, each extension is
   identified by two-octets.  Thus, the Length field MUST always be
   an even number.  Extension types listed in the ERE are listed in
   priority order, with the extensions of highest priority listed before
   those of lower priority.  One and only one ERE extension is permitted
   within a DHCP message.
6.3.1. Client Considerations


   If the client implementation supports it, the client SHOULD generate
   a Extensions Request Extension identifying which extensions it is
   interested in and include it in its DHCP Request messages.
6.3.2. Server Considerations


   If a server receives a DHCP request with an ERE extension present,
   the server SHOULD attempt to provide valid values for all the
   information requested.
6.4. Subnet Prefix Extension


   The ``Subnet Prefix'' extension is a DHCP server-only extension used
   to advertise what networks are available on the client's link.  Each
   extension carries a single subnet prefix.

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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8207         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Prefix Len (number of left-most bits) (1 octet)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Subnet Prefix Octets (variable number of octets)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   A subnet prefix is specified by the ``Subnet Prefix Octets'' field
   and the ``Prefix-Len'' field.  The ``Subnet Prefix Octets'' field
   is large enough to contain all the bits of the subnet prefix.  Any
   unused bits in the last octet of this field MUST be set to off
   (zero).


   The length of this extension is variable.
6.4.1. Client Considerations


   Clients MAY use the ``Subnet Prefix'' extension value to request
   one or more IP addresses on that network.  A client does this by
   forming an ``IP address'' extension with the value of the ``Subnet
   Prefix Octets'' field copied into the high-order portion of the ``IP
   address'' field (``C'' bit set) and the ``Prefix-len'' value copied
   into the ``prefix-len'' field of the ``IP address'' extension for
   each IP address desired on the advertised network.
6.4.2. Server Considerations


   In response to a client's DHCP Solicit message (``P'' bit set),
   a server SHOULD return one ``Prefix'' extension for each of the
   networks it is configured to manage that exist on the client's link
   in the resultant DHCP Advertise message.  A server SHOULD NOT include
   ``Prefix'' extensions in its Advertise messages if the client has not
   requested them (``P'' bit NOT set).


   If a server receives a DHCP Request message with ``Prefix''
   extension(s), that DHCP Request message MUST be dropped.
6.5. Platform Specific Information Extension


   A platform is defined as the combination of hardware and operating
   system (OS).
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   This extension is used by clients and servers to exchange
   client-platform-specific information.  The information is an opaque
   collection of data, presumably interpreted by platform-specific code
   on the clients.  The definition of this information is platform
   specific.  Clients identify their platform through the use of the
   Platform Class identifier extension (see Section 6.6).  Clients which
   do not receive platform specific information SHOULD make an attempt
   to operate without it, although they may do so (and announce that
   they are doing so) in a degraded mode.


   If a platform vendor encodes more than one item of information in
   this extension, then the vendor MUST encode the extension using
   ``Encapsulated platform-specific extensions'' as described below.
   The ``Encapsulated platform-specific extensions'' field MUST be
   encoded as a sequence of type/length/value fields of identical syntax
   to the form defined for DHCP extensions (see section 2), encapsulated
   within the ``Platform Specific Information Extension''.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8208         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Platform-specific extension information  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The minimum length for this extension is 4.


   DHCP servers which support the configuration of ``Platform Specific
   Information'' extensions, and which have been configured with
   configuration information specific to some number of ``Platform Class
   Identifiers'' MUST select and return only those platform-specific
   extensions which match the ``Platform Class Identifier'' provided by
   the DHCP client.
6.6. Platform Class Identifier Extension


   This extension is used by a DHCP client to identify the hardware type
   and operating system platform it is hosted on.  The extension value
   itself is an opaque value to a DHCP server, and is only used by the
   DHCP server to "lookup" Platform Specific Extensions associated with
   clients of a certain platform class.  DHCP servers SHOULD also allow
   the association of other extensions (Releasable, General, etc) with
   clients of a certain platform class.


   Note that unlike the ``User Class Identifier'' (see section 6.7, the
   ``Platform Class Identifier'' does not need to be echoed back to the
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   DHCP client because there can be one and only one ``Platform Class
   Identifier'' for a client.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8209         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Platform Class Identifier ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The minimum length for this extension is 4.


   The ``Platform Class Identifier'' is a string of UTF-8 characters
   of Length octets.  The ``Platform Class Identifier'' represents the
   hardware and operating system class of which the client is a member.


   In order to prevent collisions in the ``Platform Class Identifier''
   namespace, DHCP client vendors MUST prefix their ``Platform Class
   Identifiers'' with their stock symbol or some other globally
   recognized organizational identifier.  For example, ``Platform Class
   Identifiers'' for Sun Microsystems Inc platforms would be prefaced
   by ``SUNW'', the NASDAQ stock symbol for Sun.  Those associated with
   Microsoft platforms would be prefaced by ``MSFT''.
6.6.1. Client Considerations


   If the client wishes platform-specific data, it includes a platform
   class identifier extension identifying its platform type.
6.6.2. Server Considerations


   Servers not equipped to interpret the platform class identifier
   specified by a client MUST ignore it (although it may be reported
   to the DHCP administrator).  Otherwise, servers SHOULD respond with
   the set of extensions corresponding to the platform class identifier
   specified by the client.
6.7. User Class Identifier Extension


   This extension is used by a DHCP client to optionally identify the
   type or category of user or applications it represents.


   Network administrators may define specific user class identifiers to
   convey information about a client's software configuration or about

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   its user's preferences.  For example, an identifier may specify that
   a particular machine hosting a DHCP client is a member of the class
   ``accounting auditors'', which have special service needs such as a
   particular database server or printer.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8210         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     User Class Identifier ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The minimum length for this extension is 4.


   The user class identifier is a UTF-8 string of Length octets.
   The value of the ``User Class Identifier'' is selected by the
   administrator of the DHCP domain containing the all members of this
   class.  Thus, a ``User Class Identifier'' need only be unique within
   the DHCP domain, although the administrator MAY choose to prefix the
   ``User Class Identifier'' with the department name in order to reduce
   the possibility of ``User Class Identifier'' name space collisions.
6.7.1. Client Considerations


   If the client is configured to request user class-specific data, it
   includes a User Class identifier extension for each user class it is
   configured with.
6.7.2. Server Considerations


   Servers not equipped to interpret the user class identifier specified
   by a client MUST ignore it (although it may be reported to the
   network administrator).  Otherwise, servers SHOULD respond with
   the set of extensions corresponding to the user class identifier
   specified by the client.  Further, if the server responds with the
   set of extensions corresponding to the given user class identifier,
   it MUST echo the client's user class identifier extension back to the
   client.
6.8. Reconfigure Multicast Address Extension


   A DHCP server can instruct its clients to join one or more multicast
   groups for the purposes of receiving DHCP Reconfigure or DHCP
   Reconfigure-init messages.  The DHCP server accomplishes this by
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   returning a ``Reconfigure Multicast Address Extension'' for each
   multicast address associated with the group.  See the ``DHCP for
   IPv6'' document [4] for more details on the use of this extension.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8211         |           Length = 16         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Reconfigure Multicast Address                |
   |                          (16 octets)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The Length for this extension is 16.
6.9. Renumber DHCP Server Address Extension


   A DHCP server can instruct its clients to change their internal
   records to reflect the server's newly renumbered IP address, by using
   the ``Renumber DHCP Server Address Extension''.  This extension
   SHOULD be sent in the DHCP Reconfigure message.


   The server includes both its previous IP address and its new IP
   address.  Providing the previous IP address allows clients to update
   only those resource associations owned by this server.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8212         |           Length = 32         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Previous DHCP Server Address                  |
   |                          (16 octets)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   New DHCP Server Address                     |
   |                          (16 octets)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The Length for this extension is 32.
6.10. Client-Server Authentication Extension


   Exactly one ``Client-Server Authentication Extension'' MAY be present
   in any DHCP message transmitted between a client and server (or
   vice-versa).  If present, it MUST be the last extension.

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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8213         |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Security Parameters Index (SPI)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Replay Protection                      |
   |                           (8 octets)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Authenticator (variable length) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Length   (unsigned integer, variable) 4 for the SPI, plus 8 for
               the replay protection, plus the number of octets in the
               Authenticator.


      SPI      A Security Parameters Index (SPI) [2] identifying a
               security context from among those available between the
               DHCP client and server.


      Replay Protection
               A 64-bit timestamp (in Network Time Protocol (NTP) [15]
               format) (see section 7.1).


      Authenticator
               (variable length) (See Section 7.2.)


   This authentication extension remedies the inability of IPsec (RFC
   2402 [11]) to provide for non end-to-end authentication, since
   authentication is needed even when the client has no IPv6 address
   with enough scope to reach the DHCP server.  The extension can be
   originated by either the client or server to authenticate the rest of
   the data in the DHCP message.  The default authentication algorithm,
   which MUST be supported by all clients and servers, is defined in
   section 7.2.


   SPI values 0 through 255 are reserved and, if used, MUST conform
   to the security context defined by that value in the most recent
   Assigned Numbers RFC (e.g., STD1 [17]).
6.11. Client Key Selection Extension


   A DHCP server may wish to indicate to a prospective client which
   SPI it must use to authenticate subsequent messages, using the
   ``Client-Server Authentication Extension''.  In such cases, the
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   server includes the ``Client Key Selection Extension'' in its DHCP
   Advertise message.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 8214         |           Length = 4          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Security Parameters Index (SPI)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The Security Parameters Index (SPI)  [2] identifies a security
   context between a pair of nodes among the contexts available in the
   security association defined between the DHCP client and server.


   SPI values 0 through 255 are reserved and, if used, MUST conform to
   the security context defined by that value as defined in the most
   recent Assigned Numbers RFC (e.g.,STD1 [17]).
7. Security Considerations


   A security protocol is urgently needed for use with DHCP, since
   otherwise malicious parties could create numerous denial-of-service
   style attacks based on depleting available server resources or
   providing corrupted or infected data to unsuspecting clients.  The
   following sections discuss aspects of security relevant for users
   of the Client-Server Authentication extension 6.10.  See also the
   Security Considerations in the companion specification [4].
7.1. Replay Protection


   A 64-bit timestamp, in Network Time Protocol [15](NTP) format, is
   used to protect against replay of previous authenticated messages
   by malicious agents.  The NTP timestamp value used in the extension
   MUST be chosen, and verified, to be larger than values used by the
   originator in previous Client-Server Authentication extensions.
   On the other hand, the timestamp value MUST also be chosen (and
   verified) to be no greater than one year more than the last known
   value (if any) used by the originator.
7.2. Default Authentication Algorithm


   The default authentication algorithm is HMAC [12], using
   keyed-MD5 [18].  Given a secret key K, and "data" the information to
   be authenticated, HMAC_result is computed as follows:
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    1. opad := 0x36363636363636363636363636363636 (128 bits)


    2. ipad := 0x5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C (128 bits)


    3. zero_extended_key := K extended by zeroes to be 128 bits long


    4. opadded_key := zero_extended_key XOR opad


    5. ipadded_key := zero_extended_key XOR ipad


    6. HMAC_result := MD5 (opadded_key , MD5 (ipadded_key, data))


   The key K is the shared secret defined by the security association
   between the client and server and by the SPI value specified in
   the Authentication Extension.  The "data" is the stream of octets
   in all previous fields in the DHCP message and extensions.  The
   authenticator is the 128-bit value HMAC_result.
8. IANA Considerations


   This document MAY be superseded by new documents for DHCP extensions,
   which will then include the entire current list of valid extensions.
   This section details the method for specifying new extensions.


   Implementation specific use of undefined extensions (all those in the
   range 86-32767 inclusive) may conflict with other implementations,
   and registration is required.


   The following steps MUST be followed by the author of any new DHCP
   extension, in order to obtain acceptance of the extension as a part
   of the DHCP Internet Standard:


    1. The author documents the new extension as an Internet Draft.


    2. The author submits the Internet Draft for review through the
       IETF standards process as defined in "Internet Official Protocol
       Standards" [17].  The new extension will be submitted for
       eventual acceptance as an Internet Standard.


    3. The author requests a number for the new extension from IANA by
       contacting:

          Internet Assigned Numbers Authority (IANA)
          USC/Information Sciences Institute
          4676 Admiralty Way


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          Marina del Rey, California 90292-6695
          or by email as:  iana@isi.edu


    4. The new extension progresses through the IETF standards
       process; the new extension will be reviewed by the Dynamic Host
       Configuration Working Group (if that group still exists), or as
       an Internet Draft not submitted by an IETF working group.


    5. If the new extension fails to gain acceptance as an Internet
       Standard, the assigned extension number will be returned to IANA
       for reassignment.


   This procedure for defining new extensions will ensure that:


     * allocation of new extension numbers is coordinated from a single
       authority,


     * new extensions are reviewed for technical correctness and
       appropriateness, and


     * documentation for new extensions is complete and published.
9. Acknowledgements


   The original form of this internet draft was copied directly from
   RFC1533 [1], written by Steve Alexander and Ralph Droms.  Thanks to
   Mike Carney for his many helpful comments, as well as contributing
   the design of the Platform Specific Information and Platform Class
   Identifier.  Thanks to Erik Guttman for his helpful suggestions
   for the Service Location extensions.  Thanks to Ralph Droms, Matt
   Crawford, Thomas Narten, and Erik Nordmark for their careful review
   as part of the Last Call process.
10. Full Copyright Statement


   Copyright (C) The Internet Society (1997).  All Rights Reserved.


   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However,
   this document itself may not be modified in any way, such as by
   removing the copyright notice or references to the Internet Society
   or other Internet organizations, except as needed for the purpose
   of developing Internet standards in which case the procedures
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   for copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.


   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.


   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."



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References


    [1] S. Alexander and R. Droms.  DHCP Options and BOOTP Vendor
        Extensions.  Request for Comments (Proposed Standard) 1533,
        Internet Engineering Task Force, October 1993.


    [2] R. Atkinson.  IP Authentication Header.  Request for Comments
        (Proposed Standard) 1826, Internet Engineering Task Force,
        August 1995.


    [3] T. Berners-Lee, L. Masinter, and M. McCahill.  Uniform Resource
        Locators (URL).  Request for Comments (Proposed Standard) 1738,
        Internet Engineering Task Force, December 1994.


    [4] J. Bound, M. Carney, and C. Perkins.  DHCP for IPv6.
        draft-ietf-dhc-dhcpv6-15.txt, May 2000.  (work in progress).


    [5] S. Bradner.  Key words for use in RFCs to Indicate Requirement
        Levels.  Request for Comments (Best Current Practice) 2119,
        Internet Engineering Task Force, March 1997.


    [6] B. Carpenter and Y. Rekhter.  Renumbering Needs Work.  Request
        for Comments (Informational) 1900, Internet Engineering Task
        Force, February 1996.


    [7] M. Crawford, C. Huitema, and S. Thomson.  DNS Extensions to
        Support IPv6 Address Aggregation and Renumbering.
        draft-ietf-ipngwg-dns-lookups-07.txt, 2000.  (work in progress).


    [8] S. Deering and R. Hinden.  Internet Protocol, Version 6 (ipv6)
        Specification.  Request for Comments (Draft Standard) 2460,
        Internet Engineering Task Force, December 1998.


    [9] E. Guttman, C. Perkins, and J. Kempf.  Service Templates and
        Service:  Schemes.  Request for Comments (Proposed Standard)
        2609, Internet Engineering Task Force, June 1999.


   [10] E. Guttman, C. Perkins, J. Veizades, and M. Day.  Service
        Location Protocol, Version 2.  Request for Comments (Proposed
        Standard) 2608, Internet Engineering Task Force, June 1999.


   [11] S. Kent and R. Atkinson.  IP Authentication Header.  Request for
        Comments (Proposed Standard) 2402, Internet Engineering Task
        Force, November 1998.


   [12] H. Krawczyk, M. Bellare, and R. Canetti.  HMAC: Keyed-Hashing
        for Message Authentication.  Request for Comments
        (Informational) 2104, Internet Engineering Task Force,
        February 1997.
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   [13] D. Mills.  Simple Network Time Protocol (SNTP) Version 4 for
        IPv4, IPv6 and OSI.  Request for Comments (Informational) 2030,
        Internet Engineering Task Force, October 1996.


   [14] D. L. Mills.  Internet time synchronization:  The Network Time
        Protocol.  Request for Comments 1129, Internet Engineering Task
        Force, October 1989.


   [15] David L. Mills.  Network Time Protocol (version 3)
        Specification, Implementation.  Request for Comments (Draft
        Standard) 1305, Internet Engineering Task Force, March 1992.


   [16] P. V. Mockapetris.  Domain names - concepts and facilities.
        Request for comments (standard), Internet Engineering Task
        Force, November 1987.


   [17] J. Reynolds and R. Braden.  Internet Official Protocol Sandards.
        Request for comments (proposed standard), Internet Engineering
        Task Force, March 2000.


   [18] R. Rivest.  The MD5 Message-Digest Algorithm.  Request for
        Comments (Informational) 1321, Internet Engineering Task Force,
        April 1992.


   [19] S. Thomson and T. Narten.  IPv6 Stateless Address
        Autoconfiguration.  Request for Comments (Draft Standard) 2462,
        Internet Engineering Task Force, December 1998.


   [20] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan.  Service
        Location Protocol.  Request for Comments (Proposed Standard)
        2165, Internet Engineering Task Force, June 1997.


   [21] P. Vixie, Ed., S. Thomson, Y. Rekhter, and J. Bound.  Dynamic
        Updates in the Domain Name System (DNS UPDATE).  Request for
        Comments (Proposed Standard) 2136, Internet Engineering Task
        Force, April 1997.



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Chair's Addresses


   The working group can be contacted via the current chair:


         Ralph Droms
         Computer Science Department
         323 Dana Engineering
         Bucknell University
         Lewisburg, PA 17837


         Phone:  (717) 524-1145
         EMail:  droms@bucknell.edu

Authors' Addresses


   Questions about this memo can be directed to:


        Jim Bound
        Compaq Computer Corporation
        Mail Stop:  ZK03-3/U14
        110 Spitbrook Road
        Nashua, NH 03062
        USA
        Phone:  +1-603-884-0400
        Email:  bound@zk3.dec.com


        Mike Carney
        Sun Microsystems, Inc
        Mail Stop:  UMPK17-202
        901 San Antonio Road
        Palo Alto, CA 94303-4900
        USA
        Phone:  +1-650-786-4171
        Email:  mwc@eng.sun.com


        Charles E. Perkins
        Communications Systems Lab
        Nokia Research Center
        313 Fairchild Drive
        Mountain View, California 94043
        USA
        Phone:  +1-650 625-2986
        EMail:  charliep@iprg.nokia.com
        Fax:  +1 650 625-2502


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