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Network Working Group                                     Baiju V. Patel
INTERNET DRAFT                                         Intel Corporation

                                                              Munil Shah
                                                   Microsoft Corporation

                                                              March 1997


                  Multicast address allocation extensions to the
                        Dynamic Host Configuration Protocol
                        <draft-ietf-dhc-mdhcp-00.txt>

Status of this memo

   This document is an Internet-Draft. 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.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

Abstract

   The Dynamic Host Configuration Protocol (DHCP) provides a framework
   for passing configuration information to hosts on a TCP/IP network.
   The multicast extensions to DHCP add additional capability of
   dynamic allocation of the multicast addresses and additional
   configuration options.

1. Introduction

   The multicast extensions to DHCP (MDHCP) provide configuration
   parameters to the multicast applications.  MDHCP is built on a
   client-server model, where designated DHCP server allocate
   multicast addresses and deliver parameters associated with the
   address to dynamically configured hosts.  Throughout the remainder
   of this document, the term "server" refers to a host providing
   multicast address(es) and parameters through DHCP, and the term
   "client" refers to a host requesting multicast address(es) and
   parameters from a DHCP server. MDHCP server is used at times, to
   indicate a DHCP server capable of handling MDHCP extensions to the

Patel & Shah                                                    [Page 1]


   DHCP protocol and the MDHCP client is used to indicate the MDHCP
   capable DHCP client. MDHCP is not a separate protocol, but is
   simply extensions to the DHCP protocol.

   MDHCP supports two mechanisms for multicast address allocation.  In
   "automatic allocation", MDHCP assigns a permanent multicast address
   to a client.  In "dynamic allocation", MDHCP assigns a multicast
   address to a client for a limited period of time (or until the
   client explicitly relinquishes the address).  In "manual
   allocation", a client's IP address is assigned by the network
   administrator, and DHCP is used simply to convey the assigned
   address to the client.  A particular network will use one or more
   of these mechanisms, depending on the policies of the network
   administrator.

   Like DHCP, MDHCP should be a mechanism rather than a policy.  MDHCP
   must allow local system administrators control over configuration
   parameters where desired; e.g., local system administrators should
   be able to enforce local policies concerning allocation and access
   to local resources where desired.

   The MDHCP client is not required to obtain IP address from a DHCP
   server in order to use MDHCP protocol.

   The design goals specified in the DHCP RFC also apply to MDHCP.


1.1 Requirements

   Throughout this document, the words that are used to define the
   significance of particular requirements are capitalized.  These words
   are:

      o "MUST"

        This word or the adjective "REQUIRED" means that the
        item is an absolute requirement of this specification.

      o "MUST NOT"

        This phrase means that the item is an absolute prohibition
        of this specification.

      o "SHOULD"

        This word or the adjective "RECOMMENDED" means that there
        may exist valid reasons in particular circumstances to ignore
        this item, but the full implications should be understood and
        the case carefully weighed before choosing a different course.

      o "SHOULD NOT"

        This phrase means that there may exist valid reasons in
        particular circumstances when the listed behavior is acceptable

Patel & Shah                                                    [Page 2]


        or even useful, but the full implications should be understood
        and the case carefully weighed before implementing any behavior
        described with this label.

      o "MAY"

        This word or the adjective "OPTIONAL" means that this item is
        truly optional.  One vendor may choose to include the item
        because a particular marketplace requires it or because it
        enhances the product, for example; another vendor may omit the
        same item.

1.2 Terminology

   This document uses the following terms:

      o "DHCP client"

        A DHCP client is an Internet host using DHCP to obtain
        configuration parameters such as a network address.

      o "DHCP server"

        A DHCP server is an Internet host that returns configuration
        parameters to DHCP clients.

      o "MDHCP client"

        A MDHCP client is a DHCP client that supports MDHCP extensions.

      o "MDHCP server"

        A MDHCP server is a DHCP server that supports MDHCP extensions.

      o "BOOTP relay agent"

        A BOOTP relay agent or relay agent is an Internet host or router
        that passes DHCP messages between DHCP clients and DHCP servers.
        DHCP is designed to use the same relay agent behavior as
        specified in the BOOTP protocol specification.

      o "binding"

        A binding is a collection of configuration parameters, including
        at least an IP address, associated with or "bound to" a DHCP
        client.  Bindings are managed by DHCP servers.

1.3 Motivation

   The current mechanisms used for multicast address allocation are
   fairly adhoc and are specific to the applications. For example, the
   mbone tools listen to the SAP messages to determine the multicast
   addresses that are currently in use. Since there are no unified
   mechanisms for allocating multicast address, each class of

Patel & Shah                                                    [Page 3]


   applications or tools need to implement their own solution. This
   not only creates many different groups of address but also leads to
   potential conflict in address usage.  The router that filters
   multicast packets based on the administrative scopes must be
   aware of different pools of addresses used by different
   applications. If an organization intends to implement a multicast
   allocation and use policy, each of every application must be aware
   of it and know how the addresses are to be used within this
   policy. The protocols that attempt to guess multicast addresses by
   listening to SAP messages may at times lead to conflicting use of
   multicast addresses as well. The conflicts may arise due to use of
   different TTLs, Scoping, packet losses, and temporary shutdown of
   the originating system. Consider an example where a site does not
   allow any multicast to enter but allows all the out going
   multicasts. In that case, an application internal to the site has
   no way of determining which multicast address are in use outside
   the site. Similarly, if some announcement packets are lost, the
   application may incorrectly conclude that a multicast address is
   not in use. Finally, since the multicast address allocation scheme
   is adhoc, the organization cannot clearly define the policy for
   multicasts.

   Therefore, there is a need to provide a mechanism for requesting
   and assigning a multicast address. If this mechanism is based on
   client/server paradigm, the client is not responsible for ensuring
   uniqueness of multicast address. Moreover, the organization may be
   able to enforce a multicast policy through which all the multicast
   addresses are assigned. An example of a policy would be to assign
   multicast addresses from range X to be used within an organization,
   range Y to used for an entire corporation etc. The applications
   should not have to be manually configured to determine these
   policies.

   The proposed protocol does not address the specific means that are
   needed at a DHCP server to determine the address to be
   allocated. For the administratively scoped addresses, the DHCP
   server may have a block of address that it can assign to the
   application. Currently, it is not clear how will it determine the
   addresses to be allocated for the Internet (mbone). If and when
   these mechanisms become available, the DHCP server could act as a
   proxy in obtaining those addresses as well.

1.4 Protocol Summary

   From the client's point of view, MDHCP is an extension of the DHCP
   mechanisms. The MDHCP servers assigns multicast addresses to the
   hosts to be used within a specific scope, and valid for a specific
   period. A client may request multiple multicast addresses.

   The client requests a multicast address(es) to be used for a
   specific multicast scope available to it, and for a specific lease
   period. The MDHCP server would ideally assign the address from the
   requested scope or may allocate it for a different scope. However,
   if it allocates the address from a different scope, it will provide

Patel & Shah                                                    [Page 4]


   this information as an option. The DHCP server MUST provide a TTL
   value. The multicast packets using the assigned address MUST NOT
   use a TTL value larger then the one provided.  The lease period is
   defined by the duration of the lease and the time at which the
   lease becomes effective. Since the client may want to extend lease
   at a later time, the DHCP server SHOULD make every attempt at
   allocating an address which is not currently allocated to any other
   client. The DHCP server MUST NOT allocate the same addresses to
   different clients with overlapping lease period.  The multicast
   scope list is one of the DHCP configuration parameters.

   The scope list may be obtained through the DHCP option described in
   [3], or may be obtained with some other means. Similarly, the MDHCP
   server address (unicast or multicast) may also be obtained by the
   option described in [3] or by some other means.

   The MDHCP protocol uses M flag and a set of options defined below.

2   MDHCP messages and options.
   The following options and flags are used by MDHCP extensions.

2.1 M flag

   A new flag (M) is defined to differentiate the MDHCP messages from
   DHCP messages.  All the messages (DHCPDISCOVER, DHCPOFFER etc.) use
   M flag (this is a new flag) defined below to indicate multicast
   address negotiations. The second bit of the flag field (bit 1)
   defines M (multicast) flag.  The M bit must be set for all the
   message exchanges pertinent to the multicast address assignment.
   The client MUST obtain an IP address prior to requesting a
   multicast address. Therefore, B flag MUST not be set when M flag is
   set.


                                     1 1 1 1 1 1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |B|M|           MBZ             |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                B:  BROADCAST flag
                M:  Multicast address request flag.

                MBZ:  MUST BE ZERO (reserved for future use)

2.2 Multicast Scope Option

   This option is used by the client to indicate the multicast scope
   for the requested multicast address. It is also used to indicate
   the scope of the assigned address by the DHCP server. If this
   option is not specified, the DHCP server MAY allocate an address
   from a DEFAULT scope or reject the request.



Patel & Shah                                                    [Page 5]


                                     1 1 1 1 1 1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | code          | length=4      |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | Scope id                      |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | Scope id                      |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   The client may obtain the scope list through the option described in
   [3] or using some other means. The scope id is the numeric representation
   of the scope as described in [3].
   The 'code' for this option is TBD.

2.3 Start time Option

   The start time is used in a client request (DHCPDISCOVER or
   DHCPREQUEST) to allow the client to request the starting time for
   the use of the assigned address. This option allows client to
   request a multicast address for use at a future time.

                                     1 1 1 1 1 1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | code          | length=4      |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | t1            | t2            |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | t3            | t4            |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The time is the Coordinated Universal Time(UTC) in unit of seconds
   and is specified as a 32-bit integer and is specified in the network
   time format.

   The 'code' for this option is TBD.

   If IP Address Lease Time option specifies the duration of the lease
   beginning at Start Time option value.

2.4 Multicast TTL Option

   This option specifies the TTL value to be used with the multicast
   address. The TTL is specified as an octet with a value between 1
   and 255.

                                     1 1 1 1 1 1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | code          | length=1      |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | TTL           |
                +-+-+-+-+-+-+-+-+

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2.5 Multicast Block Size option

   In some cases, an application may require a group of consecutive
   addresses to be assigned. This option is used by a client to request
   n consecutive addresses. It is also used by the DHCP server to
   indicate number of consecutive addresses assigned starting at the
   address specified in ``yiaddr'' field.


                                     1 1 1 1 1 1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | code          | length=1      |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | n             |
                +-+-+-+-+-+-+-+-+

2.6 Client Port Option

   In order to facilitate implementations outside the operating system
   kernel, and to allow two separate client implementations: one for
   DHCP and one for MDHCP, if this option is specified, the MDHCP
   server MUST use the source port number used in the
   DHCPDISCOVER, DHCPREQUEST, DHCPINFORM, and DHCPRESEASE
   as the destination port number in the response messages.

                 0 1 2 3 4 5 6 7
                +-+-+-+-+-+-+-+-+
                | code          |
                +-+-+-+-+-+-+-+-+

2.7 Cookie Option

   The MDHCP server may issue a cookie along with the multicast
   address(es) so that a different user may use the cookie to renew
   lease on address(es).

                                   1 1 1 1 1 1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | code          | length=2      |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                | cookie                        |
                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This option is useful when the "owner" of the address leaves a
   multicast group and some other member decides to either renew or
   terminate the lease. If a different member of the group from the
   one who was assigned the multicast address wants to modify the
   terms of multicast address, it must use this cookie as its client
   identifier option. For example, if host X was issued a multicast
   address, who decides to leave the multicast group that is using the
   assigned the address. Then, another participant in the group

Patel & Shah                                                    [Page 7]


   determines that the work group must continue beyond the lease time
   for the multicast address, it may renew the lease by specifying the
   cookie to uniquely identify the group of multicast addresses. Note
   that cookie is not used as a security capability but is used to
   simplify the client and server implementations.

3. MDHP protocol

   The client needs to obtain the IP address of the MDHCP server (this
   may be a unicast or a multicast address for MDHCP group), and the
   multicast scope list. This list may be obtained as part of the
   normal DHCP protocol using the options specified in [3] or by some
   other means.

   The client selects one of multicast scopes and requests multicast
   address(es) from the MDHCP servers.  The fields and options that
   are different from the normal DHCP message exchange are summarized
   in Table 1 to 3.  details on rest of the parameters, please
   consult DHCP RFC[1].  The mutlicast addresses are renewed or
   released using the DHCP exchanges for network addresses as defined
   in the DHCP RFC[1].

   Note that all the messages in this exchange have their M flag set
   and B flag not set.

   The MDHCP Client MUST provide client identifier option when sending
   messages for multicast address assignment. The client generates a
   unique key and uses that as a client identifier in the DHCPDISCOVER
   message. When the server responds to this with DHCPOFFER, it also
   provides a cookie along with it. This cookie is generated on the
   server and it uniquely idenfies the transaction associated with the
   multicast addresss(es) being offered to this client. For all the
   subsequent messages, client uses this cookie as a client identifier.
   Each client may be running several different multicast enabled
   applications, and each application may require separate multicast
   address(es). Client MUST use separate unique client identifier when
   requesting separate multicast address(es) for each application.

   A client implementation may choose to use hardware address, hardware
   type and application instance number to generate unique client
   identifier


  Field      DHCPOFFER            DHCPACK             DHCPNAK
  -----      ---------            -------             -------
  'ciaddr'   'ciaddr' from        'ciaddr' from       0
             DHCPDISCOVER or 0    DHCPREQUEST or 0
  'yiaddr'   Starting address of  Starting address of 0
             the multicast block  the multicast block
             assigned to client   assigned to client
  'siaddr'   Server's IP address  Server's IP address 0
             reachable from the   reachable from the
             client.              client.


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  'chaddr'   'chaddr' from        'chaddr' from       'chaddr' from
             client DHCPDISCOVER  client DHCPREQUEST  client DHCPREQUEST
             message              message             message
  'file'     may contain options  may contain options    (unused)
  'options'  options              options

  Option                    DHCPOFFER    DHCPACK            DHCPNAK
  ------                    ---------    -------            -------
  IP address lease time     MUST         MUST (DHCPREQUEST) MUST NOT
  Server identifier         MUST         MUST               MUST
  Multicast TTL             MUST         MUST               MUST NOT
  Multicast Block size      MAY          MAY                MUST NOT
  Cookie                    MUST         MAY                MUST NOT

           Table 1:  Fields and options that are different in
                        multicast DHCP server messages.



     Field      DHCPDISCOVER      DHCPREQUEST           DHCPDECLINE,
                                                        DHCPRELEASE
     -----      ------------      -----------           -----------
     'flags'    Set 'M' Bit.      set 'M' Bit           set 'M' bit
                BROADCAST bit 0   BROADCAST bit 0       BROADCAST bit 0
     'ciaddr'   client's network  client's network      0
                addr reachable    addr reachable
                from the server   from the server
     'chaddr'   may contain       may contain           may contain
                hardware address  hardware address      hardware address
     'options'  options           options               (unused)

     Option                     DHCPDISCOVER  DHCPREQUEST   DHCPDECLINE,
                                                            DHCPRELEASE
     ------                     ------------  -----------   -----------
     Requested IP address       MAY           MUST (in      MUST
                                              SELECTING or  (DHCPDECLINE),
                                              INIT-REBOOT)  MUST NOT
                                              MUST NOT (in  (DHCPRELEASE)
                                              BOUND or
                                              RENEWING)
     Start time                 MAY           MAY           MUST NOT
     Client identifier          MUST          MUST          MAY

             Table 2:  Fields and options that are different in
                               multicast DHCP client messages










Patel & Shah                                                   [Page 10]


   ---------------------------------------------------------------------
   |              |INIT-REBOOT  |SELECTING         |RENEWING|REBINDING |
   ---------------------------------------------------------------------
   |multi/unicast |multicast    |multicast if      |unicast |multicast |
   |              |             |multicast DISCOVER|        |          |
   |              |             |unicast otherwise |        |          |
   |server-ip     |MUST NOT     |MUST              |MUST NOT|MUST NOT  |
   |requested-ip  |MUST         |MUST              |MUST NOT|MUST NOT  |
   |ciaddr        |IP addr      |IP addr           |IP addr |IP address|
   ---------------------------------------------------------------------

              Table 3: Client messages from different states


3.1 DHCPDISCOVER Message.

   If the unicast address of a MDHCP server is known and it supports
   the desired multicast scope, the MDHCP client SHOULD send a
   DHCPDISCOVER address to the MDHCP server. If the MDHCP server fails
   to allocate address(es) or fails to respond, the DHCP client SHOULD
   send a multicast DHCPDISCOVER message to the group address
   (multicast) of the MDHCP server. In both cases, if the client uses
   non-standard DHCP port number, it MUST specify the client port
   option. The client MUST also specify its IP address in the ciaddr
   field so that the MDHCP server and respond to the client request
   with a unicast message. The B flag must not be set and M flag MUST
   be set.

   The client MUST include client identifier option.
   In addition, the DHCPDISCOVER option SHOULD include the following
   options:

   o DHCP Scope,
   o Start time,
   o Lease time (duration)

   If any of these options are not specified, the DHCP server
   may assume default values.

3.2 DHCPOFFER Message.

   The DHCP server may respond to a DHCPDISCOVER message with a
   unicast DHCPOFFER the client. This message MUST includes an
   available multicast address using ``yiaddr'' field. The
   MDHCP server SHOULD reserve the offered address. When allocating
   the address, the server MUST make every effort to ensure that the
   address is not in use for the lease period.

   The server MUST include configuration parameters such as DHCP
   scope, start and lease time, in the DHCPOFFER message, if different
   from the ones requested. The MDHCP server must specify a cookie
   value in this message and this cookie MUST be specified in all the
   subsequent messages exchanged between the MDHCP clients and server
   pertaining to associated address(es). The MDHCP server MUST use the

Patel & Shah                                                   [Page 11]


   cookie to identify the addresses instead of the client IP
   address.

3.3 DHCPREQUEST

   The client will select a multicast address(es) from a DHCPOFFER
   response. The client SHOULD send a unicast DHCPREQUEST message
   indicating the selected multicast address(es) to the MDHCP server,
   when the DHCPOFFER was in response to a unicast DHCPDISCOVER
   message, and using a multicast message, when the DHCPOFFER was in
   response to a multicast address. It MUST include multicast address
   option field in the response. If the number of address selected are
   different from the number of offerred address, the client MUST also
   include the multicast block size option.

   The M flag MUST be set and B flag MUST NOT be set.

3.4 DHCPACK.

   If the multicast address(es) are still available, the MDHCP server
   MUST reserve the address and send a DHCPACK message. Any
   configuration parameters in the DHCPACK message SHOULD NOT conflict
   with the ones in earlier DHCPOFFER message. The M flag MUST be set
   and B flag MUST NOT be set.

3.5 DHCPNACK

   The server MAY choose to mark the multicast address in DHCPOFFER
   unavailable to the client. In that case it will send DHCPNACK
   message. The M flag MUST be set and B flag MUST NOT be set.

3.6 Renewing and termination of lease

   The client may choose to release address(es) before the lease time
   has expired. The usual DHCP messages are used for this purpose.

   The M flag MUST be set and B flag MUST not be set. Moreover, the
   client port option SHOULD be specified, if the client is using a
   port different from the standard DHCP port. The cookie MUST be
   specified with RENEW and RELEASE messages.




4. Examples of usage

   The MDHCP server is not required to be co-located with a DHCP
   server. Therefore, in a typical deployment, there may be fewer
   MDHCP servers then the DHCP servers. We consider specific examples
   of DHCP configurations and the use of MDHCP protocol extensions.

4.1 One MDHCP server

   There is one MDHCP server which is configured to allocate multicast

Patel & Shah                                                   [Page 12]


   addresses to a client and there may be many DHCP servers. The DHCP
   servers should be configured to provide the address of the MDHCP
   server capable of allocating multicast address to the MDHCP client,
   and should include a multicast scope list supported by the MDHCP
   server. The client may obtain the DHCP server address and scope
   list through DHCP client configuration procedure (and may use
   DHCPINFORM message). The client then selects a multicast scope from
   which the multicast address is to be requested and sends out a
   unicast DHCPDISCOVER address and includes multicast scope, start
   time, and lease time information using DHCP options. It
   may also specify multicast block size. The MDHCP server
   responds with an DHCPOFFER for multicast address and includes a TTL
   value to be used with this address. The client sends out a
   DHCPREQUEST message and includes the selected. If the address is
   still available, the server responds with an DHCPACK message, else
   responds with a NACK message.

   Since the DHCP messages are directly send to the MDHCP server, the
   server is capable of interpreting M flag and therefore, there will
   be no conflict between the interpretation of DHCP and MDHCP
   messages.


































Patel & Shah                                                   [Page 13]


                                Client          Server
                                              (selected)

                                  v               v
                                  |               |
                        Obtain IP address         |
                                  |               |
                                  |               |
                   Begin multicast address request|
                                  |               |
                                  |               |
                                  |\_____________ |
                                  | DHCPDISCOVER \|
                                  |               |
                                  |          Determines
                                  |          address(es)
                                  |               |
                                  |  ____________/|
                                  | /DHCPOFFER    |
                                  |/              |
                                  |               |
                                 \|               |
                        Selects Address(es)       |
                                  |               |
                                  |\_____________ |
                                  |  DHCPREQUEST \|
                                  |               |
                                  |     Commits address(es)
                                  |               |
                                  | _____________/|
                                  |/ DHCPACK      |
                                  |               |
                    assignment complete           |
                                  |               |
                                  .               .
                                  .               .
                                  |               |
                         Graceful release         |
                                  |               |
                                  |\_____________ |
                                  |  DHCPRELEASE \|
                                  |               |
                                  |        Discards lease
                                  |               |
                                  v               v
     Figure 1: Timeline diagram of messages exchanged between MDHCP
               client and servers when allocating multicast
               address(es) using unicast messages to a MDHCP capable
               server.





Patel & Shah                                                   [Page 14]


4.2 One or more MDHCP servers

   If one or more MDHCP servers are available to a MDHCP client for
   the purpose of assigning multicast addresses, the DHCP scope list
   option SHOULD specify an administratively scoped group address used
   by the MDHCP servers to receive DHCPDISCOVER messages. Each scope
   in the scope list MUST be supported by atleast one server listening
   to the group multicast address used by MDHCP servers.

   The client SHOULD select a scope and send out a DHCPDISCOVER,
   DHCPREQUEST messages to the group multicast address. The multicast
   DHCPREQUEST message is only received by the MDHCP capable DHCP
   servers, and therefore, there is no conflict between the MDHCP and
   DHCP messages. Further, the messages for renewing and releasing
   lease are sent directly to the MDHCP servers only, and therefore,
   there is conflict between DHCP and MDHCP message interpretation by
   a non-MDHCP capable server.


   A summary of fields of MDHCP in messages that are different from
   the corresponding DHCP [1] messages are specified in Tables 1 to 3.

   In some cases, the client may be aware of the unicast address of an
   MDHCP capable server, and may also be aware of the group multicast
   address of the MDHCP capable servers. In that case, the client
   SHOULD first try to use the unicast address, and if unsuccessful,
   SHOULD try the group multicast address for MDHCP servers.




























Patel & Shah                                                   [Page 15]


                Server          Client          Server
            (not selected)                    (selected)

                  v               v               v
                  |               |               |
                  |     Obtain IP address         |
                  |               |               |
                  |               |               |
                  |Begin multicast address request|
                  |               |               |
                  |               |               |
                  | _____________/|\_____________ |
                  |/ DHCPDISCOVER | DHCPDISCOVER \|
                  |               |               |
              Determines          |          Determines
              address(es)         |          address(es)
                  |               |               |
                  |\              |  ____________/|
                  | \_________    | /DHCPOFFER    |
                  |  DHCPOFFER\   |/              |
                  |            \  |               |
                  |       Collects replies        |
                  |              \|               |
                  |     Selects Address(es)       |
                  |               |               |
                  | _____________/|\_____________ |
                  |/ DHCPREQUEST  |  DHCPREQUEST \|
                  |               |               |
                  |               |     Commits address(es)
                  |               |               |
                  |               | _____________/|
                  |               |/ DHCPACK      |
                  |               |               |
                  | assignment complete           |
                  |               |               |
                  .               .               .
                  .               .               .
                  |               |               |
                  |      Graceful release         |
                  |               |               |
                  |               |\_____________ |
                  |               |  DHCPRELEASE \|
                  |               |               |
                  |               |        Discards lease
                  |               |               |
                  v               v               v
     Figure 2: Timeline diagram of messages exchanged between MDHCP
               client and servers when allocating multicast
               address(es) using group multicast address for MDHCP
               capable servers.





Patel & Shah                                                   [Page 16]


5. Acknowledgements

  The authors would like to thank Thomas Pfenning, Rajeev Byrisetty,
  and Ramesh Vyaghrapuri for their assistance in creating this
  document.

6. References

   [1] Droms, R., "Dynamic Host Configuration Protocol", RFC1541,
   October 1993


   [2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor
       Extensions", RFC 1533, Lachman Technology, Inc., Bucknell
       University, October 1993.

   [3] Patel, B., and Shah, M., ``Multicast address allocation
       extensions options''
       <draft-ietf-dhc-multopt-00.txt>

   [4] Meyer, D., ``Administratively scoped IP Multicast'', Internet draft,
       <draft-ietf-mboned-admin-ip-space-01.txt>

7. Author's Address

   Baiju V. Patel
   Intel Corp.
   2111 NE 25th Ave.
   Hillsboro, OR 97124

   Phone: 503 264 2422
   EMail: baiju@ibeam.intel.com

   Munil Shah
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone:206 703 3924
   Email:munils@microsoft.com

   This document will expire on Sept, 1997














Patel & Shah                                                    [Page 17]


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