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Versions: 02 03 04 05 06 RFC 2730

MALLOC working group                        Baiju V. Patel, Intel Corp.
Internet Draft                              Munil Shah, Microsoft Corp.
February 1999                  Stephen R. Hanna, Sun Microsystems, Inc.
Expires: August 1999                    draft-ietf-malloc-madcap-04.txt

     Multicast Address Dynamic Client Allocation Protocol (MADCAP)

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   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

   This document defines a protocol, Multicast Address Dynamic Client
   Allocation Protocol (MADCAP), that allows hosts to request multicast
   addresses from multicast address allocation servers.

1. Introduction

   Multicast Address Dynamic Client Allocation Protocol (MADCAP) is a
   protocol that allows hosts to request multicast address allocation
   services from multicast address allocation servers. This protocol is
   part of the Multicast Address Allocation Architecture being defined
   by the IETF Multicast Address Allocation Working Group. However, it
   may be used separately from the rest of that architecture as
   appropriate.








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1.1. Protocol Overview

   MADCAP is built on a client-server model, where hosts request address
   allocation services from address allocation servers. See Appendix A
   for examples of typical protocol exchanges.

1.2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

   Throughout the rest of this document, the words "server" or "MADCAP
   server" refer to a host providing multicast address allocation
   services via MADCAP. The words "client" or "MADCAP client" refer to a
   host requesting multicast address allocation services via MADCAP.

1.3. Motivation and Protocol Requirements

   For multicast applications to be deployed everywhere, there is a need
   to define a protocol that any host may use to allocate multicast
   addresses. Here are the requirements for such a protocol.

   Quick response: The host should be able to allocate a multicast
   address and begin to use it promptly.

   Low network load: Hosts that are not allocating or deallocating
   multicast addresses at the present time should not need to send or
   receive any network traffic.

   Support for intermittently connected or power managed systems: Hosts
   should be able to be disconnected from the network, powered off, or
   otherwise inaccessible except during the brief period during which
   they are allocating a multicast address.

   Multicast address scopes: The protocol must be able to allocate both
   the administratively scoped and globally scoped multicast addresses.

   Efficient use of address space: The multicast address space is fairly
   small. The protocol should make efficient use of this scarce
   resource.

   Authentication: Because multicast addresses are scarce, it is
   important to protect against hoarding of these addresses. One way to
   do this is by authenticating clients.

   Policy neutral: Allocation policies (such as who can allocate
   addresses) should not be dictated by the protocol.



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1.4. Relationship with DHCP

   MADCAP was originally based on DHCP. There are still some
   similarities and it may be possible to share some code between a DHCP
   implementation and a MADCAP implementation. However, MADCAP is
   completely separate from DHCP, with no dependencies between the two
   and many significant differences.

2. Protocol Description

   The MADCAP protocol is a client-server protocol. In general, the
   client unicasts or multicasts a message to one or more servers, which
   optionally respond with messages unicast to the client.

   Messages are always sent via UDP. A reserved port number dedicated
   for MADCAP is used on the server (port number 2535, as assigned by
   IANA). Any port number may be used on client machines. When a MADCAP
   server sends a message to a MADCAP client, it MUST use a destination
   port number that matches the source port number provided by the
   client in the message that caused the server to send its message.

   MADCAP is a mechanism rather than a policy. MADCAP allows local
   system administrators to exercise control over configuration
   parameters where desired. For example, MADCAP servers may be
   configured to limit the number of multicast addresses allocated to a
   single client. Properly enforcing such a limit requires cryptographic
   security, as described in the Security Consideration section.

   All MADCAP messages have the same format. Each message includes a
   version, a message type, a transaction id, and a type-length-value
   encoded options field.

   The next few sections describe the MADCAP message format and message
   types. A full list of MADCAP options is provided in section 3.

2.1. Message Format

   Figure 1 gives the format of a MADCAP message and Table 1 describes
   each of the fields in the MADCAP message. The numbers in parentheses
   indicate the size of each field in octets. The names for the fields
   given in the figure will be used throughout this document to refer to
   the fields in MADCAP messages.

   All multi-octet quantities are in network byte-order.

   Any message whose UDP data is too short to hold this format (at least
   12 bytes) MUST be ignored.




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   +-+-+-+-+-+-+-+-+
   |  version (1)  |
   +---------------+
   |  msgtype (1)  |
   +---------------+
   |  addrfamily   |
   |     (2)       |
   +---------------+
   |               |
   |    xid (4)    |
   |               |
   |               |
   +---------------+
   |               |
   |   options     |
   |  (variable)   |
   |      ...      |
   +---------------+

                 Figure 1:  Format of a MADCAP message


   FIELD      OCTETS       DESCRIPTION
   -----      ------       -----------

   version       1  Protocol version number (zero for this specification)
   msgtype       1  Message type (DISCOVER, INFORM, etc.)
   addrfamily    2  Address family (IPv4, IPv6, etc.)
   xid           4  Transaction ID
   options     var  Options field

           Table 1:  Description of fields in a MADCAP message

2.1.1. The version field

   The version field must always be zero for this version of the
   protocol.  Any messages that include other values in this field MUST
   be ignored.

2.1.2. The msgtype field

   The msgtype field defines the "type" of the MADCAP message.

   For more information about this field, see section 2.2.

2.1.3. The addrfamily field

   The addrfamily field defines the default address family (such as IPv4



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   or IPv6) for this MADCAP message. Unless otherwise specified, all
   addresses included in the message will be from this family.  The set
   of address families is defined by the IANA, with IPv4 being 1 and
   IPv6 being 2.

2.1.4. The xid field

   The xid field is a transaction id. This is a number chosen by the
   client so that the combination of xid, msgtype, and Client Identifier
   option value is unique over some short period of time.

   The xid field is used by the client and server to associate messages
   and responses between a client and a server. Before a client sends a
   message, it chooses a number to use as an xid. When the server
   responds to a client message, it MUST use the same xid value in the
   response that the client used in the request. This allows the client
   to associate responses with the message that they are responding to.

   When retransmitting messages (as described in section 2.3), the
   client MUST retransmit them without changing them, thereby using the
   same xid, msgtype, and Client Identifier option. The client SHOULD
   NOT use the same xid, msgtype, and Client Identifier together for two
   different messages.

   If a server receives a message with the same xid value, msgtype
   value, and Client Identifier option value as one received within
   [RESPONSE_CACHE_INTERVAL], it SHOULD treat this message as a
   retransmission of the previously received one and retransmit the
   response, if any. After [RESPONSE_CACHE_INTERVAL], the server may
   forget about the previously received message and treat any
   retransmissions of this message as if they were new messages.

   This avoids retransmissions causing multiple allocations.  An
   appropriate value for [RESPONSE_CACHE_INTERVAL] would be sixty
   seconds, but it may have any value (including zero seconds) and may
   be adjusted dynamically according to resource constraints on the
   server.

2.1.5. The options field

   The options field consists of a list of tagged parameters that are
   called "options". All options consist of a two octet option code and
   a two octet option length, followed by the number of octets specified
   by the option length. In the case of some options, the length field
   is a constant but must still be specified.

   The option field MUST contain any number of options that are not an
   end option followed by an end option (code 0). Any message whose



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   options field does not conform to this syntax MUST be ignored.

   Any MADCAP client or server sending a MADCAP message MAY include any
   of the options listed in section 3, subject to the restrictions in
   Table 3 and elsewhere in this document. They MAY also include other
   MADCAP options that are defined in the future. A MADCAP client or
   server MUST NOT include more than one option with the same option
   type in one MADCAP message.

   All MADCAP clients and servers MUST recognize all options listed in
   this document and behave in accordance with this document when
   receiving and processing any of these options. Any unrecognized
   options MUST be ignored. If a message is received that does not
   conform to the requirements of this document (for instance, not
   including all required options) it MUST be ignored. The order of
   options within a message has no significance and any order MUST be
   supported in an equivalent manner, with the exception that the End
   option must occur once per message, as the last option in the option
   field.

   New MADCAP options may only be defined by IETF Consensus, as
   described in [7]. Basically, this means that they are defined by RFCs
   approved by the IESG.

2.2. Message Types

   The msgtype field defines the "type" of a MADCAP message. Legal
   values for this field are:

           Value   Message Type
           -----   ------------
             1     DISCOVER
             2     OFFER
             3     REQUEST
             4     RENEW
             5     ACK
             6     NAK
             7     RELEASE
             8     INFORM

      Table 2:  MADCAP message types

   Throughout this document, MADCAP messages will be referred to by the
   type of the message; e.g., a MADCAP message with a message type of 0
   will be referred to as an INFORM message.

   Here are descriptions of the MADCAP message types.  Table 3, which
   appears at the end of this section, summarizes which options are



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   allowed with each message type.

   MADCAP clients and servers MUST handle all MADCAP message types
   defined in this document in a manner consistent with this document.
   If a client or server receives a message whose message type it does
   not recognize, it MUST ignore this message. Note, however, that under
   some circumstances this document requires or suggests that clients or
   servers ignore certain messages. For instance, clients that do not
   send DISCOVER messages SHOULD ignore OFFER messages.  Also, secure
   servers SHOULD ignore DISCOVER messages and all servers SHOULD ignore
   DISCOVER or INFORM messages that they cannot satisfy.

   New MADCAP message types may only be defined by IETF Consensus, as
   described in [7]. Basically, this means that they are defined by RFCs
   approved by the IESG.

2.2.1. INFORM

   The INFORM message is used by a MADCAP client that wants to acquire
   configuration parameters, especially a multicast scope list.  This
   message also allows a client to determine which servers are likely to
   be able to handle future requests.

   The MADCAP client sends out an INFORM message. The message may be
   unicast to a particular MADCAP server or multicast to a MADCAP Server
   Multicast Address. For more details about the MADCAP Server Multicast
   Address, see section 2.9.

   If a server receives an INFORM message and it can process the request
   successfully, it SHOULD unicast an ACK message to the client. If the
   INFORM message includes an Option Request List option, the server
   SHOULD try to include the specified options in its response, but is
   not required to do so. If the Option Request List option is not
   included, the server SHOULD include the Multicast Scope List option.

   If a server receives an INFORM message and it can not process the
   request successfully, it SHOULD ignore the INFORM message.

   If a client sends an INFORM message and does not receive any ACK
   messages in response after an appropriate delay, the client MAY
   resend its INFORM message, as described in section 2.3.

   When a MADCAP client sends an INFORM message, it MAY include the
   Requested Language option, which specifies which language the client
   would prefer for the zone names in the Multicast Scope List. The
   proper way to handle this tag with respect to zone names is discussed
   in the definition of the Multicast Scope List option.




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2.2.2. DISCOVER

   The DISCOVER message is a multicast message sent by a MADCAP client
   that wants to discover MADCAP servers that can probably satisfy a
   REQUEST.

   MADCAP clients are not required to use the DISCOVER message.  They
   MAY employ other methods to find MADCAP servers, such as sending a
   multicast INFORM message, caching an IP address that worked in the
   past or obtaining a DNS name or IP address from DHCP or prior
   configuration. Using the DISCOVER message has the particular
   advantage that it allows clients to automatically move to another
   server if one fails.

   The MADCAP client begins by sending a multicast DISCOVER message to a
   MADCAP Server Multicast Address. Any servers that wish to assist the
   client respond by sending a unicast OFFER message to the client. If a
   server can process the request with a shorter lease time or later
   start time than the client requested, it SHOULD send an OFFER message
   with the lease time or start time that it can offer.  However, it
   MUST NOT offer a lease time shorter than the minimum lease time
   specified by the client or a start time later than the maximum start
   time specified by the client.

   For more details about the MADCAP Server Multicast Address, see
   section 2.9.

   After a suitable delay [DISCOVER-DELAY], the client MAY select the
   server it wants to use (if any) and send a multicast REQUEST message
   identifying that server. See section 2.2.4 for more information about
   the REQUEST message.

   The value of [DISCOVER-DELAY] and the mechanism used by the client in
   selecting the server it wants to use are implementation dependent.

   When a MADCAP client sends a DISCOVER message, it MAY include the
   Lease Time, Minimum Lease Time, Start Time, Maximum Start Time,
   Number of Addresses Requested, and List of Address Ranges options,
   describing the addresses it wants to receive. However, it need not
   include any of these options. If one of these options is not
   included, the server will provide the appropriate default (maximum
   available for Lease Time, no minimum for Minimum Lease Time, as soon
   as possible for Start Time, no maximum for Maximum Start Time, one
   for Number of Addresses Requested, and any addresses available for
   List of Address Ranges). The Multicast Scope option MUST be included
   in the DISCOVER message so that the server knows what scope should be
   used. The Current Time option MUST be included if the Start Time or
   Maximum Start Time options are included.



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   If a client sends a DISCOVER message and does not receive any OFFER
   messages after an appropriate delay, the client MAY retransmit its
   DISCOVER message, as described in section 2.3.

2.2.3. OFFER

   The OFFER message is a unicast message sent by a MADCAP server in
   response to a DISCOVER message that it can probably satisfy.

   A MADCAP server is never required to send an OFFER message in
   response to a DISCOVER message. For instance, it may not be able to
   satisfy the client's request or it may have been configured to
   respond only to certain types of DISCOVER messages or not to respond
   to DISCOVER messages at all.

   If a MADCAP server decides to send an OFFER message, it MUST include
   the Lease Time and Multicast Scope options, describing the addresses
   it is willing to provide. However, it need not include the List of
   Address Ranges option. If the List of Address Ranges Allocated option
   is not included, it is assumed that the server is willing to provide
   the number of addresses that the client requested. If the Start Time
   option is not included, it is assumed that the server is willing to
   provide the start time requested by the client (if any). The Current
   Time option MUST be included if the Start Time option is included.

   If a server can process the request with a shorter lease time or
   later start time than the client requested, it SHOULD send an OFFER
   message with the lease time or start time that it can offer.
   However, it MUST NOT offer a lease time shorter than the minimum
   lease time specified by the client or a start time later than the
   maximum start time specified by the client.

   If the server sends an OFFER message, it SHOULD attempt to hold
   enough addresses to complete the transaction. If it receives a
   multicast REQUEST message with the same xid and Client Identifier
   option as the DISCOVER message for which it is holding these
   addresses and a Server Identifier option that does not match its own,
   it SHOULD stop holding the addresses.  The server SHOULD also stop
   holding the addresses after an appropriate delay [OFFER-HOLD] if the
   transaction is not completed. The value of this delay is
   implementation-specific.

   As with all messages sent by the server, the xid field MUST match the
   xid field included in the client request to which this message is
   responding. The Client Identifier option MUST be included, with the
   value matching the one included in the client request. The Server
   Identifier option MUST be included, with the value being the server's
   IP address. And the packet MUST NOT be retransmitted.



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2.2.4. REQUEST

   The REQUEST message is used by a MADCAP client that wants to allocate
   one or more multicast addresses. It is not used for renewing an
   existing lease. The RENEW message is used for that.

   If a REQUEST message is completing a transaction initiated by a
   DISCOVER message, the following procedure MUST be followed so that
   all MADCAP servers know which server was selected. The same xid value
   used in the DISCOVER message MUST be used in the REQUEST message.
   Also, the Server Identifier option MUST be included, using the Server
   Identifier of the server selected. Also, the REQUEST message MUST be
   multicast to the MADCAP Server Multicast Address.

   If a REQUEST message is not completing a transaction initiated by a
   DISCOVER message, the REQUEST message MUST be unicast to the MADCAP
   server that the client wants to use. In this case, the Server
   Identifier option MAY be included, but need not be.

   If the selected server can process the request successfully, it
   SHOULD unicast an ACK message to the client. Otherwise, it SHOULD
   unicast an NAK to the client. If a server can process the request
   with a shorter lease time or later start time than the client
   requested, it SHOULD send an ACK message with the lease time or start
   time that it can offer. However, it MUST NOT offer a lease time
   shorter than the minimum lease time specified by the client or a
   start time later than the maximum start time specified by the client.

   When a MADCAP client sends a REQUEST message, it MAY include the
   Lease Time, Minimum Lease Time, Start Time, Maximum Start Time,
   Number of Addresses Requested, and List of Address Ranges options,
   describing the addresses it wants to receive. However, it need not
   include any of these options. If one of these options is not
   included, the server will provide the appropriate default (maximum
   available for Lease Time, no minimum for Minimum Lease Time, as soon
   as possible for Start Time, no maximum for Maximum Start Time, one
   for Number of Addresses Requested, and any addresses available for
   List of Address Ranges). The Multicast Scope option MUST be included
   in the REQUEST message so that the server knows what scope should be
   used. The Current Time option MUST be included if the Start Time or
   Maximum Start Time options are included.

   If a client sends a REQUEST message and does not receive any ACK or
   NAK messages in response after an appropriate delay, the client MAY
   resend its REQUEST message, as described in section 2.3.

   If the server responds with a NAK or fails to respond within a
   reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the



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   client MAY try to find another server by sending a DISCOVER message
   with another xid or sending a REQUEST message with another xid to
   another server.

2.2.5. ACK

   The ACK message is used by a MADCAP server to respond affirmatively
   to an INFORM, REQUEST, or RELEASE message. The server unicasts the
   ACK message to the client from which it received the message to which
   it is responding.

   The set of options included with an ACK message differs, depending on
   what sort of message it is responding to.

   If the ACK message is responding to an INFORM message, it SHOULD
   include any options requested by the client using the Option Request
   List option in the INFORM message. If the Option Request List option
   was not included in the INFORM message, the server SHOULD include the
   Multicast Scope List option.

   If the ACK message is responding to a REQUEST message, it MUST
   include Lease Time, Multicast Scope, and List of Address Ranges
   options.  It MAY include a Start Time option. If a Start Time option
   is included, a Current Time option MUST also be included. If no Start
   Time option is included, the lease is assumed to start immediately.

   If the ACK message is responding to a RENEW message, it MUST include
   Lease Time, Multicast Scope, and List of Address Ranges options.  It
   MAY include a Start Time option. If a Start Time option is included,
   a Current Time option MUST also be included. If no Start Time option
   is included, the lease is assumed to start immediately.

   If the ACK message is responding to a RELEASE message, it MUST only
   include Server Identifier and Client Identifier options.

   As with all messages sent by the server, the xid field MUST match the
   xid field included in the client request to which this message is
   responding. The Client Identifier option MUST be included, with the
   value matching the one included in the client request. The Server
   Identifier option MUST be included, with the value being the server's
   IP address. And the packet MUST NOT be retransmitted.

2.2.6. NAK

   The NAK message is used by a MADCAP server to respond negatively to a
   REQUEST, RELEASE, or RENEW message. The server unicasts the NAK
   message to the client from which it received the message to which it
   is responding.



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   As with all messages sent by the server, the xid field MUST match the
   xid field included in the client request to which this message is
   responding. The Client Identifier option MUST be included, with the
   value matching the one included in the client request. The Server
   Identifier option MUST be included, with the value being the server's
   IP address. And the packet MUST NOT be retransmitted.

2.2.7. RELEASE

   The RELEASE message is used by a MADCAP client that wants to
   deallocate one or more multicast addresses before their lease
   expires.

   The client unicasts the RELEASE message to the MADCAP server from
   which it allocated the addresses. If the selected server can process
   the request successfully, it SHOULD unicast an ACK message to the
   client. Otherwise, it SHOULD unicast a NAK to the client.

   The lease to be released is whichever one was allocated with a Client
   Identifier option matching the one provided in the RELEASE message.
   It is not possible to release only part of the addresses in a single
   lease.

   If a client sends a RELEASE message and does not receive any ACK or
   NAK messages in response after an appropriate delay, the client MAY
   resend its RELEASE message, as described in section 2.3.

   If the server responds with a NAK or fails to respond within a
   reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
   client MAY send a RELEASE message with another xid to another server,
   provided that the Server Mobility feature was used in the original
   REQUEST message and that this feature is required for the subsequent
   RELEASE message sent to another server. For more information about
   the Server Mobility feature, see section 2.12.1.

2.2.8. RENEW

   The RENEW message is used by a MADCAP client that wants to renew a
   multicast address lease, changing the lease time or start time.

   The client unicasts the RENEW message to a MADCAP server. If the
   server can process the request successfully, it SHOULD unicast an ACK
   message to the client. Otherwise, it SHOULD unicast a NAK to the
   client.

   The lease to be renewed is whichever one was allocated with a Client
   Identifier option matching the one provided in the RENEW message.
   The Lease Time, Start Time, and Current Time options MAY be included,



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   depending on whether the client wishes to change the lease time, the
   start time, or both.

   If a client sends a RENEW message and does not receive any ACK or NAK
   messages in response after an appropriate delay, the client MAY
   resend its RENEW message, as described in section 2.3.

   If the server responds with a NAK or fails to respond within a
   reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the
   client MAY send a RENEW message with another xid to another server,
   provided that the Server Mobility feature was used in the original
   REQUEST message and that this feature is required for the subsequent
   RENEW message sent to another server. For more information about the
   Server Mobility feature, see section 2.12.1.

2.2.9. Options Allowed

   Table 3 summarizes which options are allowed with each message type.

   Option                  INFORM         ACK (in response to INFORM)
   ------                  ------         ---------------------------
   Lease Time              MUST NOT       MUST NOT
   Server Identifier       MUST NOT       MUST
   Client Identifier       MUST           MUST
   Multicast Scope         MUST NOT       MUST NOT
   Option Request List     MAY            MUST NOT
   Start Time              MUST NOT       MUST NOT
   Number of Addresses
     Requested             MUST NOT       MUST NOT
   Requested Language      MAY            MUST NOT
   Multicast Scope List    MUST NOT       MAY
   List of Address Ranges  MUST NOT       MUST NOT
   Current Time            MUST NOT       MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MUST NOT
   Minimum Lease Time      MUST NOT       MUST NOT
   Maximum Start Time      MUST NOT       MUST NOT

   Option                  DISCOVER       OFFER
   ------                  --------       -----
   Lease Time              MAY            MUST
   Server Identifier       MUST NOT       MUST
   Client Identifier       MUST           MUST
   Multicast Scope         MUST           MUST
   Option Request List     MUST NOT       MUST NOT
   Start Time              MAY            MAY
   Number of Addresses
     Requested             MAY            MUST NOT



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   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MAY            MAY
   Current Time            MAY            MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MUST NOT
   Minimum Lease Time      MAY            MUST NOT
   Maximum Start Time      MAY            MUST NOT

   Option                  REQUEST        ACK (in response to REQUEST)
   ------                  -------        ----------------------------
   Lease Time              MAY            MUST
   Server Identifier       MUST (if       MUST
                             multicast)
   Client Identifier       MUST           MUST
   Multicast Scope         MUST           MUST
   Option Request List     MUST NOT       MUST NOT
   Start Time              MAY            MAY
   Number of Addresses
     Requested             MAY            MUST NOT
   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MAY            MUST
   Current Time            MAY            MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MAY
   Minimum Lease Time      MAY            MUST NOT
   Maximum Start Time      MAY            MUST NOT

   Option                  RENEW          ACK (in response to RENEW)
   ------                  -----          --------------------------
   Lease Time              MAY            MUST
   Server Identifier       MUST NOT       MUST
   Client Identifier       MUST           MUST
   Multicast Scope         MUST NOT       MUST
   Option Request List     MUST NOT       MUST NOT
   Start Time              MAY            MAY
   Number of Addresses
     Requested             MUST NOT       MUST NOT
   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MUST NOT       MUST
   Current Time            MAY            MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MAY
   Minimum Lease Time      MAY            MUST NOT
   Maximum Start Time      MAY            MUST NOT




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   Option                  RELEASE        ACK (in response to RELEASE)
   ------                  -------        ----------------------------
   Lease Time              MUST NOT       MUST NOT
   Server Identifier       MUST NOT       MUST
   Client Identifier       MUST           MUST
   Multicast Scope         MUST NOT       MUST NOT
   Option Request List     MUST NOT       MUST NOT
   Start Time              MUST NOT       MUST NOT
   Number of Addresses
     Requested             MUST NOT       MUST NOT
   Requested Language      MUST NOT       MUST NOT
   Multicast Scope List    MUST NOT       MUST NOT
   List of Address Ranges  MUST NOT       MUST NOT
   Current Time            MUST NOT       MAY
   Feature List            MAY            MAY
   Retry Time              MUST NOT       MUST NOT
   Minimum Lease Time      MUST NOT       MUST NOT
   Maximum Start Time      MUST NOT       MUST NOT

   Option                  NAK
   ------                  ---
   Lease Time              MUST NOT
   Server Identifier       MUST
   Client Identifier       MUST
   Multicast Scope         MUST NOT
   Option Request List     MUST NOT
   Start Time              MUST NOT
   Number of Addresses
     Requested             MUST NOT
   Requested Language      MUST NOT
   Multicast Scope List    MUST NOT
   List of Address Ranges  MUST NOT
   Current Time            MUST NOT
   Feature List            MAY
   Retry Time              MUST NOT
   Minimum Lease Time      MUST NOT
   Maximum Start Time      MUST NOT

             Table 3:  Options allowed in MADCAP messages

2.3. Retransmission

   MADCAP clients are responsible for all message retransmission. The
   client MUST adopt a retransmission strategy that incorporates an
   exponential backoff algorithm to determine the delay between
   retransmissions.  The delay between retransmissions SHOULD be chosen
   to allow sufficient time for replies from the server to be delivered
   based on the characteristics of the internetwork between the client



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   and the server.

   For example, in a 10Mb/sec Ethernet internetwork, the delay before
   the first retransmission SHOULD be at least 4 seconds. The delay
   before the next retransmission SHOULD be at least 8 seconds. The
   retransmission delay SHOULD be doubled with subsequent
   retransmissions up to a maximum of 64 seconds. The client MAY provide
   an indication of retransmission attempts to the user as an indication
   of the progress of the process. The client MAY halt retransmission at
   any point.

2.4. The Client Identifier

   The Client Identifier option is included in each message sent by an
   MADCAP client. Its value is used to identify a particular client and
   MUST be unique across all clients in a multicast address allocation
   domain. In addition, for each lease requested by a client, the client
   must use a different client identifier. This allows the lease to be
   identified using just the client identifier.

   The first octet of the client identifier is the client identifier
   type.  If the client identifier type is 0, the remainder of the
   client identifier is a long random number (such as 128 bits from a
   decent random number generator). If the client identifier type is 1,
   the remainder of the client identifier is a two octet IANA-defined
   address family number, an address from the specified address family,
   and a client-specific identifier (such as a sequence number or the
   current time).

   New MADCAP client identifier types may only be defined by IETF
   Consensus, as described in [7]. Basically, this means that they are
   defined by RFCs approved by the IESG.

   The MADCAP server does not need to parse the client identifier. It
   SHOULD use the client identifier only as an opaque identifier, which
   must be unique for each lease. The purpose of defining different
   client identifier types is to allow MADCAP clients that already have
   a globally unique address to avoid the possibility of client ID
   collisions by using this address together with a client-specific
   identifier. MADCAP clients that do not have a globally unique address
   SHOULD use client identifier type 0.

   In addition to associating client and server messages (along with the
   xid field, as described in the next section), the Client Identifier
   is used to control who can renew or release a lease. MADCAP servers
   MUST require that the client identifier used in a RENEW or RELEASE
   message match the client identifier used in the initial REQUEST
   message. If not, the server MUST ignore the message. To provide true



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   security with this technique, the confidentiality of the client
   identifier must be maintained by encrypting all messages that contain
   it.

2.5. Associating Client and Server Messages

   Messages between clients and servers are associated with one another
   using the Client Identifier option and the xid field.  For each
   transaction initiated by a client, the client MUST generate an xid
   value that is unique for that client identifier and likely to be
   unique across all client identifiers. For instance, a client might
   start with a random xid and increment from there. The client
   identifier option and xid field MUST be included in each message sent
   by the client or the server.

   The client MUST check the client identifier option and xid field in
   each incoming message to ensure that they match the client identifier
   and xid for an outstanding transaction. If not, the message MUST be
   discarded. The server MUST check the client identifier option and xid
   field in each incoming message to establish the proper context for
   the message. If the message is inappropriate for its context, it
   SHOULD be discarded.

   A transaction can be an attempt to allocate a multicast address
   (consisting of DISCOVER, OFFER, REQUEST, ACK, and NAK messages), an
   attempt to renew a lease (consisting of RENEW, ACK, and NAK
   messages), an attempt to release a previously allocated multicast
   address (consisting of RELEASE, ACK, and NAK messages), or an attempt
   to acquire configuration parameters (consisting of INFORM and ACK
   messages).

2.6. Multicast Scopes

   RFC 2365 [3] provides for dividing the multicast address space into a
   number of administratively scopes. Routers should be configured so
   that each scope corresponds to a particular partition of the network
   into disjoint regions. Messages sent to a multicast address that
   falls within a certain administrative scope should only be delivered
   to hosts that have joined that multicast group *and* fall within the
   same region as the sender. For instance, packets sent to an address
   in the organization-local scope should only be delivered to hosts
   that have joined that group and fall within the same organization as
   the sender.

   Different sets of scopes may be in effect at different places in the
   network and at different times. Before attempting to allocate an
   address from an administrative scope (other than global or link-
   level scope, which are always in effect), a MADCAP client SHOULD



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   determine that the scope is in effect at its location at this time.
   Several techniques that a MADCAP client may use to determine the set
   of administrative scopes in effect (the scope list) are: manual
   configuration or configuration via MADCAP (using the Multicast Scope
   List option).

   If a MADCAP client is unable to determine its scope list using one of
   these techniques, it MAY temporarily assume a scope list consisting
   of a single scope. If it is using IPv4, it SHOULD use IPv4 Local
   Scope (239.255.0.0/16), with a maximum TTL of 16.  If it is using
   IPv6, it SHOULD use SCOP 3, with a maximum TTL of 16. Using this
   temporary scope list, it MAY attempt to contact a MADCAP server that
   can provide a scope list for it.

   When a MADCAP client requests an address with a DISCOVER or REQUEST
   message, it MUST specify the administrative scope from which the
   address should be allocated. This scope is indicated with the
   Multicast Scope option. Likewise, the server MUST include the
   Multicast Scope option in all OFFER messages and all ACK messages
   sent in response to REQUEST messages.

2.7. Multicast TTL

   Another way to limit propagation of multicast messages is by setting
   the TTL field before sending them. This technique has several
   disadvantages in comparison to administratively scoped multicast
   addresses, but it is currently in widespread usage.

   In an environment where administrative scoping is not in use and TTL
   scoping is, a MADCAP server MAY return a scope list that includes
   artificial scopes with TTLs less than 255. The MADCAP client MAY then
   allocate addresses from these scopes, but MUST NOT set the TTL field
   of any packet sent to such an address to a value greater than the
   maximum TTL indicated in the scope list. In particular, it is
   recommended that the MADCAP Server Multicast Addresses associated
   with the IPv4 Local Scope (or SCOP 3 for IPv6) be blocked so that
   packets sent to them with TTL of 16 are not sent outside an
   appropriate boundary.  This will allow MADCAP clients to use their
   default behavior for finding MADCAP servers.

   In an environment where administrative scoping is in use, the maximum
   TTLs in the scope list SHOULD be 255. The admin scope zone boundary
   routers will prevent leakage of MADCAP packets beyond appropriate
   limits.

2.8. Locating MADCAP Servers

   There are several ways for a MADCAP client to locate a MADCAP server.



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   For instance, the client may obtain a DNS name or IP address from
   DHCP or manual configuration.

   One particularly convenient technique (and the recommended one) is
   for the client to send an INFORM message to a MADCAP Server Multicast
   Address and wait for ACK responses. This technique is described in
   more detail in the next section.

2.9. MADCAP Server Multicast Address

   Each multicast scope has an associated MADCAP Server Multicast
   Address. This address has been reserved by the IANA as the address
   with a relative offset of -1 from the last address of a multicast
   scope.

   A MADCAP client looking for servers that can provide multicast
   allocation services MAY send an INFORM message to a MADCAP Server
   Multicast Address. Any MADCAP servers listening to this address
   SHOULD respond with a unicast ACK message to the client if they wish
   to offer a response.

   The MADCAP Server Multicast Address used by a client MAY be
   established by configuration (manually or via DHCP). If a client has
   no such configuration, it SHOULD use the MADCAP Server Multicast
   Address associated with IPv4 Local Scope (or SCOP 3 for IPv6) with
   maximum TTL of 16, unless otherwise configured.

2.10. Going Beyond the Local Scope

   If a client receives no response to a message sent to a MADCAP Server
   Multicast Address (after retransmission), it MAY send the message to
   a larger scope and repeat this process as necessary. However, the
   client MUST NOT send a MADCAP message to the MADCAP Server Multicast
   Address associated with the global scope.

   This technique allows MADCAP servers to provide services for scopes
   in which they do not reside. However, this is a dangerous and
   complicated technique and is *not* recommended at this time.
   Therefore, MADCAP clients SHOULD only send multicast messages to the
   MADCAP Server Multicast Address corresponding to the IPv4 Local Scope
   (or SCOP 3, if using IPv6), unless configured otherwise.

   MADCAP servers that wish to provide services for scopes in which they
   do not reside MUST make special efforts to ensure that their services
   meet clients' needs for largely conflict-free allocation and accurate
   scope list information.  In particular, coordinating with other
   servers that provide services for this scope may be difficult. Also,
   establishing which scope the client is in may be difficult. If a



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   MADCAP server is not prepared to provide services for scopes in which
   it does not reside, it SHOULD ignore REQUEST, RENEW, and RELEASE
   messages whose scope does not match (or is enclosed by) the scope of
   the MADCAP Server Multicast Address on which the request was
   received. It SHOULD also ignore INFORM messages that are not received
   on the MADCAP Server Multicast Address for IPv4 Local Scope.

2.11. Clock Skew

   The Current Time option is used to detect and handle clock skew
   between MADCAP clients and servers. This option MUST be included in
   any MADCAP message that includes an absolute time (such as the Start
   Time option). It MAY be included in any DISCOVER, OFFER, REQUEST, or
   ACK message.

   Clock skew is a situation where two systems have clocks that are not
   synchronized. MADCAP servers SHOULD expect and tolerate a small
   amount of clock skew with their clients by ensuring that multicast
   addresses are allocated for an extra period of time
   [CLOCK_SKEW_ALLOWANCE] on either side of the lease given to the
   client. However, large amounts of clock skew require special
   handling. The value of [CLOCK_SKEW_ALLOWANCE] is implementation
   dependent. A reasonable value might be one hour.

   The Current Time option contains the sender's opinion of the current
   time in UTC at or about the time the message was assembled. Because
   of delays in transmission and processing, this value will rarely
   match the receiver's opinion of the current time at the time the
   option is processed by the receiver.

   If a MADCAP server detects clock skew greater than
   [CLOCK_SKEW_ALLOWANCE], it SHOULD ignore the client. The server MAY
   log a message. While it would be possible for the server to correct
   for the clock skew, this would probably cause other problems, such as
   the client advertising a session with an incorrect time. This is an
   error condition and should be handled as such.

2.12. Optional Features

   Each MADCAP client or server MAY implement one or more optional
   features.  Optional features of MADCAP are identified with a two
   octet feature code.

   A MADCAP client MAY request, require, or indicate support for an
   optional feature by including a Feature List option in a message. A
   MADCAP server MUST either provide all required features for this
   message or ignore that message. For more information about optional
   features, see the description of the Feature List option.



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   Table 4 lists the feature codes defined at this time and sections
   2.12.1 and 2.12.2 describe how these features work.

   New MADCAP feature codes may only be defined by IETF Consensus, as
   described in [7]. Basically, this means that they are defined by RFCs
   approved by the IESG.

           Feature Code   Feature Name
           ------------   ------------
                0         Server Mobility
                1         Retry After

      Table 4:  MADCAP message types

2.12.1. Server Mobility

   The Server Mobility feature allows an address allocated on one MADCAP
   server to be renewed or released on a different MADCAP server. This
   requires communication and coordination among MADCAP servers. The
   primary benefits are immunity to the failure of a single MADCAP
   server and perhaps greater performance through load balancing.

   In order to take advantage of the Server Mobility feature, a MADCAP
   client must ensure that the feature is implemented by both the server
   that is used for the original allocation and the server that is used
   for the renewal or release. The best way to ensure this is to include
   the Server Mobility feature in the required list of a Feature List
   option in the REQUEST message used to allocate the address (and the
   DISCOVER message, if one is used). When the time comes to renew or
   release the address, the client may, of course, send a unicast RENEW
   or RELEASE message to the server from which it allocated the address.
   However, if this server is unavailable, the client may send the RENEW
   or RELEASE message to any other server that includes the Server
   Mobility feature in its list of supported features. The client may
   find such a server by (for instance) sending an INFORM message with a
   required Server Mobility feature.

   If the MADCAP client does not want to require this feature when
   allocating addresses, it may include the feature in the requested
   list of a Feature List option and see if the server includes the
   feature in the supported list of a Feature List option in the ACK
   message.

   Even if the Server Mobility feature is used, there is no guarantee
   that a server will be available to perform the renewal or release or
   that the renewal or release will succeed. Server connectivity may
   have failed, for instance.




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2.12.2. Retry After

   The Retry After feature allows a MADCAP server to ask the MADCAP
   client to retry its request later, as may be required when allocating
   large numbers of addresses or allocating addresses for a long period
   of time.

   If a MADCAP client includes the Retry After feature in the supported
   list of a Feature List option in a REQUEST message, a MADCAP server
   that also implements the Retry After feature MAY decide to perform a
   future allocation. In this case, the MADCAP server will include an
   empty List of Address Ranges option in its ACK message, a Feature
   List option that includes the Retry After feature in the supported
   list, and a Retry Time option with a time after which the client
   should retry the REQUEST.

   The client MUST NOT include the Retry After feature in the requested
   or required list of a Feature List option, since the decision about
   whether Retry After is desirable should be left to the MADCAP server.

   At some later time (preferably after the time indicated in the Retry
   Time option), the client MAY send a REQUEST message with all the same
   options as the original REQUEST message (especially the Client
   Identifier option), but with a new xid value.  The server MAY return
   a normal ACK or NAK message at this point or it MAY continue the
   transaction to a later time by including an empty List of Address
   Ranges option in its ACK message, a Feature List option that includes
   the Retry After feature in the supported list, and a Retry Time
   option with a later time after which the client should retry the
   REQUEST.

   At any point after receiving the initial ACK message with the Retry
   Time option, the client MAY terminate the allocation process and any
   accompanying lease by sending to the server performing the allocation
   (or another server if the Server Mobility feature is also in effect)
   a RELEASE message with the Client Identifier included in the original
   REQUEST message.

   The Retry After feature may also be used when renewing a lease.  In
   this case, the description above applies except that the client sends
   a RENEW message instead of a REQUEST message.

   If a client sends a RENEW message with a Client Identifier that
   matches a lease which is currently undergoing allocation with the
   Retry After feature in response to a REQUEST message, the server
   SHOULD ignore the RENEW message.  Also, if a client sends a RENEW
   message with a Client Identifier that matches a lease which is
   currently undergoing allocation with the Retry After feature in



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   response to a RENEW message, but the options supplied with the two
   RENEW messages do not match, the server SHOULD ignore the second
   RENEW message.

3. MADCAP Options

   The following options are defined for use in MADCAP messages. The
   options are listed in numerical order.

3.1. End

   The End option marks the end of valid information in the options
   field. This option MUST be included at the end of the options field
   in each MADCAP message.

   The code for this option is 0, and its length is 0.

        Code        Len
   +-----+-----+-----+-----+
   |     0     |     0     |
   +-----+-----+-----+-----+

3.2. Lease Time

   This option is used in a client request (DISCOVER, REQUEST, or RENEW)
   to allow the client to request a lease time for the multicast
   address. In a server reply (OFFER or ACK), a MADCAP server uses this
   option to specify the lease time it is willing to offer.

   The time is in units of seconds, and is specified as a 32-bit
   unsigned integer.

   The code for this option is 1, and its length is 4.

        Code        Len            Lease Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |     1     |     4     |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.3. Server Identifier

   This option contains the IP address of a MADCAP server. A two octet
   address family (as defined by IANA) is stored first, followed by the
   address.  The address family for this address is not determined by
   the addrfamily field so that addresses from one family may be
   allocated while communicating with a server via addresses of another
   family.




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   All messages sent by a MADCAP server MUST include a Server Identifier
   option with the IP address of the server sending the message.

   MADCAP clients MUST include a Server Identifier option in multicast
   REQUEST messages in order to indicate which OFFER message has been
   accepted.

   The code for this option is 2, and its minimum length is 3.

        Code        Len    Address Family     Address
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |     2     |     n     |   family  |  a1 |  ...            |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

3.4. Client Identifier

   This option is used by MADCAP clients to specify their unique
   identifier. For more information about this option and how it is
   used, see section 2.4.

   The code for this option is 3, and its minimum length is 1.

        Code        Len     Client Identifier
   +-----+-----+-----+-----+-----+-----+---
   |     3     |     n     |  i1 |  i2 | ...
   +-----+-----+-----+-----+-----+-----+---

3.5. Multicast Scope

   The multicast scope option is used by the client to indicate the
   requested multicast scope in a DISCOVER or REQUEST message. It is
   also used by the MADCAP server to indicate the scope of an assigned
   address.

   The client may obtain the scope list through the Multicast Scope List
   option or using some other means. The scope id is the first multicast
   address in the scope. The address family of the Scope ID is
   determined by the addrfamily field.

   The code for this option is 4, and its minimum length is 1.

        Code        Len        Scope ID
   +-----+-----+-----+-----+-----+-----
   |     4     |     n     |  i1 |  ...
   +-----+-----+-----+-----+-----+-----






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3.6. Option Request List

   This option is used by a MADCAP client in an INFORM message to
   request that certain options be included in the server's ACK
   response. The server SHOULD try to include the specified options in
   its response, but is not required to do so.

   The format of this option is a list of option codes.

   The code for this option is 5 and the minimum length is 2.

        Code        Len      Requested Options
   +-----+-----+-----+-----+-----+-----+---...
   |     5     |     n     |  Option1  |
   +-----+-----+-----+-----+-----+-----+---...

3.7. Start Time

   The Start Time option specifies the starting time for a multicast
   address lease.

   A client may include this option in a DISCOVER, RENEW, or REQUEST
   message to request a multicast address for use at a future time. A
   server may include this option in an OFFER message or in an ACK in
   response to REQUEST or RENEW message to indicate that a lease has
   been granted which starts at a future time.

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

   If the Start Time option is present, the Current Time option MUST
   also be present, as described in section 2.11.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP timestamp by adding decimal 2208988800.
   This time format will not wrap until the year 2106.

   The code for this option is 6 and the length is 4.

        Code        Len      Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |     6     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+






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3.8. Number of Addresses Requested

   This option specifies the minimum and desired number of addresses
   requested by the client. It is only used in DISCOVER and REQUEST
   messages and is only sent by the client.

   The minimum and desired number of addresses requested are unsigned 16
   bit integers in network byte order. The minimum MUST be less than or
   equal to the desired number. If a message is received where this is
   not the case, the message MUST be ignored.

   The client MAY obtain more than one address either by repeating the
   protocol for every address or by requesting several addresses at the
   same time via this option. When the client is requesting only one
   address, this option SHOULD not be included. A MADCAP server
   receiving a DISCOVER or REQUEST packet including this option MUST
   include between minimum and desired number of addresses in any OFFER
   or ACK response.

   The code for this option is 7 and the length is 4.

        Code        Len      Minimum     Desired
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |     7     |     4     | min       | desired   |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.9. Requested Language

   This option specifies the language in which the MADCAP client would
   like strings such as zone names to be returned. It is only included
   in an INFORM message sent by the client. It is an RFC 1766 [6]
   language tag. The proper way to handle this tag with respect to zone
   names is discussed further in the definition of the Multicast Scope
   List option.

   The code for this option is 8 and the minimum length is 0.

        Code        Len      Language Tag
   +-----+-----+-----+-----+-----+-...-+-----+
   |     8     |     n     | L1  |     | Ln  |
   +-----+-----+-----+-----+-----+-...-+-----+

3.10. Multicast Scope List

   This option is sent by the server in an ACK message in response to an
   INFORM message sent by the client.

   If the client did not include a Requested Language option in its



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   INFORM message, the MADCAP server SHOULD return all zone names for
   each zone. If the client included a Requested Language option in its
   INFORM message, the MADCAP server MUST return no more than one zone
   name for each zone. For each zone, the MADCAP server SHOULD first
   look for a zone name that matches the requested language tag (using a
   case-insensitive ASCII comparison). If any names match, one of them
   should be returned. Otherwise, the MADCAP server SHOULD choose
   another zone name to return (if any are defined). It SHOULD give
   preference to zone names that are marked to be used if no name is
   available in a desired language.

   The code for this option is 9 and the minimum length is 0.

   The format of the multicast scope list option is:

        Code        Len     Count  Scope List
   +-----+-----+-----+-----+-----+-----+-...-+-----+
   |     9     |     p     | m   | L1  |     | Lm  |
   +-----+-----+-----+-----+-----+-----+-...-+-----+

   The scope list is a list of m tuples, where each tuple is of the
   form,

           Scope ID      Last Address   TTL   Name  Encoded Name List
                                              Count
       +---+--...--+---+---+--...--+---+-----+-----+-----+-...-+-----+
       |  ... ID ...   | ... Last ...  | T   | n   | EN1 |     | ENn |
       +---+--...--+---+---+--...--+---+-----+-----+-----+-...-+-----+


   where Scope ID is the first multicast address in the scope, Last
   Address is the last multicast address in the scope, TTL is the
   multicast TTL value for the multicast addresses of the scope, and
   Name Count is the number of encoded names for this zone (which may be
   zero). The address family of the Scope ID and Last Address is
   determined by the addrfamily field. Each encoded name is of the form


        Name  Lang   Language Tag      Name   Name
        Flags Length                   Length
       +-----+-----+-----+-...-+-----+-----+-----+-...-+-----+
       | F   | q   | L1  |     | Lq  | r   | N1  |     | Nr  |
       +-----+-----+-----+-...-+-----+-----+-----+-...-+-----+

   where Name Flags is a flags field with flags defined below, Lang
   Length is the length of the Language Tag in octets (which MUST NOT be
   zero), Language Tag is a language tag indicating the language of the
   zone name (as described in [6]), Name Length is the length of the



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   Name in octets (which MUST NOT be zero), and Name is a UTF-8 [5]
   string indicating the name given to the scope zone.

   The high bit of the Name Flags field is set if the following name
   should be used if no name is available in a desired language.
   Otherwise, this bit is cleared. All remaining bits in the octet
   SHOULD be set to zero and MUST be ignored.

   The scope IDs of entries in the list MUST be unique and the scopes
   SHOULD be listed from smallest (topologically speaking) to largest.

   Example:

   There are two scopes supported by the multicast address allocation
   server: Inside abcd.com with addresses 239.192.0.0-239.195.255.255,
   and world with addresses 224.0.1.0-238.255.255.255. Then this option
   will be given as:

            Code        Len     Count
       +-----+-----+-----+-----+-----+...
       |     9     |     51    | 2   |
       +-----+-----+-----+-----+-----+...

           Scope ID     Last Address    TTL Name  Name  Lang   Language
                                            Count Flags Length Tag
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
       |239|192| 0 | 0 |239|195|255|255|10 | 1   | 128 |  2   | en  |
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...

        Name
        Length Name
       +------+--+--+-...-+--+--+...
       |  15  | Inside abcd.com |
       +------+--+--+-...-+--+--+...

           Scope ID     Last Address    TTL Name  Name  Lang   Language
                                            Count Flags Length Tag
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...
       |224| 0 | 1 | 0 |238|255|255|255|16 | 1   | 128 |  2   | en  |
       +---+---+---+---+---+---+---+---+---+-----+-----+------+-...-+...

        Name
        Length Name
       +------+--...--+
       |  5   | world |
       +------+--...--+





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3.11. List of Address Ranges

   This option is used by the server to provide the list of all the
   address ranges allocated to the client.

   This option is also used by the client when requesting a lease for a
   specific set of addresses.

   The address family of the addresses is determined by the addrfamily
   field.

   The code for this option is 10 and the minimum length is 0.

        Code        Len       Address Range List
   +-----+-----+-----+-----+-----+-----+-...-+-----+
   |    10     |     n     | L1  | L2  |     | Ln  |
   +-----+-----+-----+-----+-----+-----+-...-+-----+

   where the Address Range List is of the following format.

           StartAddress1  BlockSize1 StartAddress2 BlockSize2 ...
           +---+---+---+---+---+---+---+---+---+---+---+---+--...--+
           |  ... S1 ...   |B11|B12|  ... S2 ...   |B21|B22|       |
           +---+---+---+---+---+---+---+---+---+---+---+---+--...--+

3.12. Current Time

   This option is used to express what the sender thinks the current
   time is. This is useful for detecting clock skew. This option MUST be
   included if the Start Time or Maximum Start Time options are used, as
   described in section 2.11.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP [4] timestamp by adding decimal
   2208988800. This time format will not wrap until the year 2106.

   The code for this option is 11 and the length is 4.

        Code        Len        Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    11     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.13. Feature List

   This option lists optional MADCAP features supported, requested, or
   required by the sender. This option MAY be included in any message



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   sent by a MADCAP server or client.

   Optional features of MADCAP are identified with a two octet feature
   code.  New MADCAP feature codes may only be defined by IETF
   Consensus, as described in [7]. Basically, this means that they are
   defined by RFCs approved by the IESG.

   The Feature List option consists of three separate lists: supported
   features, requested features, and required features. Each list
   consists of an unordered list of feature codes. Features in the
   supported list are features that the sender supports. Features in the
   requested list are features that the sender would like the receiver
   to support and use. However, it is OK if the receiver does not
   support or use those features. Features in the required list are
   features that the sender requires. If the receiver does not support
   those features, it MUST ignore the message.

   If a MADCAP client includes the Feature List option in a message, it
   MAY include features in any of the lists: supported, requested, and
   required.  A MADCAP server that receives a message containing the
   Feature List option from a client MUST ignore the entire message if
   it does not support all of the features in the required list. If it
   supports all of the features in the required list, it MUST implement
   them as appropriate for this message.  It SHOULD try to implement the
   features in the requested list and it MAY implement any of the
   features in the supported list.  If an optional feature (such as
   Retry After) is not included in any part of the Feature List option
   included in the client's message (or if the client does not include a
   Feature List option in its message), the server MUST NOT use that
   feature in its response. If the server does respond to a client's
   message that includes a Feature List option, it MUST include a
   Feature List option listing the features that it supports and with an
   empty requested and required features list.

   The code for this option is 12 and the minimum length is 6.

        Code        Len      Supported   Requested   Required
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |    12     |     n     |    FL1    |    FL2    |    FL3    |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

   where each of the Feature Lists is of the following format:

          Feature     Feature           Feature
           Count      Code 1            Code m
       +-----+-----+-----+-----+-...-+-----+-----+
       |     m     | FC1       |     |    FCm    |
       +-----+-----+-----+-----+-...-+-----+-----+



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3.14. Retry Time

   The Retry Time option specifies the time at which a client should
   retry a REQUEST or RENEW message when using the Retry After feature.

   This option should only be sent by a MADCAP server in an ACK when
   responding to a REQUEST or RENEW message that includes the Retry
   After feature in the supported, requested, or required list. For more
   discussion of Retry After, see section 2.12.2.

   If the Retry Time option is present, the Current Time option MUST
   also be present.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP timestamp by adding decimal 2208988800.
   This time format will not wrap until the year 2106.

   The code for this option is 13 and the length is 4.

        Code        Len      Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    13     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.15. Minimum Lease Time

   This option is used in a client request (DISCOVER, REQUEST, or RENEW)
   to allow the client to specify a minimum lease time for the multicast
   address. If a server cannot meet this minimum lease time, it SHOULD
   ignore a DISCOVER or send a NAK in response to a RENEW or REQUEST
   (unless it's a multicast REQUEST with a Server Identifier that
   doesn't match the server's, which should be ignored altogether).

   The time is in units of seconds, and is specified as a 32-bit
   unsigned integer.

   The code for this option is 14, and its length is 4.

        Code        Len            Lease Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    14     |     4     |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+-----+-----+

3.16. Maximum Start Time

   The Maximum Start Time option specifies the latest starting time that
   the client is willing to accept for a multicast address lease.



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   A client may include this option in a DISCOVER, RENEW, or REQUEST
   message to specify that it does not want to receive a lease with a
   starting time later than the specified value. If a server cannot meet
   this maximum start time, it SHOULD ignore a DISCOVER or send a NAK in
   response to a RENEW or REQUEST (unless it's a multicast REQUEST with
   a Server Identifier that doesn't match the server's, which should be
   ignored altogether).

   If the Maximum Start Time option is present, the Current Time option
   MUST also be present, as described in section 2.11.

   The time value is an unsigned 32 bit integer in network byte order
   giving the number of seconds since 00:00 UTC, 1st January 1970. This
   can be converted to an NTP timestamp by adding decimal 2208988800.
   This time format will not wrap until the year 2106.

   The code for this option is 15 and the length is 4.

        Code        Len      Time
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |    15     |     4     | t1  | t2  | t3  | t4  |
   +-----+-----+-----+-----+-----+-----+-----+-----+

4. Security Considerations

   MADCAP has relatively basic security requirements. At present there
   is no way of enforcing authorized use of multicast addresses in the
   multicast routing/management protocols.  Therefore, it is not
   possible to identify unauthorized use of multicast address by an
   adversary. Moreover, a multicast address allocated to a user/system
   can be used by other systems without violating terms of the multicast
   address allocation. For example, a system may reserve an address to
   be used for a work group session where each and every member of the
   work group is allowed to transmit packets using the allocated group
   address. In other words, the multicast address allocation protocol
   does not dictate how the address should be used, it only dictates the
   time period for which it can be used and who gets to release it or
   renew it. When an address is allocated to a system/user, it basically
   means that no other user/system (most likely) will be allocated that
   address for the time period, without any restrictions on its use.

   To protect against rogue MADCAP servers (mis-configured servers and
   intentional), clients in certain situations would like to
   authenticate the server. Similarly, for auditing or book-keeping
   purposes, the server may want to authenticate clients. Moreover, in
   some cases, the server may have certain policies in place to restrict
   number of addresses that are allocated to a system or a user. This
   feature is of much value when a well behaved but naive user or client



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   requests a large number of addresses, and therefore, inadvertently
   impacts other users or systems. Therefore, an administrator may want
   to exert a limited amount of control based on the client
   identification.  The client identification could be based on the
   system or user identity. In most practical situations, system
   identification will suffice, however, particularly in case of
   multi-user systems, at times, user identification will play an
   important role. Therefore, authentication capabilities based on user
   identification may be desirable. As usual, data integrity is a strong
   requirement and if not protected, can lead to many problems including
   denial of service attacks.

   In the case of MADCAP, confidentiality is not a strong requirement.
   In most of the cases, at least when a multicast address is in use, an
   adversary will be able to determine information that was contained in
   the MADCAP messages. In some cases, the users/systems may want to
   protect information in the MADCAP messages so that an adversary is
   not able to determine relevant information in advance and thus, plan
   an attack in advance. For example, if an adversary knows in advance
   (based on MADCAP messages) that a particular user has requested large
   number of address for certain time period and scope, he may be able
   to guess the purpose behind such request and target an attack. If
   MADCAP servers are configured to allow renewal or release purely on
   the basis of knowledge of the Client Identifier option (without
   consideration of authentication), preserving the confidentiality of
   MADCAP messages becomes more important. Also, there may be features
   added to the protocol in future that may have stronger
   confidentiality requirements.

   The IPSEC protocol [8] meets client/server identification and
   integrity protection requirements stated above, requires no
   modification to the MADCAP protocol, and leverages extensive work in
   IETF and industry. Therefore, when security is a strong requirement,
   IPSEC SHOULD be used for protecting all the unicast messages of
   MADCAP protocol. When IPSEC based security is in use, all the
   multicast packets except INFORM MUST be dropped by the MADCAP server.
   The prevalent implementations of IPSEC support client identification
   in form of system identification and do not support user
   identification. However, when desired, IPSEC with appropriate API's
   may be required to support user identification.

5. IANA Considerations

   This document defines several number spaces (MADCAP options, MADCAP
   message types, MADCAP client identifier types, and MADCAP features).
   For all of these number spaces, certain values are defined in this
   specification. New values may only be defined by IETF Consensus, as
   described in [7]. Basically, this means that they are defined by RFCs



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   approved by the IESG.

6. Acknowledgments

   The authors would like to thank Rajeev Byrisetty, Steve Deering,
   Peter Ford, Mark Handley, Van Jacobson, David Oran, Thomas Pfenning,
   Dave Thaler, Ramesh Vyaghrapuri and the participants of the IETF for
   their assistance with this protocol.

   Much of this document is based on [1] and [2]. The authors of this
   document would like to express their gratitude to the authors of
   these previous works. Any errors in this document are solely the
   fault of the authors of this document.

7. References

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

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

   [3]  Meyer, D., "Administratively Scoped IP Multicast", RFC 2365,
        July 1998.

   [4]  Mills, D., "Network Time Protocol (Version 3) Specification,
        Implementation and Analysis", RFC 1305, March 1992.

   [5]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [6]  Alvestrand, H., "Tags for the Identification of Languages", RFC
        1766, March 1995.

   [7]  Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
        Considerations Section in RFCs", RFC 2434, October 1998.

   [8]  R. Atkinson, S. Kent, "Security Architecture for the Internet
        Protocol", RFC 2401, November 1998.

8. Authors' Addresses

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

      Phone: 503 264 2422



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      EMail: baiju.v.patel@intel.com

      Munil Shah
      Microsoft Corporation
      One Microsoft Way
      Redmond, WA 98052

      Phone: 425 703 3924
      Email: munils@microsoft.com

      Stephen R. Hanna
      Sun Microsystems, Inc.
      One Network Drive
      Burlington, MA 01803

      Phone: +1.781.442.0166
      Email: steve.hanna@sun.com

APPENDIX A: Examples

   This appendix includes several examples of typical MADCAP protocol
   exchanges.

1. Multicast Scope List Discovery

   In this example, a MADCAP client wants to determine the scope list in
   effect. The client is using IPv4, so it starts with by multicasting
   an INFORM packet to the MADCAP Server Multicast Address corresponding
   to IPv4 Local Scope. This packet includes the Client Identifier
   option, an Option Request List including the Multicast Scope List
   option code, and a Requested Language option containing the string
   "en", since the client is configured to prefer the English language.

   Two MADCAP servers respond by sending ACK messages. These ACK
   messages include the Client Identifier option and xid supplied by the
   client, the server's Server Identifier, and the Multicast Scope List
   with one name per scope (the one that most closely matches the
   language tag "en").

   The following figure illustrates this exchange.











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                   Server          Client          Server
                     v               v               v
                     |               |               |
                     |               |               |
                     | _____________/|\_____________ |
                     |/   INFORM     |    INFORM    \|
                     |               |               |
                     |               |               |
                     |\              |  ____________/|
                     | \_________    | /   ACK       |
                     |      ACK  \   |/              |
                     |            \  |               |
                     |               |               |
                     v               v               v

        Figure 2: Timeline diagram of messages exchanged
                  in Multicast Scope List Discovery example

2. Multicast Discovery and Address Allocation

   In this example, the MADCAP client wants to allocate a multicast
   address from the global scope for use during the next two hours.

   The client begins by multicasting a DISCOVER packet to the MADCAP
   Server Multicast Address associated with IPv4 Local Scope.  This
   packet includes the Lease Time, Client Identifier, and Multicast
   Scope options.

   Any servers that receive the DISCOVER packet and can satisfy this
   request temporarily reserve an address for the client and unicast an
   OFFER packet to the client. These packets contain the Lease Time,
   Server Identifier, Client Identifier, and Multicast Scope options.

   After a suitable delay, the client multicasts a REQUEST packet to the
   MADCAP Server Multicast Address. This packet contains all of the
   options included in the DISCOVER packet, but also includes the Server
   Identifier option, indicating which server it has selected for the
   request.

   The server whose Server Identifier matches the one specified by the
   client responds with an ACK packet containing the options included in
   the OFFER packet, as well as a List of Address Ranges option listing
   the address allocated. All the other servers that had sent OFFER
   packets stop reserving an address for the client and forget about the
   whole exchange.

   The client now has a two hour "lease" on the multicast address.




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   If the client had not received an ACK from the server, it would have
   retransmitted its REQUEST packet for a while. If it still received no
   response, it would start over with a new DISCOVER message.

   The following figure illustrates this exchange.

                   Server          Client          Server
               (not selected)                    (selected)
                     v               v               v
                     |               |               |
                     |Begin multicast address request|
                     |               |               |
                     | _____________/|\_____________ |
                     |/   DISCOVER   |   DISCOVER   \|
                     |               |               |
                 Reserves            |           Reserves
                 Address             |           Address
                     |               |               |
                     |\              |  ____________/|
                     | \_________    | /    OFFER    |
                     |     OFFER \   |/              |
                     |            \  |               |
                     |       Collects replies        |
                     |              \|               |
                     |     Selects Server            |
                     |               |               |
                     | _____________/|\_____________ |
                     |/   REQUEST    |    REQUEST   \|
                     |               |               |
                     |               |     Commits address
                     |               |               |
                     |               | _____________/|
                     |               |/    ACK       |
                     |               |               |
                     |     assignment complete       |
                     |               |               |
                     v               v               v

        Figure 3: Timeline diagram of messages exchanged
                  in Multicast Address Allocation example

3. Lease Extension

   This is a continuation of the previous example. The client has
   already allocated a multicast address from the global scope for use
   during the next two hours. Half way through this two hour period, it
   decides that it wants to extend its lease for another hour.




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   The client unicasts a RENEW packet to the server from which it
   allocated the address. This packet includes the Lease Time and Client
   Identifier options. The Client Identifier matches the one used for
   the original allocation. The time included in the Lease Time is two
   hours, since the client wants the lease to expire two hours from the
   current time.

   The server responds with an ACK packet indicating that the lease
   extension has been granted. This packet includes the Lease Time,
   Server Identifier, Client Identifier, Multicast Scope, and List of
   Address Ranges options.

   If the server did not want to grant the requested lease extension, it
   would have responded with a NAK packet with the Client Identifier
   option.

   The following figure illustrates this exchange.

                   Client          Server
                     v               v
                     |               |
                     |\_____________ |
                     |    RENEW     \|
                     |               |
                     |        Extends lease
                     |               |
                     | _____________/|
                     |/    ACK       |
                     |               |
                     |               |
                     v               v

        Figure 4: Timeline diagram of messages exchanged
                  in Lease Extension example

4. Address Release

   This is a continuation of the previous example. The client has
   already allocated a multicast address and extended its lease for
   another two hours. Half an hour later, the client finishes its use of
   the multicast address and wants to release it so it can be reused.

   The client unicasts a RELEASE packet to the server from which it
   allocated the address. This packet includes the Client Identifier
   option. The Client Identifier matches the one used for the original
   allocation. When the server receives this packet, it cancels the
   client's lease on the address and sends an ACK packet to the client
   indicating that the lease has been released. This packet includes the



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   Server Identifier and Client Identifier options.

   The following figure illustrates this exchange.

                   Client          Server
                     v               v
                     |               |
                     |\_____________ |
                     |    RELEASE   \|
                     |               |
                     |        Cancels lease
                     |               |
                     | _____________/|
                     |/    ACK       |
                     |               |
                     v               v

        Figure 5: Timeline diagram of messages exchanged
                  in Address Release example

5. Unicast Address Allocation

   This is a continuation of the previous example. At some later time,
   the client decides to allocate another multicast address. Since it
   has recently worked with a server, it decides to try sending a
   unicast REQUEST to that server. If this doesn't work, it can always
   try a multicast DISCOVER, as illustrated in example 2.

   The client unicasts a REQUEST packet to the server from which it
   wants to allocate the address. This packet includes the Lease Time,
   Client Identifier, and Multicast Scope options.

   The server responds with an ACK packet containing the Lease Time,
   Client Identifier, and Multicast Scope options from the REQUEST
   packet, as well as the Server Identifier option and a List of Address
   Ranges option listing the address allocated.

   The client now has a lease on the multicast address.

   If the client had not received an ACK from the server, it would have
   retransmitted its REQUEST packet for a while. If it still received no
   response, it would start over with a multicast DISCOVER message.

   The following figure illustrates this exchange.







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                   Client          Server
                     v               v
                     |               |
                     |\_____________ |
                     |    REQUEST   \|
                     |               |
                     |        Allocates address
                     |               |
                     | _____________/|
                     |/    ACK       |
                     |               |
                     v               v

        Figure 6: Timeline diagram of messages exchanged
                  in Unicast Address Allocation example




































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