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Network Working Group                                          S. HannaRequests for Comments: 2730                      Sun Microsystems, Inc.Category: Standards Track                                      B. Patel                                                            Intel Corp.                                                                M. Shah                                                        Microsoft Corp.                                                          December 1999Multicast Address Dynamic Client Allocation Protocol (MADCAP)Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.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.1.1. 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 inRFC 2119 [9].   Constants used by this protocol are shown as [NAME-OF-CONSTANT], and   summarized inAppendix B.Hanna, et al.               Standards Track                     [Page 1]

RFC 2730                         MADCAP                    December 19991.2. Definitions   This specification uses a number of terms that may not be familiar to   the reader. This section defines some of these and refers to other   documents for definitions of others.   MADCAP client or client     A host requesting multicast address allocation services via MADCAP.   MADCAP server or server     A host providing multicast address allocation services via MADCAP.   Multicast     IP Multicast, as defined in [11] and modified in [12].   Multicast Address     An IP multicast address or group address, as defined in [11] and     [13].  An identifier for a group of nodes.   Multicast Scope     A range of multicast addresses configured so that traffic sent to     these addresses is limited to some subset of the internetwork. See     [3] and [13].   Scope ID     The lowest numbered address in a multicast scope. This definition     applies only within this document.   Scope Zone     One multicast scope may have several instances, which are known as     Scope Zones or zones, for short.     For instance, an organization may have multiple sites. Each site     might have its own site-local Scope Zone, each of which would be an     instance of the site-local Scope. However, a given interface on a     given host would only ever be in at most one instance of a given     scope.  Messages sent by a host in a site-local Scope Zones to an     address in the site-local Scope would be limited to the site-local     Scope Zone containing the host.   Zone Name     A human readable name for a Scope Zone. An ISO 10646 character     string with anRFC 1766 [6] language tag. One zone may have several     zone names, each in a different language. For instance, a zone for     use within IBM's locations in Switzerland might have the names "IBM     Suisse", "IBM Switzerland", "IBM Schweiz", and "IBM Svizzera" with     language tags "fr", "en", "de", and "it".Hanna, et al.               Standards Track                     [Page 2]

RFC 2730                         MADCAP                    December 1999   Multicast Scope List     A list of multicast scope zones.     Since it can be difficult to determine which multicast scope zones     are in effect, MADCAP clients can ask MADCAP servers to supply a     Multicast Scope List listing all of the zones available to the     client. For each scope zone, the list includes the range of     multicast addresses for this scope, a maximum TTL or hop count to     be used for this scope, and one or more zone names for this scope     zone.     This definition applies only within this document.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. This is also a key prerequisite   for establishing policies.Hanna, et al.               Standards Track                     [Page 3]

RFC 2730                         MADCAP                    December 1999   Policy neutral: Allocation policies (such as who can allocate   addresses) should not be dictated by the protocol.   Conferencing support: When allocating an address for use in a   conferencing environment, members of the conference should be able to   modify a multicast address lease used for the conference.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.1.5. Protocol Overview   MADCAP is built on a client-server model, where hosts request address   allocation services from address allocation servers. When a MADCAP   client wishes to request a service, it unicasts or multicasts a   message to one or more MADCAP servers, each of which optionally   responds with a message unicast to the client.   All messages are UDP datagrams. The UDP data contains a fixed length   header and a variable length options field. Options are encoded in a   type-length-value format with two octets type and value fields.  The   fixed fields are version, msgtype (message type), addrfamily (address   family), and xid (transaction identifier).   Retransmission is handled by the client. If a client sends a message   and does not receive a response, it may retransmit its request a few   times using an exponential backoff. To avoid executing the same   client request twice when a retransmitted request is received,   servers cache responses for a short period of time and resend cached   responses upon receiving retransmitted requests.   Each request contains a msgtype, an xid, and a Lease Identifier   option.  Clients must ensure that this triple is probably unique   across all MADCAP messages received by a MADCAP server over a period   of [XID-REUSE-INTERVAL] (10 minutes). This allows the MADCAP server   to use this triple as the key in its response cache.   Messages sent by servers include the xid included in the original   request so that clients can match up responses with requests.   The msgtype field is a single octet that defines the "type" of a   MADCAP message. Currently defined message types are listed in Table   2. They are: DISCOVER, OFFER, REQUEST, RENEW, ACK, NAK, RELEASE, andHanna, et al.               Standards Track                     [Page 4]

RFC 2730                         MADCAP                    December 1999   GETINFO.  DISCOVER, REQUEST, RENEW, RELEASE, and GETINFO messages are   only sent by a client. OFFER, ACK, and NAK messages are only sent by   a server.   The REQUEST, RENEW, and RELEASE messages are used to request, renew,   or release a lease on one or more multicast addresses. A client   unicasts one of these messages to a server and the server responds   with an ACK or a NAK.   The GETINFO message is used to request information, such as the   multicast scope list, or to find MADCAP servers. A client may unicast   an GETINFO message to a MADCAP server. However, it may not know the   IP address of any MADCAP server. In that case, it will multicast an   GETINFO message to a MADCAP Server Multicast Address and all servers   that wish to respond will send a unicast ACK or NAK back to the   client.   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. MADCAP clients use this address to find MADCAP servers.   The DISCOVER message is a message used to discover MADCAP servers   that can probably satisfy a REQUEST. DISCOVER messages are always   multicast.  Servers that can probably satisfy a REQUEST corresponding   to the parameters supplied in the DISCOVER message temporarily   reserve the addresses needed and send a unicast OFFER back to the   client. The client selects a server with which to continue and sends   a multicast REQUEST including the server's Server Identifier to the   same multicast address used for the DISCOVER. The chosen server   responds with an ACK or NAK and the other servers stop reserving the   addresses they were temporarily holding.   For detailed descriptions of typical protocol exchanges, consultAppendix A.   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.   MADCAP requests from a single host may be sent on behalf of different   applications with different needs and requirements. MADCAP servers   MUST NOT assume that because one request from a MADCAP client   supports a particular optional feature (like Retry After), future   requests from that client will also support that optional feature.Hanna, et al.               Standards Track                     [Page 5]

RFC 2730                         MADCAP                    December 19992. 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.   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.   The next few sections describe the MADCAP message format and message   types. A full list of MADCAP options is provided insection 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.Hanna, et al.               Standards Track                     [Page 6]

RFC 2730                         MADCAP                    December 1999                +-+-+-+-+-+-+-+-+                |  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, GETINFO, etc.)  addrfamily    2  Address family (IPv4, IPv6, etc.)  xid           4  Transaction ID  options     var  Options field          Table 1:  Description of fields in a MADCAP message2.1.1. 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, seesection 2.2.Hanna, et al.               Standards Track                     [Page 7]

RFC 2730                         MADCAP                    December 19992.1.3. The addrfamily field   The addrfamily field defines the default address family (such as IPv4   or IPv6) for this MADCAP message, using the address family numbers   defined in by the IANA (including those defined in [10]). Unless   otherwise specified, all addresses included in the message will be   from this family.2.1.4. The xid field   The xid field is a transaction identifier. This number MUST be chosen   by the client so that the combination of xid, msgtype, and Lease   Identifier is unique across all MADCAP messages received by a MADCAP   server over a period of [XID-REUSE-INTERVAL] (10 minutes).   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. The technique used to   choose an xid is implementation-dependent, but whatever technique is   used MUST make it unlikely that the same combination of xid, msgtype,   and Lease Identifier will be used for two different messages within   [XID-REUSE-INTERVAL] (even across multiple clients which do not   communicate among themselves).  This allows enough time for the   message to be dropped from all server response caches (as described   in the next few paragraphs) and for any network delays to be   accomodated.   The RECOMMENDED technique for choosing an xid is to choose a random   four octet number as the first xid in a session and increment this   value each time a new xid is needed.  The random number chosen need   not be cryptographically random. The random number may be chosen via   any suitable technique, such as the one described in section A.6 ofRFC 1889 [14].   When a 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 insection 2.3), the   client MUST retransmit them without changing them, thereby using the   same xid and and Lease Identifier.   If a server receives a message with the same xid, msgtype, and Lease   Identifier as one received within [RESPONSE-CACHE-INTERVAL], it MUST   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 treatHanna, et al.               Standards Track                     [Page 8]

RFC 2730                         MADCAP                    December 1999   any retransmissions of this message as if they were new messages. Of   course, a server need not cache a message if it ends up ignoring that   message. However, performance gains may be achieved by doing so.   This avoids retransmissions causing multiple allocations, since   requests are not idempotent.  An appropriate value for [RESPONSE-   CACHE-INTERVAL] would be sixty seconds, but it may have any value   from zero seconds to 300 seconds (five minutes) and may be adjusted   dynamically according to resource constraints on the server.   However, using a value less than sixty seconds is NOT RECOMMENDED   because this is the normal client retransmission period.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 a sequence of options with the last one   being the End option (option code 0). Any message whose 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 insection 3, subject to the restrictions in   Table 5 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 and the rest of the message processed as if   the unknown options were not present. If a MADCAP server receives a   message that does not conform to the requirements of this document   (for instance, not including all required options), an Invalid   Request error MUST be generated and processed in the manner described   insection 2.6. If a MADCAP client receives a message that does not   conform to the requirements of this document, it MUST ignore the   message.   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.Hanna, et al.               Standards Track                     [Page 9]

RFC 2730                         MADCAP                    December 1999   New MADCAP option codes may only be defined by IETF Consensus, as   described insection 5.2.2. 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     GETINFO      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 8   will be referred to as an GETINFO message.   Here are descriptions of the MADCAP message types.  Table 5, which   appears at the beginning ofsection 3, summarizes which options are   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 MADCAP server receives a message whose message type it does not   recognize, an Invalid Request error MUST be generated and processed   in the manner described insection 2.6. If a MADCAP client receives a   message whose message type it does not recognize, it MUST ignore the   message.   Note, however, that under some circumstances this document requires   or suggests that clients or servers ignore messages with certain   message types even though they may be recognized. 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 messages that they cannot satisfy.   New MADCAP message types may only be defined by IETF Consensus, as   described insection 5.Hanna, et al.               Standards Track                    [Page 10]

RFC 2730                         MADCAP                    December 19992.2.1. GETINFO   The GETINFO 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 GETINFO 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, seesection 2.10.   If a server receives an GETINFO message and it can process the   request successfully, it MUST unicast an ACK message to the client.   All GETINFO messages MUST include an Option Request List option. The   server SHOULD try to include the specified options in its response,   but is not required to do so (especially if it does not recognize   them).   If a server receives an GETINFO message and it does not process the   request successfully, it MUST generate and process an error in the   manner described insection 2.6.   If a client sends an GETINFO message and does not receive any ACK   messages in response, it SHOULD resend its GETINFO message, as   described insection 2.3.   When a MADCAP client sends an GETINFO 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.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 GETINFO message, caching an IP address that worked in the   past or being configured with an IP address. Using the DISCOVER   message has the particular advantage that it allows clients to   receive responses from all servers that can satisfy the request.Hanna, et al.               Standards Track                    [Page 11]

RFC 2730                         MADCAP                    December 1999   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 only 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, seesection 2.10.   If a client sends a DISCOVER message and does not receive any OFFER   messages in response, the client SHOULD retransmit its DISCOVER   message, as described insection 2.3.   If a client sends a DISCOVER message and receives one or more OFFER   messages in response, it SHOULD select the server it wants to use (if   any) and send a multicast REQUEST message identifying that server   within [DISCOVER-DELAY] after receiving the first OFFER message.  Seesection 2.2.4 for more information about the REQUEST message.   The mechanism used by the client in selecting the server it wants to   use is implementation dependent.  The client MAY choose the first   acceptable response or it MAY wait some period of time (no more than   [DISCOVER-DELAY]) and choose the best response received in that   period of time (if the first response has a smaller lease time than   requested, for instance).   The value of [DISCOVER-DELAY] is also implementation dependent, but   the RECOMMENDED value is the current retransmit timer, as specified   insection 2.3. Waiting too long (approaching [OFFER-HOLD]) may cause   servers to drop the addresses they have reserved.   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. The Lease Identifier optionHanna, et al.               Standards Track                    [Page 12]

RFC 2730                         MADCAP                    December 1999   MUST always be included.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 Lease 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,   but a value of at least 60 seconds is RECOMMENDED.   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 Lease 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.Hanna, et al.               Standards Track                    [Page 13]

RFC 2730                         MADCAP                    December 19992.2.4. 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 client MUST   multicast a REQUEST message to the same MADCAP Server Multicast   Address that the DISCOVER message was sent to. The same Lease   Identifier 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.   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   generate and process an error in the manner described insection 2.6.   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, the client SHOULD resend its REQUEST   message, as described insection 2.3.Hanna, et al.               Standards Track                    [Page 14]

RFC 2730                         MADCAP                    December 1999   If the server responds with a NAK or fails to respond within a   reasonable (implementation-dependent) delay [NO-RESPONSE-DELAY], the   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. The RECOMMENDED value for [NO-RESPONSE-DELAY] is 60   seconds.2.2.5. ACK   The ACK message is used by a MADCAP server to respond affirmatively   to an GETINFO, 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 GETINFO message, it SHOULD   include any options requested by the client using the Option Request   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 Lease 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 Lease 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   message. The server unicasts the NAK message to the client from which   it received the message to which it is responding.Hanna, et al.               Standards Track                    [Page 15]

RFC 2730                         MADCAP                    December 1999   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 Lease 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. The Error option MUST be included with an error code   indicating what went wrong. And the packet MUST NOT be retransmitted.2.2.7. 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 MUST generate and process an   error in the manner described insection 2.6.   The lease to be renewed is whichever one was allocated with a Lease   Identifier option matching the one provided in the RENEW message.   When a MADCAP client sends a RENEW message, it MAY include the Lease   Time, Minimum Lease Time, Start Time, and Maximum Start Time options,   describing the new lease 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, and no maximum for Maximum Start Time).   The Current Time option MUST be included if the Start Time or Maximum   Start Time options are included.   If a client sends a RENEW message and does not receive any ACK or NAK   messages in response, the client SHOULD resend its RENEW message, as   described insection 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, seesection 2.13.1. The RECOMMENDED value   for [NO-RESPONSE-DELAY] is 60 seconds.Hanna, et al.               Standards Track                    [Page 16]

RFC 2730                         MADCAP                    December 19992.2.8. 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 MUST unicast an ACK message to the   client. Otherwise, it MUST generate and process an error in the   manner described insection 2.6.   The lease to be released is whichever one was allocated with a Lease   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, the client SHOULD resend its RELEASE   message, as described insection 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, seesection 2.13.1. The RECOMMENDED   value for [NO-RESPONSE-DELAY] is 60 seconds.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   and the server.   The RECOMMENDED algorithm is to use a 4 second delay before the first   retransmission and to double this delay for each successive   retransmission, with a maximum delay of 16 seconds and a maximum of   three retransmissions. If an initial transmission was sent at time   (in seconds) t and no responses were received, subsequent   transmissions would be at t+4, t+12, and t+28. If no response has   been received by t+60, the client would stop retransmitting and take   another course of action (such as logging an error or sending aHanna, et al.               Standards Track                    [Page 17]

RFC 2730                         MADCAP                    December 1999   message to another address.   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 Lease Identifier   The Lease Identifier option is included in each MADCAP message.  Its   value is used to identify a lease and MUST be unique across all   leases requested by all clients in a multicast address allocation   domain.   The first octet of the Lease Identifier is the Lease Identifier type.   Table 3 lists the Lease Identifier types defined at this time and   sections2.4.1 and2.4.2 describe these Lease Identifier types.   New MADCAP Lease Identifier types may only be defined by IETF   Consensus, as described insection 5.           Lease Identifier Type   Name           ---------------------   ----                     0             Random Lease Identifier                     1             Address-Specific Lease Identifier      Table 3:  MADCAP Lease Identifier Types   The MADCAP server does not need to parse the Lease Identifier. It   SHOULD use the Lease Identifier only as an opaque identifier, which   must be unique for each lease. The purpose of defining different   Lease Identifier types is to allow MADCAP clients that already have a   globally unique address to avoid the possibility of Lease Identifier   collisions by using this address together with a client-specific   identifier. MADCAP clients that do not have a globally unique address   SHOULD use Lease Identifier type 0.   In addition to associating client and server messages (along with the   msgtype and xid fields, as described in the next section), the Lease   Identifier is used to determine which lease a RENEW or RELEASE   request refers to. MADCAP servers SHOULD match the Lease Identifier   included in a RENEW or RELEASE message with the Lease Identifier used   in an initial REQUEST message. If the Lease Identifier does not   match, a MADCAP server MUST generate and process a Lease Identifier   Not Recognized error in the manner described insection 2.6.Hanna, et al.               Standards Track                    [Page 18]

RFC 2730                         MADCAP                    December 1999   For conferencing applications, it may be desirable to allow   conference participants to modify a lease used for the conference.   The Shared Lease Identifier feature code is used to support this   requirement.  If this feature code was requested by the client and   implemented by the server when the lease was allocated, the server   SHOULD disable any authentication requirements and allow any client   that knows the Lease Identifier to modify the lease.   As described in the Security Considerations section, MADCAP security   is not terribly useful without admission control in the multicast   routing infrastructure. However, if MADCAP security is desired when   using the Shared Lease Identifier feature, the confidentiality of the   Lease Identifier MUST be maintained by encrypting all messages that   contain it. A Lease Identifier that includes a long cryptographically   random number (at least eight octets in length) MUST be used in this   circumstance so that it is not easy to guess the Lease Identifier.2.4.1. Random Lease Identifier   The first octet of a Random Lease Identifier is the Lease Identifier   type (0 to indicate Random Lease Identifier). After this come a   sequence of octets, which SHOULD represent a long random number (at   least 16 octets) from a decent random number generator.   A Random Lease Identifier does not include any indication of its   length.  It is assumed that this may be determined by external means,   such as a length field preceding the Lease Identifier.    Lease ID      Type    Random Number   +---------+-------------...   |    0    |   +---------+-------------...2.4.2. Address-Specific Lease Identifier   The first octet of an Address-Specific Lease Identifier is the Lease   Identifier type (1 to indicate Address-Specific Lease Identifier).   After this comes a two octet IANA-defined address family number   (including those defined in [10]), an address from the specified   address family, and a client-specific identifier (such as a sequence   number or the current time).   An Address-Specific Lease Identifier does not include any indication   of its length. It is assumed that this may be determined by external   means, such as a length field preceding the Lease Identifier.Hanna, et al.               Standards Track                    [Page 19]

RFC 2730                         MADCAP                    December 1999    Lease ID     Address Family      Address    Client-specific      Type           Number                       Identifier   +---------+---------+---------+-----...-----+-----...-----+   |    1    |     addrfamily    |   address   | cli-spec id |   +---------+---------+---------+-----...-----+-----...-----+2.5. Associating Client and Server Messages   Messages between clients and servers are associated with one another   using the Lease Identifier and the xid field. As described insection2.1.4, the client MUST choose an xid so that it is unlikely that the   same combination of xid, msgtype, and Lease Identifier will be used   for two different messages within [XID-REUSE-INTERVAL] (even across   multiple clients which do not communicate among themselves).  The   Lease Identifier option, msgtype, and xid field MUST be included in   each message sent by the client or the server.   The client MUST check the Lease Identifier option and xid field in   each incoming message to ensure that they match the Lease Identifier   and xid for an outstanding transaction. If not, the message MUST be   ignored. The server MUST check the Lease Identifier option and xid   field in each incoming message to establish the proper context for   the message. If a server cannot process a message because it is   invalid for its context, the server MUST generate and process an   Invalid Request error, as described insection 2.6.  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 GETINFO, ACK, and NAK   messages).2.6. Processing Errors   If a MADCAP server encounters an error while processing a message,   there are two different ways to process this error. If it is clear   that the message is not a NAK, the server SHOULD respond with a NAK   containing the appropriate Error option. However, the server MAY   decide to completely ignore chronic offenders. If the message is a   NAK or it is not clear whether the message is a NAK (for instance,   the message is garbled or has an incorrect version number), the   server SHOULD ignore the message. This avoids NAK loops.   If a MADCAP client encounters an error while processing a message, it   MUST ignore the message.Hanna, et al.               Standards Track                    [Page 20]

RFC 2730                         MADCAP                    December 19992.7. Multicast ScopesRFC 2365 [3] provides for dividing the multicast address space into a   number of administrative 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 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 hop count 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.2.8. Multicast TTL   Another way to limit propagation of multicast messages is by using   TTL scoping. This technique has several disadvantages in comparison   to administratively scoped multicast addresses (as described in [3]),   but it is currently in widespread usage.   With TTL scoping, areas of the network are designated as scopes.   Routers on the edges of these areas are configured with TTL   thresholds so that multicast packets are not forwarded unless theirHanna, et al.               Standards Track                    [Page 21]

RFC 2730                         MADCAP                    December 1999   remaining TTL exceeds this threshold. A packet which should be   restricted to a given TTL scope should have an initial TTL less than   that scope's TTL threshold. Similar techniques may be used with IPv6,   using the Hop Count field instead of the TTL field.   MADCAP may be used in an environment where administrative scoping is   not in use and TTL scoping is. Under these circumstances, a MADCAP   server MAY return a scope list that includes 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 such an environment, it is recommended that the MADCAP   Server Multicast Addresses associated with the IPv4 Local Scope (or   SCOP 3 for IPv6) be configured using TTL thresholds so that packets   sent to these addresses 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.2.9. Locating MADCAP Servers   There are several ways for a MADCAP client to locate a MADCAP server.   For instance, the client may be configured with an IP address.   The RECOMMENDED technique is for the client to send an GETINFO   message to a MADCAP Server Multicast Address and wait for ACK   responses. This technique is described in more detail in the next   section.2.10. 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 GETINFO 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.Hanna, et al.               Standards Track                    [Page 22]

RFC 2730                         MADCAP                    December 1999   The MADCAP Server Multicast Address used by a client MAY be   established by configuration. 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.2.11. 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   MADCAP server is not prepared to provide services for scopes in which   it does not reside, it SHOULD ignore DISCOVER and REQUEST messages   whose scope does not match or enclose the scope of the MADCAP Server   Multicast Address on which the request was received. It SHOULD also   ignore GETINFO messages that are not received on the MADCAP Server   Multicast Address for IPv4 Local Scope.2.12. 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,   RENEW, or ACK message.   Clock skew is a situation where two systems have clocks that are not   synchronized. Many protocols (such as DHCP) ignore clock skew by   using relative times. MADCAP could use a similar technique, but this   leads to nasty situations due to the way multicast addresses are   used.Hanna, et al.               Standards Track                    [Page 23]

RFC 2730                         MADCAP                    December 1999   For example, assume that at 1 PM UTC a client whose clock is one hour   fast requests a lease for one hour starting in one hour. If we were   using relative times for MADCAP, the server, whose clock is set   correctly, would reserve a multicast address for 2 to 3 PM UTC and   grant the request. If the client was the only one using the lease,   everything would be OK. The client would start using the lease in one   hour and continue for one hour. This would coincide with the time the   server had reserved (although the client would think it was 3 to 4 PM   UTC).   However, multicast addresses are usually used by several parties at   once.  The client would probably use SAP (or some other mechanism for   conveying SDP) to advertise a session using the multicast address   just leased. SDP uses absolute times, since it may be sent via email,   web, or other store-and-forward mechanisms. So the client would   advertise the session as running from 3 to 4 PM UTC. Any clients   whose clocks are set correctly would use the address during this   interval. Since the server only reserved the address from 2 to 3 PM   UTC, this might cause the address to be used for multiple sessions   simultaneously.   MADCAP cannot solve all clock skew problems. That is the domain of   NTP [4].  However, it does attempt to detect substantial clock skew   between MADCAP clients and servers so that this clock skew does not   cause massive collisions in multicast address usage later on.   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. However, difference greater than   a minute or two probably indicate clock skew between the sender and   the receiver.   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 [EXTRA-ALLOCATION-TIME] on   either side of the lease given to the client. However, large amounts   of clock skew require special handling. The value of [EXTRA-   ALLOCATION-TIME] MUST be a configurable parameter, since local   circumstances may vary.  The RECOMMENDED default is one hour.   However, large amounts of clock skew will cause problems later when   sessions are advertised. If a MADCAP server detects clock skew   greater than [CLOCK-SKEW-ALLOWANCE], it MUST generate and process an   Excessive Clock Skew error, as described insection 2.6. The server   MAY also log a message. The value of [CLOCK-SKEW-ALLOWANCE] MUST be a   configurable parameter, since local circumstances may vary.  TheHanna, et al.               Standards Track                    [Page 24]

RFC 2730                         MADCAP                    December 1999   RECOMMENDED default is 30 minutes.2.13. 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. For   more information about optional features, see the description of the   Feature List option.   Table 4 lists the feature codes defined at this time and sections   2.13.1 and 2.13.2 describe how these features work.   New MADCAP feature codes may only be defined by IETF Consensus, as   described insection 5.           Feature Code   Feature Name           ------------   ------------                0         Server Mobility                1         Retry After                2         Shared Lease Identifier      Table 4:  MADCAP Feature Codes2.13.1. 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 SHOULD 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 can   find such a server by (for instance) sending an GETINFO message withHanna, et al.               Standards Track                    [Page 25]

RFC 2730                         MADCAP                    December 1999   an Option Request List option that includes the Feature List option   code.   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 required 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.2.13.2. 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.   For instance, if a MADCAP client requests 1000 addresses,   administrative approval may be required or allocation of more   addresses from another MASC domain may be necessary. This may take   several hours or several days.  If the MADCAP client and server both   support the Retry After feature, the MADCAP server can send back an   ACK message with a Retry Time option indicating when the addresses   may be ready. The client can retry its request after the Retry Time   to get the addresses.   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 supports the Retry After feature MAY decide to begin a lengthy   allocation process. 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 required   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.Hanna, et al.               Standards Track                    [Page 26]

RFC 2730                         MADCAP                    December 1999   At some later time (preferably after the time indicated in the Retry   Time option), the client SHOULD send a REQUEST message with all the   same options as the original REQUEST message (especially the Lease   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 required 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 Lease 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 Lease Identifier that   matches a lease which is currently undergoing allocation with the   Retry After feature in response to a REQUEST message, the server MUST   generate and process an Invalid Request error in the manner described   insection 2.6.  Also, if a client sends a RENEW message with a Lease   Identifier that matches a lease which is currently undergoing   allocation with the Retry After feature in response to a RENEW   message, but the options supplied with the two RENEW messages do not   match, the server MUST generate and process an Invalid Request error   in the manner described insection 2.6.   Note that the Retry After feature may complicate the application API.   For this reason, a MADCAP client may request the Retry After feature   for some messages and not for others. This should not cause problems   for a robust MADCAP server. In general, servers should not expect   consistent behavior from clients except as required by this   specification. This also applies to clients' expectations.2.13.3. Shared Lease Identifier   For conferencing applications, it may be desirable to allow   conference participants to modify a lease used for the conference.   The Shared Lease Identifier feature code is used to support this   requirement.Hanna, et al.               Standards Track                    [Page 27]

RFC 2730                         MADCAP                    December 1999   If this feature code was requested by the client and implemented by   the server when the lease was allocated, the server SHOULD disable   any authentication requirements pertaining to this lease, allowing   any client that knows the Lease Identifier to modify the lease.   A MADCAP client wishing to use the Shared Lease Identifier feature   should include this feature in the requested or required lists of the   Feature List option of a REQUEST message when first allocating the   lease. If the feature was required, the server SHOULD try to   implement it for this request and include the feature in the required   list of the response. If the server can not implement the feature for   this request, it MUST generate and process a Required Feature Not   Supported error in the manner described insection 2.6. If the   feature was requested, the server SHOULD try to implement the feature   and include the feature in the required list of the response.   However, if the server cannot implement the feature, it may simply   skip it.   Subsequent requests pertaining to a lease for which the Shared Lease   Identifier feature was implemented at allocation time MAY include the   Shared Lease Identifier feature in the requested or required lists of   the Feature List option. In this case, the server SHOULD try to   implement the feature by disabling any authentication requirements   pertaining to this lease, allowing any client that knows the Lease   Identifier to modify the lease, and including the feature in the   required list of the response.  If the server cannot implement the   feature, it SHOULD skip it if the feature was requested. But if the   feature was required and the server cannot implement it, the server   MUST generate and process a Required Feature Not Supported error in   the manner described insection 2.6.3. MADCAP Options   As described earlier, each MADCAP message includes an options field   consisting 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.   This section defines a set of option codes for use in MADCAP   messages.  New options may be defined using the process defined insection 5. The options are listed in numerical order.Hanna, et al.               Standards Track                    [Page 28]

RFC 2730                         MADCAP                    December 1999   Table 5 summarizes which options are allowed with each message type.   Option                  GETINFO        ACK (in response to GETINFO)   ------                  ------         ---------------------------   Lease Time              MUST NOT       MUST NOT   Server Identifier       MUST NOT       MUST   Lease Identifier        MUST           MUST   Multicast Scope         MUST NOT       MUST NOT   Option Request List     MUST           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   Error                   MUST NOT       MUST NOT   Option                  DISCOVER       OFFER   ------                  --------       -----   Lease Time              MAY            MUST   Server Identifier       MUST NOT       MUST   Lease 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            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   Error                   MUST NOT       MUST NOTHanna, et al.               Standards Track                    [Page 29]

RFC 2730                         MADCAP                    December 1999   Option                  REQUEST        ACK (in response to REQUEST)   ------                  -------        ----------------------------   Lease Time              MAY            MUST   Server Identifier       MUST (if       MUST                             multicast)   Lease 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   Error                   MUST NOT       MUST NOT   Option                  RENEW          ACK (in response to RENEW)   ------                  -----          --------------------------   Lease Time              MAY            MUST   Server Identifier       MUST NOT       MUST   Lease 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   Error                   MUST NOT       MUST NOTHanna, et al.               Standards Track                    [Page 30]

RFC 2730                         MADCAP                    December 1999   Option                  RELEASE        ACK (in response to RELEASE)   ------                  -------        ----------------------------   Lease Time              MUST NOT       MUST NOT   Server Identifier       MUST NOT       MUST   Lease 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       MUST NOT   Feature List            MAY            MAY   Retry Time              MUST NOT       MUST NOT   Minimum Lease Time      MUST NOT       MUST NOT   Maximum Start Time      MUST NOT       MUST NOT   Error                   MUST NOT       MUST NOT   Option                  NAK   ------                  ---   Lease Time              MUST NOT   Server Identifier       MUST   Lease 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   Error                   MUST             Table 5:  Options allowed in MADCAP messages3.1. 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.Hanna, et al.               Standards Track                    [Page 31]

RFC 2730                         MADCAP                    December 1999   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 number (as defined by IANA, including those defined in   [10]) is stored first, followed by the address.  The address family   for this address is not determined by the addrfamily field in the   fixed header so that addresses from one family may be allocated while   communicating with a server via addresses of another family.   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 |  ...            |   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+Hanna, et al.               Standards Track                    [Page 32]

RFC 2730                         MADCAP                    December 19993.4. Lease Identifier   This option is used by MADCAP clients to specify a unique lease   identifier. For more information about this option and how it is   used, seesection 2.4.   The code for this option is 3, and its minimum length is 1.           Code        Len     Lease 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 in the fixed header.   The code for this option is 4, and its minimum length is 1.        Code        Len        Scope ID   +-----+-----+-----+-----+-----+-----   |     4     |     n     |  i1 |  ...   +-----+-----+-----+-----+-----+-----3.6. Option Request List   This option is used by a MADCAP client in an GETINFO 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  |   +-----+-----+-----+-----+-----+-----+---...Hanna, et al.               Standards Track                    [Page 33]

RFC 2730                         MADCAP                    December 19993.7. 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 insection 2.12.   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  |   +-----+-----+-----+-----+-----+-----+-----+-----+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 MADCAP server MUST generate and process an Invalid   Request error in the manner described insection 2.6.   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 serverHanna, et al.               Standards Track                    [Page 34]

RFC 2730                         MADCAP                    December 1999   receiving a DISCOVER or REQUEST packet including this option MUST   include between the 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 GETINFO message sent by the client. It is anRFC 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   GETINFO message sent by the client.   If the client did not include a Requested Language option in its   GETINFO message, the MADCAP server SHOULD return all zone names for   each zone. If the client included a Requested Language option in its   GETINFO 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.Hanna, et al.               Standards Track                    [Page 35]

RFC 2730                         MADCAP                    December 1999   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 in the fixed header.  Note that a   particular MADCAP server may be allocating addresses out of some   subset of the scope.  For instance, the addresses in the scope may be   divided among several servers in some way.   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   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.Hanna, et al.               Standards Track                    [Page 36]

RFC 2730                         MADCAP                    December 1999   The Scope IDs of entries in the list MUST be unique and the scopes   SHOULD be listed from smallest (topologically speaking) to largest.   This makes it easier to display a consistent user interface, with   scopes usually keeping the same order. However, scopes may not be   strictly nested. In this circumstance, there is no strict ordering   from smallest to largest and the server must use another technique   for ordering the scope list.   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 |       +------+--...--+Hanna, et al.               Standards Track                    [Page 37]

RFC 2730                         MADCAP                    December 19993.11. 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. This feature should be needed only rarely,   such as when a lease is accidentally allowed to expire and it needs   to be reallocated.   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 insection 2.12.   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  |   +-----+-----+-----+-----+-----+-----+-----+-----+Hanna, et al.               Standards Track                    [Page 38]

RFC 2730                         MADCAP                    December 19993.13. Feature List   This option lists optional MADCAP features supported, requested, or   required, by the sender. This option MAY be included in any message   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 insection 5.   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. The supported list is   used by MADCAP clients or servers to indicate the features that the   sender supports.  The requested and required lists are used by MADCAP   clients to indicate which features are requested of or required from   a MADCAP server.  The required list is used by MADCAP servers to   indicate which features were implemented by the MADCAP server in   processing this message. Messages sent by MADCAP servers MUST NOT   include any feature codes in the requested list.   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.  If a MADCAP server receives a message containing the   Feature List option and it does not support all of the features in   the required list, it MUST generate and process a Required Feature   Not Supported error in the manner described insection 2.6. If the   server 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 a MADCAP server does respond to a client's message that includes a   Feature List option, the server MUST include a Feature List option   with a supported features list that lists the features that it   supports, a required features list that lists the features that it   implemented in responding to this message (which must be included in   the supported features list of the client's Feature List option), and   an empty requested features list.Hanna, et al.               Standards Track                    [Page 39]

RFC 2730                         MADCAP                    December 1999   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    |       +-----+-----+-----+-----+-...-+-----+-----+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, seesection 2.13.2.   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 MUST   generate and process a Valid Request Could Not Be Completed error in   the manner described insection 2.6.Hanna, et al.               Standards Track                    [Page 40]

RFC 2730                         MADCAP                    December 1999   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.   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 MUST generate and process a Valid Request   Could Not Be Completed error in the manner described insection 2.6.   If the Maximum Start Time option is present, the Current Time option   MUST also be present, as described insection 2.12.   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  |   +-----+-----+-----+-----+-----+-----+-----+-----+3.17. Error   A MADCAP server includes this option in a NAK message to indicate why   the request failed. A MADCAP server MUST include an Error option in   each NAK message.   The first two octets of an Error option contain a MADCAP error code.   Several MADCAP error codes are defined later in this section.  New   MADCAP error codes may only be defined by IETF Consensus, as   described insection 5.Hanna, et al.               Standards Track                    [Page 41]

RFC 2730                         MADCAP                    December 1999   Any remaining octets in the Error option contain extra data about the   error. The format of this data depends on the error code. The   definition of a MADCAP error code must include a definition of the   extra data to be included with that error code.   A client that receives a NAK message containing an Error option MAY   log or display a message indicating the error code and extra data   received.  The client MUST NOT retransmit a message once a NAK   response to that message has been received. The client MAY adjust the   message to correct the error and send the corrected message or send a   message to a different server.   The code for this option is 16, and the minimum length is 2.        Code        Len      Error Code  Extra Data   +-----+-----+-----+-----+-----+-----+-----+-----+ ...   |    16     |     n     |   ecode   |  d1    d2   +-----+-----+-----+-----+-----+-----+-----+-----+ ...3.17.1. Valid Request Could Not Be Completed   MADCAP error code 0 indicates that the request was valid, but could   not be completed with the available addresses and the current   configuration.  The extra data is a two octet option code indicating   which option caused the problem. A value of 0xFFFF indicates that the   problem was not with a specific option.3.17.2. Invalid Request   MADCAP error code 1 indicates that the request was malformed or   invalid in some other manner. The extra data is a two octet option   code indicating which option caused the problem. A value of 0xFFFF   indicates that the problem was not with a specific option.3.17.3. Excessive Clock Skew   MADCAP error code 2 indicates excessive clock skew (seesection2.12).  The extra data consists of a four octet time value   representing the server's idea of the current time, 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.Hanna, et al.               Standards Track                    [Page 42]

RFC 2730                         MADCAP                    December 19993.17.4. Lease Identifier Not Recognized   MADCAP error code 3 indicates that the Lease Identifier was not   recognized (usually in response to a RENEW or RELEASE message). There   is no extra data.3.17.5. Required Feature Not Supported   MADCAP error code 4 indicates that at least one feature included in   the required list of the Feature List option is not supported. The   extra data contains a list of the feature codes in the required list   that are not supported.3.17.6. Experimental Use   MADCAP error codes 1024-2047 are reserved for experimental use. The   format of the extra data included with these error codes is not   defined.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   the 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 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 basedHanna, et al.               Standards Track                    [Page 43]

RFC 2730                         MADCAP                    December 1999   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 a   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.   When the Shared Lease Identifier feature is used, preserving the   confidentiality of MADCAP messages becomes more important. Also,   there may be features added to the protocol in the 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 GETINFO 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 Lease Identifier types, MADCAP features, and   MADCAP error codes).  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 approved by the IESG.Hanna, et al.               Standards Track                    [Page 44]

RFC 2730                         MADCAP                    December 19996. 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",BCP 23,RFC2365, 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",RFC2279, January 1998.   [6]  Alvestrand, H., "Tags for the Identification of Languages",RFC1766, March 1995.   [7]  Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA        Considerations Section in RFCs",BCP 26,RFC 2434, October 1998.   [8]  Atkinson, R. and S. Kent, "Security Architecture for the        Internet Protocol",RFC 2401, November 1998.   [9]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [10] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,RFC 1700,        October 1994.   [11] Deering, S., "Host Extensions for IP Multicasting", STD 5,RFC1112, August 1989.Hanna, et al.               Standards Track                    [Page 45]

RFC 2730                         MADCAP                    December 1999   [12] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)        Specification",RFC 2460, December 1998.   [13] Hinden, R. and S. Deering, "IP Version 6 Addressing        Architecture",RFC 2373, July 1998.   [14] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,        "RTP: A Transport Protocol for Real-Time Applications",RFC1889, January 1996.8. Authors' Addresses   Stephen R. Hanna   Sun Microsystems, Inc.   One Network Drive   Burlington, MA 01803   Phone: +1.781.442.0166   EMail: steve.hanna@sun.com   Baiju V. Patel   Intel Corp.   Mail Stop: AG2-201   5200 NE Elam Young Parkway   Hillsboro, OR 97124   Phone: 503 696 8192   EMail: baiju.v.patel@intel.com   Munil Shah   Microsoft Corporation   One Microsoft Way   Redmond, WA 98052   Phone: 425 703 3924   EMail: munils@microsoft.comHanna, et al.               Standards Track                    [Page 46]

RFC 2730                         MADCAP                    December 1999APPENDIX 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 by multicasting an   GETINFO packet to the MADCAP Server Multicast Address corresponding   to IPv4 Local Scope. This packet includes the Lease 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 Lease 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.                    Server          Client          Server                      v               v               v                      |               |               |                      |               |               |                      | _____________/|\_____________ |                      |/   GETINFO    |    GETINFO   \|                      |               |               |                      |               |               |                      |\              |  ____________/|                      | \_________    | /   ACK       |                      |      ACK  \   |/              |                      |            \  |               |                      |               |               |                      v               v               v         Figure 2: Timeline diagram of messages exchanged                   in Multicast Scope List Discovery example2. 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.Hanna, et al.               Standards Track                    [Page 47]

RFC 2730                         MADCAP                    December 1999   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, Lease 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, Lease Identifier, and Multicast Scope options.   After an appropriate 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.   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.Hanna, et al.               Standards Track                    [Page 48]

RFC 2730                         MADCAP                    December 1999   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 example3. 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.   The client unicasts a RENEW packet to the server from which it   allocated the address. This packet includes the Lease Time and Lease   Identifier options. The Lease Identifier matches the one used for the   original allocation. The time included in the Lease Time is twoHanna, et al.               Standards Track                    [Page 49]

RFC 2730                         MADCAP                    December 1999   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, Lease 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 Lease 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 example4. 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 Lease Identifier   option. The Lease 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   Server Identifier and Lease Identifier options.Hanna, et al.               Standards Track                    [Page 50]

RFC 2730                         MADCAP                    December 1999   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 example5. 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,   Lease Identifier, and Multicast Scope options.   The server responds with an ACK packet containing the Lease Time,   Lease 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.Hanna, et al.               Standards Track                    [Page 51]

RFC 2730                         MADCAP                    December 1999   The following figure illustrates this exchange.                    Client          Server                      v               v                      |               |                      |\_____________ |                      |    REQUEST   \|                      |               |                      |        Allocates address                      |               |                      | _____________/|                      |/    ACK       |                      |               |                      v               v         Figure 6: Timeline diagram of messages exchanged                   in Unicast Address Allocation exampleAPPENDIX B: Recommended Constant Values   Table 6 lists recommended values for constants defined in this   specification.       Constant Name             Recommended Value       -------------             -----------------       [CLOCK-SKEW-ALLOWANCE]    30 minutes       [DISCOVER-DELAY]          current retransmit delay       [EXTRA-ALLOCATION-TIME]   1 hour       [NO-RESPONSE-DELAY]       60 seconds       [OFFER-HOLD]              at least 60 seconds       [RESPONSE-CACHE-INTERVAL] at least 60 seconds (5 minutes maximum)       [XID-REUSE-INTERVAL]      10 minutes (required)          Table 6:  Recommended Constant ValuesHanna, et al.               Standards Track                    [Page 52]

RFC 2730                         MADCAP                    December 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Hanna, et al.               Standards Track                    [Page 53]