Movatterモバイル変換

Network Working Group                                     R. FinlaysonRequest for Comments: 2771                                    LIVE.COMCategory: Informational                                  February 2000An Abstract API for Multicast Address AllocationStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.Abstract   This document describes the "abstract service interface" for the   dynamic multicast address allocation service, as seen by   applications.  While it does not describe a concrete API (i.e., for a   specific programming language), it describes - in abstract terms -   the semantics of this service, including the guarantees that it makes   to applications.   Additional documents (not necessarily products of the IETF) would   describe concrete APIs for this service.1. Introduction   Applications are the customers of a multicast address allocation   service, so a definition of this service should include not only the   inter-node network protocols that are used to implement it, but also   the 'protocol' that applications use to access the service.  While   APIs ("application programming interfaces") for specific programming   languages (or operating systems) are outside the domain of the IETF,   it is appropriate for us to define - in abstract terms - the semantic   interface that this service presents to applications.  Specific APIs   would then be based upon this abstract service interface.   Note that it is possible to implement the multicast address   allocation service in at least two different ways.  The first (and   perhaps most common) way is for end nodes to allocate addresses by   communicating with a separate "Address Allocation Server" node, using   the "Host to Address Allocation Server" network protocol (MADCAP)   [1][7].  Alternatively, an "Address Allocation Server" implementationFinlayson                    Informational                      [Page 1]

RFC 2771     Abstract API for Multicast Address Allocation February 2000   might be co-located (along with one or more applications) on an end   node, in which case some other, internal, mechanism might be used to   access the server.  In either case, however, the abstract service   interface (and, presumably, any specific APIs) would remain the same.   The remainder of this document describes the abstract interface.   Note that this interface is intended only for the allocation of   dynamic multicast addresses, as used by the traditional multicast   service model [2].  Future multicast service models might allocate or   assign multicast addresses in other ways, but this is outside the   scope of this document.2. Abstract Data Types   The interface described below uses the following abstract data types:   -  AddressFamily: e.g., IPv4 or IPv6   -  MulticastAddress: An actual multicast address (i.e., that could      subsequently be used as the destination of a datagram)   -  MulticastAddressSet: A set of "MulticastAddress"es   -  LanguageTag: The code for a (human) language, as defined in [4]   -  Scope: An "administrative scope" [3] from which multicast             addresses are to be allocated.  Each scope is a             "MulticastAddressSet", with an associated set of             (character-string) names - indexed by "LanguageTag".  (Each             language tag has at most one corresponding name, per             scope.)  For each scope, a (language tag, name) pair may be             defined to be the 'default' name for this scope. (See the             section "Querying the name of a scope" below.)             (An implementation of this abstract data type might also             include other information, such as a default TTL for the             scope.)   -  Time: An (absolute) event time.  This is used for specifying the            "lifetime" of multicast addresses: the period of time during            which allocated multicast addresses are guaranteed to be            available.  (It is also used to specify the desired start            time for an "advance allocation".)Finlayson                    Informational                      [Page 2]

RFC 2771     Abstract API for Multicast Address Allocation February 2000            Note that a concrete API might prefer to specify some of            these times as relative times (i.e., relative to the current            time-of-day), rather than absolute time.  (Relative times            have the advantage of not requiring clock synchronization.)   -  Lease: A compound data type that describes the result of a             (successful) multicast address allocation.  It consists of:             -  [MulticastAddressSet] The set of addresses that were                allocated;             -  [AddressFamily] The address family of these addresses             -  [Time] The lifetime of these addresses (the same for                each address)             -  [Time] The "start time" of the allocation.  (See the                discussion of "advance allocation" below.)  (A concrete                API would likely also include a MADCAP "Lease                Identifier" [1].)   -  NestingRelationship: A binary data type that describes whether or                           not two scopes nest. Two scopes nest if                           traffic sent sent to a multicast group within                           one scope could be seen by all hosts present                           within the other scope were they to join the                           multicast group within the first scope. This                           value would be "False" for overlapping scopes                           where only some (or none) of the hosts within                           the second scope could see traffic sent to an                           address due to the presence of an                           administratively scoped boundary. In cases                           where the first and second scopes are                           topologically identical this value would be                           "True."   -  Status: A result code.Finlayson                    Informational                      [Page 3]

RFC 2771     Abstract API for Multicast Address Allocation February 20003. The Abstract Interface3.1 Allocating multicast addresses:   alloc_multicast_addr(in AddressFamily family,                        in Scope scope,                        in Integer minDesiredAddresses,                        in Integer maxDesiredAddresses,                        in Time minDesiredStartTime,                        in Time maxDesiredStartTime,                        in Time minDesiredLifetime,                        in Time maxDesiredLifetime,                        out Lease multicastAddressSetLease,                        out Status status)   This operation attempts to allocate a set of multicast addresses (the   size of this set is in the range [minDesiredAddresses,   maxDesiredAddresses]) within the given address family and scope, and   within a given range of desired lifetimes.  ("minDesiredStartTime"   and "maxDesiredStartTime" are used to specify "advance allocation";   this is described in more detail below.)   If the address allocation succeeds, the result is returned in   "multicastAddressSetLease" (with "status" = OK).   During the lifetime of this lease, the allocation service will make a   "best-effort" attempt to not allocate any of these addresses to   others.  (However, once the lease's lifetime has expired, any of its   addresses can be allocated to others.)   Multicast addresses are allocated for a limited lifetime.  An   application may attempt to extend this lifetime, but this operation   may fail.  Therefore, an application must be prepared for the   possibility it will not be able to use the same addresses for as long   as it desires.  In particular, the application must be prepared to   either quit early (because its original multicast address assignments   have expired), or, alternatively, to occasionally 'renumber' its   multicast addresses (in some application or higher-level-protocol   dependent way), by making a new allocation.  However, if an   application needs to consider 'renumbering', it will always know this   in advance, at the time it acquired its current address(es) - by   checking the lifetime in the returned lease.  An application will   never need to be notified asynchronously of the need to 'renumber'.Finlayson                    Informational                      [Page 4]

RFC 2771     Abstract API for Multicast Address Allocation February 2000   Possible errors:         - bad address family         - bad scope         - bad desired number of addresses (e.g., max < min)         - bad desired lifetimes (e.g., max < min)         - errors with the two "start time" parameters (see           "Advance allocation" below)         - no addresses can be allocated (for the requested parameters)   An allocation attempt can also fail with a result "status" code of   TRY_LATER, indicating that the requested allocation cannot be made at   this time, but that it might succeed if the caller retries the   attempt at some future time.  (This future time is returned in the   "start time" field of the   "multicastAddressSetLease";           the other parts of this lease are undefined.)   Note that a concrete (i.e., programming language-specific) API for   multicast address allocation will probably include additional,   specialized variants of this general allocation operation.  For   instance, it may include separate operations for:           - allocating only a single address             (i.e., minDesiredAddresses = maxDesiredAddresses = 1);           - (attempting to) allocate an address with a single, fixed              lifetime (i.e., minDesiredLifetime = maxDesiredLifetime);           - (attempting to) allocate an address for immediate use             (i.e., minDesiredStartTime = maxDesiredStartTime = 'now')3.2 Changing multicast addresses' lifetime:    change_multicast_addr_lifetime(in Lease multicastAddressSetLease,                                   in Time minDesiredLifetime,                                   in Time maxDesiredLifetime,                                   out Time lifetime)   This operation attempts to change the lifetime of previously   allocated multicast addresses.  Unless an error occurs, it returns   the new lifetime (which might remain unchanged).   Possible errors:           - bad address family           - bad durations (e.g., max < min)Finlayson                    Informational                      [Page 5]

RFC 2771     Abstract API for Multicast Address Allocation February 2000           - the addresses' lifetime could not be changed             (and the existing lifetime was not in the requested range             [minDesiredLifetime,maxDesiredLifetime])           - the addresses were not ones that we had allocated             (seesection 5.9) - or they have already expired3.3 Deallocating multicast addresses:   deallocate_multicast_addr(in Lease multicastAddressSetLease) This   operation attempts to deallocate previously allocated multicast   addresses.   Possible errors:           - bad address family           - the addresses were not ones that we had allocated                   (or they have already expired)3.4 Querying the set of usable multicast address scopes:    get_multicast_addr_scopes(in AddressFamily family,                             out "set of" Scope)   This operation returns the set of administrative multicast address   scopes that are defined for this node.   Possible errors:           - bad address family3.5 Querying the name of a scope:    get_scope_name(in Scope scope,                   in LanguageTag language,                   out String name,                   out LanguageTag languageForName)   This operation returns a character-string name for a given scope.  If   the scope has a name in the specified "language", then this name (and   language) is returned.  Otherwise, the scope's default (language,   name) pair is returned.   Possible errors:           - bad scope.Finlayson                    Informational                      [Page 6]

RFC 2771     Abstract API for Multicast Address Allocation February 20003.6 Querying the nesting state of known usable multicast address scopes:    get_scope_nesting_state(in "set of" Scope,                            out "matrix of" NestingRelationship)   Possible errors:       - bad scope.       - nesting state undetermined at this time.   This operation would return a matrix that shows the   current nesting relationships between the supplied   set of scopes which would have previously been supplied   via the get_multicast_addr_scopes(...) function.3.7 Querying the set of scopes that a given scope is known to nest inside:    get_larger_scopes(in Scope,                      out "set of" Scope)   This operation returns the set of administrative multicast   address scopes that are known to encompass the supplied   Scope.   Possible errors:       - bad scope.       - nesting state undetermined at this time.3.8 Querying the set of scopes that are known to nest inside a given scope:    get_smaller_scopes(in Scope,                      out "set of" Scope)   This operation returns the set of administrative multicast address   scopes that are known to nest inside the supplied Scope (NB this   would include those scopes that are topologically identical to the   supplied scope).   Possible errors:       - bad scope.       - nesting state undetermined at this time.Finlayson                    Informational                      [Page 7]

RFC 2771     Abstract API for Multicast Address Allocation February 20003.9 Note: The decision as to who is allowed to deallocate (or change    the lifetime of) a previously allocated multicast address set lease    is implementation-specific, and depends upon the security policy of    the host system.  Thus it is not specified in this abstract API.    One possible starting point, however, is the following:      A previously allocated multicast address can be deallocated (or      have its lifetime queried or changed) by the same "principal", and      on the same node, as that which originally allocated it.      ("principal" might, for example, be a "user" in the host operating      system.)3.10 Advance allocation      By specifying "minDesiredStartTime = maxDesiredStartTime = 'now'",      the address allocation operation - "alloc_multicast_addr" -      described above can be used to request a set of multicast      addresses that can be used *immediately* (and until their lifetime      expires).  During this whole time, the addresses are not available      for allocation to others.      It is also possible - using the "minDesiredStartTime" and      "maxDesiredStartTime" parameters - to allocate multicast addresses      *in advance* - i.e., so that they have a future "start time" as      well as an expiration time.  Before the start time, the multicast      addresses may be allocated to others.      Advance allocation is convenient for allocating addresses for      events that begin far in the future - e.g., several weeks or      months away.  Without advance allocation, it would be necessary to      allocate addresses for a long period of time - even when it will      not be used.  Such a request would not only be a wasteful use of      the multicast address space, but it may also be difficult to      implement (especially since address allocations are expected to      remain valid in spite of topology changes).      Advance allocation requests can produce the following errors (in      addition to those defined earlier):      - bad start time durations (e.g., max < min)      - requested start times conflict with requested lifetimes        (i.e., min start time > max lifetime)Finlayson                    Informational                      [Page 8]

RFC 2771     Abstract API for Multicast Address Allocation February 2000   The following operation is also defined:      change_multicast_addr_start_time(in Lease multicastAddressSetLease,                                       in Time minDesiredStartTime,                                       in Time maxDesiredStartTime,                                       out Time startTime)      This operation attempts to change the start time of previously      allocated multicast addresses.  Unless an error occurs, it returns      the new start time (which might remain unchanged).      Possible errors: the same as "change_multicast_addr_lifetime"4. Security Considerations   As noted insection 5.9 above, each implementation of this abstract   API should define a security policy that specifies when (and by whom)   previously allocated multicast addresses can be deallocated (or   queried, or have their lifetime changed).   Because multicast addresses are a finite resource, there is a   potential for a "denial of service" attack by allocating a large   number of multicast addresses without deallocating them.  Preventing   such an attack, however, is not the role of the API, but rather by   the underlying MAAS ("Multicast Address Allocation Server(s)" [6]).5. Acknowledgements   Many thanks to other participants in the "MALLOC" working group - in   particular Steve Hanna, Dave Thaler, Roger Kermode, and Pavlin   Radoslavov - for their valuable comments.6. References   [1] Hanna, S., Patel, B. and M. Shah, "Multicast Address Dynamic       Client Allocation Protocol (MADCAP)",RFC 2730, December 1999.   [2] Deering, S., "Host Extensions for IP Multicasting", STD 5,RFC1112, August 1989.   [3] Meyer, D., "Administratively Scoped IP Multicast",BCP 23,RFC2365, July, 1998.   [4] Alvestrand, H., "Tags for the Identification of Languages",RFC1766, March 1995.Finlayson                    Informational                      [Page 9]

RFC 2771     Abstract API for Multicast Address Allocation February 2000   [5] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",RFC 2327, April 1998.   [6] Estrin, D., Handley, M. and D. Thaler, "The Internet Multicast       Address Allocation Architecture", Work in Progress.   [7] Kermode, R.,"MADCAP Multicast Scope Nesting State Option", Work       in Progress.7. Author's Address   Ross Finlayson,   Live Networks, Inc. (LIVE.COM)   EMail: finlayson@live.com   WWW:http://www.live.com/Finlayson                    Informational                     [Page 10]

RFC 2771     Abstract API for Multicast Address Allocation February 20008. Full Copyright Statement   Copyright (C) The Internet Society (2000).  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.Finlayson                    Informational                     [Page 11]