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Network Working Group                                         D. ThalerRequest for Comments: 2908                                    MicrosoftCategory: Informational                                      M. Handley                                                                  ACIRI                                                              D. Estrin                                                                    ISI                                                         September 2000The Internet Multicast Address Allocation ArchitectureStatus 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 proposes a multicast address allocation architecture   (MALLOC) for the Internet.  The architecture is modular with three   layers, comprising a host-server mechanism, an intra-domain server-   server coordination mechanism, and an inter-domain mechanism.Table of Contents   1: Introduction ................................................2   2: Requirements ................................................2   3.1: Address Dynamics ..........................................4   3: Overview of the Architecture ................................5   4: Scoping .....................................................7   4.1: Allocation Scope ..........................................8   4.1.1: The IPv4 Allocation Scope -- 239.251.0.0/16 .............9   4.1.2: The IPv6 Allocation Scope -- SCOP 6 .....................9   5: Overview of the Allocation Process ..........................9   6: Security Considerations .....................................10   7: Acknowledgments .............................................11   8: References ..................................................11   9: Authors' Addresses ..........................................12   10: Full Copyright Statement ...................................13Thaler, et al.               Informational                      [Page 1]

RFC 2908                  MALLOC Architecture             September 20001.  Introduction   This document proposes a multicast address allocation architecture   (MALLOC) for the Internet, and is intended to be generic enough to   apply to both IPv4 and IPv6 environments.   As with unicast addresses, the usage of any given multicast address   is limited in two dimensions:   Lifetime:      An address has a start time and a (possibly infinite) end time,      between which it is valid.   Scope:      An address is valid over a specific area of the network.  For      example, it may be globally valid and unique, or it may be a      private address which is valid only within a local area.   This architecture assumes that the primary scoping mechanism in use   is administrative scoping, as described inRFC 2365 [1].  While   solutions that work for TTL scoping are possible, they introduce   significant additional complication for address allocation [2].   Moreover, TTL scoping is a poor solution for multicast scope control,   and our assumption is that usage of TTL scoping will decline before   this architecture is widely used.2.  Requirements   From a design point of view, the important properties of multicast   allocation mechanisms are robustness, timeliness, low probability of   clashing allocations, and good address space utilization in   situations where space is scare.  Where this interacts with multicast   routing, it is desirable for multicast addresses to be allocated in a   manner that aids aggregation of routing state.   o  Robustness/Availability      The robustness requirement is that an application requiring the      allocation of an address should always be able to obtain one, even      in the presence of other network failures.   o  Timeliness      From a timeliness point of view, a short delay of up to a few      seconds is probably acceptable before the client is given an      address with reasonable confidence in its uniqueness.  If the      session is defined in advance, the address should be allocated as      soon as possible, and should not wait until just before theThaler, et al.               Informational                      [Page 2]

RFC 2908                  MALLOC Architecture             September 2000      session starts.  It is in some cases acceptable to change the      multicast addresses used by the session up until the time when the      session actually starts, but this should only be done when it      averts a significant problem such as an address clash that was      discovered after initial session definition.   o  Low Probability of Clashes      A multicast address allocation scheme should always be able to      allocate an address that can be guaranteed not to clash with that      of another session.  A top-down partitioning of the address space      would be required to completely guarantee that no clashes would      occur.   o  Address Space Packing in Scarcity Situations      In situations where address space is scarce, simply partitioning      the address space would result in significant fragmentation of the      address space.    This is because one would need enough spare      space in each address space partition to give a reasonable degree      of assurance that addresses could still be allocated for a      significant time in the event of a network partition.  In      addition, providing backup allocation servers in such a hierarchy,      so that fail-over (including partitioning of a server and its      backup from each other) does not cause collisions would add      further to the address space fragmentation.      Since guaranteeing no clashes in a robust manner requires      partitioning the address space, providing a hard guarantee leads      to inefficient address space usage.  Hence, when address space is      scarce, it is difficult to achieve constant availability and      timeliness, guarantee no clashes, and achieve good address space      usage.  As a result, we must prioritize these properties.  We      believe that, when address space is scarce, achieving good address      space packing and constant availability are more important than      guaranteeing that address clashes never occur.  What we aim for in      these situations is a very high probability that an address clash      does not occur, but we accept that there is a finite probability      of this happening.  Should a clash occur (or should an application      start using an address it did not allocate, which may also lead to      a clash), either the clash can be detected and addresses changed,      or hosts receiving additional traffic can prune that traffic using      source-specific prunes available in IGMP version 3, and so we do      not believe that this is a disastrous situation.      In summary, tolerating the possibility of clashes is likely to      allow allocation of a very high proportion of the address space in      the presence of network conditions such as those observed in [3].Thaler, et al.               Informational                      [Page 3]

RFC 2908                  MALLOC Architecture             September 2000      We believe that we can get good packing and good availability with      good collision avoidance, while we would have to compromise      packing and availability significantly to avoid all collisions.      Finally, in situations where address space is not scarce, such as      with IPv6, achieving good address space usage is less important,      and hence partitioning may potentially be used to guarantee no      collisions among hosts that use this architecture.2.1.  Address Dynamics   Multicast addresses may be allocated in any of three ways:   Static:      Statically allocated addresses are allocated by IANA for specific      protocols that require well-known addresses to work.  Examples of      static addresses are 224.0.1.1 which is used for the Network Time      Protocol [13] and 224.2.127.255 which is used for global scope      multicast session announcements.  Applications that use multicast      for bootstrap purposes should not normally be given their own      static multicast address, but should bootstrap themselves using a      well-known service location address which can be used to announce      the binding between local services and multicast addresses.      Static addresses typically have a permanent lifetime, and a scope      defined by the scope range in which they reside.  As such, a      static address is valid everywhere (although the set of receivers      may be different depending on location), and may be hard-coded      into applications, devices, embedded systems, etc.  Static      addresses are also useful for devices which support sending but      not receiving multicast IP datagrams (Level 1 conformance as      specified inRFC 1112 [7]), or even are incapable of receiving any      data at all, such as a wireless broadcasting device.   Scope-relative:RFC 2365 [1] reserves the highest 256 addresses in every      administrative scope range for relative assignments.  Relative      assignments are made by IANA and consist of an offset which is      valid in every scope.  Relative addresses are reserved for      infrastructure protocols which require an address in every scope,      and this offset may be hard-coded into applications, devices,      embedded systems, etc.  Such devices must have a way (e.g. via      MZAP [9] or via MADCAP [4]) to obtain the list of scopes in which      they reside.Thaler, et al.               Informational                      [Page 4]

RFC 2908                  MALLOC Architecture             September 2000      The offsets assigned typically have a permanent lifetime, and are      valid in every scope and location.  Hence, the scope-relative      address in a given scope range has a lifetime equal to that of the      scope range in which it falls.   Dynamic:      For most purposes, the correct way to use multicast is to obtain a      dynamic multicast address.  These addresses are provided on demand      and have a specific lifetime.  An application should request an      address only for as long as it expects to need the address.  Under      some circumstances, an address will be granted for a period of      time that is less than the time that was requested.  This will      occur rarely if the request is for a reasonable amount of time.      Applications should be prepared to cope with this when it occurs.      At any time during the lifetime of an existing address,      applications may also request an extension of the lifetime, and      such extensions will be granted when possible.  When the address      extension is not granted, the application is expected to request a      new address to take over from the old address when it expires, and      to be able to cope with this situation gracefully.  As with      unicast addresses, no guarantee of reachability of an address is      provided by the network once the lifetime expires.      These restrictions on address lifetime are necessary to allow the      address allocation architecture to be organized around address      usage patterns in a manner that ensures addresses are aggregatable      and multicast routing is reasonably close to optimal.  In      contrast, statically allocated addresses may be given sub-optimal      routing.3.  Overview of the Architecture   The architecture is modular so that each layer may be used, upgraded,   or replaced independently of the others.  Layering also provides   isolation, in that different mechanisms at the same layer can be used   by different organizations without adversely impacting other layers.   There are three layers in this architecture (Figure 1).  Note that   these layer numbers are different from the layer numbers in the   TCP/IP stack, which describe the path of data packets.Thaler, et al.               Informational                      [Page 5]

RFC 2908                  MALLOC Architecture             September 2000   +--------------------------+         +------------------------+   |                          |         |                        |   |       to other peers     |         |   to other peers       |   |          ||   //         |         |      ||  //   ||       |   |          Prefix          |         |    Prefix     Prefix   |   |       Coordinator        |         |Coordinator  Coordinator|   +------------||------------+         +-------||----//---------+                ||Layer 3                       ||   //   +------------||------------------------------||--//-----------+   |          Prefix                          Prefix             |   |       Coordinator=======================Coordinator         |   |             ^                              ^                |   |             +----------------+-------------+                |   |             |       Layer 2  |             |                |   |     MAAS<---/                |             +---> MAAS       |   |     ^   ^                    v                    ^         |   |     .    .                 MAAS                   .         |   |     .     .Layer 1           ^                    .Layer 1  |   |     v      v                 .Layer 1             v         |   | Client   Client              v                 Client       |   |                           Client                            |   +-------------------------------------------------------------+  Figure 1: An Overview of the Multicast Address Allocation Architecture   Layer 1      A protocol or mechanism that a multicast client uses to request a      multicast address from a multicast address allocation server      (MAAS).  When the server grants an address, it becomes the      server's responsibility to ensure that this address is not then      reused elsewhere within the address's scope during the lifetime      granted.      Examples of possible protocols or mechanisms at this layer include      MADCAP [4], HTTP to access a web page for allocation, and IANA      static address assignments.      An abstract API for applications to use for dynamic allocation,      independent of the Layer 1 protocol/mechanism in use, is given in      [11].   Layer 2      An intra-domain protocol or mechanism that MAAS's use to      coordinate allocations to ensure they do not allocate duplicate      addresses.  A MAAS must have stable storage, or some equivalent      robustness mechanism, to ensure that uniqueness is preserved      across MAAS failures and reboots.Thaler, et al.               Informational                      [Page 6]

RFC 2908                  MALLOC Architecture             September 2000      MAASs also use the Layer 2 protocol/mechanism to acquire (from      "Prefix Coordinators") the ranges of multicast addresses out of      which they may allocate addresses.      In this document we use the term "allocation domain" to mean an      administratively scoped multicast-capable region of the network,      within which addresses in a specific range may be allocated by a      Layer 2 protocol/mechanism.      Examples of protocols or mechanisms at this layer include AAP [5],      and manual configuration of MAAS's.   Layer 3      An inter-domain protocol or mechanism that allocates multicast      address ranges (with lifetimes) to Prefix Coordinators.      Individual addresses may then be allocated out of these ranges by      MAAS's inside allocation domains as described above.      Examples of protocols or mechanisms at this layer include MASC [6]      (in which Prefix Coordinators are typically routers without any      stable storage requirement), and static allocations by AS number      as described in [10] (in which Prefix Coordinators are typically      human administrators).   Each of the three layers serves slightly different purposes and as   such, protocols or mechanisms at each layer may require different   design tradeoffs.4.  Scoping   To allocate dynamic addresses within administrative scopes, a MAAS   must be able to learn which scopes are in effect, what their address   ranges and names are, and which addresses or subranges within each   scope are valid for dynamic allocation by the MAAS.   The first two tasks, learning the scopes in effect and the address   range and name(s) of each scope, may be provided by static   configuration or dynamically learned.  For example, a MAAS may simply   passively listen to MZAP [9] messages to acquire this information.   To determine the subrange for dynamic allocation, there are two cases   for each scope, corresponding to small "indivisible" scopes, and big   "divisible" scopes.  Note that MZAP identifies which scopes are   divisible and which are not.   (1) For small scopes, the allocation domain corresponds to the entire       topology within the administrative scope.  Hence, all MAASs       inside the scope may use the entire address range (minus the lastThaler, et al.               Informational                      [Page 7]

RFC 2908                  MALLOC Architecture             September 2000       256 addresses reserved as scope-relative addresses), and use the       Layer 2 mechanism/protocol to coordinate allocations.  For small       scopes, Prefix Coordinators are not involved.       Hence, for small scopes, the effective "allocation domain" area       may be different for different scopes.  Note that a small,       indivisible scope could be larger or smaller than the Allocation       Scope used for big scopes (see below).   (2) For big scopes (including the global scope), the area inside the       scope may be large enough that simply using a Layer 2       mechanism/protocol may be inefficient or otherwise undesirable.       In this case, the scope must span multiple allocation domains,       and the Layer 3 mechanism/protocol must be used to divvy up the       scoped address space among the allocation domains.  Hence, a MAAS       may learn of the scope via MZAP, but must acquire a subrange from       which to allocate from a Prefix Coordinator.       For simplicity, the effective "allocation domain" area will be       the same for all big scopes, being the granularity at which all       big scopes are divided up.  We define the administrative scope at       this granularity to be the "Allocation Scope".4.1.  Allocation Scope   The Allocation Scope is a new administrative scope, defined in this   document and to be reserved by IANA with values as noted below.  This   is the scope that is used by a Layer 2 protocol/mechanism to   coordinate address allocation for addresses in larger, divisible   scopes.   We expect that the Allocation Scope will often coincide with a   unicast Autonomous System (AS) boundary.   If an AS is too large, or the network administrator wishes to run   different intra-domain multicast routing in different parts of an AS,   that AS can be split by manual setup of an allocation scope boundary   that is not an AS boundary.  This is done by setting up a multicast   boundary dividing the unicast AS into two or more multicast   allocation domains.   If an AS is too small, and address space is scarce, address space   fragmentation may occur if the AS is its own allocation domain.   Here, the AS can instead be treated as part of its provider's   allocation domain, and use a Layer 2 protocol/mechanism to coordinate   allocation between its MAAS's (if any) and those of its provider.  An   AS should probably take this course of action if:Thaler, et al.               Informational                      [Page 8]

RFC 2908                  MALLOC Architecture             September 2000   o  it is connected to a single provider,   o  it does not provide transit for another AS, and   o  it needs fewer than (say) 256 multicast addresses of larger than      AS scope allocated on average.4.1.1.  The IPv4 Allocation Scope -- 239.251.0.0/16   The address space 239.251.0.0/16 is to be reserved for the Allocation   Scope.  The ranges 239.248.0.0/16, 239.249.0.0/16 and 239.250.0.0/16   are to be left unassigned and available for expansion of this space.   These ranges should be left unassigned until the 239.251.0.0/16 space   is no longer sufficient.4.1.2.  The IPv6 Allocation Scope -- SCOP 6   The IPv6 "scop" value 6 is to be used for the Allocation Scope.5.  Overview of the Allocation Process   Once Layer 3 allocation has been performed for large, divisible   scopes, and each Prefix Coordinator has acquired one or more ranges,   then those ranges are passed to all MAAS's within the Prefix   Coordinator's domain via a Layer 2 mechanism/protocol.   MAAS's within the domain receive these ranges and store them as the   currently allowable addresses for that domain.  Each range is valid   for a given lifetime (also acquired via the Layer 3   mechanism/protocol) and is not revoked before the lifetime has   expired.  MAAS's also learn of small scopes (e.g., via MZAP) and   store the ranges associated with them.   Using the Layer 2 mechanism/protocol, each MAAS ensures that it will   exclude any addresses which have been or will be allocated by other   MAAS's within its domain.   When a client needs a multicast address, it first needs to decide   what the scope of the intended session should be, and locate a MAAS   capable of allocating addresses within that scope.   To pick a scope, the client will either simply choose a well-known   scope, such as the global scope, or it will enumerate the available   scopes (e.g., by sending a MADCAP query, or by listening to MZAP   messages over time) and allow a user to select one.Thaler, et al.               Informational                      [Page 9]

RFC 2908                  MALLOC Architecture             September 2000   Locating a MAAS can be done via a variety of methods, including   manual configuration, using a service location protocol such as SLP   [12], or via a mechanism provided by a Layer 1 protocol itself.   MADCAP, for instance, includes such a facility.   Once the client has chosen a scope and located a MAAS, it then   requests an address in that scope from the MAAS located.  Along with   the request it also passes the acceptable range for the lifetimes of   the allocation it desires.  For example, if the Layer 1 protocol in   use is MADCAP, the client sends a MADCAP REQUEST message to the MAAS,   and waits for a NAK message or an ACK message containing the   allocated information.   Upon receiving a request from a client, the MAAS then chooses an   unused address in a range for the specified scope, with a lifetime   which both satisfies the acceptable range specified by the client,   and is within the lifetime of the actual range.   The MAAS uses the Layer 2 mechanism/protocol to ensure that such an   address does not clash with any addresses allocated by other MAASs.   For example, if Layer 2 uses manual configuration of non-overlapping   ranges, then this simply consists of adhering to the range configured   in the local MAAS.  If, on the other hand, AAP is used at Layer 2 to   provide less address space fragmentation, the MAAS advertises the   proposed allocation domain-wide using AAP.  If no clashing AAP claim   is received within a short time interval, then the address is   returned to the client via the Layer 1 protocol/mechanism.  If a   clashing claim is received by the MAAS, then it chooses a different   address and tries again.  AAP also allows each MAAS to pre-reserve a   small "pool" of addresses for which it need not wait to detect   clashes.   If a domain ever begins to run out of available multicast addresses,   a Prefix Coordinator in that domain uses the Layer 3   protocol/mechanism to acquire more space.6.  Security Considerations   The architecture described herein does not prevent an application   from just sending to or joining a multicast address without   allocating it (just as the same is true for unicast addresses today).   However, there is no guarantee that data for unallocated addresses   will be delivered by the network.  That is, routers may drop data for   unallocated addresses if they have some way of checking whether a   destination address has been allocated.  For example, if the border   routers of a domain participate in the Layer 2 protocol/mechanism and   cache the set of allocated addresses, then data for unallocatedThaler, et al.               Informational                     [Page 10]

RFC 2908                  MALLOC Architecture             September 2000   addresses in a range allocated by that domain can be dropped by   creating multicast forwarding state with an empty outgoing interface   list and/or pruning back the tree branches for those groups.   A malicious application may attempt a denial-of-service attack by   attempting to allocate a large number of addresses, thus attempting   to exhaust the supply of available addresses.  Other attacks include   releasing or modifying the allocation of another party.  These   attacks can be combatted through the use of authentication with   policy restrictions (such as a maximum number of addresses that can   be allocated by a single party).   Hence, protocols/mechanisms that implement layers of this   architecture should be deployable in a secure fashion.  For example,   one should support authentication with policy restrictions, and   should not allow someone unauthorized to release or modify the   allocation of another party.7.  Acknowledgments   Steve Hanna provided valuable feedback on this document.  The members   of the MALLOC WG and the MBone community provided the motivation for   this work.8.  References   [1]  Meyer, D., "Administratively Scoped IP Multicast",BCP 23,RFC2365, July 1998.   [2]  Mark Handley, "Multicast Session Directories and Address        Allocation", Chapter 6 of PhD Thesis entitled "On Scalable        Multimedia Conferencing Systems", University of London, 1997.   [3]  Mark Handley, "An Analysis of Mbone Performance", Chapter 4 of        PhD Thesis entitled "On Scalable Multimedia Conferencing        Systems", University of London, 1997.   [4]  Hanna, S., Patel, B. and M. Shah, "Multicast Address Dynamic        Client Allocation Protocol (MADCAP)",RFC 2730, December 1999.   [5]  Handley, M. and S. Hanna, "Multicast Address Allocation Protocol        (AAP)", Work in Progress.   [6]  Estrin, D., Govindan, R., Handley, M., Kumar, S., Radoslavov, P.        and D. Thaler, "The Multicast Address-Set Claim (MASC)        Protocol",RFC 2909, September 2000.Thaler, et al.               Informational                     [Page 11]

RFC 2908                  MALLOC Architecture             September 2000   [7]  Deering, S., "Host Extensions for IP Multicasting", STD 5,RFC1112, August 1989.   [8]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",RFC 1771, March 1995.   [9]  Handley, M., Thaler, D. and R. Kermode, "Multicast-Scope Zone        Announcement Protocol (MZAP)",RFC 2776, February 2000.   [10] Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8",RFC 2770,        February 2000.   [11] Finlayson, R., "Abstract API for Multicast Address Allocation",RFC 2771, February 2000.   [12] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service        Location Protocol, Version 2",RFC 2608, June 1999.   [13] Mills, D., "Network Time Protocol (Version 3) Specification,        Implementation and Analysis",RFC 1305, March 1992.9.  Authors' Addresses   Dave Thaler   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052-6399   EMail: dthaler@microsoft.com   Mark Handley   AT&T Center for Internet Research at ICSI   1947 Center St, Suite 600   Berkeley, CA 94704   EMail: mjh@aciri.org   Deborah Estrin   Computer Science Dept/ISI   University of Southern California   Los Angeles, CA 90089   EMail: estrin@usc.eduThaler, et al.               Informational                     [Page 12]

RFC 2908                  MALLOC Architecture             September 200010.  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.Thaler, et al.               Informational                     [Page 13]