Network Working Group                                         D. Thaler
Request for Comments: 2908                                    Microsoft
Category: Informational                                      M. Handley
                                                              D. Estrin
                                                         September 2000

         The Internet Multicast Address Allocation Architecture

Status 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.


   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 -- .............  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 ................................... 13

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1.  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:

      An address has a start time and a (possibly infinite) end time,
      between which it is valid.

      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 in RFC 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 the

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      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

   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].

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      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:

      Statically allocated addresses are allocated by IANA for specific
      protocols that require well-known addresses to work.  Examples of
      static addresses are which is used for the Network Time
      Protocol [13] and 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 in RFC 1112 [7]), or even are incapable of receiving any
      data at all, such as a wireless broadcasting device.

      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.

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      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.

      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

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.

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   +--------------------------+         +------------------------+
   |                          |         |                        |
   |       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

      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

   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.

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      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 last

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       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

   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:

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   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 --

   The address space is to be reserved for the Allocation
   Scope.  The ranges, and
   are to be left unassigned and available for expansion of this space.
   These ranges should be left unassigned until the 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.

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   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

   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 unallocated

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   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, RFC
        2365, 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.

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RFC 2908                  MALLOC Architecture             September 2000

   [7]  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
        1112, 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


   Mark Handley
   AT&T Center for Internet Research at ICSI
   1947 Center St, Suite 600
   Berkeley, CA 94704


   Deborah Estrin
   Computer Science Dept/ISI
   University of Southern California
   Los Angeles, CA 90089


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10.  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

   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


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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