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The Internet Multicast Address Allocation Architecture

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2908.
Authors Dave Thaler , Mark J. Handley , Dr. Deborah Estrin
Last updated 2013-03-02 (Latest revision 2000-07-07)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
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IESG IESG state Became RFC 2908 (Historic)
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MALLOC Working Group                                     D. Thaler
INTERNET-DRAFT                                           Microsoft
June 21, 2000                                           M. Handley
Expires December 2000                                        ACIRI
Category: Informational                                  D. Estrin

      The Internet Multicast Address Allocation Architecture

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups.  Note that
other groups may also distribute working documents as Internet-

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time.  It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in

The list of current Internet-Drafts can be accessed at

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Copyright Notice

Copyright (C) The Internet Society (2000).  All Rights Reserved.

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1.  Abstract

This document proposes a multicast address allocation architecture
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.

2.  Introduction

This document proposes a multicast address allocation architecture
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.

3.  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.

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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
   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

   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

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   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].  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.

3.1.  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

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     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.

     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

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     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.

4.  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.

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

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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.

     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

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     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.

5.  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

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 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

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       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".

5.1.  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:

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.

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5.1.1.  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.

5.1.2.  The IPv6 Allocation Scope -- SCOP 6

The IPv6 "scop" value 6 is to be used for the Allocation Scope.

6.  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.

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.

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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.

7.  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 addresses in a range
allocated by that domain can be dropped by creating multicast

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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.

8.  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.

9.  References

[1]  D. Meyer, "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

[5]  Handley, M., Hanna, S., "Multicast Address Allocation
     Protocol (AAP)", Work in progress, draft-ietf-malloc-

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     aap-04.txt, June 2000.

[6]  Estrin, D., Govindan, R., Handley, M., Kumar, S., Radoslavov,
     P., and D. Thaler, "The Multicast Address-Set Claim (MASC)
     Protocol", Work in progress, draft-ietf-malloc-masc-05.txt,
     January 2000.

[7]  Deering, S., "Host Extensions for IP Multicasting", 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] R. Finlayson, "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] D. Mills, "Network Time Protocol (Version 3) Specification,
     Implementation and Analysis", RFC 1305, March 1992.

10.  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

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     University of Southern Calif.
     Los Angeles, CA 90089

11.  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

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Table of Contents

1: Abstract .................................................    2
2: Introduction .............................................    2
3: Requirements .............................................    2
3.1: Address Dynamics .......................................    4
4: Overview of the Architecture .............................    6
5: Scoping ..................................................    8
5.1: Allocation Scope .......................................    9
5.1.1: The IPv4 Allocation Scope -- ..........   10
5.1.2: The IPv6 Allocation Scope -- SCOP 6 ..................   10
6: Overview of the Allocation Process .......................   10
7: Security Considerations ..................................   11
8: Acknowledgments ..........................................   12
9: References ...............................................   12
10: Authors' Addresses ......................................   13
11: Full Copyright Statement ................................   14

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