Network Working Group D. Thaler
Request for Comments: 2908 Microsoft
Category: Informational M. Handley
ACIRI
D. Estrin
ISI
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.
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 ................................... 13
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:
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 in RFC2365 [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
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].
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 in RFC1112 [7]), or even are incapable of receiving any
data at all, such as a wireless broadcasting device.
Scope-relative:
RFC2365 [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.
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.
+--------------------------+ +------------------------+
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.
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
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:
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.
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 unallocated
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)", RFC2730, 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", RFC2909, 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)",
RFC1771, March 1995.
[9] Handley, M., Thaler, D. and R. Kermode, "Multicast-Scope Zone
Announcement Protocol (MZAP)", RFC2776, February 2000.
[10] Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", RFC2770,
February 2000.
[11] Finlayson, R., "Abstract API for Multicast Address Allocation",
RFC2771, February 2000.
[12] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2", RFC2608, June 1999.
[13] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC1305, 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.edu
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