INTERNET-DRAFT Supratik Bhattacharyya
Expires 18 November 2001 Christophe Diot
Sprint ATL
Leonard Giuliano
Juniper Networks
Rob Rockell
Sprint E|Solutions
John Meylor
Dave Meyer
Cisco Systems
Greg Shepherd
Juniper Networks
Brian Haberman
Nortel Networks
18 May 2001
An Overview of Source-Specific Multicast(SSM) Deployment
<draft-ietf-ssm-overview-00.txt>
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-
Drafts.
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 progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
Bhattacharyya et. al. [Page 1]
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Abstract
This document provides an overview of the Source-Specific Multicast
(SSM) service and its deployment using the PIM-SM and IGMP/MLD
protocols. The network layer service provided by SSM is a "channel",
identified by an SSM destination IP address (G) and a source IP
address S. The IP address range 232/8 has been designated as SSM
addresses by IANA for IPv4. An SSM address range already exists for
IPv6. A source S transmits IP datagrams to an SSM address G. A
receiver can receive these datagrams by subscribing to the channel
(S,G). Channel subscription is supported by version 3 of the IGMP
protocol for IPv4 and version2 of the MLD protocol for IPv6. The
interdomain tree for forwarding UDP datagrams is rooted at the source
S. Although a number of protocols exists for constructing source-
rooted forwarding trees, this document discusses the most widely
implemented one - PIM Sparse Mode [PIM-SM-NEW].
This document is intended as a starting point for deploying SSM
services. It provides an architectural overview of SSM and describes
how it solves a number of problems faced in the deployment of inter-
domain multicast. It outlines changes to protocols and applications
both at end-hosts and routers for supporting SSM, with pointers to
more detailed documents where appropriate. Issues of interoperability
with the existing multicast service model (as defined by RFC 1112)
are also discussed.
1. Terminology
This section defines some terms that are used in the rest of this
document :
Any-Source Multicast (ASM) : This is the IP multicast service model
defined in RFC 1112 [RFC1112]. An IP datagram is transmitted to a
"host group", a set of zero or more hosts identified by a single IP
destination address (224.0.0.0 through 239.255.255.255 for IPv4). This
model supports one-to-many and and many-to-many multicast groups.
Hosts may join and leave the group any time. There is no restriction
on the location or number of receivers, and a source need not be a
member of the host group it transmits to.
Source-Specific Multicast (SSM) : This is the multicast service model
defined in [SSM-ARCH]. An IP datagram is transmitted by a source S to
an SSM address G, and receivers can receive this datagram by
subscribing to channel (S,G). SSM is derived from EXPRESS [EXPRESS]
and supports one-to-many multicast.The address range 232/8 has been
assigned by IANA [IANA-ALLOC] for SSM service in IPv4. For IPv6, the
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range FF2::/11 through FF3x::/11 is defined for SSM services [SSM-
IPV6].
Source-Filtered Multicast (SFM) : This is a variant of the multicast
service model defined in RFC 1112. A source transmits IP datagrams to
a host group address in the range of 224.0.0.0 to 239.255.255.255.
However, each "upper layer protocol module" can now request data sent
to a host group G by only a specific set of sources, or can request
data sent to host group G from all BUT a specific set of sources. Such
support for source filtering is provided by version 3 of the Internet
Group Management Protocol (or IGMPv3) [IGMPv3] for IPv4, and version 2
of the Multicast Listener Discovery (or MLD) protocol for IPv6
[MLDv2]. We shall henceforth refer to these two protocols as "SFM-
capable". Earlier versions of these protocols - IGMPv1/IGMPv2 and
MLDv1 - do not provide support for source-filtering, and are referred
to as "non-SFM-capable".
2. Current Interdomain Multicast Architecture
The current interdomain multicast architecture is based on the ASM
service model. To become a member of a particular host group end-
hosts register multicast group membership with querier routers
handling multicast group membership function using the IGMP version 2
(IGMPv2) protocol [RFC2236] for IPv4 or the MLD version 1 (MLDv1)
protocol [RFC2710] for IPv6. These protocols are non-SFM-capable,
hence source-filtering capabilities are unavailable to receivers.
Multicast-capable routers then exchange messages with each other
according to a routing protocol to construct a distribution tree
connecting all the end-hosts. A number of different protocols exist
for building multicast forwarding trees, which differ mainly in the
type of delivery tree constructed [IPMULTICAST,PIM-ARCH, RFC2362,
PIM-SM-NEW, PIM-DM]. Of these, the Protocol Independent Multicast
Sparse-Mode (PIM-SM) protocol [PIM-SM-NEW] is the most widely
deployed in today's public networks. PIM-SM, by default, constructs a
single spanning multicast tree rooted at a core rendezvous point or
RP for all group members within a domain. Local sources then send
their data to this RP which forwards the data down the shared tree to
interested local receivers. A receiver joining a host group can only
specify interest in the entire group and therefore will receive data
for any source to this group forwarded on the shared tree.
Distribution via a shared tree can be effective for certain types of
traffic, e.g., where the number of sources is large since forwarding
on the shared tree is performed via a single multicast forwarding
entry. However, there are many cases (e.g., Internet broadcast type
streams) where forwarding from a source to a receiver is most
efficient via the shortest path. PIM-SM also allows a designated
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router serving a particular subnet to switch to a source-based
shortest path tree for a given source once the source's address is
learned from data arriving on the shared tree. This capability
provides for distribution of data from local sources to local
receivers both sharing a common RP inside a given PIM domain.
It is also possible for RP's to learn about sources in other PIM
domains by using the Multicast Source Discovery Protocol (MSDP)
[MSDP]. Once an active remote source is identified, an RP can join
the shortest path tree to that source and obtain data to forward down
the local shared tree on behalf of interested local receivers.
Designated routers for particular subnets can again switch to a
source-based shortest path tree for a given remote source once the
source's address is learned from data arriving on the shared tree.
The IGMPv2/PIM-SM/MSDP-based interdomain multicast architecture is
widely deployed in IPv4 networks and can be particularly effective
for groups where sources are not known in advance by hosts joining a
group, or when sources come and go dynamically, or when forwarding on
a common shared tree is found to be operationally beneficial.
3. Problems with Current Architecture
There are several deployment problems associated with current
multicast architecture:
A) Inefficient handling of well-known sources :
In cases where the address of the source is well known in advance
of the receiver joining the group, and when the shortest
forwarding path is the preferred forwarding mode, then shared tree
mechanisms and MSDP only are not necessary.
B) Lack of access control :
In the ASM service model, a receiver can not specify which
specific sources it would like to receive when it joins a given
group. A receiver will be forwarded data sent to a host group by
any source.
C) Address Allocation :
Address allocation is one of core deployment challenges posed by
the ASM service model. The current multicast architecture does not
provide an adequate solution to prevent address collisions among
multiple applications. The problem is more serious for IPv4 than
IPv6 since the total number of multicast addresses is smaller. A
static address allocation scheme, GLOP [GLOP00] has been proposed
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as an interim solution for IPv4; however, GLOP addresses are
allocated per registered AS, which is inadequate in cases where
the number of sources exceeds the AS numbers available for
mapping. Proposed longer-term solutions such as the Multicast
Address Allocation Architecture (MAAA) are generally perceived as
being too complex (with respect to the dynamic nature of multicast
address allocation) for widespread deployment. However, the
unicast-prefix-based multicast architecture of IPv6 [HABE1]
expands on the GLOP approach, simplifies the multicast address
allocation solution and incorporates support for source-specific
multicast addresses.
4. Source Specific Multicast (SSM) : Benefits and Requirements
As mentioned before, Source Specific Multicast (SSM) defines a
service model for a "channel" identified by an (S,G) pair, where S is
a source address and G is an SSM address. This model can be realized
by a protocol architecture, where packet forwarding is restricted to
shortest path trees rooted at specific sources, and channel
subscriptions are described using an SFM-capable group management
protocol such as IGMPv3 or MLDv2.
The SSM service model alleviates all of the deployment problems
described earlier :
4.1 SSM lends itself to an elegant solution to the access control
problem. Only a single source S can transmit to a channel (S,G)
where G is an SSM address. This makes it significantly more
difficult to spam an SSM channel than an ASM host group. In
addition, data from unrequested sources need not be forwarded by
the network, which prevents unnecessary consumption of network
resources.
4.2 SSM defines channels on a per-source basis; hence SSM
addresses are "local" to each source. This averts the problem of
global allocation of SSM addresses, and makes each source
independently responsible for resolving address collisions for the
various channels that it creates.
4.3 The distribution tree for an SSM channel (S,G) is always
rooted at the source S. Thus there is no need for a shared tree
infrastructure. In terms of the IGMPv2/PIM-SM/MSDP architecture,
this implies that neither the RP-based shared tree infrastructure
of PIM-SM nor the MSDP protocol is required. Thus the complexity
of the multicast routing infrastructure for SSM is low, making it
viable for immediate deployment and more efficient for well-known
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sources.
4.4 It is widely held that point-to-multipoint applications such
as Internet TV will dominate the Internet multicast application
space in the near future. The SSM model is ideally suited for such
applications.
A protocol architecture for SSM requires the following :
A) Source specific host membership reports : A SFM-capable
protocol is needed to allow a host to describe specific sources
from which it would like to receive data.
B) Shortest path forwarding. DR's must be capable of recognizing
receiver-initiated, source specific host reports and initiating
(S,G) joins directly and immediately as result.
C) Elimination of shared tree forwarding. In order to achieve
global effectiveness of SSM, all networks must agree to restrict
data forwarding to source trees (i.e., prevent shared tree
forwarding) for SSM addresses. The address range 232/8 has been
allocated by IANA for deploying source-specific IPv4 multicast
(SSM) services. In this range, SSM is the sole service model. For
IPv6, a source-specific multicast address range has been defined
in [HABE1], as a special case of unicast prefix-based multicast
addresses.
5. SSM Framework
Figure 1 illustrates the elements in an end-to-end SSM framework.
--------------------------------------------------------------
IANA assigned 232/8 for IPv4 ADDRESS ALLOCATION
SSM range exists for IPv6
--------------------------------------------------------------
|
v
+--------------+ session directory/web page
| source,group | SESSION DESCRIPTION
--------------------------------------------------------------
^ |
Query | | s,g
| v
+-----------------+ host
| SSM-aware app | CHANNEL DISCOVERY
--------------------------------------------------------------
| SSM-aware app | SSM-AWARE APPLICATION
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--------------------------------------------------------------
| IGMPv3/MLDv2 | IGMPv3/MLDv2 HOST REPORTING
+---------------+
|(source specific host report)
|
--------------------------------------------------------------
v
+-----------------+ Querier Router
| IGMPv3/MLDv2 | QUERIER
--------------------------------------------------------------
| PIM-SSM | PIM-SSM ROUTING
+------------+ Designated Router
|
| (S,G) Join only
v
+-----------+ Core Router
| PIM-SSM |
+-----------+
|
| (S,G) Join only
V
Figure 1 : SSM Framework: elements in end-to-end model
We now discuss the framework elements in detail :
5.1 Address Allocation
For IPv4, the address range of 232/8 has been assigned by IANA for
SSM. Sessions expecting SSM functionality MUST allocate addresses
from the 232/8 range. To ensure global SSM functionality in 232/8,
including in networks where routers run non-SFM-capable protocols,
operational policies are being proposed [SSM-BCP] which prevent data
sent to 232/8 from being delivered via shared trees.
Note that it is possible to achieve the benefit of direct and
immediate (S,G) joins in response to IGMPv3 reports in other ranges
than 232/8.However, non-SSM address ranges allow for concurrent use
of both the ASM and SSM service models. Therefore, while we can
achieve the PIM join efficiency in the non-SSM address range with
IGMPv3, it is not possible to prevent the creation of shared trees or
shared tree data delivery, and thus cannot provide for certain types
of access control or assume per-source unrestricted address use as
with the SSM address range.
In case of IPv6, [HABE1] has defined an extension to the addressing
architecture to allow for unicast prefix-based multicast addresses.
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In this case, bytes 0-3 (starting from the least significant byte) of
the IP address is used to specify a multicast group id, bytes 4-11 is
be used to specify a unicast address prefix (of up to 64 bits) that
owns this multicast group id, and byte 12 is used to specify the
length of the prefix. A source-specific multicast address can be
specified by setting both the prefix length field and the prefix
field to zero. Thus IPv6 allows for 2^32 SSM addresses per scope for
every source, while IPv4 allows 2^24 addresses per source.
5.2 Channel Discovery
In case of ASM, receivers need to know only the group address for
a specific session. In the IGMPv2/PIM-SM/MSDP architecture,
designated routers discover an active source via PIM-SM and MSDP,
and then graft themselves to the multicast forwarding tree rooted
at that source.
In case of the SSM, an application on an end-host must know both
the SSM address G and the source address S before subscribing to a
channel. Thus the function of channel discovery becomes the
responsibility of applications. This information can be made
available in a number of ways, including via web pages, sessions
announcement applications, etc. The exact mechanisms for doing
this is outside the scope of this framework document.
5.3. SSM-Aware Applications
-- For applications sourcing content expected to be available to
receivers via SSM channels, the session must be advertised
including a source address as well as an SSM address.
-- Applications expecting to subscribe to an SSM channel must be
capable of specifying a source address in addition to an SSM
address. In other words, the application must be "SSM-aware".
Specific API requirements are identified in [THAL00].
5.4. IGMPv3 for SSM
The currently deployed version of IGMP (IGMPv2) allows end-hosts
to register their interest in a multicast group by specifying a
class-D IP address for IPv4. However in order to implement the SSM
service model, an end-host must specify a source's unicast address
as well as an SSM address. This capability is provided by the
recently proposed IGMP version 3 (IGMPv3). IGMPv3 supports "source
filtering", i.e., the ability of an end-system to express interest
in receiving data packets sent only by SPECIFIC sources, or from
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ALL BUT some specific sources. Thus, IGMPv3 provides a superset of
the capabilities required to realize the SSM service model. Hence
an upgrade from IGMPv2 to IGMPv3 is an essential change for
implementing SSM.
IGMPv3 requires the API to provide the following operation (or its
logical equivalent) [CAIN99]:
IPMulticastListen (Socket, IF, G, filter-mode, source-list)
As explained in the IGMPv3 specifications [CAIN99], the above
IPMulticastListen() operation subsumes the group-specific join and
leave operations of IGMPv2. Performing (S,G)-specific joins and
leaves is also trivial. A join operation is equivalent to :
IPMulticastListen (Socket,IF,G,INCLUDE,{S})
and a leave operation is equivalent to
IPMulticastListen (Socket,IF,G,EXCLUDE,{S})
There are a number of backward compatibility issues between IGMP
versions 2 and 3 which have to be addressed. There are also some
additional requirements for using IGMPv3 for the SSM address
range. A detailed discussion of these issues is provided in [SSM-
IGMPv3].
5.5 MLDv2 for SSM
The Multicast Listener Discovery (MLD) protocol used by an IPv6 router
to discover the presence of multicast listeners on its directly attached
links, and to discover the multicast addresses that are of interest to
those neighboring nodes. Version 1 of MLD [DEER99] is derived from
IGMPv2 and allows a multicast listener to specify the multicast group(s)
that it is interested in. Version 2 of MLD [VIDA01] is derived from, and
provides the same support for source-filtering as, IGMPv3.
5.6. PIM-SM Modifications for SSM
PIM-SM [PIM-SM-NEW] itself supports two types of trees, a shared tree
rooted at a core (RP), and a source-based shortest path tree. Thus
PIM-SM already supports source-based trees; however, PIM-SM is not
designed to allow a router to choose between a shared tree and a
source-based tree. In fact, a receiver always joins a PIM shared tree
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to start with, and may later be switched to a per-source tree by its
adjacent edge router.
A key to implementing SSM is eliminate the need for starting with a
shared tree and then switching to a source-specific tree. This
involves several changes to PIM-SM as described in [PIM-SM-NEW]. The
resulting PIM functionality is described as PIM-SSM. The most
important changes to PIM-SM with respect to SSM are as follows:
-- When a DR receives an (S,G) join request with the address G in
the SSM address range, it must initiate a (S,G) join and NEVER a
(*,G) join.
--Core routers (i.e. routers that do not have directly attached
hosts) must not propagate (*,G) joins for group addresses in the
SSM address range.
--Rendezvous Points (RPs) must not accept PIM Register messages or
(*,G) Join messages in the SSM address range.
The specific architectural issues associated with PIM-SSM and
IGMPv3/MLDv2 are detailed in [SSM-ARCH].
6. Interoperability with Existing Multicast Service Models
Interoperability with ASM is one of the most important issues in
moving to SSM deployment. ASM and SSM will always coexist; hence
there will be two service models for Internet multicast. SSM is the
ONLY service model for the SSM address range (232/8 for IPv4 and
FF::/8 for IPv6) - the correct protocol behaviour for this range is
specified in [SSM-ARCH]. The ASM service model will be offered for
the non-SSM adddress range, where receivers can issue (*,G) join
requests to receive multicast data. A receiver is also allowed to
issue an (S,G) join request in the non-SSM address range; however, in
that case there is no guarantee that it will receive service
according to the SSM model.
Another backward compatibility issue concerns the MSDP protocol,
which is used between PIM-SM rendezvous points (RPs) to discover
multicast sources across multiple domains. SSM obviates the needs for
MSDP, but MSDP is still required to support ASM for non-SSM class-D
IPv4 addresses. In order to ensure that SSM is the sole forwarding
model in 232/8, RPs must not accept, originate or forward MSDP SA
messages for the SSM address range [SSM-BCP].
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7. Security Considerations
SSM does not introduce new security considerations for IP multicast.
It can help in preventing denial-of-service attacks resulting from
unwanted sources transmitting data to a multicast channel (S, G).
However no guarantee is provided.
8. Acknowledgments
We would like to thank Gene Bowen, Ed Kress, Bryan Lyles, Sue Moon
and Timothy Roscoe at Sprintlabs, Hugh Holbrook, Isidor Kouvelas,
Tony Speakman and Nidhi Bhaskar at Cisco Systems for participating in
lengthy discussions and design work on SSM, and providing feedback on
this document. Thanks are also due to Mujahid Khan and Ted Seely at
SprintLink, Tom Pusateri at Juniper Networks, Bill Fenner at AT&T
Research, Kevin Almeroth at the University of California Santa
Barbara, Brian Levine at the University of Massachusetts Amherst,
Brad Cain at Cereva Networks and Hugh LaMaster at NASA for their
valuable insights and continuing support.
9. References:
[EXPRESS] H. Holbrook and D.R. Cheriton. IP Multicast Channels :
EXPRESS Support for Large-scale Single-Source Applications. In
Proceedings of SIGCOMM 1999.
[IANA-ALLOCATION] Internet Assigned Numbers Authority.
http://www.isi.edu/in-notes/iana/assignments/multicast-addresses.
[RFC2236] W. Fenner. Internet Group Management Protocol, Version 2.
Request For Comments 2236.
[IGMPv3] B. Cain and S. Deering, I. Kouvelas and A. Thyagarajan.
Internet Group Management Protocol, Version 3. Work in Progress.
[SSM-IGMPv3] H. Holbrook and B. Cain. IGMPv3 for SSM. Work in
Progress.
[SSM-ARCH] H. Holbrook and B. Cain. Source-Specific Multicast for
IP. Work in Progress.
[IPMULTICAST] S. Deering and D. Cheriton. Multicast Routing in
Datagram Networks and Extended LANs. ACM Transactions on Computer
Systems, 8(2):85-110, May 1990.
[PIM-ARCH] S. Deering et al. PIM Architecture for Wide-Area
Bhattacharyya et. al. [Page 11]
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Multicast Routing. IEEE/ACM Transaction on Networking, pages 153-162,
April 1996.
[RFC2362] D. Estrin et al. Protocol Independent Multicast - Sparse
Mode (PIM-SM) : Protocol Specification. Request for Comments, 2362.
[PIM-SM] Bill Fenner, et al. Protocol Independent Multicast - Sparse
Mode (PIM-SM) : Protocol Specifications (Revised). Work in Progress.
[PIM-DM] S. Deering et al. Protocol Independent Multicast Version 2
Dense Mode Specification. Work in Progress.
[MSDP] Farinacci et al. Multicast Source Discovery Protocol. Work in
Progress.
[MCAST-DEPLOY] C. Diot, B. Levine, B. Lyles, H. Kassem and D.
Balensiefen. Deployment Issues for the IP Multicast Service and
Architecture. In IEEE Networks Magazine's Special Issue on
Multicast, January, 2000.
[SSM-RULES] H. Sandick and B. Cain. PIM-SM Rules for Support of
Single-Source Multicast. Work in Progress.
[MSF-API] Dave Thaler, Bill Fenner and Bob Quinn. Socket Interface
Extensions for Multicast Source Filters. Work in Progress.
[RFC2770] GLOP Addressing in 233/8. Request For Comments 2770.
[RCVR-INTEREST] B. Levine et al. Consideration of Receiver Interest
for IP Multicast Delivery. In Proceedings of IEEE Infocom, March
2000.
[SSM-BCP] G. Shepherd et al. Source-Specific Protocol Independent
Multicast in 232/8. Work in Progress.
[RFC2710] S. Deering, W. Fenner and B. Haberman. Multicast Listener
Discovery for IPv6. Request for Comments 2710.
[MLDv2] R. Vida, et. al.
Multicast Listener Discovery Version 2 (MLDv2) for IPv6.
Work in progress.
[SSM-IPv6] B. Haberman and D. Thaler.
Unicast-Prefix-Based IPv6 Multicast Addresses. Work in
Progress.
[IPSEC] S. Kent, R. Atkinson.
Security Architecture for the Internet Protocol. Request for
Bhattacharyya et. al. [Page 12]
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Comments 2401.
[IPv6-ALLOC] B. Haberman.
Dynamic Allocation Guidelines for IPv6 Multicast Addresses.
Work in Progress.
12. Authors' Address:
Supratik Bhattacharyya
Christophe Diot
Sprint Advanced Technology Labs
One Adrian Court
Burlingame CA 94010 USA
{supratik,cdiot}@sprintlabs.com
http://www.sprintlabs.com
Leonard Giuliano
Greg Shepherd
Juniper Networks, Inc.
1194 North Mathilda Avenue
Sunnyvale, CA 94089 USA
{lenny,shep}@juniper.net
Robert Rockell
Sprint E|Solutions
Reston Virginia USA
rrockell@sprint.net
John Meylor
Dave Meyer
Cisco Systems
San Jose CA USA
{jmeylor,dmm,shep@cisco.com}
Brian Haberman
Nortel Networks
haberman@nortelnetworks.com
Bhattacharyya et. al. [Page 13]
