INTERNET-DRAFT Supratik Bhattacharyya
Expires 04 May 2003 Christophe Diot
Sprint ATL
Leonard Giuliano
Juniper Networks
Rob Rockell
Sprint
John Meylor
Cisco Systems
David Meyer
Sprint
Greg Shepherd
Juniper Networks
Brian Haberman
Caspian Networks
04 November 2002
An Overview of Source-Specific Multicast (SSM)
<draft-ietf-ssm-overview-04.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.
Bhattacharyya et. al. [Page 1]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
Abstract
The purpose of this document is to provide an overview of Source-
Specific Multicast (SSM) and issues related to its deployment. It
discusses how the SSM service model addresses the challenges faced in
inter-domain multicast deployment, changes needed to routing protocols
and applications to deploy SSM and interoperability issues with current
multicast service models.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
1.Introduction
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. An IPv4 address range has been reserved by IANA for use
by the SSM service. An SSM destination address range already exists
for IPv6. A source S transmits IP datagrams to an SSM destination
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 IP multicast datagrams is rooted
at the source S, and is constructed using the PIM Sparse Mode [9]
protocol.
This document is not intended to be a standard for Source-Specific
Multicast (SSM). Instead, its goal is to serve as an introduction to
SSM and and its benefits for anyone interested in deploying SSM
services. It provides an overview of SSM and and 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
multicast service model defined by RFC 1112 are also discussed.
Bhattacharyya et. al. [Page 2]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
2. 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 [27]. An IP datagram is transmitted to a "host
group", a set of zero or more end-hosts (or routers) identified by a
single IP destination address (224.0.0.0 through 239.255.255.255 for
IPv4). End-hosts may join and leave the group any time, and there is
no restriction on their location or number. Moreover, this model
supports multicast groups with arbitrarily many senders - any end-host
(or router) may transmit to a host group, even if it is not a member
of that group.
Source-Specific Multicast (SSM) : This is the multicast service model
defined in [5]. An IP datagram is transmitted by a source S to an SSM
destination address G, and receivers can receive this datagram by
subscribing to channel (S,G). SSM provides host applications with a
"channel" abstraction, in which each channel has exactly one source
and any number of receivers. SSM is derived from earlier work in
EXPRESS [1].The address range 232/8 has been assigned by IANA for SSM
service in IPv4. For IPv6, the range FF3x::/96 is defined for SSM
services [23].
Source-Filtered Multicast (SFM) : This is a variant of the ASM service
model, and uses the same address range as ASM
(224.0.0.0-239.255.255.255). It extends the ASM service model as
follows. 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.
Support for source filtering is provided by version 3 of the Internet
Group Management Protocol (or IGMPv3) [3] for IPv4, and version 2 of
the Multicast Listener Discovery (or MLDv2) [22] protocol for IPv6.
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". Note that while SFM is a different model than ASM from a
receiver standpoint, there is no distinction between the two for a
sender.
For the purpose of this document, we treat the scoped multicast model of
[12] to be a variant of ASM since it does not explicitly restrict the
number of sources, but only requires that they be located within the
scope zone of the group.
Bhattacharyya et. al. [Page 3]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
3. The IGMP/PIM-SM/MSDP/MBGP Protocol Suite for ASM
As of this writing, all multicast-capable networks support the ASM
service model. One of the most common multicast protocol suites for
supporting ASM consists of IGMP version 2 [28], PIM-SM [8,9], MSDP
[13] and MBGP [29] protocols. IGMPv2 [2] is the most commonly used
protocol for hosts to specify membership in a multicast group, and
nearly all multicast routers support (at least) IGMPv2. In case of
IPv6, MLDv1 [21] is the commonly used protocol.
Although a number of protocols such as PIM-DM [10], CBT [26,11],
DVMRP [6], etc. exist for building multicast tree among all receivers
and sources in the same administrative domain, PIM-SM [8,9] is the
most widely used protocol. PIM-SM builds a spanning multicast tree
rooted at a core rendezvous point or RP for all group members within
a single administrative domain. A 'first-hop' router adjacent to a
multicast source sends the source's traffic to the RP for its domain.
The RP forwards the data down the shared spanning tree to all
interested receivers within the domain. PIM-SM also allows receivers
to switch to a source-based shortest path tree.
As of this writing, multicast end-hosts with SFM capabilities are not
widely available. Hence a client can only specify interest in an
entire host group and receives data sent from any source to this
group.
Inter-domain multicast service (i.e., where sources and receivers are
located in multiple domains) requires additional protocols - MSDP
[13] and MBGP [29] are the most commonly used ones. An RP uses the
MSDP [13] protocol to announce multicast sources to RPs in other
domains. When an RP discovers a source in a different domain
transmitting data to a multicast group for which there are interested
receivers in its own domain, it joins the shortest-path source based
tree rooted at that source. It then redistributes the data received
to all interested receivers via the intra-domain shared tree rooted
at itself.
The MBGP protocol [29] defines extensions to the BGP protocol to
support the advertisement of reachability information for multicast
routes. This allows an autonomous system (AS) to support incongruent
unicast and multicast routing topologies, and thus implement separate
routing policies for each.
4. Problems with Current Architecture
There are several deployment problems associated with current
Bhattacharyya et. al. [Page 4]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
multicast architecture:
A) Address Allocation :
Address allocation is one of core deployment challenges posed by
the ASM service model. The current multicast architecture does not
provide a deployable solution to prevent address collisions among
multiple applications. The problem is much less serious for IPv6
than for IPv4 since the size of the multicast address space is
much larger. A static address allocation scheme, GLOP [18] has
been proposed 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 [14] are generally perceived as
being too complex (with respect to the dynamic nature of multicast
address allocation) for widespread deployment.
B) Lack of Access control :
In the ASM service model, a receiver cannot 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. Moreover, even when a source is allocated a multicast
group address to transmit on, it has no way of enforcing that no
other source will use the same address. This is true even in the
case of IPv6, where address collisions are less likely due to the
much larger size of the address space.
C) 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 are not necessary.
5. Source Specific Multicast (SSM) : Benefits and Requirements
As mentioned before, the Source Specific Multicast (SSM) service
model defines a "channel" identified by an (S,G) pair, where S is a
source address and G is an SSM destination address. Channel
subscriptions are described using an SFM-capable group management
protocol such as IGMPv3 or MLDv2. Only source-based forwarding trees
are needed to implement this model.
The SSM service model alleviates all of the deployment problems
described earlier :
Bhattacharyya et. al. [Page 5]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
A) Address Allocation : SSM defines channels on a per-source
basis, i.e., the channel (S1,G) is distinct from the channel
(S2,G), where S1 and S2 are source addresses, and G is an SSM
destination address. This averts the problem of global allocation
of SSM destination addresses, and makes each source independently
responsible for resolving address collisions for the various
channels that it creates.
B) Access Control : SSM lends itself to an elegant solution to the
access control problem. When a receiver subscribes to an (S,G)
channel, it receives data sent by a only the source S. In
contrast, any host can transmit to an ASM host group. At the same
time, when a sender picks a channel (S,G) to transmit on, it is
automatically ensured that no other sender will be transmitting on
the same channel (except in the case of malicious acts such as
address spoofing). This makes it much harder to "spam" an SSM
channel than an ASM multicast group.
C) Handling of well-known sources : SSM requires only source-based
forwarding trees; this eliminates the need for a shared tree
infrastructure. In terms of the IGMP/PIM-SM/MSDP/MBGP protocol
suite, 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. Note that MBGP is
still required for distribution of multicast reachability
information.
6. SSM Framework
Figure 1 illustrates the elements in an end-to-end implementation
framework for SSM :
--------------------------------------------------------------
IANA assigned 232/8 for IPv4 ADDRESS ALLOCATION
FF3x::/96 for IPv6
--------------------------------------------------------------
|
v
+--------------+ session directory/web page
| source,group | SESSION DESCRIPTION
--------------------------------------------------------------
^ |
Query | | (S,G)
| v
+-----------------+ host
| SSM-aware app | CHANNEL DISCOVERY
Bhattacharyya et. al. [Page 6]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
--------------------------------------------------------------
| SSM-aware app | SSM-AWARE APPLICATION
--------------------------------------------------------------
| 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
+-----------+ Backbone 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 :
6.1 Address Allocation
For IPv4, the address range of 232/8 has been assigned by IANA for
SSM. To ensure global SSM functionality in 232/8, including in
networks where routers run non-SFM-capable protocols, operational
policies are being proposed [20] which recommend that routers should
not send SSM traffic to parts of the network that do not have channel
subscribers.
Note that IGMPv3/MLDv2 does not limit (S,G) joins to only the 232/8
range. However, SSM service, as defined in [5], is available only in
this address range for IPv4.
In case of IPv6, [25] has defined an extension to the addressing
architecture to allow for unicast prefix-based multicast addresses.
Bytes 0-3 (starting from the least significant byte) of the IP
address are used to specify a multicast group id, bytes 4-11 are 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 is specified by setting
Bhattacharyya et. al. [Page 7]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
both the unicast address prefix field and the prefix length field to
zero.
6.2 Session Description and Channel Discovery
An SSM receiver application must know both the SSM destination
address G and the source address S before subscribing to a
channel. Channel discovery is the responsibility of applications.
This information can be made available in a number of ways,
including via web pages, sessions announcement applications, etc.
This is similar to what is used for ASM applications where a
multicast session needs to be announced so that potential
subscribers can know of the multicast group adddres, encoding
schemes used, etc. In fact, the only additional piece of
information that needs to be announced is the source address for
the channel being advertised. However, the exact mechanisms for
doing this is outside the scope of this framework document.
6.3. SSM-Aware Applications
There are two main issues in making multicast applications "SSM-
aware":
-- An application that wants to received an SSM session must first
discover the channel address in use. Any of the mechanisms
described in Section 5.2 can be used for this purpose.
-- A receiving application must be able to specify both a source
address and a destination address to the network layer protocol
module on the end-host. In other words, the application must be
"SSM-aware".
Specific API requirements are identified in [17]. [17] describes a
recommended application programming interface for a host operating
system to support the SFM service model. Although it is intended
for SFM, a subset of this interface is sufficient for supporting
SSM.
6.4. IGMPv3/MLDv2 Host Reporting and Querier
In order to use SSM service, an end-host must be able to specify a
channel address, consisting of a source's unicast address and an
SSM destination address. IGMP version 2 [28] and MLD version 1
[21] allows an end-host to specify only a destination multicast
address. The ability to specify an SSM channel address c is
provided by IGMP version 3 [3] and MLD version 2 [22]. These
Bhattacharyya et. al. [Page 8]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
protocols support "source filtering", i.e., the ability of an end-
system to express interest in receiving data packets sent only by
SPECIFIC sources, or from ALL BUT some specific sources. In fact,
IGMPv3 provides a superset of the capabilities required to realize
the SSM service model.
A detailed discussion of the use of IGMPv3 in the SSM destination
address range is provided in [4].
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
[21] is derived from IGMPv2 and does not provide the source
filtering capability required for the SSM service model. Version 2
of MLD [22] is derived from, and provides the same support for
source-filtering as, IGMPv3. Thus IGMPv3 (or MLDv2 for IPv6)
provides a host with the ability to request the network for an SSM
channel subscription.
6.5. PIM-SSM Routing
[9] provides guideliness for how a PIM-SM implementation should
handle source-specific host reports as required by SSM. Earlier
versions of the PIM protocol specifications did not describe how to
do this.
The router requirements for operation in the SSM range are detailed
in [5]. These rules are primarily concerned with preventing ASM-style
behaviour in the SSM address range. In order to comply with [5]
several changes to the PIM-SM protocol are required, as described in
[9].The most important changes in PIM-SM required for compliance with
[5] are :
-- 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.
--Backbone 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.
Note that only a small subset of the full PIM-SM protocol
functionality is needed to support the SSM service model. This subset
is explicitly documented in [9].
Bhattacharyya et. al. [Page 9]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
7. Interoperability with Existing Multicast Service Models
Interoperability with ASM is one of the most important issues in
moving to SSM deployment, since both models are expected to be used
at least in the foreseeable future. SSM is the ONLY service model for
the SSM address range - the correct protocol behaviour for this range
is specified in [5]. 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 interoperability issue concerns the MSDP protocol, which is
used between PIM-SM rendezvous points (RPs) to discover multicast
sources across multiple domains. MSDP is not needed for SSM, but is
needed if ASM is supported. [20] specifies operational
recommendations to help ensure that MSDP does not interfere with the
ability of a network to support the SSM service model. Specifically,
[20] states that RPs must not accept, originate or forward MSDP SA
messages for the SSM address range [20].
8. 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.
9. Acknowledgments
We would like to thank Gene Bowen, Ed Kress, Bryan Lyles 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.
Bhattacharyya et. al. [Page 10]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
10. References:
[1] H. Holbrook and D.R. Cheriton, "IP Multicast Channels : EXPRESS
Support for Large-scale Single-Source Applications", In Proceedings
of SIGCOMM 1999.
[2] W. Fenner, "Internet Group Management Protocol, Version 2", RFC
2236.
[2] B. Cain and S. Deering, I. Kouvelas and A. Thyagarajan, "Internet
Group Management Protocol, Version 3.", Work in Progress.
[4] H. Holbrook and B. Cain, "IGMPv3 for SSM", Work in Progress.
[5] H. Holbrook and B. Cain, "Source-Specific Multicast for
IP", Work in Progress.
[6] S. Deering and D. Cheriton,"Multicast Routing in Datagram
Networks and Extended LANs", ACM Transactions on Computer Systems,
8(2):85-110, May 1990.
[7] S. Deering et al., "PIM Architecture for Wide-Area Multicast
Routing", IEEE/ACM Transaction on Networking, pages 153-162, April
1996.
[8] D. Estrin et al., "Protocol Independent Multicast - Sparse Mode
(PIM-SM) : Protocol Specification", RFC 2362.
[9] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas, "Protocol
Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", Work In Progress, 2000.
[10] S. Deering et al., "Protocol Independent Multicast Version 2
Dense Mode Specification", Work in Progress.
[11] A. Ballardie, "Core-Based Trees (CBT) Multicast Routing
Architecture", RFC 2201.
[12] D. Meyer, "Adminstratively Scoped IP Multicast", RFC 2365.
[13] Farinacci et al., "Multicast Source Discovery Protocol", Work in
Progress.
[14] M. Handley, D. Thaler and D. Estrin, "The Internet Multicast
Address Allocation Architecture", Work in Progress.
[15] C. Diot, B. Levine, B. Lyles, H. Kassem and D. Balensiefen,
"Deployment Issues for the IP Multicast Service and Architecture", In
Bhattacharyya et. al. [Page 11]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
IEEE Networks Magazine's Special Issue on Multicast, January, 2000.
[16] H. Sandick and B. Cain, "PIM-SM Rules for Support of Single-
Source Multicast", Work in Progress.
[17] Dave Thaler, Bill Fenner and Bob Quinn, "Socket Interface
Extensions for Multicast Source Filters", Work in Progress.
[18] D. Meyer and P. Lothberg, "GLOP Addressing in 233/8", Request
For Comments 2770.
[19] B. Levine et al., "Consideration of Receiver Interest for IP
Multicast Delivery", In Proceedings of IEEE Infocom, March 2000.
[20] G. Shepherd et al., "Source-Specific Protocol Independent
Multicast in 232/8", Work in Progress.
[21] S. Deering, W. Fenner and B. Haberman, "Multicast Listener
Discovery for IPv6", RFC 2710.
[22] R. Vida, et. al., "Multicast Listener Discovery Version 2(MLDv2)
for IPv6", Work in progress.
[23] B. Haberman and D. Thaler, "Unicast-Prefix-Based IPv6 Multicast
Addresses", Work in Progress.
[24] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", Request for Comments 2401.
[25] B. Haberman, "Dynamic Allocation Guidelines for IPv6 Multicast
Addresses", Work in Progress.
[26] A. Ballardie, "Core-Based Trees (CBT Version 2) Multicast
Routing -- Protocol Specification", RFC 2189.
[27] S. Deering, "Host Extensions for IP Multicasting", RFC 1112.
[28] W. Fenner, "Internet Group Management Protocol, Version 2", RFC
2236.
[29] T. Bates, R. Chandra, D. Katz, and Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC 2283.
Bhattacharyya et. al. [Page 12]
INTERNET-DRAFT An Overview of SSM 4 04 November 2002
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
David Meyer
Sprint E|Solutions
Reston Virginia USA
{rrockell,dmm}@sprint.net
John Meylor
Cisco Systems
San Jose CA USA
jmeylor@cisco.com
Brian Haberman
Caspian Networks
bkhabs@nc.rr.com
Bhattacharyya et. al. [Page 13]
