INTERNET-DRAFT Supratik Bhattacharyya Expires 18 February 2002 Christophe Diot Sprint ATL Leonard Giuliano Juniper Networks Rob Rockell Sprint E|Solutions John Meylor Cisco Systems David Meyer Sprint E|Solutions Greg Shepherd Juniper Networks Brian Haberman Nortel Networks 18 August 2001 An Overview of Source-Specific Multicast(SSM) Deployment 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] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 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 IPv4 address range 232/8 has been reserved by IANA fo 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. Although a number of protocols exists for constructing source-rooted forwarding trees, this document discusses one of 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 multicast service model 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 end-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. End-hosts may join and leave the group any time, and there is no restriction on their location or number. Moreover, any end-host 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 [SSM-ARCH]. 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 is derived from EXPRESS [EXPRESS] and supports one-to-many multicast.The address range 232/8 has been Bhattacharyya et. al. [Page 2] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 assigned by IANA [IANA-ALLOC] for SSM service in IPv4. For IPv6, the range FF3x::/12 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. The IGMP/PIM-SM/MSDP/MBGP Architecture for ASM All multicast-capable networks of today support the ASM service model. One of the most common multicast protocol architectures for supporting ASM in wide-area backbones consists of IGMP version 2 [IGMPv2], PIM-SM [PIM-SM,PIM-SM-NEW], MSDP [MSDP] and MBGP [MBGP] protocols. To become a member of a particular host group end-hosts report 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. 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, PIM-SM, PIM- SM-NEW, PIM-DM]. For scalability reasons, sparse-mode protocols (e.g., PIM-SM) are preferred over dense-mode protocols (e.g., DVMRP, PIM-DM) for deployment in large backbone networks (though many smaller networks deploy dense-mode protocols). PIM-SM, most widely deployed sparse-mode protocol, builds a spanning multicast tree rooted at a core rendezvous point or RP for all group members within a single administrative domain. Multicast sources within this domain send their data to this RP which forwards the data down the shared tree to interested receivers within the domain. 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. PIM-SM also allows receivers to switch to a source-based shortest path tree. An RP uses the MSDP [MSDP] protocol to announce multicast sources to Bhattacharyya et. al. [Page 3] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 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 [MBGP] defines extensions to the BGP protocol [BGP] 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. 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 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 a deployable 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 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. Bhattacharyya et. al. [Page 4] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 4. 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 : 4.1 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. Hence, it is more difficult to spam an SSM channel than an ASM host group. 4.2 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. 4.3 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. 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. 5. 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::/12 for IPv6 -------------------------------------------------------------- Bhattacharyya et. al. [Page 5] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 | 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 -------------------------------------------------------------- | 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 : 5.1 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 [SSM-BCP] which prevent data sent to 232/8 from being delivered 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 Bhattacharyya et. al. [Page 6] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 range. However, SSM service, as defined in [SSM-ARCH], is available only in this address range for IPv4. In case of IPv6, [HABE1] has defined an extension to the addressing architecture to allow for unicast prefix-based multicast addresses. 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. 5.2 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. 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 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 destination address. In other words, the application must be "SSM- aware". Specific API requirements are identified in [THAL00]. 5.4. IGMPv3/MLDv2 Host Reporting and Querier IGMP version 2 [IGMPv2] allows end-hosts to report 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 destination address. This capability is provided by 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 ALL BUT some specific sources. Thus, IGMPv3 provides a superset of the capabilities required to Bhattacharyya et. al. [Page 7] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 realize the SSM service model. There are a number of backward compatibility issues between IGMP versions 2 and 3 which have to be addressed. A detailed discussion of the use of IGMPv3 in the SSM destination address range is provided in [SSM-IGMPv3]. 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. 5.5. PIM-SSM Routing 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. The original PIM-SM [PIM-SM] did not allow a router to choose between a shared tree and a source-based tree. In fact, a receiver always joined a PIM shared tree to start with, and may later be switched to a per-source tree by its adjacent edge router. However, the more recent PIM-SM specification [PIM-SM-NEW] has support for source-specific join. Supporting SSM with PIM-SM involves several changes to PIM-SM as described in [PIM-SM-NEW]. The resulting PIM functionality is described as PIM-SSM. The specific architectural issues associated with PIM-SSM and IGMPv3/MLDv2 are detailed in [SSM-ARCH]. 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. --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. Bhattacharyya et. al. [Page 8] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 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 - 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]. 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. Bhattacharyya et. al. [Page 9] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 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 Multicast Routing. IEEE/ACM Transaction on Networking, pages 153-162, April 1996. [PIM-SM] D. Estrin et al. Protocol Independent Multicast - Sparse Mode (PIM-SM) : Protocol Specification. Request for Comments, 2362. [PIM-SM-NEW] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas. Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", Work In Progress, 2000. . [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. [MAAA] M. Handley, D. Thaler and D. Estrin. The Internet Multicast Address Allocation Architecture. Work in Progress (draft-ietf- malloc-arch-**.txt) June 2000. [MCAST-DEPLOY] C. Diot, B. Levine, B. Lyles, H. Kassem and D. Bhattacharyya et. al. [Page 10] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 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 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 Bhattacharyya et. al. [Page 11] INTERNET-DRAFT An Overview of SSM Deployment 18 May 2000 Leonard Giuliano Greg Shepherd Juniper Networks, Inc. 1194 North Mathilda Avenue Sunnyvale, CA 94089 USA {lenny,shep}@juniper.net Robert Rockell David Mayer Sprint E|Solutions Reston Virginia USA {rrockell,dmm}@sprint.net John Meylor Cisco Systems San Jose CA USA {jmeylor@cisco.com} Brian Haberman Nortel Networks haberman@nortelnetworks.com Bhattacharyya et. al. [Page 12]