Network Working Group Dino Farinacci Internet Draft Cisco Systems David Meyer University of Oregon Yakov Rekhter Cisco Systems Expiration Date: February 1997 August 1997 Intra-LIS IP multicast among routers over ATM using Sparse Mode PIM 1. Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 2. Abstract This document describes how intra-LIS IP multicast can be efficiently supported among routers over ATM without using the Multicast Address Resolution Server (MARS). The method described here is specific to Sparse Mode PIM [PIM-SM], and relies on the explicit join mechanism inherent in PIM-SM to notify routers when they should create group specific point-to-multipoint VCs. Dino Farinacci, David Meyer, Yakov Rekhter [Page 1] Internet Draft draft-ietf-ion-intralis-multicast-01.txt August 1997 3. Overall model This document focuses on forwarding of multicast traffic among PIM-SM routers connected to an ATM network. Routers on an ATM network are partitioned into Logical IP Subnets, or LISs. This document deals with handling multicast within a single LIS. Handling inter-LIS multicast traffic, including handling shortcuts, is outside the scope of this document. In addition, this document does not address forwarding of multicast traffic to or from hosts connected to an ATM network. 4. Router behavior This document requires that each router within a LIS knows IP and ATM addresses of all other routers within the LIS. The mapping between IP and ATM addresses may be provided by an ARP server [RFC1577], or by any other means (e.g., static configuration). Each PIM router within a LIS is required to maintain a single (shared) point-to-multipoint distribution VC rooted at the router with all other PIM routers in the LIS as the leaf nodes. The VC is expected to be used for forwarding of multicast traffic (both data and control) among routers within the LIS. For example, this VC would be used for distributing PIM [PIM-SM] control messages (Join/Prune messages). In addition, if a PIM router receives a IGMP report from an non-PIM neighbor, then the router may add the reporter to the existing shared distribution VC or to the group specific distribution VC (if it exists). The PIM router may also create a specific VC for this IGMP proxy. 4.1. Establishing Dedicated, Per Group Point-to-Multipoint VCs Routers may also maintain group specific, dedicated point-to- multipoint VCs. In particular, an upstream router for a group may choose to become the root of a group specific point-to-multipoint VC whose leaves are the downstream routers that have directly connected or downstream receivers for the group. While the criteria for establishing a group specific point-to-multipoint VC are local to a router, issues such as the volume of traffic associated with the group and the fanout factor within the LIS should be considered. Finally, note that a router must minimally support a single shared point-to-multipoint VC for distribution of control messages and data (to all group addresses). Dino Farinacci, David Meyer, Yakov Rekhter [Page 2] Internet Draft draft-ietf-ion-intralis-multicast-01.txt August 1997 A router can choose to establish a dedicated point-to-multipoint VC (or add another leaf to an already established dedicated point-to- multipoint VC) when it receives a PIM Join or IGMP report messages from another device in the same LIS. When a router that is the root of a point-to-multipoint VC receives PIM Prune message or IGMP leave, it removes the originator of the message from its dedicated point- to-multipoint VC. 4.2. Switching to a Source-Rooted Tree If at least one of the routers within a LIS decides to switch to a source-rooted tree (by sending (S,G) PIM Joins), then all other routers within the LIS that have downstream members for G should switch to that source-rooted tree as well. Since a router that switches to a source-rooted tree sends PIM Join messages for (S,G) over its shared point-to-multipoint VC, the other routers within the LIS are able to detect this. Once a router that has downstream members for G detects this, the router should send (S,G) PIM Join message as well (otherwise the router may receive duplicate traffic from S). Note that it is possible for a non-PIM router in the LIS to fail to receive data if the injection point moves to router to which there is not an existing VC. 4.2.1. Adding New Members to a Source-Rooted Tree As mentioned above, this document requires that once one router in a LIS decides to switch to the source tree for some (S,G), all routers in the LIS that have downstream members must also switch to the (S,G) source tree. Now, when a new router wants to receive traffic from G, it starts sending (*,G)-Joins on it's shared point-to-multipoint VC toward the RP for G. The root of the (S,G)-source-rooted tree will know to add the new router to the point-to-multipoint VC servicing the (S,G)-source-rooted tree by observing the (*,G)-joins on it's shared point-to-multipoint VC. However, the new router must also switch to the (S,G)-source-rooted tree. In order to accomplish this, the newly added router must: (i). Notice that it has been added to a new point-to-multipoint VC (ii). Notice (S,G) traffic coming down this new point-to-multipoint VC Dino Farinacci, David Meyer, Yakov Rekhter [Page 3] Internet Draft draft-ietf-ion-intralis-multicast-01.txt August 1997 (iii). Send (S,G) joins toward S, causing it to switch to the source-rooted tree. The router learns that the VC is used to distribute (S,G) traffic in the previous steps. 4.3. Handing the "Packet Reflection" Problem When a router receives a multicast packet from another router in its own LIS, the router should not send the packet on any of the routers distribution point-to-multipoint VCs associate with the LIS. This eliminates the problem of "packet reflection". Sending the packet on the routers' distribution VCs associated with other LISs is controlled by the multicast routing procedures. 5. Brief Comparison with MARS The intra-LIS multicast scheme described in this document is intended to be a less complex solution to an important subset of the functionality provided by the Multicast Address Resolution Server, or MARS [MARS]. In particular, it is designed to provide intra-LIS multicast between routers using PIM-SM, and does not consider the case of host-rooted point-to-multicast multicast distribution VCs. Although MARS supports both of the current schemes for mapping the IP multicast service model to ATM (multicast server and meshes of point-to-multipoint VCs), it does so at at cost and complexity higher than of the scheme described in this document. In addition, MARS requires new encapsulations, whereas this proposal works with either LLC/SNAP or with NLPID encapsulation. Another important difference is that MARS allows point-to-multipoint VCs rooted either at a source or at a multicast server (MCS). The approach taken here is to constrain complexity by focusing on PIM-SM (taking advantage of information available in explicit joins), and by allowing point-to-multipoint VCs to be rooted only at the routers (which is roughly analogous to the complexity and functionality of rooting point-to-multipoint VCs at the sources). In summary, the method described in this document is designed for the router-to-router case, and takes advantage of the explicit-join mechanism inherent in PIM-SM to provide a simple mechanism for intra-LIS multicast between routers. MARS, on the other hand, accepts different tradeoffs in complexity-functionality design space. In particular, while the MARS paradigm provides a general neighbor discovery mechanism, allows host to participate, and is protocol Dino Farinacci, David Meyer, Yakov Rekhter [Page 4] Internet Draft draft-ietf-ion-intralis-multicast-01.txt August 1997 independent, it does so at considerable cost. 6. Security Considerations Security issues are not discussed in this document. 7. References [MARS] G. Armitage, "Support for Multicast over UNI 3.0/3.1 based ATM Networks.", RFC2022, November 1996. [PIM-SM] Estrin, D, et. al., "Protocol Independent Multicast Sparse Mode (PIM-SM): Protocol Specification", draft-ietf-idmr-PIM-SM-spec-09.ps, October, 1996. 8. Acknowledgments Petri Helenius provided several insightful comments on earlier versions of this document. 9. Author Information Dino Farinacci, David Meyer, Yakov Rekhter [Page 5] Internet Draft draft-ietf-ion-intralis-multicast-01.txt August 1997 Dino Farinacci Cisco Systems 170 Tasman Dr. San Jose, CA 95134 Phone: (408) 526-4696 e-mail: dino@cisco.com David Meyer University of Oregon 1225 Kincaid St. Eugene, OR 97403 Phone: (541) 346-1747 e-mail: meyer@antc.uoregon.edu Yakov Rekhter cisco Systems, Inc. 170 Tasman Dr. San Jose, CA 95134 Phone: (914) 528-0090 email: yakov@cisco.com