Proto col Indep endent MulticastSparse Mo de PIMSM Motivation and Arc hitecture Stephen Deering Deb orah Estrin arinacci Dino F Xerox PARC Computer Science DepartmentISI Cisco Systems Inc Coyote Hill Road University of Southern California West Tasman Drive Palo Alto CA Los Angeles CA San Jose CA deeringparcxeroxcom estrinuscedu dinociscocom Handley Ahmed Helmy Van Jacobson Mark Department of Computer Science Lawrence Berkeley Lab oratory Computer Science Department University College London Cyclotron Road University of Southern California wer Street Go Berkeley CA Los Angeles CA BT vaneelblgov London WCE ahelmycatarinauscedu UK mhandleycsuclacuk Chinggung Liu Puneet Sharma David Thaler Computer Science Department Computer Science Department EECS Department higan University of Southern California University of Southern California University of Mic Ann Arb or MI Los Angeles CA Los Angeles CA thalerdeecsumichedu charleycatarinauscedu puneetcatarinauscedu Liming W ei Cisco Systems Inc West Tasman Drive San Jose CA lweiciscocom draftietfidmrpimarc hps Octob er Status of This Memo This do cument is an Internet Draft Internet Drafts are working do cuments of the Internet Engineering Task orce IETF its Areas and its Working Groups Note that other groups may also distribute working do cuments F as Internet Drafts Internet Drafts are draft do cuments valid for a maximum of six months Internet Drafts may b e up dated replaced or obsoleted by other do cuments at any time It is not appropriate to use Internet Drafts as orking draft or work in progress reference material or to cite them other than as a w draftietfidmrpimarchps Please check the ID abstract listing contained in each Internet Draft directory to learn the current status of this or any other Internet Draft draftietfidmrpimarchps Abstract Traditional multicast routing mechanisms eg DVMRP and MOSPF were intended for use within regions where groups are widely represented or bandwidth is universally plentiful When group memb ers and senders to those group memb ers are distributed sp arsely across a wide area these schemes are not ecient data packets or memb ership rep ort information are p erio dically sent over lead to receivers or senders resp ectively This characteristic lead the Internet many links that do not community to investigate multicast routing architectures that eciently establish distribution trees across widearea internets where many groups are sparsely represented and where bandwidth is not uniformly plentiful due to the distances and multiple administrations traversed Eciency is evaluated across the entire in terms of the state control message pro cessing and data packet pro cessing required network in order to deliver data packets to the memb ers of the group Mo de PIMSM architecture The Proto col Indep endent MulticastSparse a maintains the traditional IP multicast service mo del of receiverinitiated memb ership that propagate hopbyhop from memb ers directly connected routers toward b uses explicit joins the distribution tree c builds a shared multicast distribution tree centered at a Rendezv ous Point and then builds trees for those sources whose data trac warrants it sourcesp ecic d is not dep endent on a sp ecic unicast routing proto col and e uses softstate mechanisms to adapt to underlying network conditions and group dynamics y and scaling prop erties of this architecture make it well suited to large The robustness exibilit heterogeneous internetworks This do cument motivates and describ es the PIMSM architecture Companion do cuments describ e the detailed proto col mechanisms for PIMSM and PIMDM resp ectively draftietfidmrpimarchps Intro duction This do cument describ es an architecture for eciently routing to multicast groups that may span wide area and interdomain internets We refer to the approach as Proto col Indep endent MulticastSparse Mo de PIMSM b ecause it is not dep endent on any particular unicast routing proto col Throughout this do cument we will use the shorter term PIM to mean PIMSM When we are referring to the PIM Dense Mo de proto col we will say PIMDM explicitly t innovation in this architecture is the ecient supp ort of sparse wide area groups The most signican This sp arse mode SM of op eration complements the traditional densemode approach to multicast as develop ed by Deering and implemented in MOSPF and DVMRP routing for campus networks These traditional dense mo de multicast schemes were intended for use within regions where a group is widely represented or bandwidth is universally plentiful Ho wever when group memb ers and senders across a wide area these schemes are not ecient data packets to those groups are distributed sparsely in the case of DVMRP or memb ership rep ort information in the case of MOSPF are o ccasionally sent over many links that do not lead to receivers or senders resp ectively The purp ose of this work is to develop a multicast routing architecture that eciently establishes distribution trees even when memb ers is evaluated in terms of the state control message and data packet are sparsely distributed Eciency across the entire network in order to deliver data packets to the memb ers of the group overhead required Denition of Terms Glossary Following is a list of terms and denitions used throughout this do cument in alphab etical order This is a subset of the glossary list that app ears in the proto col sp ecication Asserts The pro cess of cho osing a single router to forward multicast packets from a particular segment The need for Asserts arises when a LAN segment has source onto a particular LAN multiple directlyconnected routers with routes to the source Bo otstrap router BSR A BSR is a dynamically elected router within a PIM domain It is resp onsible for constructing the RPSet and originating Bo otstrap messages CandidateBSR CBSR A CBSR is a router congured to participate in the BSR election and act as BSRs if elected Densemo de DM A generic term referring to a multicast routing proto col that is optimized for dense groups DVMRP MOSPF and Densemo de PIM are examples Designated Router DR The DR is the highest IP addressed PIM router on a multiaccess LAN Normally the DR sets up multicast route entries and sends corresp onding JoinPrune and Register messages on b ehalf of directlyconnected receivers and sources resp ectively The DR may or may not b e the same router as the IGMP Querier The DR may or may not b e the longterm lasthop router for the group or a particular source that is sending to the group a router on the LAN that has a lower metric route to the data source or to the groups RP may take over that role Incoming interface iif The iif of a multicast route entry indicates the interface from which multicast data packets are accepted for forwarding The iif is initialized when the entry is created Join list The Join list is one of two lists of IP unicast addresses that is included in a JoinPrune message each address refers to a source or RP It indicates those sources or RPs to which down stream receivers wish to join draftietfidmrpimarchps Lasthop router The lasthop router is the router which forwards multicast data packets to directlyconnected memb er hosts In general the lasthop router is the DR for the LAN However under various conditions describ ed in this do cument a parallel router connected to the same LAN may take over as the lasthop router in place of the DR Memb er A host that desires to receive multicast datagrams for a group This host need not b e a sender to the group A Memb er is synonymously called a R eceiver interface oif list Each multicast route entry has an oif list containing the outgoing Outgoing interfaces to which multicast packets matching that entry should b e forwarded Prune List The Prune list is the second list of IP unicast addresses that is included in a JoinPrune message It indicates those sources or RPs from which downstream receivers wish to prune PIM Multicast Border Router PMBR A PMBR connects a PIM domain to other multicast routing domains Rendezvous Point RP Each multicast group has a sharedtree via which receivers hear of sources The RP is the ro ot of this p ergroup shared tree called the RPTree CandidateRPs are routers congured to participate as RPs for some or all groups RPSet The BSR for a PIM region constructs a set of RP IP addresses based on CandidateRP advertisements received The RPSet information is distributed to all PIM routers in a domain in a Bo otstrap message Reverse Path Forwarding RPF RPF is used to select the appropriate incoming interface for is the the nexthop router used a multicast route entry The RPF neighb or for an IP address X to forward packets toward X The RPF interface is the interface to that RPF neighb or In the common case this is the next hop used by the unicast routing proto col for sending unicast packets toward X For example in cases where unicast and multicast routes are not congruent it can b e t dieren A multicast route entry is state maintained in a router along the distribution tree Route entry and is created and up dated based on incoming control messages and in some cases data packets The route entry may b e dierent from the forwarding entry the latter is used to forward data packets in real time Typically a forwarding entry is not created until data packets arrive the forwarding entrys iif and oif list are copied from the route entry and the forwarding entry may b e and recreated at will ushed Shared Tree RP tree The set of paths connecting all receivers of a group