Protocol-Independent Multicast: Difference between revisions

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[[image:IGMP basic architecture.png|thumb|400px|Example of a multicast network architecture]]

'''Protocol-Independent Multicast''' ('''PIM''') is a family of [[multicast routing protocolsprotocol]]s for [[Internet Protocol]] (IP) networks that provide [[Point-to-multipoint communication|one-to-many]] and [[many-to-many]] distribution of data over a [[LAN]], [[Wide area network|WAN]] or the [[Internet]]. It is termed ''protocol-independent'' because PIM does not include its own [[network topology|topology discovery]] mechanism, but instead uses routing information supplied by other [[routing protocolsprotocol]]s. PIM is not dependent on a specific unicast routing protocol; it can make use of any unicast routing protocol in use on the network. PIM does not build its own [[routing tablestable]]s. PIM uses the unicast routing table for [[reverse-path forwarding]].<ref name="Cisco Multicast">{{citation |url=https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/15-2_2_e/multicast/configuration_guide/b_mc_1522e_3750x_3560x_cg/b_mc_3750x_3560x_chapter_011.html |title=IP Multicast Routing Configuration Guide |publisher=[[Cisco]] |access-date=2017-05-27}}</ref>{{rp|56-57}}

There are four variants of PIM:

* '''PIM Sparse Mode''' (PIM-SM) explicitly builds unidirectional shared trees rooted at a ''rendezvous point'' (RP) per group, and optionally creates shortest-path trees per source. PIM-SM generally scales fairly well for wide-area usage.<ref>{{cite web |url=https://technet.microsoft.com/en-us/library/bb742462.aspx |title=PIM-SM Multicast Routing Protocol |publisher=[[Microsoft]] |access-date=2014-03-26}}</ref>

* '''PIM Dense Mode''' (PIM-DM) uses [[dense multicast]] routing. It implicitly builds shortest-path trees by flooding [[multicast]] traffic ___domain wide, and then pruning back branches of the tree where no receivers are present. PIM-DM is straightforward to implement but generally has poor scaling properties. The first multicast routing protocol, [[DVMRP]] used dense-mode multicast routing.<ref>{{cite web |url=http://www.multicasttech.com/faq/ |title=Frequently Asked Questions (FAQ) File for Multicasting |publisher=Multicast Tech |archive-url=https://web.archive.org/web/20110614164202/http://www.multicasttech.com/faq/ |archive-date=2011-06-14}}</ref> See the PIM Internet Standard <nowiki>RFC 3973</nowiki>.

* '''Bidirectional PIM''' (Bidir-PIM) explicitly builds shared bi-directional trees. It never builds a shortest path tree, so may have longer end-to-end delays than PIM-SM, but scales well because it needs no source-specific state.<ref name="Cisco Multicast"/>{{rp|70-73}} See Bidirectional PIM Internet Standard <nowiki>RFC 5015</nowiki>.

* '''PIM Source-Specific Multicast''' (PIM-SSM) builds trees that are rooted in just one source, offering a more secure and scalable model for a limited number of applications (mostly broadcasting of content). In SSM, 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). See informational <nowiki>RFC 3569</nowiki>.

PIM-SM is commonly used in [[IPTV]] systems for routing multicast streams between VLANs[[VLAN]]s, [[Subnetwork|Subnets]] or local area networks.<ref>{{cite web |url=https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Y.Sup16-201202-I!!PDF-E&type=items |title=Supplement on guidelines on deployment of IP multicast for IPTV content delivery |publisher=[[ITU-T]] |access-date=2014-03-23}}</ref>

==Versions==

There are two PIM versions. The versions are not directly compatible though may coexist on the same network. Network equipment may implement both versions. PIMv2 has the following improvements over PIMv1: A single RP is used per group. RP discovery is accomplished by a '''Bootstrap Router''' (BSR). Groups are either sparse or dense mode; Interfaces can be either. General improvements to protocol flexibility and efficiency.<ref name="Cisco Multicast">{{citation|title=IP Multicast Routing Configuration Guide|url=https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/15-2_2_e/multicast/configuration_guide/b_mc_1522e_3750x_3560x_cg/b_mc_3750x_3560x_chapter_011.html|publisher=[[Cisco]]|access-date=2017-05-27}}</ref>{{rp|59}}

==Sparse mode==