Editing Protocol-Independent Multicast (section)

{{short description|Multicast routing protocol}}
[[image:IGMP basic architecture.png|thumb|400px|Example of a multicast network architecture]]

'''Protocol-Independent Multicast''' ('''PIM''') is a family of [[multicast routing protocol]]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 protocol]]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 table]]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 |date=9 December 2009 |publisher=[[Microsoft]] |access-date=2014-03-26}}</ref>{{ref RFC|4601}}
* '''PIM Dense Mode''' (PIM-DM) 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 RFC 3973. 
* '''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 RFC 5015.
* '''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 {{IETF RFC|3569}}.

PIM-SM is commonly used in [[IPTV]] systems for routing multicast streams between [[VLAN]]s, [[subnet]]s 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>