Multicasting is a technique that lets a single data source like a software update, large data file or online audiovisual stream reach a large number of recipients on the Internet simultaneously. DVMRP, or Distance Vector Multicast Routing Protocol, and PIM-DM, or Protocol-Independent Multicast Dense Mode, are two multicasting protocols used by network routers to efficiently connect a data source to all the recipients who request it.
DVMRP is the older of the two multicast protocols. It was introduced as Internet Engineering Task Force Request For Comments -- or IETF RFC 1075 -- in November 1998 by Stanford University. DVMRP was a reworking of the older Routing Information Protocol or RIP multicast protocol. PIM Dense was presented in January 2005 as RFC 3973 by wireless company NextHop Technologies. PIM Dense was not based upon DVMRP, but it shares some multicast techniques with the older protocol.
Both PIM Dense and DVMRP routers find network paths by flooding the network downstream from the data source with data packets. PIM Dense and DVMRP routers use different methods to remember which downstream targets are willing to accept data from the source. DVMRP only sends its packets to other DVMRP routers or child nodes which, in turn, try to distribute the data stream further downstream. A child mode can refuse the first packet sent by a DVMRP router by issuing a "prune" message. PIM Dense, on the other hand, sends data packets to every device on the network, including routers. A PIM Dense router will stop sending packets to a downstream device only if it receives a prune message that tells it a particular downstream target doesn't want any more data.
Routers using either PIM Dense or DVMRP protocol have to be able to communicate with the upstream data source or other networked devices in unicast mode. DVMRP uses a special unicast protocol that only works with other DVMRP routers. The PIM Dense protocol, however, does not have built-in unicasting and can work with whatever unicast protocol is used in your network by default.
Network Scaling Issues
PIM Dense is not the most efficient multicasting protocol, but it does scale better with large networks than DVMRP. While DVMRP calculates and records the network path of all its child nodes, PIM Dense only keeps track of those downstream devices it has had to prune. A DVMRP router also periodically refreshes its list of routes by reflooding downstream with more data packets. This reduces DVMRP's scalability even more. In addition, because PIM Dense is unicast protocol-independent, it can use more efficient techniques to find paths back to the upstream data source.
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