IP-Multicasting Glossary of Terms

Broadcast

One-to-all transmission where the source sends one copy of the message to all nodes, whether they wish to receive it or not. See Unicast, Multicast.

Class D IP addresses

Used to specify multicast host groups. In Internet standard "dotted decimal" notation, host group addresses range from 224.0.0.0 to 239.255.255.255.

Core Based Trees (CBT) Routing Protocol

Unlike DVMRP or MOSPF, which construct spanning trees for each source/group pair, CBT protocol constructs a single tree that is shared by all members of the group. Multicast traffic for the entire group is sent and received over the same tree, regardless of the source. A CBT shared tree has a small number of core routers (called cores) that are used to construct the tree. Other routers may join the tree by sending a join message to the core. CBT trees are bi-directional.

Dense-mode multicast routing protocols

Dense-mode routing protocols assume that the multicast group members are densely distributed throughout the network and bandwidth is plentiful, i.e., almost all hosts on the network belong to the group. Dense-mode routing protocols Distance Vector Multicast Routing Protocol (DVMRP), Multicast Open Shortest Path First (MOSPF), and Protocol-Independent Multicast - Dense Mode (PIM-DM). See also Sparse-mode Routing Protocols.

Distance Vector Multicast Routing Protocol (DVMRP)

The first protocol that was developed to support multicast routing is called Distance Vector Multicast Routing Protocol (DVMRP), described in RFC 1075. It is used extensively on the MBONE. The approach used by DVMRP is to assume initially that every host on the network is part of the multicast group. Multicast messages are transmitted over every possible router interface as they proceed across the network, forming a spanning tree to all possible members of the multicast group. DVMRP maintains a current image of the network topology using a distance-vector routing protocol such as the Routing Information Protocol RIP. The distance metric used for RIP and DVMRP is the number of hops in the path.

Host Group

All hosts belonging to a multicast session. The membership of a host group is dynamic; that is, hosts may join and leave groups at any time. There is no restriction on the location or number of members in a host group. A host may be a member of more than one group at a time.

Internet Group Management Protocol (IGMP)

IGMP is used by multicast routers to learn the existence of host group members on their directly attached subnets. See RFC 1112.

IP Multicast

A one-to-many transmission, in contrast to Unicast, Broadcast.

An extension to the standard IP network-level protocol. RFC 1112, Host Extensions for IP Multicasting, authored by Steve Deering in 1989, laid the groundwork for IP Multicasting. The RFC describes IP Multicasting as: "the transmission of an IP datagram to a 'host group', a set of zero or more hosts identified by a single IP destination address. A multicast datagram is delivered to all members of its destination host group with the same 'best-efforts' reliability as regular unicast IP datagrams. ... The membership of a host group is dynamic; that is, hosts may join and leave groups at any time. There is no restriction on the location or number of members in a host group. A host may be a member of more than one group at a time."

IP Multicast Datagram

A datagram delivered to all members of the multicast host group with the same 'best-efforts' reliability as regular unicast IP datagrams.

IP Multicast Router

A router supporting IGMP and one or more routing protocols, including Distance Vector Multicast Routing Protocol (DVMRP), Multicast Open Shortest Path First (MOSPF), and Protocol-Independent Multicast - Dense Mode (PIM-DM), Core Based Trees (CBT) and Protocol-Independent Multicast - Sparse Mode (PIM-SM).

MBONE

The MBONE (Multicast Backbone) is a virtual network layered on top of the physical Internet to support routing of IP Multicast packets. It has been in existence for about 5 years. See www.mbone.com for more information.

Multicast Open Shortest Path First (MOSPF)

The MOSPF routing protocol is the IP Multicast extension of the Open Shortest Path First (OSPF) unicast routing protocol. It is defined in RFC 1584. OSPF routes messages along least-cost paths, where cost is expressed in terms of a link-state metric, as opposed to hops, used by RIP and DVMRP. Each router can calculate a spanning tree with the multicast source at the root and the group members as leaves. This tree is the path that is used to route multicast traffic from the source to each of the group members.

Multicast

To transmit information to a group of recipients via a single transmission by the source, in contrast to Unicast, Broadcast. See also IP Multicast.

Protocol-Independent Multicast (PIM) Routing Protocols

The Protocol Independent Multicast (PIM) routing protocol is currently under development by an IETF working group. The objective of the designers of PIM is to develop a standard multicast routing protocol that can provide scalable inter-domain multicast routing across the Internet that is not dependent on the mechanisms provided by any particular unicast routing protocol. In contrast, DVMRP is based on RIP and MOSPF on OSPF. PIM has two modes, dense and sparse, discussed below.

PIM- Dense Mode (PIM-DM) Routing Protocol

PIM also defines a new dense-mode protocol for "dense" groups, instead of relying on existing dense-mode protocols such as DVMRP and MOSPF. See the entry for dense-mode multicast routing protocols. PIM-DM control message processing and data packet forwarding is integrated with PIM-SM operation so that a single router can run different modes for different groups.

PIM- Sparse Mode (PIM-SM) Routing Protocol

PIM-SM differs from existing dense-mode multicast algorithms in two essential ways. Routers with directly attached or downstream members are required to join a sparse mode distribution tree by transmitting explicit join messages. If a router does not become part of the pre-defined distribution tree, it will not receive multicast traffic addressed to the group.

Real-Time Streaming Protocol (RTSP)

RTSP is an application-level protocol for control over the delivery of data with real-time properties to enable controlled, on-demand delivery of real-time data, such as audio and video.

Real-Time Transport Protocol (RTP)

RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services.

Reliable multicast protocols

Reliable multicast protocols overcome the limitations of unreliable multicast datagram delivery and expand the uses of IP Multicast.

ReSerVation Protocol (RSVP)

RSVP, the ReSerVation Protocol, enhances the current Internet architecture with support requests for a specific quality of service (QoS) from the network for particular data streams or flows.

Sparse-mode multicast routing protocols

A selective one-to-many transmission, in contrast to Unicast, Broadcast. See also IP Multicast.

Sparse-mode routing protocols assume that the multicast group members are sparsely distributed throughout the network and bandwidth is not necessarily widely available, for example across many regions of the Internet. It is important to note that sparse-mode does not imply that the group has a few members, just that they are widely dispersed. Sparse-mode routing protocols include Core Based Trees (CBT) and Protocol-Independent Multicast - Sparse Mode (PIM-SM).

Spanning Tree

For efficient transmission, multicast routers construct a spanning tree from the multicast source at the root of the tree to all the multicast receivers as leaves of the tree. A spanning tree has just enough connectivity so that there is only one path between every pair of LAN's, and it is loop-free.

Tunneling

An interim deployment strategy to connect islands of multicast routers separated by links which do not support IP Multicast. Tunneling is used extensively in the MBONE. Tunneling is discussed in the IP Multicast Initiative white paper Introduction to IP Multicast Routing.

Unicast

Point-to-point transmission requiring the source to send an individual copy of a message to each requester.