Multicasting Techniques and protocol

Basic Definition and techniques

Multicasting in computer network is a group communication, where a sender(s) send data to multiple receivers simultaneously. It supports one – to – many and many – to – many data transmission across LANs or WANs. Through the process of multicasting, the communication and processing overhead of sending the same data packet or data frame in minimized.

Ethernet Multicast

Ethernet multicast constitutes multicasting at the data link layer of the OSI model for Ethernet networks. Ethernet frames for multicasting are identified by a 1 bit in the LSB (least significant bit) of the first byte of the destination address.

IP Multicast

IP multicast provides one-to-many communication over an IP network. The destination nodes send join and leave messages that informs the routers whether they are correct recipients of the messages. The sender sends the data packet only once irrespective of the number of users. The routers in the network performs necessary replications so that the packet can reach multiple receivers simultaneously

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Intradomain Multicast Protocols�

• Intradomain Multicast Protocols
• Intra domain routing protocols carry out the multicast function within domains. The implementation of multicast routing faces the following particular challenges:
• Dynamic change in the group membership
• Minimizing network load and avoiding routing loops
• Finding concentration points of traffic
• In practice, several protocols play major roles in establishing multicast connections. The Distance Vector Multicast Routing Protocol (DVMRP) and the Internet Group Management Protocol (IGMP) are the two original protocols forming the early version of the multicast backbone (MBone). Other protocols, such as Multicast Open Shortest Path First (MOSPF), core -based trees (CBT), and protocol-independent multicast (PIM) enhance MBone performance.

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• Distance Vector Multicast Routing Protocol (DVMRP)
• The Distance Vector Multicast Routing Protocol (DVMRP) is one of the oldest multicast protocols. It is based on a concept of exchanging routing table information among directly connected neighboring routers. The MBone topology can enable multiple tunnels to run over a common physical link. Each participating router maintains information about all the destinations within the system. DVMRP creates multicast trees, using the dense-mode algorithm. A multicast router typically implements several other independent routing protocols besides DVMRP for multicast routing, such as RIP or OSPF for unicast routing.
•  Internet Group Management Protocol (IGMP)
• The Internet Group Management Protocol (IGMP) is used for TCP/IP between a receiver and its immediate multicast-enabled routers reporting multicast group information. This protocol has several versions and is required on all machines that receive IP multicast. As the name suggests, IGMP is a group-oriented management protocol that provides a dynamic service to registered individual hosts in a multicast group on a particular network.

Multicast OSPF (MOSPF) Protocol

The Multicast Open Shortest Path First (MOSPF) protocol, an extension to the unicast model of OSPF. constructs a link-state database with an advertisement mechanism. Let's explore what new features a link-state router requires to become capable of multicast functions.

Link-State Multicast

ink-state routing occurs when a node in a network has to obtain the state of its connected links and then send an update to all the other routers once the state changes. On receipt of the routing information, each router reconfigures the entire topology of the network. The link-state routing algorithm uses Dijkstra's algorithm to compute the least-cost path.

Begin MOSPF Protocol

Protocol-Independent Multicast (PIM)�

• Protocol-independent multicast (PIM) is an excellent multicast protocol for networks, regardless of size and membership density. PIM is "independent" because it implements multicasting independently of any routing protocol entering into the multicast routing information database. PIM can operate in both dense mode and sparse mode . Dense-mode is a flood-and-prune protocol and is best suited for networks densely populated by receivers and with enough bandwidth. This version of PIM is comparable to DVMRP.

Core-Based Trees (CBT) Protocol�

• In sparse mode, forwarding traffic to the rendezvous router and then to receivers causes delay and longer paths. This issue can be partially solved in the core-based tree (CBT) protocol, which uses bidirectional trees. Sparse-mode PIM is comparable to CBT but with two differences. First, CBT uses bidirectional shared trees, whereas sparse-mode PIM uses unidirectional shared trees. Clearly, bidirectional shared trees are more efficient when packets move from a source to the root of the multicast tree; as a result, packets can be sent up and down in the tree. Second, CBT uses only a shared tree and does not use shortest-path trees.

Multicast Backbone (MBone)�

• The first milestone in the creation of a practical multicast platform was the development of the multicast backbone (MBone), which carried its first worldwide event when several sites received audio simultaneously . The multicast routing function was implemented using unicast-encapsulated multicast packets. The connectivity among certain receivers was provided using point-to-point IP-encapsulated tunnels . Figure 15.7 shows an example of tunneling among routers in the early version of MBone. Each tunnel connects two end points via one logical link and crosses several routers. In this scenario, once a packet is received, it can be sent to other tunnel end points or broadcast to local members. The routing in earlier version of MBone was based on DVMRP and IGMP.
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Interdomain Multicast Protocols�

• Interdomain multicast protocols are designed for hierarchical and Internet-wide multicast purposes. Within a domain, a network manager can implement any routing protocol desired. The challenge in interdomain multicast administration is choosing the best external link to route to hosts in an external domain. Among the protocols in this category are multiprotocol Border Gateway Protocol (MBGP), Multicast Source Discovery Protocol (MSDP), and Border Gateway Multicast Protocol (BGMP).

Multiprotocol BGP (MBGP)�

• Multiprotocol BGP (MBGP) is an extension to its unicast version, Border Gateway Protocol (BGP). In Figure 15.8, three domains are connected through two types of paths: a unicast path handled by BGP and a multicast path handled by MBGP. With BGP, the multicast routing hierarchy operates the same way as multiple unicast routing does. Between each two domains, two corresponding border routers compute the set of domain parameters that should be traversed to reach any network. Typically, the parameters in one domain are not known or trusted by the others.

Figure MBGP: interdomain multicast routing�

Multicast Source Discovery Protocol (MSDP)�

• The Multicast Source Discovery Protocol (MSDP) has potential solutions to these issues. Figure 15.9 shows how this protocol operates. A unique feature of this protocol is that it assigns representatives in each domain. A representative reports to other domains the existence of active sources. A new source for a group must first register with the domain's rendezvous router.

Border Gateway Multicast Protocol (BGMP)�

• The Border Gateway Multicast Protocol (BGMP) is based on the construction of bidirectional shared trees among domains using a single root. Finding the best domain to place the root of such shared trees is a challenge, but several solutions are available. One of the methods of address resolution is the Multicast Address-Set Claim (MASC) protocol, which guarantees the immediate resolution of address collisions.

Node-Level Multicast Algorithms�

• Multicasting techniques can also be used at the router level. To implement a multicast connection, a binary tree is normally constructed with the source switch port at the root and the destination switch ports at the leaves . Internal nodes act as relay points that receive packets and make copies. A number of such multicast methods are used. One is a tree-based multicast algorithm using a separate copy network. The Boolean splitting multicast algorithm is used for multistage switches. The third technique the packet recirculation multicast algorithm . The fourth is multicasting in three-dimensional switches .

Tree-Based Multicast Algorithm�

• Imagine that a multicast tree algorithm must be applied on a multistage switch fabric, as shown in the expansion switch in Figure 15.10. Multicasting is implemented as a tree structure. A source generates a packet and sends it out to the first crossbar switch. The packet may have a field that specifies how many copies of this packet are to be made. All copies may be destined for other crossbars, in which more copies of the packet are made, and some packets may move to their destinations.

Tree-based multicasting in an expansion switch�

Boolean Splitting Multicast Algorithm�

• The Boolean splitting multicast algorithm is a method of copying packets in any space-division switch fabric with n inputs or outputs constructed with 2 x 2 switch elements and therefore with log ₂ n stages. Consider a Banyan network in which switching nodes replicate packets based on 2-bit header information. The following algorithm presents multicast steps within a copy network

Packet Recirculation Multicast Algorithm�

• Packet recirculation ( recycling ) is another feasible method for constructing large switching fabrics for broadband switching applications. To implement a multicast connection, a binary tree is constructed with the source port at its root and the destination switch port at its leaves. This technique can be used for almost all kinds of space-division switch fabrics.

Multicasting in Three-Dimensional Switches

IPP –input port processor

OPP-output port processor

Multicasting in Three-Dimensional Switches�

• This switch uses a Clos network as a building block module. An n -port Clos network is a nonblocking network if k  2 d - 1, where d and k are the sizes of the first-stage crossbar switch elements. The three-dimensional structure of the Clos network consists of m parallel planes of the same type and same size Clos network in Figure 15.15. An incoming packet can be demultiplexed among m planes. The demultiplexer does the work of distributing the input packets. In other words, the three-dimensional switching system accepts packets coming from different planes and time multiplexes them onto the same output port. The multicast algorithm for each plane is described as follows .