Multicasting

=Multicasing=

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Definition
Multicast is communication between a single sender and multiple receivers on a network (Wikipedia). It is normally used in the updating of mobile personnel from a home office and the periodic issuance of online newsletters. When used with anycast and unicast, multicast is one of the packet types in the Internet Protocol Version 6.

Multicast is the delivery of information to a group of destinations simultaneously. It using the most efficient strategy to deliver the messages over each link of the network only once and only creates copies when the links to the destinations get split up (Wikipedia).

Multicast is usually used to refer to IP Multicast. That is a protocol for efficiently sending to multiple receivers at the same time on TCP/IP networks, by using a multicast address. It is also commonly associated with audio/video protocols like the RTP.

The term multicasting is sometimes used interchangeably with narrowcasting.

Unicast
Unlike multicast, the usual point-to-single-point delivery is called unicast. A copy of the data is sent from the sender to each recipient when unicast is used to deliver to several recipients, which results in unproductive and badly scalable duplication at the sending side. Some popular protocols like XMPP accept this inadequacy but, limit the number of possible recipients (Wikipedia).

It's Uses
Multicast is supported through wireless data networks as part of the Cellular Digital Packet Data (CDPD) technology (Wikipedia).

Multicast is also used for programming on the MBone, which is a system that allows high-bandwidth users to receive live video and sound programming. Mbone multicast also uses a protocol that allows signals to be comprised into TCP/IP packets when passing through parts of the Internet as it can not handle the multicast protocol directly (Wikipedia).

But there are also other protocols that implement the multicast concept. Connection-oriented link layers have built-in mechanisms for "point-to-multipoint" or "multipoint-to-multipoint" connections and an example of this is Asynchronous Transfer Mode. This model normally assumes that the participants in the communication are known ahead of time. This way the distribution trees can be produced and resources allocated by network elements in advance of actual data traffic. Dave Clark descirbes the IP Multicast model by saying, "You put packets in at one end, and the network conspires to deliver them to anyone who asks (Wikipedia)."

There are other kinds of multicast technologies used in the Internet. The Explicit multicast (Xcast) uses the unicast addresses of all the destinations, instead of the designated multicast address. Because IP packet sizes are limited, the Explicit multicast can not be used for multicast groups of large number of destinations (Wikipedia).

**IP Multicast**
//Internet Protocol (IP) multicast// is, as mentioned above, a bandwidth-conserving technology that decreases traffic by concurrently delivering a single stream of information to thousands of recipients. Applications that take advantage of multicast include videoconferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news (Documentation).

IP Multicast delivers source traffic to multiple receivers without adding any additional burden on the source or the receivers even if they are using the least network bandwidth of any competing technology. Multicast packets are replicated in the network by Cisco routers along with Protocol Independent Multicast (PIM) and other supporting multicast protocols. This enables the delivery of data to multiple receivers possible. All alternatives require the source to send more than one copy of the data, while some require the source to send an individual copy to each receiver. With thousands of receivers, the majority of the applications benefit from using Cisco IP Multicast; like the low bandwidth applications. High-bandwidth applications, such as MPEG video, may require a large portion of the available network bandwidth for a single stream. In these applications, the only way to send to more than one receiver simultaneously is by using IP Multicast (Documentation).



//The figure above shows how data from one source is delivered to several interested recipients using IP multicast.//

**Issues**
Multicast security is a major issue. Standard, practical, communications security solutions normally employ symmetric cryptography. But applying that to IP Multicast traffic would enable any of the receivers to position themselves as the sender. This is clearly unacceptable. The IETF MSEC workgroup is developing security protocols to solve this problem, mostly within the architectural framework of the IPsec protocol suite (Wikipedia).

References:
Multicast. Wikipedia. Accessed February 22, 2006. [|] Documentation. IP Multicast. 2002. Accessed February 22, 2006. [|]