Multicast Fundamentals n The communication ways of the hosts n IP multicast n Application level multicast

The Communication Ways of the Hosts

Needs for Multicast n Classic communication protocols (e.g. HTTP) are client-server oriented –They resemble one-to-one connections: »a client sends a request and »the server answers with his reply –Although a web site directs its content to a mass audience, »each connection is a connection between a single user's client and the website's server n The same applies to the streaming protocols used by internet radios –A web radio really runs into problems when it becomes popular: »it has to pay too much for its increasing upstream bandwidth –No wonder that we do not see many web-TV stations until now: »the technology simply does not allow it –Huge amounts of data directed to a mass audience must be distributed differently now: »TV programs are being broadcasted via satellite or cable »movies are distributed physically on DVD

IP Address Classes Three fundamental types of IPv4 addresses : - Unicast - Broadcast - Multicast 1. Unicast: is designed to transmit a packet to a single destination 2. Broadcast: is used to send a datagram to an entire subnetwork 3. Multicast: is designed to enable the delivery of datagrams to a set of hosts that have been configured as members of Multicast group in various scattered subnetworks

Broadcast/Unicast/Multicast/Concast  Unicast: a separate IP packet is sent from a source to a destination participating in a connection.  Broadcast: When a packet is broadcast the same packet is sent to all clients on the network.  Multicast: General idea behind multicast is to send single copy of an IP packet to all of those of clients that requested it, and not to send multiple copies of a packet over the same portion of the network.  Basic idea in multicasting IP packets is to construct a tree structure data delivery path through the network.  This tree is rooted at the source of the multicast traffic and its leaves are subnetworks containing receivers of that traffic  Concast:It has mainly theoretical importance only. It can be imagined as a reversed multicast.

Broadcast BROADCAST Source transmits one stream of data for all the receivers without exception Replication happens inside routers and switches Broadcast is limited for LANs

Unicast n n UNICAST n n With 4 receivers, sender must replicate the stream 4 times

Multicast MULTICAST Source transmits one stream of data for n receivers Replication happens inside routers and switches WAN links only need one copy of the data, not n copies.

Concast CONCAST Source recieves one stream of data from n receivers Aggregation happens inside routers and switches

Concast Aggregation of the data is necessary for bandwidth saving

Multicast application Video Conference Distance Learning ConferenceXP: An Example of Multicast application  Further applications:  Audio conference  IP TV, Video on Demand  Advertisement, Stock  Synchronizing of distributed database, websites

The IP Multicast n AnySource Multicast (ASM) –STEVE DEERING estabilished in 1989 (RFC 1112) –Group communication model –Dynamic and anonym group membership –The source does not know the places or addresses of destinations; it send data packets to the group address only –The care of the network is to perform the necessary routing and packet multiplication –The client does not know from where the information comes, it joins only to a multicast group and then receives all data packets, which are sent to this address

The IP Multicast n Source Specific Multicast (SSM) –In its case the explicit knowledge of the sources is necessary –Source is identified by its unicast ip address –All the sources (mainly only one) have to be known by the recievers –Less trouble for the routing –Source routed trees can be used easily based on unicast routing information

The IP Multicast Multicast Groups Individual hosts are free to join or leave a Multicast group at any time. There are no restrictions on the physical location or the number of members in a Multicast group Group Membership Protocol A group membership protocol is employed by routers to learn about the presence of group members on their directly attached subnetworks Multicast Routing Protocol Multicast routers execute a Multicast routing protocol to define delivery paths that enable the forwarding of Multi- cast datagrams across an internetwork

Multicast Group and Service Model I  The key difference between a multicast IP packet and a unicast IP packet is the presence of a GROUP ADDRESS in the Destination Address field of the IP header of multicast IP packet  A multicast address is designed to enable a delivery of IP multicast packets to a set of clients that have been configured as members of a multicast group in various scattered subnetworks  Individual clients are free to join or leave multicast group at any time  A client may be a member of more than one multicast group at any given time and does not have to belong to a group to send message to members of a group

Multicast Group and Service Model II  Multicast Address IP reserved class D addresses for multicast ~ Base address: is reserved ~ are devoted to multicast routing and group maintenance protocols Multicast addresses can only be used as destination

Multicast Group and Service Model III  Mapping IP Multicast to Ethernet Multicast: Place the lower 23 bits of the IP multicast address into the lower 23 bits of special Ethernet multicast address E multicast groups may be mapped into the same address. Probability is small, but receivers should check the datagram

Mapping the Multicast MAC- & IP-Addresses

Reserved Multicast Addresses

Scope Control n Distance-based scope control –Distance (or time-to-live) based –Additional meaning of the TTL comparing to the IP unicast n Administrative scope control

TTL Domains

Administrative Scope-control n Demand: geographical and bandwidth limitations parallel n The range is reserved for administrative scoping n Similar to the local unicast address ranges ( , 10. etc.) n Reuse: a multicast address used by an application inside a scope controlled region can be used by any application outside of the scope n The applications can totally use the available bandwidth without the traffic flushing the smaller bandwidth connections and arising a glitch in the whole communication session

Application Level Multicast n The multicast communication session is possible in the application level, too n Host applications forward data by unicast n Disadvantages: –The propagation time is increased –The hosts have not inherent responsibility in the internet (oppositely of the routers) –Only sub-optimal bandwith usage (double or more on the last hop) n Presently operating solutions: –System of network news (Usenet) –Virus dissemination through the network (regrettable)