Multicast Fundamentals

The communication ways of the hosts IP multicast Application level multicast

The Communication Ways of the Hosts

Type Name Description

from one point - toone point

unicast One host is in a communication relation with another host

from one point - tomany points

multicast One host and a group of host are in communication relation, where thesender sends the data once, and that data is received by all members

from many points -to many points

multipointmulticast

In one communication relation there are more than one sender, whichcan individually send data to the members of the group

from many points -to one point

concast A certain group of the hosts are in communication relation, where allmembers send data to that one

from one point-toall points

broadcast One host sends data to all other hosts

Needs for Multicast 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 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

UNICAST 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

Legend

router host connection

path of concast packets with increasing bandwidth

Destination

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

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

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 224.0.0.0~239.255.255.255

• Base address: 224.0.0.0 is reserved• 224.0.0.1~224.0.0.255 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 01.00.5E.00.00.00. 32 multicast groups may be mapped into the same address. Probability is small, but receivers should check the datagram

Mapping the Multicast MAC- &IP-Addresses

nonused

0 010111100000000000000001

32 bits D-class IP-address:

48 bits MAC-address

8 716 15 024 2331

Low 23 bits of the multicast groupaddress is copied to the MAC-address

1110 xxxx x

Reserved Multicast AddressesIP address Multicast group name

224.0.0.1 All system which have IP address in a LAN

224.0.0.2 All IP multicast router in a LAN

224.0.0.4 All DVMRP router

224.0.0.5 All MOSPF router in a LAN

224.0.0.6 All designated MOSPF router in a LAN

224.0.0.13 All PIM router

224.0.0.15 All CBT router

224.0.0.0 - 224.0.0.255(excluding the above addresses)

Reserved for routing purposes

224.0.1.1 Network Time Protocol (NTP)

224.2.2.2 Session Description Protocol (SDP)

224.2.127.254 Used by the Session Directory application

232/8(232.0.0.0-232.255.255.255)

Reserved for Source Specific Multicast (SSM)

239/8(239.0.0.0 - 239.255.255.255)

Administrative scoping area (not Internet-wide, but locallyused)

Scope Control

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

Administrative scope control

TTL Domains

TTL Scope

0 Restricted to the same host

1 Restricted to the same subnetwork. Nem továbbítja az útválasztó

< 32 Restricted to the same site, organisation, or campus

< 64 Restricted to the same region

< 128 Restricted to the same continent

128 ... 255 Worldwide, not limited

Administrative Scope-control

Demand: geographical and bandwidth limitations parallel The range 239.0.0.0 - 239.255.255.255 is reserved for

administrative scoping Similar to the local unicast address ranges (192.168., 10. etc.) Reuse: a multicast address used by an application inside a scope

controlled region can be used by any application outside of the scope

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

The multicast communication session is possible in the application level, too

Host applications forward data by unicast 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) Presently operating solutions:

– System of network news (Usenet)– Virus dissemination through the network (regrettable)