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 - to
one point
unicast
One host is in a communication relation with another host
from one point - to
many points
multicast
One host and a group of host are in communication relation, where the
sender sends the data once, and that data is received by all members
from many points - multipoint
to many points
multicast
In one communication relation there are more than one sender, which
can 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 all
members send data to that one
from one point-to
all 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
Concast
Legend
router
host
connection
path of concast
packets with
increasing
bandwidth
Destination
Aggregation of the data is
necessary for bandwidth
saving
Multicast application
ConferenceXP: An Example of Multicast application
Distance Learning
Video Conference

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 Multicast 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
31
32 bits D-class IP-address:
24 23
8 7
0
1110 xxxx x
non
used
48 bits MAC-address
00000001
16 15
00000000 01011110 0
Low 23 bits of the multicast group
address is copied to the MAC-address
Reserved Multicast Addresses
IP 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 locally
used)
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)