Post on 17-Dec-2015
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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)