1. Implementing Multicast
Explaining Multicast

2. Why Multicast? Used when sending same data to multiple receivers
Better bandwidth utilization
Less host/router processing
Used when addresses of receivers unknown
Used when simultaneous delivery for a group of receivers is required (simulcast)
Multicast may be used to send the same data packets to multiple receivers.
By sending to multiple receivers the packets are not duplicated for every receiver, but are sent in a single stream where downstream routers take care for packet multiplication over receiving links.
Routers process less packets since they receive only single copy of the packet. This packet is then multiplied and sent on outgoing interfaces where there are receivers.
Because downstream routers take care for packet multiplication and delivery to receivers, sender or source of multicast traffic does not have to know about receivers unicast addresses.
Simulcast – simultaneous delivery for a group of receivers - can be used for deploying software upgrades or patches.

3. Unicast vs. Multicast
Unicast transmission sends multiple copies of data, one copy for each receiver:
The top example shows a host transmitting 3 copies of data and a network forwards each packet to 3 separate receivers. The host can only send to one receiver at a time since it has to create a different packet destination address for each receiver.
Multicast transmission sends a single copy of data to multiple receivers:
The lower example shows a host transmitting 1 copy of data and a network replicates the packet at the last possible hop for each receiver, each packet exists only in a single copy on any given network. The host can send to multiple receivers simultaneously since it’s sending only one packet.

4. Multicast Advantages
Enhanced efficiency: Controls network traffic and reduces server and CPU loads
Optimized performance: Eliminates traffic redundancy
Distributed applications: Makes multipoint applications possible
Multicast transmission affords many advantages over unicast transmission in a one-to-many or many-to-many environment:
Enhanced Efficiency: available network bandwidth is utilized more efficiently since multiple streams of data are replaced with a single transmission
Optimized Performance: less copies of data require forwarding and processing
Distributed Applications: multipoint applications will not be possible as demand and usage grows because unicast transmission will not scale (traffic level and clients increase at a 1:1 rate with unicast transmission)

5. Multicast Disadvantages
Multicast is UDP-based.
Best-effort delivery: Drops are to be expected. Multicast applications must not expect reliable delivery of data and should be designed accordingly. Reliable multicast applications will address this issue.
No congestion avoidance: The lack of TCP windowing and “slow-start” mechanisms can result in network congestion. If possible, multicast applications should attempt to detect and avoid congestion conditions.
Duplicates: Some multicast protocol mechanisms result in the occasional generation of duplicate packets. Multicast applications should be designed to expect occasional duplicate packets.
Out-of-sequence delivery: Network topology changes affect the order of delivery—the application must properly address the issue.
There are also some disadvantages of multicast that need to be considered
Most Multicast Applications are UDP based. This results in some undesirable side-effects when compared to similar unicast, TCP applications.
Best Effort Delivery results in occasional packet drops. Many multicast applications that operate in real-time (e.g. Video, Audio) can be impacted by these losses. Also, requesting retransmission of the lost data at the application layer in these sort of real-time applications is not feasible.
Heavy drops on Voice applications result in jerky, missed speech patterns that can make the content unintelligible when the drop rate gets high enough.
Moderate to Heavy drops in Video is sometimes better tolerated by the human eye and appear as unusual “artifacts” on the picture. However, some compression algorithms can be severely impacted by even low drop rates; causing the picture to become jerky or freeze for several seconds while the decompression algorithm recovers.
No Congestion Control can result in overall Network Degradation as the popularity of UDP based Multicast applications grow.
Duplicate packets can occasionally be generated as multicast network topologies change.
Applications should expect occasional duplicate packets to arrive and should be designed accordingly.
Efficient filtering and security for multicast flows is a big issue since senders randomly select UDP ports and very often multicast addresses as well. This kind of behavior prevents effective static filtering.

6. Types of Multicast Applications
One to many: A single host sending to two or more (n) receivers
Many to many: Any number of hosts sending to the same multicast group; hosts are also members of the group (sender = receiver)
Many to one: Any number of receivers sending data back to a source (via unicast or multicast)
There are different types of multicast applications. Two most common models are
one-to-many model where one sender sends data to many receivers or
many-to-many model where a host can be a sender as well as receiver simultaneously
Other models (e.g. many-to-one where many receivers are sending data back to one sender) are used less frequently.

7. IP Multicast Applications
Corporate Broadcasts
Live TV and Radio Broadcast
to the Desktop
Distance Learning
Multicast File Transfer
Data and File Replication
Many new multipoint applications are emerging as demand for them grows:
Real-time applications include live broadcasts, financial data delivery, whiteboard collaboration, and video conferencing.
Non-real-time applications include file transfer, data and file replication, and video-on-demand.
Note also that the latest version of Novell Netware uses Ipmc for file and print service announcements.
Training
Videoconferencing
Video-on-Demand
Whiteboard/Collaboration
Real-Time Data Delivery—Financial

8. IP Multicast Basic Addressing
IP group addresses:
Class D address (high-order three bits are set)
Range from through
Well-known addresses assigned by IANA
Reserved use: through
(all multicast systems on subnet)
(all routers on subnet)
(all DVMRP routers)
(all PIMv2 routers)
, , , and used by unicast routing protocols
Multicast IP addresses use the Class D address space. Class D addresses are denoted by the high 4 bits set to 1110.
The multicast IP address space is separated into following address groups:
Local Scope Addresses are addresses through and are Reserved by IANA for network protocol use. Multicasts in this range are never forwarded off the local network regardless of TTL and usually the TTL is set to 1. Examples of local multicast addresses are:
All Hosts
All Multicast Routers
All DVMRP Routers
All OSPF Routers
All OSPF DR
Global Scope Addresses are addresses through and are allocated dynamically throughout the Internet
Administratively Scoped Addresses are addresses through and are reserved for use inside of private domains.

9. IP Multicast Basic Addressing (Cont.)
Transient addresses, assigned and reclaimed dynamically (within applications):
Global range:
224.2.X.X usually used in MBONE applications
Limited (local) scope: /8 for private IP multicast addresses (RFC-2365)
Site-local scope: /16
Organization-local scope: to
Part of a global scope recently used for new protocols and temporary usage
Global scope multicast addresses administratively scoped multicast addresses are transient addresses. They are assigned and reclaimed dynamically within applications.
Administratively scoped multicast address space is divided into following scopes:
Local scope ( /16 and grows downward to /16, /16...)
Organization local scope ( /14 with possible expansion to ranges /10, /10 and /10)
Parts of a global scope are also used for new protocols and temporary usage:
ST Multicast Groups
Multimedia Conference Calls
SAPv1 Announcements
SAPv0 Announcements (deprecated)
SAP Dynamic Assignments
DIS transient groups
VMTP transient groups

10. Layer 2 Multicast Addressing
IP Multicast MAC Address Mapping Ethernet
Ethernet & FDDI Multicast Addresses
The low order bit (0x01) in the first octet indicates that this packet is a Layer 2 multicast packet. Furthermore, the “0x01005e” prefix has been reserved for use in mapping L3 IP multicast addresses into L2 MAC addresses.
When mapping L3 to L2 addresses, the low order 23 bits of the L3 IP multicast address are mapped into the low order 23 bits of the IEEE MAC address. Notice that this results in 5 bits of information being lost.
A bit of History
It turns out that this loss of 5 bits worth of information was not originally intended. When Dr. Steve Deering was doing his seminal research on IP Multicast, he approached his advisor with the need for 16 OUI’s to map all 28 bits worth of Layer 3 IP Multicast address into unique Layer 2 MAC addresses.
Note: An OUI (Organizationally Unique Identifier) is the high 24 bits of a MAC address that is assigned to “an organization” by the IEEE. A single OUI therefore provides 24 bits worth of unique MAC addresses to the organization.
Unfortunately, at that time the IEEE charged $1000 for each OUI assigned which meant that Dr. Deering was requesting that his advisor spend $16,000 so he could continue his research. Due to budget constraints, the advisor agreed to purchase a single OUI for Dr. Deering. However, the advisor also chose to reserve half of the MAC addresses in this OUI for other graduate research projects and granted Dr. Deering the other half.
This resulted in Dr. Deering having only 23 bits worth of MAC address space with which to map 28 bits of IP Multicast addresses. (It’s too bad that it wasn’t known back then how popular IP Multicast would become. If they had, Dr. Deering might have been able to “pass the hat” around to interested parties and collected enough money to purchase all 16 OUI’s. :-) )

11. Learning About Multicast Sessions
Potential receivers have to learn about multicast streams or sessions available before multicast application is launched. Possibilities:
Another multicast application sending to a well-known group whose members are all potential receivers
Directory services
Web page,
Session Announcement Protocol
Whenever a multicast application is started on a receiver the application has to know to which multicast group to join. The application has to learn about the available sessions / streams which typically map to one ore more IP multicast groups.
There are several possibilities for applications to learn about the sessions:
The application may join a well known predefined group to which announcements about available sessions are done
Some type of directory services are available and the application contacts the appropriate directory server
The application can be launched from a web page on which the sessions are listed as URLs; even can be used

12. A Cisco IP/TV Example Cisco IP/TV application
Clients (viewers) use program listing
Contact the server directly
Listen to SAP announcements
The SDR mechanisms are shown via sample IP multicast application – Cisco IP/TV is used and generally there are three components in it:
Server (the source)
Content Manager (the “directory server”)
Viewer (the receiver)
The Viewers can either:
Contact the Content Manager directly (via unicast) and request the list of available programs (sessions, streams) from it
Listen to periodic SAP announcements
The standard SDR format for session description is used.

13. Summary
IP multicast is a much more efficient means of delivering content where a single sender needs to deliver the content to multiple receivers. This task may be achieved through the use of multicast groups.
IP multicasts are designated by the use of a specific Class D IP address range. This is achieved through global scope addresses, which are assigned dynamically, and administratively scoped, which are assigned locally and are reserved for use inside private domains.