Multicast

• IP Multicast is a technology that allows data to be delivered over networks to a group of destinations as efficiently as possible.
• IP Multicast delivers source traffic to multiple recievers without additionaly burden on the source or the receivers while using the least network bandwidth of any competing technology.
• Data is sent from the source as one stream; this single data stream travels as far as it can in the network.
• Devices only replicate the data if they need to send it out on multiple interfaces to reach al members of the destination group.
  • Mulitcast packets are replicated in the network by Cisco routers enabled with Protocol Independent Multicast (PIM) and other multicast protocols.

Multicast vs. Unicast

• In Multicast, packets are not duplicated when sending to multiple receivers. Instead, they are sent in a single stream.
  • Downstream routers replicate the packets only on links where receiving hosts exist.
  • The source of multicast traffic (the sender) does not have to know the addresses of the receivers.
• Unicast transmission sends multiple copies of data packets; one copy for each receiver.

Multicast Applications

• One-to-Many
  • One sender sends data to many receivers.
  • May be used for audio or video distribution, push-media, announcements, monitoring, etc.
  • May become many-to-many if feedback is required from the receivers.
• Many-to-Many
  • Any number of hosts send to the same multicast groups.
  • Two or more receivers also act as senders and a host can be a sender and a receiver simultaneously.
• Realtime Applications include:
  • TV, Radio, corporate broadcasts, financial data delivery, whiteboard collabaration, e-learning, video-conferencing.
• Non-realtime Applications include:
  • File transfer, data and file replication, and video on demand (VoD)

Advantages of Multicast

• Enhanced effieciency – multiple streams of data can be replaced with a single transmission. Server and CPU loads are also reduced.

Reduced traffic load: Example of all clients listening to a the same 8-kbps audio stream

• Optimized performance – Eliminates traffic redundancy because fewer copies of the data require forwarding and processing.
• Support for distributed applications.

Disadvantages of Multicast

• Most multicast applications user the User Datagram Protocol (UDP) transport mechanism.
  • As a result, there is no insurance for reliable delivery of data due to the best-effort delivery mechanism that is true of UDP. Therefore, reliability must lie at the application layer itself.
    • An example of this would be packet drops in a voice application. A drop in a voice packet cannot benefit from retransmission of the lost data because once a voice data is lost, it doesn’t make sense to recreate the lost packet for real-time use such as VoIP.
  • Because of UDP’s inherent lack of a windowing mechanism present in TCP, network congestion and degradation could occur.
• Duplicate packets may occur when multicast topologies change.
• Out-of-sequence delivery of packets to the application can also occur if the topology changes. The Mulicast application design should take this into account in the planning process.

IP Multicast Addresses

IP Class D Address

• IANA has assigned the Class D IPv4 address space range of 224.0.0.0 through 239.255.255.255.
• The Internet Assigned Numbers Authority (IANA) hands out the assignment of multicast addresses.

Reserved Link Local Addresses

• 224.0.0.0 through 244.0.0.255
• The IANA has reserved the range 224.0.0.0/24 for use by network protocols on a local network segment.
• Packets with these addresses are not to be forwarded by a routers.
• They have TTL value of 1.
• This range is also known as local network control block.
• Some well known IP multicast addresses are:
  • 224.0.0.1 – All hosts
  • 224.0.0.2 – All multicast routers
  • 224.0.0.5 – OSPF routers
  • 224.0.0.6 – OSPF DRs
  • 224.0.0.9 – RIPv2 routers
  • 224.0.0.10 – EIGRP routers
  • 224.0.0.12 – DHCP server/relay agent

Globally Scoped Addresses

• 224.0.1.0 through 238.255.255.255
• These addresses are used to multicast data between organizations and across the Internet.
• The IANA has reserved some of these addresses for multicast applicationsm such as Network Time Protocol (224.0.1.1)

Limited Scope Addresses

• 239.0.0.0 through 239.255.255.255
• Also known as Administratively Scoped Addresses.
• They are defined by RFC 2365.
• They are reserved for use inside private domains – local group or organizations.
• Routers are typically configured with filters to prevent multicast traffic in this address range from flowing outside of an AS or any user defined domain.
• The IANA further subdivides this group into the following scopes:
  • Site Local Scope
    
    • 239.255.0.0/16
      
    • 239.252.0.0/16
      
    • 239.253.0.0/16
      
    • 239.254.0.0/16
  • Organizational Local Scope
    • 239.192.0.0 to 239.251.255.255

Layer 2 Mulitcast Address

• In 802.3 standard, bit 0 of the first octet is used to indicate a broadcast and/or multicast frame.

• This bit 0 is an indication of the frame’s destination towards an arbitrary group of hosts (mulitcast) or, in the case of broadcast, all hosts on the network (address 0xFFFF.FFFF.FFFF)
  • IP multicast makes use of this bit to transmit IP packets to a group of hosts on a LAN segment.

Ethernet MAC Address Mapping

• The IANA owns a block of Ethernet MAC addresses that start with 01:00:5E in hexadecimal.
• The lower half of this block is allocated for multicast addresses:
  • 0100.5e00.0000 – 0100.5e7f.ffff available for MAC addresses.
• The low-order 23 bits of the IP mulitcast address is mapped into the low-order 23 bits of the MAC address, shown in the figure below:

• In the figure above, there are 28 bits of unique address space available for an IP multicast address:
  • 32bits minus the first 4 bits containing the 1110 Class D prefix.
• As mentioned earlier, there are 23 bits mapped into the IEEE MAC Addresses.
  • Therefore, there are five (28-23 = 5) bits of overlap.
  • 2^5 = 32 addresses
• There is a 32:1 overlap of IP addresses to MAC addresses. In other words 32 IP multicast addresses map to the same MAC multicast address.

Resources

1. Internet Protocol IP Multicast Technology – Cisco Systems
2. IP Multicast Technology Overview – Cisco Systems
3. Iana.org – Internet Multicast Addresses

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet; Routing TCP/IP, Volume 1 (2nd Edition) (CCIE Professional Development) by Jeff Doyle and Jennifer Carroll; as well as following the links on the resources section of this entry.