Forget Unicast, Broadcast, and Anycast … the Future is Multicast
IP Multicasting is an evolving technology that is used extensively nowadays for fulfilling the needs of streaming applications such as Voice and Video over IP.
Multicasting is proven to be very beneficial to network administrators due to its bandwidth-conserving behavior and broadcast-limitation achievement. IP Multicast allows a source to transmit a stream of data to multiple destinations without the need of sending multiple copies.
IP multicast is responsible for replicating the data stream at appropriate locations so that it reaches all interested recipients.
Replicating packets at the edge, at places where potential recipients are situated, results in conserving bandwidth which is a scarce resource nowadays.
Applications that make use of multicasting include video streaming, videoconferencing, distance learning, etc. Multicasting has no physical boundaries. It is based on the "group" concept in the sense that hosts that are interested in receiving a particular data stream need to join the respective group. This is achieved using Internet Group Management Protocol (IGMP).
Today I’ll cover the basic multicasting concepts. I will introduce IGMP and focus on layer 2 switching multicasting issues. A brief description of basic multicast routing will be presented as well.
Multicasting Address Range
IP addresses within the range 126.96.36.199 through 188.8.131.52 are reserved for multicast groups. Internet Assigned Numbers Authority (IANA) has reserved addresses 184.108.40.206 through 220.127.116.11 to be used by network protocols within a local area network.
For example 18.104.22.168 is used by OSPF routers, 224.0.018 is used by VRRP. Moreover IANA has reserved a range of multicast addresses within the range of addresses from 22.214.171.124 through 126.96.36.199 for multicast applications. The whole range of reserved multicast addresses can be found at www.iana.org.
IGMP version 2
IGMP version 2 is specified in RFC 2236. Three main types of IGMP messages are defined:
- Membership report: Hosts that are interested in joining a specific multicast group send out IGMP membership reports for that particular group.
- Membership query: Multicast aware routers on local segments periodically send IGMP membership queries to find out if there are still hosts interested in receiving traffic from a particular multicast group. If not, then traffic towards not interested local segments is stopped.
- Leave group: In addition to IGMP version 1, hosts are able to send their intention to leave the group to their local multicast router so that unnecessary traffic is stopped much earlier.
A sample IGMP membership report is presented below. This specific host is requesting to join the VRRP group on 188.8.131.52.
Layer 2 switches treat multicast traffic as broadcast, therefore, they flood multicast frames out every port within a VLAN. This behavior is unacceptable because it wastes bandwidth.
IGMP snooping enables layer 2 switches to "snoop" on layer 3 and monitor IGMP packets traversing between hosts and routers. With IGMP snooping, switches are able, based on the received membership report for a particular multicast group, to associate hosts’ port numbers with multicast groups.
As a consequence, only hosts that are group receivers actually receive multicast traffic. IGMP snooping is most appropriate on layer 3 switches with ASICs that can perform IGMP snooping in hardware.
The following figure shows that without IGMP snooping, switches forward multicast traffic out all host ports.
With IGMP snooping multicast traffic is forwarded only to desired recipients.
Introducing Basic Multicast Routing Protocols
In order to be able to deliver IP multicast traffic, multicast capable routers build and maintain distribution trees to follow the traffic path. The concept of forwarding multicast traffic is called reverse path forwarding (RPF) and is based on the concept that multicast routers are able to forward multicast traffic away from the source and down the distribution tree towards the multicast recipient.
Upon receiving a multicast packet, the router performs a source lookup in its rooting table to find out if the received packet’s source address matches the interface on which packets from this subnet are expected. Only when RPF check is successful packets are forwarded, otherwise they are dropped.
PIM Dense Mode
PIM Dense Mode (PIM-DM) is a multicast routing protocol designed with the idea that receivers for any multicast group have dense distribution throughout the network. In other words, subnets throughout the network are members of almost all multicast groups and need to receive traffic from all different sources.
With PIM-DM, multicast traffic is sent to all hosts in the network and only those routers that are not interested in a particular traffic stream send special PIM Prune messages in order to be removed from the distribution tree. Eventually, traffic is transmitted to only those parts of the network that require it.
PIM Sparse Mode
PIM Sparse Mode (PIM-SM) is a multicast routing protocol designed on the assumption that recipients for any particular multicast group will be sparsely distributed throughout the network. In other words, it is assumed that most subnets in the network will not require multicast packets from any group.
In order to receive multicast data, routers must explicitly tell their upstream neighbors about their interest in particular groups. Routers use PIM Join and Prune messages to join and leave multicast distribution trees.
A Few Things to Remember …
Multicasting is a broad subject. I tried to give you a very basic knowledge of the multicasting concept without going into too much detail.
Nevertheless, multicasting is part of our daily routine. Your routing protocols use multicasting to achieve communication between routers. Your streaming applications use multicasting. Multicast traffic is crucial not only for the operability of your services and applications but also for the persistence of your network’s reliability.
Beware how you design your network and especially where and how you set your access lists. If you accidentally block a multicast stream in your network, you may notice undesirable effects like inconsistent routing tables or even loss of redundancy.
Multicasting can seriously preserve bandwidth and provide the opportunity to use the conserved bandwidth space for carrying additional traffic streams.
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