Multicasting and multicast address

Question

I have confusion about the multicast addresses, I have read an example which is given by.

Suppose two applications have been built to send audio over a network. One application accepts and digitizes an audio input stream, and then sends the resulting frame across the network to other application. The second application receives the digitized audio from the network, converts it back to the audio signal and plays the result over a speaker. Unless the two applications use broadcast to send frames, no other computers on the network will receive a copy of the frame. Multicasting provides an excellent solution to the problems of allowing some computers to participate in audio transmission. To use multicasting , a multicast address must be chosen for the audio application. And the receiving application passes the multicast address to the network interface. The interface begins to accept the packets sent to that address.

Question: how this multicast address is chosen, how the receiving application knows that the sender using this specific destination address for the audio frames.

Answer 1

The multicast address is chosen arbitrarily out of the 239.0.0.0/8 range (if the application is enterprise-internal, at least). It is then configured on the source(s) and all subscribers.

So, there is in general no "directory service" within the network, it relies on human interaction, to configure the applications correctly.

Answer 2

First off let me state that multicasting is evil. It is extremely hard to set up and really tricky to troubleshoot effectively.

That being said, I will attempt to answer your question. The sender chooses what multicast IP address that it uses to send traffic on. The reserved range of multicast IP addresses is 224.0.0.0 through 239.255.255.255.

Most ISPs do not support multicasting across their public internet networks, so if you are looking to multicast between locations you will need to have VPNs of some sort set up between them. Each router that the multicast transverses will need to be configured for multicasting.

If you are configuring your own equipment, I will try to provide an overview of the processes involved, but for detailed information you will need to do some reading. Fortunately wikipedia has decent articles on IP multicasting. The sending device sends out multicast traffic, you will need to set up each network device (routers, switches, firewalls, etc) you want the multicast traffic to pass over. Each manufacturer's equipment is going to be different so I can't provide much input on how specifically to set it up.

You will then need to set up each client that wants to recieve multicast traffic with the Internet Group Management Protocol (IGMP) this is usually built into whatever software package you are running on the client for the multicasting application. What IGMP does is go out to it's local network device and inform it that that client is interested in receiving multicast traffic from a particular multicast address. So, for instance, your sending is set up to send on 224.10.10.123, the client who wants to listen would set up IGMP to connect to it's local network device and request any multicast traffic coming to that device on 224.10.10.123.

In order to reduce unnecessary multicast traffic, you will probably also want to look into Protocol Independent Multicast (PIM). You set PIM up on your network devices. What PIM does is keep track of if anyone connected to that network device is actually listening to a multicast, and if no one is, then it will prune the traffic. If you have a large network with mutliple multicasting sources, you will definately want to look into implementing PIM.

Answer 3

Windows Media Services has the option of broadcasting live events over multicast. As Lloyd Baker pointed out, this is something that ends up being local to a network. On our University network we would multicast things like Commencement and speeches by the President, which would allow anyone on the network to tune in (which could be thousands) without hammering the snot out of the media server itself. Those who are off-campus would be stuck with unicast to get the live broadcast.

The IP we pick to use for our live broadcast is selected from the multicast range, which is goverened by RFC3171. The 239/8 block is for 'internal' use and is not routed over the internet, similar to RFC1918 addresses. Multicast works best when your network has been designed for it. Ours was since we had a dependency on the Service Location Protocol for a long time, and that's a multi-cast based protocol.

Once we pick an address for our broadcast we then assign a DNS name for it. At that point publishing the broadcast is pretty simple, whether it is an all-campus email or posted to the front of our web-page.