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Multicast the chatwaves

Use IP multicast along with custom stream classes to implement a cool, yet simple, peer-to-peer networked chat system

By Merlin Hughes, JavaWorld.com, 10/20/99

Page 3 of 4

So, how do you know which group is yours? Well, that's a big picture issue. You can apply to the Internet Architecture and Naming Authority (IANA), if you want; however, it's doubtful that IANA would assign you a group number just for a chat room. More often, if the group will be restricted to just your organization, you can ask your system administrator to assign one of the 16 million private-use group numbers (those beginning with 239).

To make administration easier, multicast supports a special time-to-live feature whereby you can limit how far messages will travel. You can restrict a chat group to just your department intranet, for instance, or perhaps to your local Ethernet wire. There are two aspects to this. First, you must configure the routers and tunnels of your network appropriately. Usually, this is performed by your system administrator. Second, you must choose an appropriate time-to-live for the messages that you send; the value will depend on your local network configuration. A time-to-live of 1 will restrict packets to your local Ethernet wire, while 63 typically restricts it to a single multicast island. You can find more details in the Resources.

We now have an overall design for the chat system:

• Configure your network to be multicast-enabled. If you have multiple machines on a single hub or switch, they should be able to communicate using multicast without any configuration.
  
  
• Choose a multicast group and a time-to-live that lets all the desired users connect to your chat session, but prevents messages from travelling too far. For testing purposes, any multicast group in the 239.x.x.x range will do, as will a time-to-live of 1.
  
  
• Join the multicast group so that you receive messages destined for it. Start a thread that listens for these messages and display them in a text area.
  
  
• Open a text field for the user to enter messages. To transmit a message to the multicast group, simply drop it on the network bound to the multicast address.

Data communications

IP multicast is built on top of the User Datagram Protocol (UDP). This type of network protocol is packet-based; you construct a packet of information (up to 65,508 bytes long) and dispatch the packet (a datagram) into the network, bound for a particular destination (identified by an IP address and a port). The network will then try to deliver the packet to the remote address. Upon successful delivery, the recipient will receive the exact payload that you initially dispatched. However, the network sometimes loses the datagrams, sometimes duplicates them, and sometimes misorders them (that is, packets may arrive in a different order than the one in which they were sent). Neither the sender nor the recipient will receive notification of any such network errors. Rather, it is up to the application to identify and surmount such problems. This is one of the complexities of datagram-based networking. The datagram protocols only guarantee that, if a packet is delivered, it will be delivered intact.

 

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