MBONE: Multicasting Tomorrow's Internet

How do TTLs limit the life of MBONE packets?

An infinitely loud megaphone, one that could be heard anywhere in the world, would be a bad thing: once a few dozen of us had them, we wouldn't be able to get useful information from the din. The MBONE is no exception. Broadcast packets need to have a finite life lest they bounce around the network forever. The MBONE includes procedures for limiting how far multicast packets may travel, to prevent them from saturating the entire Internet. Each packet has a time to live (TTL) value, a counter that is decremented every time the packet passes through an mrouter. Because of TTLs, each multicast packet is a ticking time bomb.

If an MBONE broadcast were a TV station (as in the analogy at the beginning of this chapter), TTLs would be the station's signal area -- the limitation of how far the information can travel before petering out.

Those multicasting something that's of little interest to the outside world (for example, a company board meeting) might produce a packet that had a small time to live (perhaps 10). As the packet moved around the company's internal network, its TTL would be notched down every time it passed through an mrouter. When the packet's TTL reached 0, the packet would die and not be passed further. With careful planning, those multicasting can keep their multicast packets on their internal networks or within their state or country.

Events of interest to the world at large are generally multicast with long TTLs -- perhaps 200 -- to guarantee that the information will reach around the world.

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