Saturday, December 22, 2012

TTL (Time to Live) Field in IP Header

The purpose of the TTL field is to catch packets that have been going around in routing loops and discard them, rather than let them consume resources indefinitely. Hence this field is used to avoid infinite looping of packets.

What does the value of TTL field in the IP header indicate? Is it number of seconds, or is it number of hops?

Originally, TTL was set to a specific number of seconds that the packet would be allowed to live, and routers along the path would decrement this field until it reached 0. However, since it was rare for a packet to sit for as long as 1 second in a router, and routers did not all have access to a common clock, most routers just decremented the TTL by 1 as they forwarded the packet. Thus, it became more of a hop count than a timer, which is still a perfectly good way to catch packets that are stuck in routing loops. 

The default value currently used is 64.

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