Hal Murray
Fri Oct 15 18:51:15 1999

Yes, it does stand for Time To Live.  The main idea is to prevent 
packets from bouncing around forever if the routing tables get into 
a loop. 

In the old days, it was the number of additional seconds that a packet 
should be allowed to live.  But each router counts as at least 1 
second.  I'm not sure any router ever bumped the TTL down by more 
than 1 but it would make sense on a slow link with long queues.

Now, it's really a hop count.  The name is historic.

The sending host initializes that field.  Usually it's hard coded 
into the protocol stack.  Older systems used 16.  That stopped working 
several years ago when the internet got too big.  I think most modern 
systems just set it to 255 - the max that will fit.

[I might be off on some of the details above, but the general idea 
is correct.]