Performance

[Robert G. Brown]

On Fri, 15 Oct 1999, Omar wrote:

> 
> Anyone know what the TTL means when you ping someone?
> 
> I also noticed that my TTL is in the 200's while every other customer I 
> ping is in the 100's
> 
> even my win98 computer on the same cable modem is in the 100's
> 
> i think it stands for time to live...but that doesn't seem to tell me much.....
> 

It does indeed mean time to live.  Every packet sent out on the network
has to have a TTL in a field in the packet header.  This entry is
decremented every time the packet makes a router hop.  If/when it gets
to zero, the packet dies.  If it weren't for TTL, there might well be
"deathless" ethernet packets bouncing here and there along long, indrect
circular routes, clogging up networks and routers and generally annoying
people.

They're the direct analog of teliomeres in cells (which also get
"decremented" every time a cell divides until they go away, at which
point the cell is tremendously weakened or dies).  I guess
God/Evolution/Both (take your pick) thought a TTL field was a good thing
as well, perhaps to keep individuals like certain Senators from North
Carolina from clogging up the network of humanity for all time.

   rgb

Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb@phy.duke.edu