[ih] TTL [was Exterior Gateway Protocol]

[Łukasz Bromirski]

Jack,

I was reading a lot of old BBN PDFs thanks to all good souls on
this list that post nice URLs from time to time.

I remember reading in at least one of them, that apparently first
TCP/IP implementations were indeed using TTL as literally “time”,
not hop count. I believe there somewhere there between PDP docs
and ARPANET docs I’ve read something to the effect “and from this
time we changed from measuring time to simply count routing hops”.
Of course, right now google-fu is failing me.

Quoting RFC 1009 that was already brought up, there’s quite
direct “definition” of the field:

"4.8.  Time-To-Live

 The Time-to-Live (TTL) field of the IP header is defined to be a
 timer limiting the lifetime of a datagram in the Internet.  It is
 an 8-bit field and the units are seconds.  This would imply that
 for a maximum TTL of 255 a datagram would time-out after about 4
 and a quarter minutes.  Another aspect of the definition requires
 each gateway (or other module) that handles a datagram to
 decrement the TTL by at least one, even if the elapsed time was
 much less than a second.  Since this is very often the case, the
 TTL effectively becomes a hop count limit on how far a datagram
 can propagate through the Internet."

Were there any implementations that survived somewhere and actually
did exactly that - counted actual time/processing delay, not hops?
And if it took 2s to process packet, did they really decrement TTL
by two?

Thanks for any pointers,
-- 
Łukasz Bromirski