Here is a thread about TTL on the Internet History mailing list:

https://elists.isoc.org/pipermail/internet-history/2020-Sept...

A good summary from Jack Haverty:

https://elists.isoc.org/pipermail/internet-history/2020-Sept...

>My recollection, from the discussions when the IP4 header was being
defined, was that TTL was included for two reasons:

>1/ we couldn't be sure that packets would never loop, so TTL was a last
resort that would get rid of old packets. The name "Time To Live"
captured the desire to actually limit packet lifetime, e.g., in
milliseconds, but hops were the best metric that could actually be
implemented. So TTL was a placeholder for some future when it was
possible to measure time. TTL did not prevent loops, but it did make
them less likely to cause problems.

>2/ TCP connections could be "confused" if an old packet arrived that
looked like it belonged to a current connection, with unpredictable
behavior. The TTL maximum at least set some limits on how long such a
window of vulnerability could be open. Computers had an annoying
tendency to crash a lot in those days, but took rather long times to
reboot. So the TTL made it less likely that an old packet would cause
problems. (Note that this did not address the scenario when a gateway
crashed, was subsequently restarted possibly hours later, and sent out
all the packets it still had in its queues.)

/Jack

https://elists.isoc.org/pipermail/internet-history/2020-Sept...

>I just realized that I should have also said:

>3/ Although TTL did not prevent loops, it was a mechanism that
detected loops. When a packet TTL dropped to zero, an ICMP message
(something like "TTL Time Exceeded") was supposed to be generated to
tell the source of the failure to deliver. Gateways could also report
such events to some monitoring/control center by use of SNMP, where a
human operator could be alerted.

>4/ TTL was also intended for use with different TOS values, by the
systems sending voice over the Internet (Steve Casner may remember
more). The idea was that a packet containing voice data was useless if
it didn't get to its destination in time, so TTL with a "fastest
delivery" TOS enabled the sender to say "if you can't deliver this in
200 milliseconds, just throw it away and don't waste any more
bandwidth". That of course wouldn't work with "time" measured in hops,
but we hoped to upgrade soon to time-based measurements. Dave Mills
was working hard on that, developing techniques for synchronizing clocks
across a set of gateways (NTP was intended for more than just setting
your PC's clock).

>I've noticed that researchers, especially if they're not involved in
actually operating a network, often don't think about the need for tools
to be used when things are not going well - both to detect problems and
to take actions to fix the problem.

>Without that operational perspective, some of the protocol functions and
header fields may not seem to be necessary for the basic operation of
carrying user traffic. TTL is one such mechanism. Source route was
another, in that it might provide a way to get control messages
delivered to a misbehaving host/router/whatever that was causing routing
failure. This involved using source routing as a somewhat "out of band"
signal mechanism that might not be affected by whatever was going on at
the time.

>All of this was considered part of the "Internet Experiment", providing
instrumentation to monitor the experiment to see what worked and what
didn't, and evolve into tools for use in dealing with problems in
operational networks.

>At one point I remember writing, sometime in the mid 80s, a rather
large document called something like "Managing Large Network Systems"
for some government contract, where these kinds of tools were
described. But I haven't been able to find it. Probably in a dusty
warehouse somewhere....

>/Jack