[ih] Origin of the loopback interface

[Brian E Carpenter]

On 22/10/2017 02:54, Miles Fidelman wrote:
> On 10/20/17 11:44 PM, Jack Haverty wrote:
> 
>> IIRC, loopback was a term used in modems at least as early as the late
>> 60s.  Maybe before.  When a line was put "in loopback" all the data
>> going out onto the line was reflected directly back to the sender.
>>
>> ....
>>
>> So, the specific term "loopback interface" probably depends on the
>> context.  It may have been first used in Unix, but the concept of
>> "looping an interface" was much older.  It existed in modems, and in the
>> ARPANET, and in the Internet, and was used primarily for debugging and
>> fault isolation during operations.  When something works, you just keep
>> using it.....
>>
>>
> The term "loopback" goes all the way back to early electrical circuits - 
> bridging two wires with a cliplead, at various points, to test connectivity.

For sure. I would expect that it was standard practice in the days of
teleprinters and telegraphs, probably back into the 19th century. For the
notion of the loopback interface as a TCP/IP software construct, it seems
that BSD in 1981 is the origin.

We could have another little chat about the loopback address in IP. I reached
the conclusion last night that it was never really necessary. All the TCP/IP
stacks that I know will happily send a message to any of their own assigned
addresses, without putting it on the wire. So having a dedicated address for
loopback tests seems useless today.

Thanks for all the feedback.

    Brian