[ih] Origin of the loopback interface

[Dave Walden]

Search for "loop test" at 
http://www.dtic.mil/dtic/tr/fulltext/u2/686811.pdf , an early (the 
earliest?) BBN ARPANET IMP development report to ARPA.

Bob Kahn was our modem expert and knew what the (Bell 303?) modem could 
do.  See
    http://www.walden-family.com/impcode/bbn-report-1877.pdf
and search for "loop test".   While this is a 1973 revision of the 
report, I believe this description was in the original version of the 
report in 1969. My further memory is that such testing could be done 
remotely under software control.

On 10/20/2017 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.
>
> Looping was used as a primary operations tool in the ARPANET.  By
> "looping a line" the NOC operator could determine what was likely to
> have failed - the modem at either end, the line itself (backhoe attack),
> or the interface card in the IMPs at either end of the line.    The IMPs
> could trigger the "looping" capability of the modems to cause loops to
> be established at the various points.  That was important to determine
> whose Field Service to call to get a failure fixed.
>