[ih] Origin of the loopback interface

[Brian E Carpenter]

> So, I'd advise caution in concluding that some features are not needed
> now, and were never really necessary.  I'd advise the IPV6 crew to think
> about not only how things should work when they work, but also how to
> deal with the situation when they don't.

Absolutely, as a general principle.

Thanks very much,
   Brian Carpenter

On 22/10/2017 09:29, Jack Haverty wrote:
> Hi Brian,
> 
> When we were designing those initial capabilities into TCP/IP
> implementations, we were thinking not only of how everything would be
> used in everyday operation, but also how you would deal with problems -
> when things were not working as expected.
> 
> Especially in those early days, IIRC things were more often not working
> than working...  So tools, techniques, hooks, etc., for "fault
> isolation" were introduced.  The early Internet was explicitly
> considered an "experiment", so lots of functions were needed to control
> and monitor that experiment.
> 
> Loopback functionality is an example.  So are other things, especially
> in ICMP, e.g., Ping, Source Routing, timestamping, etc.  The IP
> "options" in particular was a means for hopefully adding new tools no
> one had thought of yet.
> 
> Still-popular tools like Traceroute were possible because of the hooks
> we put in place.  For example, IP addressing was specifically set up to
> provide each physical interface with an address.  So a gateway/router,
> for example, had a separate IP address for each wire attached to it.
> That made it possible to use Ping, source-routing, etc., to "loop back"
> at many different points and determine exactly where a problem was
> occurring.
> 
> The downside of that architectural choice of IP address per physical
> port was one of the items in the list of "things to work on" - namely
> "Multi-homed hosts".  Our conclusion at the time (circa 1978) was that
> the advantages of having fault analysis tools exceeded the disadvantages
> of not dealing well with IP nodes that had multiple physical ports -
> something to be fixed "next year" as IPV4 phased into the next version.
> Didn't quite happen that way...
> 
> So, I'd advise caution in concluding that some features are not needed
> now, and were never really necessary.  I'd advise the IPV6 crew to think
> about not only how things should work when they work, but also how to
> deal with the situation when they don't.
> 
> /Jack
> 
> 
> On 10/21/2017 12:36 PM, Brian E Carpenter wrote:
>> 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
>>  
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