[ih] Overlay networks (1980s SRI Reconstitution Protocol)

[Jack Haverty]

The ICCB discussed the "reconstitution" need back in the early 1980s.  
The concept was that the Internet could be used to re-establish 
host-host connectivity when an underlying network, e.g., the ARPANET, 
was partitioned.  This was thinking beyond research to operational 
military usage, when the underlying network, i.e., the Defense Data 
Network, might be partitioned due to enemy activity.

The mechanism would require multiple paths through the Internet. That 
situation existed at the time in the research installations. For 
example, the WBNET provided connectivity between the US east and west 
coasts, and could serve as a backup to the ARPANET connectivity.   
Similarly, SATNET provided connectivity to Europe, and the X.25 public 
network provided a second path.

I don't know any details of what was actually tested but the notion was 
that the relevant gateways could notice that one of their underlying 
networks (e.g., the ARPANET) had become partitioned. They could detect 
that situation by their sudden inability to communicate with other 
gateways on a particular underlying network.

A similar procedure had been used internally within the ARPANET, to 
detect and route around circuit failures.  Gateways talked to each other 
much as IMPs did, to detect failures and route around them. But they 
assumed at that time that networks were either working or not working.  
In the ARPANET, circuits never "partitioned", and the gateway system had 
used the same assumptions about its underlying "circuits", i.e., the 
underlying networks.  "Reconstitution" was possible in the Internet, 
where it wasn't in the ARPANET.

The gateways could then exchange routing information, using the paths 
between gateways that still were functional, to begin to treat the 
broken ARPANET as two separate networks.   Routing changes would then 
direct Internet traffic as needed to re-establish communications between 
host pairs on the broken network(s).

This of course required hosts to be using TCP/IP, and also a robust 
connectivity within the Internet, so that there were alternate paths to 
be used.

The underlying "broken" network was not itself patched back together.  
But assuming the individual pieces of that network continued 
functioning, traffic through the Internet could be resumed, passing 
around the breaks.  The ARPANET pieces would continue to work even when 
partitioned.

I recall an incident in the early 1980s when an operator from the 
ARPANET NOC stuck his head in my office and frantically asked if the 
Internet could be used to get to some west-coast computer from our 
east-coast location, without using the ARPANET.  Couldn't happen - there 
was no such "reconstitution" mechanism in place at the time. Curious, I 
wandered down to the NOC to see what was going on.  Some combination of 
buggy software and possibly an errant backhoe had partitioned the 
ARPANET, and the operators were trying to figure out how to put it back 
together.  Turned out bugs and backhoes had similar effects to bombs and 
missilies.

"Reconstitution" was about re-establishing communications through the 
Internet, not fixing broken underlying networks.

/Jack Haverty


On 8/21/25 12:38, John Day via Internet-history wrote:
> This topic has come up several times here. Could someone please explain it to me.
>
> My understanding is that this was a protocol to repair network partitions.
> If there is a network partition, there are two issues:
> 1) it can’t be known that it is a partition until it is over. From within the network, a network partition is indistinguishable from a number of hosts being down for some other reason.
> 2) If it is a network partition, there is no communication between the partitions so how can there be a protocol to repair it?
>
> Thanks,
> John Day
>
>> On Aug 21, 2025, at 15:32, John Gilmore via Internet-history<internet-history at elists.isoc.org> wrote:
>>
>> Barbara, thank you for reminding us of the Network Reconstitution Protocol.
>>
>> Here are two copies of their final report:
>>
>>   https://apps.dtic.mil/sti/tr/pdf/ADA184755.pdf
>>   https://archive.org/details/ADA184755/
>>
>> They appear to come from the same (microfiche) source.
>>
>> Also here's a brief note by Greg Skinner about this work and report:
>>
>>   https://gregbo.medium.com/network-reconstitution-protocol-79bc61067f8e
>>
>> 	John
>>
>> PS: Due to Greg Skinner's comment at the bottom of this Computer History
>> Museum story, I ran across this today, from back when the Internet
>> itself was barely an overlay network:
>>
>>   "Born in a Van: Happy 40th Birthday to the Internet!
>>   40th Anniversary of the First Major TCP Internetwork Demonstration, November 22, 1977"
>>   by Marc Weber
>>   Nov 22, 2017
>>   https://medium.com/chmcore/born-in-a-van-happy-40th-birthday-to-the-internet-d81287f172bf
>> -- 
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