[ih] History from 1960s to 2025 (ARPANET to TCP)

[Barbara Denny]

 A little busy right now but I wanted to mention that I think people should consider how packet radio might have motivated the new architecture. I think support for mobility (nodes and hosts) certainly had an impact on not offering reliability  in the network.  I think there was practical constraints for how much functionality could be placed in the radio (network node).  I don't remember which version of the radio (second generation????) had a dual processor but I think it was because more processing power was required.  Also I believe there was a requirement that packet radio had to operate without any fixed infrastructure (Military necessity).
BTW. I also think that Jim Mathis slimmed down the TCP implementation in the LSI-11 because of memory(?) constraints. 

barbara


    On Saturday, January 3, 2026 at 12:42:06 PM PST, Jack Haverty via Internet-history <internet-history at elists.isoc.org> wrote:  
 
 On 1/1/26 01:26, Lars Brinkhoff wrote:
>  I'd like to say one thing we have observed running this
> ARPANET reconstruction is how resilient and self organizing the IMP
> subnet[*] is.

I totally agree with Lars.   When I did a "deep dive" in 2012 into parts 
of the 1973 IMP code, I was impressed at how it did its functions, 
especially given the severe constraints of the computer hardware of the era.

But thinking about the transition from ARPANET to Internet brought up 
some questions.....

The ARPANET nurtured the Internet and eventually was decommissioned.  
Reading the original ARPANET proposal by BBN, there were a number of 
arguments made about the use of an external computer (the IMP) instead 
of adding the required network functions to the "host" computers.  For 
example, owners of those expensive host computers didn't like the idea 
of added overhead from network operations consuming their CPU cycles.  
Another argument was that maintenance and evolution of the network 
mechanisms would be easier with a uniform set of IMPs, operated and 
maintained by a single organization.

When TCP appeared, its architecture placed much of the work of network 
functions on the host computers, which were now responsible for the 
mechanisms to counteract errors during network transits. Checksumming, 
retransmissions, re-ordering, and related functions previously performed 
in the IMPs were now performed in the host computers.    The new 
architecture pushed the "ends" of the network machinery closer to the 
users, which better conformed to the "end-to-end" principle which was 
the popular goal at the time.  But the new architecture also removed the 
clean boundary between the network and the hosts (the "1822 
specification"), as well as the possibility of having "the network" 
operated and managed by a single organization.

Some TCP implementers in the 1980s chose to use a "front end" approach, 
placing all of the TCP mechanisms in a separate processor somehow 
attached to their main computer.   AFAIK, such implementations have 
mostly disappeared.

I was at BBN from 1977 to 1990 in the same group that had built the 
ARPANET.   During that time, the internal mechanisms of the IMP "subnet" 
changed significantly.   I remember the introduction of "PSN7" and the 
lengthy and elaborate process (analysis, simulations, tests, etc.) to 
assure that the transition went well (it did, PSN6 was replaced by 
PSN7).  More info on the process is in DTIC.   One such report is 
https://apps.dtic.mil/sti/pdfs/ADA121350.pdf

In the new Internet architecture today, TCPV4 is still in use, even 
though TCPV6 was "on the shelf" decades ago.   Has the delay been a 
result of the change in architecture?  Are we missing the "process" for 
evolution of the networking mechanisms?  Is such a process even possible 
given the size and breadth of the Internet?

So, my basic question for History is "Why did the architecture change?"  
  Were the arguments for a separate network switch (e.g., an IMP) no 
longer applicable?    Did the technology explosion during the 70s have 
some effect?  What was the reasoning behind the decision to move the 
"virtual circuit" mechanisms from the network (IMPs) to the hosts?

/Jack Haverty
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