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

[Vint Cerf]

inline response at end

On Sat, Jan 3, 2026 at 3:42 PM 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?
>

the point was the we were supporting end/end communication between
processes that lived in the operating systems of the hosts.
If the network appliance (e.g. IMP) failed for some reason, the source host
might never know unless the receiving host confirmed receipt. It was not
enough that the "IMP" (or attached processor) got the data - the source
host needed to know that the destination process or at least the host-based
TCP actually had delivered the data to the receiving process.  Moreover,
the networks were not all "reliable" - ethernet, packet radio and packet
satellite had network losses, so we moved the "end" into the receiving
host's operating system (and in some cases, like QUIC, into the user space).

v


>
> /Jack Haverty
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