[ih] when did APRANET -TIPs become known as -TACs

[Jack Haverty]

On TIPs, TACs, IMPs, C/30s, ...  FYI, what I remember:

The original IMPS were standard Honeywell minicomputers with additional 
hardware for I/O and real-time clock functionality designed by BBN.  
TIPs were IMPs, but with more memory installed and additional hardware 
to support I/O to terminals (or modems), and some more software of course.

So, IMPs and TIPs were essentially the same basic machine.  Hosts 
connected to both IMPs and TIPs through an "1822 interface" defined in 
BBN Report 1822.

X.25 was added to the IMPs.   I suspect that occurred only with the C/30 
hardware but can't remember the timing.   DDN desired X.25 since that 
was part of OSI, was encouraged by the DoD GOSIP efforts, and was more 
likely to be available on all sorts of computers than the obscure 1822 
interface.   IIRC, the X.25 addition to the IMP code was funded by DCA.

X.25 was supported by gateways in the US and UK over the public X.25 
networks.   So the public X.25 global network could be considered one of 
the networks underlying The Internet.  But I don't remember if TACs ever 
used X.25 to carry TCP/IP, or whether or not X.25 was ever used as DDN 
expanded inside DoD.

Of course BBN also took advantage of the availability of X.25 for sales 
into the commercial marketplace.   There were lots of "ARPANET Clones" 
using X.25 in corporate networks through the 1980s.

Much of the work in the 1980+ timeframe was in preparation for the 
January 1, 1983 conversion of the ARPANET from NCP to TCP.  There was 
lots of planning for that transition.  At the time, I was on the ICCB 
and one of its tasks was to help figure out what needed to be done in 
order to effect that transition.  The planning and implementation 
literally took several years, with DCA, ARPA, and several other parts of 
the US government involved in putting all the pieces in place.

One task in that timeline was to get TCP implemented in the TIPs. Bob 
Hinden is probably the authority on that part of the evolution.   IIRC, 
he wrote the code to put TCP into the TIPs, before moving to lead the 
project to make the "core gateways" into a 24x7 operational service.

In addition, there was another program called TACACS (TAC Access Control 
System) which implemented a password scheme for controlling access to 
the TACs.   It was a follow-on to an earlier project called "TIP 
Login".   TACACS was considered a necessary component for DDN to control 
who could use the 'net(s).

Much of this history was probably well-documented in the reports 
submitted by BBN, as well as all sorts of other materials from the 
government or other contractors.  It may be available on 
discover.dtic.mil -- e.g., search for "ddn tac bbn"

/Jack Haverty


On 9/26/25 17:26, Vint Cerf via Internet-history wrote:
> At some point, the BBN Cxx machines also offered an X.25 interface because
> around 1983, I was using them to build MCI Mail and the network interfaces
> were X.25 vs BBN 1822, for example.
>
> v
>
>
> On Sat, Sep 27, 2025 at 1:00 AM Noel Chiappa via Internet-history <
> internet-history at elists.isoc.org> wrote:
>
>>      > From: Jorge Amodio
>>
>>      > TIP (Terminal Interface Processor) ~= TAC (Terminal Access
>> Controller)
>>
>> The biggest difference between the two was that the TIP used the older
>> Network Control Protocol protocol family, whereas the TAC was also able to
>> use the newer TCP/IP family. (I see from "Getting Started Computing at the
>> AI
>> Lab":
>>
>>    https://dspace.mit.edu/bitstream/handle/1721.1/41180/AI_WP_235.pdf
>>
>> that a TAC user could switch back and forth - pg. 17 of the PDF.)
>>
>> The TIP also included IMP functionality, and TIPs could be ARPANET nodes;
>> the
>> 'MIT-TIP' machine used to be one of the two IMPs at MIT: IMP #44, to be
>> exact.
>> Not so for TACs.
>>
>> Does anyone remember how TACs which used to be TIPs connected to the
>> network,
>> though? (I mean using what hardware?) I see that the host 'MIT-TIP' used to
>> be 2/44; i.e. host 2 on IMP 44. That machine would have conncted to the
>> rest
>> of the ARPANET via phone lines (so through a modem port). However in its
>> later 'MIT-TAC' incarnation, it was 10.2.0.77; i.e. it was connected to IMP
>> 77 - I would assume via an 1822 port? Did ex-TIP machines use one of their
>> one-time 1822 'IMP' ports as a 'host' 1822 port?
>>
>> I should know that, because I was the MIT 'IMP liaison', and I do remember
>> a
>> bunch of events with them (e.g. the move of MIT-MC, which involved
>> converting
>> an LH port on a C/30 IMP to a DH), but I have no memory of the switch to
>> the
>> C/30 IMPs.
>>
>> Also, were there any TACs which were not one-time TIPs? If so, what was the
>> hardware base? Maybe a C/30 with a multiple serial line controller?
>>
>>          Noel
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