[ih] More topology

[Noel Chiappa]

    > From: Jack Haverty

    > I never did learn how the PE handled RFNMs, in particular how it
    > interacted with its associated NCP host that it was "stealing" RFNMs
    > from.

I know a bit about the Port Expander; we were planning on using it at MIT at
one point, since MIT had no spare IMP ports for an IP gateway (router). (We
didn't get an IMP port for the MIT gateway until MIT got its third IMP, one
of the first C/30's.) That didn't work out, as I'll explain later.

The PE didn't share the NCP 'host' among connected hosts; all NCP traffic
coming in from the IMP is sent to the 'main' subsidiary host's port:

  ; WHEN A TYPE 0 OR TYPE 3 MESSAGE IS RECEIVED, FIRST CHECK THE MESSAGE'S
  ; LINK NUMBER.  IF THE MSG IS NOT ON AN INTERNET LINK, THEN SEND THE MSG TO
  ; THE PORT THAT RECEIVES ALL NON-INET TRAFFIC (PORT INDEX IS IN NCPPRT)

For IP traffic, the PE acts as a gateway (i.e. router), and there's a table
which says which downstream port various IP hosts are on.

The way it handles RFNM's is that it has a database of "CONNECTION BLOCK"s
which record messages sent out to the IMP; when a RFNM arrives, it uses the
CB database to work out the downstream host which originated the message the
RFNM is for; the RFNM is then handed to it.


As the above excerpt probably made clear, I still have the PE code (it had
been squirreled away on the MIT-CSR Unix - I made a full dump of that machine
before it croaked, so we now have access to all that history; I guess I was
concerned about history even back then).

I don't think I have the _original_, unmodified PE code; what I have is a
bodged version that I hacked to act as a gateway to the MIT 1 Mbit/sec ring
LAN. I.e. it did't have any subsidiary hosts attached to 1822 ports; just the
main 1822 port (connected to the IMP) and the LAN. I'm too lazy to
see exactly what I did with RFNM's there; probably just pitched them
(no RFNM's on a LAN :-).

While I was looking for that, I ran across some other old code that
might be interesing:

- the TIU (kind of a predecessor to the TAC, a _very_ early implementation of
  TCP in Macro-11 for the PDP-11, written by Jim Mathis, which I believe
  was the basis for Jack's first UNIX TCP at BBN);
- a couple of modules from the BCPL gateway code from BBN (the one that
  ran under ELF); historically interesting, as it was the very first
  IP router code _ever_.

If anyone is interested in any of this stuff, let me know and I'll look
into getting it uploaded and made available.


The reason we couldn't get the PE to work was that the SRI 1822 interface
(which is what were planning to use on our PE) didn't _exactly_ electrically
duplicate the IMP 1822 interface; the latter used optp-isolators on the DH
interface, and the SRI interface didn't have them.

The plan was to put the PE in from of the DM ITS machine, but when
we tried it, it didn't work. Ken Pogran looked into the issue, and
discovered that the person who did DM's IMP interface (I wonder who
that was :-) had done some 'trick' (the exact details of which now
escape me - it was something to do with the ground he used for the
DH interface signals),and without the opto-isolators the SRI
1822 interface wouldn't talk to it.


	Noel