[TUHS] PK protocol

[Paul Ruizendaal]

I ran a search for ‘Datakit’ on the archive of this maling list and came across the below message from Norman Wilson (Sep 2017). Having spent quite a bit of time recently on figuring out Datakit details and 8th Edition source, I now much better understand what he was saying — or at least I think I do.

It made me take another look at the /dev/pk[0123].c files in the V7 source code. I’d seen it before, but always thought it was UUCP code.

Now I’m wondering. It looks like UUCP packet "protocol g” is maybe much the same as the original (“Chesson”) packet algorithm for Datakit, and if so it would be “dual use”. It would seem that in V7 line discipline ‘0’ was normal tty handling, discipline ‘1’ was PK protocol over serial and line discipline ‘2’ was PK protocol over something with CRC in the driver - whatever that was.

If the above thought is correct, then it shines a light on network buffering in V7: it uses buffer space in blocks of n*32 bytes, carved out from a pool of disk buffers (see pk3.c); it pre-allocates space for one full receive window.

I have not fully figured it out, but at first glance it seems that the PK line discipline was only integrated with the DH-11 driver in the public V7 source. That would make sense in a networking context, as that board offered input buffering / DMA output to reduce the interrupt load. In 1979 Datakit seems to have connected over a DR-11C board, but there is no driver for that in the V7 source tree.

Am I on the right track?

=====

The point of the stream I/O setup with
stackable line disciplines, rather than the old single
line-discipline switch, was specifically to support networking
as well as tty processing.

Serial-device drivers in V7 used a single line-discipline
driver, used variously for canonical-tty handling and for
network protocols.  The standard system as used outside
the labs had only one line discipline configured, with
standard tty handling (see usr/sys/conf/c.c).  There were
driver source files for what I think were internal-use-only
networks (dev/pk[12].c, perhaps), but I don't think they
were used outside AT&T.

The problem Dennis wanted to solve was that tty handling
and network protocol handling interfered with one another;
you couldn't ask the kernel to do both, because there was
only one line discipline at a time.  Hence the stackable
modules.  It was possible to duplicate tty handling (probably
by placing calls to the regular tty line discipline's innards)
within the network-protocol code, but that was messy.  It also
ran into trouble when people wanted to use the C shell, which
expected its own special `new tty' line discipline, so the
network code would have to know which tty driver to call.
It made more sense to stack the modules instead, so the tty
code was there only if it was needed, and different tty
drivers could exist without the network code knowing or caring.

When I arrived at the Labs in 1984, the streams code was in
use daily by most of us in 1127.  The terminals on our desks
were plugged into serial ports on Datakit (like what we call
a terminal server now).  I would turn on my terminal in the
morning, tell the prompt which system I wanted to connect to,
and so far as I could tell I had a direct serial connection.
But in the remote host, my shell talked to an instance of the
tty line module, which exchanged data with a Datakit protocol
module, which exchanged data with the low-level Datakit driver.
If I switched to the C shell (I didn't but some did), csh would
pop off the tty module and push on the newtty module, and the
network code was none the wiser.

Later there was a TCP/IP that used the stream mechanism.  The
first version was shoehorned in by Robert T Morris, who worked
as a summer intern for us; it was later cleaned up considerably
by Paul Glick.  It's more complicated because of all the
multiplexers involved (Ethernet packets split up by protocol
number; IP packets divided by their own protocol number;
TCP packets into sessions), but it worked.  I still use it at
home.  Its major flaw is that details of the original stream
implementation make it messy to handle windows of more than
4096 bytes; there are also some quirks involving data left in
the pipe when a connection closes, something Dennis's code
doesn't handle well.

The much-messier STREAMS that came out of the official System
V people had fixes for some of that, but at the cost of quite
a bit more complexity; it could probably be done rather better.
At one point I wanted to have a go at it, but I've never had
the time, and now I doubt I ever will.

One demonstration of virtue, though: although Datakit was the
workhorse network in Research when I was there (and despite
the common bias against virtual circuits it worked pretty well;
the major drawback was that although the underlying Datakit
fabric could run at multiple megabits per second, we never had
a host interface that could reliably run at even a single megabit),
we did once arrange to run TCP/IP over a Datakit connection.
It was very simple in concept: make a Datakit connection (so the
Datakit protocol module is present); push an IP instance onto
that stream; and off you go.

I did something similar in my home V10 world when quickly writing
my own implementation of PPP from the specs many years ago.
The core of that code is still in use in my home-written PPPoE code.
PPP and PPPoE are all outside the kernel; the user-mode program
reads and writes the serial device (PPP) or an Ethernet instance
that returns just the desired protocol types (PPPoE), does the
PPP processing, and reads and writes IP packets to a (full-duplex
stream) pipe on the other end of which is pushed the IP module.

All this is very different from the socket(2) way of thinking,
and it has its vices, but it also has its virtues.

Norman Wilson
Toronto ON