SPECIFICATION OF THE INTERNAL LLP

I. ASSUMPTIONS

The internal LLP assumes that a transaction is the exchange of one
pair of messages. The `request message' is passed from the client to
the server, which sends a `response message' back to the client.
It is meant to meet the following needs:

1. be as simple as possible
2. signal the end of a message as an `end of file'
3. allow a site to cancel and restart the message that it
   is currently about to send

The internal LLP is supposed to use UNIX domain stream type sockets as
it's channel and probably will not run on any other channel. This is
because the protocol depends on specific properties of the channel
that only the UNIX domain stream type socket has: Reliable sequenced
unduplicated transport of data bytes governed by a stream semantic
along with the immediate notification of the existence of out-of-band
data. The latter property is not guaranteed by Internet domain TCP
type sockets. It is because of this, that the internal LLP is never an
option for transactions between different machines.

Unfortunately out-of-band messages are not supported on 4.3BSD (Net2)
and 4.4BSD (lite) UNIX domain sockets. In fact, I am not aware of any
Unix system that does implement oob data for Unix domain sockets and I
suspect there is no such implementation at all. However oob data is
supported in Internet domain sockets, with the limitation that
transmission may be delayed.


II. NORMAL OPERATION

	     CLIENT			     SERVER

A.			CONNECTION SETUP

1.	connect to server		accept connections

			     yields

		newly opened, bidirectional, stream
		type socket

2.	write				read

		from-address as a character string
		delimited by ASCII STX


B.			REQUEST MESSAGE

3.	write				read

			message data

4.	shutdown write part of		read returns 0 delivered as
	socket				`end of file' condition to
					higher layers


C.			RESPONSE MESSAGE

5.	read				write

			message data
	
6.	read returns 0 delivered as	shutdown write part of
	`end of file' condition to	socket
	higher layers


D.			CONNECTION SHUTDOWN

7.		socket became unusable since it is 
		mutually widowed on both ends

	close socket			close socket


III. EXCEPTIONS

1. RESTART MESSAGE

The site which is about to send a message may restart the transmission
of the same message by sending the single ASCII character STX out of
band.  The other site is notified of the exception by a SIGURG signal.
Upon reception of SIGURG the receiving site discards all data up to
the mark, which indicates when the exception occured. The receiver
restarts reception of the message immediately after that mark.

2. CANCEL TRANSACTION

The site which is about to send a message may cancel the whole
transaction by sending the single ASCII character CAN out of band. The
other site is notified of the exception by a SIGURG signal. Any site
may abort the transaction without further notification when it is not
currently about to send a message. The notificateion is only necessary
in order to allow the receiving process to distinguish between normal
end of message block and abort.