Trailing-Edge - PDP-10 Archives - BB-H348C-RM_1982 - swskit-v21/pmr/psthru.mem
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	In a phase II DECnet network, or a network with phase II nodes,
or in a phase III DECnet network partioned in such a way that some nodes
are invisible from other nodes, it happens that physical links can
exist between two nodes where logical links can't exist. Consider
this example:

	A --- B --- C

	If nodes A, B, and C are phase II nodes, then node A and node
C cannot easily communication. However, if one runs a program in node
B to which programs in  node A can connect and this program connects
to programs in node C, then A and C can talk.
	The pass through program does just this. Any program in any node
can connect to the passthru program in an adjacent node. The passthru
program is then told what path should be taken to the target node
and what object to connect to on the target node. If the target node
is adjacent, the passthru task connect to the desired object on the
target node. If not, the passthrough task connects to the next
passthrough task and passes on the path or "routing" information.

	To connect to a passthrough task make a connection to object
type 123. Then pass an 8-bit byte record of the following format
on the logical link:

BYTE 1:	1
BYTE 2-N:	8 bit byte ascii string of the form:

	Next, read a record from the logical link. This record has the
following format and indicates whether the connection was made successfully
or not:

BYTE 1:	1 to indicate success, 2 to indicate failure
BYTE 2-N:	If success, this is the route taken in the same form
		as the previous record.
		If failure, this is an error message string.

	Then proceed with your application as if you had made a local
connection. Note that this algorithm requires any active task to have
code to connect to object 123 and send the passthrough record and receive
the status reply. This code is in HOST, network MS, and the maintenance
release of NFT/FAL on TOPS-20. The paths for MS and NFT are gotten from
the file SYSTEM:DECNET-HOSTS.TXT. The path for HOST can be specified
on the command line, or gotten from the system file. Following is
an example of such a file:

!Sample routing file from SYSTEM:DECNET-HOSTS.TXT

	To build and install pass through on a DECsystem-20 do the following.
Compile link and save PSTHRU.MAC. In system startup somewhere run
PSTHRU.EXE. This program will support up to 8 pass through links at once.
	PSTHRU.EXE runs the program SYSTEM:-PTS-.EXE. To build this
EXE file compile snolib.mac. Then type the following:


	When you do this SNOLIB.REL must be on disk and the snobol
must be the one from this directory.