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! D I G I T A L ! INTEROFFICE MEMORANDUM
+---------------+
TO: TOPS-10 Network Group DATE: 12 JUL 78
Karen Campbell (2) FROM: Ric Werme Doc # EJW-78-008-00-U
SWE Tech Archive DEPT: LCG Comm/Nets Engr.
EXT: 6059
CC: George Conant LOC/MAIL STOP: MR1-2/E89
Tony Lauck
Rob Ryan
SUBJ: NCL, the TOPS-10 Network Logical Link Protocol
Due to the few systems involved, the TOPS-10 network project has
survived without a specification for it's logical link level protocol.
However, protocol specs have never been known to hurt a project and the
lack of a spec has limited the impact of the TOPS-10 network
architecture on DECnet.
Therefore, I cannot resist the temptation to archive these three
documents as an informal spec of NCL.
The first document is a talk I presented at 10/20 Spring DECUS this
year. It's main value is to show how messages interact.
A fortuitous foulup results in two people preparing talks for the NCL
session. The second document is Scott Robinson's presentation which
will also be in the DECUS proceedings. It does a good job of covering
message details.
The third document was lost for a year or two and was found when I moved
my office - after DECUS. Patterned after the NSP V1 spec it was going
to become a formal spec but never made it.
COMPANY CONFIDENTIAL
RW/skb
1.0 NCL
The key to TOPS-10 networks is its NCL protocol. Developed late in 1974
it has undergone only minor revisions since then. I will not describe
the protocol and timing problems in entirity, but will emphasize the
types of messages and how they are used in common dialogs.
N E T W O R K
C O N T R O L
L A N G U A G E
THE ANF-10 LOGICAL LINK PROTOCOL
Creation Date
July 1978
Page 2
2.0 PROTOCOL LAYERING
This shows one interpretation of the protocol sturcture implemented in
the TOPS-10 networks. The bottom three layers are implemented in system
software. The physical link level provides an error free path between
two machines, the logical link level (NCL) provides an error free path
between any two nodes in a network and all the support to learn about
nodes in the network. The device control layer is a dialect of DECnet's
DAP version 1 and is mainly used to control terminals and unit record
devices. Customers may write their own application protocols to
implement systems that carry on a dialog across task to task links.
D E C N E T - 1 0 P R O T O C O L L A Y E R I N G
APPLICATION PROTOCOLS (USER WRITTEN)
DEVICE CONTROL (A VARIANT OF DAP V1)
LOGICAL LINK (NCL)
PHYSICAL LINK (DDCMP, DTE20 QUEUED PROTOCOL)
Page 3
3.0 NCL MESSAGE HEADER
All NCL messages have a header so I will describe it here and refer back
to as necessary.
The NCT field provides some information about the message type. The
important part is a three bit field that gives the message type. A zero
value means that the message is numbered. In this case, the type of
numbered message is supplied after the header. A non-zero value means
that the message is an unnumbered control message which I will cover in
the next slide.
The DNA and SNA fields contain the node numbers of the destination and
source of the message. NCL implementations scan all messages they
receive. When they see a message that is not for their node they pass
it on to the transmit code who routes the message to another node closer
to the destination.
I've mentioned message numbering. Unnumbered messages contain data that
can easily be regenerated so that their loss will do no more than slow
down the network. Numbered messages contain data that must be delivered
to the destination and processed in order. To guarantee delivery a
numbered message is saved after transmission until a positive
acknowledgement is returned. If a negative acknowledgement returns then
the message is retransmitted. The message number is incremented for
each message so that the receiver may order incoming messages. This
also allows several messages to be outstanding at once and lets the NCA
field acknowledge any number of them.
The next two fields handle most of this. The NCA field acknowledges
processing messages up to that number. In a numbered message, the
message number is contained in the NCN filed.
N C L M E S S A G E H E A D E R
NCT DNA SNA NCA NCN
| | | | |
| | | | |___ MESSAGE NUMBER OF THIS MESSAGE
| | | |
| | | |_________ ACKNOWLEDGEMENT NUMBER
| | |
| | |_______________ SOURCE NODE ADDRESS (BINARY NUMBER)
| |
| |_____________________ DESTINATION NODE ADDRESS (BINARY NUMBER)
|
|___________________________ MESSAGE TYPE AND FLAGS
Page 4
4.0 UNNUMBERED CONTROL MESSAGES
If your're familiar with DDCMP, you've probably noticed that NCL and
DDCMP use the same message numbering scheme. In fact, that and most of
the unnumbered control messages are taken directly from DDCMP! The ACK,
NAK, and REP messages are similar to DDCMP's. They extend the
information in the header but do not supply new data.
The ACK is really a NOP. Processing the NCA field leaves no work to do
here.
NAK messages will positively acknowledge messages if the NCA field is
the biggest seen so far. Any messages left on the acknowledge wait
queue are retransmitted.
REP messages are sent when an NCL implementation hasn't received a reply
to some unnumbered messages and wants to know what happened to them. A
node that receives a REP must respond with an ACK or NAK.
START and STACK are extensions of their DDCMP couteraprts. Along with
the NODEID message, they consitute most of the startup mechanism between
NCL nodes. These messages tell the destination who and what they are.
On TOPS-10 all this information is returned by the first line of NODE
command output. I'll describe how these are used in a later slide.
U N N U M B E R E D C O N T R O L M E S S A G E S
FORMAT:<HEADER>,<INFORMATION>
NCT NAME INFORMATION
1 ACK
2 NAK
3 REP
4 START NODE NUMBER, NAME,
5 STACK IDENTIFICATION,
6 NODEID AND CREATION DATE
Page 5
5.0 NUMBERED CONTROL MESSAGE
There are only four NCL messages that deal directly with logical links,
CONNECT and DISCONNECT are two of them. These messages control the
creation and destruction of logical links. Another message, DATA
REQUEST, provides flow control on the link. I'll explain how all of
these work in another slide.
The NEIGHBORS message provides routing information in the form of a list
of all the node's next door neighbors and the cost level of using the
physical links to them. Whenever a node's neighborhood changes, NCL
sends updated NEIGHBORS messages to all other nodes in the network.
Whenever a node receives a NEIGHBORS message it saves the new
neighborhood and calls its routing algorithm to determine which neighbor
is the best to use to get messages to each of the other nodes in the
net. The DN8x code also determines which neighbor is second best. This
secondary path will be used if its output queue length is much shorter
than the primary's.
REQ CONFIG messages are sent when a node wants to learn what network
devices are in the rest of the network. A node returns its
configuration in the CONFIG message. Users access this information by
NODE, UUOs or by reading the second line of NODE command output.
I could spend most of a session on STATION CONTROL messages. For now
let's just call them a separate protocol. Suffice it to say that they
provide utility functions such as examining or depositing memory at the
destination node or one of its neighbors via DDCMP maintenance messages.
N U M B E R E D C O N T R O L M E S S A G E S
FORMAT: <HEADER>.0.(CNT,TYP,<INFORMATION>)
TYP NAME INFORMATION
1 CONN DLA,SLA,DPN,SPN,MML
2 DISC DLA,SLA,RSN
3 NGH (NNM,LVL)
4 REQ CONFIG
5 CONFIG (OBJ,NDV,PID)
6 DATA REQ DLA,DRQ
7 STAT CTRL <STATION CONTROL MESSAGE>
Page 6
6.0 DATA
How about that - NCL even has a DATA message! it differs from a
numbered control message by this DLA field. In those, the DLA field is
zero.
NOTE
describe fields
A special form of the data message is called the interrupt message. It
bypasses the data request mechanism and may be sent anytime. However,
there is no guarantee that the destination can find the memory to
process it, so it is not used as a reliable form of data transfer.
Page 7
7.0 ADJACENT NODE INITIALIZATION
We've seen all the NCL messages and what they do. The next few slides
show how they work together. Please don't get concerned if I ignore
some of the timing conditions networks have to handle, we don't have
time to consider all of them.
Once communication is established on a physical link at the physical
link protocol level, NCL gets into the act and tries to start
communicating with its new neighbor.
describe
NODEID - no DNA, SNA
delay before ACK
ACKs will not be shown again
A D J A C E N T N O D E I N I T I A L I Z A T I O N
NODE A NODE B
NODEID -------------------------- > NODEID
/
< ------------------------/
STACK < --------------------------- START
\
\ -------------------------- >
REQ CONFIG ------------------------ > REQ CONFIG
/
< -------------------------/
CONFIG < --------------------------- CONFIG
\
\------------------------->
< ----------------------------- ACK
Page 8
8.0 NONADJACENT NODE INITIALIZATION
On the previous slide NODEID messages initiated a START/STACK exchange.
Between nonadjacent nodes NEIGHBORS messages do the same job. In this
slide node C has just come up and is joining nodes A and B.
describe
B,C startup timing
A's START beating B's NEIGHBORS
REQ CONFIG and CONFIG messages not shown
N O N - A D J A C E N T N O D E
I N I T I A L I Z A T I O N
NODE A NODE B NODE C
NODEID NODEID
START START
NGH STACK
STACK
START NGH
STACK
Page 9
9.0 LOGICAL LINK INITIALIZATION
After NCL starts between a pair of nodes in a network, one can access
resources on the other. This slide shows a simple example.
describe
1: 0 DLA;DPN, SPN
2: DLA, SLA
3: RSN
The DISCONNECT message is used both to refuse connection requests and to
break complete connections.
NOTE
note show how DISC would be used.
There is one special reason code I should mention here called reconnect.
When a node receives this at the beginning of the disconnect sequence it
returns the answering disconnect and then initiates a connection to a
new node named in the first disconnect message. This is used to
implement the SET HOST command.
L O G I C A L L I N K I N I T I A L I Z A T I O N
CONN[DLA(0),SLA(LINK#),DPN(3,177),SPN(0,LPTSPL[1,2]),MML(?)]
------------------------------------>
CONN[DLA(LINK#),SLA(THIS NODE'S LINK #),...]
< -----------------------------------
-- OR --
DISC[DLA(LINK #),SLA(0),RSN(#)]
< -----------------------------------
Page 10
10.0 DATA FLOW
Once a connection is setup you would expect data to flow. To prevent
the receiver from overflowing his buffers the DATA REQUEST message is
used to request data when buffer space is probably available.
describe
2 CONNECTs
DRQ(2)
delay, what can be done
D A T A F L O W
NODE A NODE B
CONN
CONN
DRQ(2)
DATA
DATA DRQ(1)
DRQ(1)
DATA
DATA DRQ(1)
DRQ(1)
DATA
DATA DRQ(1)
Page 11
11.0 TERMINAL PROTOCOL
We have enough time to take a peek at this last slide which shows the
terminal protocol used in the TOPS-10 networks. This will be much
better described in the DECUS proceedings.
NOTE
first three DAP types used in unit record
control
Uses of control types.
T E R M I N A L P R O T O C O L D A T A
M E S S A G E S
FORMAT:<HEADER, DLA,(CNT,DAPTYP,<INFORMATION>)
DAPTYP NAME INFORMATION
1 DATA WITHOUT EOR <DATA>
2 DATA WITH EOR <DATA>
3 STATUS <STATUS BITS>
4 CONTROL CTLTYP,<INFORMATION>
CONTYP NAME INFORMATION
0 ECHO MARKER SERIAL #
1 CHARACTER GOBBLER
2 CHARACTERISTICS BYTE DATA (SPEEDS, FILL
TIMES, ETC.)
3 AUTO DIAL 6 BYTES OF DIGIT PAIRS