A Technical History of National Physical Laboratories (NPL) Network
Architecture - A Technical Tour
Technical Detailed Tour
PDU
- Protocol Data Unit (PDU), Name: packet, Length: Variable Length, 128-bits
to 1024 bits
The protocol data unit (PDU) is called a packet. It is Davies
who coined the term, which represents the broken-up message blocks used
in Paul Baran's distributed network, the Cyclades network, the ARPANET,
and today's Internet.
A packet is a short message with a fixed format made up
of 128-bit segments. The length of a packet can vary from 128 bits to
1024 bits. The small segments are designed to improve the efficiency of
transmitting short packets. The first segment of a packet, called a header,
differs from other segments because it contains a header information about
the message transmission.
The format of each message is described in the following
figure:
Connection
- Type of network service provided, either connection-oriented or
connectionless
The network implements connectionless protocol because there
does not exist an initial handshaking or connection establishment procedure.
Addressing
- Scheme used to identify a computer on the network
The network implements a flat addressing scheme. Each computer
on the network has a unique identifier.
Fragmentation and Reassembly
- Breaking up messages into smaller transmittable units
The interface computer handles all fragmentation and reassembly
of messages. The source computer fragments and encapsulates a message
into packets, while the destination interface computer reassembles incoming
packets.
Multiplexing
- Gathering data from different processes and encapsulating each piece
of data with a header
Demultiplexing
- Using information contained in a packet header to direct it upward through
a protocol stack.
Multiplexing took place at the interface computer using
a demand sorter. Based on it's priority, a byte arriving at one of the
eight inputs of a demand sorter would be transfered to the deamnd at the
next level. At the top level data was transferred directly to the memory
of the computer without software intervention. A three-bit address corresponding
to the input of the demand sorter was attached to an incoming byte to
help determine the origin at each stage by the program. A scheme of depth
3 multiplexing would allow a maximum of 512 devices to be connect. This
was sufficient to handle the 200 devices initially estimated by the Davies.
(Campbell-Kelly p231)
Routing
- Determining a good path from a source to a destination
There are three programs within the node computer that works
concurrently to effectively manage the routing of the packets: the Main
Processor (MP), Input Processor (IP) and the Output Processor (OP). The
MP includes the 'Output Queue Subroutine' (OQS) and the 'Input Queue Subrouting'
(IQS).
These programs inside the node computers are responsible
for routing the packets through the network by using an adaptive routing
strategy. First, the IQS selects the top packet in the input queue. Then
it increments and checks the handover number, changes the packet indicator
field and finds an appropriate output queue. The subroutine uses routing
tables to decide which link a packet should be transmitted through. The
tables include output link numbers, which represent the destination of
each link. These link numbers are ranked from the highest preference to
the lowest. The IQS selects the link with the highest preference whose
queue is not empty and link is not down.
The OQS takes the top item off the output queue and checks
to see if the outgoing link is available. If the link is available, then
the indicator of the packet is placed into trace mode. The OQS checks
the trace queue to see if it has room for the new packet. If there is
space, then it initiates transmission.
The IP is responsible for placing packets in the input queue
and putting the packet either at the end or the beginning of the queue
based on it's priority. The OP's task is to signal when the hardware is
free and to transmit outgoing packets.
Protocol
- Defines format, order of messages sent and received among network entities,
and actions taken on message transmission and reception.
An alternating bit protocol was used to transmit data reliably
over a serial link. Each unit of data transmitted is identified with a
single bit. This bit alternates with between each successful transmission
of units of data. (Barlett, Scantlebury, and Wilkinson p260, 13)
Congestion Control
- Network management strategy whose goal is to alleviate or avoid congestion
It is possible under heavy load conditions that a packet
may be caused to circle around its required destination node. To prevent
a packet from orbiting forever under these conditions, an 8-bit handover
number is incremented at each node. When the handover number reaches a
certain value, the packet can either be returned to its source or dropped.
A Shut-up message tells a node at the far end to
stop sending because the receiving end is busy or broken.
Flow Control
- A mechanism in which the receiver of the data changes the transmission
rate of the sender so data will not arrive too quickly to process
Multiplexing at the network level implements flow control.
A 'round robin' scheme examines each of the eight input channels for incoming
data. In the event that the buffers which holds incoming data are full,
the multiplexer continues to 'take and envelope' from each channel, but
does not receive data. Then the network reduces the rate of the fastest
channel. These steps help resolve the congestion in the network.
Error Detection
- Noticing corrupted or lost data during the transmission over the network
Error detection is the responsibility of each link's receiving
node computer. A checksum, which is included in every segment, is used
by the hardware in the node computer to detect link errors. Once an error
has been detected, a trace message is sent back to the sending node computer
along the originating links using special hardware. The receiving computer
then request for the sender to re-transmit the packet. After special hardware
in the sending node computer receives the trace message, it re-transmit
the packet.
Error Recovery
A trace queue holds the three most recent packets that have
been sent out on the network. It re-sends lost packets or retransmit packets
that have detected errors. If the new packet is the 4th packet of the
trace queue, then the first packet of the queue was successfully at the
next node. If a node computer detects a trace message, the sender takes
appropriate action to re-transmit the packet.
Security
As an experimental architecture, security features such
as authentication and privacy were only conjectured. Over connectivity
provided safeguards against broken or damaged links.
