Un poco de Investigación Histórica
¡Gestación del protocolo Internet!
The Postman Analogy
The
switching process in any store-and-forward system is analogous to a postman
sorting mail. A postman sits at each switching node. Messages arrive
simultaneously from all links. The postman records bulletins describing the
traffic loading status for each of the outgoing links. With proper status
information, the postman is able to determine the best direction to send out
any letters. So far, this mechanism is general and applicable to all
store-and-forward communication systems.
Assuming
symmetrical bi-directional links, the postman can infer the "best"
paths to transmit mail to any station merely by looking at the cancellation
time or the equivalent handover number tag. If the postman sitting in the
center of the United States received letters from San Francisco, he would find
that letters from San Francisco arriving from channels to the west would come
in with later cancellation dates than if such letters had arrived in a
roundabout manner from the east. Each letter carries an implicit indication of
its length of transmission path. The astute postman can then deduce that the
best channel to send a message to San Francisco is probably the link associated
with the latest cancellation dates of messages from San Francisco. By observing
the cancellation dates for all letters in transit, information is derived to
route future traffic. The return address and cancellation date of recent
letters is sufficient to determine the best direction in which to send
subsequent letters.
Hot-Potato Heuristic Routing Doctrine
To
achieve real-time operation it is desirable to respond to change in network
status as quickly as possible, so we shall seek to derive the network status
information directly from each message block.
Each
standardized message block contains a "to" address, a
"from" address, a handover number tag, and error detecting bits
together with other housekeeping data. The message block is analogous to a
letter. The "from" address is equivalent to the return address of the
letter.
The
handover number is a tag in each message block set to zero upon initial
transmission of the message block into the network. Every time the message
block is passed on, the handover number is incremented. The handover number tag
on each message block indicates the length of time in the network or path
length. This tag is somewhat analogous to the cancellation date of a
conventional letter.
The Handover Number Table. While cancellation dates could
conceivably be used on digital messages, it is more convenient to think in
terms of a simpler digital analogy--a tag affixed to each message and
incremented every time the message is relayed. Figure 11 shows the handover
table located in the memory of a single node. A row is reserved for each major
station of the network allowed to generate traffic. A column is assigned to
each separate link connected to a node. As it was shown that redundancy levels
on the order of four can create extremely "tough" networks and
additional redundancy brought little, only about eight columns are really
needed.
Digital Simulation
This
basic routing procedure was tested by a Monte Carlo simulation of a 7x7 array
of stations. All tables were started completely blank to simulate a worst-case
starting condition where no station knew the location of any other station.
Within 1/2 second of simulated real world time, the network had learned the
locations of all connected stations and was routing traffic in an efficient
manner. The mean measured path length compared very favorably to the absolute
shortest possible path length under various traffic loading conditions.
Preliminary results indicate that network loadings on the order of 50 per cent
of link capacity could be inserted without undue increase of path length. When
local busy spots occur in the network, locally generated traffic is
intermittently restrained from entering the busy points while the potential
traffic jams clear. Thus, to the node, the network appears to be a variable
data rate system, which will limit the number of local subscribers that can be
handled. If the network is carrying light traffic, any new input line into the
network would accept full traffic, perhaps 1.5 million bits per second. But, if
every station had heavy traffic and the network became heavily loaded, the
total allowable input data rate from any single station in the network might
drop to perhaps 0.5 million bits per second. The absolute minimum guaranteed data
capacity into the network from any station is a function of the location of the
station in the network, redundancy level, and the mean path length of
transmitted traffic in the network. The "choking" of input procedure
has been simulated in the network and no signs of instability under overload
noted. It was found that most of the advantage of store-and-forward
transmission can be provided in a system having relatively little memory
capacity. The network "guarantees" very rapid delivery of all traffic
that it has accepted from a user (see ODC-II, -III).
Forgetting and Imperfect Learning
We
have briefly considered network behavior when all links are working. But, we
are also interested in determining network behavior with real world links--some
destroyed, while others are being repaired. The network can be made rapidly
responsive to the effects of destruction, repair, and transmission fades by a
slight modification of the rules for computing the values on the handover
number table.
Learning
In the previous
example, the lowest handover number ever encountered for a given origination,
or "from" station, and over each link, was the value recorded in the
handover number table. But, if some links had failed, our table would not have
responded to the change. Thus, we must be more responsive to recent
measurements than old ones. This effect can be included in our calculation by
the following policy. Take the most recently measured value of handover number;
subtract the previous value found in the handover table; if the difference is
positive, add a fractional part of this difference to the table value to form
the updated table value. This procedure merely implements a
"forgetting" procedure--placing more belief upon more recent
measurements and less on old measurements. This device would, in the case of
network damage, automatically modify the handover number table entry so as to
exponentially and asymptotically approach the true shortest path value. If the
difference between measured value minus the table value is negative, the new
table value would change by only a fractional portion of the recently measured
difference.
This
implements a form of sceptical learning. Learning will take place even with
occasional errors. Thus, by the simple device of using only two separate
"learning constants," depending whether the measured value is greater
or less than the table value, we can provide a mechanism that permits the
network routing to be responsive to varying loads, breaks, and repairs. This
learning and forgetting technique has been simulated for a few limited cases
and was found to work well (see ODC-II, -III).
Adaptation to Environment
This
simple simultaneous learning and forgetting mechanism implemented independently
at each node causes the entire network to suggest the appearance of an adaptive
system responding to gross changes of environment in several respects, without
human intervention. For example, consider self-adaptation to station location.
A station, Able, normally transmitted from one location in the network, as
shown in Fig. 12(a). If Able moved to the location shown in Fig. 12(b), all he
need do to announce his new location is to transmit a few seconds of dummy
traffic. The network will quickly learn the new location and direct traffic
toward Able at his new location. The links could also be cut and altered, yet
the network would relearn. Each node sees its environment through myopic eyes
by only having links and link status information to a few neighbors. There is
no central control; only a simple local routing policy is performed at each
node, yet the overall system adapts.
Fuentes consultadas
http://www.isoc.org/internet/history/index.shtml
Historias de Internet
http://www.isoc.org/internet-history/brief.html
Breve Historia de Internet
http://www.intec.edu.do/~bistec/Internet/historia/Una_historia_abreviada_del_Internet.htm
Breve Historia en Español
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