[ih] Internet-history Digest, Vol 50, Issue 6

[Noel Chiappa]

    > From: Vint Cerf

    > the last point about speed adaptation is a key value in packet switching

    > From: John Day 

    > Packet switching had many advantages .. was the improvement over
    > message switching

Packet switching - at least non-virtual-circuit packet switching (I'm not
sure what the current technical term is for that now - would 'datagram packet
switching' be it?), the flavour of packet switching everyone takes for
granted these days, when 'packet switching' is used - has one key advantage,
for a global-scale network, in addition to the speed/delay issues you all
mention. Moreover, it's an advantage that IIRC we didn't appreciate at the
time - and an advantage that nobody has really thought about since, because
it _solves a problem that never happened_ - because NVC packet switching
solved it before anybody realized it _was_ a problem!

Moreover, it's an advantage over actual circuit networks, as well as virtual
circuit neworks - which makes it clear that it's not (just) the 'packets'
alone that are the solution to our never-problem, it's a _particular kind_ of
packets.

I refer to the growth of state in intermediate switches. If a data
communication system involves _any_ state in intermediate switches related to
a particular communication flow, the growth of that state, as the network
gets bigger, is going to be a real problen. The (obvious, in retrospect)
solution is to move all such state out to the leaves (hosts); since the
number of leaves will grow roughly at the same rate as the network itself -
problem solved - built-in scaling.

Maybe one of you smart people realized this beore I showed up, so it was not
necessary to talk about it - because I certainly don't recall any discussion
at any point after my arrival. (Then again, we were up to our chins - and
later noses - in real, pressing problems! :-) I remember reading a memo
written by Dave Clark, soon after I joined, which made the point that moving
the state out to the edges was good for _robustness_. Something much like it
was printed some years later (1988), a copy here:

  http://ccr.sigcomm.org/archive/1995/jan95/ccr-9501-clark.pdf

I wonder if anyone has a copy of the original, it would have been circa 1978
or so -  ISTR the phase 'they will all go together if they go', of
the connection state and the application. However, that note says nothing
about _scaling_.

So either it was so obvious it didn't need to be talked about - or it hadn't
been thought about - perhaps because the choice of NVC packets meant it was
(accidentally?) never a problem.


This was brought home to me forcefully a year or so ago, when people on the
UNIX history list, TUHS, started reminiscing about Datakit, and how nice it
had been, and roughly (from memory) 'Gee, it's too bad the Internet didn't use
Datakit'. My reaction be imagined. (OK, maybe not! :-) I did a quick 'back of
the envelope' calculation and asked them, given the average HTTP TCP
connection length, how many connection setups per second a Datakit backbone
switch with a couple of OC48 links would have to do. (It's _many_ milions per
second, or something like that.) Nothing more of how wonderful Datakit would
be for the Internet was heard on that thread.

(I am perhaps more attuned to this issue because controlling state growth
_was_ an issue for Nimrod, with flows; I invented stacked flow-ids - RFC 1753
- as part of the solution. This mechanism was ater recycled as the 'label
stack' in MPLS.)



PS: > From: John Day 

    > Packet switching had many advantages, but from the point of view of the
    > inventors (Baran and Davies)

I'm putting this down here, so it won't distract from my main point (above),
I would like to point out that an abstract of Baran's 1964 IEEE ToCS paper
(Paul Baran, "On Distributed Communications Networks", IEEE Transactions on
Communications Systems, Vol. CS-12 No. 1, pp. 1-9, March 1964) had been
published in "IEEE Spectrum" (circulation about 160,000 in those days) in
August 1964, so Baran's basic idea had been circulated very widely well
before Davies started to think about the problem.

Which is not to say that Davies _didn't_ genuinely completely independently
re-invent the concept of packet switching! But it's also _possible_ that the
germ for the idea came to him, say, in a lunch-time conversation with someone
who had either i) read about it in IEE Spectrum, or ii) had themselves heard
about it from a third person.

At this point, we'll never know for absolute sure. All we _can_ say, _for
sure_, was that Baran's ideas were published in the open literature in 1964.

       Noel