[e2e] Port numbers in the network layer?

Noel Chiappa jnc at mercury.lcs.mit.edu
Fri Apr 26 06:26:19 PDT 2013

    > From: Detlef Bosau <detlef.bosau at web.de>

    > X.25 to my understanding is circuit switched. ... The same holds true
    > for Frame Relay in a sense, however in FR the whole packets are switched
    > IIRC and not subdivided into smaller pieces.

That's not the most important characteristic of 'circuit switched', and in
fact, to me 'circuit switched' is characterized by a number of things (i.e.
it's not a single characteristic, but a suite of them).

So, for instance, when I head 'circuit switched', I think of things like
'per-connection state _in the network_ (not just at the hosts)', 'setup
required _in the network_ before communication can happen', 'reliability is
_in the network_, not done by the hosts'.

An important corollary is that there are a whole range of different things
which are not "circuit switched", but still have _some_ of the characteristics
that define that model. So Frame Relay has some of these aspects - but so did

At one point, Dave Clark started this concept that 'pure packet switched
models have some advantages, but some disadvantges, and pure circuit switched
models have some advantages, but some disadvantges, and the way to get all the
advantages and (mostly) none of the disadvantages is to have something which
is mid-way between the two'.

So things like RSVP, for instance, bring some (not all!) of the hallmarks of
circuit switching into the packet switching world. But "circuit switched" !=
"not packet switched"! The world is bigger than that...


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