[e2e] Port numbers in the network layer?

Detlef Bosau detlef.bosau at web.de
Fri Apr 26 07:45:55 PDT 2013

Am 26.04.2013 15:26, schrieb Noel Chiappa:
>      > 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'.

And AFAIK exactly this holds true for X.25 and FR.

And yes, as far as I dealt with FR, any connection was to be set up 
throughout the whole network before it could be used.
There was no such thing like a "routing table" but there was a per 
connection switching table.
> 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
> the ARPANET.
> 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 it is a trade off ;-) As always ;-)

> 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...

Of course.

Your point to RSVP is interesting here, particularly from the aspect 
that RSVP did not really invent something that ground breaking new, but 
mapped existing mechanisms into the world of packet switching.

And with respect to the context of our discussion, we talked about 
Ethertypes here and to which degree some (switching) node must analyze 
packet headers in order to properly process a packet, and in a somewhat 
broader context the discussion was whether port numbers should be placed 
on transport layer or on network layer, it shows that perhaps not only 
me tends to oversimplify things - while at the same time dealing with 
too much details wouldn't make thins easier.

Sometimes, we have to make agreements. And, e.g., port numbers on the 
transport layer have worked fine for about 35 years now. (Is this 
correct?) So there must be extremely compelling reasons to restart this 

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