[e2e] Port numbers in the network layer?

Detlef Bosau detlef.bosau at web.de
Fri Apr 26 03:01:14 PDT 2013

Am 26.04.2013 03:00, schrieb l.wood at surrey.ac.uk:
>> On the other hand: Do you know a current technology which is actually
>> being used that does not use Ethertypes?
> CANbus, SpaceWire, CCSDS, RapidIO, (A)X.25...

Oh, yes :-)

I'm sorry about that...

Additional question: Can you tell me which car uses TCP over CANbus, 
e.g. to control his lamps? ;-)

But you made an important point: My view on this matter is too simplistic.
> But you can always  layer (Cisco) HDLC or HDLC/ Frame Relay across any
> of these to get an Ethertype. Or lobby SpaceWire to put a value in
> their single-byte not-an-Ethertype field.

At least we have do agree on talking about "packet switching networks" 
in a quite narrow sense here, when it comes to Ethertypes.

E.g. X.25 to my understanding is circuit switched. The packet switching 
view is mounted upon the top ;-) 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.

However, this is in fact a discussion of implementation issues.
> (The CCSDS community finds the thought of layering HDLC over CCSDS
> especially abhorrent, because it cuts down their custom engineering,
> and any layering or modularity is considered to be inefficiency.)

At least, it is not a "holy cow". Layers should assist network design. 
And not the other way round.

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