This had ended up in my spambox for some reason. Sorry for the late
reply, and I'll just make a couple of short comments...
On 2015-10-07 15:54, Paul Koning wrote:
On Oct 6, 2015, at 9:08 PM, Clem Cole <[email protected]> wrote:
On Tue, Oct 6, 2015 at 8:36 PM, Johnny Billquist <[email protected]> wrote:
Right. I suspect Rich was actually thinking of the 7 layer OSI model, which DEC
tried really hard to implement.
insert >>eventually<< in there and I agree.
In the first few generations of DECnet, as Rich points out, DEC was no different than
anyone else and had a closed networking system and it was not that clean. By DECnet
phase III many lessons had been learn and it really was a nice subsystem. However, I've
always felt that one of the failures of DECnet was the dogged adherence to ISO protocol
later made them ignore the "IP" part of the the Internet protocol for too long
- because in fact OSI did not really have it in its model.
You're mixing up layer models and specific protocols. TCP/IP is (roughly) an
instance of the OSI model, just as DECnet Phase III and IV are. Specifically,
DECnet routing layer, IP, and OSI IS/IS are all network layer (layer 3)
protocols.
That is like saying that any network stack is roughly an instance of the
OSI layer, which then becomes a rather meaningless comment. I think we
need to take OSI a little more literally than just "a network stack".
For example, IP is both layer 3 and layer 4 of the OSI stack. Layer 3 in
OSI deals with the next hop forwarding, while layer 4 deals with end to
end communication.
IP do the next hop processing, at the routing layer.
However, fragmentation is done end-to-end (well, it's even more
complicated, as any hop can fragment a packet, but only the endnode will
reassemble).
So here is a fundamental difference in the view of how the work is
split, and who do what.
It's very common for people to say that IP is OSI layer 3, and TCP is
OSI layer 4. But, as I note above, IP really covers both layer 3 and 4.
Also, TCP could be argued to be both layer 4 and 5 (since TCP could be
said deals with sessions).
Also, if we start talking about things like IPSEC, it's something done
at the IP layer, but in OSI it's layer 6.
But in TCP/IP, you can also have encyrption with SSL, for example, which
would be above TCP.
So a lot of things are placed differently, the responsibilities are
split in other ways, and other things are grouped together than in OSI.
Also, the isolation between the layers are no way as strict in TCP/IP as
the OSI model advocates.
Also, UDP is pretty much impossible to place in the OSI model. It is
clearly not layer 3, since it deals with end-to-end communication.
But it's also clearly not layer 4, which in OSI should be "reliable and
error free transmissions".
In short, it do not make sense to try and fit TCP/IP into the OSI model.
They are not "compatible". They are both views on a networking stack,
but they are different views.
By the time the ISO guys added intra-networking (network of networks - or what I referred
to as "Dave Clark's observation"), Metcalfe's law (the value of a
communications network is exponentally proportional to the number of things connected to
it) it was too late. And by then the US Gov had paid for enough IP/TCP implementations
and there were so many on the Internet that even IBM and Microsoft could not catch up
(although they too tried).
True. What DEC missed is the speed of adoption of all the stuff that came with
Unix, once Unix became a force in the commercial world (which, I believe, is
due to the success of Sun).
It's sad, since a lot of early TCP/IP work and so on was done on DEC
systems. But the PDP-10 wasn't the hot thing inside DEC by then.
(And no, TCP/IP do *not* follow the 7 layer OSI model.)
Amen, not by a >>long<< shot. You could sort of map it, and we all tried to
explain the IP stack in those terms, but you are so right.
I'm not sure anything ever precisely followed the 7 layer OSI model. In any
case, that's not particularly interesting. The 7 layer model is nothing more
than a guideline to protocol designers. What actually matters in reality is
the protocols.
I know that Phase V tried really hard to follow the OSI model, and there
were specific protocols designed within that framework, that was
expected to be the future. Except noone really accepted them.
DEC poured a lot of resources into it. I'm sure you know this better
than I do.
Johnny
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