[Simh] Comments WRT to IP vs ISO and OSI model

[Johnny Billquist]

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 <clemc at ccc.com> wrote:
>>
>>
>> On Tue, Oct 6, 2015 at 8:36 PM, Johnny Billquist <bqt at softjar.se> 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