[ih] principles of the internet

[Alex McKenzie]

I disagree with John's definition of what it means to be a datagram network. In my opinion, all that is required is the independent routing of packets.  In this sense, the ARPANET presented a classic datagram interface to its users (Hosts), except that the units crossing the Host-IMP interface were called "messages".  Messages were truly independently routed. HOWEVER, the ARPANET did go to great lengths to insure that messages, once accepted, were  correctly delivered to the recipient with high probability.  ARPANET also kept messages between a given pair of Hosts in order. These two design decisions put a great deal of complication into the IMPs. It should be remembered, though, that the original concept of the ARPANET was that each Host would contain a program to do all the store-and-forward functions.  It was Wes Clark's idea that a minicomputer should sit next to each Host to do all the hard jobs (routing, ordering, error detection/correction, etc)
 so that each Host did not have to write programs to get these jobs done. Larry Roberts was enthusiastic about this idea because it provided a more cost-effective way of getting the programming done, done on time, and done correctly. So it was a design decision that the complexity SHOULD all go in the IMPs.

An additional source of possible confusion is that inside the ARPANET, the IMPs broke messages into shorter units that were called "packets", and from internally the network treated the packets independently (eg, as datagrams in either John's sense or my sense).

Finally, there is a very real sense in which the ARPANET was designed to build a celiable message communication system from unreliable parts. Each circuit and each IMP was an unreliable part, yet the Host messages were delivered correctly, and acknowledged by RFNMs, with very high reliability.

We can all see now that the Internet we know couldn't be built out of a bunch of independently designed and implemented ARPANET-like networks and offer reasonable bandwidth and delay characteristics. We can also see that it could be (and was) built out of independently designed and implemented Cyclade-like (actually, for purists, Cigale-like) networks.  But that wasn't the perspective in 1968.

Regards,
Alex McKenzie

--- On Tue, 6/1/10, John Day <jeanjour at comcast.net> wrote:
> 
> Sorry, but neither Baran nor the ARPANET were a datagram
> network. 
> There are two aspects to being a datagram network:  1)
> the 
> independent routing of the packets, and 2) the network does
> not try 
> to recover all failures, but leaves most of that to the
> hosts. 
> There is nothing about the IMP subnet that was "building
> reliable 
> systems from unreliable parts." Also Baran's report and the
> ARPANET 
> had much more in common with the virtual circuit approach
> to packet 
> switching than the datagram approach.