[ih] principles of the internet

[Bernie Cosell]

On 1 Jun 2010 at 17:31, Noel Chiappa wrote:

> The big change going from the ARPANET (not Host-Host Protocol, see above) to
> the Internet (in terms of the _placement_ of function - the Internet of course
> added other capabilities, such as being able to use a diverse range of
> technologies, but that's different) was to make the hosts responsible for
> reliable transmission (checksums, sequence numbers, timeouts,
> retransmissions).

I have an odd perspective on this and it seemed to me that the thrust for 
this kind of change was that the phone lines were *MUCH* better than we 
expected them to be [and the current fiberoptic links are even better].

When Bob Kahn cobbled up the checksum equation the IMPs were to use over 
the 50Kb circuits, it was assumed that the lines would be pretty much 
full all the time and that the lines would perform according to AT&T's 
specs.  (and indeed we even calculated [but I can't remember the details 
any more] how often a broken-but-undetected packet would get through)  It 
was very conservative and checksummed hop-to-hop [since if you expect a 
lot of errors, that's more efficient that sending it all the way to the 
other end only to discover that it got broken back at hop-1].  But we 
quickly discovered that the reality was we had almost *no* 
retransmissions and while that didn't cause any change right-off [since 
the modems were still generating and checking their 24-bit checksums and 
the IMPs were dutifully retransmitting hop-by-hop] it *DID* indicate that 
a change in the direction of end-to-end would likely result in much 
better throughput than hop-to-hop.

  /Bernie\

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Bernie Cosell                     Fantasy Farm Fibers
mailto:bernie at fantasyfarm.com     Pearisburg, VA
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