[ih] ARPAnet Type 3 packets (datagrams)

[Bernie Cosell]

On 26 Nov 2009 at 11:12, Noel Chiappa wrote:

>     > did IMP-level retransmission of packets at link layer.
> 
> To be clear, I believe this was hop-by-hop, not end-to-end, but this is
> from memory; I'm too lazy to go pull out the IFIPS IMP paper (and in any
> case, this may have changed after the paper was written, but I would
> classify that as unlikely, though).

That's correct: IMPs *only* buffered packets at the modem-output queue.  
There was no source-IMP buffering of data for the destination-IMP.

>     > No new MESSAGE (from the host) was permitted until the host received
>     > a Request for Next Message (RFNM) from its serving IMP.
> 
> Umm, not quite; a host was allowed to have up to 8 packets 'in flight' to
> a given destination at a time (basically - there are more details).

I don't think that hosts knew about packets -- hard to remember [and I've 
loaned out my copy of the listing] but I believe that the host just sent 
a "message" and the packetizing happened in the IMP, invisibly to the 
host.

>     > The IMPS did not themselves use RFNM between each other
> 
> The wording here could be confusing. According to 1822 (5/1978 edition,
> pp. 3-2/3-3), IMPs did look for the RFNM from the far end (i.e. on an
> end-end basis), and on a timeout would query the destination IMP
> (repeatedly so) until it saw either a RGNM or Incomplete Transmission,
> giving up only after 30 seconds or so.

And it isn't quite the same, but the "fake hosts", of course, used the 
same RFNM machinery as the real hosts.

>     > I don't remember now whether lack of a RFNM triggered a
>     > retransmission from the originating IMP
> 
> My supposition is that this would have been impossible, as it seems (see
> above) that the source IMP discarded its copy of the message once the next-hop
> IMP had acknowledged receipt of all the frames of it. (Whether this discarding
> was frame-by-frame, or message-at-a-time, I have no idea.)

That's correct.  It was up to the host to resend the message.

  /Bernie\

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