Re: PSN 7 End-to-End question.


Ken Pogran (pogran@ccq.bbn.com)
Thu, 28 Jan 88 19:05:07 EST


Robert,

One of your questions has an easy answer; the answer to the other
is a lot more complex.

X.25 was chosen to replace 1822 for new IMP connections because
it is an international standard that has broad support among host
vendors. It's DoD policy to adopt commercial standards whenever
possible. This says nothing one way or the other, of course,
about the relative technical merits of X.25 and 1822 as access
protocols to a wide-area packet network.

Why was it deemed necessary for an End-to-End protocol in the
network? Well, first: the network has ALWAYS had an end-to-end
protocol. It's not something that's been made a big deal of.
The new end-to-end protocol introduced in PSN 7 is the first
change in this protocol since the relatively early days of the
deployed ARPANET.

The purpose of the end-to-end (EE) protocol is, primarily, to
manage the INTERNAL resources of the network in response to the
demand for services from the network's hosts. It's called an
end-to-end protocol because it operates between the source PSN
(the PSN to which the host originating a given message is
attached) and the destination PSN (the PSN to which the host
which is the destination of the message is attached). The EE
functionality of the PSNs of the network is in addition to the
"store-and-forward" functionality that occurs from one PSN to the
next.

The "IMP-to-IMP" protocol accomplishes reliable transmission from
one PSN to the next; the EE protocol manages resource utilization
for a flow of data "across" the network from source to
destination PSN. It also provides the mechanism by which a PSN
is able to enform a source host about what happened to his
message -- whether it was delivered to the destination (host gets
a RFNM, in 1822 parlance), or not (host gets a Destination Dead,
or Transmission Incomplete, in the event something in the network
failed while the message was in transit).

EE ACKs are on a per-host-message basis, which in an IP world
translates into per-IP-packet. (Under the "new EE" of PSN 7, EE
ACKS can be aggregated when being sent across the net for
efficiency, but are sorted out at the source PSN for proper
presentation -- via individual RFNMS in 1822, for example -- to
the source host.

In the ARPANET, messages from hosts can be up to (approx) 8K bits
long and are fragmented by the PSN into packets of (approx) 1K
bits.

The EE protocol is employed for all host traffic in the network;
after all, it's used to manage the resources of the network
itself. Which answers the question of why the EE protocol was
deemed necessary in a network that already has PSN-to-PSN
reliability.

Hope this helped.

Regards,
 Ken Pogran



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