TOP (ISO) addressing


Charles Hedrick (HEDRICK@RED.RUTGERS.EDU)
4 Feb 86 02:07:01 EST


Apparently a few people on this group are interested in the ISO
protocols. At least there has been enough interest to post some
of the standards as RFC's (for which I am grateful). So I hope
there may be someone who can answer a question:

I have been looking at the specifications for TOP. (For those who
don't know what this is, this is the most likely candidate among
the ISO protocols for a replacement to TCP/IP in the environment
where most of us use it. ISO is a large family of protocols.
It provides alternatives at many steps, and leaves a number of
things unspecified. MAP and TOP are specifications that make
choices where choices are needed, and that fill in the unspecified
details. MAP is intended for manufacturing, whereas TOP is
intended for the office or research environment. MAP and TOP are
really a family, and make similar choices where there is no
specific reason to do otherwise.) If I undestand all of the
verbiage correctly (and the probability is high that I do not),
it looks like TOP is likely to run out of address space. As I
read the spec, an address contains two major parts: a two-byte
subnetwork number and a variable component which for most of us
will turn out to be the host's Ethernet address. It seems to
me that 16 bits is not very much for the subnetwork number.
As I understand it, the subnetwork number will have to be
globally unique (i.e. no other subnetwork in the world can
have the same subnetwork number). Even if that is not said in
the spec, it seems clear that it is going to have to be the
case if we are going to allow for the possibility that subnetworks
will communicate with each other over common-carrier X.25 networks
or the Arpanet. Furthermore, it is said that routing through
gateways is to be based entirely on the subnetwok number. This
seems to imply that places like Rutgers that are class B
Internet sites will need to use a separate TOP subnetwork number
for each of our internal subnets. So in practice, it seems
like we are going to need roughly one subnetwork number for
every Ethernet that runs TOP. I find it hard to believe that
16 bits will be enough.

Does anyone see an error in this argument?
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