Re: IBM TCP.


Charles Hedrick (hedrick@topaz.rutgers.edu)
Thu, 6 Aug 87 13:43:53 EDT


No, DEC does not preempt other vendors' addresses. Let's try this
again. The Ethernet spec requires that a vendor allocate each
Ethernet interface an address which is globally unique. That is,
there is no other interface in the world with the same number. DEC
interfaces have such an address. However when you turn on DECnet, the
processor causes the interface to change addresses. It no longer uses
its globally unique address, but instead uses an address that is
produced algorithmically from the DECnet address. The first 4 bytes
of the address are a constant. The last two are the DECnet address
with various bit twiddling. (The exact algorithm is documented in the
DECnet manual.) This address is obviously unique within your site,
and even within the set of sites that are connected by DECnet, but is
no longer globally unique, because any other site with the same DECnet
address will now have the same Ethernet address. The Ethernet address
is otherwise legal. The vendor prefix is DEC's. However the address
as a whole is not unique across the whole world.

Perhaps the confusion is whether DECnet addresses are globally unique?
A DECnet address has two parts: area and host. There are 64 possible
area numbers and 1024 possible host numbers. This is not a large enough
address space to allow DECnet addresses to be globally unique. Again,
they are obviously unique for any given DECnet network. But your
DECnet network, DEC's, GM's, etc., may and probably do use the same
addresses. The physics net, in which both of us participate, controls
allocation of area numbers for the schools that participate. So among
that set of schools DECnet addresses are unique. But there can only
be 62 schools in that network, because the network itself uses one
area number, and 0 is not allowable. There are certainly more than 62
universities in the country. Indeed there was a message on dcom.lan
not long ago from someone who couldn't join the network because their
local DECnet needed several areas, and therefore their address allocations
conflicted with the physics nets'. There are other large organizations,
such as DEC's engineering network, that use most of the possible DECnet
addresses. So the conclusion is: DECnet addresses are unique within
the specific DECnet network, but not on a world-wide basis. Furthermore,
the address space is small enough that there could not be a DECnet
network as large as the IP internet. Simple arithmetic will show
that. DECnet uses 16 bits for its address. In general 16 bit addresses
do not provide enough address space to allow globally unique addresses.
Thus the 16-bit networks, including chaos, PUP, and DECnet, are used
primarily as local networks, or between specific sets of institutions.
For a large network, at least 32 bits is needed, and even that may
be marginal. IP uses 32 bits, DECnet phase V uses 64 bits, and
XNS uses 96 bits. All of these are designed to be used with large-scale
networks.



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