Re: Questions on TCP-IP (vs. DECNET)


Charles Hedrick (hedrick@topaz.rutgers.edu)
15 Jul 88 06:20:12 GMT


We use both DECnet and TCP/IP. (Indeed I wrote the DECnet code for
the cisco gateways, and also do a lot of TCP/IP software work.) I no
longer feel quite as strongly about TCP/IP's advantages as I used to,
though I'd still choose it in most situations. What it mostly comes
down to is this: If you have a moderate size network, particularly
with a fairly simple topology, and you are willing to lock yourselves
into DEC, then their network products work very well. (Indeed in many
cases DECnet is noticably easier to configure than IP, though part of
that is because any single-vendor system is easier to deal with than a
multi-vendor system.) Otherwise, you should use IP. I see the
comparison as follows:

DECnet:

It is fairly easy to configure. It is well-integrated into the VAX
system, so some remote system management stuff is much easier to do,
and remote file access is more transparent. It is generally cheaper.
This is the up side, and is generally sufficent for small institutions
that are locked into DEC, or into combinations that DEC supports (e.g.
DEC and IBM). For the down side:

DECnet routing tends to be slightly unstable. In many configurations
it doesn't matter, but I have heard reports from a site that has many
overseas lines that routing changes cause a cascade of routing
broadcasts that makes life very unpleasant for a while. I have also
heard a report from Berkeley that having many routers on a single
Ethernet causes trouble, because the hellos synchronize, and produces
spikes of broadcast traffic that can exceed the capacity of some
Ethernet controllers to handle.

Another problem for large configurations is that 16 bits is not enough
address space. There are several national or international DECnet
networks. Their growth is limited by the availability of area
numbers. A given machine can talk to at most one such network. If
your campus wants to be involved in more than one, you'll probably
have to do some sort of ad hoc partition of your routing.

DECnet does not have some of the robustness features of IP, e.g.
the ability to deal with packet corruption. (This should not be
an issue, as serial lines are done on top of DDCMP, which does
good error control. But in large networks, you really do want
end to end error control.)

And obviously DECnet will tend to lock you into DEC. The specs are
public, and there are now implementations for various other systems.
But you'll still probably find yourself using DEC routers, terminal
servers, etc. (By the way, LAT is particularly noxious in this
respect. The protocol for it is not public. This means that even
other vendors that support DECnet do not support LAT. So I recommend
avoiding LAT even in networks that use DECnet.)

IP:

The up side of IP is that just about everybody supports it, or at
least can point you to a software house that does. It seems to have a
wider variety of services available (finger, rwho, etc.). Its mail
systems tend to be designed to handle more complex configurations.
(Interfacing DEC's mail system into an environoment with DECnet, IP,
and BITnet is always a real trip.) Some of the newer services seem to
have no equivalent on DECnet, e.g. the network file system. I haven't
heard or X or NeWS over DECnet, though I'd think DEC might have an X
over DECnet for its VMS-based workstations. The community of people
who are doing interesting projects in workstation system software
seems to be working mostly with IP. IP is designed to handle very
large and complex networks. 32 bits of address space is enough for
the next few years.

The downside is in many ways the converse of the upside. Because it
is supported by many vendors, and because it is designed for complex
networks, you have more chance that vendors' implementations will not
communicate, and more setup options. In general interoperability is
much better than it was 5 years ago. I haven't had to fix bugs that
caused interoperability problems for several years. The main
remaining problem is that not all implementations support subnets, and
as a corrollary they may not agree on a broadcast address. This can
be solved, as the new implementations can be set to use the old
broadcast address, and commercial gateways use proxy ARP to deal with
implementations that don't understand subnets. However you do have to
know something about your implementations, so it complicates setup.

Setting up IP is a bit more complex, because routing isn't as
automatic. There is no standard routing protocol. (There is one that
most people implement -- RIP -- but it isn't an official standard.)
This means that you have to think about your routing design. Once you
know what you are doing, it's easy to set up machines, but it does
mean you have to spend time figurng out the technology and deciding
how to use it in your case. The serious IP gateways have a zillion
setup options. However a DECnet router does too. I think they're
about comparable in complexity. However they tend to be used in
different cases. The complex IP gateway options are normally used
when you are setting up a network that connects many organizations
with differing routing strategies and contraints about who will talk
to whom.

To answer your specific questions: (1) I doubt that patching is needed
these days, as long as you get current, supported software. (2) The
smoothest DEC/IP bridge is an Ultrix system with software intended to
do just that. However any machine that has both DECnet and TCP/IP can
act as a bridge if you don't expect too much. (3) We are probably a
good reference site, but you might be better off finding a place that
has used Ultrix's DECnet/IP gatewaying software. (4) I think you can
find IP software that will do what you are doing, but I'm not sure
you'd find the changeover worth doing. I'd add IP to a reasonable set
of machines, and start using it for new things, but I'm not sure I'd
erase all my DECnet software. In particular, I'd modify your proposal
by putting IP on the 8650 also, and one at least one PC just so you
can play with it. There are a number of IP implementations for the
PC, so you should talk to someone who knows more about them. (5) We
still use Wollongong for VMS. They have gotten a lot better recently,
and are now aggessively following the latest Berkeley technology.

Ideologically pure solutions sound real neat, but probably aren't the
best in the long run. DECnet is just fine for the machines it
supports, particularly in a small network. Unless you have to spend a
lot of time administering it, there's no reason to remove it. But if
you depend solely on DECnet, there's a lot of good technology you
won't be able to use. The IP community is a lot of fun, gets you a
lot of flexibility in a number of directions, but it also takes a
commitment of time to learn the technology.



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