smart@ditmela.oz (Robert Smart) (05/20/88)
Helpppp! I am trying to understand where networking is going. I
suspect I'm not the only one. As an investigative measure I present
a guess based largely on rumour. Will the people who know please
comment.
The OSI CLNS (ConnectionLess Network Service) is basically just
DoD's IP with minor changes (like a bigger address field). The
objective is obviously to replace IP with CLNS. Presumably a subset
of CLNS addresses is reserved for (and in simple arithmetic relation
to) IP addresses. This will allow a mix of CLNS and IP as long as
the backbone is CLNS. We can imagine that during the cutover there
will be arrangements like this:
IP CLNS IP
IP1----IP/CLNS-gateway------IP/CLNS-gateway----IP2
Where IP1 and IP2 are ethernets still using IP. They shouldn't see
any change talking to each other. The question then is how they will
talk to people who have made the cutover. Presumably the gateways
will have to have tables (presumably built dynamically by querying
relevant servers) of CLNS to IP addresses: CLNS hosts which wish to
be able to talk to IP hosts will have to have their IP-equivalent
address registered so the gateway has a replacement IP address to
use for that CLNS address when packets enter the IP world (and
contrari-wise). Those gateways will need fast processors in them.
If CLNS hosts are going to talk to IP hosts (and this is absolutely
essential if the whole thing is going to get off the ground) then
obviously the first middle level protocols to be implemented will be
TCP/CLNS and UDP/CLNS, and the associated application level
protocols. This should be a trivial modification of existing TCP/IP
and UDP/IP code.
The nice thing about going to CLNS is that you can then start
running other high level protocols. In particular CLNS/TP4 will give
access to the OSI higher level protocols [I know the X.400 standard
says that it must run only over TP0 and only over X.25 but this is
just a pathetic attempt by the PTTs to force people to use X.25. No
sensible implementations will enforce this restriction. DECs
implementation (MRX) currently allows TP4 over ethernet].
The next interesting thing is that DEC says DECNET Phase V will work
over CLNS. So presumably VAXes (and other computers) on the CLNS
Internet will be able to talk DECNET to each other.
That's all very well, but I would like to understand how CLNS over
X.25 coexists with the existing CONS (ConnectionOriented Network
Service) over X.25. I guess they just use different Subaddress or
Protocol-id or Call User Data. I am also not sure how CLNS over
802.3 fits in with existing OSI over 802.3. Presumably the packets
will have CLNS sender and destination addresses in them (the new IP
addresses). That certainly isn't what is in them now.
Bob Smart, CSIRO Division of Information Technology, Australiafrg@jfcl.dec.com (Fred R. Goldstein) (05/24/88)
In article <1097@ditmela.oz> smart@ditmela.oz.au (Robert Smart) writes: >The OSI CLNS (ConnectionLess Network Service) is basically just >DoD's IP with minor changes (like a bigger address field). The Well, not exactly... but they're rather similar. Note, though, that the proper terminology is that OSI IP (ISO 8473) is the protocol which implements the OSI CLNS (service description in ISO 8348/AD1. In the OSI world, services and protocols are usually described separately. >That's all very well, but I would like to understand how CLNS over >X.25 coexists with the existing CONS (ConnectionOriented Network >Service) over X.25. I guess they just use different Subaddress or >Protocol-id or Call User Data. I am also not sure how CLNS over >802.3 fits in with existing OSI over 802.3. Presumably the packets >will have CLNS sender and destination addresses in them (the new IP >addresses). That certainly isn't what is in them now. X.25-1984 provides the CONS, and is equal to ISO8208. Of course X.25 uses X.121 addresses and CONS allows the address to be the "long form" in ISO8348/AD2. ISO IP (CLNS provider) can run on top of X.25 or 802.3 or, for that matter, practically anything else. The procedure for using X.25 (called a Subnetwork Dependent Convergence Protocol) is within the text of ISO 8473. The IP (CLNS) packet sits inside an X.25 packet. This is just an example of the structure of the Network Layer, which is not one protocol but in practice, potentially a stack of "roles". Whatever the transport layer sees (the Network Service) is provided by the top Subnetwork Independent Convergence Protocol. (Sometimes called an Internet protocol, esepcially if connectioneless!) Whatever the underlying comms medium provides (wire, Ethernet, X.25, CONS, LAP-D, etc.) is the Subnetwork. Whatever has to go in between them to make the SNICP work is the SNDCP. Put 'em together right and you can make all sorts of combinations play. Not all with equal efficiency, though! > >Bob Smart, CSIRO Division of Information Technology, Australia fred -- usual disclaimers apply