X.25, X.31 vs Q.921/Q.921 and Networking Style in General
mart...@bacchus.uucp.uucp
work over ISDN primary and basic rate interfaces. The system was
originally designed to use only PSDN (PDNs) as the communications
subnet. An end to end protocol was never designed so that individual
applications have their own end to end protocols. Eventually, the
developers put the communications system over a proprietary token
ring and ethernet, as well as point to point leased lines.
Since they wanted to modify the networking software as little as
possible, they basically used a slightly modified version of X.25
level 2 for the token ring and ran X.25 level 3 on top of it. On
ethernet they substituted 802.3 for X.25 level 2 and ran X.25 level 3
on top of that. And for point to point leased lines they run straight
X.25. Of course, they had some sort of procedure whereby one party
becomes the DCE and the other party becomes the DTE. Thus X.25 has
basically become the end to end transport protocol in the network
unless a PSDN is the communications subnet.
Now they want to put their network over ISDN using LAPD, LAPB, X.25
level 3, X.31 with Q.931/Q.921 signaling and I see problems with how
people use the X.25 family of protocols and what Q.931/Q.921 seems to
provide.
Full Q.931 seems to allow for "hold" on a given CSC (Circuit-Switched
Circuit), and in fact I think I really want to do this because I could
easily see an individual user using 3 CSCs or more for a given session
(e.g. remote terminal session from A to B, while on B doing a remote
copy from C to B -- the user could easily be taking up 3 CSCs). With
an average of 40 users on the machine with a full-blown remote file
system and RPCs (Remote procedure calls), I think it is fairly easy
for even a PRI to get fully loaded really fast, and I would want the
PBX to send indication of incoming calls even when there are no
available channels, so that the controller could put one call on-hold
and then accept the new call. For outgoing, if there are no channels
available I would like to put one call on hold and then place the new
outgoing call, and then round-robin (with prioritization for calls
which have outgoing data) the calls on the available channels.
In the case of a (2B+D or 1B+D) BRI on a workstation, the scenario
described above becomes even more problematic.
The problem comes in with X.25 restart and rest procedures as outlined
in the redbook and as typically implemented.
Deasington says in X.25 Explained (p.52) says:
The restart procedure is used to ensure that both the DCE and DTE
consider that all the PVCs and SVCs are in a known state. A restart
causes all PVCs to be reset and all SVCs to be cleared and free to be
used. The Network level will need to be restarted if the Link level
has failed for some reason, for example the line between the DTE and
DCE was disconnected. If the Link level has timed out and has been
restarted by the exchange of SABM, UA, etc., as previously described,
the Network level will be restarted by the DCE sending to the DTE a
Network level Restart Indication frame; in PSS the Restart Indication
frame will be sent to the DTE at intervals of six seconds until a
Restart Confirmation is received. The DTE may at any time transmit a
Restart Indication to the DCE if it needs to restart the Network level
completely, for example if it has completely lost track of the state
of the active SVCs.
According to the X.25 specification, resets should be generated on VCs
if data has been lost or a protocol error has occurred.
I have no problems with either restart or reset procedures as long as
disconnection of level 2 is not considered a protocol error because I
don't intend for my hosts to loose data in this case and I can set up
the following scenario:
1) non-alien host (i.e one of my hosts which form a part of my
contractee's proprietary network) connects to non-alien host.
2) calling party exchanges XID with called party,
establishes calling party as DCE and establishes
automatic call-back in the event of disconnections while
VCs are still active (sometimes CSCs just drop).
3) Level 2 gets connected. DCE does a network level restart.
4) VCs get started and normal data communications proceeds.
5) Called party interface fills and incoming call arrives,
Called party interface selects channel to put on-hold (probably
least-recently used algorithm), and stat muxes the active calls
among physical channels.
6) Possibly, the level 2 of one of the calls on hold times out and
becomes disconnected, when the call goes off-hold.
Now from the above in Deasington, apparently many automatically
restart the interface upon disconnection of the logical link under the
assumption this means the foreign interface had been turned off. I
think in the case of hold this should be utterly unnecessary and the
level 2 should just be reconnected transparently to the level 3.
6) Alternatively, the call drops for unspecified reason, the DCE
replaces the call, reconnects the link-layer, but does not restart the
network interface.
7) Eventually the number of VCs on the link goes to zero. Level three
on both hosts requests to controller to disconnect the logical level 2
and then to disconnect the call.
In the case of alien hosts who do not understand the proprietary
protocols, the alien host is always the DTE and the non-alien host is
always the DCE. Hold and auto-redial could be negotiated in the XID
but I would tend to make the default to disallow hold and neither
expect nor perform auto-redial in case of disconnect but rather inform
level 3 of termination of level 2 so that the VCs could be abnormally
terminated from the host point of view.
When one of my hosts calls a PSDN or is called by a PSDN, the PSDN
would of course be the DCE while my (non-alien) host would be the DCE.
Procedures about hold and auto-redial would be negotiated with the
PSDN at subscription time and could be exchanged in the XID.
To me this all seems like a perfectly reasonable way to do data
communications via ISDN (I mean real resource sharing not kermit)
however all the network "experts" at my employer totally freaked at my
suggest and scenarios.
I would like opinions. If you comment, I would be interested in your
background and experience because I guess I am conducting a survey and
I would like to have some sort of guide to weight opinions. If you
prefer, you may send your comments to me by private mail at
mart...@athena.mit.edu
..!ihnp4!mit-eddie!athena!martillo
If there is desire, I can post the results to the net.
Yaqim Martillo