Subject: Loop Prevention

From: "R. C. (Roy) Dixon" <[email-masked]>
Date: Mon, 23 Aug 93 08:07:09 EDT

Hi Choon Lee,

This is Roy Dixon and I have been monitoring your exchange of notes on
the subject of loop prevention.  Although I have not had time to dig
through all the technical details, I believe that Scott Adams' proposal
is on the right track to a solution for this problem.

I agree that the use of the IEEE 802.1 spanning tree protocol does
not seem appropriate to the DLSw configuration.  Also, I like the
idea of introducing a LAN "heartbeat" function for DLSw (Scott's TWIK
packet).  This is very much akin to the mechanisms used by various
routing protocols for their router-to-router protocols (i.e., IS/IS Hello
in OSI protocols).  The heartbeat would be sent to a multicast address
assigned to DLSw and would use a well known Link SAP, yet to be defined.

The use of the heartbeat on the local LAN could also be beneficial for
solving the problem of establishing transport connections, a problem
under consideration by another SIGlet.

  o For example, the heartbeat could include parameters such as a region
  identifier associated with the bridged LAN.  This information would be
  predefined at each DLSW and the exchange of heartbeats would allow each
  DLSw to verify the configuration.  The region identifier would allow
  the mechanism used for setting up transport connections to understand
  that more than one DLSw is attached to the same bridged LAN.

  o Also, the heartbeat could include an identifier for the transport
  network, thereby allowing a single bridged LAN to be connected though
  data link switches to multiple transport networks.  In this case, the
  protocols for loop prevention would function independently for each
  transport network accessible from a given bridged LAN.

Now the details of this proposal have not been worked out completely, but
I wanted to let you know that I have been thinking along similar lines.

So to summarize:  if the heartbeat mechanism proves useful in solving the
loop-prevention problem, the addition of a region identifier and a
transport network identifier might then be used as an aid in establishing
and using the transport connections.

Regards, Roy

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From: Choon Lee <[email-masked]>
Date: Fri, 27 Aug 1993 11:03:25 -0700

Hello, Roy,

>> and using the transport connections.

One concern that I have about the region identifier is that it intoduces yet
another parameter that need to be configure/synchronised on all LANs in the
network. Ensuring that its uniqueness (and simultaneously have the same value
for DLSw attached to the same LAN) could be a chore.

It looks like the heartbeat has other uses as you suggested. Whether or not
the heartbeat can be used for loop prevention need to be worked on further.

Regards,
Choon Lee
3Com Corp