Subject: [Isis-wg] Potential for impossible state in the 3-way text

From: Neal Castagnoli <[email-masked]>
Date: Fri, 1 Feb 2002 18:21:13 -0800

The current 3-way text states:

            Neighbor System ID if known (zero to eight octets)
            Neighbor Extended Local Circuit ID (four octets, if Neighbor
              System ID is present)

It is possible to know the Neighbor System ID, but not know the Neighbor
Extended Local Circuit ID, and in that event you are commanded to put
information into the packet you don't have (at least one of the common
implementations sends only the state, and it is also possible the other side
may not support 3-way).

Furthermore, if you know the neighbor extended local circuit ID, you'll
always know the neighbor system ID, and so  

       "if Neighbor System ID is present"

is superfluous.

It seems to me that the encoding rules should state 

	 Neighbor System ID if Neighbor Extended Local Circuit ID known (six
octets)
       Neighbor Extended Local Circuit ID if known (four octets)

	-or-
	
      Neighbor System ID if known (six octets)
	Neighbor Extended Local Circuit ID if known (four octets)

The former has the advantage that there are less encoding formats.

  --Neal Castagnoli
    Allegro Networks
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From: Les Ginsberg <[email-masked]>
Date: Sun, 3 Feb 2002 12:02:21 -0800

In general I agree with Neal's comments. 
It can be debated (not too vehemently I think) that there may still be some
value in sending neighbor System ID even when you don't know neighbor
extended circuit ID just to catch the case where due to physical layer
changes you are suddenly connected to "somebody else". This would only be of
value if the neighbor sends only the 1 byte state value in its 3 way option
but supports on reception the full 17 byte option. Possible I suppose, but
seems not very likely. But if we stick to the tried and true mantra of being
generous in what we receive, then there seems no reason to disallow this
form. I would therefore suggest a slightly modified version of Neal's second
proposal:

  Neighbor System ID if known (0-8 octets)
  Neighbor Extended Local Circuit ID if known (four octets)

(BTW - Neal I assume your specifying "6" as the system ID length was an
unintentional slip due to common practice.)

   Les

-----Original Message-----
From: Neal Castagnoli
To: [email-masked]'
Sent: 2/1/02 6:21 PM
Subject: [Isis-wg] Potential for impossible state in the 3-way text

It is possible to know the Neighbor System ID, but not know the Neighbor
Extended Local Circuit ID, and in that event you are commanded to put
information into the packet you don't have (at least one of the common
implementations sends only the state, and it is also possible the other
side
may not support 3-way).

is superfluous.

	 Neighbor System ID if Neighbor Extended Local Circuit ID known
(six
octets)
       Neighbor Extended Local Circuit ID if known (four octets)

	-or-

  --Neal Castagnoli
    Allegro Networks