Subject: re:Names in ACTPU/ACTLU

From: "jim (j.) cobban" <[email-masked]>
Date: Tue, 12 Dec 1995 12:38:00 -0500

In message "Names in ACTPU/ACTLU", Stan Ratliff writes:

>We could leave PU2 alone, and use bit settings in the type 3 XID 
>to indicate desire to see the names. That way, implementations
>wanting the names could be setup as a LEN node. We would be making
>the assumption that LEN node implementations aren't quite as 
>brain-dead as old PU2's (which seems reasonable to me).

I just noticed something fascinating as I looked over the back copies of the
SNA Formats manual.  I have kept these back copies (-5, -6, -8, -10, and
-12) around just because of the wierd things that change or are dropped from
time to time.  As late as -10 the description of the Format 3 XID read
"variable format (for T2.1 to T2.1/BF and T2.0 to T5 node exchanges)", but
in -12 it reads "variable format (for T2.1 to T2.1/4/5 node exchanges)".
Why would anyone remove the statement about T2.0 at precisely the same time
that byte 10 bit 3, a bit which only has meaning for a T2.0, was added to
the XID?

There does not appear to be any architectural necessity for restricting
format 3 to T2.1 nodes.  A T2.0 could just send an XID 3 in which almost all
of the bits are zero.  The exceptions are:

  Byte 8 bit 0 set to 1 to indicate no support for INIT-SELF
  Byte 8 bits 2,3 set to 1 to indicate no support for BIND segmentation
  Byte 10 bit 3 set to 1 to indicate support for PU Cap. CV on ACTPU
  Byte 10-14 bit ? set to 1 to indicate support for additional CVs on
		   ACTPU and ACTLU
  Byte 17 set to indicate the DLC type, and assuming SDLC ...
  Bytes 21,22 set to contain BTU length
  Byte 27 set to contain I-frame window

The Product Set ID control vector has been mandatory since the first
introduction of the format 3 XID.  However although the current
documentation insists that the Network Name type F4 is also mandatory, this
was not always true, and as I recall the S/36 does not include it.  In any
event from a LEN node this CV doesn't do anything except possibly provide a
more convenient unique identifier in a switched major node.

Jim Cobban   |  [email-masked]                        |  Phone: (613) 763-8013
BNR Ltd.     |  bnrgate.bnr.ca!bcarsf1f!jcobban       |  FAX:   (613) 763-5199
|                                                                             |
|   The opinions expressed are those of the author and do not necessarily     |
|   represent the opinions of Bell-Northern Research or Nortel.               |

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From: Dave Finnie <[email-masked]>
Date: Wed, 13 Dec 1995 15:50:01 +1100

> that byte 10 bit 3, a bit which only has meaning for a T2.0, was added to

As my memory recalls, DDDLU is a function available to T2.1 nodes
supporting local dependent LUs (eg our own implementation) - so this
bit *does* have relevance to T2.1 nodes.

But I take your point - why was it dropped ?

Dave