Subject: Re: Elimination of LLC Connection in HPR

From: "jim (j.) cobban" <[email-masked]>
Date: Thu, 3 Aug 1995 09:41:00 -0400

In message "Re: Elimination of LLC Connection in HPR", Dave writes:

>Well, the definitely is not correct, since we implement TR LLC2 spec for Ethernet,
>FDDI and TR, and have had not problems in this area. Surely the following need to
>apply:
>
>  * Since these LAN protocols can easily be bridged, the method should theoretically
>    be the same for all of them
>
>  * CS *does* in fact expect the XID to be delivered when it tells DLC to.

Read the ISO 8802.2 specification before you criticise my statement.  Also I
would talk to the developers of the 8209 before you make wild claims such as
that "these LAN protocols can easily be bridged".  Lastly, could you point
out to me the location of the statement in the APPN architecture that CS
expects the XID to be reliably delivered.  The closest I can find is on page
9-19 where there is a sentence "Moreover, when retrying an XID command that
has not been responded to within an implementation dependent limit, the XID
sent will be the one that was not responded to, rather than an XID3 that
reflects the current state of the XID exchange."  Since this is within a
section entitled "Rules for DLC Operation on ABM Links" this could be
interpreted as an instruction to the DLC, however most of the other
statements in this section describe the behavior of CS, so this remains
ambiguous.  However I am being legalistic, since I do know that reliable
delivery is expected.

>I'm a little confused by your use of DLC. DLC in this context actually consists
>of 2 sublayers - LLC, and MAC. Since you are saying that XID Retry is not the
>role of the LLC, does that mean you believe it is the role of the MAC ? I do

This is the APPN Implementor's Workshop.  When I use the term DLC I am
referring to how that term is used in the APPN architecture, that is as the
process with which CS (and PC) communicate.  So in this "context" DLC
consists of THREE layers: MAC, LLC, and the CS/PC interface.  Since CS
expects reliable delivery of XIDs , and since the ISO 8802.2 definition of
LLC2 does NOT guarantee that reliable delivery, and in fact some LAN adapters
do NOT retransmit unacknowledged XIDs, in those situations it would be
necessary for the CS interface portion of the DLC to handle this
retransmission.  This would not be any different in the LLC1 case.  If the CS
architecture for HPR specifies that CS expects DLC to provide reliable
delivery of XIDs, then that remains true regardless of the nature of the data
transport.

My bottom line is that I cannot afford to tie up 1/255th of a scarce resource
just to do keep-alives on the line.  I have one customer who has had to
deploy FIFTY (50) TRN adapters at their host site just to obtain enough LLC
connections, even though the traffic volumes could be handled by 1/10th that
many.

Jim Cobban   |  [email-masked]                        |  Phone: (613) 763-8013
BNR Ltd.     |  bnrgate.bnr.ca!bcars153!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 Northern Telecom.     |

---

From: Dave Finnie <[email-masked]>
Date: Wed, 2 Aug 1995 23:18:51 +1000

Jim,

Please don't take my comments as criticism. The bottom line is that I
totally agree that it doesn't make a lot of sense to have 2 logical
connections when only one is used for transport most of the time
(except that this was done for backward compatibility). I also
understand your concern about use of SAPs.

I now understand your use of DLC, and, yes, that would be a good place
to put XID retransmission. However, my original comment, which you
seemed to object to, still stands. We both know - even if the
reference doesn't state it very well - that CS requires reliable
delivery of XIDs. My comment was that we have to ensure that this
actually happens so that CS continues to operate in the case of
dropped XIDs. I apologise for not being aware that some
implementations use the 8802.2, which does not retry XIDs. In that
case, we still need to make it clear that the APPN DLC layer must
retransmit XIDs for CS - since anyone that has implemented the TR LLC2
will be used to LLC2 doing it automatically.

Dave