Subject: Use of SAPs in HPR

From: "jim (j.) cobban" <[email-masked]>
Date: Thu, 8 Feb 1996 10:19:00 -0500

There is an apparent ambiguity in the specification of how SAPs are used by
HPR implementations which is causing additional difficulty for our data link
layer gateways.  Section 6.2.2 of the HPR specification repeatedly talks
about the APPN SAP and the HPR SAP, but does not clearly specify how they are
used.  The description of CV 61 subfield 80 in section A.1.11 specifies that
the HPR SAP is used as the destination SAP.  Neither section specifies what
the sender should put in the origin SAP.  The problem is that in 802.2 LLC
the link connection is identified by the 4-tuple of the two MACs and the two
SAPs.  In order to uniquely identify a particular link connection it is
necessary to know both the destination and the source SAP.

Because of this ambiguity some implementations are sending frames with the
source SAP set to their HPR SAP, and others are sending frames with the
source SAP set to their APPN SAP.  If the HPR SAP and the APPN SAP are not
equal, and either end of the link is capable of supporting multiple SNA
protocol stacks, which is always the case with data link layer gateways, then
it is essential that the source SAP be predictable.

I therefore propose that the language of section 6.2.2 be clarified to
indicate that the source SAP on HPR frames is set to the HPR SAP of the
sender, just as the destination SAP is set to the HPR SAP of the receiver.

At the risk of beating a dead horse:  This is another example of the stupid
problems that arise because of the insistence by IBM on using separate SAPs
for HPR and ISR.

Jim Cobban   |  [email-masked]                        |  Phone: (613) 763-8013
Nortel Tech. |  bnrgate.bnr.ca!bcarsf1f!jcobban       |  FAX:   (613) 763-5199
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|   represent the opinions of Nortel or Nortel Technology.                    |