Subject: Interoperability Problems cause by XID Command/Response

From: "jim (j.) cobban" <[email-masked]>
Date: Sat, 27 Jan 1996 10:34:00 -0500

It would appear that almost nobody out there has bothered to read the
Token-Ring Architecture manual before writing their XID support for APPN.
The result is interoperability problems.  We at Nortel are getting a little
weary having to program around some of the weird behavior we have seen.  We
have actually had to add configuration parameters to permit our customers to
get their networks to communicate with various pieces of SNA equipment.

Is there some way that we at the AIW can demand that everyone must comply to
the TRN Architecture before they can claim to be APPN compliant?

The most flagrant offender is VTAM/NCP.  VTAM/NCP has chosen to ignore any
XID which arrives as a Command when a Response is outstanding.  It is obvious
that this was seen as some sort of optimization, since it avoided handling an
XID command which had merely crossed in the network with a Command which NCP
had just sent out.  However you will search the entire TRN reference in vain
looking for any state in which it is permitted to ignore an XID command.

The worst part of this offense is that it has caused many other SNA product
developers to perform weird perversions of the LLC2 protocol, just in order
to keep VTAM happy.

For example we have been having difficulty connecting to implementations
using DCLs SNAP-LLC2.  DCL, noting that VTAM has this penchant for ignoring
XID Commands, has chosen to send all of its XIDs as RESPONSES!  The problem
is that if you look at the Token-Ring Architecture reference you will see
that a compliant implementation is required to IGNORE responses which are
received when none is expected because the station has not recently sent out
an XID Command.  I am confident that the only reason DCL is behaving in this
strange way is because it was the easiest way to keep VTAM happy.

Nortel therefore requests that the APPN Seal of Approval
be withheld from all implementations of APPN, including VTAM, which fail to
comply with the Token-Ring Architecture Reference on the handling of XID
commands and responses, because this creates a major interoperability
problem with any implementation which DOES comply to the architecture.

---

From: Dave Finnie <[email-masked]>
Date: Mon, 29 Jan 1996 09:17:35 +1100

Jim,

> XID Commands, has chosen to send all of its XIDs as RESPONSES!  The problem

Our implementation would also have problems dealing with an XID RSP
only approach. As you mentioned, the normal action is to discard RSPs
received that have final set when no CMD is outstanding, which is
indeed what we would do. If, however, final is not set, it would be
passed to CS. I believe this is as per the TR referrence, although it
is sometime difficult to tell, because the suggested implementation
includes vague text such as "inform higher layer". 

Do these XIDs have final set ?

Dave