Subject: Re ODAI being reserved for dependent LU traffic

From: "Right theory, wrong universe! 29-Apr-1994 1001 -0400" <[email-masked]>
Date: Fri, 29 Apr 94 11:33:16 EDT

Regarding the proposed update to the APPN architecture to make the ODAI bit
reserved (and hence zero) for dependent LU sessions - I think there is a 
problem with doing this, as follows; 

If the system containing the dependent LUs ends up adopting the primary role on
a LAN connection, then it will use ODAI of zero for Independent LU sessions
where it sends BIND.

Now, if we take dependent LU address 2 for the sake of example, and if ODAI is
reserved and set to zero, then SSCP-LU RUs this system sends for this LU
will contain:

	ODAI=0, DAF'=0, OAF'=2

in the TH. BUT, the first independent session that is started by this node will
use:

	ODAI=0, SIDH=2, SIDL=0

and, as defined in Figure 10-9 of the -3 APPN FAPL, this will be put into the TH
as:

	ODAI=0, DAF'=0, OAF'=2

The upshot of this is that the system at the other end can't tell the difference
between dependent SSCP-LU traffic and RUs sent on independent LU-LU sessions!!

Now, you might say that this is all theoretical and won't happen in practice,
since NCP and VTAM always end up as the primary on LAN connections to systems
with dependent LUs. However, the AS/400 with dependent LU passthrough configured
can *very* easily end up as the secondary, since it uses a fixed Block ID of
X'056', so there are an awful lot of block id/num values that the other system
can supply that will have a higher value (and hence it will end up as primary on
the link). It is also possible to force VTAM to be secondary, although you have
to fudge this by using a very high block-id/num value.

btw: both VTAM and OS/400 do set ODAI to 1 for RUs they send to dependent
     LUs when they end up as primary on the link.

I believe that the only solution here is to specify that BOTH ODAI=0 and
ODAI=1 with SIDH=0 or 1 are reserved for dependent LUs, AND the setting
of ODAI for dependent LUs should always be the ODAI of the ACTPU/ACTLU/BIND
sender, just like it is for independent LUs.

Simon Graham
Digital Equipment Corporation