[Css-csts] multiple instances of BDD procedure

[John Pietras]

CSTSWG colleagues ---

When I have read through the Buffered Data Delivery (BDD) definition in
the past, I have tended to think in terms of a single BDD instance per
service instance. Of course, however, it is possible to have multiple
instances of any procedure (including BDD) in a service. Reading through
the BDD specification with this in mind exposes some issues.

 

The first issue involves sequence preservation. The statement "it is
ensured that data units and notifications are always delivered without
discarding data" is true for a single BDD instance, but not necessarily
for a service that uses multiple instance of BDD. As an example,
consider a CSTS version of RCF that allows the user to select multiple
VCs by allowing separate BDD instances per VC. For this example, assume
that VC1 generates 10 frames for every frame in VC0: i.e., VC1 has a
frame at T0, T1, T2, etc., and VC0 has a frame at T0, T10, etc.  Also
assume that both BD instances have transfer buffers of size 3 and
latency limits sufficiently long to allow the VC0-instance transfer
buffer to fill before the release timeout (i.e., 30 time units).
Clearly, the VC0 frames for T0 and T10 will be delivered after the VC1
frames for T0 through T27. So while sequence preservation can be
guaranteed within the transfer associated with a single BDD procedure
instance, it can't be guaranteed across the whole service instance. 

 

Understanding that this behavior is on a per-procedure instance basis is
clouded by the "Service Provider" terminology that is used throughout
the FW. Use of this term makes it ambiguous as to whether the
requirement applies  to the particular procedure instance or across the
service. 

 

The above example also introduces the second issue - are the managed
parameters for a procedure to be specified on a per-instance basis, are
they common across all instances of a service instances, or is there a
mix? I would guess that all BDD instances would be either real-time of
complete, but should each BDD instance be able to have its own transfer
buffer size or latency limit? The simple answer is to make all procedure
instances use the same managed parameter values, but I think that it is
worth asking the question (and making clear in the FW whatever we decide
is the answer).

 

Best regards,
John  

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