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Subject: RE: [ws-rx] New proposal for i019 and i1028



Thank you for your careful review of this proposal. I have some comments regarding some of the points you make and I’m also including answers to some of the detailed questions you’ve made.


I will propose a new issue relating to gaps.


Cancel relates to i019 and i028 to the same degree as Close does because it addresses them in the same way; by providing a mechanism that an RMS can use to end a sequence with a final and accurate ack set. Attached is an example of how to do this using Cancel.


Cancel addresses a superset of the issues addressed by Close; i019, i028 and the yet-to-be-named issue related to gaps.


I don't expect Cancel to change the protocol at all but rather to just add to the protocol. In fact, it could plausibly be its own document (I not advocating for or against that, I'm just making a point).


I acknowledge that the amount of sequence state Cancel adds is a concern. I think the benefits of factoring the mechanisms used to end sequences and resolve doubt independently outweigh these concerns. This clean factoring could potentially allow for different levels of support allowing for someone to support only sequence termination with a final accurate ack set and not incur any extra state management. I have to say I need to give this “different levels of support” idea some further thought since I just thought of this as I read your comments.


If a new issue is opened and accepted, then, since Cancel addresses i019, i028 and i0<whatever>, I don’t see why we’d need Close as well.


Answers to some of the other detailed questions:


Q: How does an RMD "never accept messages"?

A: It ignores them.


Q: Is the SeqCancelAck header on all messages that have a SeqAck header?

A: No.


Q: What if the SeqCancelAck is lost? Do we need a CancelAckRequested?

A: RMS can resend SeqCancel.


Q: How does an RMS know whether a particular message was filled-in or cancelled?

A: The RMS is driving; if it wants to know if a particular message was captured or not then it should include that message in a SeqCancel before including it in a SeqFill.


Q: What is the distinction between canceling and gap-filling?

A: Canceling, by itself, resolves doubt about the transmission of messages. Gap-filling, by itself, fills gaps in the ack set, possibly incurring doubt. Canceling followed by gap-filling resolves doubt and compacts the sequence state.


Q: Does filling a gap remove those messages from future SeqCancelAck headers?

A: Yes.


Thanks again.



From: Doug Davis [mailto:dug@us.ibm.com]
Sent: Wednesday, September 14, 2005 6:56 AM
To: ws-rx@lists.oasis-open.org
Subject: Re: [ws-rx] New proposal for i019 and i1028


  I'm glad that you guys finally see the need for the proposal.  I
must admit though, I don't see how this proposal actually relates to
the specified issues.  Both issues relate to how an RMS can get a final
and accurate accounting of the sequence from the RMD - in one case
because of a fault and in another because the RMS decides to stop
using the sequence. This proposal, if viewed as a complete replacement
for mine does not address these at all.
  Your proposal focuses more on gap-filling (2 variants of it) and
neither of these is mentioned in the issues.  And as such I think a new
issue should be opened and this can be offered up as a potential
solution for it.  Once that is done, I then have some comments and
questions about it:
 - How does an RMD "never accept messages" that have been cancelled?
   Does it throw a fault? Ignore them?
 - Does a SeqCancelAck header get carried on all messages that have a
   SeqAck header?  In order for the RMS to get an accurate view of the
   sequence it would need all of this info each time.
 - If 'no', then are you proposing an CancelAckRequested header as
   well since the SeqCancelAck headers could be lost and there needs to
   be some way for the RMS to ask for it.
 - The SeqCancel and SeqFill headers are not information - like the
   AckReq and SeqAck headers.  As Dan noted in [1] dealing with these
   headers when piggy-backed on other messages could be problematic.  
   Dan's proposal is that any fault generated by these headers should not
   influence the processing of the original message - and that implies
   that in some cases the fault may be simply dropped.  Which is no
   big deal since these headers are mainly informational anyway.  The
   new headers you've introduced are not simply information though - they
   are critical to the success resolving doubt.  In the past I said that
   the Close() in my proposal could be a header - its just a syntax
   difference.  You're proposal makes me realize that it really can't be.
   The Close(), much like your SeqCancel and SeqFill headers, are much
   more serious than the informationally piggy-backed headers we have
   now.  And as such they deserve to be treated with more seriousness.
 - After the RMD receives a SeqFill header how does the RMS know whether
   any particular message was 'filled-in' or the message was just late
   in arriving at the RMD? Does this mean that to fill a gap, the RMS
   should send a SeqCancel, then wait for a SeqCancelAck, then send
   a SeqFill?  Unless the RMS never sent the message, or doesn't care
   that it go there, I don't see what else it could do.
 - The distinction between Cancelling and Gap-Filling a hole in a sequence
   seems almost arbitrary and redundant.  There is no clear way for an
   RMS to know which one to use when.
 - As I read it, the distinction between SeqFill and SeqCancel isn't clear
   and almost arbitraty.
 - Actually, perhaps you believe that gap-filling is for cases where the
   RMS never actually sent the message.  And while I don't dispute this
   might be a valid use-case my initial reaction is that this should not
   be something the spec deals with.  We've specifically avoided having
   the spec talk about things outside of what goes on the wire - things
   like how the DA might influence the protocol - so following that same
   logic it doesn't seem right to take into account some processing
   that takes place before the RM logic. If inOrder processing across
   sequences is out of scope then this feels like it should be too.  But
   I need to think more about it.
 - You've introduced quite a lot more data for the RMD and RMS to manage.
   The RMD must now maintain an ACK list and a CancelList - and both
   must be kept to inform the RMS of the sequence state.
   The RMS must now maintain an ACK list, a CancelList and a Gap-List -
   and all three must be kept in order for the RMS to have an accurate
   accounting of what happened to each message.
   And merging/reconciling these lists could be quite
   expensive (remember, we introduced Nacks just because one list could
   be hard for some impls - imagine what 3 overlapping ones will do).
 - You claim that the Close proposal introduces another way of terminating
   a sequence - this is false.  Please explain this by showing me where in
   the proposal it does this.  Terminate does not change at all.  The
   proposal simply allows for the RMS to tell the RMD to not accept any
   more messages for the sequence - that is very different than Terminate
   which involved resource reclaimation.
 - There is still no mention of how to know for sure which Ack received by
   the RMS is the final one - which is the point of the Close proposal.
 - Given the only reason for gap-filling in this proposal is to shrink the
   SeqAck header, does filling a gap remove it from the SeqCancelAck? If
   not, this could also continue to grow.
 - This change means that it is impossible to examine a SeqAck header and
   see which messages were delivered, due to gap-filling.  Whilst this
   doesn't break the protocol, it also doesn't help in terms of debugging
   problems.  It doesn't feel right for the SeqAck header to lie about
   which messages were really received, just so it has less entries in it.

Overall, I think this proposal is addressing a very different set
of issues from the original one proposed - and does so in a way that
significantly changes the protocol.  I would suggest that you rework it,
as Paul Cotton suggested during one of our previous phone calls, into a
form that would allow the TC to see the specific spec changes you want.  
I think that will draw out even more questions than what I've listed above.  
And, once that happens I think it will be even more obvious that this is
a separate issue from the other proposal so please open a new issue with
the use-cases this proposal addresses so the TC can get a more complete
view of what is driving this. And in the end I suspect you'll find that
both could be needed and we should proceed with the other proposal as is.


[1] http://www.oasis-open.org/apps/org/workgroup/ws-rx/email/archives/200509/msg00048.html

"Stefan Batres" <stefanba@microsoft.com> wrote on 09/13/2005 03:49:46 PM:

> All,

> After much discussion Doug has managed to make me understand his reasons for
> proposing his Close/FinalAck mechanism for addressing i019 and i028. We now agree,
> an accurate acknowledgement state can be helpful in order to resolve doubt and in
> addition, for cases where a sequence must be ended, that acknowledgement state must
> be final. These are preconditions for recovery and the protocol can aid in
> establishing these preconditions.

> However, we still have an issue with the current proposal [1]. We think it
> conflates the notions of resolving doubt and closing sequences. Attached is an
> alternate proposal that we believe addresses this issue, allows for the exact
> semantics [1] provides and has other advantages. Details are in the document.

> [1] http://www.oasis-open.org/apps/org/workgroup/ws-rx/email/archives/200509/msg00084.html
> I look forward to the group’s comments.
> Thanks,
> --Stefan
>  [attachment "WSRM-Cancel.doc" deleted by Doug Davis/Raleigh/IBM]


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