Issue 25 - form of seqAck when RMD has received no message

I’d like to propose an alternate resolution for issue 25.

 

In the meeting yesterday, we discussed issue 25 and the
group expressed its opinion that they preferred a definite statement to be made
in this boundary case of no messages received yet.

 

The remaining proposal being considered last night was to
send “Ack 0..0” for the case where no messages have been received.

 

<wsrm:SequenceAcknowledgement
xmlns:wsrm="http://docs.oasis-open.org/whatever">

       
<wsrm:Identifier>http://example.org/mysequence/1234</wsrm:Identifier>

       
<wsrm:AcknowledgementRange Upper="0" Lower="0">

</wsrm:SequenceAcknowledgement>

 

My intellectual concerns with this approach are that we are
making an incorrect statement – we are acknowledging receipt of a message
[#0] that (a) we haven’t actually received, (b) doesn’t actually
exist, and (c) will never be sent

 

My practical concerns with this approach are that I fear this
change may cause a large ripple effect on implementations. 

I suspect implementations will need to change to explicitly recognize
and exclude / ignore the dummy message #0 anywhere they use a sequence range (although
I haven’t actually checked into the details yet). 

Also changing the schema to say #0 is in the message number
range makes it harder to do effective validation of message content on the wire
(should you wish to do that) – specifically is Ack 0..10 valid or just
Ack 0..0?

 

I believe we already have existing features of the RM spec that
can be used to solve this problem and make a definite but correct statement to
handle this boundary condition – We can assert “I’m still
waiting for you to send me the first message” – eg. “Nack 1”

 

<wsrm:SequenceAcknowledgement
xmlns:wsrm="http://docs.oasis-open.org/whatever">

       
<wsrm:Identifier>http://example.org/mysequence/1234</wsrm:Identifier>

       
<wsrm:Nack>1</wsrm:Nack>

</wsrm:SequenceAcknowledgement>

 

The advantages of this approach are:

 

 
It makes a definite statement
     rather than relying on the absence of a statement to convey meaning

 
The statement made is accurate –
     “I haven’t received message #1”

 
We don’t need to change
     the message number range to accommodate this boundary condition –
     the schema can show the real 1..unbounded condition to allow better
     validation of on-the-wire messages

 
We don’t need to introduce
     and document the concept of the fictitious “Message #0” and
     explain where and when it is valid to use that message number

 
I _suspect_ the impact on existing implementation will be
     less by not introducing lots of special-case handling code

 
We are re-using existing functionality
     in the spec rather than introducing new functionality to handle special cases
     – which should ultimately lead to fewer code paths and a smaller
     surface area.

 
This approach is basically backward
     compatible with exiting spec versions (both syntactically and
     semantically) – it represents a clarification for this boundary case
     rather than introducing completely new functionality.

 

 

So I propose the following resolution for Issue 25

Recommend that an RMD be
required to respond with a SequenceAcknowledgement element containing a Nack
child element with content “1” to signify that the first message has
not yet been received for a given Sequence. e.g. 

<wsrm:SequenceAcknowledgement
xmlns:wsrm="http://docs.oasis-open.org/whatever">

       
<wsrm:Identifier>http://example.org/mysequence/1234</wsrm:Identifier>

       
<wsrm:Nack>1</wsrm:Nack>

</wsrm:SequenceAcknowledgement>