[AMG] Need for XMLP_UnitData.forward Correlation parameter

Doesn't XMLP_UnitData.forward need to take a (mandatory) Correlation
parameter so
that the XMLP layer can correlate requests and replies when
intermediaries are present?

The following scenario hopefully makes this clear, but please accept my
apologies for the
level of detail - as a newcomer to this area, I wasn't sure how much to
assume.

Consider an extension of the scenario in [1] (or [2]) to include a reply
which flows
from the 'receiving app.' back to the 'sending app.' via the
intermediary as follows:

    From    To       Primitive
 1. S       X        send m1
 2. X       I        receive m1'
 3. X       S        status
 4. I       X        forward m2
 5. X       R        receive m2'
 6. X       I        status
 7. R       X        send m3 (m2' reference)
 8. X       I        receive m3' (m2 reference)
 9. X       R        status
10. I       X        forward m4
11. X       S        receive m4' (m1 reference -- but how?)
12. X       I        status

where S=sending app., I=intermediary app., R=receiving app., X=XMLP
layer,
mn=message, mn'=new local copy of mn.

The XMLP layer can infer the correlation of m3' to m2 in (8) from the
correlation
of m3 and m2' in (7). We can see this with the help of a bit of
notation. Let '=' be
equality of messages and '~' be the 'correlates to' relation. Then:

m1=m1'
m2=m2'~m3=m3'
m4=m4'

So m2~m3'.

However, the XMLP layer does not appear to have enough information to
correlate m4' to m1 in (11). It would need:

m1'~m2
m3'~m4

which could be provided by specifying a Correlation parameter on (4) and
(10), thus:
 4. I       X        forward m2 (m1' reference)
10. I       X        forward m4 (m3' reference)

Glyn Normington
IBM Hursley

[1] http://www.w3.org/TR/2001/WD-xmlp-am-20010709/#Sec3.1.2
[2] http://www.w3.org/2000/xp/Group/1/08/14-am/xmlp-am.html#Sec3.1.2