Proper value for embedded marker when FPDU is preceded by a marker

Clause 7.1 in MPA states:

   "An FPDUPTR value of 0x0000 is a special case - it is used when the
   marker falls exactly between FPDUs.  In this case, the marker MUST be
   placed in the following FPDU and viewed as being part of that FPDU
   (e.g. for CRC calculation). Thus an FPDUPTR value of 0x0000 means
   that immediately following the marker is an FPDU header."

When a marker falls between FPDUs, and is "viewed as being part of that
FPDU", is it considered to be part of the FPDU in terms of the values
placed in other markers? That is, if I have a second marker in the FPDU
that has a marker right before the start of the FPDU, is the value in my
second marker point me to the start of the first marker or to the start
of the "real" FPDU.

By example:

|FPDU-start....FPDU-end|MARKER|FPDU-start....MARKER2....FPDU-end|
                       ^      ^
                       |      |
                       |      |
                       A      B

Does the back pointer in MARKER2 point to A or to B?


This issue has already been discussed a bit on another reflector - and
Pat Thaler has observed the following:

+++++++++++++++++++++++++++++++++++

I don't think we thought about this issue. It may be better to have the
marker point to location A. My rationale: 

We are always having to keep an eye on which location has a marker and
skip it so it doesn't save anything to have the markers point at B. 

On the other hand, if we get data out of sequence and unaligned so we
are having to use the markers to find the start of data, we will have to
know where to start the CRC check on the marker we find for that frame.
If the rule is the marker has to point at B, then we will always have to
check to see if B is just after a marker location to know where to start
the CRC calculation.

Another point: We are using markers to find out where we are right? I
expect one would look at the earliest marker one has in the stream
first. Looking at the stream, if what we have received starts here or
earlier,
                         |
                         V
> |FPDU-start....FPDU-end|MARKER1|FPDU-start....MARKER2....FPDU-end|
>                        ^      ^
>                        |      |
>                        |      |
>                        A      B

Then we will have looked at MARKER1 anyway and MARKER2 won't matter and
we wouldn't do any extra math. Lets say instead that what we got started
at B. If we get MARKER2 and it points to B, it would be natural to try
to process the frame because we have the start. But we can't do that
because we don't have MARKER1 so we can't do the CRC. We would have to
do the math to check whether there was a marker immediately proceeding
before we could even decide whether to process the frame.

If markers always point to the frame start (meaning start including the
marker position), then we never have to do an extra checking step.

Regards,
Pat
Barry Reinhold
Lamprey Networks
[email protected]
(603) 868-8411