RE: Proper understanding of

bbr wrote:
> Folks,
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> I'm looking at the changes in MPA from draft 02 (against
> which the conformance suite was written) to draft 08. I would
> like to understand how to take the following text from page 25
> (clause 6).=20
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> "To detect the start of the FPDU unambiguously one of the following
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> MUST be used:
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> 1: In an ordered TCP stream, the ULPDU Length field in the current
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> FPDU when FPDU has a valid CRC, can be used to identify the
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> beginning of the next FPDU.
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> 2: For optimized MPA/TCP receivers that support out of order
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> reception of FPDUs (see section 4.3 MPA Markers on page 15) a
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> Marker can always be used to locate the beginning of an FPDU (in
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> FPDUs with valid CRCs). Since the location of the Marker is
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> known in the octet stream (sequence number space), the Marker can
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> always be found.
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> 3: Having found an FPDU by means of a Marker, an optimized MPA/TCP
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> receiver can find following contiguous FPDUs by using the ULPDU
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> Length fields (from FPDUs with valid CRCs) to establish the next
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> FPDU boundary."
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> Question: Is an implementation allowed to use the marker, as
> described below, to "unambiguously" detect the start of a FPDU?
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> Condition under which marker was received:
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> 1.    Markers being used (negotiated on)
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> 2.    Large FPDU being sent which consumes three TCP segments (not
> desired but allowed)=20
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> 3.    Second TCP segment lost
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> 4.    Marker located in third TCP segment
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> Issue: In item two above it would appear that the
> implementation would have to validate the FPDU in which the
> marker is received (perform CRC check) before it can use it.
> However, since a portion of the FPDU is missing this would
> not be possible to do. Therefore a conformant device MUST NOT
> use this marker to detect the start of the FPDU.
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> Is this the correct understanding of this requirement?
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The probable FPDU is not complete, therefore a conformant
receiver MUST NOT provide the DDP Segment to the DDP layer.

Whether the MPA receiver "used" the marker in the third segment
in an attempt to validate the FPDU is irrelevant, since no action
would result from merely testing the hypothesis. It would still
conclude that the FPDU was no ready for delivery to the DDP layer.