RE: Proper understanding of

Thanks Mike,

I understand you to be in agreement that the conformant device MUST NOT use
the marker received in the 3rd segment.

BTW - I am unclear as to why the buffer only has to be 512B, wouldn't it
need to be large enough to cover whatever size the FPDU might be? What is
gained by catching the next marker?

-----Original Message-----
From: Mike Seufert [mailto:[email protected]] 
Sent: Tuesday, January 02, 2007 10:05 AM
To: [email protected]
Cc: RDDP
Subject: Re: [rddp] Proper understanding of

Barry,

In the situation you described, the receiver could not use the marker 
information in the 3rd segment until the 2nd segment had also arrived.
This means the receiver must be prepared to do some amount of 
(per-session) buffering. Fortunately the amount of this buffering is 
small since the marker period is only 512B.

Mike Seufert
www.neterion.com

bbr wrote:
>
> Folks,
>
> 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).
>
> "To detect the start of the FPDU unambiguously one of the following
>
> MUST be used:
>
> 1: In an ordered TCP stream, the ULPDU Length field in the current
>
> FPDU when FPDU has a valid CRC, can be used to identify the
>
> beginning of the next FPDU.
>
> 2: For optimized MPA/TCP receivers that support out of order
>
> reception of FPDUs (see section 4.3 MPA Markers on page 15) a
>
> Marker can always be used to locate the beginning of an FPDU (in
>
> FPDUs with valid CRCs). Since the location of the Marker is
>
> known in the octet stream (sequence number space), the Marker can
>
> always be found.
>
> 3: Having found an FPDU by means of a Marker, an optimized MPA/TCP
>
> receiver can find following contiguous FPDUs by using the ULPDU
>
> Length fields (from FPDUs with valid CRCs) to establish the next
>
> FPDU boundary."
>
> Question: Is an implementation allowed to use the marker, as described 
> below, to "unambiguously" detect the start of a FPDU?
>
> Condition under which marker was received:
>
> 1. Markers being used (negotiated on)
>
> 2. Large FPDU being sent which consumes three TCP segments (not 
> desired but allowed)
>
> 3. Second TCP segment lost
>
> 4. Marker located in third TCP segment
>
> 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.
>
> Is this the correct understanding of this requirement?
>
> Barry Reinhold
>
> (603) 868-8411
>
> [email protected] <mailto:[email protected]>
>
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