Subject: [rddp] Need some clarification on how to "locate the start of the FPDU unambiguously"

From: "Barry Reinhold" <[email-masked]>
Date: Tue, 30 Nov 2004 07:49:13 -0500

If markers are being used is it mandatory to also be using CRCs?
This question arises out of the requirements of clause 5.4:

Clause 5.4 of MPA states the following:

============

An MPA receiver MUST first verify the FPDU before passing the ULPDU
   to DDP.  To do this, the receiver MUST:

   *   locate the start of the FPDU unambiguously,

   *   verify its CRC (if CRC checking is enabled).

   If the above conditions are true, the MPA receiver passes the ULPDU
   to DDP.

   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 receivers that support out of order reception of FPDUs (see
       section 5.1 MPA Markers on page 16) 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, following contiguous
       FPDUs can be found by using the ULPDU Lengths (from FPDUs with
       valid CRCs) to establish the next FPDU boundary.
=======

If we have an out of order FPDU, and support markers, we use the next
available marker to recover stream sync. By item two above, this
requires that we validate the FPDU which has the marker in it with a
CRC.

Hence it appears that an implementation planning on using markers must
also require that CRCs be supported. Is this the intent?

Barry Reinhold
Lamprey Networks
[email-masked]
(603) 868-8411

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