Subject: RE: [Tsvwg] Another view of the SCTP error check problem

From: [email-masked]
Date: Fri, 29 Jun 2001 19:41:09 -0400

> While it is clear that the iSCSI recovery stack would be
> exercised less often with an SCTP plus CRC transport than with a TCP
> plus 16-bit Check Sum transport, the number of errors will
> be the same.  None of these transport mechanisms support ECC and
> all use similar retransmission mechanisms.
> I am  assuming that the transport layer recovery
> algorithms are about as expensive as the application
> layer recovery, so the impact on performance is approximately
> the same in both cases.

That's a bad assumption when the application is SCSI.  SCSI level
recovery is horrendously expensive by comparison to a TCP
or SCTP retransmit.  FCIP will discard the corrupt frame and
leaves SCSI to pick up the pieces (assuming that the
application is SCSI over FCP over FCIP) - this will almost
always be much slower than a TCP or SCTP retransmit.

--David

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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
+1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
[email-masked]       Mobile: +1 (978) 394-7754
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From: "Douglas Otis" <[email-masked]>
Date: Sat, 30 Jun 2001 16:50:11 -0700

David,

Unless there is some method of restoring synchronization, the recovery
becomes more than just a dropped FCIP encapsulated frame if synchronizing on
known corrupt data.  FCIP does not use CRC based headers to allow
restoration that may be required many times a day and thus cause the loss of
state between connections and the subsequent dropping of all potentially
outstanding frames.  Considering the level of impact this problem could
exhibit together with overstating the level of protection afford by the
summation, albeit small, only furthers the potential difficulty, and greatly
overlooks the benefits offered by the transport being robust.

Doug