RE: ISCSI: Urgent Flag requirement violates TCP.

["Douglas Otis" <dotis@sanlight.net>]

David, Matt,

The use of the urgent flag actually slows a normal TCP stack in both the
added single byte send and the added execution for handling the urgent
pointer.  In addition to differences in interpretation, there may also be
byte position errors due to machine pointer alignments.  As the urgent
pointer was never intended as a record pointer nor is the position of the
pointer directly passed to the application, such errors easily go unnoticed
just as differences between stacks.

Your practical use for this flag would be to implement non-standard TCP
designed specifically to locate a record mark useful to software
specifically designed to interpret SCSI transport which could not be used
for standard TCP streams.  By making this urgent flag use a Must, you are
benefiting designs using the modified TCP while standard TCP implementations
suffer degraded performance as a result.

The practical use of this feature would be to add a running list of record
positions that may point to the being of some records.  Should there be a
lost TCP segment, data transfers contained in the following segments could
be placed within application space pending recovery of the missing segment.
Status messages would be held but data, which should occupy the greatest
volume, could be stored without double buffering during the segment recovery
period.  A noble goal but only possible with very non-standard TCP
implementations.

As there would be no way to identify the use of this feature in the general
sense, you will be making a royal mess for those wishing to make dedicated
hardware to accelerate "standard" protocols.  As the charter for this WG is
to avoid modifying TCP, why make the use of a standard TCP transport a
handicap or to muddy the waters for those wishing to make standard
accelerated hardware?

Doug