RE: ISCSI: Urgent Flag requirement violates TCP.

["Y P Cheng" <ycheng@advansys.com>]

> From: Douglas Otis [mailto:dotis@sanlight.net]
> Sent: Monday, November 13, 2000 10:24 AM
>
> Y.P.
>
> TCP does not technically allow zero copy as PDU headers may be
> split across segments and urgent pointers may coalesce beyond
> received segments.  You are advocating modification to TCP
> specifically for iSCSI and appear to advocate segment alignment
> for PDU headers with a one packet at a time standard.
> Do you wish to see the TCP sender and receiver modified as
> indicated within the proposal?

The TOE is designed to deal the segment alignment and coalescing of
segments.  All I said was that the urgent pointer helps the parsing of the
segment not changing the alignment.  Without the urgent pointer we need to
wait for all missing segments before parsing.  Missing data segments do not
affect the parsing at all.  We know the missing segments are data, the
buffering requirements for the TOE is dramatically reduced.  For 10 Gb
Ethernet, there are gigantic TCP windows when latency time on transmission
is in hundreds of milliseconds.  (All data segments are passed through to
the application software directly.  Only segments containing iSCSI messages
are important to the TOE adapter.)  No, I don't wish the TCP sender or
receiver to be modified as I have said that the TOE adapter would work with
all clients and servers implemented in traditional TCP.  Only the
implementation of the TCP software where the TOE adapter is installed needs
to be modified for adopting the zero-copy function.  Since there is no TCP
changes, this WG proposal requires no descriptions beyond the iSCSI itself.

> In your view TCP software must be modified so why stop at
> specifying the urgent pointer, include frame alignment.  Clearly, you view
> wire compatibility the only constraint.  Is TCP just a wire specification?

There are many TCP implementations.  As Matt has said many times, these
implementations are "inside the box".  TCP does not specify "inside the box"
implementations.  Our discussions herein is limited to "inside the box."
Yes, the adapter does do out-of-order and missing frame detection, frame
alignment, flow control and congestion avoidance for TCP.  However, in
dealing with the data buffers pre-allocated by application software, the
flow control is greatly simplified.  Congestion avoidance and timeouts for
missing segments are totally different stories.  All TCP implementations,
including the TOE adapter, must follow a number of different RFC's that were
kindly pointed out to me by many folks in this WG.

Y.P. Cheng, CTO, ConnectCom Solutions Corp.

>