RE: Framing Formats

["Mike Parkhurst" <mike@integrix.com>]

If the iSCSI protocol is going to take on the responsibility of insuring
a reliable data stream then there are only two alternatives.

	1) Fixed length commands with padding.
	or
	2) Pattern markers (and the required byte stuffing) that allow
the 
         detection of a command frame boundary. (I agree that this
approach
         is oriented more towards those with hardware solutions)

Personally I would rather drop the whole issue of Syncing and Steering
(section 1.2.8) from the specification all together. In paragraph two of
section 1.2.8.1 there are reasons given for not allowing TCP to do the
re-ordering needed when packets are either dropped or show up out of
order. Yet in paragraph three of the very same section we are told to
use the same TCP techniques to handle dropped or up out of order
packets.

Mike

-----Original Message-----
From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu] On Behalf Of
Rod Harrison
Sent: Tuesday, July 10, 2001 11:42 AM
To: ips@ece.cmu.edu
Subject: RE: Framing Formats


	Byte stuffing will kill the performance of software
implementations.

	Do we really believe re-synching in this way is necessary?
Remember that the host SCSI layer will retry failed
commands. I believe we will see error rates low enough to
make pushing recovery back onto the host a viable solution.

	- Rod

-----Original Message-----
From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On
Behalf Of
Mike Parkhurst
Sent: Tuesday, July 10, 2001 6:24 PM
To: Black_David@emc.com
Cc: ips@ece.cmu.edu
Subject: RE: Framing Formats


In my previous posting I was referring to the TCP-unaware
variety. At
the heart of the matter is whether we trust the underlying
transport
mechanism or not. If we don't, then there has to be some
sort of
error-recovery procedure for when information/data is lost
in transit.
If we do trust it, then the iSCSI specification is
simplified and we
assume that the protocol is riding on an error free link.

But to go back to the point of this thread....

Byte and word stuffing mechanisms have been proven to work
for many
years. Picking a 32 bit number reduces the chances of it
showing up in
the data stream. Even if it does the stuffing mechanism
would mask it.

Mike


-----Original Message-----
From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]
On Behalf Of
Black_David@emc.com
Sent: Tuesday, July 10, 2001 7:07 AM
To: Julian_Satran@il.ibm.com
Cc: ips@ece.cmu.edu
Subject: Framing Formats

NOTE: iSCSI tag removed because there's FCIP-related
material in this message.

> We don't want this thread to be started again in this
forum.
> For anything related to framing go to the TSWG or to the
RDMA mailing
list.

I think Julian may have jumped for that conclusion a little
too quickly.
Certainly, there are topics that belong on those mailing
lists:

- TCP-aware framing mechanisms are in the domain of TSVWG.
Use
	of URG and PSH for framing is not going to happen.
- RDMA mechanism discussions belong on the RDMA list - this
	is not an official IETF list.

Beyond these, there are a couple of topics that are fair
game
for the IPS list:

- TCP-unaware framing mechanisms intended for specification
as
	part of an IPS protocol; this includes markers, as well as
	the synchronization recovery mechanism proposed for FCIP.
- Discussion of appropriate requirements (e.g., should
markers
	be specified as "MUST implement" with some additional
	rules about which end of a connection controls their
	usage on what traffic?).

The requirements issue is definitely an open issue at this
point
in time.  As for alternatives to markers, a fairly high
level of rigor
will be expected of approaches that scan data looking for
some
distinctive pattern - it basically needs to be the case that
this
sort of scan and detect algorithm cannot get confused by any
pattern appearing in a data payload (read or write data for
iSCSI).  A motivating example to think about is where trace
data that includes the distinctive pattern (e.g., from a
sniffer)
is being written to or read from a file.
Byte and word stuffing mechanisms to ensure that
the pattern the algorithm is looking for can't appear in
data
are one way to accomplish this, but there are others.

While I'm on this topic, let me note that the algorithm in
Annex A of the -03 FCIP draft does not meet this "cannot
get confused" criterion.  For FCIP, it is possible to meet
this criterion without resorting to stuffing techniques -
the
basic idea is to scan enough incoming data so that there
have to be valid headers in there, note every possible
header found, and their relationships based on length
fields.  If the resulting situation is ambiguous/confusing,
restart and/or abandon the attempt to re-establish
synchronization.  A considerably improved algorithm
should appear in the -04 version of the FCIP draft based
on some offline discussion with some of the draft authors.

Thanks,
--David

---------------------------------------------------
David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
+1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
black_david@emc.com       Mobile: +1 (978) 394-7754
---------------------------------------------------