Subject: Re: iSCSI: Markers

From: "John Hufferd" <[email-masked]>
Date: Mon, 7 Jan 2002 11:49:47 -0800

OK, Julian,
I buy that argument.  I think that you have made the point that the value
of Markers is Greater then I was giving credit for, in iSCSI/TOE integrated
HBAs that are operating in CRC mode.   That is great, since I am afraid we
will not see the Framing stuff for some time.

So the questions are which form of markers is the best.

My vote comes down on the side of Fixed Interval Markers (FIM). For the
following reasons:

On the SW sending side, FIM works with low overhead whether CRC is being
used or not.  COWS, is a dramatic overhead unless the SW is performing CRC
computation anyway.

The COWS has a built in conflict with the Pipeline HW HBA, that has two
different needs regarding the direction of the Pointers when talking to
another partner  HW HBA that uses a Pipeline approach.

.
.
.
John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/SSG San Jose Ca
Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
Home Office (408) 997-6136, Cell: (408) 499-9702
Internet address: [email-masked]

Julian Satran/Haifa/IBM@[email-masked] on 01/07/2002 05:34:13 AM

Sent by:    [email-masked]

To:    [email-masked]
cc:
Subject:    Re: iSCSI: Markers

John,

The markers are supposed to help you get fat to the next PDU and start
assembling and placing it.
It does not alleviate the need for a PDU assembly buffer - that you need
anyhow - it alleviates the need to keep
TCP packets along when you have lost an iSCSI PDU header.

Once you have an iSCSI PDU header you need only the (as short as you want)
PDU reassembly.

If you do not have an iSCSI header you have potentially to keep around an
RTT dependent amount of data.
Markers are here to reduce the later to an amount that is not a function of
Bandwidth*RTT.

Regards,
Julo

    John                      To:        Julian Satran/Haifa/IBM@IBMIL    
    Hufferd@IB                cc:        [email-masked]                  
    MUS                       From:        John Hufferd/San               
                      Jose/IBM@IBMUS                                      
    07-01-02                  Subject:        Re: iSCSI: MarkersLink      
    12:07                                                                 

Julian,
There are two different issues with regards to CRC.

One is on the sender side, and if Markers are done in Software, of course
it matters if CRC is done, since if the sender does not use CRC, the cost
of COWS is very high.

The second point is about the Receiving Side.  I did not understand your
point, so I would appreciate you expanding on your comment that the Target
Side would still get value out of Markers even if they use CRC.  I can
understand that there would be some additional value, especially in TCP
Segment errors.  By permitting some additional TCP Segments to arrive
during the retry, but most of the value is lost because the Full Reassembly
Buffer is needed for CRC.  So now that I confess, that I do not completely
understand your point, perhaps you could step me through your view of the
value in the presents of CRC on a HW HBA/Chip.   I think it is important
that we are able to quantify the value.

With regards to iWARP, I was using that name as an alias for the technique
explained in the framing Draft (at
http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-tcp-ulp-frame-01.txt).
But except for the last part of COWS, there is little resemblance between
that and COWS.  The framing does NOT use Word replacement.  They only have
a 8 byte header in common.

That brings me to another point.  If FIM needs to double its Markers (two
words instead of one), would not COWS also need to double its Header
Markers, for the same reason?

.
.
.
John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/SSG San Jose Ca
Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
Home Office (408) 997-6136, Cell: (408) 499-9702
Internet address: [email-masked]

Sent by:        [email-masked]

To:        [email-masked]
cc:
Subject:        Re: iSCSI: Markers

Some comments in text - Julo

     John Hufferd/San                                                     
     Jose/IBM@IBMUS                  To:        [email-masked]           
     Sent by:                        cc:                                  
     [email-masked]               Subject:        iSCSI: Markers       
     .edu                                                                 
                                                                          
     06-01-02 23:57                                                       

When we were in Salt Lake City, we decided to hold a discussion on the
Reflector regarding Markers.  I think it is probably time to hold that
discussion.  Therefore, I will start it out.

First I want to thank Julian for going to the trouble to document the
proposal for COWS.  This now gives us two Marker proposals to consider.
They are the Fixed Interval Markers (FIM) and the Constant Overhead Word
Stuffing (COWS) proposals.

Having said that, lets examine each one and determine their usefulness.

1. Markers (FIM or COWS) are only needed by Hardware HBAs that are
attempting to limit the amount of Reassembly Buffers they need on-board.
This will always be a probabilistic determination by the vendors, but the
purpose of Markers are to hold down the amount of total RAM needed,
especially when factored into a probabilistic determination.
2. The use of Markers are negotiated per connection, and per direction.  It
is therefore possible for a software implementation to send markers, but
not have to accept them.
+++ that is specially important for software implementations ++
3. When CRC Digest are negotiated to be used, it does no good to send out
Markers since the hardware side will have to have Reassembly Buffers to
compute the CRC Digest.  Therefore, Markers do not help anything when CRCs
are used on a connection.
+++ That is not correct - Framing mechanisms are used to avoid an RTT
related size for the receive buffer not avoinding  a one-PDU buffer. Both
FIM and COWS will help reducing HBA ememory requirements even when using
CRC.  When Using CRC a PDU reassembly buffer is unavoidale but it can be
limited by the MaxRcvPDU. +++

4. FIM can be implemented easily in software, and with an out going
Scatter/Gather technique could send PDUs across a network to a Hardware HBA
destination without requiring additional moves or touching each byte.
5. Software iSCSI implementations should always negotiate NOT to receive
markers since Markers do not help the SW in any way.
6. Hardware HBAs can also implement FIM easily, and HW can do it easily in
both directions.  Therefore, if FIM is the approved Marker proposal, a HW
HBA should send FIM whenever it sends iSCSI Commands and/or Data to other
HW HBAs (assuming iWARP is not an available option), and the other HW HBAs
wants them.
7. I do not foresee many installations using CRC Digests with Laptops and
Desktops, since the overhead will probably be noticeable, and most
installations do not judge Desktop and Laptop data to be key corporate
assets (I am not saying that opinion is right, or that it is Universal,
just that is a prevalent opinion).
+++CRCs are not relevant +++
8. A single software node would not be fast enough to cause a significant
load or a significant  memory requirement for the Reassembly Buffers.
However, the combination of many (perhaps Desktops and Laptops spread
across a real or virtual campus) can bring on a large load and the need for
many Reassembly Buffers.  The resultant amount of small Reassembly Buffers
could make the Total requirement very High (thus raising the cost of the
HBA).
9. Given the need for Desktops and Laptops to avoid noticeable degradation,
and the probability that they will not be using CRC, means that the use of
FIM is a compatible option when working across a network from a SW Node to
an iSCSI HW HBA implemented Storage Controller.
10. COWS was also designed to be easily implemented in Software and in
Hardware.
11. COWS will require the iSCSI Software implementations to touch each
byte, as it looks for matches to its Framing Pattern within the PDUs.
+++ except when implemented within the CRC, checksum where no ADDITIONAL
touch is involved +++
12. When CRC-32 digests are being computed, the Software will have to touch
ever byte anyway, so the additional overhead to look for matches to the
Framing Pattern is negligible.  On the other hand FIM also works well with
SW CRC-32 Digest computation.
13. But the premies I set above (which is clearly up to debate), is that
Desktops and Laptops will not usually compute the CRC Digest.
14. COWS has the ability to have its pointing Markers, which point to the
Framing Pattern Matches, point either forward or backwards.  When being
processed by software, the direction does not matter.  So since the Markers
should only be used on outgoing PDUs, the approprate direction of the
pointers is what ever the destination needs.
15. Many vendors, especially at the higher speeds, are implementing a pipe
line design.  The pipeline receive process, will most likely want the COWS
pointers to be forward pointing (pointing in the direction of the end of
the PDU).  Since the SW does not care, forward pointers should be fine.
16. If the HW Pipelining iSCSI HBA is to send COWS Markers to another HW
Pipelining HBA, there may be a problem.  The outgoing pipeline would prefer
to have the pointers pointing backwards, but the receiving pipeline would
prefer to have the pointers going forwards.  Therefore with COWS, either
the sending or the receiving HW Pipelining HBA will be unhappy and pipeline
design will get very much more complicated.
17. Everyone likes iWARP better, but it requires changes to the Host SW
TCP/IP stacks in the case of SW Initiators.  Since these SW installations
are probably going to be Desktops and Laptops, these systems will very
likely have some non current version of the OS, (such as windows 2k, ME,
etc.).  Hence it is unlikely that they will have iWARP in the majority of
the SW TCP/IP Stacks).
+++iWARP needs framing as well and I would guess the technique will be
close to what COWS is.  Future convergence speaks for COWS+++
18. If, for some reason, a Server uses Software iSCSI, it will probably
have the latest and greatest OS software, and if that is true, and if iWARP
is approved by the IETF then it will probably have the TCP/IP extensions
that are required by iWARP.  It will therefore use iWARP instead of any
Marker solution.
19. If the Server uses Software iSCSI,  and if iWARP is not approved by the
IETF, then COWS or FIM are both  reasonable choices  for the Server, if the
Server also uses the CRC option.  If CRC is not used on the Server, FIM is
a better match with the Server SW iSCSI.
19. iWARP is probably going to stay in IETF "experimental mode" for some
time into the future.  And without iWARP being specified by iSCSI as -- at
least, "MAY Implement" and "MAY use", the HW to HW mode will still need a
Marker solution.
20. Therefore, we are left with Markers as the only technique we can count
on to aid and assist the iSCSI HW HBAs, keep their "on-board" RAM
requirements to a minimum.

Based on the above I come to the following conclusions:
1. Markers are needed.
2. FIM works best with the probable SW environment, and has the lowest
overhead (since they will probably not use CRC Digests).
+++CRCs are not relevant+++
3. Since if CRC is used, the HW needs its Reassembly Buffers anyway, so
there is no reason to use COWS, or any type of Marker.
+++ again the same confusion betwen the PDU buffer and the RTT related
holding are in the absence of Markers+++
4. FIM can work well between HW HBAs without significant impact to pipeline
design.
+++ Somebody already said that FIMs are like democracy - imperfect, ugly
but work+++
+++ However COWS are not that much heavier if carefully implemented and
might get us faster to converge on a framing model+++

Therefore, I recommend that the Draft continue to define FIM.  Also, I
recommend that the FIM type of Markers be "MUST Implement" on the Out-Going
direction, and "MAY Implement" on the incoming direction".   Further, I
recommend the Draft say that FIM "MAY be used" in any direction, depending
on the negotiations.

.
.
.
John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/SSG San Jose Ca
Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
Home Office (408) 997-6136, Cell: (408) 499-9702
Internet address: [email-masked]

---

From: Paul Koning <[email-masked]>
Date: Tue, 8 Jan 2002 11:59:55 -0500 (EST)

>>>>> "John" == John Hufferd <[email-masked]> writes:

 John> OK, Julian, I buy that argument.  I think that you have made
 John> the point that the value of Markers is Greater then I was
 John> giving credit for, in iSCSI/TOE integrated HBAs that are
 John> operating in CRC mode.  That is great, since I am afraid we
 John> will not see the Framing stuff for some time.

 John> So the questions are which form of markers is the best.

 John> My vote comes down on the side of Fixed Interval Markers
 John> (FIM). For the following reasons:

 John> On the SW sending side, FIM works with low overhead whether CRC
 John> is being used or not.  COWS, is a dramatic overhead unless the
 John> SW is performing CRC computation anyway.

 John> The COWS has a built in conflict with the Pipeline HW HBA, that
 John> has two different needs regarding the direction of the Pointers
 John> when talking to another partner HW HBA that uses a Pipeline
 John> approach.

I think you are making general assumptions about what is cheap in
software that may be valid for SOME software implementations but are
certainly not valid for others.

In particular, FIM is NOT necessarily low overhead.  Scatter/gather is
a reasonably cheap mechanis IF it is available for this job.  (Note
that it is not all that cheap: adding a bunch of extra descriptor
fetches to the I/O device's job is extra overhead, the scale of which
depends on the hardware.

The problem is that scatter/gather may not be available at all.  If it
is available in principle, it may have restrictions that clash with
what FIM needs.  (For example, a particular implementation may support
breaking buffers up in many pieces, but all breaks must be on cache
line boundaries.  For a cache line size of 32, clearly you cannot use
scatter/gather in that implementation to do FIM.)

So FIM may force a full buffer copy, which is a major hit.

As for COWS, it's clearly nasty.  Even if you're doing CRC, it adds a
significant increment to the processing cost.

In conclusion, I am opposed to mandating markers.  Adding such a large
increment in processing overhead doesn't make sense to me, given that
I don't see how it can be expected to save all that much memory in
implementations that use it.  (As far as I can see, the gain doesn't
come anywhere near a factor of two.  Is there any analysis available
that discusses this in any detail -- covering not just the specific
buffering needs of the reassembly side but also the other memory
requirements in a node?  It seems to me a node needs at least as much
memory in the other direction -- quite possibly more if it's the
storage end, given that a lot of the traffic is reads.)

	paul