Re: iSCSI: Markers

["Julian Satran" <[e-mail]>]

John,

FIM works now and COWS is not as bad as you paint it - as most software 
stacks touch the data at least once (no exceptions!).  As for the pipeline 
argument - it must have something to do with Alaska - as I don't get it 
:-)

If you want a vote - I can't - my heart is thorn ...

Julo




John Hufferd@IBMUS
07-01-02 21:49


        To:     Julian Satran/Haifa/IBM@IBMIL
        cc:     [e-mail]
        From:   John Hufferd/San Jose/IBM@IBMUS
        Subject:        Re: iSCSI: Markers
 





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 [phone], Tie: 276-0403,  eFax: [phone]
Home Office [phone], Cell: [phone]
Internet address: [e-mail]

Sent by:        [e-mail]
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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 Hufferd@IBMUS 

07-01-02 12:07 

        To:        Julian Satran/Haifa/IBM@IBMIL 
        cc:        [e-mail] 
        From:        John Hufferd/San Jose/IBM@IBMUS 
        Subject:        Re: iSCSI: MarkersLink 
  





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 [phone], Tie: 276-0403,  eFax: [phone]
Home Office [phone], Cell: [phone]
Internet address: [e-mail]
 

Sent by:        [e-mail] 

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Subject:        Re: iSCSI: Markers 




Some comments in text - Julo 




John Hufferd/San Jose/IBM@IBMUS 
Sent by: [e-mail] 

06-01-02 23:57 

        
        To:        [e-mail] 
        cc:         
        Subject:        iSCSI: Markers 

       

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 [phone], Tie: 276-0403,  eFax: [phone] 
Home Office [phone], Cell: [phone] 
Internet address: [e-mail]