Subject: RE: [rddp] Option or mandatory CRC in MPA

From: "Shah, Hemal" <[email-masked]>
Date: Wed, 9 Apr 2003 08:11:10 -0700

I also find myself in the same situation as John pointed out below. RDMA
over TCP/IP is optimized to be implemented on RDMA enabled NIC (RNIC). A
host SW implementation of RDMA over TCP/IP realizing real benefits of
RDMA is questionable. Thus, I also find it hard to justify some of the
arguments concerning a complete software implementation of RDMA over
TCP/IP. If some of the features that are optimized for HW can not be
implemented efficiently in SW, then we should not be changing features
that optimize SW implementation at the cost of added HW complexity.

As others have pointed out, the cost of computing/verifying CRC in the
RNIC hardware is not significant compared to the total RNIC
functionality. By not making CRC verification optional simplifies the
RNIC implementation. So, I do not think it is justified to make CRC
verification optional to optimize SW implementation of RDMA over TCP/IP.

Hemal

-----Original Message-----
From: John Hufferd [mailto:[email-masked]] 
Sent: Tuesday, April 08, 2003 8:04 PM
To: [email-masked]'; [email-masked]
Subject: RE: [rddp] Option or mandatory CRC in MPA

I find some of these arguments somewhat strange.  We have all entered
into this effort to bring optimized processing with Remote Direct Data
Placement, yet we continue to talk about, and give serious weight to,
the argument concerning an all software implementation.  It seems that
we might think about a complete software implementation, in passing, but
this is NOT the goal of RDMA or this RDDP workgroup (in my opinion).
Therefore, I do not think it is important to do things that will make
the protocol more complicated for this small segment of the market,
assuming that it exist at all other then for a testing platform.   

Most of the applications that will be using the RDMA protocols already
have existing TCP/IP implementations, and they are not enhanced or
accelerated by a software implementation of RDMA.  If systems only have
a standard NIC, then the applications should continue to use TCP/IP.   

Expanding on this point further, the work being done in various places
on Socket Direct Protocols, will permit the application to continue to
use the Socket APIs, and the OS will convert that to RDMA where possible
and reasonable.  I submit that will be when there is an RDMA chip set
available. 

There are other applications that I am aware of that are "adding"  the
RDMA capability, not taking away the TCP/IP  implementation.  They check
to see which one they can use and act according.   

So again, being overly concerned about a software implementation of RDMA
is not something on which we should spend much time. 
.
.
John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/System Group, 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] 

"Williams, Jim" <[email-masked]> 
Sent by: [email-masked] 

04/08/2003 05:17 PM 

        To:        "[email-masked]'" <[email-masked]> 
        cc:         
        Subject:        RE: [rddp] Option or mandatory CRC in MPA 

The argument for making the CRC optional
in the RDDP over TCP mapping is the following.

 A.  A large percentage of usage will be on
     highly reliable private networks where
     the TCP checksum is more that sufficient
     the achieve the desired level or reliability.

 B.  Optimizing the performance of software 
     implementations is important, and computing
     an unneeded CRC is too high a price to pay
     for avoiding the need to negotiate an option.

The input from NFS is important because it takes
into account years of field experience.  If they
determine that for most all usage, even on private
networks, the TCP checksum is NOT adequate, then
I would regard that as a refutation of point A.
If they determine that for a large enough percentage
of implementations the TCP checksum IS adequate,
then I would regard that as support of point A.

Regarding point B, if one envisions that all 
implementations of interest are hardware customized
for RDDP, then the point does not carry much weight.
However, I would argue that this may be too narrow
a view for the IETF to take.
_______________________________________________
rddp mailing list
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https://www1.ietf.org/mailman/listinfo/rddp

---

From: Stephen Bailey <[email-masked]>
Date: Wed, 09 Apr 2003 16:34:11 -0400

> So, I do not think it is justified to make CRC verification optional
> to optimize SW implementation of RDMA over TCP/IP.

The reason to provide CRC is widespread demand.

The reason to make it optional is that we can not conclusively
evaluate the impact/return tradeoff with data we have now.

The traditional reason for mandating a feature like data integrity is
not demand, but rather because it is broadly unsafe not to implement
it.  Absent experience with implementations and applications, all we
have is `hunch' (albeit strong and widely held), which probably isn't
enough.

The answer will be clear when MPA goes from Proposed to Draft.  I
don't see anything short of a miracle (or a failure by the working
group to advance the proposal at all) making it clear going from ID to
Proposed.

Steph