Re: Why FCP doesn't need RDMA? It has a better way.

["Randall R. Stewart" <randall@stewart.chicago.il.us>]

"GUPTA,SOMESH (HP-Cupertino,ex1)" wrote:
> 
> Even iSCSI does not really need RDMA. The underlying note in
> the whole debate is really the preservation of message boundaries.
> 
> If we could use one of the reserved bits in the TCP header to say,
> do the repacketize or adjust segment boundaries, and each TCP
> segment starts with an iSCSI header, there should be no need for
> another protocol (or TCP option header).
> 

I don't think you can go playing with TCP option bits. This implies
changing TCP and I don't see this in the WG charter. If bits need
to be changed then a draft needs to be written and submited to
the proper WG.. in this case I would say transport area WG...

I think in some respects this is exactly what Costa has done... Now
as far as message boundary preservation, you get this for free
with SCTP.. and has Douglas as already pointed out, you could very
easily implement in SCTP the ability to copy a specific stream
into receive buffers .. i.e. tell the sender, but the data on
stream X and then as it arrives dump it directly to user buffers.

This would take a bit of work, but so does the RDMA solution :/

R

> The iSCSI header contains enough information in it to enable
> the recepient to determine where to put the data.
> 
> Somesh
> 
> -----Original Message-----
> From: Robert Snively [mailto:rsnively@Brocade.COM]
> Sent: Thursday, September 28, 2000 8:29 AM
> To: 'Matt Wakeley'; IPS Reflector
> Subject: RE:Why FCP doesn't need RDMA? It has a better way.
> 
> >  I object to mandating iSCSI use an RDMA option because:
> >
> >  - (main reason) there isn't any standardized mechanism now, and I
> >    don't want to hold up iSCSI while one crawls through the
> >  standards process.
> >
> >  - I don't think RDMA is needed.  FCP doesn't use it, and it
> >  works great with
> >  the
> >    FC protocol chips that "accelerate" FCP.
> 
> Actually, RDMA is not needed in FCP because all protocol chips
> implemented perform a real peer-to-peer DMA straight to the
> data areas specified by the user's interaction with the operating
> systems allocation algorithms.  The combination of the FCP/SCSI
> pointer structure, task tagging, and the FC relative offset perform the
> function you would otherwise have to use RDMA to accomplish.
> 
> Bob Snively
> Brocade Communications           Phone  408 487 8135
> 1745 Technology Drive
> San Jose, CA 95110               Email   rsnively@brocade.com

-- 
Randall R. Stewart
randall@stewart.chicago.il.us or rrs@cisco.com
815-342-5222 (cell) 815-477-2127 (work)