RE: a vote for asymmetric connections in a session

[Charles Monia <cmonia@NishanSystems.com>]

Hi Randal:

> -----Original Message-----
> From: Randall R. Stewart [mailto:randall@stewart.chicago.il.us]
> Sent: Friday, September 08, 2000 4:58 AM
> To: Charles Monia
> Cc: Matt Wakeley; ips@ece.cmu.edu
> Subject: Re: a vote for asymmetric connections in a session
> 
> 
> Charles Monia wrote:
> <snip....snip>
> > I'm not sure what is meant by "congestion."  If we're talking about
> > congestion in the TCP/IP transport, I'm in agreement.  
> However, I thought we
> > were referring to the sort of congestion that  the 
> application on top of
> > iSCSI might see if it received more commands than it had room for.
> > 
> > Unless I misunderstood your point, threfore, I think there 
> might be an
> > issue. The only way I can see flow control in the tranport 
> layer being used
> > to avoid dropping commands is if higher layer congestion 
> results in back
> > pressure to the iSCSI pipe.  I believe that behavior is 
> undesirable because
> > it introduces "head-of-line" blocking, with the following 
> consequences:
> > 
> 
> Any time you get ANY retransmissions in TCP you will create a
> "head-of-line"
> blocking scenario. This does not matter if it is from flow control as
> you state or network congestion where a packet is dropped by a router.
> You will have situations where the TCP is holding data waiting for the
> retransmissions of an earlier packet... this is one of the reasons
> that sigtran developed SCTP...
> 
> 

Hi Randall:

My concern was about coupling the SCSI command service layer with a
transport layer of any kind such that congestion in the command service
layer causes transport layer back pressure.  From all the email traffic I've
seen so far, I'm satisfied that this is not an issue.

Charles
Charles Monia
Senior Technology Consultant
Nishan Systems Corporation
email: cmonia@nishansystems.com
voice: (408) 519-3986
fax:   (408) 435-8385