RE: R2TDataSN

[julian_satran@il.ibm.com]

You don't. the next R2T will tell you you have missed one.

Julo

"Somesh Gupta" <someshg@yahoo.com> on 07/03/2001 06:46:07

Please respond to someshg@yahoo.com

To:   Julian Satran/Haifa/IBM@IBMIL, ips@ece.cmu.edu
cc:
Subject:  RE: R2TDataSN




Julian,

How do I know it is an R2T if the header digest failed?
I am sorry if I am missing something very obvious here.

Let us say that a header digest failed. What information
in the header am I able to trust at this point? I don't
know if it is an R2T or not. I don't know if the length
is ok or not.

Thanks,
Somesh

> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> julian_satran@il.ibm.com
> Sent: Tuesday, March 06, 2001 4:01 PM
> To: ips@ece.cmu.edu
> Subject: RE: R2TDataSN
>
>
>
>
> Somesh,
>
>  I am still lost. R2T is fixed length and you know that you have a bad
> digest after reading it.
> No length involved.   ???
>
> Julo
>
> "Somesh Gupta" <someshg@yahoo.com> on 07/03/2001 00:26:10
>
> Please respond to someshg@yahoo.com
>
> To:   Julian Satran/Haifa/IBM@IBMIL, ips@ece.cmu.edu
> cc:
> Subject:  RE: R2TDataSN
>
>
>
>
> Julian,
>
> > -----Original Message-----
> > From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> > julian_satran@il.ibm.com
> > Sent: Tuesday, March 06, 2001 9:31 AM
> > To: ips@ece.cmu.edu
> > Subject: RE: R2TDataSN
> >
> >
> >
> >
> > Somesh,
> >
> > You've lost me. I do not propose that you look at the bad R2tT but to
> find
> > that you have missed one
> > by looking at the next.
>
> Since iSCSI PDUs define how long they are, you have to look at
> one PDU to determine where the next PDU is. (unless ofcourse
> the sync and steering layer is doing the work - see my exchange
> with Venkat on that).
>
> On the long transfer etc., I was not sure what scenarios
> an R2TDataSN was providing recovery from. Since you clarified
> that it is to recover from a header digest error, we can
> focus on that scenario.
>
>
> This interesting for long transfers that have
> > several outstanding R2Ts.
> > What is speculative here? There was never a consensus that there
> > will be no
> > more than one outstanding R2T.
> >
> > Regards,
> > Julo
> >
> > "Somesh Gupta" <someshg@yahoo.com> on 06/03/2001 17:23:04
> >
> > Please respond to someshg@yahoo.com
> >
> > To:   Julian Satran/Haifa/IBM@IBMIL, ips@ece.cmu.edu
> > cc:
> > Subject:  RE: R2TDataSN
> >
> >
> >
> >
> > I beg to disagree. If an R2T PDU (header) has bad digest, or any other
> > header has a bad digest - since you always need the PDU length from
> > the header, there is some uncertainty associated with further
> processing.
> >
> > Are you proposing that the processing machine go into a "speculative"
> > mode where the processing of the next PDU determines whether we were
> > successfuly able to skip a bad PDU header? When there is a data digest
> > error, further stream parsing is deterministic. But not when the PDU
> > header digest error.
> >
> > Also the consensus (in my interpretation) was on applications
> > not transfering very large amounts of data using a single command or
> > read/write PDU.
> >
> > > -----Original Message-----
> > > From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf
Of
> > > julian_satran@il.ibm.com
> > > Sent: Monday, March 05, 2001 5:41 PM
> > > To: ips@ece.cmu.edu
> > > Subject: Re: R2TDataSN
> > >
> > >
> > >
> > >
> > > Somesh,
> > >
> > > 1.The only consensus I heard is not to transfer a large amount of
> > > data with
> > > one PDU.
> > >
> > > 2.With DatasN and Sack we dont need any data in a bad header.
> > >
> > > 3. If an R2T is lost (received at initiator with bad digest) - the
> > > initiator will know that from
> > > the next R2T if the target has several outstanding - very likely at
> long
> > > distances - and will not have to way for a timeout.   Other uses are
> > > marginal.  Basically it is "part of a command execution" and we can
> > > painless recover
> > > from failures for this case too.
> > >
> > > Regards,
> > > Julo
> > >
> > > "Somesh Gupta" <someshg@yahoo.com> on 05/03/2001 20:40:06
> > >
> > > Please respond to someshg@yahoo.com
> > >
> > > To:   Julian Satran/Haifa/IBM@IBMIL, ips@ece.cmu.edu
> > > cc:
> > > Subject:  R2TDataSN
> > >
> > >
> > >
> > >
> > > > R2TDataSN
> > > > ----------
> > > > Sec 6.7.1 has some new content on how to handle lost R2Ts using
> > > > SACKs.  But I noticed that the SACK request (Sec 2.16) has not
> > > > changed to indicate whether the DataSN is a R2T DataSN or just
> > > > a Read PDU DataSN (D bit)
> > > > So do we demux on the read/write operation type?
> > > > And how does this affect PDU loss in bidirectional commands ?
> > > > +++ SACK is ascking for data (DataSN) the target knows
> > > >
> > >
> > > Julian,
> > >
> > > Regarding the R2TDataSN, I have a comments and a
> > > question.
> > >
> > > I think that when a PDU header fails a CRC/checksum check etc,
> > > it is a problem to depend on any information in the header (including
> > > length fields), thereby making any further processing on
> > > the connection unreliable.
> > >
> > > What scenarios do you envision where the R2TDataSN is useful.
> > > IN Orlando I think there was clear consensus that application
> > > do not try to transfer very large amounts of data using a
> > > single command.
> > >
> > > Thanks,
> > > Somesh
> > >
>
> Thanks,
> Somesh Gupta
>
>
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