iSCSI: Header Digest Failure (was R2TDataSN)

[Matt Wakeley <matt_wakeley@agilent.com>]

Julian,

What Somesh is talking about has nothing to do with R2T specifically.  His
point is, the length field in the PDU header determines how long *this* PDU
is, and where to find the *next* PDU.  If *this* PDU fails its header digest
check, you cannot trust the length field, therefore, you cannot know where the
*next* PDU is in the byte stream.

Given that, is seems like whenever there is a header digest error, you have no
choice but to terminate the TCP connection and start a new one.

-Matt Wakeley
Agilent Technologies


Somesh Gupta wrote:
> 
> 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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