Re: iSCSI: Header Digest Failure (was R2TDataSN)

[Matt Wakeley <matt_wakeley@agilent.com>]

What you say is that some method of framing is *required* to perform recovery
after a header digest failure.  So, is it then to be inferred that framing in
iSCSI is mandatory?

-Matt Wakeley


julian_satran@il.ibm.com wrote:
> 
> I understood that. But markers are meant to solve this too aren't they?
> 
> Julo
> 
> Matt Wakeley <matt_wakeley@agilent.com> on 08/03/2001 01:54:10
> 
> Please respond to Matt Wakeley <matt_wakeley@agilent.com>
> 
> To:   ips@ece.cmu.edu
> cc:
> Subject:  iSCSI: Header Digest Failure (was R2TDataSN)
> 
> 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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