RE: R2TDataSN and other recovery mechanisms

["Somesh Gupta" <someshg@yahoo.com>]

Venkat,

You are right but this does put some additional requirements
on the sync and steering layer (assuming we can agree on one
and that it is not optional).

Consider the marker mechanism as an example. If the marker
falls within the TCP byte stream region which contains the
header, and the header digest fails, then do you trust this
marker or skip to the next marker. Or do we need a sum on the
marker itself.

We do have various recovery mechanisms in the protocol without
detailed state machines or other documentation to make sure
that the mechanisms are really robust and worth including. And
even more important, that we will have interoperable
implementations that are implemented to the standard (rather
than being compliant with some defacto implementation)

I am not saying that these tools do not work, and Julian
(and other smart people)may even have worked them out.
However, the proof is not in the document.

Thanks,
Somesh

> -----Original Message-----
> From: Venkat Rangan [mailto:venkat@rhapsodynetworks.com]
> Sent: Tuesday, March 06, 2001 10:00 AM
> To: someshg@yahoo.com; julian_satran@il.ibm.com; ips@ece.cmu.edu
> Subject: RE: R2TDataSN
>
>
> Somesh,
>
> > When there is a data digest error, further stream parsing is
> > deterministic. But not when the PDU header digest error.
>
> Isn't it true that with Synch and Steering layer, you are able to
> skip past
> PDUs with header-digest errors and reach a point where you can begin
> deterministically processing the stream? (assuming that the
> headers used for
> Synch and Steering are in-tact).
>
> Of course, in the event that this option is "negotiated out" you
> don't have
> this ability.
>
> Venkat Rangan
> Rhapsody Networks Inc.
> http://www.rhapsodynetworks.com
>
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Somesh Gupta
> Sent: Tuesday, March 06, 2001 7:23 AM
> To: julian_satran@il.ibm.com; ips@ece.cmu.edu
> 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
> >
> > _________________________________________________________
> > Do You Yahoo!?
> > Get your free @yahoo.com address at http://mail.yahoo.com
> >
> >
> >
>
> _________________________________________________________
> Do You Yahoo!?
> Get your free @yahoo.com address at http://mail.yahoo.com
>


_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com