RE: R2TDataSN

["Venkat Rangan" <venkat@rhapsodynetworks.com>]

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
>
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