Re: Frame Formats

[Mike Thompson <mike.thompson@qlogic.com>]

As an implementer of a hardware implementation of iSCSI/TCP, I would like to
see format 2 from the slide set presented at IETF. The fixed location of the
total data length and AHS length will make out of order data placement
reasonable. With these two fields at the beginning of the PDU, hardware will
immediately know how much data needs to be checked to verify the header
digest and if the digest is valid, it can go on to process the next PDU,
looking for data PDUs that can be processed. In previous formats, the
hardware has to process too many fields to get to the digest.

Ideally, I would like to see a slight modification to this format where the
header digest just covers the BHS. My understanding of the header digest is
to allow for iSCSI routers to be able to modify a PDU header if it is acting
as a proxy of some type. It seems that in this case the only thing that
would be modified would be the BHS and not the AHS. With this change, I
would envision the AHS be covered by the data digest. Again, this makes
hardware processing easier, since the header that the digest covers
is always a fixed length.

I also think that the 24 bit total data length is more than adequate for the
total PDU length. In order to be able to efficiently/reliably process PDUs,
the PDU length should be on the order of 8-64kBytes in length. PDUs of 4
GBytes will require 4 Gbytes of reassembly memory in out of order cases.
This is not reasonable.