RE: iSCSI: Keep-alive traffic (was iSCSI: more on StatRN)

["Y P Cheng" <ycheng@advansys.com>]

> >A TCP-Offload-Engine will interpret the TCP/IP header in order
> >to move data payload directly to application software.
> >with changes to the TCP/IP software like that of zero-copy TCP function,
> >the adapter can generate ping-response automatically.
>
> Zero copy does not change the nature of TCP. Just because the data has
> been moved to an application buffer does not mean that the application
> can process it before out of order segments or retransmissions have
> been handled. If you do that, you do not have TCP -- you have a new
> combined iSCSI/TCP transport.

You are correct, it is a combined iSCSI/TCP transport. An iSCSI adapter
receives SCSI requests for data transfers.  It takes other TCP requests for
establishing connection and security and ping and task management matters.
For SCSI requests the adapter and its driver creates iSCSI PDUs by adding
TCP/IP headers.  On receiving, it removes the TCP/IP header and returns data
directly to application that issued SCSI requests.  For TCP requests, the
adapter with help from the device driver splits the TCP header from the
payload to achieve zero-copy.  The TCP software using a TOE adapter is aware
of the fact that data payload has already been transferred to application
software and it does not have to perform memory-to-memory copy.

Having said that, I have never suggested that an iSCSI or TOE adapter will
ever create or accept non-TCP segments or process them out-of-order.