RE: iSCSI: RE: Framing Discussion

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

> Yeesh!! Thank goodness all I have to do is drop-in an embedded processor
> into our chip. I'll let the firmware folks deal with this problem.
> Certainly, this path does not have to be high-performance since
> we are going into congestion control anyway, but we have to deal with it.
> We can keep a
> context stack for the current open TCP sessions which contain mappings of
> TCP sequence numbers to specific CDB context locations (either command or
> offset within the gather list). We can keep this stack as deep as the
> maximum number of outstanding (i.e. not ack'd) TCP segments which for
> Randy's example (1.25Gbs and RTT of 100ms) is not too bad (about 8K
> entries). We can recover the contexts needed to re-build this missing TCP
> segment and re-construct entire PDU(s) so that any necessary
> digests can be re-calculated. We can then stage this data in memory
> somewhere and pull-out the exact TCP block that we need to re-send.
> Lovely.

You forget the most important fact of an iSCSI adapter.  The microcode does
the transmit knows exactly how it creates a TCP segment.  First, it would
not mix multiple PDU's into a single segment.  Second, there is no TCP stack
in the iSCSI adapter.   While a command or status PDU is created on the fly,
all data PDUs are retransmitted from the application buffers which are not
freed until an exchange is complete.  Third, there is a separate TCP stream
to each connection.  The task of retransmit is to find the exchange number
by using the end-point and segment sequence number. The microcode keeps
track the starting sequence numbers for multiple open exchanges.  However,
one can take an easy way out by having only one open exchange per endpoint.
Of course, this slows things down.  But, don't be too surprised that some
implementation might just do that.   As I said before, to keep track of a
few thousand exchanges, there is a lot of memory needed.  However, the
algorithm to go through the exchange context to retransmit a missing segment
is straight forward and does not take a rocket scientist!  (Or should I say
an iSCSI scientist?)

In summary, transmit is easy because you got to call the shots as long as
rules are followed.  Can't say the same for receive when old TCP
implementations must be honored.