RE: iSCSI: No Framing

["Naveen Nimmu" <naveen@broadcom.com>]

I second all the points made here and vote for no framing...
Naveen Nimmu

-----Original Message-----
From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
WENDT,JIM (HP-Roseville,ex1)
Sent: Tuesday, January 29, 2002 10:47 PM
To: ips@ece.cmu.edu
Subject: iSCSI: No Framing

Perhaps we should discuss the possibility of not
specifying any framing mechanism (FIM or COWS) in the
first version of iSCSI.

The arguments for not specifying framing include
(there may be others as well):
1) The brute force approach of putting TCP receiver
reassembly memory on the iSCSI NIC will work for both
1Gbps and 10Gbps. Cost is incurred for memory chips,
ASIC pins, power, and board space. But, it is a
feasible approach.

2) I/O bus latency (or bandwidth limitations) could
mandate inbound buffering (as a 'smoothing' buffer)
from the iSCSI NIC to the host system. If this buffer
memory is large enough to have to be off-chip, then
it will require some minimum number of memory chips
to provide the necessary bandwidth, and the
memory/pins/power/space costs will be incurred
anyway.

3) Very large receive buffering on the NIC is only
required for high-bandwidth links traversing large
geographic distances (which may not be the
predominate case). These specialized implementations
can cost more (e.g. more memory/power/pins/etc) and
customers will likely be willing to pay accordingly.

4) Many NICs will likely aspire to be combo
iSCSI+TOE+Ethernet NICs allowing the host system to
use a single Ethernet port for all of its network
communications. The TOE function on this combo NIC
will already require TCP receive buffering and off-
chip buffer memory to support the existing sockets
interface receive model.  More importantly, to allow
senders to assume ownership of the buffer upon return
from a socket send call, the TOE NIC would need to
copy application send buffers into NIC memory as
well.

5) The framing/marker mechanism will be an iSCSI-only
solution. It is quite possible that the problem will
be solved via a different, and hopefully more
general, mechanism for other ULPs.

6) Both FIM and COWS are poor solutions for software
implementation. COWS requires, at a minimum, the
sender to read every word in the buffer. FIM either
imposes additional sender gather processing (iovecs)
or requires a data buffer copy (on systems that don't
support HW gather on sends).

7) Unless all senders thoroughly test framing/markers
now (i.e. unless the framing method is a MUST
implement), there is potential for future
interoperability problems as framing/marker receivers
are deployed in the future.

8) Neither FIM nor COWS is an elegant solution. Are
we comfortable with the legacy we are creating under
the umbrella of state-of-the-art IP networked
storage?

9) Is it essential to build in forward compatibility
now, or will there be a different solution in the
10Gbps or 40Gbps timeframe - perhaps involving iSCSI-
2?  Will it be reasonable to update or bridge initial
1Gbps deployments?


So, it would be good to hear from several iSCSI
NIC/chip implementors who:
- have or plan to implement FIM or COWS (or some
other framing mechanism) and take advantage of it to
minimize demands on on-NIC buffer memory
bandwidth/quantity.
- believe that all-buffers-on-chip solutions are
feasible and valid (wrt the points above, including
#2)
- can quantify the memory/pin/power/space cost
savings for all-buffers-on-chip solutions

Jim