Re: ISCSI: Urgent Flag requirement violates TCP.

[Glen Turner <glen.turner@aarnet.edu.au>]

Matt Wakeley wrote:

> The intended manner that is to be implemented is this:
> 
> 1. iSCSI sends the first byte of the iSCSI message to TCP, with the byte marked
> "urgent".  You can do this today using any off the shelf TCP stack, including MS
> windows.
> 2. iSCSI sends the rest of the bytes of the iSCSI message to TCP with normal
> delivery.
> 
> If TCP coalesces, fine.  But eventually, a TCP segment will be sent that will
> contain a byte referenced by the urgent pointer.  At this point, the remote side
> (and any LAN analyzers on the network) will be able to "sync up" on iSCSI
> messages.

Maybe I'm thick.

But when I look at the socket API for UNIX I can't see how
a receiver can use Urgent data create a synchronisation point
within a TCP stream.

Firstly I open a SOCK_STREAM (this creates a TCP socket).
Then I call recv() with the MSG_OOB option (which allows
me to receive Urgent data).

Now when I'm sent Urgent data I can collect it, but I have
no idea of its relationship with the rest of the TCP stream.

If I don't use MSG_OOB then the socket implementation may
do one of two things:
 - leave the Urgent data in-stream (effectively, not implement
   Urgent)
 - move the Urgent to the head of the receive queue.
Neither option allows the socket API to use Urgent data for
marking a message boundary.  Without using MSG_OOB you
don't even know *which* data is Urgent.

As I pointed out in my posting on FCIP, modifying the
operating system's TCP path to supply a socket service
for use by an iSCSI client isn't acceptable, either from
a commercial or from a performance point of view.

This WG also needs to sync with the Firewall Traversal WG
if it wants support for a modified TCP service.

I strongly recommend that the WG discard this approach and
use TCP as no more than a octet stream service.  This will
require an adaption layer for iSCSI over TCP, where the adaption
layer marks SCSI protocol data unit boundaries.

Regards,
Glen

-- 
 Glen Turner                                 Network Engineer
 (08) 8303 3936      Australian Academic and Research Network
 glen.turner@aarnet.edu.au          http://www.aarnet.edu.au/
--
 The revolution will not be televised, it will be digitised