Re: ISCSI: Urgent Flag requirement violates TCP.

[Matt Wakeley <matt_wakeley@agilent.com>]

Glen Turner wrote:

> 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.

For a strictly software implementation, the urgent pointer provides no benefit at all.
This framing mechanism is to help special TCP/iSCSI accelerated implementations, not
iSCSI implemented using generic off the self TCP stacks.

Sorry I forgot to mention that...

-Matt Wakeley
Agilent Technologies

>
>
> 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