Re: ISCSI: Urgent Flag requirement violates TCP.

[Matt Wakeley <matt_wakeley@agilent.com>]

Dick Gahan wrote:

> Matt
>      After a TCP segment is lost - what is the maximum and average number of
> bytes that can be received without
>      seeing an URG flag in your scheme, since you state that
>
> >We don't care if sometimes coalescing happens.  The point is, that somewhere
> >down the stream, an urgent pointer will point to the beginning of a new iSCSI
> >message and messages after this point can be processed.  All messages between
> >the point of the dropped segment and this point will of course have to be
> >retransmitted.
>
>      Dropping good segments just because they were out of order(previous missing
> one)
>       seems wrong and wasteful of bandwidth.
>      On a lossy connection you would make it even lossier!.

They don't have to be dropped.  They could be stored somewhere on the adapter.

>
>
>      Also, when TCP advertizes a window size it should be able to buffer that
> amount of data in case
>      the application cannot accept the data straight away or at the rate required.

I'll bet most TCPs lie about how much they could buffer if all open connections
suddenly experienced a burst of data and the application(s) where busy doing
something other than reading from TCP...

But at any rate, on high speed links across long distances, the window will have to
be advertised as huge to keep the pipe full, probably much more than there is
available memory on the NIC (or perhaps even the machine memory).


> In your appliaction
>      I assume the application is willing and ready to receive all the data in
> the window whenever it comes.

Yep - the way SCSI works, the application provides the buffers when it issues the
command.

-Matt Wakeley
Agilent Technologies


>
>
> Dick Gahan
> 3Com