Re: ISCSI: draft-wakeley-iscsi-msgbndry-00.txt

["Franco Travostino" <travos@nortelnetworks.com>]

IMO, 'Customized' and 'closely coupled' reads also as a protocol layering
violation. This raises concerns of architectural and practical nature.
For one, low-cost commodity TCP silicons will come in one shape, and they
better be agnostic to whatever the client(s) are. The implementation
space has historically been fragmented over urgent pointer semantics, I'd
hope that we do not raise this entropy with new iSCSI self-serving
flavors.

Alternately, it there is a contribution on urgent pointer matters that is
of general applicability, it could be submitted as a general purpose
amendment to TCP for framing (with the same general applicability
statement that was applied to the RDMA thread). Other communities (say
http 1.x) may add their support.

-franco 


At 09:14 PM 9/29/00, Matt Wakeley wrote:
> Somesh,
>
> What you describe is how current TCP stacks typically operate when it
> does not
> know anything about the protocol running on top of it.  A customized
> TCP stack
> that was closely coupled with the iSCSI protocol running on top of it
> could
> provide the appropriate urgent pointer in each segment that is
> (re)transmitted
> without "coalescing" the urgent data. Likewise a customized TCP
> inbound path
> that was closely coupled with the iSCSI protocol running on top of it
> could
> pass the urgent information to the iSCSI layer without
> "coalescing" the urgent
> data. This could be done without any modifications or violations of
> current
> TCP specifications.
>
> -Matt
>
> "GUPTA,SOMESH (HP-Cupertino,ex1)" wrote:
>
> > Matt,
> >
> > Use of the urgent pointer requires very careful investigation. If
> you
> > look at the TCP RFC it mentions that that the urgent pointer
> > location is a connection-wide pointer, both on the receiver
> side
> > and the sender side.
> >
> > From the sender side, even if TCP emits a TCP segment for 
> every
> > user send, you are not guaranteed the preservation of the
> urgent
> > pointer - e.g.
> >
> > 1. Let us say that a retransmission timeout occurs and a 
> number
> >    of segments have to be retransmitted. In this
> case, based on
> >    (my interpretation) of RFC 793, only the last
> value of the
> >    urgent pointer is meaningful.
> >
> > 2. Let us say that user data gets queued up due to flow 
> control
> >    issues. Again this will cause the last value of
> the urgent
> >    pointer to take effect.
> >
> > the second part will have an impact in the slow start case. A
> > further analysis is required, but it seems that in cases where
> > packets are being dropped, the urgent pointer setting will not
> > be the most optimal (just about when it is needed the most).
> > The urgent pointer proposal may have a benefit when you are
> > loosing exactly one packet per window size (maybe even two
> > with SACK).
> >
> > On the receive side, again the same thing happens when 
> multiple
> > segments with urg pointer are received. Since it is a
> connection-
> > wide pointer, only the last one should be provided to the 
> user.
> >
> > If the data passes through "intermediate TCP devices" then
> again
> > the urgent pointer setting will be again be suboptimal.
> >
> > I would urge some TCP expert to jump in here.
> >
> > Somesh