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

["GUPTA,SOMESH (HP-Cupertino,ex1)" <somesh_gupta@am.exch.hp.com>]

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