Subject: RE: [rddp] silent dropping of TCP segment

From: [email-masked]
Date: Thu, 25 Mar 2004 11:08:26 -0700

The reason there is a MUST in the statement from 5.1.2 is that we want to ensure that once MPA validates a segment and passes it to DDP that it won't get that segment from TCP again. 

Committed meant a commitment to MPA and not an ACK. In DDP and RDMAP ordering rules, we assume that once the received data stream is complete to a given point that no placements from before that point will be made. 

The statement needs some clarification to allow an exception for the case where an MPA aware TCP learns that MPA dropped the segment due to a CRC error.

Regards,
Pat

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Barry
Reinhold
Sent: Thursday, March 25, 2004 8:22 AM
To: RDDP
Subject: [rddp] silent dropping of TCP segment

MPA clause 9, error semantics, states that an MPA aware TCP
implementation may silently drop a TCP segment when MPA detects a CRC
error. The goal of this TCP segment dropping would be to avoid forcing
the teardown of the stream by having TCP retransmit the segment.

However, clause 5.1.2 on page 11 requires that 

"The TCP layer MUST have committed to keeping each segment before it can
be passed to the MPA." 

I assume "committed" means that TCP ACKs the segment. Thus the segment
is ACKed before it is delivered to MPA. If this is the proper
understanding I don't see how "MPA" can silently drop a segment, since
by the time it can detect a CRC error TCP has already committed to it.

It is not clear to me why there is a "MUST" in clause 5.1.2. It would
seem desirable to allow MPA some control over when the ACK is to be
generated.

I would like to see the "MUST" removed from clause 5.1.2. 

Barry Reinhold
Lamprey Networks
[email-masked]
(603) 868-8411

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