Subject: [Ips] Unlikely scenario case about SNACK retransmission

From: William Studenmund <[email-masked]>
Date: Sun, 11 Sep 2005 18:13:08 -0700

Say the target is in the process of retransmitting PDUs in response to 
a SNACK request. Say for the sake of discussion, it "takes a while"; 
the time to retransmit is long compared to other things in the session. 
Now say that while this is happening, the initiator adjusts its 
MaxRecvDataSegmentLength. What should the target do?

My main interest is in the case where MaxRecvDataSegmentLength 
decreases, causing the sent PDUs to be smaller than before. Thus the 
target can't simultaneously retransmit the PDUs as before and honor 
MaxRecvDataSegmentLength.

I'd however like to come up with something consistent for the 
MaxRecvDataSegmentLength increases case as we only discuss 
resegmentation in other parts of the spec, not resegmentation to 
smaller PDUs.

Thoughts on what to do:

1) Consider retransmission to be atomic, and just send the PDUs 
as-sent. The thought here is that the retransmission code doesn't 
"notice" the resegmentation while it's in the middle of sending.

2) Stop retransmitting. Reasonable, but I'm not sure how to inform the 
initiator why we stopped. Reject the SNACK Request PDU even though we 
started responding to it?

3) Abort the task with a "target out of resources" type error; 
something where the initiator knows it can retry the command. This 
option seems rather drastic, but it is a good "get out of the corner 
we're painted in" type option.

Take care,

Bill
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From: Julian Satran <[email-masked]>
Date: Mon, 12 Sep 2005 11:36:09 +0300

This scenario has been taken care in the text of RFC3720.
The retransmitted data PDUs do not have have to be the same as the 
original ones and the new length should be used by the target.
The only think that is "forbidden" is to ask for data that is prior to the 
last ack point.

Julo

William Studenmund <[email-masked]> 
Sent by: [email-masked]
12-09-05 04:13

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Subject
[Ips] Unlikely scenario case about SNACK retransmission

Thoughts on what to do:

Take care,

Bill

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From: William Studenmund <[email-masked]>
Date: Mon, 12 Sep 2005 10:17:57 -0700

On Sep 12, 2005, at 1:36 AM, Julian Satran wrote:

> The only think that is "forbidden" is to ask for data that is prior to 
> the last ack point.

Oh. Now I see what you mean. Looking in section 10.16, I see:

    The numbered Data-In PDUs, requested by a Data SNACK MUST be
    delivered as exact replicas of the ones that the target transmitted
    originally except for the fields ExpCmdSN and MaxCmdSN, which MUST
    carry the current values and except for resegmentation (see Section
    10.16.3 Resegmentation).

which I now take as meaning that if we are in a Data SNACK and notice 
resegmentation, we are free to change the size of the PDUs.

If I am correct in this interpretation, I think we may want a comment 
about this as "and except for resegmentation." is not clear grammar.

Take care,

Bill