RE: Error code returns for DDP tagged buffer messages

Hemal,

Your responses seem to depend on assumptions about implementation.

Depending implementation, the validity check may allow the implementation to see that the STag is valid but not associated with this stream or it may not see anything that distinguishes the two cases. Also, if the peer has tried to access an STag it shouldn't have access to, I can't think of any reason to tell the peer that the STag is possibly valid on another stream. Doing so doesn't convey any useful information to an honest peer and it potentially reveals informtion to an attacker.

Therefore, STag not associated with DDP Stream should be returned.

On the MSN errors, your answer presumes there is a fixed queue size which there may or may not be. There also isn't a value in distinguishing between these two errors. A message arrived and there was no buffer.

Also, nothing in the draft makes these distinctions between the errors. In each case the distinction is not particularly useful and I think we should consolidate to one error for STag not valid on the stream and one error for no buffer for the MSN.

Pat

-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Shah,
Hemal
Sent: Monday, April 26, 2004 12:25 PM
To: Barry Reinhold; RDDP
Subject: RE: [rddp] Error code returns for DDP tagged buffer messages


Barry,

See my comments below embedded between <hvs> and </hvs>.

Hemal

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Barry Reinhold
Sent: Friday, April 23, 2004 12:22 PM
To: RDDP
Subject: [rddp] Error code returns for DDP tagged buffer messages

Clause 9.2 DDP Error Numbers provides the error codes "Invalid STag" and
"STag not associated with DDP Stream", but the language of the first
paragraph is not clear enough for me to understand the expectation for
the following:

<hvs>
For an "Invalid STag" memory access is not allowed at all. "STag not
associated with DDP Stream" means that the STag is valid but is not
associated with the DDP Stream on which the DDP Segment was received. In
other words, memory access using the STag is not allowed on this DDP
Stream. 
</hvs>

Is it the intent to require that a receiver delineate between "Invalid
STag" and "STag not associated with DDP Stream"? That is, if STag 10 was
valid on stream A but not on stream B, MUST the receiver return "STag
not associated with DDP Stream?" if a DDP Segment is received on stream
B with STag 10, or is "Invalid STag" acceptable.
<hvs> No. The receiver MUST return "STag not associated with DDP Stream"
in this case. </hvs>

This question also applies to delineating between "Invalid MSN - no
buffer available", and "Invalid MSN - MSN range is not valid". However,
this is a bit more difficult conceptually as Clause 9.1, item 5 states:

       The legal range is defined as being between the MSN value
assigned to
       the first available buffer for a specific QN and the MSN value 
       assigned to the last available buffer for a specific QN.

Which appears to tie "invalid MSN pretty closely to no buffer available.
<hvs>
Let us say QN has a queue length QLEN. So, at any given point [MSN1,
MSN2] represents the MSN range of the entries of the queue. Here,
MSN2-MSN1 = QLEN-1 (in modulo arithmetic). Let us say the number of
buffers posted on the queue at any given time is N (0 <= N <= QLEN).

So, for N > 0, if a MSN is not in [MSN1,MSN1+N-1] and MSN1 <= MSN <=
MSN2, then it is the condition "Invalid MSN - no buffer available". For
N=0, if MSN is in [MSN1, MSN2], then it is the condition ""Invalid MSN -
no buffer available".

If MSN is not in [MSN1,MSN2], then it is "Invalid MSN - MSN range not
valid" condition.
</hvs>  


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




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