Re: Some questions related to "last segment" of a ddp message

Barry Reinhold wrote:

> Hemal,
> 
> I would like to check on a couple of issues:
> 
> I believe that it is possible to RECEIVE a sequence of DDP segments
> using the untagged buffer model as follows: Let me know if this is not
> correct.
> 
> Segment 1: QN=1, MSN=1, MO=200, L=0 (payload 100) (MO not 0 on first)
> Segment 2: QN=1, MSN=1, MO=200, L=0 (payload 100) (duplicate)
> Segment 3: QN=1, MSN=1, MO=100, L=0 (payload 100) (backward)
> Segment 4: QN=1, MSN=1, MO=000, L=0 (payload 050) (meets 6.3, MO field)
> Segment 5: QN=1, MSN=1, MO=400, L=1 (payload 000) (meets 6.1 L bit
> field)
> 
> In particular, 
> 1. If no packets have been received w/ a given MSN and QN and a packet
> arrives with a non-zero MO this is not an error.
> 2. The MO fields in intermediate segments are not constrained to be in
> order, can have gaps, and can write over buffer space already written
> to.

My understanding is yes, yes and (re write over buffer space) not convinced.

On pg 20 of draft-ietf-rddp-ddp-01.txt, "For an Untagged DDP Message, 
the combination of the QN and MSN uniquely identifies a DDP Message." 
Since an MSN doesn't make sense for a DDP _message_, I've read this to 
mean that QN & MSN uniquely identifies a DDP _segment_.

So the way I read the draft, I can't find text that says one cannot send 
overlapping DDP segments.

> Or from a test perspective, if a receiver returned an error when
> receiving the above sequence the receiver would be considered
> non-conformant.

I'd return an error for several reasons.

1) I know of no legal untagged message on QN=1 that has a payload as 
large as you describe.

2) Where are payload bytes 50-99?  300-399?

If I understand the example correctly, segment 4 provides DDP/RDMA 
message bytes 0-49; segment 3 provides 100-199, segments 1 & 2 provide 
200-300

MikeC