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

Culley, Paul wrote:

> A couple of points are included below...
> 
> Paul R. Culley
> HP Fellow
> 281-514-5543
> 
> 
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Behalf Of
> Michael C. Cambria
> Sent: Friday, January 16, 2004 8:39 AM
> To: RDDP
> Cc: Barry Reinhold
> Subject: Re: [rddp] 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_.
> [prc] I suspect some misunderstanding here.  An MSN (Message Sequence
> Number)
> specifically identifies which _message_ on the particular Queue the
> segment
> belongs to.  Think of a queue with 5 2000 byte buffers, If TCP can
> manage 
> 1500 bytes or so per segment, each buffer can hold two segments.  The
> two segments for the second buffer might be identified by QN=1, MSN=2,
> MO=0, L=1500 and QN=1, MSN=2, MO=1500, L=500.  The two segments have the
> same MSN, so your statement is not correct.
> [prc]

I was confused.  I understand what you describe above, but read more 
into it (e.g. applying to uniquely identify a DDP segment as well). 
Until know, I didn't think overlaps were possible.

Thanks,
MikeC


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

This should have been "the way the draft reads".  My reading (which is 
incorrect) ruled out overlaps.

Thanks,
MikeC