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

Barry

I believe the only ordering constraint we have in the draft is that the segment with the L bit set must have the highest message offset. We did that because we didn't want the require the receiver to scoreboard and it is therefore necessary for correct operation. It allows the receiver to determine total message length to report based on the information in the last segment.

We have no provisions in the untagged message passing for messages with holes. So, the message sequence you presented would be an incorrect sequence because of the gaps. If you added segments or changed offsets and lengths to fill the gaps, it would be a valid sequence. As it is, the gaps may be detected as an error by some receivers.

It is possible that the draft could use a clearer statement that there should be no holes in a message.

Pat

-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Barry
Reinhold
Sent: Thursday, January 15, 2004 1:37 PM
To: Hemal Shah
Cc: RDDP
Subject: [rddp] Some questions related to "last segment" of a ddp
message


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.

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

I am also a bit confused by the use of the term "last" in the draft.

If the above is considered a valid sequence it is unclear to me how the
receiver can meet the requirements of knowing that all DDP Segments of
the DDP message have been placed before it "delivers" the DDP message to
the ULP. (page 14 under L - Last flag, first bullet)

Is there some method other than the L bit that indicates that all
segments of a DDP message have been placed? (i.e. do implementations
need to keep a scoreboard?) The draft (in clause 7.4, first *, on page
20) references a requirement on the receiver to deliver only when the
last bit is set on the "last DDP Segment". How is the last segment
defined beyond the L bit?



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



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