Subject: Re: [rddp] Clarification on DDP segmentation

From: "Talpey, Thomas" <[email-masked]>
Date: Thu, 10 Oct 2002 15:47:21 -0700

The draft is unambiguous on the issue that counts - that the L bit is
set only on the final segment, where final is defined by the transport's
ordering. DDP implicitly depends on this ordering to "deliver" its
messages to RDMAP - it has no ordering of its own. So, it requires
that the L bit is injected last, but it actually doesn't matter what order
the other segments have. TCP semantics will guarantee that all their
payloads reach remote memory before the L bit causes signalling.

Certainly, most implementations will choose to inject segments
in a monotonically increasing sequence. This is an implementation
decision though, not required by the protocol.

Tom.

At 12:35 PM 10/10/2002, Larsen, Roy K wrote:
>The draft is ambiguous as to whether the DDP layer may send DDP message
>segments out of order to the LLP.  The draft does explicitly deal with OOO
>segments on receive but only as it applies to a "rototilled" implementation
>with the implication being that reordering has been done by the fabric.
>However, in describing the Last Flag bit in section 6.1, the sentence:
>
>"The DDP Segment with the L bit set to 1 MUST be posted to the LLP after all
>other DDP Segments...."
>
>This implies that DDP may send segments OOO to the LLP.  In any case, I
>would like to see one of the following statements added to section 7.2:
>
>"The Data Source MUST deliver segments of a DDP message to the LLP in
>order."
>
>or
>
>"The Data Source MAY deliver segments of a DDP message to the LLP in any
>order with the exception of the DDP segment with the L bit set which MUST be
>the last segment sent to the LLP."
>
>Non-rototilled implementations (particularly hybrid RNIC solutions) can take
>advantage of an in-order DDP segment semantic so it is important that the
>draft be clear on this subject.
>
>Roy
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