Subject: [rddp] Untagged Messages requirements for unreliable transports

From: Caitlin Bestler <[email-masked]>
Date: Mon, 21 Oct 2002 18:21:45 -0500

While discussing the absolute minimal requirements for
correct ordering of untagged message segments it occurred to
me that there are additional requirements that would be
valid for DDP over an unreliable transport.

Unreliable mapping is not a top priority right now, but it
would probably be desirable to place any simple restrictions
on the reliable modes that will make later defintion of
unreliable DDP simpler -- especially if they do not really
impose any costs on the reliable versions.

Specifically, I believe the following rule can be added at
no real cost to reliable implementations which would greatly
benefit unreliable implementations:

    Each byte within an untagged message must be presented
    to the LLP as part of exactly one untagged segment.
    
The rationale:

With a reliable LLP, the DDP receiver knows that a message
has been completely received when the LLP-ack-point passes
the point where the DDP segment with the 'L' bit was set.
By the nature of a reliable transport, all prior segments
that are part of that message are known to have been
received. No scoreboarding of any kind is required.

With an unreliable LLP, the DDP receiver must have a way of
knowing that the complete message has been received. A very
simple method for doing so would be to count the bytes
placed for the message, and include a validating count for
the last segment (this assumes a definition of unreliable
LLP that delivers each DDP segment at most once).

So this requirement would:

- only restrict the Data Source from doing something stupid     
    (redundantly sending the same data).
- simplify the logic for an unreliable DDP.
- encourage greater compatability between mainstream
    DDP (reliable) and any future unreliable mode.

Caitlin Bestler
http://asomi.com/CaitlinBestler/
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