Re: [SDLTF-Members] SDL-News: SAVE: Request for additional feedba ck

Subject: Re: [SDLTF-Members] SDL-News: SAVE: Request for additional feedba ck
From: Rick Reed TSE (
Date: Sat Jan 10 2004 - 12:25:36 GMT

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Frend Patrick-BPF005 at wrote on 01/09/04 17:22:

> A very common usage of this would be for a protocol
> that handles re-transmission of failed transmissions for
> a reliable protocol.
> If a flush were part of the language, combined with save,
> this would be very simple.
> Each time a signal is send over the protocol, it is also sent
> to a re-transmission process, where the signal is saved.
> If the ack is received, a signal is sent to this re-transmission
> process, which then flushes it's input queue atomically. Else
> after a timer, it processes the saved signals, for re-transmission.
> Yes, this CAN be handled by the process queueing stuff internally,
> I agree. The same goes for SAVE. The question should be is this a sensible
> thing for the programmer to be thinking about. I.E. thinking about handling
> queues rather than the problem domain.

Dear All,

While this initially appears to be a valid case, I do not think it would
actually work.

Assume the sent signals are S(1), S(2), S(3), s(4).

If an Ack is received, there must be some mechanism in the protocol to know
how many signals are being acknowledged.

If an Ack is sent for each signal sent, then it is just necessary to remove
one stored signal per Ack. Similarly if an Ack is sent for every n signals,
n signals are removed. Both of these functions can be done simply without
any addition to SDL.

If the Ack covers an indeterminate number of signals, there would be no way
to know how many signals to remove. In practice it seems there would need to
be some additional mechanism such as sequence numbering and the signals
would need to be filtered on this basis and the content of the signals would
need to be examined.

Rick Reed -
Tel:+44 15394 88462 Mob.:+44 7970 50 96 50

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