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Re: Persist Mode -- ** Standards bug ??

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  • Charles Esson
    I ve not implemented a TCP stack that uses the scale option ( performance over satellite links is not my problem), so I had not thought about this. I have now,
    Message 1 of 2 , Jul 5, 2001
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      I've not implemented a TCP stack that uses the scale option ( performance over
      satellite links is not my problem), so I had not thought about this.

      I have now, and I can see your point.

      For the last 7 bytes the receiver is advertising a right window position past it's
      actual right position.

      It would not be a problem if the window was calculated. n*scale - ( scale-1). I
      would argue that if that isn't the formula for the window then 1323 has a serious
      bug as it breaks some fundamentals.

      Regards Charles

      Matt Mathis wrote:

      > I think we have conflicting standards.... although I agree with your
      > recollection, I believe that strict "never retract the window" can not be
      > implemented. Consider the following scenario:
      >
      > A connection negotiates TCP window scale (e.g. 3, so the window size is
      > quantized in steps of 8 bytes).
      > After running for awhile with a reasonable window, the receiver stops
      > consuming data.
      > The sender continues to send data, 1 byte at a time, progressively filling
      > the window.
      > Under these conditions the right edge of the window can not be maintained at a
      > constant position in the sequence space. Following 7 consecutive data packets
      > it will advance by one byte. On the 8th, it will retract by 7 bytes.
      > (Alternatively, if the TCP rounds windows to MSS, the steps are MSS sized).
      >
      > Furthermore when the window finally closes all the way, it will always be due
      > to a window retraction.
      >
      > If the TCP doesn't behave this way it will violate some other part of RFC1323
      > or the base specifications. Therefore every TCP that implements 1323 violates
      > rule 1 under some conditions.
      >
      > Furthermore any TCP requires strict adherence to rule 1 is broken.
      >
      > I believe that this is "well tested" in todays Internet (it happens all the
      > time) but nobody has been looking for symptoms.
      >
      > Now a question for the readers: what might happen if this rule was formally
      > retracted or amended?
      >
      > Thanks,
      > --MM--
      >
    • Matt Mathis
      ... I beleive all implementation are careful to do the rounding such that they never announce space beyond the end of the buffer. Im my example the last
      Message 2 of 2 , Jul 6, 2001
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        On Fri, 6 Jul 2001, Charles Esson wrote:

        > I've not implemented a TCP stack that uses the scale option ( performance over
        > satellite links is not my problem), so I had not thought about this.
        >
        > I have now, and I can see your point.
        >
        > For the last 7 bytes the receiver is advertising a right window position past
        > it's actual right position.
        >
        > It would not be a problem if the window was calculated. n*scale - ( scale-1). I
        > would argue that if that isn't the formula for the window then 1323 has a
        > serious bug as it breaks some fundamentals.

        I beleive all implementation are careful to do the rounding such that they
        never announce space beyond the end of the buffer. Im my example the last
        non-zero receiver window announcement would be when there are exactly 8 bytes
        of space left (and the window field would have the value 1). Following the
        next byte sent the announced window *must* drop to 0 even though there are
        still 7 bytes of space left. Otherwise the receiver will announce space beyond
        the end of the buffer.

        Thanks,
        --MM--
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