[ih] Loss as a congestion signal [internet-history Digest, Vol 84, Issue 4]

[Noel Chiappa]

    > From: Brian E Carpenter <brian.e.carpenter at gmail.com>

    > it's surely the case that actual bit destruction causing non-congestive
    > packet loss was a much bigger worry in the 1970s than it was ten years
    > later?

I don't recall us worrying about damaged packets, to be honest. If they
happened, they were re-transmitted, and you just didn't notice.

The one exception I can remember is when a _particular packet_ could not be
sent along the CHAOSNET (think Ethernet, but over heavy CATV cable) at MIT.
Every time the source tried to send it, it got damaged. We only noticed
because the email queue (it was from a piece of email) to that machine got
wedged! :-)

Other than that, errors were rare enough, even back then, that they just
weren't an issue.


    > when actual packet loss became a significant factor with the rise of
    > wireless networks some years ago, it proved that treating it mainly as a
    > congestion signal was (and is) problematic. If you have a path that
    > includes both loss-prone and congestion-prone segments, TCP doesn't work
    > so well.

Which is a good part of why I lament the loss of SQ!

The commonly-heard reason for getting rid of it ('It increases congestion')
doesn't make sense to me, because unless something's unusual about the path
(and return path) between the source and the congestion point, that section of
the path is by definition un-congested (since the user's packet made it to the
congestion point OK). Maybe I'm missing something?

And there are several good reasons to like SQ: First, and quite importantly,
as you point out, it's an un-ambiguous congestion signal.

Second, it's a slightly/somewhat faster congestion signal (since it's only the
actual RTT from the source to the congestion point, not the end-end RTT plus a
fudge factor for variability plus (potentially) wakeup clock
delay/quantization/etc.  My memory of control theory is dim, but I seem to
remember that faster feedbacks are always better (although the response has to
be suitably damped, of course). Although it's probably a second-order effect
compared to the first one.

	 Noel