[lwip-devel] [bug #20199] TCP appears to be "Shrinking the Window"

[Kieran Mansley]

Follow-up Comment #4, bug #20199 (project lwip):

Thank you Tom for your constructive comments.  I think sweating and shaking
because your window might shrink is perhaps getting a little over excited,
but then maybe I don't understand the problem!

In the existing code the reason it has gone unfixed for so long is because
this bug has very few side-effects.  The window shrinking is bad, don't get
me wrong, but there are two possible ways another stack might react:
 i) use the old value of the window (before we shrank it) and send anyway.  I
think in this case as it is only the advertised window we've shrunk, rather
than the real value, we'll still accept the data and so everything will be
OK.
 ii) use the new value of the window and not send a packet when potentially
it could.  This will hurt performance when it happens, but if you're about to
run out of window anyway, this isn't going to be a big factor.

Note that I'm not justifying the current code or putting an argument for not
fixing this, but trying to explain why this bug has remained for so long.

As to the suggested fix being a backwards step, it's hard to work out exactly
which MUSTs, SRONGLY_DISCOURAGEDs, etc, you're refering to given that you only
quote a year as reference, but assuming you mean that we'll go from swapping
the shrinking window problem for a silly window problem I wasn't suggesting
that both would be solved by this one change, which is why I mentioned that
in comment#1.  I personally prefer to isolate issues and fix them
independently.  Sometimes you have two dependent bugs such as in this case
where fixing one makes another worse.  That's OK as long as both are tracked
and fixed.

Finally, your email follow-up could be taken as something of an attack on the
lwIP project and current developers: to paraphrase (and you might not have
intended it like this) "your lwIP sucks and is worse than some student
project from 15 years ago, why haven't you fixed all the bugs yet?  I'm
amazed it can send traffic at all!".  Sadly lwIP does have bugs, and it would
have been nice if they had all been fixed long ago.  lwIP has a few active
developers who do their best to fix things when reported, often in spare
time.  This problem has never been reported before, despite the large
community using it, which is one of the reasons it's not been fixed. 
Personally I think the most constructive way to improve the code base, which
clearly you feel strongly would be good thing, is by doing things like
reporting problems, filing this bug report (thanks! and please file any more
you find) etc, rather than describing how bad you think things are.

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