Re: [lwip-devel] [bug #50837] TCP: zero window probe doesn't timeout

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Joel Cunningham wrote:
> The idea of performing the split upon the first fire of the persist 
> timer seems a reasonable compromise

The downside is that for this to work, we'd have to start the persist 
timer before receiving a zero window. Which is what we do now already 
but which is a bit off the standard. Sending a split segment then seems 
to me like only solving cases where otherwise the connection would get 
stuck. You see I tend to shift positions here, but as arguments come to 
mind...

> I would imagine most of the time, the receiver would have a window 
> update before the timer fires, so the split doesn't end up happening muc

That was my initial thought, exactly.

> What level of optimization would it take to be ok always doing the split? Is it 
> copying the remainder payload that's the problem or more generally just doing 
> re-segmentation?

I guess it's copying the data. You could argue that we could just 
reference the remainder (much like the ip-frag code does) and wouldn't 
have to copy, but then most zero-copy MACs have to memcpy anyway due to 
bad alignment (I assume most DMAs nowadays have at least a 4-byte 
alignment requirement so statistically, a 3:1 chance that such a 
scatter-gather MAC has to memcpy).

This is all feeling however. I don't have numbers of what exactly this 
would mean.

Given all this, I still think our current approach is the best. Splittin 
segments when the persist timer expires would only mean more code. 
Unless the complete window is < 4*MSS, I don't expect problems here...


Simon