tcp: allow dctcp alpha to drop to zero
If alpha is strictly reduced by alpha >> dctcp_shift_g and if alpha is less than 1 << dctcp_shift_g, then alpha may never reach zero. For example, given shift_g=4 and alpha=15, alpha >> dctcp_shift_g yields 0 and alpha remains 15. The effect isn't noticeable in this case below cwnd=137, but could gradually drive uncongested flows with leftover alpha down to cwnd=137. A larger dctcp_shift_g would have a greater effect. This change causes alpha=15 to drop to 0 instead of being decrementing by 1 as it would when alpha=16. However, it requires one less conditional to implement since it doesn't have to guard against subtracting 1 from 0U. A decay of 15 is not unreasonable since an equal or greater amount occurs at alpha >= 240. Signed-off-by: Andrew G. Shewmaker <agshew@gmail.com> Acked-by: Florian Westphal <fw@strlen.de> Signed-off-by: David S. Miller <davem@davemloft.net>
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