Commit c80dbe04 authored by Andrew Shewmaker's avatar Andrew Shewmaker Committed by David S. Miller
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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: default avatarAndrew G. Shewmaker <>
Acked-by: default avatarFlorian Westphal <>
Signed-off-by: default avatarDavid S. Miller <>
parent ab997ad4
......@@ -209,7 +209,7 @@ static void dctcp_update_alpha(struct sock *sk, u32 flags)
/* alpha = (1 - g) * alpha + g * F */
alpha -= alpha >> dctcp_shift_g;
alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
if (bytes_ecn) {
/* If dctcp_shift_g == 1, a 32bit value would overflow
* after 8 Mbytes.
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