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authorAndrew Shewmaker <agshew@gmail.com>2015-10-18 21:59:08 -0700
committerDavid S. Miller <davem@davemloft.net>2015-10-23 02:46:52 -0700
commitc80dbe04612986fd6104b4a1be21681b113b5ac9 (patch)
treea0ddb2c94d317b163b2c5a5dbff06cf6534d1f94 /net
parentab997ad408394bcaf7f3015d4c4e38047eaf2ad6 (diff)
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>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/tcp_dctcp.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 7092a61c4dc8..7e538f71f5fb 100644
--- a/net/ipv4/tcp_dctcp.c
+++ b/net/ipv4/tcp_dctcp.c
@@ -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.