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tcp_cubic: limit delayed_ack ratio to prevent divide error 

TCP Cubic keeps a metric that estimates the amount of delayed
acknowledgements to use in adjusting the window. If an abnormally
large number of packets are acknowledged at once, then the update
could wrap and reach zero. This kind of ACK could only
happen when there was a large window and huge number of
ACK's were lost.

This patch limits the value of delayed ack ratio. The choice of 32
is just a conservative value since normally it should be range of
1 to 4 packets.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
stephen hemminger 2011-05-04 10:04:56 +00:00 committed by David S. Miller
parent 9c412942a0
commit b9f47a3aae
1 changed files with 7 additions and 2 deletions
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9
net/ipv4/tcp_cubic.c
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@ -93,6 +93,7 @@ struct bictcp {
u32 ack_cnt; /* number of acks */ u32 ack_cnt; /* number of acks */
u32 tcp_cwnd; /* estimated tcp cwnd */ u32 tcp_cwnd; /* estimated tcp cwnd */
#define ACK_RATIO_SHIFT 4 #define ACK_RATIO_SHIFT 4
#define ACK_RATIO_LIMIT (32u << ACK_RATIO_SHIFT)
u16 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */ u16 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */
u8 sample_cnt; /* number of samples to decide curr_rtt */ u8 sample_cnt; /* number of samples to decide curr_rtt */
u8 found; /* the exit point is found? */ u8 found; /* the exit point is found? */
@ -398,8 +399,12 @@ static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
u32 delay; u32 delay;
if (icsk->icsk_ca_state == TCP_CA_Open) { if (icsk->icsk_ca_state == TCP_CA_Open) {
cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT; u32 ratio = ca->delayed_ack;
ca->delayed_ack += cnt;
ratio -= ca->delayed_ack >> ACK_RATIO_SHIFT;
ratio += cnt;
ca->delayed_ack = min(ratio, ACK_RATIO_LIMIT);
} }
/* Some calls are for duplicates without timetamps */ /* Some calls are for duplicates without timetamps */