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.
Unified · Split
| @@ -93,6 +93,7 @@ struct bictcp { · 92 unmodified lines | ||
| 93 | 93 | u32 ack_cnt; /* number of acks */ |
| 94 | 94 | u32 tcp_cwnd; /* estimated tcp cwnd */ |
| 95 | 95 | #define ACK_RATIO_SHIFT 4 |
| 96 | +#define ACK_RATIO_LIMIT (32u << ACK_RATIO_SHIFT) | |
| 96 | 97 | u16 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */ |
| 97 | 98 | u8 sample_cnt; /* number of samples to decide curr_rtt */ |
| 98 | 99 | u8 found; /* the exit point is found? */ |
| @@ -398,8 +399,12 @@ static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us) · 299 unmodified lines | ||
| 398 | 399 | u32 delay; |
| 399 | 400 | |
| 400 | 401 | if (icsk->icsk_ca_state == TCP_CA_Open) { |
| 401 | − cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT; | |
| 402 | − ca->delayed_ack += cnt; | |
| 402 | + u32 ratio = ca->delayed_ack; | |
| 403 | + | |
| 404 | + ratio -= ca->delayed_ack >> ACK_RATIO_SHIFT; | |
| 405 | + ratio += cnt; | |
| 406 | + | |
| 407 | + ca->delayed_ack = min(ratio, ACK_RATIO_LIMIT); | |
| 403 | 408 | } |
| 404 | 409 | |
| 405 | 410 | /* Some calls are for duplicates without timetamps */ |